Alexandre Aubry - Academia.edu (original) (raw)

Papers by Alexandre Aubry

Zenodo (CERN European Organization for Nuclear Research), Oct 4, 2023

Label-free microscopy exploits light scattering to obtain a threedimensional image of biological ... more Label-free microscopy exploits light scattering to obtain a threedimensional 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 3D imaging. Here we develop a multi-spectral matrix approach to solve these fundamental problems. Based on an interferometric measurement of a polychromatic reflection matrix, the focusing process can be optimized in post-processing at any voxel by addressing independently each frequency component of the wave-field. A proof-of-concept experiment demonstrates the three-dimensional image of an opaque human cornea over a 0.1 mm 3field-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.

Nano Letters, 2010

A new strategy to control the flow of surface plasmon polaritons at metallic surfaces is presente... more A new strategy to control the flow of surface plasmon polaritons at metallic surfaces is presented. It is based on the application of the concept of Transformation Optics to devise the optical parameters of the dielectric medium placed on top of the metal surface. We describe the general methodology for the design of Transformation-Optical devices for surface plasmons and analyze, for proof-of-principle purposes, three representative examples with different functionalities: a beam shifter, a cylindrical cloak and a ground-plane cloak.

Journal of Geophysical Research: Solid Earth

Imaging the structure of major fault zones is essential for our understanding of crustal deformat... more Imaging the structure of major fault zones is essential for our understanding of crustal deformations and their implications on seismic hazards. Investigating such complex regions presents several issues, including the variation of seismic velocity due to the diversity of geological units and the cumulative damage caused by earthquakes. Conventional migration techniques are in general strongly sensitive to the available velocity model. Here we apply a passive matrix imaging approach which is robust to the mismatch between this model and the real seismic velocity distribution. This method relies on the cross‐correlation of ambient noise recorded by a geophone array. The resulting set of impulse responses form a reflection matrix that contains all the information about the subsurface. In particular, the reflected body waves can be leveraged to: (a) determine the transmission matrix between the Earth's surface and any point in the subsurface; (b) build a confocal image of the subsu...

Nature Communications

Matrix imaging paves the way towards a next revolution in wave physics. Based on the response mat... more Matrix imaging paves the way towards a next revolution in wave physics. Based on the response matrix recorded between a set of sensors, it enables an optimized compensation of aberration phenomena and multiple scattering events that usually drastically hinder the focusing process in heterogeneous media. Although it gave rise to spectacular results in optical microscopy or seismic imaging, the success of matrix imaging has been so far relatively limited with ultrasonic waves because wave control is generally only performed with a linear array of transducers. In this paper, we extend ultrasound matrix imaging to a 3D geometry. Switching from a 1D to a 2D probe enables a much sharper estimation of the transmission matrix that links each transducer and each medium voxel. Here, we first present an experimental proof of concept on a tissue-mimicking phantom through ex-vivo tissues and then, show the potential of 3D matrix imaging for transcranial applications.

IEEE Transactions on Medical Imaging, Dec 1, 2022

This is the first article in a series of two dealing with a matrix approach for aberration quanti... more 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 so-called focused reflection matrix that actually contains much more information than a confocal ultrasound image. In this paper, a time-frequency 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 input-output 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 state-of-the-art 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 proof-of-concept based on time-domain simulations, UMI is applied to the in-vivo study of a human calf. Beyond this specific example, UMI opens a new route for speed-of-sound and scattering quantification in ultrasound imaging.

arXiv (Cornell University), Jun 11, 2021

Fault zones are associated with multi-scale heterogeneities of rock properties. Large scale varia... more Fault zones are associated with multi-scale 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 small-scale 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 cross-correlations was extended with the introduction of aberration corrections to handle the actual lateral velocity variations in the fault zone [1]. 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 three-dimensional 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.

We investigate, both experimentally and theoretically, the spectral properties of the backscatter... more We investigate, both experimentally and theoretically, the spectral properties of the backscattering operator in random media. The experimental set up consists in an array of 64 programmable ultrasonic transducers placed in front of a random scattering medium. The impulse responses between each couple of transducers are measured and form the interelement matrix.The evolution of the singular values with time and frequency is computed by means of a short-time Fourier analysis. Our aim is to compare the mean distribution of the singular values with the predictions of Random Matrix Theory (RMT). Two kinds of random media have been investigated: a highly scattering medium in which multiple scattering predominates and a weakly scattering medium. In both cases, residual correlations that may exist between matrix elements are shown to be a key parameter. The mean distribution of singular values exhibits, surprisingly, very different behaviors in the single and multiple scattering regimes. The agreement between experimental results and RMT predictions is found to be very good in both cases. The results are applied to the detection of a target embedded in a random scattering medium with the DORT method.

