Seminars

Abstract: questionnaire DD&RSE : https://framaforms.org/avis-sur-le-schema-directeur-ddrse-pour-linstitut-langevin-1773657502

Abstract: TBA

Abstract: Dynamic wavefront shaping enables control of light propagation through complex scattering media, but its applicability to realistic outdoor dynamic environments remains limited. A central question is to determine under which conditions such control remains physically and practically relevant. In this talk, I will present recent experimental advances enabling wavefront shaping at sub-microsecond timescales, and examine the conditions under which this approach becomes effective in dynamic regimes. From a photometric standpoint, I will introduce the notion of a vanishing distance, defined as the point where diffuse and ballistic luminances become comparable. This provides a practical way to locate the transition between regimes dominated by ballistic transport and those where multiple scattering must be exploited. These considerations will be illustrated with experimental results demonstrating wavefront shaping at sub-microsecond timescales, where the correction operates close to the intrinsic temporal decorrelation of the medium under a constrained photon budget. Together, these results delineate the conditions under which dynamic wavefront shaping becomes practically relevant for imaging and optical communication through complex media.

Abstract: Optomechanics investigates the interactions between mechanical and electromagnetic degrees of freedom. Originally proposed as a plateform for investigating the fundamental limits in interferometric measurements, this concept has been thriving for the past 40 years, contributing to important milestones both in classical and quantum sensing. However to date, those fundamental limits have remained out of reach. In this presentation, we argue that this explains because of the combination of technologically-driven evolutions, and a major epistemological loophole, namely the miniaturization of optomechanical systems to the nanoscale, and the failure to take into account the multimode character of the optomechanical interaction. Using both estimation theory and wavefront shaping, we report a novel approach enabling the enhancement of the optomechanical sensitivity by several orders of magnitude compared to the state of the art.

Abstract: When at rest, granular materials are trapped in metastable configurations. However, when subjected to strong enough mechanical vibrations, they begin to flow and rearrange. These vibrofluidized granular systems exhibit unusual mechanical and rheological properties, as well as spontaneous pattern formation. First, I will present our experimental study of the self-assembly processes of granular binary mixtures vibrated on a substrate. Starting from a disordered configuration, and depending on the size ratio and proportions of the two grain species, we observed the formation of both quasi-crystalline structures [1] and periodic square crystals [2]. Remarkably, this follows predictions from the phase diagram of a colloidal system obtained through numerical simulations under the assumption of thermodynamic equilibrium. We have also established experimental conditions that allow for stable coexistence between the square crystal phase and a granular amorphous fluid. We demonstrated that the observed phenomena resemble those of a first-order phase transition. However, the two phases coexist at different effective temperatures. Unexpectedly, the denser crystalline phase is hotter than the less dense liquid phase. This phenomenon can be explained by the coupling between local structure and energy transfer mechanisms. In the final part of my talk, I will discuss another phenomenon observed in the same setup when considering more dilute systems. This involves a phase transition between absorbing states with no horizontal motion and active states with highly energetic horizontal motion. I will present some recent experimental findings based on previous theoretical and numerical results [3]. [1]: A. Plati, R. Maire, E. Fayen, F. Boulogne, F. Restagno, F.Smallenburg and G. Foffi, Nat. Phys. 20, 465–471 (2024) [2]: A. Plati, R. Maire, F. Boulogne, F. Restagno, F. Smallenburg and G. Foffi, J. Chem. Phys. 163, 054509 (2025) [3]: R. Maire, A. Plati, M. Stockinger, E. Trizac, F. Smallenburg and G. Foffi, Phys. Rev. Lett. 132, 238202 (2024)

Abstract: The manipulation of optical properties in materials often relies on ordered media like photonic crystals, metamaterials, or metasurfaces. Despite their promise for the design of optical properties, less attention has been paid to disordered media, such as randomly distributed nanoparticles. The talk will be split into 3 parts : 1) I will demonstrate a way to achieve spectral selectivity in random particulate media. The strategy is based on engineering the cooperative scattering between two kinds of particles, one of which is resonant and the other lossless. By relying on statistical parameters only — size distribution and volume fraction of the particles — the electromagnetic interferences between the two populations are adjusted so that near-perfect absorption emerges at a prescribed frequency. 2) I will present recent results regarding experimental realisations based on this approach. A substantial increase in the resonant peak displayed by a deposition of polar particles is observed when suitable lossless particles are incorporated into it. The critical points allowing the improvement of the device’s performance will be discussed. 3) More generally and beyond these theoretical and experimental achievements, light-matter interaction in random media may exhibit quite complex behaviours in resonant regimes. I will put the focus on the homogenisation of such configurations, and discuss it from the vantage points of Bergman’s theory and spectral representation of the effective dielectric function.

Abstract: In this talk, I will present my thesis work on the first observation of the thermal melting of a vortex lattice in a rotating quasi-2D superfluid. The presence of this melting scenario in 2D crystals is well explained by the theory of Kosterlitz, Thouless, Halperin, Nelson, and Young (KTHNY). According to the KTHNY theory, melting in 2D crystals occurs through two consecutive phase transitions: from solid to hexatic, and from hexatic to liquid. These two transitions are characterized by the thermal activation and unbinding of lattice defects. I will discuss the observation of the phase transition from hexatic to liquid as a function of increasing rotation rate in our experiment. In the end, I will also talk about the recent implementation of a new non-destructive imaging setup in our experiment, which will facilitate future studies of the in situ dynamics of a trapped Bose-Einstein condensate. Below is the link for the article about thermal melting in case you are curious to know more: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.143401

Abstract: few years ago we established a laboratory in the basement of the Facultad de Ciencias in Montevideo dedicated to the experimental study of wave propagation in granular media. While the project is inspired by geophysical phenomena on granular asteroids, the core research focuses on the fundamental mechanical behavior of these complex materials. The need for this research was highlighted by events like NASA's DART mission, where an impact triggered not just a direct ejecta plume, but also a prolonged, slow tail of material. This suggests that seismic waves propagating through the asteroid's granular interior played a key role in redistributing momentum and ejecting material from the surface. To accurately model such events, we require robust experimental data on how waves travel through and interact with granular matter. This presentation is structured in two parts: Wave Speed under Confinement: I will present an experiment using glass beads to measure the speed of acoustic waves as a function of confining pressure. This allows us to simulate conditions at different depths within a granular body. We then use the Discrete Element Method to numerically replicate these findings, creating a validated model. This model is crucial for extrapolating how seismic waves would propagate in the extremely low-pressure environment of an asteroid. Wave Phenomena in Soft Granular Media: I will discuss experiments using soft, rubber grains. In these materials, mechanical perturbations propagate slowly, allowing us to observe fascinating dynamics—such as the ejection of surface material by subsurface waves—using high-speed cameras at manageable sampling rates. Furthermore, we are developing ultrasound imaging techniques to non-invasively monitor fast wave propagation phenomena within the volume of a granular sample.

Abstract: Optical coherence tomography (OCT) is a technology invented in 1991 to image small critical tissue structures throughout the body with micrometer resolution. It is widely used in the management of eye and coronary heart diseases. OCT received broad attention when its inventors received the prestigious Lasker-DeBakey Clinic Medical Research Award and the National Medal of Technology and Innovation. For me, it was the culmination of 3 decades of work as an engineer, clinician, and translational researcher. I will present OCT from an inventor’s perspective. The physical principles will be explained with illustrations on measuring the time-of-flight of light with interferometry. I will tell the story of the aha moment when the idea of OCT came to my mind, as well as the rapid pace of development that made OCT a clinical reality. The biggest applications of OCT in the management of eye diseases will be shown. Recent advances made at OHSU that enable OCT to advance beyond the imaging of tissue structure to the detection of blood flow and photoreceptor function will be described. OCT is still a rapidly developing technology. The technical capabilities have improved in many aspects, but the most astounding has been the continual improvement in imaging speed, which has doubled approximately every 2.5 years over the past 3 decades. The technological advances have made more and more clinical applications feasible. I will present a vision for the broader applications of OCT, which includes imaging the eye to assess brain and cardiovascular diseases (“Oculomics”), as well as direct OCT imaging of other target organs such as the heart, skin, digestive tract, brain, ear, and teeth.

Abstract: Since the very beginning of medicine, tissue stiffness has been used to aid in diagnosis. Elastography, which enhances medical imaging with mechanical property information, therefore holds strong clinical potential. Here, we present the NCi (Noise Correlation Inspired) method along with our latest results in optical elastography.

Abstract: Scattering limits our ability to see inside biological tissue, as light penetration is severely distorted by tissue components with varying refractive indices. One promising method to overcome scattering aberration is wavefront shaping. This technique involves placing a spatial light modulator (SLM) in the microscope's optical path to correct the wavefront emitted from a point deep within the tissue. The goal is to bring light photons from a single target point to a single sensor point, despite tissue aberrations. Wavefront shaping is particularly beneficial for imaging weak biological fluorescent sources because the correction is performed optically rather than digitally. When modulation is successful, all light photons are combined into a single detector pixel, resulting in a higher signal-to-noise ratio (SNR). This technique has the potential to revolutionize tissue imaging by enabling high-SNR imaging deep within scattering biological targets. However, estimating wavefront-shaping modulations in practice is challenging, since the modulations must be estimated in real time, using non-invasive feedback, and under a low photon budget. In the first part of this talk, I will discuss efforts to derive noise-robust score functions that can identify effective modulation corrections using non-invasive feedback. I will review previous approaches and introduce a new, simple, noise-robust method that uses confocal correction of both incoming and outgoing light with linear single-photon fluorescent excitation. We show that modulations leading to high confocal intensity at the detector outside the tissue also focus light into a spot inside the tissue. Given a score function, estimating the desired modulation becomes an optimization problem. However, since the desired modulation depends on the unknown tissue structure, typical optimization strategies involve slow sequential scanning, where each modulation parameter is queried independently. In the second part of this talk, I will present a novel approach for rapid modulation optimization. This method leverages optical computing ideas and uses the optical system to directly measure the gradient of the score function, allowing simultaneous updates of all modulation parameters from a single measurement.

Abstract: Time-varying photonics have recently emerged as a bustling field of research in the nanophotonics community with promising applications such lasing, optical rerouting, analogue computing and the exploration of fundamental physics such as time reversal, Zel’dovich amplification or breaking the Rozanov bound. Through a set of experiments aiming at the implementation of time-varying media at optical frequencies, starting with all-optical pumping of Indium Tin Oxide, we will discuss how ultrafast electron dynamics enable the exploration of new physics beyond conventional nonlinear optics. We will then consider current challenges in the field and the exciting new materials and physics that could emerge as alternative platforms.

Abstract: It is key to understand how cells and tissues respond to mechanical stimuli. Such cues include the stiffness of the environment, notably involved in pathological morphogenesis, but also the forces and deformation that are exerted on the sample. At the heart of these processes is the ability of cells to convert mechanical stimuli into biochemical signals driven, in part, by their mechanical properties, i. e. stiffness and viscosity. A partial approach to this problem is to observe the biochemical cascade triggered by mechanical perturbation (either a force or a deformation) with biomolecular tools. If however one wants to further characterize the inherent mechanical properties of cells, it necessary to probe imposed force and the resulting deformation simultaneously. To do this, techniques mostly inherited from material sciences have been favoured, using microrheometers based on parallel plates or rotating beads for instance. In the subcellular realm, Atomic force microscopy (AFM) remains king due to its ability to image at a nanometer scale. At the nanoscale too, but until recently limited to solid-state physics, Brillouin light scattering (BLS) has emerged as a promising tool in mechanobiology. Since the first reports of its underlying physical principles in 1922, BLS has been developed and widely utilized in the context of material science, where it became a powerful and well-established technique for the investigation of condensed matter. It provides of the sound velocity and attenuation in the GHz frequency range, which can be converted into an elastic modulus as well. In biology, the recent introduction of confocal Brillouin microscopy based on a fully parallel spectrometer has allowed the imaging of single cells with a micrometer resolution. These developments stimulated the booming field of mechanobiology, with the promise of directly imaging viscoelastic properties of living matter in 3D and in a noncontact, label-free and high-resolution fashion. Because AFM and BLS share the ability to uncover mechanical material properties within living samples, and because BLS applications to life sciences remains in its infancy, it prompted a direct comparison. Since AFM data is usually interpreted as the response of the underlying elastic network of biopolymers, a similar conclusion was drawn for BLS. Subsequent works later pointed out that water content also plays a crucial role in the response of biorelevant hydrogels, and that the coordinated response of biopolymer network and intracellular fluids could lead to intricate non-linear acoustic behaviour in multicellular tissues. To elucidate these nuances, we designed an experiment in which we vary the water content in cells, and identify a more complex behaviour.

Abstract: Ultrasound tomography has potential across many areas of NDE, including corrosion mapping and materials characterisation. Traditional methods have transplanted ideas from radiographic CT into ultrasound, missing refraction and diffraction present and hence heavily restricting the quality of the resulting image. This talk will highlight the importance of correctly capturing the physics, incorporating refraction, diffraction and other phenomena in order to generate high resolution, accurate images. This will be applied to guided wave tomography for thickness mapping, as well as some illustrations from breast ultrasound tomography.

Abstract: Anderson localization is a ubiquitous wave phenomenon in which diffusive transport in disordered systems is suppressed due to destructive interference. For electromagnetic waves, it has already been observed in lower-dimensional systems, but its experimental confirmation in 3D is still elusive. This scenario drove the search for alternative photonic media as well as investigations on the obstacles to achieving light localization. In this seminar, I will present contributions to both of these challenges. In the first part, I will discuss an approach to localization with aperiodic deterministic media. In particular, I will present our numerical results for Vogel spirals, whose predictions of localized states were recently confirmed in a microwave setup. In the second part, I will revisit the problem of localization in 3D disordered media and address the existence of a fractal phase for electromagnetic waves.

