Seminars

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: It is an exciting time for ultrasound research which is currently seeing a surge of imaging and therapeutic advancements that have transformed the field. These advancements have been enabled by the new technical capabilities of programmable ultrasound scanners which acquire massive amounts of data at a high frame-rate and volumetrically. As a consequence this new generation of ultrasound has seen a broadly expanded range of scientific interrogation, pre-clinical applications, and it is on the cusp of clinical applications that could fundamentally change diagnostic imaging and clinical practice. This talk addresses scientific challenges and opportunities especially as they apply to the brain in super-resolution imaging, functional imaging, and shock wave imaging, but also and to other organs, such as lung ultrasound imaging, and liver cancer imaging.

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: Graphene nanostructures, such as graphene quantum dots (G-QDs), graphene nanoribbons (G-NRs) and carbon nanotubes (C-NTs), combine the unique mechanical and electronical transport properties of sp2- hybridized carbon materials and the optical properties of direct semiconductors provided by the optical gap resulting from the reduction of dimensionally. Among them, the recent developments within the well-known synthesis of G-QDs though bottom-up approach [1] have led to exceptionally well-controlled nanostructures in terms of size, shape and dispersion [2]. The resulting graphene nanoflakes provide tunable emission in the red range, with fluorescence quantum yield close to 1. Furthermore, these nanostructures have revealed to be promising stable emitters of single photons, as shown in our laboratory [2-5]. Here we use transient absorption of 30 fs temporal resolution with polarization-controlled configuration to probe the hot exciton relaxation (internal conversion, Sn→S1) in rectangular G-QDs of various lateral lengths. The nanoflakes are composed of exactly 96, 114 and 132 conjugated carbons (respectively 2.30, 2.71 and 3.11 nm). While the ultrafast electronic dynamics in graphene nanostructures are often being blurred by large broadband photo-induced absorption signals [6-8] (in particular involving triplet states, T1→Tn), here the suppressed aggregation in the studied graphene nanoflakes allows a clear observation and identification of the discrete ground state bleaching and photo-induced emission signals. We selectively excite the different samples at the second optically active electronic transition and, thought the appearance of a photo-induced emission signal at the energy corresponding to the bandedge and red-shifted vibrational replica (i.e. at the position of the steady-state photoluminescence peaks), the dynamics of relaxation were unveiled. The resulting relaxation times range from 100 fs to 175 fs. These results allowed to discuss the mechanism of relaxation, with the effect of the length of the graphene nanoflakes and of the fluence excitation [Quistrebert et al., in preparation]. References: [1] A. Narita, X.-Y. Wang, X. Feng, K. Müllen. Chem. Soc. Rev. 44, 6616 (2015). [2] D. Medina-Lopez, T. Liu, S. Osella, H. Levy-Falk, N. Rolland, C. Elias, G. Huber, P. Ticku, L. Rondin, B. Jousselme, D. Beljonne, J.-S. Lauret, S. Campidelli. Nat. Commun. 14:4728 (2023). [3] T. Liu, B. Carles, C. Elias, C. Tonnelé, D. Medina-Lopez, A. Narita, Y. Chassagneux, C. Voisin, D. Beljonne, S. Campidelli, L. Rondin, J.-S. Lauret. J. Chem. Phys. 156, 104302 (2022) [4] T. Liu, C. Tonnelé, S. Zhao, L. Rondin, C. Elias, D. Medina-Lopez, H. Okuno, A. Narita, Y. Chassagneux, C. Voisin, S. Campidelli, D. Beljonne, J.-S. Lauret. Nanoscale 14, 3826 (2022). [5] S. Zhao, J. Lavie, L. Rondin, L. Orcin-Chaix, C. Diederichs, P. Roussignol, Y. Chassagneux, C. Voisin, K. Müllen, A. Narita, S. Campidelli, J.-S. Lauret. Nat. Commun. 9 :3470 (2018). [6] M.L. Mueller, X. Yan, B. Dragnea, L.s. Li. Nano Lett. 11, 56-60 (2011). [7] D. Sebastian, A. Pallikkara, H. Bhatt, H.N. Ghosh, K. Ramakrishnan. J. Phys. Chem. C 126, 11182-11192 (2022). [8] M. Reale, A. Sciortino, M. Cannas, E. Maçoas, A.H.G. David, C.M. Cruz, A.G. Campana, F. Messina. Materials 16, 835 (2023). Co-authors: Sébastien QUISTREBERT, Daniel MEDINA-LOPEZ, Stéphane CAMPIDELLI, Jean-Sébastien LAURET, Elsa CASSETTE 1 LuMIn (UMR 9024), Université Paris-Saclay, ENS Paris-Saclay, CNRS, CentraleSupélec, 91400 Orsay, France, 2 NIMBE/LICSEN (UMR 3685), Université Paris-Saclay, CEA, CNRS, 91191 Gif-sur-Yvette, FRANCE