Waves scattered by a weakly scattering random medium contain a predominant single scattering cont... more Waves scattered by a weakly scattering random medium contain a predominant single scattering contribution as well as a multiple scattering contribution which is usually neglected. But its investigation can be fruitful for characterization purposes because it provides measurements of statistical parameters such as the scattering mean free path. Our aim is to extract the multiple scattering contribution in a weakly scattering random medium. The experimental set up consists in an array of programmable transducers placed in front of the sample. The impulse responses between each couple of transducers are measured and form the interelement matrix. Our technique allows the extraction of multiple scattering signals by taking advantage of their randomness, contrary to single scattering contributions which are shown to exhibit a deterministic coherence along the antidiagonals of the array response matrix, whatever the distribution of scatterers. To illustrate the interest of this technique, we applied it to a synthetic medium (Agar gel) and to breast tissues. Surprisingly, the multiple scattering contribution is far from negligible in the breast around 4.3 MHz. The temporal evolution of its intensity can provide a new tool to complete information provided by standard echography; it also constitutes an experimental test of the Born approximation.

Journal of the Acoustical Society of America, Mar 1, 2023

Conventional ultrasound imaging generally relies on a single scattering assumption and a constant... more Conventional ultrasound imaging generally relies on a single scattering assumption and a constant sound hypothesis. However, in dense granular sediments where the glass beads diameter is comparable to the wavelength (typically 500 μm), both hypothesizes are no longer valid, resulting in a loss of resolution and contrast, up to the situation where a target to image could totally vanish in the fog. To better characterize this strongly scattering medium, we have conducted several ultrasonic experiments in reflection and in transmission, either on the diffused wave or on the coherent one. Both the transport and scattering mean free paths are estimated in the imaging bandwidth (1–5 MHz), alongside with the phase velocity and extinction length of the coherent wave. These results are compared with an analytical model, i.e. Generalized Coherent Potential Approximation*, which considers a coated sphere buried in an effective medium. Finally, we show a dispersive beamformer which takes into account the measured phase and group velocity. This allows us to obtain a more resolved and contrasted image in such a scattering medium. * Jing X, Sheng P, Zhou M. Phys Rev Lett. 1991

In 2018, the company St1 Oy performed an Enhanced Geothermal System (EGS) experiment using near-r... more In 2018, the company St1 Oy performed an Enhanced Geothermal System (EGS) experiment using near-real-time seismic monitoring as feedback for the pumping procedure and thus allowing the control of the induced seismicity. Between 4 June and 22 July (49 days), ~18,000 cubic meters of water was injected at around 6 km depth beneath the University campus in Otaniemi, Espoo, Finland (Kwiatek el al. 2019). The hydraulic stimulation and post-stimulation stages were monitored by a temporary ~100 three-components geophone network installed by the Institute of Seismology, University of Finland (Hillers et al. 2020). If the requirements for induced seismicity have been successfully met, the medium changes produced by the hydraulic stimulation remain unclear. Especially with regard to the activation or opening of cracks. Recent application of the distortion matrix concept (Badon et al. 2020) in seismology (Touma et al. 2021) allows us to consider resolving some of these questions by imaging the distribution of the scatterers in the medium during the different injection stages. The distortion matrix operator makes it possible to correct a large part of the aberrations present in images obtained by focusing in depth a reflection matrix recorded at the surface. The phase distortions of the seismic wavefield due to complex velocity distributions can be recovered for any virtual source in the medium by comparing the recorded reflection matrix to the ideal geometric wave-front corresponding to this virtual source. By taking advantage of this powerful approach this study present 3D images of the volume surrounding the injection well of the EGS experiment.

Digital Holography and 3-D Imaging 2022, 2022

We report on a reflection matrix approach of optical microscopy that pushes back the fundamental ... more We report on a reflection matrix approach of optical microscopy that pushes back the fundamental limit of aberrations and multiple scattering. This allows an in-depth diffraction-limited imaging of biological media over a wide field-of-view.