Abstract: A seemingly widely accepted conjecture in the physical literature states that classical wave-fields propagating in random media over large distances eventually follow a complex circular Gaussian distribution. In this limit, the wave intensity becomes exponentially distributed, which corroborates the speckle patterns of, e.g., laser light observed in experiments. This talk reports on recent results settling the conjecture in the weak-coupling, paraxial regime of wave propagation, which is accurate in the application of laser light propagation in turbulent atmospheres. The limiting macroscopic Gaussian wave-field is fully characterized by a correlation function that satisfies an unusual diffusion equation. The derivation of the limiting model is first obtained in the Itô-Schrödinger regime, where the random medium is replaced by its white noise limit. The resulting stochastic PDE has the main advantage that finite dimensional statistical moments of the wave-field satisfy closed form equations. The proof of the derivation of the macroscopic model is based on showing that these moment solutions are asymptotically those of the Gaussian limit, on obtaining a stochastic continuity (and tightness) result, and on establishing that moments in the paraxial and the Itô-Schrödinger regimes are asymptotically close.

Abstract: Additive manufacturing, and in particular the Laser Powder Bed Fusion process, allows the elaboration of geometrically complex parts by successively melting thin layers of metal powder, with applications in various industrial domains. A limitation for the development of the technology is the need to ensure that the produced parts are dense and defect free, which motivates a number of research efforts for the set-up of in situ diagnostic tools. Since the evolution of a part internal temperature field is a key point in the mechanisms of powder melting, part solidification and microstructure formation, real-time monitoring of this temperature field could contribute to this quality assessment of the process. However, the estimation of this field in LPBF machines is still a complex challenge from an experimental perspective. Infrared sensors and thermocouples can be installed in a machine to measure temperature on a point or surface basis, but they are not able to estimate the complete internal field of a part during fabrication. On the other hand, the study of elastic wave propagation has led to the development of thermometry techniques in a wide variety of fields. In this respect, the objective of this study is to develop a technique based on an ultrasonic time of flight measurement to estimate the temperature field within a manufactured part. The first aspect of this work is experimental and consists in carrying out initial pulse-echo time of flight measurements on parts being manufactured. Temperatures on the part surface are measured in parallel, in order to correlate the observations on times of flight and temperatures. Analysis of machine-recorded time signals shows that, while the times of flight are governed by geometry changes at the scale of the full fabrication, they are sensitive to temperature variations at the time scale of a single melting-solidification cycle. A numerical second part focuses on the study of the evolution of the time of flight as a function of the temperature field. A thermal and ultrasonic numerical model, allowing to account for the main physical phenomena governing heat transfer within the parts (laser absorptivity, exchange coefficients with atmosphere and below fabrication plate) has been proposed. The agreement between the simulated temperature field and experimental data supports the potential of time of flight measurement for in situ monitoring of the temperature field in LPBF machines.

Abstract: Optical imaging of biological tissues is currently limited by the strong scattering of light. However, the use of auxiliary ultrasonic waves for imaging allows overcoming this limitation and imaging in depth. However, there are several limitations to this so-called "acousto-optic" method. When so-called "classical" light is used for imaging, the limit is then the photon noise, a limit that is also found in the world's most powerful optical detectors, such as interferometers used for gravitational wave detection. Therefore, in this presentation, I will discuss the possibilities that arise when "non-classical" light is used to improve the signal-to-noise ratio of a measurement, whether it be in medical imaging or gravitational wave detection. Quantum noise is one of the limitations to strain sensitivity in laser interferometric Gravitational Wave Detectors (GWDs). Recently, LIGO has demonstrated a scheme involving the injection of frequency dependent squeezing to reduce both the influence of shot noise and quantum back-action in a wide frequency bandwidth. In 2018, E. Polzik and F.Ya. Khalili proposed to resort to an atomic spin ensemble to perform a similar noise reduction measurement. In this presentation, I will describe this approach by going over the physics and formalism of Cesium atomic spin ensemble and see how the noise evading measurement can be performed.

Abstract: I will describe a construction of a reduced order model from wave scattering data collected by an array of sensors. The construction is based on interpreting the wave propagation as a dynamical system that is to be learned from the data. The states of the dynamical system are the snapshots of the wave at discrete time intervals. We only know these snapshots at the sensors in the array. The reduced order model is a Galerkin projection of the dynamical system that can be calculated from such knowledge. I will describe the construction and some properties of the reduced order model and I will show how it can be used for solving inverse wave scattering problems. Joint work with Josselin Garnier, Alexander Mamonov and Joern Zimmerling

Abstract: Computed Ultrasound Tomography in Echo Mode (CUTE) maps the tissue’s speed-of-sound (SoS) using conventional handheld echo ultrasound (US) probes. The technique consists of a sequence of steps that can be performed in real time: beamforming of US images under varying transmit- and receive steering angles, detection of echo shift between different angle combinations, SoS inversion from the echo shift data. After a short recap of the basic principles and the recent clinical results, this talk will focus on discussing critical aspects of CUTE: (i) the role of regularization for quantitative imaging: Despite the ill-posedness of the SoS inversion, quantitative imaging is in principle feasible if the pseudo-inverse is developed from the perspective of a machine learning problem. (ii) the challenge of imaging real tissue: Wave aberrations at short scale heterogeneities of SoS degrade the echo shift data, leading to artifacts and – more importantly – reduced quantitative accuracy. (iii) view on deep learning based SoS inversion. This may be a clever alternative to a full-wave inversion with the goal to account for aberration effects, however, care needs to be taken to appropriately design the training data. (List of co-authors: Michael Jaeger, Naiara Korta Martiartu, Parisa Salemi, Jules Bloom)

Abstract: Dynamic elastography is an imaging method to measure the elasticity of biological tissues in a non-invasive and quantitative way. Recently, the transposition of the technique to a small scale has been called dynamic microelastography and has allowed the first measurements of cellular elasticity by shear waves using an optical microscope. My thesis aimed at understanding the limits of this technique and to develop new micro-elastography methods, to test new wave sources but also potential applications of the technique. In a first step, the dispersion of shear waves was studied on gelatin phantoms. Two distinct regimes of guided elastic waves and shear waves were identified. The high-frequency limit of wave propagation was also explored, establishing the existence of a cutoff frequency which explains the absence of ultrasonic shear imaging. The same approach was then applied to visco-elastic fluids, revealing two cutoff frequencies and revisiting previous studies on rheology and wave propagation in this type of medium. Then, the initial objective being to carry out micro-elastography on single cells and the experiments previously carried out with micro-pipettes presenting certain defects, an original method of cellular micro-elastography was developed. An oscillating microbubble is used as a contactless shear wave source at 15 kHz to perform experiments on blood cells whose diameter is about 15 μm. These are the smallest objects ever explored by elastography. Larger objects, cell clusters of a few tens of thousands of cells have also been studied. Indeed, since ultrasound elastography of these tumour models of about 800 μm in diameter is impossible, optical micro-elastography is a suitable technique. These samples contain magnetic nanoparticles, so a magnetic pulse could be used as a wave source. Previously, proofs of concept on both macroscopic (in ultrasonic elastography) and microscopic (in optical micro-elastography) phantoms were conducted to validate the use of this diffuse field source. Finally, pulse wave measurements were performed on retinal arteries of about 50 μm in diameter using laser Doppler holography acquisitions performed in vivo. The application of monochromatic correlation algorithms allowed the measurement of guided wave velocities, finally revealing the existence of a second pulse wave, an antisymmetric bending wave. This guided wave, much slower than the axisymmetric pulse wave studied so far, was also observed on the carotid artery thanks to ultrafast ultrasound acquisitions.

Abstract: The early detection and characterisation of damage such as cracks, disbonds or bonding defects (particularly adhesive defects) in a bonded assembly is of paramount importance in ensuring the safety of structures or installations. Non-linear ultrasound has aroused considerable interest in the detection and non-destructive evaluation (NDT) of these types of defects, which are deemed undetectable by conventional linear ultrasound methods. Ultrasonic waves are affected by the non-linearity of materials or by contact non-linearity. Several non-linear acoustic phenomena can be observed when elastic waves interact with contact interfaces: DC effect, generation of harmonics, sub-harmonics, modulation and mixing of waves, hysteresis effects, etc. These non-classical effects result from complex contact dynamics localised at the interface, known as contact acoustic non-linearity. This seminar offers a synthesis of the work I have done on this subject: involving a numerical study of these non-linear effects on the non-linear observable (i.e. the second harmonic generated from a monochromatic excitation) is proposed. The behaviour of a contact interface may involve various phenomena that are not always easy to model and observe experimentally. Also, in this approach, I tackle the problem on a simple system where the only non-linearity is included in the contact interface, taking into account contact effects (clapping without adhesion and roughness, adhesion, roughness). Based on this understanding, I will then present an experimental analysis of a crack with contact non-linearity. Finally, this presentation will conclude with an overview of work in progress on the linear and non-linear analysis of local fault resonance (LDR).

Abstract: Since the theory of quantum mechanics was established, it has been confirmed by many experimental demonstrations. However, decoherence limits the observation of quantum dynamics to extremely isolated systems and thus to small systems (i.e., atoms). To increase the size of a quantum system, it would therefore be necessary to use experimental platforms that allow unprecedented isolation from the environment. Such a platform would make it possible to answer questions such as the existence of a maximum size for a quantum system or the reconciliation of the theories of quantum mechanics and gravity. Levitated optomechanics in vacuum offers a promising solution. Freed from vibrations due to mechanical clamping or gas collisions, the 100/200 nm diameter levitated particle combines both high isolation requirements and "macroscopic" properties. Moreover, the control over the particle, held in an optical tweezer, has recently made it possible to bring it to its quantum ground state. To go further and increase the wave function of the particle, one would have to avoid photon recoil decoherence and strong confinement of the particle in a narrow potential. In this talk we will discuss the characteristics of a recently developed Paul-optical (hybrid) trap and the results of a so-called "frequency jump" protocol realised with this system. The above mentioned hybrid trap combines the well known optical and Paul traps to take advantage of both systems: the good control of the particle in the optical trap and the photon-free evolution in the Paul trap. Moreover, the mechanical frequencies of the particle in the two traps are different enough to allow specific experimental protocols to be carried out that lead to the mechanical squeezing of a state. This protocol, called "frequency jump", is based on the sequential transfer of a particle to traps of different stiffness, in this case the optical trap and the Paul trap.

Abstract: Self-sustained musical instruments such as bowed-string instruments, flutes, brass and reed instruments, are nonlinear dynamical systems producing a wide diversity of sound regimes. This includes a number of periodic regimes with different amplitude, frequency and spectral content that most often correspond to musical notes, but also quasiperiodic and chaotic sound regimes. The existence, stability and properties of these dynamical regimes depend on a possibly large number of design parameters, which relate to the instrument geometry and are fixed at the making stage, and playing parameters which are adjusted continuously by musicians while playing. Here we first investigate the physical mechanisms responsible for the emergence of non periodic sounds - which are often considered as a defect of the instrument - in flute-like musical instruments. In particular, a bifurcation analysis unveils the crucial role played by the inharmonicity of the instrument, that is to say the detuning between resonance frequencies. Such a bifurcation analysis shows that, similarly to many other dynamical systems in a variety of applications ranging from biology to climate models and optics, musical instruments can display a high degree of multistability, that is to say that several stable regimes coexist for a given set of parameters. Which regime is accessed in practice among the coexisting stable solutions depends on initial conditions and relates to the question of basins of attraction, defined as the set of initial conditions leasing to a particular regime. In musical instruments, multistability means that a careful selection of the control parameters by the musician is not sufficient to reach a particular regime. Basins of attraction therefore relate to the practical playability of a given sound regime. We will illustrate how the boundary between basins can be computed in the case of a simple model of reed musical instrument. The dependance of basins on playing parameters is investigated, showing the crucial role of attack transients of the blowing pressure on the playability of the different notes.

Abstract: Optoelectronic devices operating in the mid-wave infrared range are required for several applications, including night vision, free-space communications, light detection and ranging (LiDAR), spectroscopy, and observational astronomy. Unipolar quantum devices exploiting optical transitions between confined states in semiconductor quantum wells, like quantum cascade lasers (QCLs) [1] and quantum well detectors [2,3], offer unique possibilities for room temperature and high frequency operation. More recently, the realization of amplitude modulators, based on the Stark effect in tunnel coupled asymmetric quantum wells, has extended the set of available optoelectronic unipolar quantum devices, leading to the demonstration of high bitrate free space communications [4]. Amplitude and phase modulators can also play an important role for highly sensitive coherent detection [5]. In this talk, I will present recent results on unipolar quantum optoelectronic systems and devices, and discuss how their implementation into metamaterial architectures [6] can improve their performances and leverage important degrees of freedom for their design. [1] J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, et A. Y. Cho, Quantum Cascade Laser, Science 264, 553 (1994) [2] B. F. Levine, Quantum-well infrared photodetectors, J. Appl. Phys. 74, R1 (1993) [3] L. Gendron, M. Carras, A. Huynh, V. Ortiz, C. Koeniguer, et V. Berger, Quantum cascade photodetector, Appl. Phys. Lett. 85, 2824 (2004) [4] H. Dely, T. Bonazzi et al., 10 Gbit s−1 Free Space Data Transmission at 9 μm Wavelength With Unipolar Quantum Optoelectronics, Laser Photonics Rev. 16, 2100414 (2022) [5] M. Saemian, L. Del Balzo et al., Ultra-sensitive heterodyne detection at room temperature in the atmospheric windows, in press (2024) [6] D. Palaferri et al., Room-temperature nine-µm-wavelength photodetectors and GHz-frequency heterodyne receivers, Nature 556, 85 (2018)

Abstract: LEDs have revolutionized the fields of lighting and display with their energy efficiency and small size. However, modifying the directionality, the polarization or the spectrum of the emitted light involves adding bulky secondary optical elements (filters, lenses, polarizers, mirrors), which increase production costs, tend to reduce energy efficiency, and complicate the integration of sources into other systems. Integrating emitters into a metasurface (a periodic array of nanostructures) would enable to fabricate ultra-thin light sources with arbitrary emission properties, but this requires a quantitative emission model. In this presentation, I will demonstrate how to use the local Kirchhoff's law to design a bright and directional photoluminescent metasurface.

