Using fluorescent beads to emulate single fluorophores AlemánCastañeda, L. A., S. Y. T. Feng, R. GutiérrezCuevas, I. Herrera, T. G. Brown, S. Brasselet, and M. A. Alonso Journal of the Optical Society of America A: Optics and Image Science, and Vision 39, no. 12, C167C178 (2022)
Résumé: We study the conditions under which fluorescent beads can be used to emulate single fluorescent molecules in the calibration of optical microscopes. Although beads are widely used due to their brightness and easy manipulation, there can be notable differences between the point spread functions (PSFs) they produce and those for singlemolecule fluorophores, caused by their different emission patterns and sizes. We study theoretically these differences for various scenarios, e.g., with or without polarization channel splitting, to determine the conditions under which the use of beads as a model for single molecules is valid. We also propose methods to model the blurring due to the size difference and compensate for it to produce PSFs that are more similar to those for single molecules.


Acoustic Localization of an Intruder in a Strongly Scattering Medium Van Den Wildenberg, S., X. Jia, J. L. Gennisson, and A. Tourin Physical Review Applied 18, no. 6 (2022)
Résumé: Localizing an intruder submerged in a strongly scattering medium, such as a dense granular suspension, is a practical challenge. Here we extract the coherent ultrasonic echo from a steel ball submerged in a dense glassbead packing saturated by water, by using a standard singleelement ultrasonic transducer thanks to a configuration averaging process. Different configurations of the granular packing are created by the nonaffine motion of the beads with a mixing blade, akin to the Brownian motion, in the vicinity of the intruder. We investigate the efficiency of this process to reduce the socalled material noise from multiply scattered ultrasound, as a function of the configuration number, the shear rate and the bladeintruder distance. Nonaffine motions of the beads in the shear zone induced by the blade are then analyzed on the basis of a splitbottom rotating shear cell. This method helps to develop not only ultrasonic imaging tools of buried objects in turbid marine sediments, but also the local rheology based on a ball falling monitored by ultrasonic tracking.


Dimers of Plasmonic Nanocubes to Reach SingleMolecule Strong Coupling with High Emission Yields Heintz, J., F. Legittimo, and S. Bidault The Journal of Physical Chemistry Letters 13, no. 51, 1199612003 (2022)
Résumé: Reaching reproducible strong coupling between a quantum emitter and a plasmonic resonator at room temperature, while maintaining high emission yields, would make quantum information processing with light possible outside of cryogenic conditions. We theoretically propose to exploit the high local curvatures at the tips of plasmonic nanocubes to reach Purcell factors of >106 at visible frequencies, rendering singlemolecule strong coupling more easily accessible than with the faceted spherical nanoparticles used in recent experimental demonstrations. In the case of gold nanocube dimers, we highlight a tradeoff between coupling strength and emission yield that depends on the nanocube size. Electrodynamic simulations on silver nanostructures are performed using a realistic dielectric constant, as confirmed by scattering spectroscopy performed on single nanocubes. Dimers of silver nanocubes feature Purcell factors similar to those of gold while allowing emission yields of >60%, thus providing design rules for efficient strongly coupled hybrid nanostructures at room temperature.


Modeling fourdimensional metamaterials: a Tmatrix approach to describe timevarying metasurfaces Garg, P., A. G. Lamprianidis, D. Beutel, T. Karamanos, B. Verfürth, and C. Rockstuhl Optics Express 30, no. 25, 4583245847 (2022)
Résumé: Exploring the interaction of light with materials periodically structured in space and time is intellectually rewarding and, simultaneously, a computational challenge. Appropriate computational tools are urgently needed to explore how such upcoming photonic materials can control light on demand. Here, we introduce a semianalytical approach based on the transition matrix (also known as Tmatrix) to analyze the optical response of a spatiotemporal metasurface. The metasurface consists of a periodic arrangement of timevarying scattering particles. In our approach, we depart from an individual scatterer’s Tmatrix to construct the effective Tmatrix of the metasurface. From that effective Tmatrix, all observable properties can reliably be predicted. We verify our semianalytical approach with fullwave numerical simulations. We demonstrate a speedup with our approach by a factor of more than 500 compared to a finiteelement simulation. Finally, we exemplify our approach by studying the effect of time modulation on a Huygens’ metasurface and discuss some emerging observable features.


Sliding friction perturbed by shear ultrasound vibrations: dynamic lubrication and overaging Léopoldès, J. The European Physical Journal E 45, no. 12 (2022)


Latest trends in bioimaging and building a proactive network of earlycareer young scientists around bioimaging in Europe Valenta, H., N. Quiblier, V. Laghi, C. Cabriel, and J. Riti Biology open 11, no. 12 (2022)
Résumé: Biological research is in constant need of new methodological developments to assess organization and functions at various scales ranging from whole organisms to interactions between proteins. One of the main ways to evidence and quantify biological phenomena is imaging. Fluorescence microscopy and labelfree microscopy are in particular highly active fields of research due to their compatibility with living samples as well as their versatility. The Imabio Young Scientists Network (YSN) is a group of young scientists (PhD students, postdocs and engineers) who are excited about bioimaging and aim to create a proactive network of researchers with the same interest. YSN is endorsed by the bioimaging network GDR Imabio in France, where the initiative was started in 2019. Since then, we aim to organize the Imabio YSN conference every year to expand the network to other European countries, establish new collaborations and ignite new scientific ideas. From 68 July 2022, the YSN including researchers from the domains of life sciences, chemistry, physics and computational sciences met at the Third Imabio YSN Conference 2022 in Lyon to discuss the latest bioimaging technologies and biological discoveries. In this Meeting Review, we describe the essence of the scientific debates, highlight remarkable talks, and focus on the Career Development session, which is unique to the YSN conference, providing a career perspective to young scientists and help to answer all their questions at this career stage. This conference was a truly interdisciplinary reunion of scientists who are eager to push the frontiers of bioimaging in order to understand the complexity of biological systems.


Ultrasound Matrix Imaging  Part II: The Distortion Matrix for Aberration Correction Over Multiple Isoplanatic Patches Lambert, W., L. A. Cobus, J. Robin, M. Fink, and A. Aubry IEEE Transactions on Medical Imaging 41, no. 12, 39213938 (2022)
Résumé: This is the second article in a series of two which report on a matrix approach for ultrasound imaging in heterogeneous media. This article describes the quantification and correction of aberration, i.e. the distortion of an image caused by spatial variations in the medium speedofsound. Adaptive focusing can compensate for aberration, but is only effective over a restricted area called the isoplanatic patch. Here, we use an experimentallyrecorded matrix of reflected acoustic signals to synthesize a set of virtual transducers. We then examine wave propagation between these virtual transducers and an arbitrary correction plane. Such wavefronts consist of two components: (i) An ideal geometric wavefront linked to diffraction and the input focusing point, and; (ii) Phase distortions induced by the speedofsound variations. These distortions are stored in a socalled distortion matrix, the singular value decomposition of which gives access to an optimized focusing law at any point. We show that, by decoupling the aberrations undergone by the outgoing and incoming waves and applying an iterative strategy, compensation for even highorder and spatiallydistributed aberrations can be achieved. After a numerical validation of the process, ultrasound matrix imaging (UMI) is applied to the invivo imaging of a gallbladder. A map of isoplanatic modes is retrieved and is shown to be strongly correlated with the arrangement of tissues constituting the medium. The corresponding focusing laws yield an ultrasound image with drastically improved contrast and transverse resolution. UMI thus provides a flexible and powerful route towards computational ultrasound.


