Computation of leaky waves in layered structures coupled to unbounded media by exploiting multiparameter eigenvalue problems Gravenkamp, H., B. Plestenjak, D. A. Kiefer, and E. Jarlebring Journal of Sound and Vibration 596 (2025)
Résumé: We present a semi-analytical approach to compute quasi-guided elastic wave modes in horizontally layered structures radiating into unbounded fluid or solid media. This problem is of relevance, e.g., for the simulation of guided ultrasound in embedded plate structures or seismic waves in soil layers over an elastic half-space. We employ a semi-analytical formulation to describe the layers, thus discretizing the thickness direction by means of finite elements. For a free layer, this technique leads to a well-known quadratic eigenvalue problem for the mode shapes and corresponding horizontal wavenumbers. Incorporating the coupling conditions to account for the adjacent half-spaces gives rise to additional terms that are nonlinear in the wavenumber. We show that the resulting nonlinear eigenvalue problem can be cast in the form of a multiparameter eigenvalue problem whose solutions represent the wave numbers in the plate and in the half-spaces. The multiparameter eigenvalue problem is solved numerically using recently developed algorithms. Matlab implementations of the proposed methods are publicly available.
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Single-emitter super-resolved imaging of radiative decay rate enhancement in dielectric gap nanoantennas Córdova-Castro, R. M., B. Van Dam, A. Lauri, S. A. Maier, R. Sapienza, Y. De Wilde, I. Izeddin, and V. Krachmalnicoff Light: Science and Applications 13, no. 1 (2024)
Résumé: High refractive index dielectric nanoantennas strongly modify the decay rate via the Purcell effect through the design of radiative channels. Due to their dielectric nature, the field is mainly confined inside the nanostructure and in the gap, which is hard to probe with scanning probe techniques. Here we use single-molecule fluorescence lifetime imaging microscopy (smFLIM) to map the decay rate enhancement in dielectric GaP nanoantenna dimers with a median localization precision of 14 nm. We measure, in the gap of the nanoantenna, decay rates that are almost 30 times larger than on a glass substrate. By comparing experimental results with numerical simulations we show that this large enhancement is essentially radiative, contrary to the case of plasmonic nanoantennas, and therefore has great potential for applications such as quantum optics and biosensing.
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Reflection Measurement of the Scattering Mean Free Path at the Onset of Multiple Scattering Goïcoechea, A., C. Brütt, A. Le Ber, F. Bureau, W. Lambert, C. Prada, A. Derode, and A. Aubry Physical Review Letters 133, no. 17 (2024)
Résumé: Multiple scattering of waves presents challenges for imaging complex media but offers potential for their characterization. Its onset is actually governed by the scattering mean free path ℓs that provides crucial information on the medium microarchitecture. Here, we introduce a reflection matrix method designed to estimate this parameter from the time decay of the single scattering rate. Our method is first validated by an ultrasound experiment on a tissue-mimicking phantom before being applied in vivo to a human liver. This Letter opens important perspectives for quantitative imaging of heterogeneous media with waves, whether it be for nondestructive testing, biomedical, or geophysical applications.
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Shearmetry of Fluids with Tunable Rheology by Polarized Luminescence of Rare Earth-Doped Nanorods Wang, Z., Q. Zou, L. Magermans, G. Amselem, C. A. Dessalles, B. Louis, M. Filoche, T. Gacoin, and J. Kim ACS Nano (2024)
Résumé: Shear stress plays a critical role in regulating physiological processes within microcirculatory systems. While particle imaging velocimetry is a standard technique for quantifying shear flow, uncertainty near boundaries and low resolution remain severe restrictions. Additionally, shear stress determination is particularly challenging in biofluids due to their significant non-Newtonian behaviors. The present study develops a shearmetry technique in physiological settings using a biomimetic fluid containing rare earth-doped luminescent nanorods acting in two roles. First, they are used as colloidal additives adjusting rheological properties in physiological media. Their anisotropic morphology and interparticle interaction synergistically induce a non-Newtonian shear-thinning effect emulating real biofluids. Second, they can probe shear stress due to the shear-induced alignment. The polarized luminescence of the nanorods allows for quantifying their orientational order parameter and thus correlated shear stress. Using scanning confocal microscopy, we demonstrate the tomographic mapping of the shear stress distribution in microfluidics. High shear stress is evident near the constriction and the cellular periphery, in which non-Newtonian effects can have a significant impact. This emerging shearmetry technique is promising for implementation in physiological and rheological environments of biofluids.
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Multiregion Light Control in Diffusive Media via Wavefront Shaping Shaughnessy, L., R. E. Mcintosh, A. Goetschy, C. W. Hsu, N. Bender, H. Yllmaz, A. Yamilov, and H. Cao Physical Review Letters 133, no. 14 (2024)
Résumé: Wavefront shaping allows focusing light through or inside strongly scattering media, but the background intensity also increases which reduces the target's contrast. By combining transmission or deposition matrices for different regions, we construct joint operators to achieve spatially resolved control of light in diffusive systems. The eigenmode of a contrast operator can maximize the power contrast between a target and its surrounding. A difference operator enhances the power delivery to a target while avoiding the background increase. This work opens the door to coherent control of nonlocal effects in wave transport for practical applications.
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Multi-spectral reflection matrix for ultrafast 3D label-free microscopy Balondrade, P., V. Barolle, N. Guigui, E. Auriant, N. Rougier, C. Boccara, M. Fink, and A. Aubry Nature Photonics 18, no. 10, 1097-1104 (2024)
Résumé: Label-free microscopy exploits light scattering to obtain a three-dimensional image of biological tissues. However, light propagation is affected by aberrations and multiple scattering, which drastically degrade the image quality and limit the penetration depth. Multi-conjugate adaptive optics and time-gated matrix approaches have been developed to compensate for aberrations but the associated frame rate is extremely limited for three-dimensional imaging. Here we develop a multi-spectral matrix approach to solve these fundamental problems. On the basis of a sparse illumination scheme and an interferometric measurement of the reflected wave field at multiple wavelengths, the focusing process can be optimized in post-processing for any voxel by addressing independently each frequency component of the reflection matrix. A proof-of-concept experiment shows a three-dimensional image of an opaque human cornea over a 0.1 mm3 field of view at a 290 nm resolution and a 1 Hz frame rate. This work paves the way towards a fully digital microscope allowing real-time, in vivo, quantitative and deep inspection of tissues.
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