Abstract: Unexpected high mobility with disastrous consequences is often reported in large landslides and in earthquake fault ruptures. No consensus position on a cause has emerged to date [1,2]. In this study, we used high-speed rotary shear experiments to show that all dry dense grain flows with crushable grains are intrinsically highly mobile. They quickly weaken to a viscosity like peanut butter, which is independent of grain composition. We found that when flow begins, grains are crushed to a special fractal structure in which every larger grain is surrounded by much smaller grains. This special structure provided a favourable condition for generating and propagating acoustic energy from the abrasion indicated by the scratches on grains. The scratching generates much very high-frequency elastic energy (vibration) in the larger grains. This profoundly weakens all grain contacts so that the grain mass flows like a giant flood of peanut butter. Chatter-marked scratches made by smaller grains scouring across boulders in rock-avalanche deposits confirm that the same processes occur in nature. This finding is important for the explaination of the behaviour of dense grain flows at large strains and high strain rates. [1] Hu et al. High time-resolved studies of stick–slip show similar dilatancy to fast and slow earthquakes. PNAS 120, e2305134120 (2023). [2] Gou et al. Stick-slip nucleation and failure in uniform glass beads etected by acoustic emissions in ring-shear experiments. J. Geophys. Res. Solid Earth 128, B026612 (2023).

Abstract: In this seminar, I will briefly expose the fundamentals of time-domain Brillouin scattering and provide recent examples of its applications in our research group at Le Mans Université (specifically, at the Laboratoire d’Acoustique de l’Université du Mans - LAUM). This technique operates on a pump-probe principle. Initially, an ultrashort laser pulse (the "pump") is employed to generate coherent acoustic pulses. Subsequently, a second laser pulse, ultrafast and time-delayed (the "probe"), is used to monitor these acoustic waves. The extent to which the sample interacts with the probe light wavelength determines the detection of acoustic pulses. If the sample absorbs this wavelength, the detection is confined to the proximity of the absorbing surface. However, when the sample is transparent to the probe light wavelength, it offers the opportunity to trace the acoustic pulses throughout their propagation path, enabled by the photoelastic effect. This capability of time-domain Brillouin scattering allows the depth profiling of materials, providing both mechanical and optical contrast with axial resolution defined by the acoustic pulse length, typically sub-optical. My talk will focus on the application for time-domain Brillouin scattering in three-dimensional imaging of polycrystalline materials under high pressure and the characterization of their mechanical properties. Additionally, I will touch upon our recent theoretical investigation of the interaction of non-collinear probe light and acoustic Gaussian beams that could occur after reflection/transmission at an inclined material interface. Finally, I will discuss the potential avenues for further advancing such imaging technique, that emerge from the synergy of experimental findings, theoretical considerations and fast acquisition, going towards three-dimensional imaging of transient processes.

Abstract: Many physical, chemical, and biological phenomena are determined by dynamical processes occurring at the nanoscale. Therefore, tracking (bio)molecules and nanoparticles with nanometric precision and high temporal resolution is a truly powerful discovery tool in various fields. Over the years, many optical methods have been successful in tracking nanoscopic motion, and remarkable advances have been made. Recently, it has been established that localizing a photon emitter with a sequence of exposures to a minimum (ideally a zero) of light is highly efficient in terms of the number of photons needed to achieve a certain localization precision. In turn, this enables super fast tracking, in principle only limited by the count-rate of the emitter. In this talk, I will present the first experiments in this direction, a common conceptual framework to benchmark different localization methods of this kind, and the most recent advances on towards simultaneous co-tracking of single molecules and super-fast tracking of nanoparticles. 1. Balzarotti, F. et al. Nanometer resolution imaging and tracking of fluorescent molecules with minimal photon fluxes. Science 355, 606–612 (2017). 2. Masullo, L. A. et al. Pulsed Interleaved MINFLUX. Nano Lett. 21, 840–846 (2021). 3. Masullo, L. A., Lopez, L. F. & Stefani, F. D. A common framework for single-molecule localization using sequential structured illumination. Biophysical Reports 2, 100036 (2022). 4. Masullo, L. A. et al. An alternative to MINFLUX that enables nanometer resolution in a confocal microscope. Light Sci Appl 11, 199 (2022). 5. Cole, F. et al. Super-Resolved FRET and Co-Tracking in pMINFLUX. Submitted 6. Stefani, F. D. Tracking nanoscopic motion with minima of light. 17, 552-5553 (2023).

Abstract: The ability to manipulate wave transport phenomena and to enhance light-matter interactions using silicon-compatible, dispersion-engineered materials, epsilon-near-zero (ENZ) platforms, and complex photonic media with desired radiation properties is at the heart of current nanophotonics and metamaterials technologies. For example, the dramatic enhancement of the nonlinear optical interactions of transparent conductive oxides (TCOs) provides unique opportunities to engineer novel optoelectronic devices with order-of-unity (non-perturbative) refractive index changes on sub-picosecond time scales for dynamically tunable metasurfaces, broadband optical modulators, optical switching, and time-varying photonics applications on the chip. Moreover, recent progress in the theory, inverse-design, fabrication, and characterization of high-refractive index, low-loss, diffractive optical elements and dielectric nanostructures with tailored disorder and hyperuniform geometries established novel strategies to engineer ultra-compact imaging devices and nanostructures with desired wave transport and localization properties over targeted spectral- and spatial- frequency bandwidths. In this talk, I will discuss our work on the design, fabrication, and characterization of highly nonlinear Si-compatible materials and nanostructures based on the indium tin oxide (ITO) platform with tunable ENZ responses across the near-infrared spectral range. In particular, I will address the non-perturbative Kerr-type optical nonlinearity of fabricated materials and resonant devices driven by the excitation of optical Tamm states. Building on this platform, I will illustrate the potential of topologically optimized high-Q nonlinear dielectric nanocavities for extreme sub-wavelength field confinement potentially enabling photon-blockade and strong-coupling effects. Next, I will present our recent work on the inverse-design of ultra- compact and multifunctional spectroscopic imaging devices based on diffractive optical networks (a-DONs) and the adjoint optimization of high-refractive index functionalized scattering arrays for imaging and radiation engineering on the chip. Finally, I will provide a perspective on anomalous diffusion and light localization in multifractal photonic environments that encode the characteristic multiscale complexity of number-theoretic sequences and algebraic number fields beyond random lasing device applications. A. Capretti, Y. Wang, N. Engheta, L. Dal Negro “Enhanced third-harmonic generation in Si-compatible epsilon-near-zero indium tin oxide nanolayers”, Opt. Lett., Vol. 40, Issue 7, 1500-1503, (2015) 2) W. Britton, F. Sgrignuoli, L. Dal Negro, “Structure-dependent optical nonlinearity of indium tin oxide”, Appl. Phys. Lett. 120, 101901 (2022) 3) W. Britton, Y. Chen, F. Sgrignuoli, L. Dal Negro, “Phase-Modulated Axilenses As Ultracompact Spectroscopic Tools”, ACS Photonics, 7, 10, 2731–2738 (2020) 4) W. Britton, Y. Chen, F. Sgrignuoli, L. Dal Negro, “Compact Dual-Band Multi-Focal Diffractive Lenses”, Laser Photonics Rev. 15, 2000207 (2021) 5) Y. Chen, Y. Zhu, W. A. Britton, and L. Dal Negro, “Inverse design of ultracompact multi-focal optical devices by diffractive neural networks”, Opt. Lett., Vol. 47, No. 11, 2842-2845, (2022). 6) Y. Zhu, Y. Chen, and L. Dal Negro, “Design of ultracompact broadband focusing spectrometers based on diffractive optical networks”, Opt. Lett., Vol. 47, No. 24, 6309-6312, (2022). 7) S. Gorsky; W. A. Britton; Y. Chen, J. Montaner; A. Lenef, M. Raukas, L. Dal Negro, “Engineered hyperuniformity for directional light extraction”, APL Photonics 4, 110801 (2019) 8) F. Sgrignuoli, S. Gorsky, W. A. Britton, R. Zhang, F. Riboli, and L. Dal Negro, “Multifractality of light in photonic arrays based on algebraic number theory” Communications Physics, 3, 106 (2020). 9) Y. Chen, F. Sgrignuoli, Y. Zhu, T. Shubitidze, and L. Dal Negro, “Enhanced wave localization in multifractal scattering media”, Phys. Rev. B 107, 054201 (2023). 10) L. Dal Negro, “Waves in Complex Media”, Cambridge University Press (2022).

Abstract: Light scattering couples the many degrees of freedom in the electromagnetic fields. In this talk, I will present recent work in our group in three directions: 1) compensating light scattering to image deeper inside biological tissue, 2) developing fast solvers to compute the multi-channel response of complex optical systems, and 3) designing high-performance multi-channel nanophotonic structures. All of them concern the scattering matrix of the system, which relates the incident wavefront to the outgoing wavefront. On the imaging part, we develop a scheme that uses the measured scattering matrix to digitally perform spatiotemporal focusing, guidestar-free input/output wavefront correction, and dispersion compensation through numerically optimizing an image quality metric. Doing so enables noninvasive volumetric imaging with one-micron isotropic resolution at one millimeter beneath mouse brain tissue; the depth-over-resolution ratio exceeds 900. On the fast solver, we present a full-wave simulation method called "augmented partial factorization" (APF). By augmenting the Maxwell operator with the input and output profiles, APF computes the scattering matrix without solving for the internal field and without looping over the inputs, achieving 3 to 7 orders of magnitude speed-up compared to existing frequency-domain solvers. We made this code open-source and use it to demonstrate two-photon coherent backscattering from disorder and open channels for 3D vectorial waves. In the last part, we use APF to realize the inverse design of nonlocal metasurfaces.

Abstract: This talk describes experiments and simulations on two different ways to confine surface acoustic waves in two-dimensions on microscopic scales: by the use of a surface phononic crystal cavity [1] or by the use of zero-group-velocity waves in a thin plate [2]. In the former case, a quasi-hexagonal cavity in a honeycomb-lattice surface phononic crystal formed in crystalline silicon is imaged by means of an ultrafast technique, and the acoustic energy confinement is quantified. In the latter case, Lamb waves are similarly imaged in a micron-scale thickness silicon-nitride plate coated with a titanium film. We discuss the dispersion relations and spatial localization in detail in these two cases. Applications include sensing and the testing of bonded nanostructures. [1] P. H. Otsuka, R. Chinbe, M. Tomoda, O. Matsuda, Y. Tanaka, D. M. Profunser, S. Kim, H. Jeon, I. A. Veres, A. A. Maznev and O. B. Wright, (Photoacoustics, in press) - [2] Q. Xie, S. Mezil, P. H. Otsuka, M. Tomoda, J. Laurent, O. Matsuda, Z. Shen and O. B. Wright, Nat. Comm. 10, 2228 (2019).

Abstract: Improving the health of populations has always been a major concern of advanced human societies. Since the beginning of the 20th century, considerable efforts have been made to improve disease prevention and access to quality care. In particular, the considerable development of biomedical imaging has led to significant improvements in the detection and treatment of many types of pathologies, including cancer. Indeed, several imaging techniques (CT-scan, PET-scan, magnetic resonance, etc.) are commonly used in medical practice for this purpose. In addition, new optical techniques (fluorescence, second harmonic generation, optical coherent tomography, etc.) have been tested for a decade to develop new imaging systems able to detect diseases at an early stage or to improve the definition of surgical resection margins of pathological areas. Among all these optical techniques, polarimetric imaging has shown great promise for biomedical diagnosis. Indeed, it has been extensively tested for the examination of biological tissues, optically very complex systems where strong scattering and anisotropy effects can occur simultaneously. In particular, Mueller polarimetric imaging allows the complete polarimetric characterization of biological tissues, providing unique information on their microstructure, which can greatly improve the diagnosis of many diseases. Indeed, this technique has proven to be very sensitive for detecting changes in cell structure and density, as well as in the organization of extracellular matrix fibers, such as collagen, due to the development of a disease, even at an early stage. Furthermore, it is a non-contact technique that can be implemented with a macroscopic field of view (several tens of cm²), while providing sensitive contrasts to the sample microstructure at a scale smaller than the actual spatial resolution of the image. In addition, it uses conventional light sources (LED, halogen lamps and others) and relatively inexpensive optics. However, although Mueller polarimetric imaging has shown great potential for improving the analysis of biological tissues, the implementation of this technique in vivo is considered extremely difficult for all research groups working in this field and most studies have been conducted ex vivo. In this talk, I will present the main results and future perspectives of my research activity, whose main objective is the design, fabrication and implementation of innovative Mueller polarimetric imaging systems with unmatched performance for ex vivo and in vivo exploration of biological tissues. After a description of the main scientific and societal challenges of this research activity, an overview of the different imaging systems developed will be presented as well as the main results obtained in different biomedical applications ranging from gynecology to neurosurgery. These systems can represent a significant advance in the early and reliable detection of various pathologies, paving the way for more effective patient management strategies.

