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.