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: Due to their exceptional coherence properties, cryogenically-cooled rare-earth ion ensembles embedded in crystals are at the core of numerous research domains, ranging from signal processing and biological imaging to quantum memories and quantum computing. The success of these applications critically relies on the effective control and mitigation of decoherence phenomena. Amongst the many sources for decoherence, the coupling between the atomic lines and the mechanical strain (referred to as optomechanical coupling) has remained relatively unexplored. In this seminar, I will highlight the significant role played by optomechanical coupling, inherent to the monolithic nature of the rare-earth-doped crystals, and describe its influence through several mechanisms. First, the sensitivity of the atomic lines to internal stress renders them highly susceptible to vibrations, requiring great precautions when resorting to closed-cycle cryogenics. This bears major consequences for the industrial development of advanced quantum technologies. Second, the excitation of the ions themselves has been shown to lead to local crystal strain, which in turn can create excitation-induced decoherence in the neighbouring ions. I will conclude with a few exciting perspectives for optomechanics in rare-earth ion-doped crystals, including the generation and control of macroscopic quantum states of motion.

Abstract: Imaging technologies rooted in quantum principles, known as quantum imaging, can surpass classical methods or offer novel imaging capabilities otherwise unattainable. For instance, some of these methods exceed the diffraction limit or enable sub-shot-noise imaging. In our work, we harness the quantum properties of entangled photon pairs within the realm of adaptive optics. Specifically, we demonstrate that measuring spatial correlations between photon pairs allows for more effective aberration correction in a label-free imaging system compared to conventional methods. This approach could play a pivotal role in the development of future quantum microscopes. In this presentation, we'll delve into quantum entanglement, imaging, adaptive optics, and there will even be cat pictures — you should come and see!

Abstract: Colors in organisms can be produced either chemically by pigments or physically by the interference of light scattered from photonic nanostructures or sometimes by both. Fade-proof, vivid, saturated structural colors that have evolved over millions of years are an ideal source to look for natural solutions to our current technological challenges and can provide facile biomimetic routes for eco-friendly materials synthesis towards functional applications from sensors, photonics, energy harvesting to catalysis. However, given that the underlying nanostructures are overwhelmingly diverse in form and function, their characterization has lagged for over a century. I have pioneered the use of synchrotron Small Angle X-ray Scattering (SAXS) as a high throughput technique to structurally and optically characterize biophotonic nanostructures from hundreds of species, in a comparative framework. This has led to the understanding that all these diverse, mesoscale nanostructures share a unifying theme – they appear to be self-assembled within cells by bottom-up and/or directed processes. For instance, I led the discovery of single gyroid photonic crystals in the iridescent green wing scales of certain lycaenid and papilionid butterflies whose growth beautifully pre-empt our current engineering approaches. In this seminar, I will broadly summarize our current state of knowledge about the structure, function, development and evolution of organismal structural colors in insects, highlighting our recent progress on the development of butterfly wing scale nanostructures using time-resolved confocal and super-resolution microscopy.