Abstract: TBA

Abstract: Amorphous materials have high heat capacity but low thermal conductivity compared to crystals with the same composition. The understanding of these properties is based on the study of acoustic attenuation and more generally on the specific vibrational properties of glasses. After a general description of the vibrational eigenmodes in disordered materials, we will show the link between the dynamics of vibrational wave packets and the expression of thermal conductivity at the atomic scale. We will then discuss the different acoustic attenuation mechanisms and their expression at different scales: from the atomic scale to the continuous scale. Finally, we will focus on the effect of crystal/amorphous interfaces on the thermal properties of nanostructured materials such as glass-ceramics. We will show how the presence of amorphous parts reinforces the diffusive contribution to heat transport, and allows controlling not only the orientation, but also the direction of the heat flux. [1] Y. Beltukov, D. A. Parshin, V.M. Giordano and A. Tanguy Physical Review E 98 023005 (2018): Propagative and diffusive regimes of acoustic damping in bulk amorphous material. [2] P. Desmarchelier, A. Carré, K. Termentzidis and A. Tanguy Nanomaterials 11, 1982 (2021): Heat Transport in Nanocomposite: The Role of the Shape and Interconnection of Nanoinclusions [3] H. Luo, V.M. Giordano, A. Gravouil and A. Tanguy Journal of Non-Crystalline Solids 583, 121472 (2022): A continuum model reproducing the multiple frequency crossovers in acoustic attenuation in glasses place=Amphithéâtre

Abstract: The purpose of this seminar is to show a model inversion framework for the estimation of material properties from acoustic measurements. Two applications are presented, namely the reconstruction of rough surfaces from sound scattering and the estimation of porous material properties from sound reflection. In the case of rough surface characterisation, the property of interest is the depth profile of the surface, and the measurement consists of scattered sound pressure retrieved at a microphone array. The application to porous material characterisation aims at determining the transport properties of a homogeneous porous sample from its sound absorption spectrum. The first step of the methodology is a deterministic model inversion, which is formulated as a non-linear optimisation problem and provides an initial estimate of the unknown model parameters. The inverse problem is then formulated in a statistical sense and solved using Bayesian inference, which enables the estimation of credible intervals on the unknowns. It is shown that the sequential use of deterministic and statistical inversion allows to reduce computational time while yielding a feasible solution and its uncertainty range.

Abstract: Recent studies showed that surface phonon-polaritons, i.e. evanescent electromagnetic waves propagating along the surface of polar dielectric materials [1] [2], may potentially serve as novel heat carriers to enhance the thermal performance in micro- and nanoscale devices. This seminar will expose the significant contribution of these carriers to thermal conductivity in ultra-thin (below 100nm) films [3], but also to radiation, yielding SuperPlanckian emission and absorption between surfaces of larger scales (10mm). [1] D.-Z Chen, A. Narayanaswamy, and G. Chen, Phys. Rev. B 72, 155435 (2005). [2]J. Ordonez-Miranda, L. Tranchant, T. Tokunaga, B. Kim, B Palpant, Y. Chalopin, T. Antoni, and S. Volz, J. Appl. Phys. 113, 084311 (2013). [3] Y. Wu, J. Ordonez-Miranda, S. Gluchko, R. Anufriev, D. De Sousa Meneses, L. Del Campo, S. Volz, and M. Nomura, Science Advances 6(40):eabb4461, (2020).

Abstract: TBA

Abstract: Light diffusion in a complex medium is a ubiquitous phenomenon in nature. The transport of photons becomes fascinating when the medium has χ(2) - nonlinearity. In this type of medium, fundamental light gets multiply scattered and subsequently generates second harmonic (SH) light. Again, these SH photons undergo multiple scattering events before coming out of the sample. In this talk, I will be discussing different aspects of second-harmonic generation in complex media which span from a few particles system to 3D disorder. With out-of-plane imaging, we show interference in the SH light among the sub-micron size harmonic particles. In a quasi-3D disordered medium, we have studied intensity-dependent speckle decorrelation in the fundamental and SH light. Further, the spatial and temporal diffusion of both the harmonics in a finite, 3D complex medium will be discussed. Finally, I will conclude my talk with the possible applications and future directions in this field.

Abstract: Optical metasurfaces are thin large-area structures with aperiodic subwavelength patterns, designed for focusing light and a variety of other wave transformation. Because of their irregularity and large scale, they are one of the most challenging tasks for computational design. This talk will present ways to harness the full computational power of modern large-scale optimization in order to design metasurfaces with thousands or millions of free parameters. To that end, we exploit domain-decomposition approximations and “surrogate” models. We will also review some traditional techniques such as adjoint simulations and the principle of equivalence. We will also present some recent experimental results on lenses. Finally, if time permits, we will discuss recent progress combining traditional numerical methods and machine learning for data-efficient surrogate models and how they will impact inverse design in nanophotonics and beyond.

Abstract: We report a new method for electron spin detection at millikelvin temperatures. Analogous to the optical fluorescence detection of atoms or molecule, the method consists in detecting the microwave photons spontaneously emitted by the spins when they relax radiatively to their equilibrium ground state after being excited by a pulse (ie, their microwave fluorescence) [1]. To enhance the radiative relaxation rate [2], the spins are inductively coupled to a small-mode-volume, high-quality-factor, superconducting resonator patterned on top of the sample. The microwave fluorescence photons are then routed towards a single-microwave-photon detector [3] based on a superconducting qubit. The method applies to all paramagnetic species with sufficiently low non-radiative decay rate. Here, we report the detection of rare-earth-ion spins (Er3+ in a scheelite CaWO4 host matrix) by their microwave fluorescence. We perform a complete spectroscopic characterization of a small erbium ensemble. We also report the first microwave detection of individual erbium ion spins, and their coherent manipulation [4]. [1] E. Albertinale et al., Nature 600, 434 (2021) - [2] A. Bienfait et al., Nature 531, 74 (2016) - [3] R. Lescanne et al., Phys. Rev. X 10, 021038 (2020) - [4] Z. Wang et al., in preparation (2022)

Abstract: The question of the origin of homochirality of living matter, or the dominance of one handedness for all molecules of life across the entire biosphere, is a long-standing puzzle in the research on the Origin of Life. In the fifties, Frank proposed a mechanism to explain homochirality based on properties of simple autocatalytic networks containing only a few chemical species. Following this work, chemists struggled to find experimental realizations of this model, possibly due to a lack of proper methods to identify autocatalysis [1]. In any case, a model based on a few chemical species seems rather limited, because prebiotic earth is likely to have consisted of complex ‘soups’ of chemicals. To include this aspect of the problem, we recently proposed a mechanism based on properties of large out-of-equilibrium chemical networks [2]. We showed that a homochiral phase transition is likely to occur as the number of chiral species in the system becomes large or as the amount of free energy injected into the system increases. Our approach, based on random matrix theory, can account for the sparsity of chemical networks, and points towards a robust mechanism for the emergence of homochirality. Furthermore, the chiral symmetry of the relevant matrices has implications for the various conventions used to measure chirality and for the relative chiral signs adopted by different groups of molecules in prebiotic chemistry [3]. References: [1] A. Blokhuis, D. Lacoste, and P. Nghe, PNAS (2020), 117, 25230. [2] G. Laurent, D. Lacoste, and P. Gaspard, PNAS (2021) 118 (3) e2012741118. [3] G. Laurent, D. Lacoste, and P. Gaspard, Proc. R. Soc. A 478:20210590 (2022).

Abstract: This talk discusses two goals that can be achieved with elastic waves travelling in soft materials. First, we see how tracking the changes in wave speed with stress or strain gives access to linear and nonlinear material parameters in a non-destructive manner. These can then be used to design biomaterials or to create meaningful Finite Element simulations. Examples include brain matter, muscles, and stretched soft plates. Then, we find that the state of stress and strain existing in a loaded material can be accessed directly from wave speed measurements, without having to determine, or even know, its material properties beyond its mass density and geometry. These techniques are expected to have important applications in health monitoring of loaded structures. Examples include stressed rail steel, the human skin in situ, and thin membranes such as a stretched rubber sheet, a piece of cling film (~10 μm thick) and the animal skin of a bodhrán, a traditional Irish drum.

Abstract: Contrary to the more widespread acousto-optic devices, based on diffraction of light, last years seen increased interest in their refraction based counterparts. In this talk I will discuss the recent developments in this field and highlight the newly opened capabilities of such devices. Namely, light focusing, focus translation and light waveguiding inside scattering media by tailored ultrasonic fields will be highlighted.

Abstract: TBA

Abstract: Buckling of elastic structures is an effective way to produce rapid motion in a fluid at any scale. Encapsulated microbubbles, which are currently used as ultrasound contrast agents, can deform and collapse under an external load from an acoustic wave. They reinflate when the pressure decreases. The shape hysteresis associated with this deformation cycle makes this simple object a good candidate to become an ultrasound controlled micro-swimmer. I will explore this possibility through experiments at macro and micro scales and numerical simulations. The coupling between the acoustic wave and the self-oscillation of the deformed shell leads to complex - sometimes chaotic - dynamics with direct consequences on the direction and efficiency of the swimming.

Abstract: TBA

Abstract: In less than a decade, topological phase physics has spread frantically over all fields of physics. Among the rich class of topological phases, of particular interest are chiral phases. Chiral symmetry, although originally introduced in quantum mechanical systems, is naturally found at the macroscale, in systems as diverse as photonic arrays, acoustic metamaterials, and mechanical networks. In this talk, using mechanics as a guiding line, I will lay out a generic framework to characterise chiral metamaterials. I will first introduce the concept of chiral polarization, a material property that distinguishes distinct topological phases. Combining theory, simulations and experiments, I will show how to measure the chiral polarization and thereby detect zero energy excitations in chiral structures. I will in particular stress on a practical method based on local excitations, which provides a quantitative and local probe of topological excitations in periodic and amorphous metastructures. I will close my talk by generalising our results to the flourishing field of higher-order topological insulators. I will show how to detect and design zero-energy corner states without relying on any apriori modelling of a mechanical, photonic or acoustic metamaterial.

Abstract: Transit-time ultrasonic flow meters measure the fluid flow rate through a pipe by exploiting the non-reciprocity due to flow. Robust operation in a wide temperature range from 0°C to 100°C is, thereby, a major practical challenge. A second difficulty arises when conveniently insonifying the pipe's interior from the outside, in which case the pipe wall acts as a mechanical waveguide. The interaction with the fluid medium results in quasi-guided (leaky and trapped) waves in the sense that their energy is no longer strictly confined within the pipe wall – an effect that is desired in this context. These quasi-guided waves are described by an intricate nonlinear eigenvalue problem. From a theoretical point of view, I will outline a procedure to reliably obtain the full set of solutions. Although the obtained waves are very convenient conceptually, their physical nature is – to date – not fully understood. From a more practical point of view, I will present how these waves can be used to conveniently model transit-time ultrasonic flow sensors, thereby including the effect of temperature. Compared to conventional meters, we find that the time of flight of guided wave based sensors exhibits a decreased cross-sensitivity to temperature. This is a consequence of the temperature-dependent radiation angle of leaky waves, which tends to compensate corresponding changes of the fluid wave speed. Finally, I will give an overview of accompanying projects: Schlieren photography of ultrasound, electromagnetic-acoustic transducers for waveguide excitation, a dip-stick sensor for fluid characterization and ultrasonic holography.

Abstract: Wave propagation at the surface of soft elastic solids brings up questions about the existence of forces able to compete with bulk elasticity. I will discuss the interplay between elastic, capillary and gravity forces on surface and guided waves, and how wave propagation measurements could reveal the nature of the interface of soft solids. I will show that for guided waves, capillary effects become visible by affecting the balance of in-plane and out-of-plane displacements at the interface and that they are enhanced for small thicknesses. I will also discuss how using gravity as a destabilising force enables us to dramatically affect the elasto-gravity wave dispersion relation.

Abstract: We present a self-consistent theory of the steady-state electron distribution in metals under continuous-wave illumination which treats, for the first time, both thermal and non-thermal effects on the same footing. We show the number of non-thermal electrons (i.e., the deviation from thermal equilibrium) is very small, so that the power that ends up generating these non-thermal electrons is many orders of magnitude smaller than the amount of power that leads to regular heating. Using this theory, we re-examine the exciting claims on the possibility to enhance chemical reactions with these non-thermal electrons. We identify a series of rather astounding errors in the temperature measurements in some of the most famous papers on the topic which led their authors to under-estimate regular heating effects. As an alternative, we show that a very simple 19th century theory, based on just simple heating, can explain the published experimental data with excellent accuracy.

Abstract: TBA

Abstract: Here, we show that the well-known and simple system of a resonant cavity with slightly aberrated curved mirrors is analogous to a Bose-Hubbard dimer, a well-known model in condensed matter physics. The modes of the aberrated cavity, namely the Ince-Gauss beams, present two topologically distinct regimes: the HG-like regime where astigmatism dominates, and the LG-like regime where spherical aberration dominates. These are analogous to the Rabi and Fock regimes of the Bose-Hubbard system. By considering the ray-optical limit, we further show that, as the ratio between the two aberrations changes, the ray structure undergoes a topological transition. This transition can be visualized in the Poincaré/Bloch sphere where the ray structure goes from being describable in terms of a single loop to splitting into two loops. Additionally, we derive and model the dynamical behavior in the ray limit, where we show that the evolution of a ray inside the aberrated cavity follows the meanfield two-mode Gross-Pitaevskii equation. We also consider the wave model and show that the aberrated cavity can emulate pure quantum behavior such as the squeezing of a coherent state.