Ultrasound Matrix Imaging  Part I: The Focused Reflection Matrix, the FFactor and the Role of Multiple Scattering Lambert, W., J. Robin, L. A. Cobus, M. Fink, and A. Aubry IEEE Transactions on Medical Imaging 41, no. 12, 39073920 (2022)
Résumé: This is the first article in a series of two dealing with a matrix approach for aberration quantification and correction in ultrasound imaging. Advanced synthetic beamforming relies on a double focusing operation at transmission and reception on each point of the medium. Ultrasound matrix imaging (UMI) consists in decoupling the location of these transmitted and received focal spots. The response between those virtual transducers form the socalled focused reflection matrix that actually contains much more information than a confocal ultrasound image. In this paper, a timefrequency analysis of this matrix is performed, which highlights the single and multiple scattering contributions as well as the impact of aberrations in the monochromatic and broadband regimes. Interestingly, this analysis enables the measurement of the incoherent inputoutput point spread function at any pixel of this image. A fitting process enables the quantification of the single scattering, multiple scattering and noise components in the image. From the single scattering contribution, a focusing criterion is defined, and its evolution used to quantify the amount of aberration throughout the ultrasound image. In contrast to the stateoftheart coherence factor, this new indicator is robust to multiple scattering and electronic noise, thereby providing a contrasted map of the focusing quality at a much better transverse resolution. After a validation of the proofofconcept based on timedomain simulations, UMI is applied to the invivo study of a human calf. Beyond this specific example, UMI opens a new route for speedofsound and scattering quantification in ultrasound imaging.


Speckle Decorrelation in Fundamental and SecondHarmonic Light Scattered from Nonlinear Disorder Samanta, R., R. Pierrat, R. Carminati, and S. Mujumdar Physical Review Applied 18, no. 5 (2022)
Résumé: Speckle patterns generated in a disordered medium carry a lot of information despite the apparent complete randomness in the intensity pattern. When the medium possesses ?(2) nonlinearity, the speckle is sensitive to the phase of the incident fundamental light, as well as the light generated within. Here, we examine the speckle decorrelation in the fundamental and secondharmonic transmitted light as a function of the varying power in the fundamental beam. At low incident powers, the speckle patterns produced by successive pulses exhibit strong correlations, which decrease with increasing power. The average correlation in the secondharmonic speckle decays faster than in the fundamental speckle. Next, we construct a theoretical model, backed up by numerical computations, to obtain deeper physical insights into the faster decorrelations in the secondharmonic light. While providing excellent qualitative agreement with the experiments, the model sheds light on the contribution of two effects in the correlations, namely, the generation of secondharmonic light and the propagation thereof.


In vivo ultrasound modulated optical tomography with a persistent spectral hole burning filter Thai, Q. M., G. Kalot, C. Venet, J. Seguin, M. Bocoum, N. Mignet, F. Ramaz, and A. LouchetChauvet Biomedical Optics Express 13, no. 12, 64846496 (2022)
Résumé: We present in vivo ultrasound modulated optical tomography (UOT) results on mice, using the persistent spectral hole burning (PSHB) effect in a Tm3+:YAG crystal. Indocyanine green (ICG) solution was injected as an optical absorber and was clearly identified on the PSHBUOT images, both in the muscle (following an intramuscular injection) and in the liver (following an intravenous injection). This demonstration also validates an experimental setup with an improved level of performance combined with an increased technological maturity compared to previous demonstrations.


Effect of Amplitude and Duration of Cyclic Loading on Frictional Sliding Instability in Granular Media: Implication to Earthquake Triggering of Landslides Hu, W., H. Luo, Q. Xu, M. Mcsaveney, R. Huang, J. Zheng, Y. Wang, and X. Jia Journal of Geophysical Research: Solid Earth 127, no. 11 (2022)
Résumé: Strong earthquakes with larger magnitude and longer durations trigger many landslides, however, how magnitude and duration affect landslides is still unclear. Many factors could contribute to this, including additional shear stress provided by strong ground motion, or “seismogenic liquefaction”; herein, we hypothesize that the dynamic weakening of sliding zone gouge is important. We explored the influence of earthquake magnitude and duration on landslide triggering by simulating the seismic response of sliding zone gouge using a dynamic ringshear device and glass spheres. The experiments showed that vibration with larger amplitudes and longer durations more easily trigger deformation and even instability in dry granular materials. We used a dynamic triaxialbender system to find that the shear modulus of these materials decreased with the increase in duration and amplitude of cyclic loading. We suggest that this universal decrease in shear modulus is an important landslidetrigger mechanism. Our results revealed how magnitude and duration of earthquakes affect coseismic landslides and why earthquakes with larger magnitude and long durations can trigger more coseismic landslides.


Statistical Approach to Estimating Audience from MACRandomized WiFi Probe Requests Yang, F., I. Ahriz, and B. Denby Sensors 22, no. 22, 8679 (2022)
Résumé: In the past few years, the ability of wireless network operators to monitor audience using control frames emitted by client devices has been compromised, both by legislation treating client MAC addresses as private information and by the difficulty of distinguishing genuine client frames from those arising from the Internet of Things or from certain enhanced services. Here, a deterministic model, based on characteristics of human activity and on seasonal trends, is used to reveal underlying client statistics in raw MACrandomized WiFi Probe Request data. The method proposes a candidate conversion factor, X, between probe request counts and the client population, which offers plausible predictions on realworld datasets.


Tailoring Instantaneous Time Mirrors for Time Reversal Focusing in Absorbing Media Wu, C. T., N. M. Nobre, E. Fort, G. D. Riley, and F. Costen IEEE Transactions on Antennas and Propagation 70, no. 10, 96309640 (2022)
Résumé: The time reversal symmetry of the wave equation allows wave refocusing back at the source. However, this symmetry does not hold in lossy media. We present a new strategy to compensate wave amplitude losses due to attenuation. The strategy leverages the instantaneous time mirror (ITM) which generates reversed waves by a sudden disruption of the medium properties. We create a heterogeneous ITM whose disruption is unequal throughout the space to create waves of different amplitude. The timereversed waves can then cope with different attenuation paths as typically seen in heterogeneous and lossy environments. We consider an environment with biological tissues and apply the strategy to a 2D digital human phantom from the abdomen. A stronger disruption is introduced where the forward waves suffer a history of higher attenuation, with a weaker disruption elsewhere. Computer simulations show heterogeneous ITM is a promising technique to improve time reversal refocusing in heterogeneous, lossy, and dispersive spaces.


Thirdorder nonlinear femtosecond optical gating through highly scattering media Bocoum, M., Z. Cheng, J. Kaur, and R. LopezMartens Physical Review A 106, no. 5 (2022)
Résumé: Discriminating between ballistic and diffuse components of light propagating through highly scattering media is not only important for imaging purposes but also for investigating the fundamental diffusion properties of the medium itself. Massively developed to this end over the past 20 years, nonlinear temporal gating remains limited to ∼1010 transmission factors. Here, we report nonlinear timegated measurements of highly scattered femtosecond pulses with transmission factors as low as ≈1012. Our approach is based on the thirdorder nonlinear crosscorrelation of femtosecond pulses, a standard diagnostic used in highpower laser science, applied to the study of fundamental light scattering properties.


High rejection and frequency agile optical filtering of RF signals using a rare earth iondoped crystal Ulrich, L., S. Welinski, A. LouchetChauvet, J. D. Rosny, D. Dolfi, C. Vaneph, P. Berger, and L. Morvan Journal of Lightwave Technology 40, no. 20, 69016910 (2022)
Résumé: While photonics is broadly considered as a leading solution for analog RF processing, photonic RF filters often show a limited outofband rejection. In this work we demonstrate a high rejection and high steepness photonic RF bandpass filter based on spectral hole burning in a rare earth iondoped crystal. The filter is obtained by programming a frequencyselective absorption grating in the crystal. With an experimental and theoretical study we identify the optimal parameters for this filter. We achieve a remarkable rejection of 60 dB and a rejection of 45 dB at 50 MHz from the edge of the filter, compliant with most demanding applications.