Abstract: The propagation of elastic waves in heterogeneous media is a fundamental research topic that concerns composite materials, porous materials, colloids and emulsions, with the aim of characterising these materials or optimising their performance. This topic has benefited from renewed interest with the development in the 2000s of metamaterials and in particular locally resonant metamaterials. These composite materials consist of a fluid or solid matrix containing an ordered or random distribution of scatterers, whose resonances alter wave propagation. In some cases, the macroscopic mechanical properties of these materials are modified in non-ordinary ways, opening the way to wave control or damping. Other strategies have been implemented to control or guide waves, such as phononic crystals or topological metamaterials. Recently, the control of spatial correlations in random scatterer distributions has emerged as a new approach, and in particular the use of hyperuniform distributions has proven to be particularly relevant. In this context, the objective of the presented work is to study the influence of positional correlations between scatterers and strong sub-wavelength resonances on the propagation of coherent elastic waves. To achieve this, the work is based on the systematic comparison of results from statistical models, numerical simulations and experiments. The numerical results are obtained using the MuScat calculation code, developed in the laboratory. This code is based on the resolution of the multiple scattering equations as well as on the development of the incident and scattered fields on the cylindrical or spherical harmonics. It allows the treatment of various wave interaction problems with many scatterers, without any restriction on the distribution of the particles, nor on their polydispersity in size or elasticity. The MuScat code is used to study the influence of positional correlations between scatterers on coherent waves. Two types of correlation are investigated: short-range correlations, which are based simply on an exclusion distance between each pair of scatterers, and long-range correlations, which apply to the whole distribution. One type of material that exhibits these long-range correlations is hyperuniform media, which are characterised by the cancellation of density fluctuations as the volume of the medium tends towards infinity. Numerical simulations highlight the elastic wave transparency regime of hyperuniform media, as well as the existence of complete stop bands. Short-range correlations, which are much simpler to implement, lead to similar results. In a second step, we investigate the propagation of coherent elastic waves in a distribution of dense beads embedded in a solid matrix. This particle-matrix pair exhibits two sub-wavelength dipolar resonances: a translational resonance that affects both longitudinal and transverse waves and a rotational resonance that affects only transverse waves. Longitudinal coherent waves are then strongly influenced by the translational resonance and the study of statistical models shows that wave conversions are particularly important. Transverse coherent waves are influenced by translational and rotational dipole resonances, and increasing concentration leads to the simultaneous propagation of two coherent waves. The influence of spatial correlations, especially short-range correlations, finally allows to optimise these non-ordinary effects.

Abstract: We're interested in wave propagation in a medium with imperfect contacts, describing cracks, disbonds, etc. We'll start by describing families of nonlinear contact laws and their consequences for wave propagation (e.g. generation of harmonics). First, we'll describe families of nonlinear contact laws and their consequences for wave propagation (e.g. harmonic generation). Then, we'll consider a periodic network of nonlinear interfaces, which we'll homogenize. We'll show the emergence of a nonlinear behavior law, which leads to the emergence of shock waves. Finally, a recent generalization of homogenization to randomly distributed cracks will be presented. In particular, it will be shown that the correctors converge to a normal distribution.

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: Earthquakes in nature follow several statistical properties. In particular, the distribution of energy released by an earthquake (Gutenberg-Richter's law) and the frequency of aftershocks after a large event (Omori's law) are both power-laws. By studying several earthquake models, we show that these properties result from the spatial distribution of the stress field. This field can be described as a random curve for one-dimensional models and a random surface for two-dimensional models. Using this analogy, a series of predictions is made that includes the Gutenberg-Richter law, the b-value, and, for two-dimensional models, the existence of aftershocks and their temporal distribution following Omori's law.

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: Full-field imaging of Lamb waves is a technique used for the nondestructive evaluation of engineered thin-walled structures. It involves the excitation of Lamb waves (usually at a single point) and then measuring the time histories of propagating Lamb waves at multiple spatial points using non-contact vibration sensors, such as laser vibrometers or air-coupled transducers. With the use of signal processing techniques, it is possible to estimate the properties of the measured waves, such as energy or wavenumber. Based on these properties, it is possible to detect and characterize damage in the structure, as well as estimate the elastic properties of the material. During this seminar, I will present the principles of full-field imaging of Lamb waves and their application for the nondestructive evaluation of thin-walled samples. I will also discuss the possibility of using full-field imaging for quantifying the effect of surface contamination on the propagating Lamb waves.

Abstract: Compressed imaging aims at maximizing the amount of collected information while minimizing the number of measurements, relying on some sample-based priors. The main advantages of this approach can be to speed up the acquisition rate, allow single shot measurements, improve resolution or the signal to noise ratio, gathering multimodal contrasts, minmizing the invasiveness of potentially photo-toxic probing beams etc. To achieve so, it turns out that random functions are almost ideal to probe samples. Light speckles have then been extensively used to provide compressed and multiplexed images, in particular using scattering samples. In this seminar, I will present our contribution to compressive imaging through super-resolution microscopy as well as wavefront sensing. We investigated 3D super-resolution fluorescence microscopy both by saturating fluorescence excitation and performing stimulated emission with speckles. I will also show our results about high-resolution multiplexed, multispectral and polarimetric wavefront imaging and their future application to optical metrology and bio-imaging.