Abstract: Single-photon sources are a key building block for secured quantum telecommunications. In view of integration in long range telecommunication networks near-infrared emission and room temperature operation are current challenges. Carbon nano-structures and especially carbon nanotubes have strong assets in this perspective. They can be operated as high quality single-photon emitters. In addition, their emission wavelength can be chosen over a wide range using the variety of their geometry (the chiral family) or external chemical functionalization. Nevertheless, the reported brightness is consistently small and the spectral purity is not well controlled. Moreover, the origin of the photon antibunching (usually a fingerprint of atom-like emitters) in this 1D system has long been controversial. We address this issue using hyper-spectral super-resolution techniques inspired by bio-imaging. These key properties can be drastically improved by coupling the nanostructure to a small volume, high-finesse optical cavity through the so-called Purcell effect. In fact, light-matter interaction at the scale of a single emitter can be strongly modified when the electromagnetic field in confined to volumes of the order of lambda^3 , which is the realm of cavity quantum electrodynamics. Originating from atomic physics, this field has become technologically relevant when solid state emitters (quantum wells, quantum dots…) were first coupled to integrated micro-cavities. Here, we used an original tunable cavity design using a scanning optical fiber in order to tackle the spatial and spectral mode matching issues. We show that the emission rate of the nanotube can be enhanced by a factor 60 leading to an effective brightness of about 0.4 and a coupling factor close to 100%. Furthermore, our original design brings a wide tuning range for the single-photon source by exploiting indirect emission processes assisted by acoustic phonons. This new feature is very attractive for multiplexing approaches or for indistinguishability engineering from remote nano-sources for quantum information processing.

Abstract: Wave transport in complex media is widely studied through calibrated model systems ranging from nano-powder (in optics) to millimetric beads aggregates (in acoustics). Those samples are designed to strongly scatter waves and achieve complex transport phenomena such as wave sub-diffusion. In this seminar, I will show how model systems achieved by millifluidics are good candidates to study such phenomena with ultrasounds. Then, we will see how similar observations can be done in a natural disordered system namely: a shoal of fish. The parallel between laboratory and in situ experiments gives i) new characterization tools for fisheries acoustics and ii) leads for the bio-inspired design of disordered systems.

Abstract: The failure of materials under load is widespread, occurring from geological scales, as in earthquakes, to biological and soft-matter systems, with huge implications to everyday life and material science. Despite consistent efforts in this field, failure is very often undesired and unpredicted: surprisingly, or maybe unsurprisingly, it turns out that what is so common in our daily experience is based on profound science that is not yet fully understood. In this talk I will discuss experiments in which we investigate the microscopic signature of failure in various soft materials, by simultaneously measuring their mechanical response and microscopic dynamics. Our experiments show that material failure is associated with qualitative and quantitative changes of the dynamics, from reversible particle displacements to bursts of irreversible plastic rearrangements.

Abstract: The effects of structural anisotropy on wave transport can be quite intriguing. I will show that, in a disordered anisotropic material, anisotropy in the propagation of multiply scattered waves is significantly reduced and may even vanish as the Anderson localization transition is approached, by capitalizing on advantages of ultrasonic experiments. Initial results using a random matrix approach to describe wave transport in an anisotropic system also support this experimental observation. These findings are expected to be very general, as the underlying physics is relevant to all types of waves (electrons, matter waves, and optical and acoustic waves), and they also resolve a puzzling question raised by previous conflicting theoretical predictions.

Abstract: Lossless linear wave propagation is symmetric in time, a principle which can be used to create time reversed waves. Such waves are special “pre-scattered” spatiotemporal fields, which propagate through a complex medium as if observing a scattering process in reverse, entering the medium as a complicated spatiotemporal field and arriving after propagation as a desired target field, such as a spatiotemporal focus. Time reversed waves have previously been demonstrated for relatively low frequency phenomena such as acoustics, water waves and microwaves. Many attempts have been made to extend these techniques into optics. However, the much higher frequencies of optics make for very different requirements. A fully time reversed wave is a volumetric field with arbitrary amplitude, phase and polarisation at every point in space and time. The creation of such fields has not previously been possible in optics. We demonstrate time reversed optical waves with a device capable of independently controlling all of light’s classical degrees of freedom simultaneously. Such a class of ultrafast wavefront shaper is capable of generating a sequence of arbitrary 2D spatial/polarisation wavefronts at a bandwidth limited rate of 4.4 THz. This ability to manipulate the full field of an optical beam could be used to control both linear and nonlinear optical phenomena.

Abstract: Transcription by RNA polymerase (RNAP) is interspersed with sequence-dependent pausing. The processes through which paused states are accessed and stabilized occur at spatiotemporal scales beyond the resolution of previous methods, and are poorly understood. Here, we combine high-resolution optical trapping with improved data analysis methods to investigate the formation of paused states at enhanced temporal resolution. We find that pause sites reduce the forward transcription rate of nearly all RNAP molecules, rather than just affecting the subset of molecules that enter long-lived pauses. We propose that the reduced rates at pause sites allow time for the elongation complex to undergo conformational changes required to enter long-lived pauses. We also find that backtracking occurs stepwise, with states backtracked by at most one base pair forming quickly, and further backtracking occurring slowly. Finally, we find that nascent RNA structures act as modulators that either enhance or attenuate pausing, depending on the sequence context.

Abstract: For decades, wave chaos has been an attractive field of fundamental research concerning a wide variety of physical systems such as quantum physics , room acoustics or ocean acoustics, elastodynamics, guided-wave optics, microwave cavities. The success of wave chaos is mainly due to its ability to describe such a variety of complex systems through a unique formalism, the so-called random matrix theory (RMT), which permits us to derive a universal statistical behaviour. More recently, chaotic cavities have been involved in a large variety of applications ranging from computational imaging to electromagnetic (EM) compatibility testing, as well as wavefront shaping for telecommunication or wave-based analog computation. Among all the universal statistics of chaotic cavities, the most important one for these applications is field ergodicity meaning that the fields in chaotic systems are statistically equivalent to an appropriate random superposition of plane waves. To implement a wave chaotic system experimentally, traditionally cavities of elaborate geometries (bowtie shapes, truncated circles, parallelepipeds with hemispheres) are employed because the geometry dictates the wave field’s characteristics. In the first part of this talk, I will present a radically different paradigm: a cavity of regular geometry with tunable boundary conditions, experimentally implemented by leveraging electronically reconfigurable metasurfaces (ERM) developed by GreenerWave. The latter are able to locally imposed a π-phase shift on the reflected field. In a way, the ERMs are EM equivalent of Spatial Light Modulators used in optics or Schroeder's diffusers used in room acoustics with the difference in this case that the ERM are reconfigurable at will. In the second part of this talk, I will present three applications using reconfigurable cavity.

Abstract: Granular material has been a model experimental system to probe to what extent standard statistical mechanics apply to out-of equilibrium systems. Grains are characterized by a negligible thermal fluctuations and dissipative interparticle interactions. Thus, static steady states are necessarily at rest, and any dynamical steady state like granular gases necessarily involves injection of energy, and this flow of energy shifts the material towards an out-of equilibrium steady state. On the contrary, colloidal suspensions are subject to thermal fluctuations and evolve at equilibrium similarly to molecular fluids, but it is also possible to make it out of equilibrium by driving externally the material e.g. shear or under any external potential energy field, or internally like in active matter or self-propelled particles. These systems have focused the attention of many researchers, in particular because it can mimick collective behaviours we can observe in living systems, like swarming of bacteria, or flock of birds. In this talk, I will briefly present my background and the context of the experimental work I did during my PhD on driven granular suspensions. Then I will present in more detail the work of my former postdoc on imaging of 3d dense suspension of self-propelled colloidal particles. Finally, I will shift towards the work I am currently doing in the Institute with Xiaoping Jia and Arnaud Tourin on wave propagation in a disordered elastic lattice - a numerical toy model to understand sound propagation in granular media. I will especially present some preliminary results concerning the relation that can exist between open and close eigenchannels of a disordered medium - obtained from its scattering matrix - and its structure e.g. the vibrational modes.

Abstract: Using the “memory effect”, we propose and implement a broadband, compact, and cost-effective Wavefront Sensing scheme with a simple thin diffuser. We experimentally demonstrate its various capabilities to provide quantitative phase imaging, nanoparticle tracking and super-localization.

Abstract: During this seminar, I will describe and explain a set of surprising observations made a few years ago in zebrafish embryos. The cerebrospinal fluid (CSF) was found to flow bidirectionally in a single channel in the center of the spinal cord, and to form several vortices, despite the small Reynolds number. In this work, we combined biology experiments with modeling and simulations to explain the origin of this flow and predict its impact on zebrafish embryogenesis. We demonstrated that CSF flow is established thanks to an active force, generated by small motile cilia beating only on one side of the channel. We found that this active force imposes a pressure gradient inside the channel that probably help to maintain the channel structure. We also demonstrate that the bidirectional flow accelerates the transport of particles in the CSF via a coupled convective-diffusive transport process, in a regime similar to the Taylor-Aris diffusion. Finally, I will turn to some (more) biology, and show how CSF circulation contributes to body axis formation and brain development. We now have several clues that pathological CSF circulation can lead to various diseases such as idiopathic scoliosis or meningitis.

Abstract: Liquid foams, i.e. aqueous foams or concentrated emulsions, are made of a dense packing of soft particles in a surfactant solution. These structures are intrinsically unstable and age with time via intermittent local particle reorganisations. Besides, their mechanical response goes from solid-like to liquid-like when the applied load is large enough to trigger particle rearrangements. Local structural changes are thus the key-processes underlying both the ageing and the mechanical behaviour. Foams strongly scatter light and tend to be opaque, which prevents direct observations of internal rearrangements. However, multiple light scattering can be turned into benefit to design new non-invasive probes. I will present studies that illustrate how coarsening- or strain- induced local dynamics in liquid foams can be investigated using dedicated coherent light scattering experiments.

Abstract: Here, we report our recent progress on acoustic and elastic Willis media, achieving the analogy of bianisotorpy in electromagnetism. We have demonstrated Willis coupling on a structured beam with resonating cantilever structures and its resultant asymmetric propagation of flexural waves on such a beam. Next, we experimentally demonstrate how the passive bound of Willis coupling can be surpassed by going to an active Willis medium.

Abstract: In wave physics, random media are characterized by the existence of multiple degrees of freedom as well as a subsequent multiplicity of configurations. In this talk, I will present different approaches that I developed to harness the versatility of disordered systems and force the emergence of collective order out of random ensembles. First, I will consider the case of strongly scattering media filled with optical gain that create random lasing emissions (and are referred to as random lasers). While the spectrum of random lasers is typically multimode—composed of many frequencies, here I will demonstrate that the interaction amongst modes can be controlled through wave-front-shaping technique in order to collectively build-up a singlemode emission at tunable frequency. Then, I will show that random populations of mobile particles can be driven to self-organize into out-of-thermodynamic-equilibrium crystalline structures through the controlled dissipation of coherent energy. The elements individually synchronize their responses to dynamically develop collective properties such as resiliency abilities or the self-adaptation to the environment.

Abstract: We present a new method to solve the linear elasticity inverse problem, i.e to reconstruct the Lamé parameters from the measurement of the displacement field in a medium. We will show why this method allows for stable inversion, then show how it can be numerically implemented and how it performs on simulated and experimental data.

Abstract: Quantum technologies hold great promise for revolutionizing photonic applications such as cryptography and imaging. Yet their implementation in real-world scenarios is held back, mostly due to sensitivity of quantum states of light to scattering. Recent developments in shaping of single photons introduce new ways to control scattering of quantum light. Here we cancel scattering of entangled photons, by shaping the classical laser beam that stimulates their creation. We show that when the laser beam and the entangled photons pass through the same diffuser, focusing the laser using classical wavefront shaping recovers the unique correlations of entangled photons, which were scrambled by scattering. Since the shaping is done exclusively on the classical laser beam, it does not introduce loss to the entangled photons, and it is not limited by the low signal-to-noise ratios associated with quantum light.

Abstract: Fast, volumetric imaging over large scales has been a long-standing challenge in biological microscopy. Camera-based microscopes are typically hampered by the problem of out-of-focus background which undermines image contrast. This background must be reduced, or eliminated, to achieve volumetric imaging. Alternatively, scanning techniques such as confocal and multiphoton microscopy can provide high contrast and high speed, but their generalization to volumetric imaging requires an axial scanning mechanism, which, in general, drastically reduces speed. I will describe a variety of strategies we have developed to enable fast, high-contrast, volumetric imaging over large length scales. These strategies include targeted-illumination widefield microscopy, multi-z confocal microscopy and reverberation multiphoton microscopy. I will discuss the principles of these strategies and present experimental validations.

Abstract: A wireless contact lens, incorporating various functions such as data transfer, energy collection, eye movement extraction, is presented. It measures the direction of gaze, vergence, blinks and jerks, combining optical and RF technologies. Their implementation is discussed from flexible substrates, encapsulable in scleral lenses, such as micro-batteries and graphene-based biofuel cells. Preliminary results will be presented.

Abstract: In this talk I will speak about the Wigner-Smith time-delay operator and its multiple applications in complex scattering problems. In particular, I will focus on wave propagation through disordered media, where this operator can be used to derive an interesting mean path-length invariance or for focusing waves on a target embedded inside the disorde. Finally, I will report on recent progress in using a suitably modified Wigner-Smith operator for the creation of scattering states that feature an optimal performance for micro-manipulation.