Transit Time of Lamb WaveBased Ultrasonic Flow Meters and the Effect of Temperature Kiefer, D. A., A. Benkert, and S. J. Rupitsch IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 69, no. 10, 29752983 (2022)
Résumé: Transittime flow meters need to compensate for crosssensitivity to temperature. We show that Lamb wavebased setups are less affected by temperature. An optimality criterion is derived that allows to tune the meter into a zero local sensitivity to temperature. For this end, the flowinduced change in ultrasonic transit time is revisited first. While wetted piston transducer meters are directly sensitive to the change in propagation speed, the change in time of flight of Lamb wavebased systems is due to the beam displacement. Second, the effect of temperature is incorporated analytically. It is found that the temperaturedependent radiation angle of Lamb waves is able to compensate for changes in the speed of sound, leading to an (almost) unaffected overall time of flight. This effect is achievable with any fluid and in a wide temperature range. As an example, we discuss a water meter in the range from 0°C to 100°C. The model is validated against temperature and flow ratedependent measurements obtained on a prototype. The measured data fits well to the developed model and confirms the reduced crosssensitivity to temperature. Although an inline meter is considered here, the results extend to clampon devices.


Pseudogap and Anderson localization of light in correlated disordered media Monsarrat, R., R. Pierrat, A. Tourin, and A. Goetschy Physical Review Research 4, no. 3 (2022)
Résumé: Among the remarkable scattering properties of correlated disordered materials, the origin of pseudogaps and the formation of localized states are some of the most puzzling features. Fundamental differences between scalar and vector waves in both these aspects make their comprehension even more problematic. Here we present an indepth and comprehensive analysis of the ordertodisorder transition in 2D resonant systems. We show with exact ab initio numerical simulations in finitesize hyperuniform media that localization of 2D vector waves can occur in the presence of correlated disorder, in a regime of moderate density of scatterers. On the contrary, no signature of localization is found for white noise disorder. This is in striking contrast with scalar waves, which localize at high density whatever the amount of correlation. For correlated materials, localization is associated with the formation of pseudogap in the density of states. We develop two complementary models to explain these observations. The first one uses an effective photonic crystaltype framework and the second relies on a diagrammatic treatment of the multiple scattering sequences. We provide explicit theoretical evaluations of the density of states and localization length in good agreement with numerical simulations. In this way, we identify the microscopic processes at the origin of pseudogap formation and clarify the role of the density of states for wave localization in resonant correlated media. The generality of our framework makes possible to apply our predictions for a large variety of scattering systems including dielectric structures with high quality factor, cold atoms, artificial atoms, as well as microwave resonators.


Coherent enhancement of optical remission in diffusive media Bender, N., A. Goetschy, C. W. Hsu, H. Yilmaz, P. J. Palacios, A. Yamilov, and H. Cao Proceedings of the National Academy of Sciences of the United States of America 119, no. 41 (2022)
Résumé: Remitted waves are used for sensing and imaging in diverse diffusive media from the Earth's crust to the human brain. Separating the source and detector increases the penetration depth of light, but the signal strength decreases rapidly, leading to a poor signaltonoise ratio. Here, we show, experimentally and numerically, that wavefront shaping a laser beam incident on a diffusive sample enables an enhancement of remission by an order of magnitude at depths of up to 10 transport mean free paths. We develop a theoretical model which predicts the maximal remission enhancement. Our analysis reveals a significant improvement in the sensitivity of remitted waves to local changes of absorption deep inside diffusive media. This work illustrates the potential of coherent wavefront control for noninvasive diffuse wave imaging applications, such as diffuse optical tomography and functional nearinfrared spectroscopy.


Wavefront Shaping for Wireless Communications in Complex Media: From Time Reversal to Reconfigurable Intelligent Surfaces Lerosey, G., and M. Fink Proceedings of the IEEE 110, no. 9, 12101226 (2022)
Résumé: Reconfigurable intelligent surfaces (RISs) are gaining huge momentum in the field of wireless communications due to the paradigm shift that they bring. Indeed, they allow making any environment electromagnetically smart and dynamically reconfigurable for more efficient and greener wireless communications. As physicists, we proposed to use electronically tunable metasurfaces to shape the electromagnetic waves carrying our wireless communications in reflection almost ten years ago, inspired by some works that we and colleagues did in the field of wave control in complex media. In this article, we review the seminal works that led us to propose this concept, starting from the original one that is time reversal. Then, we propose a physicist's point of view of RISs using a comparison with phase conjugation. Finally, we highlight what we think are their limitations, relying on both our knowledge of wave control and our study of them over a decade.


Weight of single and recurrent scattering in the reflection matrix of complex media Brütt, C., A. Aubry, B. Gérardin, A. Derode, and C. Prada Physical Review E 106, no. 2 (2022)
Résumé: In a heterogeneous medium, the wave field can be decomposed as an infinite series known as the Born expansion. Each term of the Born expansion corresponds to a scattering order, it is thus theoretically possible to discriminate single and multiple scattering contribution to the field. Experimentally, what is actually measured is the total field in which all scattering orders interfere. Conventional imaging methods usually rely on the assumption that the multiple scattering contribution can be disregarded. In a backscattering configuration, this assumption is valid for small depths, and begins to fail for depths larger than the scattering meanfree path s. It is therefore a key issue to estimate the relative amount of single and multiple scattering in experimental data. To this end, a singlescattering estimator ρ computed from the reflection matrix has been introduced in order to assess the weight of single scattering in the backscattered wave field. In this paper, the meaning of this estimator is investigated and a particular attention is given to recurrent scattering. In a diffractionlimited experiment, a multiple scattering sequence is said to be recurrent if the first and last scattering events occur in the same resolution cell. Recurrent scattering is shown to be responsible for correlations between single scattering and higher scattering orders of the Born expansion, inducing a bias to the estimator ρ that should rather be termed confocal scattering ratio. Interestingly, a more robust estimator is built by projecting the reflection matrix in a focused basis. The argument is sustained by numerical simulations as well as ultrasonic data obtained around 1.5 MHz in a model medium made of nylon rods immersed in water. From a more general perspective, this work raises fundamental questions about the impact of recurrent scattering on wave imaging.


Diffraction grating with spacetime modulation Pham, K., and A. Maurel Journal of Computational Physics 469, 111528 (2022)
Résumé: We present a theoretical and numerical analysis of the diffraction of acoustic waves by spacetime modulated gratings with rigidtype modulations. This is done by deriving the form of the modes which are exact, uncoupled, solutions of the problem in the unbounded regions, inside and outside the grating. The dispersion of the modes is studied as a function of the ratio of the modulation speed to the speed of sound which shows that each spatial diffraction order is associated with a single temporal diffraction order. For a grating of finite extend, the solution is obtained as a superposition of these modes, which couple at the grating interfaces. This provides a numerical, multimodal, method when considering a truncated version of the solution. We provide analysis of the solutions in the harmonic and in the transient regimes.


Transition from Phononic to Geometrical Mie Modes Measured in Single Subwavelength Polar Dielectric Spheres AbouHamdan, L., L. Coudrat, S. Bidault, V. Krachmalnicoff, R. Haïdar, P. Bouchon, and Y. De Wilde ACS Photonics 9, no. 7, 22952303 (2022)
Résumé: Spherical dielectric resonators are highly attractive for light manipulation, thanks to their intrinsic electric and magnetic resonances. Here, we present measurements of the midinfrared farfield thermal radiation of single subwavelength dielectric spheres deposited on a gold substrate, of radii ranging from 1 to 2.5 μm, which agree quantitatively with simulated absorption cross sections. For SiO2microspheres, we evidence the excitation of both surface phononpolariton (SPhP) modes and geometrical electric and magnetic Mie modes. The transition from a phononmodedominated to a Miemodedominated emission spectrum is observed, with a threshold radius of ∼1.5 μm. We also show that the presence of the metallic substrate augments the computed spheres absorption crosssection due to increased local field enhancement, arising from the nearfield interaction of the spheres oscillating charges with their image in the metallic mirror. In contrast, measurements of single subwavelength SPhPinactive PTFE spheres reveal that the midinfrared response of such lossy spheres is dominated by their bulk absorption. Our results demonstrate how engineering the geometrical and dielectric properties of subwavelength scatterers can enable the control of thermal emission near room temperature, with exciting perspectives for applications such as radiative cooling.