Abstract: In physical acoustic laboratories, wave propagation experiments often suffer from unwanted reflections at the boundaries of the experimental setup. We propose using multidimensional deconvolution (MDD) to post-process recorded experimental data such that the scattering imprint related to the domain boundary is completely removed and only the Green's functions associated with a scattering object of interest are obtained. The MDD results consist of the Green's functions between any pair of points on the closed recording surface, fully sampling the scattered field. We apply the MDD algorithm to post-process laboratory data acquired in a 2D acoustic waveguide to characterize the wavefield scattering related to a rigid steel block while removing the scattering imprint of the domain boundary. The experimental results corroborate that MDD is an effective and general method to obtain the experimentally desired Green's functions for arbitrary inhomogeneous scatterers.

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: Les propriétés de creusement de trou spectral (Spectral Hole Burning, SHB) rendent les ions de terres rares particulièrement attractifs pour le traitement large bande de signaux RF et les mémoires quantiques. Ces applications exploitent en effet les longs temps de cohérence des transitions optiques de ces systèmes lorsqu’ils sont refroidis à température cryogénique (3K). Nous démontrons ainsi un analyseur de spectre de radiofréquence (RF) avec une bande passante instantanée de plusieurs dizaines de gigahertz. Nous avons implémenté l'architecture dite « arc-en-ciel », dans laquelle les composantes spectrales RF portées optiquement subissent une dispersion angulaire géante (6 rad / nm) dans le cristal. Cette dispersion gigantesque est obtenue en stockant un motif spectro-spatial dans les sous-niveaux Zeeman de l'état fondamental 3H6 de Tm: YAG avec deux faisceaux laser périodiquement balayés en fréquence et angulairement balayés. La bande passante hyperfréquence d'intérêt étant alors étalée sur plus de 200 milliradians, elle peut être imagée et acquise avec un photo-détecteur pixélisé à haut débit. Les techniques optiques (balayages et stabilités de fréquence laser, opto-mécanique) ont été maitrisées pour réaliser des essais opérationnels hors laboratoire. Ce démonstrateur permet d’obtenir le spectre de signaux micro-ondes complexes sur une bande passante instantanée de 40 GHz, avec une résolution temporelle de l'ordre du µs, une résolution de fréquence adaptable jusqu'au MHz, sur une grande plage de dynamique multi-signaux. L'analyse est continue, conduisant à une probabilité de 100% d'interception des signaux RF, qu’ils soient continus, modulés en fréquence, courts ou impulsionnels, et ceci est réalisé avec une faible latence. Cette architecture ne nécessite pas de conversion analogique-numérique avec une dynamique élevée, ni d’imposants calculs de densité spectrale de puissance, qui sont les principaux goulots d'étranglement dans les techniques d'analyse spectrale ultra-large bande standard.

Abstract: DNA and RNA has emerged as an exceptional molecular building block for nano-construction due to its predictable conformation and programmable intra- and inter-molecular base pairing interactions. A variety of convenient design rules and reliable assembly methods have been developed to engineer DNA nanostructures of increasing complexity. The ability to create designer DNA architectures with accurate spatial control has allowed researchers to explore novel applications in many directions, such as directed material assembly, structural biology, biocatalysis, DNA computing, nano-robotics, disease diagnosis, and drug delivery. In this talk I will discuss some of our work in the field of structural nucleic acid nanotechnology, and present some of the challenges and opportunities that exist in DNA and RNA based molecular design and programming. Specifically, I will discuss some of the new designs for 3D DNA crystals and the use of the crystals as host to organize guest molecules and visualize their atomic level structures. I will discuss the use of DNA template to organize dye molecules for long range energy transfer over sub-micron distances for potential light harvesting applications. I will also discuss our progress in using DNA and RNA nanotechnology for biomedical applications.

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: With the increasing use of wireless communications, exposure to electromagnetic waves has become a perennial concern where the risk or the perception of risk are omnipresent. With each new technological development, the possible health effects are analyzed and challenged as well as the associated levels of protection. The 5G debate is a telling example. In this approach, tools to measure or calculate exposure to electromagnetic waves are essential. For nearly 30 years, the tools used to measure or calculate exposure levels have evolved and adapted to new protocols, new frequencies and new uses. The deterministic, experimental or numerical methods used in dosimetry have been confronted with the complexity and versatility of the systems. Stochastic approaches and artificial intelligence have supplemented deterministic approaches in dosimetry. The challenges and different methods used will be developed in this presentation.