HAL (Le Centre pour la Communication Scientifique Directe), Mar 29, 2022

Imaging and Applied Optics Congress, 2020

In optical imaging, light propagation is affected by the medium inhomogeneities. Adaptive optics ... more In optical imaging, light propagation is affected by the medium inhomogeneities. Adaptive optics has been employed to compensate for sample-induced aberrations but the field-of-view is often limited to a single isoplanatic patch. Here, we propose a non-invasive approach based on the distortion matrix concept. This matrix basically connects any focusing point with the distorted part of its wave-front in reflection. Its time- reversal and entropy analysis allows to correct for high-order aberrations over multiple isoplanatic areas. We demonstrate a Strehl ratio enhancement up to 2500 and a diffraction- limited resolution until a depth of ten scattering mean free paths through an opaque cornea.

Reflection seismic imaging usually suffers from a loss of resolution and contrast because of the ... more Reflection seismic imaging usually suffers from a loss of resolution and contrast because of the fluctuations of the wave velocities in the Earth's crust. In the literature, phase distortion issues are generally circumvented by means of a background wave velocity model. However, it requires a prior tomography of the wave velocity distribution in the medium, which is often not possible, especially in depth. In this paper, a matrix approach of seismic imaging is developed to retrieve a three-dimensional image of the subsoil without any prior knowledge of the background wave speed. To do so, passive noise cross-correlations between geophones of a seismic array are investigated under a matrix formalism. They form a reflection matrix that can be leveraged to synthesize virtual sources and detectors at any point in the medium. From this focused reflection matrix, a novel mathematical object is introduced: the distortion matrix. This operator essentially connects any virtual source inside the medium with the distortion that a wavefront, emitted from that point, experiences due to heterogeneities. A time reversal analysis of the distortion matrix enables the estimation of the transmission matrix that links each real geophone at the surface and each virtual geophone in depth. Phase distortions can then be compensated for any point of the underground. Applied to seismic data recorded along the Clark Branch of the San Jacinto Fault Zone, the present method is shown to provide an image of the fault until a depth of 4 km over the frequency range 10-20 Hz with a transverse resolution of 80 m. Strikingly, this resolution is almost one eighth below the diffraction limit imposed by the geophone array aperture. The heterogeneities of the subsoil play the role of a scattering lens which increases drastically the array aperture. The contrast is also optimized since most of the incoherent noise is eliminated by the iterative time reversal process. Beyond the specific case of the San Jacinto Fault Zone, the reported approach can be applied to any scales and areas for which a reflection matrix is available at a spatial sampling satisfying the Nyquist criterion.

arXiv (Cornell University), Oct 17, 2017

The propagation of waves in complex media can be harnessed either by taming the incident wave-fie... more The propagation of waves in complex media can be harnessed either by taming the incident wave-field impinging on the medium or by forcing waves along desired paths through its careful design. These two alternative strategies have given rise to fascinating concepts such as time reversal or negative refraction. Here, we show how these two processes are intimately linked through the negative reflection phenomenon. A negative reflecting mirror converts a wave of positive phase velocity into its negative counterpart and vice versa. In this article, we experimentally demonstrate this phenomenon with elastic waves in a 2D billiard and in a disordered plate by means of laser interferometry. Despite the complexity of such configurations, the negatively reflected wave field focuses back towards the initial source location, thereby mimicking a phase conjugation operation while being a fully passive process. The super-focusing capability of negative reflection is also highlighted in a monochromatic regime. The negative reflection phenomenon is not restricted to guided elastic waves since it can occur in zero-gap systems such as photonic crystals, chiral metamaterials or graphene. Negative reflection can thus become a tool of choice for the control of waves in all fields of wave physics.

We report on the experimental measurement of the backscattering matrix of a weakly scattering med... more We report on the experimental measurement of the backscattering matrix of a weakly scattering medium in optics, composed of a few dispersed gold nanobeads. The DORT method (Decomposition of the Time Reversal Operator) is applied to this matrix and we demonstrate selective and efficient focusing on individual scatterers, even through an aberrating layer. Moreover, we show that this approach provides the decomposition of the scattering pattern of a single nanoparticle. These results open important perspectives for optical imaging, characterization and selective excitation of nanoparticles.