Roadmap on wavefront shaping and deep imaging in complex media Gigan, S., O. Katz, H. B. De Aguiar, E. R. Andresen, A. Aubry, J. Bertolotti, E. Bossy, D. Bouchet, J. Brake, S. Brasselet, Y. Bromberg, H. Cao, T. Chaigne, Z. Cheng, W. Choi, T. čižmár, M. Cui, V. R. Curtis, H. Defienne, M. Hofer, R. Horisaki, R. Horstmeyer, N. Ji, A. K. Laviolette, J. Mertz, C. Moser, A. P. Mosk, N. C. Pégard, R. Piestun, S. Popoff, D. B. Phillips, D. Psaltis, B. Rahmani, H. Rigneault, S. Rotter, L. Tian, I. M. Vellekoop, L. Waller, and Wan Journal of Physics: Photonics 4, no. 4, 042501 (2022)
Résumé: The last decade has seen the development of a wide set of tools, such as wavefront shaping, computational or fundamental methods, that allow us to understand and control light propagation in a complex medium, such as biological tissues or multimode fibers. A vibrant and diverse community is now working in this field, which has revolutionized the prospect of diffractionlimited imaging at depth in tissues. This roadmap highlights several key aspects of this fast developing field, and some of the challenges and opportunities ahead.


Crossover from renormalized to conventional diffusion near the threedimensional Anderson localization transition for light Cobus, L. A., G. Maret, and A. Aubry Physical Review B 106, no. 1 (2022)
Résumé: We report on anomalous light transport in the strong scattering regime. Using lowcoherence interferometry, we measure the reflection matrix of titanium dioxide powders, revealing crucial features of strong optical scattering which cannot be observed with transmission measurements: (i) a subdiffusive regime of transport at early times of flight that is a direct consequence of predominant recurrent scattering loops and (ii) a crossover to a conventional, but extremely slow, diffusive regime at long times. These observations support previous predictions that nearfield coupling between scatterers prohibits Anderson localization of light in threedimensional disordered media.


Label free optical transmission tomography for biosystems: intracellular structures and dynamics Mazlin, V., O. Thouvenin, S. Alhaddad, M. Boccara, and C. Boccara Biomedical Optics Express 13, no. 8, 41904203 (2022)
Résumé: There is an increasing need for label free methods that could reveal intracellular structures and dynamics. In this context, we develop a new optical tomography method working in transmission  fullfield optical transmission tomography (FFOTT). The method can measure the forward scattering signals and reveals the timedependent metabolic signals in living cells. FFOTT is a common path interferometer taking advantage of the Gouy phase shift  a π phase shift that the light wave experiences around the focus. By modulating the position of the focus one can alter the phase of the scattered light. Demodulation of images with different phases rejects the background and enhances the light from the depthoffield, thus producing an optical section. We test FFOTT by imaging singlecell diatoms and ex vivo biological samples. In fresh samples, we show that the intracellular motions create visible intensity fluctuations in FFOTT so that the method is able to reveal a metabolic dynamic contrast. FFOTT was found to be an efficient label free technique that can be readily implemented thanks to a robust commonpath specklefree interferometer design using an incoherent light source.


Characterization and Analysis of Retinal Axial Motion at High Spatiotemporal Resolution and Its Implication for RealTime Correction in Human Retinal Imaging Cai, Y., K. Grieve, and P. Mecê Frontiers in Medicine 9 (2022)
Résumé: Highresolution ophthalmic imaging devices including spectraldomain and fullfield optical coherence tomography (SDOCT and FFOCT) are adversely affected by the presence of continuous involuntary retinal axial motion. Here, we thoroughly quantify and characterize retinal axial motion with both high temporal resolution (200,000 Ascans/s) and high axial resolution (4.5 μm), recorded over a typical data acquisition duration of 3 s with an SDOCT device over 14 subjects. We demonstrate that although breathholding can help decrease largeandslow drifts, it increases smallandfast fluctuations, which is not ideal when motion compensation is desired. Finally, by simulating the action of an axial motion stabilization control loop, we show that a loop rate of 1.2 kHz is ideal to achieve 100% robust clinical invivo retinal imaging.


Efficient and lightweight detection of PPG onset and systolic peaks using implementable timedomain strategies Chakraborty, A., D. Sadhukhan, and M. Mitra Measurement: Journal of the International Measurement Confederation 200, 111628 (2022)
Résumé: A majority of clinically reliable Photoplethysmogram (PPG) signal features necessitate exact location of PPG onset and systolic peaks. So far, stateoftheart researches in the field of PPG peak detection is lagging due to the use of amplitude thresholding, complicated techniques or pathophysiologically limited datasets. In this paper, a robust algorithm is developed that primarily uses a novel combination of PPGderivative, trigonometric area of the curve (TAOC), slope reversal and timedomain based lightweight methodologies to uncover the precise location of PPG onset and systolic peaks. Rigorous evaluation of the algorithm is conducted over 2,16,374 beats, collected from standard databases together with the PPG signal acquired from healthy and cardiac subjects. Superiority of the algorithm is presented by high average accuracy, sensitivity, predictivity and error as 99.69%, 99.77%, 99.92% and 0.31%. The adopted lightweight methodologies and highspeed execution over a wide pathophysiological variety presents immense promises for implementation in standard healthcare applications.


Passive detection in water pipelines using ambient noise II: Field experiments Li, Z., P. Lee, M. Fink, R. Murch, and M. Davidson Mechanical Systems and Signal Processing 181, 109524 (2022)
Résumé: A passive detection method has been proposed in a prior paper to extract key parameters and detect faults using the ambient noise present in water pipeline networks. This paper presents field experiments and data processing results to provide systematic experimental validation of this method. Field experiments were carried out in operational water pipeline networks at the University of Canterbury campus and the Waimakariri District, New Zealand, during which ambient noise was measured by pairs of pressure sensors installed at selected hydrants on pipelines of different materials, network topologies and simulated faults. Autocorrelation and crosscorrelation analysis of noise at a single sensor and sensor pairs were carried out to estimate the wave speed and to locate faults in the networks. Data processing results indicate that water usage generating pressure transients are the dominant sources of ambient noise in operational water pipeline networks. This type of ambient noise can also be utilized by the passive detection method to achieve similar wave speed estimation accuracy and fault detection performance as the conventional active pressure wave detection methods.


Design and thermal conductivity of 3D artificial crosslinked random fiber networks Kallel, H., and K. Joulain Materials and Design 220, 110800 (2022)
Résumé: We propose a novel procedure to design threedimensional (3D) printable crosslinked random fiber networks. Our procedure describes how to implement, in a novel way, a modified version of randomwalk based algorithm using a combination of different free and opensource software programs. Second, we describe in detail a method based on an opensource finiteelement software, to analyse the steadystate heat conduction and to compute the effective thermal conductivity for crosslinked fibrous structures immersed in vacuum. We apply this method to examine how the orientations of the fibers and the fiber volume fraction influence the effective thermal conductivity of 3D artificial crosslinked random glass fiber networks. Our numerical results show explicitly the direct link between the effective thermal conductivity and the percolating conduction paths through a 3D crosslinked fiber network. The proposed procedure for creating the 3D drawing and the one for volume meshing as well as the method for solving the steadystate heat conduction problem described in this paper can be extended to other 3D complex structures with closed surfaces.