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: This presentation will explore some basic concepts and principles in quantum computing, from quantum gates to the Quantum Fourier Transform (QFT). Shor's algorithm, a polynomial-time algorithm for integer factorization, will be presented, alongside a detailed study of its step-by-step implementation on a multi-qubit machine. Finally, an in-depth look at the computational complexity of this implementation, as well as simulation results, will be presented.

Abstract: How far can we go in shaping the random spontaneous emission of quantum emitters? The designs rules that have been established over the years are based on leveraging the Purcell effect—the dependence of the fluorescence rate of point-source emitters to their surrounding environment. Here, we experimentally show that this framework is not necessarily relevant for ensembles of interacting emitters and that a different rule governs their interactions with their environment—the local Kirchhoff law recently introduced by Greffet et al, which arises when the carriers are in a thermodynamic quasi-equilibrium. We discuss the specificities of this regime by considering assemblies of PbS nanocrystals in direct contact with metal nanostructures. We illustrate the new opportunities of these findings by showing: (i) that the emitters are not quenched by the metal, even if the latter is a very lossy material. In fact, the radiation efficiency is maximum when the emitters are touching the metal. (ii) that the carrier quasi-equilibrium imprints its signature in the photoluminescence and electroluminescence spectra. (iii) that the isotropic and unpolarised luminescence of the PbS nanocrystals can be turned into vector beams and other non-trivial light forms associated with fruitful developments in fluorescence imaging, optical trapping, high-speed telecommunications and quantum technologies.

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: In this talk, I will discuss some of the works I conducted in my last two postdoctoral positions. In a first part, I will present a novel method in laser ultrasonic using ultrashort pulses that allows to image any arbitrary frequency at GHz frequency by intensity-modulating the pump beam. Whispering-Gallery Modes and Zero-Group Velocity Modes are tracked and imaged. In a second part, I will describe a microendoscope, with a 125x250 µm2 footprint, and performing photoacoustic and fluorescent imaging through a multimode fibre using wavefront shaping. Finally, I will present experimental evidences of a novel memory effect in peculiar multimode fibres. A simple model is associated to interpret this phenomenon.

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: All-dielectric nonlinear metasurfaces have recently brought harmonic generation to sub-wavelength level, with spectral and polarization control unachievable in bulk crystals. Not only does meta-optics define a field for investigating nonlinear physics at the nanoscale, but it also opens promising application perspectives. Compared to plasmonics, where the electric field is strongly confined close to the metal surface, the electric field of the resonant modes in dielectric nanoparticles penetrates deep inside their non-dissipative volume, enhancing intracavity light-matter interactions and their nonlinear optical response. In this seminar, I will briefly review the fundamentals of this new research field, from modeling to technology and experimental techniques, with a focus on χ(2) nonlinear nanoantennas and metasurfaces. Then I will comparatively discuss the use of quasi-normal-mode formalism and multipolar expansion of the scattering efficiency to describe nonlinear frequency conversion at the nanoscale. Finally, based on the χ(2) fully tensorial features of AlGaAs metasurfaces, I will present our recent demonstration of 0-2π phase control on an harmonic field, generated for the first time with a sufficient efficiency for practical purposes.

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: 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: Among fluorescence microscopy techniques, single molecule localization microscopy (SMLM) is a powerful high resolution tool commonly used in biology for both structural and functional imaging. Provided samples are adequately labelled, multiple proteins can be imaged down to the 10-nm scale. Most 3D SMLM strategies resort to Point Spread Function (PSF) shaping to encode the axial information in the shapes of the PSFs. Although they are relatively straightforward to implement and offer a good compromise between precision and capture range, these approaches not only rely on a critical calibration step, but also share a common drawback—the axial information retrieved in relative to the focal plane. As such, they suffer from many experimental non-idealities, which include axial drift, axial chromatic aberration and sample tilt. This lack of reliability has hindered wider use of 3D SMLM. Here, we propose to exploit the near-field part of the fluorescence emission, which yields intrinsic information about the absolute axial position of emitters in the sample. Indeed, at the biological sample/glass coverslip interface, the index mismatch causes a part of the evanescent emission of the molecules to become coupled into propagative waves as they reach the interface. This fluorescence component consequently propagates above the critical angle, and is called Supercritical Angle Fluorescence (SAF). It can be collected through a high numerical aperture objective, and pupil plane filtering allows its extraction from the rest of fluorescence. As the SAF intensity decays with the distance from the emitter to the coverslip, its axial position can be obtained independently from the position of the focal plane. While this detection modality has a shallow capture range (~200 nm), it can be used as a complementary absolute reference in PSF shaping-based SMLM. We use a cross-correlation algorithm that takes both the SAF and PSF shaping axial positions of each molecule as inputs to determine the position of the focal plane and correct the 3D SMLM data. Characterization of the results evidences the efficient correction of chromatic aberration, axial drift and sample tilt. Finally, we provide examples of biological applications to multicolor cytoskeleton, neurons and bacteria samples imaging.