Abstract: Spatial Modulation Multiple Input Multiple Output (SM-MIMO) appeared to address both the needs of growing data rate and energy-efficient of smart devices for Internet-of-Things (IoT) and wireless networks (5G, Wi-Fi, etc.…) applications. In the first part of the presentation, we propose to take benefit of coupling effects between resonators to generate different radiation patterns that stand for SM-MIMO symbols. The first reconfigurable antenna results from the coupling of meander monopoles and L-shaped resonators are discussed. The experimental prototype generates efficiently between 4 and 8 patterns. The concept of spatial diversity that is a key feature in SM-MIMO is discussed. The second reconfigurable antenna is a metamaterial-like structure based on split ring resonators (SRR). A semi-analytic model that describes the magneto-electric coupling between SRR is developed in agreement with the numerical and experimental results proposed model. In a close collaboration with IETR, these antennas have been successfully implemented in a SM-MIMO device to transmit information. Most of the wireless communication devices used an active antenna to transmit information. However, researchers have been investigating the potential of backscatter communications to take the benefit of the ambient electromagnetic waves by modulating them with a backscatterer to a reader. This technique allows to lower down the power consumption just by recycling the ambient electric fields (DVB-T, 4G, etc…). In the second part of the presentation, we show how the modulated field is strongly sensitive of interference effects between the direct path and the scattered one. Within a collaboration with Orange Labs, we propose a systematic survey of this effect of interferences. Fading patterns are observed even in a Line of Sight (LOS) configuration. Experimental results are validated by an analytical model. Finally, it is demonstrated that the Binary Error Rate (BER) is directly driven by these interferences.

Abstract: We utilize a generalization of the Wigner-Smith time-delay operator to manipulate a target embedded inside a disordered structure by shaping the incident wavefront. Such a manipulation involves, e.g., applying a well-defined torque onto the target or strongly focusing onto it. Our technique relies on the generalization of the Wigner-Smith time-delay operator and was successfully tested in a microwave setup featuring a waveguide containing a disordered medium.

Abstract: Nanophotonic structures are capable of generating field hotspots, which can enhance quantum light-matter interactions by many orders of magnitude. However, numerical simulations for applications such as radiative heat transfer, electron energy loss spectroscopy, van der Waals forces, Purcell factor throughout a volume, and many others are challenging and often computationally prohibitive. Common to these simulations is that the Green’s function or local photonic density of states must be known at each point across a volume of space, necessitating the solution of Maxwell’s equations perhaps many thousands of times. We propose a modal solution, which requires just a single simulation to find the modes of the nanophotonic system, from which we immediately obtain the Green’s function everywhere in space. This not only reduces simulation time by approximately 2 orders of magnitude, but also offers ready physical insight into the spatial variation of Green’s function. Modal methods have long been used for closed systems, where the formulation is exceedingly simple. We have generalized modal methods to open systems while maintaining this simplicity, catering to the explosion of research interest in nanophotonics. We furthermore present a highly-efficient exponentially-convergent method of generating the modes themselves.

Abstract: Light-matter interfaces play a crucial role in the context of quantum information networks, enabling for instance the reversible mapping of quantum state of light onto quantum states of matter. A promising approach for the realization of such interfaces is based on ensemble of neutral atoms. A critical figure of merit of such interfaces is the overall storage-and-retrieval efficiency, which is mainly determined by technical losses and atomic decoherence, and depends on the storage mechanism and matter properties. Collective and cooperative effects manifistable in an atomic ensemble could provide essential enhancement of the coupling strength between the light and atomic systems. In this context, one of the strongest requirements to obtain a high efficiency is a large optical depth, which can be achieved by increasing the size of the atomic system or atomic density in the system. Moreover, recent experimental advances in the trapping technique have made it possible to create 1D, 2D or 3D spatially ordered atomic configurations, where the collective effects play a very important role. In addition, the interaction between light and atoms can be enhanced by trapping atoms in the vicinity of a nanoscale waveguide due to strong confinement of the light. In this context, in this talk I will discuss light propagation in a spatially dense atomic ensemble, where the average distance between atoms is comparable with the resonant wavelength. In such dense atomic configurations dipole-dipole interaction play an important role and can lead to manifestation of super and subradiance effects. I will consider the light propagation in both free space and trapped near nanofiber surface atomic ensembles. The light scattering in such dense atomic configuration is described in terms of microscopic approach based on the standard scattering matrix and Resolvent operator formalism. We show theoretically and experimentally that spatially dense atomic ensembles allow obtaining effective light-matter interface and reliable light storage with essentially fewer atoms than it can be achieved in dilute gases. Furthermore, we show that the presence of an optical nanofiber modifies the character of atomic interaction and results in long-range dipole-dipole coupling between atoms not only via the free space, but also through the waveguide mode.

Abstract: Bright squeezed vacuum (BSV) is a quantum state of light that can be produced from a strongly pumped unseeded parametric amplifier, its brightness can be comparable with lasers (more than 10 trillion photons per mode). In this seminar I will present two applications of bright squeezed light. First I will report on interferometric measurements beyond the shot noise limit using a so-called SU(1,1) interferometer. I will then describe experiments using bright squeezed light as a pump to generate nonlinear effects. The sensitivity of an interferometric measurement on a phase shift depends on the state of light used as a probe and the measurement scheme. A ‘standard’ precision is provided by a coherent state fed into a Mach-Zender interferometer, the so-called shot noise limit (SNL). A measurement beating this limit is said to be supersensitive. In order to make super-sensitive phase measurements, quantum resources can be used. For example, squeezed light is now implemented in gravitational wave detectors. Besides the input state and the detection scheme, one can also modify the interferometer. Two optical parametric amplifiers (OPAs) can be used instead of the passive beam-splitters of conventional interferometric setups to form an SU(1,1) interferometer, the phase sensitive response of the OPAs giving rise to interference patterns. Using this interferometer, we demonstrate a phase sensitivity overcoming the shot noise limit by 2.3 dB and show the remarkable robustness of this scheme against detection losses. I will then show that BSV, i.e. amplified quantum fluctuations, turns out to be very useful for pumping multiphoton effects despite the common opinion that large intensity noise is detrimental for any applications. In particular, BSV can be used to enhance the occurence of extreme events and rogue waves in nonlinear optics and to create states with exotic photon statistics.

Abstract: Granular matter display both solidlike and fluidlike properties, and are ubiquitous in nature and industrial applications. A variety of phenomena observed in driven granular matter can be attributed to glassy dynamics -- namely, local contact changes and rearrangements at loose spots. In this talk, I present an overview of the Shear-Transformation-Zone (STZ) theory, a statistical description of granular flow and the dynamics of flow heterogeneities in unconsolidated granular matter, consistent with the principles of nonequilibrium thermodynamics. The propensity for flow heterogeneities or "shear transformation zones" to rearrange and produce nonaffine strain is given by a thermodynamically-defined structural effective temperature known as the compactivity. I will discuss applications of the STZ theory in describing stick-slip instabiltiies in granular flow, and resonance shifts in nonlinear acoustic experiments. If time permits, I will also discuss an STZ description of relaxation in a granular glass bead pack.

Abstract: Semi-conductor lasers invented in 1962 are vital to our modern daily life. For example, they generate the optical impulses that carry ever-greater amounts of information in fiber-optic networks over great distances. The emergence of irregular and unpredictable pulsations and dynamical instabilities from a laser were first noted during the very early stages of the development of lasers. Pulses with amplitude varying in an erratic manner were reported in the output of the ruby solid-state laser. However, the lack of knowledge of what would later be termed butterfly effect i.e. deterministic chaos resulted in these initial observations being either left unexplained or wrongly attributed to noise. This presentation will highlight the fundamental physics underpinning the butterfly effect in semiconductor lasers and also the opportunities in harnessing it for potential applications.

Abstract: Multiple diffusion resulting from the propagation of waves in disordered environments, such as light passing through biological tissues or a fine layer of paint, is an extremely complex process but remains linear. In fact, with spatially discrete inputs and outputs, it can perform the equivalent of a random projection, i.e. the multiplication of the input vector by an iid random matrix. In this context, we have shown that these environments act precisely as "compressed sensing" model systems, allowing signal acquisition with a number of measurements driven by the actual amount of information. Conversely, one can see this physical system as an optimal mixer of information, performing instantaneously in the (physical) analog domain an elementary computation brick of many Machine Learning schemes. We will present a series of proof of concept experiments in Machine Learning, and discuss recent technological developments of optical co-processors within the startup LightOn, co-founded with Igor Carron, Sylvain Gigan & Florent Krzakala.

Abstract: Maîtriser la propagation des ondes acoustiques dans l’espace est un récent enjeu scientifique. Avec l’avènement des métamatériaux, de nombreuses architectures ont été proposées ces dernières années pour briser le principe de réciprocité ou pour permettre la focalisation spatiale de l’énergie acoustique dans des régions de taille inférieure à la longueur d’onde. Ce séminaire propose de présenter les dernières avancées sur ces sujets effectuées à l’Université du Mans en France ainsi qu’à l’Université de Hokkaido au Japon. Après un bref rappel des travaux existants et des perspectives envisagées pour la propagation asymétrique des ondes acoustique, la première partie présente un concept de rectificateur acoustique s’appuyant sur une structure composée d’un partie sélectionnante composée d’un cristal phononique et d’une partie convertissante basée sur le phénomène non-linéaire d’auto-démodulation d’un milieu granulaire non consolidé. La seconde partie de ce séminaire est dévouée au principe de la transmission extraordinaire des ondes acoustiques (EAT) qui permet de concentrer l’énergie dans une région de taille sub-longueur d’onde avec une quantité d’énergie transmise supérieure à celle attendue par les seules considérations géométriques. Une architecture est proposée afin de montrer la possibilité d’obtenir cet effet pour des ondes de volumes à des régimes de l’ordre du GHz en couplant les résonances acoustiques de Fabry-Pérot à l’intérieur d’un tube de taille nanométrique avec une structure rainurée permettant la conversion des ondes longitudinales en ondes de surface.

Abstract: Gaps formed between metal surfaces control the coupling of localized plasmons, thus allowing gap-tuning targeted to exploit the enhanced optical fields for different applications. Classical electrodynamics fails to describe this coupling across sub-nm gaps, where quantum effects become important owing to non-local screening and spill-out of electrons. The advantages of narrow gap antennas have mostly been demonstrated for processes like SERS that are excited optically, but promising new phenomena appear when such antennas are fed by electric generators. However, the extreme difficulty of engineering and probing an electrically driven optical nanogap antenna has limited experimental investigations of physical concepts at stake in these conditions. The feasibility of structuring electron-fed antennas as nano-light sources has been recently demonstrated; however, the suggested configuration remains very limited: too much power was lost as heat when operating the optical antenna, and the antenna operation time was limited by the structure lifetime to sustain a bias voltage for a few hours. The innovative structure that I propose in my talk will cope with all these limitations: ALD dielectric materials substitute the air gap to improve the antenna stability; a quantum efficiency of 0.1 is targeted owing to a significantly efficient antenna (2 orders of magnitude higher field enhancement). The resulting source will operate at room temperature and have a tunable spectral response (ranging from visible frequencies to THz regime) defined by the antenna geometry and the applied bias. Also, this source will be compact, Si-compatible, and will not request specific emitting materials (e.g. III-V semi-conductors) to operate.

Abstract: In this talk I present two different systems that are built up of resonantly coupled subwavelength particles. The first system is a disordered cloud of cold atoms, whereas the second system is a hexagonal array of silicon nanopillars. In both cases the individual entities are resonantly coupled to each other, which leads to an interaction with light that is different than the sum of individual particles. In a wavelength-sized cloud of cold atoms we theoretically show that this disordered system actually shows order in its optical response by the presence of some particular collective modes. We also predict a special regime of light scattering from dense, subwavelength-sized clouds of atoms. The ensemble of atoms scatters less light than a single atom does, precisely due to their strong near-field interactions. In the second part of my talk, I focus on hexagonal lattices of silicon nanopillars. These arrays show collective modes which we can this time experimentally visualize with 10 nm resolution thanks to cathodoluminescence spectroscopy. With this technique, we focus a 30-keV electron beam on the sample. The electrons have a strong electric field surrounding them and serve as a localized coherent broadband optical excitation source that excites modes inside the nanopillars at the nanometer scale. The periodic repetition of a resonant nanopillar leads to the creation of a band diagram; it is a photonic crystal slab. With angle-resolved cathodoluminescence spectroscopy we have access to that band diagram in a single measurement. We found experimental evidence of the existence of modes with an effective refractive index very close to one.

Abstract: Optical coherence tomography (OCT) enables volumetric rendering and the generation of fundus images that precisely register OCT images to fundus features. Yet, there is little data about how a child’s retina develops. This limits our knowledge of how diseases affect a child’s vision early in life and makes diagnosis of these diseases more difficult. The introduction of OCT to pediatric applications has been impeded by several factors, among them limited speed of data acquisition and analysis, difficulty in attaining stable fixation of the pediatric patient on a target over a period of time long enough to allow reliable analysis, short working distance, etc. The system described here integrates three major components: a) a computer-controlled video player that plays attention attracting movies and directs the subject’s fixation to a central point target, b) a retinal birefringence scanning (RBS) subsystem for fast detection of central fixation by detecting the position of the fovea, and c) a long-working-distance optical coherence tomography subsystem for acquiring 3D images from the retina. Significant issues need to be resolved. These include separating the two systems spectrally, presenting suitable visual targets on a small LCD screen, communication between the two systems in real time, precise alignment, simultaneous aiming of the two systems, etc. Of particular importance is the fact that most optical components in the combined path can also affect the polarization state of light in the RBS path, as does the human cornea. To address this issue, a computer model was employed, to optimize system performance. The hybrid system integrating OCT and RBS acquires and analyzes data only during moments of central fixation. This is expected to significantly reduce the image processing time and shorten overall exam duration.