Limits to the sensitivity of a rareearthenabled cryogenic vibration sensor LouchetChauvet, A., and T. Chanelière AVS Quantum Science 4, no. 2, 024401 (2022)
Résumé: Cryogenics is a pivotal aspect in the development of quantum technologies. Closedcycle devices have recently emerged as an environmentally friendly and lowmaintenance alternative to liquid helium cryostats. Yet the larger level of vibrations in dry cryocoolers forbids their use in most sensitive applications. In a recent work, we have proposed an inertial, broadband, contactless sensor based on the piezospectroscopic effect, i.e., the natural sensitivity of optical lines to strain exhibited by impurities in solids. This sensor builds on the exceptional spectroscopic properties of rare earth ions and operates below 4 K, where spectral hole burning considerably enhances the sensitivity. In this paper, we investigate the fundamental and technical limitations of this vibration sensor by comparing a rigid sample attachment to the cold stage of a pulsetube cryocooler and a customdesigned exchange gas chamber for acoustic isolation.


Extended Hybridization and Energy Transfer in Periodic MultiMaterial Organic Structures in Strong Coupling with Surface Plasmon Bard, A., S. Minot, C. Symonds, J. M. Benoit, A. Gassenq, F. Bessueille, B. Andrioletti, C. Perez, K. Chevrier, Y. De Wilde, V. Krachmalnicoff, and J. Bellessa Advanced Optical Materials (2022)
Résumé: The strong light−matter coupling, occurring when the light−matter interaction prevails on the damping, has found applications beyond the domain of optics in chemistry or transport. These advances make the development of various structures in strong coupling crucial. In this paper, a new way to hybridize two materials and transfer energy through a surface plasmon over micrometric distances is proposed. For this purpose, two patterned interlocked dye arrays, one donor and one acceptor, are deposited on a silver surface by successive microcontact printing, leading to a pattern of 5 microns period. The dispersion relation of the structure is measured with reflectometry experiments, showing the hybridization with the plasmon, and the formation of states that mix both excitons and the plasmon with similar weights. The mixing in these polaritonic metasurfaces enables an energy transfer mechanism in the strong coupling, which is observed with luminescence experiments. As the donor and acceptor are spatially separated by a distance larger than the diffraction limit the excitation transfer is directly measured and evaluated by comparison with dye arrays without silver.


Method to measure the refractive index for photoluminescence modelling Bailly, E., K. Chevrier, C. P. De La Vega, J. P. Hugonin, Y. De Wilde, V. Krachmalnicoff, B. Vest, and J. J. Greffet Optical Materials Express 12, no. 7, 27722781 (2022)
Résumé: Light emission by fluorophores can be computed from the knowledge of the absorption spectrum. However, at long wavelengths, the calculated emission may diverge if the decay of the imaginary part of the permittivity is not modelled with precision. We report a technique to obtain the permittivity of fluorophores such as dye molecules from fluorescence measurements. We find that the BrendelBormann model enables to fit the emission spectra accurately.


Dynamic fullfield optical coherence tomography allows live imaging of retinal pigment epithelium stress model Groux, K., A. Verschueren, C. Nanteau, M. Clémençon, M. Fink, J.A. Sahel, C. Boccara, M. Paques, S. Reichman, and K. Grieve Communications Biology 5, no. 1 (2022)


Ambient Backscatter Communications in Mobile Networks: CrowdDetectable ZeroEnergyDevices PhanHuy, D.T., D. Barthel, P. Ratajczak, R. Fara, M. Di Renzo, and J. De Rosny IEEE Journal of Radio Frequency Identification, 11 (2022)


Dynamic fullfield optical coherence tomography as complementary tool in fungal diagnostics Maldiney, T., J. M. Chassot, C. Boccara, M. Blot, L. Piroth, P. E. Charles, D. GarciaHermoso, F. Lanternier, F. Dalle, and M. Sautour Journal of Medical Mycology 32, no. 4, 101303 (2022)
Résumé: Histopathology and microscopic examination of infected tissue are the gold standards to prove the diagnosis of invasive fungal infection (IFI). Yet, they suffer from essential limitations that hamper rapid diagnosis and require the future development of new imaging tools dedicated to fungal diagnostics. To this end, the present work introduces the first use of dynamic fullfield optical coherence tomography (DFFOCT) for the visualization of microscopic filamentous fungi. Data collected from the observation of three different fungal species (Nannizzia gypsea, Aspergillus fumigatus and Rhizopus arrhizus) confirm the ability of DFFOCT to visualize not only the main structures of all selected fungal species (hyphae, spores, conidia, sporulating structures), but also the metabolic activity of the organisms, which could provide additional help in the future to better characterize the signature of each fungal structure. These results demonstrate how DFFOCT could serve as potential complementary tool for rapid diagnosis of IFI in both intensive and nonintensive care units.


Soft elastomers: A playground for guided waves Delory, A., F. Lemoult, M. Lanoy, A. Eddi, and C. Prada The Journal of the Acoustical Society of America 151, no. 5, 33433358 (2022)
Résumé: Mechanical waves propagating in soft materials play an important role in physiology. They can be natural, such as the cochlear wave in the inner ear of mammalians, or controlled, such as in elastography in the context of medical imaging. In a recent study, Lanoy, Lemoult, Eddi, and Prada [Proc. Natl. Acad. Sci. U.S.A. 117(48), 3018630190 (2020)] implemented an experimental tabletop platform that allows direct observation of inplane guided waves in a soft strip. Here, a detailed description of the setup and signal processing steps is presented as well as the theoretical framework supporting them. One motivation is to propose a tutorial experiment for visualizing the propagation of guided elastic waves. Last, the versatility of the experimental platform is exploited to illustrate experimentally original features of wave physics, such as backward modes, stationary modes, and Dirac cones.


Timereversal of SubTHz Pulses in Complex Media Mokh, A., R. Khayatzadeh, A. Ourir, M. Kamoun, A. Tourin, M. Fink, and J. De Rosny Progress In Electromagnetics Research B 95, 141162 (2022)
Résumé: AbstractFor the last 20 years, the timereversal (TR) process has been successfully applied to focus pulses in the microwave frequency range and in complex media. Here we examine the specic conditions to obtain the same results but in the subTHz frequency range. Because of the stronger attenuation at this much higher frequency, it is more challenging to exploit the TR selffocusing property. The TR of pulses is studied in two kinds of complex media: metallic waveguide and leaky reverberating cavity. For each medium, we propose one or two models to assess the quality of the focusing. For the waveguide, we show that the angle of incidence is an important parameter. Based on these results, we perform TR experiments at 273 GHz with a bandwidth that can be as large as 2 GHz. TR experiments are successfullyrst conducted in a 1m long and 10mm diameter straight hollow cylinder and then in a 5m long and 12mm diameter curved waveguide. Finally, we present results obtained in a cavity of 72 cm3 that leaks through a copper grid. The best focusing is observed with the longer waveguide.


Labelfree, noninvasive, and repeatable cell viability bioassay using dynamic fullfield optical coherence microscopy and supervised machine learning Park, S., V. Veluvolu, W. S. Martin, T. Nguyen, J. Park, D. L. Sackett, C. Boccara, and A. Gandjbakhche Biomedical Optics Express 13, no. 6, 31873194 (2022)
Résumé: We present a novel method that can assay cellular viability in realtime using supervised machine learning and intracellular dynamic activity data that is acquired in a labelfree, noninvasive, and nondestructive manner. Cell viability can be an indicator for cytology, treatment, and diagnosis of diseases. We applied four supervised machine learning models on the observed data and compared the results with a trypan blue assay. The cell death assay performance by the four supervised models had a balanced accuracy of 93.92 ±0.86%. Unlike staining techniques, where criteria for determining viability of cells is unclear, cell viability assessment using machine learning could be clearly quantified.