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: The retina is the innermost, light-sensitive tissue of the eye which translates the image of the visual world into electrical neural impulses to the brain to create visual perception. Imaging the retina with a cellular resolution allows an additional inside of the visual perception process, ophthalmic pathologies, but also to neurodegenerative diseases since the retina is an optical window to the brain. However, the desired cellular resolution is hardly achievable mainly because of the ocular aberrations induced by the eye’s optics. Here, we present different strategies to achieve high-resolution in the retina using full-field imaging techniques. These strategies include the use of adaptive optics, dark-field illumination, structured illumination, and spatially incoherence light source combined with optical coherence tomography (OCT). We show how these different strategies could be used to guide retinal laser surgery and to study important retinal features in the living human eye such as neurons and vasculature, helping early disease diagnosis and monitoring of disease evolution.

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: We report a series of experiments on the softening and volumetric compaction of a dense granular pack through acoustic pressure and shear waves. Softening is manifested by a reduction of standing-wave resonance frequency, and of the traveling-wave speed, as the amplitude of the disturbance increases beyond some threshold. We explain these observations using a theoretical model, based on shear transformation zones (STZs), that directly attributes these observations to dynamical heterogeneities and slipping contacts in the granular pack. We also discuss the slow relaxation in granular matter after the cessation of wave disturbance, and attribute the aging process to a broad spectrum of time scales and barrier heights in the STZ dynamics. In so doing, we demonstrate the fundamental connection between nonaffine granular rearrangements, mesoscopic glassy dynamics, jamming and unjamming, and matter-wave interactions.

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: Acoustic-resolution photoacoustics (AR-PA) permits spectral-sensitive imaging in deep tissues by detecting acoustic waves resulting from light absorption. The resolution in AR-PA has been so far limited by acoustic diffraction. Inspired by super-resolution methods introduced in optics, such as PALM, STORM and STED, we seek to overcome the diffraction limit in AR-PA. We demonstrate experimentally in vitro that super-resolution in AR-PA is achievable by such methods as super-localisation, fluctuation-based imaging, and model-based reconstruction. In addition, we show that model-based reconstruction can lead to super-resolution in sparse-array photoacoustic or ultrasound imaging. At the end, we determine strong and weak points of each method and provide an outlook for super-resolution photoacoustic imaging.

Abstract: Hyperbolic materials are anisotropic media which exhibit metallic or dielectric behaviour depending on polarisation. Natural hyperbolic materials as well as hyperbolic metamaterials, composites with engineered optical properties that opened up new avenues for light manipulation, have an unprecedented ability to concentrate light on deeply subwavelength scales which promises a wide variety of new applications in nanophotonic technologies. I developed three new material platforms for the realisation and control of hyperbolic dispersion and described their optical properties. These include a metamaterial based on an array of plasmonic nanocones, heterostructured metamaterial based on Au-ZnO-Au meta-atoms and a natural hyperbolic material CuS. A combined experimental and theoretical study of the optical properties of CuS colloidal nanocrystals show that they exhibit anisotropic plasmonic behaviour in the infrared and support optical modes with hyperbolic dispersion in the visible spectral range. Heterostructured, layered Au-ZnO-Au nanorod metamaterials supporting guided modes were developed with the introduction of a nanoscale dielectric gap in the the meta-atoms. The role of the shape of meta-atoms forming the array has been studied on the example of transformation of nanorods forming the metamaterial into nanocones. The plasmonic mode structure of the individual nanocones and pronounced coupling effects between them provide multiple degrees of freedom to engineer both the field enhancement and the optical properties of the metamaterials. These metamaterials are the first so-called gradient refractive index metamaterials that behave as a medium with elliptic optical dispersion in the region of the nanocone apexes and hyperbolic optical dispersion in the region of the bases. A scalable manufacturing process for these metamaterials allowing mass-production at the centimeter scales has been proposed and developed. The introduced natural and engineered plasmonic hyperbolic materials bring about new opportunities for future exploration and applications of these unusual systems in nanophotonics for linear and nonlinear light manipulation, fluorescence control, surface enhanced Raman spectroscopy as well as hot-carrier plasmonics and photocatalysis.