Abstract: La manipulation de la lumière à l’échelle de la longueur d’onde (et sublongueur d’onde) a été repensée par l’introduction des milieux périodiques tels que les cristaux photoniques et plus récemment les métamatériaux. La dispersion spatiale et temporelle est ici présentée en tant qu’élément de contrôle de la lumière, qui permet d’atteindre des fonctionnalités non-triviales dans les dispositifs nano photoniques basés sur des milieux périodiques. En particulier, dans le cadre de ce séminaire, seront discutées : la mise en forme spatiale et temporelle des faisceaux autocollimatés dans des directions arbitraires dans les cristaux photoniques mésoscopiques (composés par différentes périodes), la gestion de l’adaptation d’impédance, et la réalisation de guides, miroirs et cavités avec des interfaces planaires. La caractérisation expérimentale de ces cavités et la possibilité de les exploiter pour la réalisation de capteurs et de pince optiques innovants seront également présentées. La conception de pinces optiques plasmoniques en champ proche et ultra-compactes sera finalement exposée.

Abstract: This contribution will start with a brief introduction to nanophotonics and plasmonics. In this context, different nanofabrication techniques will be discussed with an emphasis on the DNA-Origami approach. In particular, we employ this technique as a platform where metallic nanoparticles as well as single organic fluorophores can be organized with nanometer precision in three dimensions. With these hybrid structures we initially study the nanoparticle-fluorophore interaction in terms of the distance-dependent fluorescence quenching and angular dependence around the nanoparticle. Based on these findings, we build highly efficient nano-antennas based on 100 nm gold dimers which are able to strongly focus light into the sub-wavelength region where the fluorophore is positioned and produce a fluorescence enhancement of more than three orders of magnitude. Using this highly confined excitation field we were able to perform single molecule measurements in solution at concentrations as high as 25 µM in the biologically relevant range ( larger than 1µM). Additionally, we report on a controlled increment of the radiative rate of organic dyes in the vicinity of gold nanoparticles with the consequent increment in the number of total emitted photons. We also employ the nanoantennas to mediate the fluorophore emission and thus to shift the apparent emission origin: A single molecule mirage. We will discuss how DNA-Origami can also improve the occupation of other photonic structures, the zeromode waveguides (ZMWs). These structures, which consist of small holes in aluminum films can serve as ultra-small observation volumes for single-molecule spectroscopy at high, biologically relevant concentrations and are commercially used for real-time DNA sequencing. To benefit from the single-molecule approach, each ZMW should be filled with one target molecule which is not possible with stochastic immobilization schemes by adapting the concentration and incubation time. We present DNA origami nano-adapters that by size exclusion allow placing of exactly one molecule per ZMW. The DNA origami nano-adapters thus overcome Poissonian statistics of molecule positioning and furthermore improve the photophysical homogeneity of the immobilized fluorescent dyes. Finally, we will discuss future potential applications of this technology on smartphone-based point of care diagnostic platforms and diagnostics.

Abstract: Les signaux RF sont omniprésents aujourd'hui dans les domaines de la télédétection, de la communication, et de la guerre électronique. En particulier dans le domaine radar, le traitement des signaux détectés doit alors se faire instantanément, ce qui requiert des performances en termes de bande passante, de temps d'analyse et de dynamique que les technologies purement électroniques peinent à satisfaire. En particulier, l'étape de conversion analogique-numérique fait appel à de nombreuses étapes de transposition, amplification, filtrage, multiplexage, qui dégradent les performances du traitement de signaux très large bande. Des solutions alternatives doivent être recherchées. Une voie prometteuse est celle du traitement analogique de signaux placés sur porteuse optique. Dans ce contexte, les cristaux dopés aux ions de terre rare offrent des propriétés remarquables lorsqu'ils sont refroidis à la température de l'hélium liquide. Combinant une largeur inhomogène de plusieurs dizaines de GHz et une résolution spectrale en général très inférieure à 1 MHz, capables par ailleurs de mémoriser à l'échelle de la microseconde un profil spectral pendant des temps qui peuvent atteindre plusieurs jours, ces matériaux peuvent être utilisés comme processeurs optiques programmables, basés sur le phénomène de creusement spectral ou "spectral hole burning" (SHB). Dans ce séminaire je présenterai 3 architectures utilisant le SHB, appliquées à différents enjeux de traitement du signal. Je commencerai par le plus abouti: l'analyseur spectral large bande "arc-en-ciel", conçu au Laboratoire Aimé Cotton et à présent en cours de transfert technologique à Thales Research&Technology. Ce dispositif est basé sur la programmation de réseaux de diffraction dans le cristal. Puis je décrirai la première réalisations d'un processeur de renversement temporel, dans lequel une ligne dispersive est programmée. Enfin, je présenterai comment utiliser le SHB pour l'imagerie acousto-optique en milieu diffusant à l'aide d'un processeur basé sur la programmation d'un filtre passe-bande.

Abstract: The development of micro‐ and nanotechnologies has recently opened a wide range of possibilities for controlling light at the wavelength scale or below. A fine control of the light (emission, transport and detection) in small volumes is at the heart of various applications. Most of these applications rely on the use of optical resonances, which can be either localized in small volumes or delocalized over the system. Plasmonic nanoantennas (or nanoresonators) are able to confine light in extremely small volume of the order of 1/10,000 of a cubic wavelength, i.e., well below the diffraction limit. Light confinement is associated with large field exaltations that can be used to enhance non‐linearities, spontaneous emission, or absorption. In the context of spontaneous emission, these properties lead to huge enhancement factors. Unfortunately, very large enhancement factors usually correspond to emitters located only a few nanometers away from a metal surface. As a consequence, quenching through non‐radiative energy transfer to the nearby metal constitutes the dominant relaxation process. The resulting radiative efficiency is extremely low and it is thus often thought that very large spontaneous decay rates in plasmonic nanoantennas are not of practical interest. However, recent experimental demonstrations have shown that it is possible to obtain large enhancement factors (typically above 1000) with non‐negligible radiative efficiencies (a few tenths of percent). We have developed an electromagnetic theory that is able to unravel the interplay between the decay into the antenna mode and the quenching. The theory that is valid for any plasmonic nanoantenna relies on a modal formalism recently developed for photonic and plasmonic nanoresonators. In particular, we have shown that overcoming the quenching is possible in different types of metallo‐dielectric nanoantennas. Such an in‐depth quantitative analysis has allowed us to provide guidelines to design plasmonic nanoantennas that are able to overcome quenching and provide both large spontaneous decay rates and large radiative efficiencies.

Abstract: Single-photon sources are crucial to a number of applications that take advantage of the exotic phenomena stemming from the laws of quantum physics. Color centers in diamond have gained much attention in this context, essentially for their unique optical properties at room temperature, but a substantial basic research effort in nanophotonics and electronics is still required. In my seminar I will introduce nano-optical concepts to largely improve single-photon sources and the work that we have recently undertaken towards obtaining highly efficient single-photon sources based on the silicon-vacancy color center in diamond. I will present results pertaining to diamond implantation and characterization as well as on the design of antenna configurations that can significantly improve the outcoupling efficiency and the emission rate. I will also discuss some ideas on the possibility of electrical pumping and the expected performances.

Abstract: From sand dunes to Faraday crispations, granular materials, i.e., large agglomeration of macroscopic particles, are ubiquitous in nature, industry and our daily lives with widespread applications ranging from the prediction of natural disasters (e.g. snow avalanches and debris flows) through the enhancement of energy efficiency in industries (e.g. mining, civil engineering) to emerging new technologies (e.g. powder based additive manufacturing). Due to the energy dissipation at the individual particle level, granular systems are highly dissipative and consequently their stationary states are typically far from thermodynamic equilibrium. Therefore, understanding how the mobility of individual particles influences the collective behavior is crucial in describing granular materials as a continuum. At the <code class='spip_code spip_code_inline' dir='ltr'>microscopic' level of individual particles, I will introduce a recently developed microwave radar system that is capable of tracking a metallic particle continuously in three dimensions and discuss its advantages and limitations in comparison to other particle imaging approaches. At the</code>macroscopic' level of collective motion, I will talk about the pattern forming scenario of partially wet granular materials (e.g., wet sand on the beach) with a focus on how liquid mediated particle-particle interactions influence the collective behavior. In particularly, I will focus on the formation of density-wave fronts in an oscillated wet granular layer undergoing a gas-liquid-like transition and discuss how the emerging time and length scales are associated with the competition between the time scale for the collapse of particles due to short ranged attractive interactions and that of the energy injection resisting this process.

Abstract: TBA

Abstract: A central challenge in understanding extreme events in physics is to develop rigorous models linking the complex generation dynamics and the associated statistical behaviour. Quantitative studies of extreme phenomena, however, are often hampered in two ways: (i) the intrinsic scarcity of the events under study and (ii) the fact that such events often appear in environments where measurements are difficult. A particular case of interest concerns the infamous oceanic rogue or freak waves that have been associated with many catastrophic maritime disasters. Studying rogue waves under controlled conditions is problematic, and the phenomenon remains a subject of intensive research. On the other hand, there are many qualitative and quantitative links between wave propagation in optics and in hydrodynamics, because a nonlinearly-induced refractive index perturbation to an optical material behaves like a moving fluid and is described mathematically by the same propagation equation as nonlinear waves on deep water. In this context, significant experiments have been reported in optics over the last two years, where advanced measurement techniques have been used to quantify the appearance of extreme localised optical fields that have been termed "optical rogue waves". The analogy between the appearance of localized structures in optics and the rogue waves on the ocean’s surface is both intriguing and attractive, as it opens up possibilities to explore the extreme value dynamics in a convenient benchtop optical environment. The purpose of this talk will be to discuss these results that have been obtained in optics, and to consider both the similarities and the differences with oceanic rogue wave counterparts.

Abstract: Modern dense seismological deployments consisting of many hundreds of sensors allow the reconstruction of the seismic wavefield in the near-field from noise cross-correlations. The correlation approach makes it thus feasible to resolve the focus or focal spot, which is a characteristic feature of the cross-correlation wavefield at zero lag time. Based on the equivalence of time-reversal and cross-correlation, applications in acoustics, nondestructive testing, and medical imaging have long been relying on focal spot properties. I introduce focal spot based imaging in seismology on two (very) different scales using data from two dense arrays, each consisting of 1000 stations, that cover a ~1 km2 area and half of the United States, respectively. In the presentation I will provide a brief history of re-focusing in seismology, before discussing aspects of focal spot based imaging that arise in the seismological surface wave context, and that have not been at the center of research in acoustics or elastography. These aspects include the relation to Aki's 1957 spatial autocorrelation method; the shape of the surface wave focal spot depending on horizontal and vertical motion; interference of surface wave fields and body wave fields, effects on the focal spot, and strategies for wave field separation; and the resolution of anisotropy. I will emphasize implications for spatial resolution with an outlook to a transfer of the super-resolution concept to seismology.

Abstract: The eye is the only optical window to a neuro-vascular network in our body, located in the retina. Probing this retinal network at the scale of optical wavelengths (typically a micron) provides insight not only on the major eye diseases (Age-related Macular Degeneration, Glaucoma, Diabetic retinopathy), but also on the state of the brain neuro-vascular network and on the related pathologies (Alzheimer, Parkinson, Traumatic brain injuries). These developments have been triggered by the adaptation of adaptive optics to ophthalmology since 1997. I started to work on the subject more than 10 years after. Before building yet another adaptive optics ophthalmoscope, I decided to investigate the optical properties of the eye, and of the aberrations the adaptive optics was to correct for. Indeed, unlike in astronomy, no reliable statistical model of eye motion and aberration dynamics was available. I will present the results of an aberrometry campaign on 50 eyes, and discuss the link between motion and aberration. Then, I will show the latest retinal imaging results we obtained with the ECUROeil adaptive optics ophthalmoscope, a system we built and designed based on the aberrometry results. These images reveal blood flow in arteries and capilaries at 200Hz, as well as the fine structure of the optic nerve head. Some of these images prompted us to design novel imaging instruments, using high resolution structured illumination.

Abstract: On étudie les résonances plasmoniques de particules métalliques isolées à l'aide de techniques d'équations intégrales. On effectue un développement asymptotique des champs électromagnétiques par rapport au volume de la particule et on évalue les sections efficaces d'absorption et de diffusion.

Abstract: In disordered media, the absence of diffusion arising from the spatial localization of single-particle states is known as Anderson localisation (AL). In three dimensions, AL manifests itself as a phase transition which occurs at a critical energy or at a critical disorder strength (the mobility edge) separating a metallic phase where states are spatially extended, from an insulating one where states are localized. Theoretically, much efforts have been devoted to the study of the critical properties of the Anderson transition (AT), such as wave-function multifractality or critical exponents. In practice however, only a handful of experiments have found evidence for the 3D Anderson transition, among them cold atoms, and even fewer have investigated its critical features (mostly in the context of quantum-chaotic dynamical localization). In addition to the intrinsic difficulty of achieving wave localization in three dimensions, one reason for the rareness of experimental characterizations of the Anderson transition is the lack of easily measurable observables displaying criticality. In this talk, I will show that the critical properties of the AT are encoded in two emblematic interference effects observed in momentum space: the coherent backscattering (CBS) and the coherent forward scattering (CFS) peaks, the latter being an effective ``order parameter’’ of the transition. By a finite time scaling analysis of the CBS width and of the CFS contrast temporal dynamics, one can extract accurate values of the mobility edge and critical exponents of the transition in agreement with their best known values to this date.

Abstract: During this seminar, I will show you that Michelson interferometers ae not only complicated experiments to scare students during their bachelor, but can also be of crucial importance for some researchers, as they enable the detection of sub-wavelength mechanical vibrations, even “deep” inside biological tissues. I will present our development of different optical microscopes, based on interferometry that can simultaneously measure some mechanical and biochemical information in various biological samples. We could notably demonstrate that subcellular mechanical fluctuations depend on cellular metabolic activity, offering us an original contrast to detect cells and their pathologies. Finally, I will also present our attempt to detect an electromechanical coupling in mammalian neurons with our microscopes, in order to demonstrate more complete theories explaining how information propagates in the brain.