FreezeDried Microfluidic Monodisperse Microbubbles as a New Generation of Ultrasound Contrast Agents Soysal, U., P. N. Azevedo, F. Bureau, A. Aubry, M. S. Carvalho, A. C. S. N. Pessoa, L. G. D. L. Torre, O. Couture, A. Tourin, M. Fink, and P. Tabeling Ultrasound in Medicine and Biology (2022)
Résumé: We succeeded in freezedrying monodisperse microbubbles without degrading their performance, that is, their monodispersity in size and echogenicity. We used microfluidic technology to generate cryoprotected highly monodisperse microbubbles (coefficient of variation [CV] <5%). By using a novel retrieval technique, we were able to freezedry the microbubbles and resuspend them without degradation, that is, keeping their size distribution narrow (CV <6%). Acoustic characterization performed in two geometries (a centimetric cell and a millichannel) revealed that the resuspended bubbles conserved the sharpness of the backscattered resonance peak, leading to CVs ranging between 5% and 10%, depending on the geometry. As currently observed with monodisperse bubbles, the peak amplitudes are one order of magnitude higher than those of commercial ultrasound contrast agents. Our work thus solves the question of storage and transportation of highly monodisperse bubbles. This work might open pathways toward novel clinical noninvasive measurements, such as local pressure, impossible to carry out with the existing commercial ultrasound contrast agents.


Coalescence of Andersonlocalized modes at an exceptional point in 2D random media Bachelard, N., A. Schumer, B. Kumar, C. Garay, J. Arlandis, R. Touzani, and P. Sebbah Optics Express 30, no. 11, 1809818107 (2022)
Résumé: In nonHermitian settings, the particular position at which two eigenstates coalesce in the complex plane under a variation of a physical parameter is called an exceptional point. An open disordered system is a special class of nonHermitian system, where the degree of scattering directly controls the confinement of the modes. Herein a nonperturbative theory is proposed which describes the evolution of modes when the permittivity distribution of a 2D open dielectric system is modified, thereby facilitating to steer individual eigenstates to such a nonHermitian degeneracy. The method is used to predict the position of such an exceptional point between two Andersonlocalized states in a disordered scattering medium. We observe that the accuracy of the prediction depends on the number of localized states accounted for. Such an exceptional point is experimentally accessible in practically relevant disordered photonic systems.


Nonlinear Waves Passing over Rectangular Obstacles: Multimodal Method and Experimental Validation Monsalve, E., A. Maurel, V. Pagneux, and P. Petitjeans Fluids 7, no. 5, 145 (2022)
Résumé: We report a theoretical and experimental investigation of the propagation of nonlinear waves passing over a submerged rectangular step. A multimodal method allows calculating the firstand secondorder reflected and transmitted waves. In particular, at the second order, the propagation of free and bound waves is theoretically presented. A detailed analysis of the convergence of the secondorder problem shows that a finite truncation of the series of evanescent bound waves is necessary to obtain a smooth and convergent solution. The computed coefficients of the first and second harmonics are experimentally validated via a complete spacetimeresolved measurements of the wave propagation, which permits us to verify the relative amplitude, phase and spatial interference (beating) of the free and bound waves at the second order. This result can be useful in future multimodal models since it not only keeps the accuracy of the model with the inclusion of the first part of the evanescent bound terms (being also the dominants) but also ensures the convergence of the multimodal computation with an error that decreases as a function of the number of modes.


Purcell effect with extended sources: the role of the cross density of states Carminati, R., and M. Gurioli Optics Express 30, no. 10, 1617416183 (2022)
Résumé: We analyze the change in the spontaneous decay rate, or Purcell effect, of an extended quantum emitter in a structured photonic environment. Based on a simple theory, we show that the cross density of states is the central quantity driving interferences in the emission process. Using numerical simulations in realistic photonic cavity geometries, we demonstrate that a structured cross density of states can induce subradiance or superradiance, and change substantially the emission spectrum. Interestingly, the spectral lineshape of the Purcell effect of an extended source cannot be predicted from the sole knowledge of the spectral dependence of the local density of states.


Mean arc theorem for exploring domains with randomly distributed arbitrary closed trajectories HidalgoCaballero, S., A. Cassinelli, M. Labousse, and E. Fort European Physical Journal Plus 137, no. 4 (2022)
Résumé: A remarkable result from integral geometry is Cauchy’s formula, which relates the mean path length of ballistic trajectories randomly crossing a convex 2D domain, to the ratio between the region area and its perimeter. This theorem has been generalized for nonconvex domains and extended to the case of Brownian motion to find many applications in various fields including biological locomotion and wave physics. Here, we generalize the theorem to arbitrary closed trajectories exploring arbitrary domains. We demonstrate that, regardless of the complexity of this trajectory, the mean arc length still satisfies Cauchy’s formula provided that no closed trajectory is entirely contained in the domain. Below this threshold, the mean arc length decreases with the size of the closed trajectory. In this case, an approximate analytical formula can still be given for convex closed trajectories intersecting convex domains provided they are small in comparison. To validate our analysis, we performed numerical simulations of different types of trajectories exploring arbitrary 2D domains. Our results could be applied to retrieve geometric information of bounded domains from the mean first entrance–exit length.


Additive Manufacturing of 3D Luminescent ZrO2:Eu3+ Architectures Winczewski, J., M. Herrera, C. Cabriel, I. Izeddin, S. Gabel, B. Merle, A. Susarrey Arce, and H. Gardeniers Advanced Optical Materials, 2102758 (2022)
Résumé: Implementation of more refined structures at the nano to microscale is expected to advance applications in optics and photonics. This work presents the additive manufacturing of 3D luminescent microarchitectures emitting light in the visible range. A tailormade organometallic resin suitable for twophoton lithography is developed, which upon thermal treatment in an oxygenrich atmosphere allows the creation of siliconfree tetragonal (t) and monoclinic (m) ZrO2. The approach is unique because the tailormade Zrresin is different from what is achieved in other reported approaches based on sol−gel resins. The Zrresin is compatible with the Eurich dopant, a luminescent activator, which enables to tune the optical properties of the ZrO2 structures upon annealing. The emission characteristics of the Eudoped ZrO2 microstructures are investigated in detail with cathodoluminescence and compared with the intrinsic optical properties of the ZrO2. The hosted Eu has an orange−red emission showcased using fluorescence microscopy. The presented structuring technology provides a new platform for the future development of 3D luminescent devices.


Statictodynamic field conversion with timevarying media Mencagli, M. J., D. L. Sounas, M. Fink, and N. Engheta Physical Review B 105, no. 14 (2022)
Résumé: We theoretically demonstrate that a uniform static electric field distribution can be partially converted to radiation fields when a portion of the medium undergoes a temporal change of its permittivity. An indepth theoretical investigation of this phenomenon is developed for a dielectric block with a steplike temporal change located inside a waveguide charged with a DC voltage source. Closed analytical expressions are derived for the radiated electric and magnetic fields. The exchange of energy between the electrostatic and electromagnetic fields is discussed. The reconciliation between the seemingly contradictory temporal and spatial boundary conditions for the electric and magnetic fields at the interface of the timevarying dielectric block is analyzed and elucidated. Our findings may provide an alternative solution for generating electromagnetic radiation based on timevarying media.


Modeling AutlerTownes splitting and acoustically induced transparency in a waveguide loaded with resonant channels Porter, R., K. Pham, and A. Maurel Physical Review B 105, no. 13 (2022)
Résumé: We study acoustic wave propagation in a waveguide loaded with two resonant sidebranch channels. In the lowfrequency regime, onedimensional models are derived in which the effect of the channels are reduced to jump conditions across the junction. When the separation distance is on the scale of the wavelength, which is the case that is usually considered, the jump conditions involve a single channel and acoustically induced transparency occurs due to outofphase interferences between the two junctions. In contrast, when the separation distance is subwavelength, a single junction has to be considered and the jump conditions account for the evanescent field coupling the two channels. Such channel pairs can scatter as a dipole, resulting in perfect transmission due to AutlerTownes splitting. We show that combining the two mechanisms offers additional degrees of freedom to control the transmission spectra.