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: I will discuss here the recent exciting developments associated with non-Hermitian degeneracies, also known as “exceptional points”. After having been studied mostly in the domain of mathematical physics, quite a number of experiments have recently demonstrated how the presence of exceptional points leads to very counter-intuitive effects, such as loss-induced lasing, chiral field modes, topological energy transfer etc. I will try to provide an introduction to this topic as well as an overview of the many different areas of physics in which exceptional points are meanwhile being explored.

Abstract: Acousto-optic imaging coupled with ultrasound modality would be able to discriminate between healthy or diseased biological tissues thanks to the additional optical contrast. Acousto-optic imaging is a multi-wave technique which localizes light in very scattering media with acoustic waves. The main issue is the detection of the localized light. In this presentation the investigated solution is the use of a spectral filter based on spectral hole burning. The goal is to use a YAG crystal doped with thulium ions under a magnetic field, which increases the lifetime of the spectral hole from 10ms to longer than a minute. In this presentation I will first give an overview of the context of my work. Then I will present the acousto-optic images achieved with a long-lived spectral filter in Tm:YAG, in a scattering medium. Lastly, I will also raise the subject of the challenges specific to the addition of magnetic field to my setup.

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: 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.

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.

Abstract: The efficient absorption of audible sound takes an important position since excessive noise exposure becomes a major public health concern. Thus, thin and lightweight absorbers that are both easily installed and capable to absorb sound over a wide frequency range are strongly desired. In the first part of this seminar, I will present some of our recent activities in this field focusing on the perfect acoustic absorption through the interplay of the inherent losses and transparent modes with high Q factor. These modes are generated in a two-port, one-dimensional waveguide, which is side-loaded by isolated resonators of moderate Q factor. In asymmetric structures, near perfect one-sided absorption has been observed (96%) with deep sub-wavelength samples. Granular media are the second most manipulated material by man. However, the propagation of sound in granular solids remains a challenging scientific task. Important features of these media are: (i) the disorder in their packing and (ii) their nonlinear response. In the second part of this seminar, I will present our investigations about energy transport in polydisperse granular chains. After establishing the regime of sufficiently strong disorder, we focus our studies on the role of nonlinearity as this is desribed by Hertzian contact mechanics. By increasing the initial excitation amplitudes we are able to identify three distinct dynamical regimes with different energy transport properties: a near linear, a weakly nonlinear and a highly nonlinear regime. We demonstrate that in the highly nonlinear regime, the the energy is almost ballistically transported through shock-like excitations.

Abstract: Les systèmes d'imagerie polarimétrique permettent de révéler des contrastes qui n'apparaissent pas dans les images classiques. Ils disposent d'un certain nombre de degrés de liberté qui permettent de d'optimiser l'information apportée par ces images. Cette optimisation utilise des critères issus de la théorie du traitement du signal, qui sont différents selon le type d'information recherché dans l'image, par exemple : estimation d'un état de polarisation ou mise en évidence d'un contraste. Je présenterai quelques exemples d'optimisation d'information polarimétrique pour différentes applications, en m'appuyant sur des systèmes d'imagerie polarimétriques adaptatifs que nous avons construits.

Abstract: At low temperature, liquid He transitions into a superfluid state as a result of macroscopic quantum coherence. By confining liquid 4He in well-defined structures of size comparable with the coherence length, nonbulk phenomena can be revealed. For instance, nanofluidic confinement has allowed the study of finite-size effects near the superfluid transition, realizing the most precise test of scaling laws to date. In order to study the thermodynamic properties of confined liquid 4He, we have developed a new set of measurements based upon superfluid nanomechanical resonators. We made an ultrasonic analog of the Fabry-Perot resonator to measure the sound velocity of liquid 4He in a few micron gap, and a superfluid Helmholtz resonator to measure the superfluid fraction and the onset of quantum turbulence in nanofluidic channels. In addition, at very low temperature (T ∼ 10 mK) where the normal component is negligible, the mechanical quality factor of superfluid resonators can be extremely large (Q > 10 billions), which makes nanomechanical superfluid resonators very promising systems for the field of optomechanics and quantum information.