Abstract: Many optical techniques have been developed for real-time imaging of blood flow. Laser speckle contrast imaging has become one of the most widely used techniques due to its simple instrumentation and its ability to visualize blood flow over a wide range of spatial scales. However, obtaining quantitative blood flow information remains a challenge for laser speckle imaging. Recently, an extension to laser speckle imaging, called Multi-Exposure Speckle Imaging (MESI), was introduced that increases the quantitative accuracy of CBF images. This talk will describe technical developments in laser speckle imaging as well as new methods for three-dimensional visualization of blood vessels that can be used to improve our understanding of blood flow measures inferred from speckle images.

Abstract: I will present a first principles analysis of the electromagnetic response of homogeneous and isotropic media. Using such analysis I will show that space-time causality defines necessary and sufficient conditions for negative refraction. In particular no real stable media can support negative refraction in absence of spatial non-locality and dissipation, while a generic first order correction to local dissipative response is sufficient to have negative refraction. Such results provide a sound answer to the long-standing discussion on the conditions for negative refraction, and they open the way to the classification of media with negative refraction. We see many possible extensions and applications of our anlysis to other domains: such as non-homogeneus or non - isotropic material, surface waves, sound waves, etc. with both theoretical and experimental implications. We look forward to discuss together possible applications of such formalism and results.

Abstract: Random mode mixing in multimode optical fibers has long been considered a nuisance that should be circumvented. However, it was recently realized that fibers with strong mode mixing provide outstanding opportunities for studying the transport of light in disordered media, as they exhibit two unique advantages over scattering samples. First, the transmission through fibers is extremely high, even in the presence of strong mode mixing, and thus information of the input state of the light is only scrambled but not lost. Second, unlike random scattering samples, fibers allow to fully control the coupling of the input light to all the guided modes, thanks to their finite numerical aperture. We have recently used these properties of multimode fibers to study new features of coherent backscattering, also known as weak localization of light. By utilizing a magneto-optical effect, we controlled the interference between time-reversed paths inside a multimode fiber with strong mode mixing, observed for the first time the optical analogue of weak anti-localization, and realized a continuous transition from weak localization to anti-localization [1]. Surprisingly, the chaotic-like dynamics of light in multimode fiber and the extreme sensitivity to external perturbations, open the door for new types of applications. We have recently shown that multimode fibers can be used to generate and distribute secure keys for optical encryption [2]. The fast fluctuations in the fiber mode mixing provide the source of randomness for the key generation, and the optical reciprocity principle guarantees that the keys at the two ends of the fiber are identical. We experimentally demonstrated the scheme using classical light and off-the-shelf components, paving the way towards cost‑effective key establishment at the physical-layer of fiber-optic networks. [1] Y. Bromberg, B. Redding, S. M. Popoff, and H. Cao, Control of coherent backscattering by breaking optical reciprocity, Phys. Rev. A 93, 023826 (2016) [2] Y. Bromberg, B. Redding, S. M. Popoff, and H. Cao, Remote key establishment by mode mixing in multimode fibers and optical reciprocity, arXiv:1506.07892

Abstract: Biological tissues are very strong light-scattering media. As a consequence, current medical imaging devices do not allow deep optical imaging unless invasive techniques are used. Acousto-optic (AO) imaging is a light-ultrasound coupling technique that takes advantage of the ballistic propagation of ultrasound in biological tissues to access optical contrast with a millimeter resolution. Coupled to commercial ultrasound (US) scanners, it could add useful information to increase US specificity. Thanks to photorefractive crystals, we developed a bimodal AO/US imaging setup based on wavefront adaptive holography that recently showed promising ex vivo results on tumors for which acoustical contrast were not significant. However, before any clinical applications can be thought of, two major issues of in vivo imaging have to be addressed. The first one concerns current AO sequences that take several tens of seconds to form an image, far too slow for clinical imaging. The second issue concerns in vivo speckle decorrelation that occurs over less than 1 ms, too fast for photorefractive crystals. In this talk, I will present a new US sequence that allows increasing the framerate of at least one order of magnitude and an alternative light detection scheme based on spectral holeburning in rare-earth doped crystals that allows beating speckle decorrelation as first steps toward in vivo imaging.

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Abstract: We appear opaque because our tissues scatter light very strongly. Traditionally, focusing of light in biological tissues is confounded by the extreme scattering nature of tissues. Interestingly, optical scattering is time-symmetric and we can exploit optical phase conjugation methods to null out scattering effects. I will discuss our recent results in using different types of guidestar methods in combination with digital optical phase conjugation to tightly focus light deep within biological tissues. These technologies can potentially enable incisionless laser surgery, targeted optogenetic activation, high-resolution biochemical tissue imaging and more. Fourier Ptychography - Microscopes are complex and fussy creatures that are capable of delivering limited image information. This is because physical optical lenses are intrinsically imperfect. The perfect lenses we draw in high school ray diagrams simply do not exist. I will discuss our recent work on Fourier Ptychographic Microscopy - a computational microscopy method that enables a standard microscope to push past its physical optical limitations to provide gigapixel imaging ability.

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Abstract: Wave mixing is one of the most basic nonlinear optical processes. In the vast majority of cases, it is studied for one or more interacting quasi-monochromatic waves, where all temporal and spatial scales are large and similar. In contrast, there are very few studies of the mixing of wave packets having different wavelengths, spectra, and temporal and spatial profiles. Indeed, these configurations involve rather a complicated and non-intuitive wave mixing process such that it is difficult to assess, a priory, what would be the final spatial length, temporal duration and spectral width of the pulses generated by the interaction. In this talk, I will describe one of the first thorough studies of such complex configurations, and present a systematic way to predict and interpret the wave mixing process in terms of exchange of spectral and spatial Fourier components between the interacting pulses. I will focus on a simple example where an intense short pump pulse is periodically-patterned such that it induces a transient Bragg grating (TBG) whose stop gap matches the frequency of an incoming long signal pulse. I will demonstrate the validity of our interpretation using exact numerical simulations as well as a novel derivation of coupled mode theory for pulses propagating in time-varying media. Unlike previous attempts to derive such a model, our approach involves no approximation, and does not impose any restriction on the spatio-temporal profile. Moreover, the effect of modal dispersion on mode evolution and on the coupling to other modes is fully taken into account. It also avoids various artifacts of previous derivations by introducing the correct form of the solution, and can be applied to any other wave system. I will then discuss the advantages and limitations of the proposed approach and demonstrate it for generating, switching and reversing ultrashort pulses propagating in several material platforms, such as silica fibers, semiconductor waveguides and plasmonic waveguides.

Abstract: Functional ultrasound imaging (US) has the potential to provide a high resolution image of the brain. However, finding functionally relevant structures can draw limitations to this resolution if we rely on matched anatomical atlases. Those limitations can arise from the amount of regional variations across individuals (this is large in human brain and not so large in rodent brain). They can also come from the way those atlases were produced, ie. based on anatomical markers or functional observations that are not relevant to the functions we investigate. Thus, to fully benefit from our resolution and achieve a relevant functional mapping, we need to apply data driven analysis and unsupervised learning methods such as Independent Component Analysis (ICA) and Deep Neural Networks (DNN). One way to go is to identify independent signal sources in each experiment and the corresponding activations. ICA is commonly used in fMRI and here I'll show that it's equally beneficial in fUS. Resting state activity was recorded in rodents during anaesthesia. Cortical and subcortical areas were reproducibly delineated at a fine scale at group level and back-projected to individual level. Correlation maps across areas delineated with ICASSO method reveal a strong correlation in the midline as well as local and contralateral connections. Using sparse autoencoders and DNNs with stacked layer configurations resulted in improved quality maps. The latter method was able to distinguish individual barrel activations in a whisker stimulation paradigm. The technique provides access to analysis at higher resolution and in paradigms when one cannot estimate the timing of the studied activations.

Abstract: Acoustic imaging methods are useful in order to localize sound sources, examine how these radiate sound, characterize the acoustic properties of materials, and analyze complex sound fields. These methods typically rely on measurements with an array of microphones in order to characterize thespatio-temporal properties of the sound field under study. This talk has an emphasis on spherical array processing and sparsity promoting methods based on Compressive Sensing (CS). CS constitutes an interesting alternative to classical least-squares approaches. We will discuss applications where these techniques can be useful, such as sound source localization, sound absorption estimation, and sound field visualization in enclosures.

Abstract: The ear works as a remarkable sound detector. Hearing can indeed operate over six orders of magnitudes of sound-pressure levels, with exquisite sensitivity and sharp frequency selectivity to weak sound stimuli. Curiously, the ear does not work as a high-fidelity sound receiver, introducing in the auditory percept “phantom” tones that are not present in the sound input. In this talk, I will present micromechanical experiments at the level of the cellular microphone of the inner ear – the hair cell – whose function is to transduce sound-evoked vibrations into electrical nervous signals. I will discuss the origin of stiffness and drag of the mechanoreceptive hair bundle, a tuft of cylindrical protrusions that protrudes from the apical surface of each cell, and show that hair cells can power spontaneous oscillations of their hair bundles. Oscillations are thought to result from a dynamical interplay between mechanosensitive ion channels, molecular motors, and calcium feedback. We find that oscillations of the hair bundle allow the hair cell to actively resonate with its mechanical input at the expense of distortions with properties that are characteristic of hearing. Our results promote a general principle of sound detection that is based on nonlinear amplification by self-sustained “critical” oscillators in the inner ear, i.e. active dynamical systems that operate on the brink of an oscillatory instability called a Hopf bifurcation.

Abstract: The recent realization of topological phases in insulators and superconductors has given incentive to explore analogous realizations. Propagation of microwaves in an array of dielectric ceramic cylinders constitutes a flexible experimental platform to investigate properties of topological states. I will show how a defect state in a dimerized chain of resonators can be topologically protected against structural disorder and how this state can be selectively enhanced by combining topological protection with non-hermitian symmetries. Our microwaves experiment contributed to create the active field of `artificial graphene'. I will present results related to the topological phase transition experiences by a honeycomb lattice under uniaxial strain: The gap opening and, depending of the boundary shape, the emergence of specific edges states.

Abstract: En 2006, la proposition faite par John Pendry et Ulf Leonhard de mettre au point une cape d'invisibilité a connu un important retentissement médiatique. Cette dernière est basée sur l'optique transformationnelle. Fondée sur l'invariance des équations fondamentales de l'électromagnétisme par rapport aux déformations de l'espace, elle ouvre une voie novatrice pour la conception des dispositifs nano-technologiques en optique. Ainsi, une déformation de l'espace peut être traduite sous forme de relations constitutives de milieux équivalents. La non-invariance des équations de l'élastodynamique dans le cas le plus général s'ajoute aux contraintes usuelles de fabrication. Les applications potentielles restent les plus utiles (capes sismiques, capes sonores pour les sous-marins). Une étude par Milton et al. met en œuvre la possibilité de contrôle d'ondes mécaniques dans un type de solide bien particulier : les milieux pentamodes (Méta-fluides ou métamatériaux ne pouvant encaisser que la contrainte hydrostatique). Leur tenseur d’élasticité se résume au module de compressibilité comme dans le cas d’un fluide. Nous avons cherché à fabriquer ce type de matériaux ainsi qu'à les intégrer dans le dispositif de capes d'invisibilité. L'anisotropie, la clé de la réussite, a été particulièrement étudiée. Nous avons montré non seulement la possibilité de fabriquer des pentamodes mais également leur utilité plus large dans l'élastodynamique. Le contrôle des propriétés effectives telles que les vitesses d'ondes ou la densité sera également présentée. Enfin nous verrons un exemple de cape d’invisibilité pour la mécanique basé sur le principe de l’inclusion neutre ainsi que sur la transformation directe de mailles. La seconde partie de la présentation sera consacrée à la conception de capes d’invisibilité en milieu diffusif (thermique et optique) et nous parlerons enfin des métamateriaux pour le magnéto-transport avec comme application des sondes à effet Hall.

Abstract: In complex media such as white paint or biological tissue, light encounters nanoscale refractive-index inhomogeneities that cause multiple scattering. It has been considered for a long time as a serious hitch to perform microscopy at depth, as traditional techniques rely on ballistic photons (which intensity exponentially decreases with depth). In the last decade though, wavefront shaping emerged as a powerful tool to overcome this limitation. By controlling the phase pattern of the incident beam, one has been able to focus light through such an opaque highly scattering sample. Applications in imaging as well as in multimode fiber transmission control thus arose in the past few years. After presenting the basics of light scattering and wavefront shaping, we will focus on two (very) different applications. We will first present how the propagation of non classical photon light can be manipulated in such scattering media. Then we will present how to improve imaging depth in biological samples.

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Abstract: Neuromodulation, or the alteration of neuronal activity using electric, magnetic or chemical means, is becoming a therapy of growing importance in the treatment of neurological disorders. Parkinson’s disease is the best example of success of neuromodulation therapy, with over 100,000 patients implanted worldwide with a deep brain stimulation (DBS) device. In the field of epilepsy, neuromodulation therapy holds great promise for the symptomatic treatment of epilepsy, with encouraging results from studies conducted in epileptic patients using a chronically implanted device (RNS, Neuropace, USA) resulting in a mean decrease of 50% of seizures in 50% of patients. However, the mechanisms of action are still elusive, and there is still room for optimization of neuromodulation therapy in epilepsy. In order to address these issues, we are presenting an integrated approach combining electrophysiological and metabolic recordings in a mice model of epilepsy (kainate model) on the one hand, and in silico biophysical model of interictal and ictal activity on the other hand. We present experimental results on local electrical stimulation of the brain in epileptic mice, which suggest a possible decrease of pathological hyperexcitability of brain tissue. We then explore possible biological mechanisms using an in silico model of neuronal activity, which provides further insights in our experimental results. The perspectives of this work in terms of therapeutic potential and disease prevention will be presented.