Spontaneous emergence of a spin state for an emitter in a timevarying medium BernardBernardet, S., M. Fleury, and E. Fort The European Physical Journal Plus 137, no. 4 (2022)
Résumé: Timevarying media can dramatically modify the emission of embedded sources by producing time reversed waves refocusing on the source. Here, we show that such a back action can create an angular momentum by setting the source in a spontaneous spin state. We experimentally implement this coupling using selfpropelled bouncing droplets sources coupled to the surface waves they emit on a parametrically excited bath. The spin state dynamics result from a selforganized interplay between the source motion and the time reversed waves. The discrete stability analysis agrees with the experimental observations. In addition, we show that these spin states provide a unique opportunity for an experimental access to parameters enabling comparison and calibration of the various existing models.


Experimental Implementation of Wave Propagation in Disordered TimeVarying Media Apffel, B., S. Wildeman, A. Eddi, and E. Fort Physical Review Letters 128, no. 9 (2022)
Résumé: Here, we study and implement the temporal analog in time disordered sytems. A spatially homogeneous medium is endowed with a time structure composed of randomly distributed temporal interfaces. This is achieved through electrostriction between water surface and an electrode. The wave field observed is the result of the interferences between reflected and refracted waves on the interfaces. Although no eigenmode can be associated with the wave field, several common features between space and time emerge. The waves grow exponentially depending on the disorder level in agreement with a 2D matrix evolution model such as in the spatial case. The relative position of the momentum gap appearing in the time modulated systems plays a central role in the wave field evolution. When tuning the excitation to compensate for the damping, transient waves, localized in time, appear on the liquid surface. They result from a particular history of the multiple interferences produced by a specific sequence of time boundaries.


Liquid interface shaping and transport phenomena induced by spatially inhomogeneous vibrations Apffel, B., C. Wilkinson, and E. Fort European Physical Journal Plus 137, no. 3 (2022)
Résumé: Vibrations can dynamically stabilize otherwise unstable liquid interfaces and produce new dynamic equilibria, called vibroequilibria. Typically, the vibrations are homogeneous in the liquid and the liquid interface remains approximately flat. Here, we produce controlled vertical vibration gradients by taking advantage of the resonant oscillations of sinking submerged bubbles. The locally increased amplitude of the vibrations induces a local elevation of the liquid interface that can be controlled and engineered. The mean elevation of the interface can be linked theoretically with the local vibration amplitude by a simple formula that is tested experimentally. In addition, the transport of a floating body at the interface can be induced by secondary flows triggered by the amplitude gradients of the liquid vibrations.


Realtime 3D imaging with Fourierdomain algorithms and matrix arrays applied to nondestructive testing Marmonier, M., S. Robert, J. Laurent, and C. Prada Ultrasonics 124, 106708 (2022)
Résumé: Realtime 3D ultrasound imaging with matrix arrays remains a challenge in NonDestructive Testing (NDT) due to the timeconsuming reconstruction algorithms based on delayandsum operations. Other algorithms operating in the Fourier domain have lower algorithmic complexities and therefore higher frame rates at the cost of more storage space, which may limit the number of reconstruction points. In this paper, we present an implementation for realtime 3D imaging of the Total Focusing Method (TFM) and the Plane Wave Imaging (PWI), as well as of their Fourierdomain counterparts, referred to as kTFM and kPWI. For both types of acquisition, the Fourierdomain algorithms are used to increase frame rates, and they are compared to the timedomain TFM and PWI in terms of image quality, frame rates and memory requirements. In order to greatly reduce their memory requirements, a new implementation of kTFM and kPWI is proposed. The four imaging methods are then evaluated by imaging in real time a block of stainless steel containing a 3D network of spherical porosities produced by additive layer manufacturing using a powder bed laser fusion process.


Frequency Conversion Cascade by Crossing Multiple Space and Time Interfaces Apffel, B., and E. Fort Physical Review Letters 128, no. 6 (2022)
Résumé: Time varying media recently emerged as promising candidates to fulfill the dream of controlling the wave frequency without nonlinear effects. However, frequency conversion remains limited by the dynamics of the variations of the propagation properties. Here we propose a new concept of spacetime cascade to achieve arbitrary large frequency shifts by iterated elementary transformation steps. These steps use an intermediate medium in which wave packets enter and exit through noncommutative space and time interfaces. This concept avoids high frequency or subwavelength demanding metamaterials. Upward and downward frequency conversions are performed. The transmitted energy yield is given by the frequency ratio, regardless of impedence mismatch. We implement this concept with water waves controlled by electrostriction and achieve frequency conversion over 4 octaves.


Towards a remote inspection of jet engine blades using time reversal Farin, M., C. Prada, T. Lhommeau, M. El Badaoui, and J. De Rosny Journal of Sound and Vibration 525, 116781 (2022)
Résumé: Assessing the state of damage of jet engine blades is a burning issue in aeronautics. However, most nondestructive evaluation procedures require cumbersome installations and removal of the blades from the engines, which is time and money consuming. We present a nonintrusive acoustic monitoring technique that could be applied for fast remote inspection of selected blades inside a jet engine. The technique uses a time reversal mirror in the audible frequency range to selectively excite a targeted blade a few meters away. The resonance frequencies of the blade are measured at the location of the excitation using a laser vibrometer. The technique is first applied on a few individual blades and then inside a jet engine. Selective excitation of a difficulttoaccess blade among others inside a cavity is shown. In laboratory, some damage (material removal or slit) is created on a set of initially intact blades, which cause a shift in their resonance frequencies. By evaluating these frequency shifts, we are able to remotely detect millimeter size damage on the blades. Finally, the onsite applicability and the uncertainties of the method are discussed.


Timemodulated excitation for enhanced singlemolecule localization microscopy Jouchet, P., C. Poüs, E. Fort, and S. LévêqueFort Philosophical transactions. Series A, Mathematical, physical, and engineering sciences 380, no. 2220, 20200299 (2022)
Résumé: Structured illumination in singlemolecule localization microscopy provides new information on the position of molecules and thus improves the localization precision compared to standard localization methods. Here, we used a timeshifted sinusoidal excitation pattern to modulate the fluorescence signal of the molecules whose position information is carried by the phase and recovered by synchronous demodulation. We designed two flexible fast demodulation systems located upstream of the camera, allowing us to overcome the limiting camera acquisition frequency and thus to maximize the collection of photons in the demodulation process. The temporally modulated fluorescence signal was then sampled synchronously on the same image, repeatedly during acquisition. This microscopy, called ModLoc, allows us to experimentally improve the localization precision by a factor of 2.4 in one direction, compared to classical Gaussian fitting methods. A temporal study and an experimental demonstration both show that the short lifetimes of the molecules in blinking regimes impose a modulation frequency in the kilohertz range, which is beyond the reach of current cameras. A demodulation system operating at these frequencies would thus be necessary to take full advantage of this new localization approach. This article is part of the Theo Murphy meeting issue 'Superresolution structured illumination microscopy (part 2)'.


Spacetimeresolved measurements of the effect of pinned contact line on the dispersion relation of water waves Monsalve, E., A. Maurel, V. Pagneux, and P. Petitjeans Physical Review Fluids 7, no. 1 (2022)
Résumé: We report on an experimental investigation of the propagation of gravitycapillary waves in a narrow channel with a pinned contact line. By using Fourier transform profilometry we measure the static curved meniscus as well as the surface perturbation. By varying the channel width, between 7 and 15 times the capillary length, we show how edge constraints modify the surface curvature and therefore the dispersion relation. From the spacetimeresolved field, we obtain a decomposition of the linear mode onto transverse modes satisfying the condition of pinned contact line. This approach, in which we complement the theoretical model with experimental analysis, allows computations of wave numbers and natural frequencies with a robust statistics. We verify experimentally the convergence of the model and the pertinence of the linear approximation. In addition, we analyze the relative contribution of the experimentally measured static meniscus. An excellent agreement between the computed natural frequencies and the forcing frequency confirms the contribution of the actual spacetimeresolved measured surface. These experimental results are an accurate estimation of the influence of the additional restoring force exerted by the pinned contact line on the deformed surface which increases the wave celerity. The local character of this effect is evidenced by the decrease of the shift of the dispersion relation as a function of the channel width.