Abstract: The geometric phase is a general concept appearing in several domains in Physics. In wave physics, one can observe their existence in polarization, but they exist also for oscillatory mechanical systems or in quantum mechanics. During this seminar, we will present some geometric phase examples and discuss about the general conditions for their existence. We will then address the geometric phase of polarized waves in scattering media and discuss their relation with anisotropies.

Abstract: Atherosclerosis is one of the most deadly diseases in the world, killing about 17 million people each year by its sequelae, myocardial infarction and stroke. Atherosclerotic plaques are complex, heterogeneous structures and it is difficult to predict which lesion will precipitate a clinical event. Patient and lesion-specific assessment of the disease could provide better guidance to therapeutic interventions, mechanical or pharmaceutical. In this talk, I will present some of our recent work on the development of new technologies for imaging coronary and carotid atherosclerosis, based on catheter-based optical coherence tomography at 5600 frames per second, intravascular and non-invasive photoacoustic imaging, and imaging submicron displacements with ultrasound.

Abstract: Les ions de terre rare en matrice cristalline, bien connus comme matériaux à gain pour les lasers, offrent, lorsqu'ils sont refroidis à la température de l’hélium liquide, des propriétés prometteuses pour le traitement d’information classique ou quantique transposée sur porteuse optique. Combinant une largeur inhomogène de plusieurs dizaines de GHz et une résolution spectrale en général très inférieure à 1MHz lorsqu'ils sont refroidis à basse température (<5K), et capables par ailleurs de mémoriser un profil spectral pendant des temps qui parfois atteignent plusieurs jours, ces matériaux peuvent être utilisés comme processeurs optiques programmables pour une grande variété d’applications. L’intérêt pour de tels processeurs analogiques dans les domaines du radar et de la guerre électronique vient des limites rencontrées par le traitement tout-électronique en régime très large bande (plusieurs GHz). En effet, l’acquisition des signaux et leur conversion dans le domaine numérique présente de sévères limites de performances dès que les fréquences et bandes passantes dépassent quelques GHz. De plus, les débits de données deviennent extrêmement important (jusqu'à plusieurs Tb/s), et nécessitent des puissances de calcul considérables (plusieurs Tflop/s), difficiles voire impossible à implémenter dans des systèmes, en particulier embarqués. Une autre approche consiste à implémenter des pré-traitements analogiques, qui vont réduire en amont le débit de données numériques et donc la puissance de calcul requise. Les ions de terre rare en matrice cristalline sont de très bons candidats pour implémenter ces traitements. Nous avons récemment réalisé une analyse spectrale RF avec 20 GHz de bande passante instantanée, et une cadence de rafraichissement de spectre supérieure à 10 kHz, obtenus grâce à une architecture dite « arc-en-ciel » qui utilise les propriétés spectrales des ions de terre rare en matrice cristalline refroidis à <4 K. Le démonstrateur, réalisé avec des composants optoélectroniques commerciaux et un cryostat à circuit fermé, montre des performances proches de celles requises par les systèmes pour l’analyse du spectre électromagnétique.

Abstract: Transverse Anderson localization is the trapping of waves due a disordered potential which is invariant along propagation direction, canceling the effects of diffraction. It has recently been demonstrated that such form of wave trapping may be exploited to fabricate a novel generation of optical fibers which may either be obtained in plastic or in glass. Here we report the effects of nonlinearity in disordered optical fibers supporting transverse Anderson localization. The presence of localization alters the response of a disordered plastic material turning a self-defocusing behavior into a self focusing one. At high optical power, modes are found reduced in extension and multiple optical beams inside this nonlocal disordered medium are subject to a form of long range interaction. Moreover, we performed wavefront shaping experiments in disordered optical fibers in the transverse Anderson localization regime. By wavefront shaping and optimization, we observed the generation of a propagation-invariant beam, where light is trapped transversally by disorder. These localized states can be excited by extended speckled beams, and activated at a user defined target position, with a higher efficiency with respect to homogeneous (non disordered) systems.