Superresolution imaging: When biophysics meets nanophotonics Koenderink, A. F., R. Tsukanov, J. Enderlein, I. Izeddin, and V. Krachmalnicoff Nanophotonics 11, no. 2, 169202 (2022)
Résumé: Probing lightmatter interaction at the nanometer scale is one of the most fascinating topics of modern optics. Its importance is underlined by the large span of fields in which such accurate knowledge of lightmatter interaction is needed, namely nanophotonics, quantum electrodynamics, atomic physics, biosensing, quantum computing and many more. Increasing innovations in the field of microscopy in the last decade have pushed the ability of observing such phenomena across multiple length scales, from micrometers to nanometers. In bioimaging, the advent of superresolution singlemolecule localization microscopy (SMLM) has opened a completely new perspective for the study and understanding of molecular mechanisms, with unprecedented resolution, which take place inside the cell. Since then, the field of SMLM has been continuously improving, shifting from an initial drive for pushing technological limitations to the acquisition of new knowledge. Interestingly, such developments have become also of great interest for the study of lightmatter interaction in nanostructured materials, either dielectric, metallic, or hybrid metallicdielectric. The purpose of this review is to summarize the recent advances in the field of nanophotonics that have leveraged SMLM, and conversely to show how some concepts commonly used in nanophotonics can benefit the development of new microscopy techniques for biophysics. To this aim, we will first introduce the basic concepts of SMLM and the observables that can be measured. Then, we will link them with their corresponding physical quantities of interest in biophysics and nanophotonics and we will describe stateoftheart experiments that apply SMLM to nanophotonics. The problem of localization artifacts due to the interaction of the fluorescent emitter with a resonant medium and possible solutions will be also discussed. Then, we will show how the interaction of fluorescent emitters with plasmonic structures can be successfully employed in biology for cell profiling and membrane organization studies. We present an outlook on emerging research directions enabled by the synergy of localization microscopy and nanophotonics.


Depthtargeted energy delivery deep inside scattering media Bender, N., A. Yamilov, A. Goetschy, H. Yılmaz, C. W. Hsu, and H. Cao Nature Physics (2022)


Optical phase modulation by natural eye movements: application to timedomain FFOCT image retrieval Mazlin, V., P. Xiao, K. Irsch, J. Scholler, K. Groux, K. Grieve, M. Fink, and A. C. Boccara Biomedical Optics Express 13, no. 2, 902920 (2022)
Résumé: Eye movements are commonly seen as an obstacle to highresolution ophthalmic imaging. In this context we study the natural axial movements of the in vivo human eye and show that they can be used to modulate the optical phase and retrieve tomographic images via timedomain fullfield optical coherence tomography (TDFFOCT). This approach opens a path to a simplified ophthalmic TDFFOCT device, operating without the usual piezo motorcamera synchronization. The device demonstrates in vivo human corneal images under the different image retrieval schemes (2phase and 4phase) and different exposure times (3.5 ms, 10 ms, 20 ms). Data on eye movements, acquired with a spectraldomain OCT with axial eye tracking (180 Bscans/s), are used to study the influence of ocular motion on the probability of capturing highsignal tomographic images without phase washout. The optimal combinations of camera acquisition speed and amplitude of piezo modulation are proposed and discussed.


Unidirectional amplification with acoustic nonHermitian space−time varying metamaterial Wen, X., X. Zhu, A. Fan, W. Y. Tam, J. Zhu, H. W. Wu, F. Lemoult, M. Fink, and J. Li Communications Physics 5, no. 1 (2022)
Résumé: Space−time modulated metamaterials support extraordinary rich applications, such as parametric amplification, frequency conversion, and nonreciprocal transmission. The nonHermitian space−time varying systems combining nonHermiticity and space−time varying capability, have been proposed to realize wave control like unidirectional amplification, while its experimental realization still remains a challenge. Here, based on metamaterials with softwaredefined impulse responses, we experimentally demonstrate nonHermitian space−time varying metamaterials in which the material gain and loss can be dynamically controlled and balanced in the time domain instead of spatial domain, allowing us to suppress scattering at the incident frequency and to increase the efficiency of frequency conversion at the same time. An additional modulation phase delay between different metaatoms results in unidirectional amplification in frequency conversion. The realization of nonHermitian space−time varying metamaterials will offer further opportunities in studying nonHermitian topological physics in dynamic and nonreciprocal systems.


Gravitational lens effect revisited through membrane waves Catheline, S., V. Delattre, G. LaloyBorgna, F. Faure, and M. Fink American Journal of Physics 90, no. 1, 4750 (2022)
Résumé: By means of experiments and curved manifold simulations, we show that wave propagation past a topological deviation on a twodimensional flat fabric membrane is analogous to gravitational lensing. Using an ultrafast camera, we track a membrane plane wave as it crosses a local warped depression. Finite difference simulation, based on the scalar wave equation in a Schwarzschild metric, fully describes the experimental wavefront shape. Comparison between the theoretical and experimental deviation of wave geodesics from straight lines shows that (i) the nonlinear behavior of fabrics due to stretching induces second order effects only and (ii) the experimental depression is closely approximated by the Schwarzschild metric of a gravity well. The experiment demonstrates, in a simple way, how wave propagation is influenced by the topology of the transmission medium.


Modal approximation for strictly convex plasmonic resonators in the time domain: The Maxwell's equations Ammari, H., P. Millien, and A. L. Vanel Journal of Differential Equations 309, 676703 (2022)
Résumé: We study the possible expansion of the electromagnetic field scattered by a strictly convex metallic nanoparticle with dispersive material parameters placed in a homogeneous medium in a lowfrequency regime as a sum of modes oscillating at complex frequencies (diverging at infinity), known in the physics literature as the quasinormal modes expansion. We show that such an expansion is valid in the static regime and that we can approximate the electric field with a finite number of modes. We then use perturbative spectral theory to show the existence, in a certain regime, of plasmonic resonances as poles of the resolvent for Maxwell's equations with nonzero frequency. We show that, in the time domain, the electric field can be written as a sum of modes oscillating at complex frequencies. We introduce renormalised quantities that do not diverge exponentially at infinity.


Distribution of seismic scatterers in the San Jacinto Fault Zone, southeast of Anza, California, based on passive matrix imaging Touma, R., A. Aubry, Y. BenZion, and M. Campillo Earth and Planetary Science Letters 578 (2022)
Résumé: Fault zones are associated with multiscale heterogeneities of rock properties. Large scale variations may be imaged with conventional seismic reflection methods that detect offsets in geological units, and tomographic techniques that provide average seismic velocities in resolved volumes. However, characterizing elementary localized inhomogeneities of fault zones, such as cracks and fractures, constitutes a challenge for conventional techniques. Resolving these smallscale heterogeneities can provide detailed information for structural and mechanical models of fault zones. Recently, the reflection matrix approach utilizing body wave reflections in ambient noise crosscorrelations was extended with the introduction of aberration corrections to handle the actual lateral velocity variations in the fault zone (Touma et al., 2021). Here this method is applied further to analyze the distribution of scatterers in the first few kilometers of the crust in the San Jacinto Fault Zone at the Sage Brush Flat (SGB) site, southeast of Anza, California. The matrix approach allows us to image not only specular reflectors but also to resolve the presence, location and reflectivity of scatterers for seismic waves starting with a simple homogeneous background velocity model of the medium. The derived threedimensional image of the fault zone resolves lateral variations of scattering properties in the region within and around the surface fault traces, as well as differences between the Northwest (NW) and the Southeast (SE) parts of the study area. A localized intense damage zone at depth is observed in the SE section, suggesting that a geometrical complexity of the fault zone at depth induces ongoing generation of rock damage.

