arXiv subjects

Antton Goïcoechea

Publications and source records attributed to Antton Goïcoechea.

8 recordsLinked to original sources

arXiv2026

Experimental observation of three-dimensional Anderson localization of electromagnetic waves

A prominent phenomenon in contemporary condensed matter physics is Anderson localization -- suppression of wave propagation in disordered systems as a result of interference effects. Despite being observed with various types of waves over the years, all prior attempts to reach Anderson localization of light in three-dimensional systems have been hampered by experimental artifacts. Here, we report an unambiguous experimental proof of three-dimensional Anderson localization of microwaves in disordered metal aggregates. By studying samples with different metal volume fractions, we show a clear difference between diffusive and localized behaviors, and the latter is confirmed by a scaling analysis of transmitted beam width in excellent agreement with theoretical and numerical results. Our demonstration opens avenues for both fundamental studies and practical applications of this extraordinary phenomenon.

cond-mat.dis-nn↗

arXiv2026

Backscatter Communication Through Disordered Media Enabled by a Programmable Leaky Cavity

Retrieving information through disordered scattering systems remains a major challenge in wireless communication and sensing due to the severe distortion and attenuation caused by multiple scattering. Here, we demonstrate robust focusing through random media using a leaky cavity made programmable by reconfigurable metasurfaces. We show that the leaky cavity leverages backscattering from the disordered medium to enhance the degree of control over the transmitted field. We achieve an enhancement in focused intensity by an order of magnitude compared to a horn antenna. We further demonstrate robust communication through the medium using low-power amplitude-modulated signals, and finally implement a backscatter communication scheme in which the same cavity illuminates and decodes data from a passive reflectivity-modulating target embedded within the disorder. Our results open new possibilities for reliable communication and sensing in complex environments.

physics.optics↗

arXiv2025

Single-Antenna Non-Line-of-Sight Matrix Imaging via Reconfigurable Intelligent Surfaces

Modern imaging and sensing in complex environments, ranging from biomedical diagnostics to wireless communication, relies on accurately measuring and then controlling the wave propagation. Conventional approaches demand large arrays of antennas or transducers to reconstruct the full reflection or transmission matrix, enabling advanced protocols such as selective focusing or adaptive wave control. Yet, these arrays are expensive, bulky, and difficult to implement at microwave frequencies. Here, we show that a single transmitting-receiving antenna, when combined with a reconfigurable intelligent surface (RIS), can fully reconstruct the reflection matrix from far-field measurements, effectively transforming the RIS into a programmable synthetic antenna array. This approach allows high-fidelity imaging of complex scenes, selective focusing through clutter, and real-time tracking of moving targets. Our results establish RIS as a versatile, low-cost platform for matrix-based imaging, with broad implications for adaptive wave control, real-time sensing, and imaging in environments previously considered inaccessible.

physics.app-ph↗

arXiv2025

Detection and characterization of targets in complex media using fingerprint matrices

When waves propagate through a complex medium, they undergo several scattering events. This phenomenon is detrimental to imaging, as it causes full blurring of the image. Here we describe a method for detecting, localizing and characterizing any scattering target embedded in a complex medium. We introduce a fingerprint operator that contains the specific signature of the target with respect to its environment. When applied to the recorded reflection matrix, it provides a likelihood index of the target state. This state can be the position of the target for localization purposes, its shape for characterization or any other parameter that influences its response. We demonstrate the versatility of our method by performing proof-of-concept ultrasound experiments on elastic spheres buried inside a strongly scattering granular suspension and on lesion markers, which are commonly used to monitor breast tumours, embedded in a foam mimicking soft tissue. Furthermore, we show how the fingerprint operator can be leveraged to characterize the complex medium itself by mapping the fibre architecture within muscle tissue. Our method is broadly applicable to different types of waves beyond ultrasound for which multi-element technology allows a reflection matrix to be measured.

physics.app-ph↗

arXiv2025

Detecting and Focusing on a Nonlinear Target in a Complex Medium

Wavefront shaping techniques allow waves to be focused on a diffraction-limited target deep inside disordered media. To identify the target position, a guidestar is required that typically emits a frequency-shifted signal. Here we present a noninvasive matrix approach operating at a single frequency only, based on the variation of the field scattered by a nonlinear target illuminated at two different incident powers. The local perturbation induced by the nonlinearity serves as a guide for identifying optimal incident wavefronts. We demonstrate maximal focusing on electronic devices embedded in chaotic microwave cavities and extend our approach to temporal signals. Finally, we exploit the programmability offered by reconfigurable smart surfaces to enhance the intensity delivered to a nonlinear target. Our results pave the way for deep imaging protocols that use any type of nonlinearity as feedback, requiring only the measurement of a monochromatic scattering matrix.

physics.app-ph↗

arXiv2024

Reflection Measurement of the Scattering Mean Free Path at the Onset of Multiple Scattering

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 $\ell_s$ that provides crucial information on the medium micro-architecture. 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 study opens important perspectives for quantitative imaging of heterogeneous media with waves, whether it be for non-destructive testing, biomedical or geophysical applications.

physics.med-ph↗

arXiv2021

Anisotropy of localized states in an anisotropic disordered medium

We study Anderson localization of a scalar wave in an ensemble of resonant point scatterers embedded in an anisotropic background medium. For uniaxial anisotropy of moderate strength, the mobility edges and the critical exponent of the localization transition are found to be unaffected by the anisotropy provided that the determinant of the anisotropy tensor is kept equal to one upon introducing the anisotropy. Localized modes have anisotropic spatial shapes although their anisotropy is weaker than the one expected from purely geometric considerations. The modes with the longest lifetimes are found to be the most anisotropic and their anisotropy increases with the size of the disordered medium.

cond-mat.dis-nn↗

arXiv2020

Suppression of transport anisotropy at the Anderson localization transition in three-dimensional anisotropic media

We study the transport of classical waves through three-dimensional (3D) anisotropic media close to the Anderson localization transition. Time-, frequency-, and position-resolved ultrasonic measurements are performed on anisotropic slab-shaped mesoglass samples to probe the dynamics and the anisotropy of the multiple scattering halo, and hence to investigate the influence of disorder on the nature of wave transport and its anisotropy. These experiments allow us to address conflicting theoretical predictions that have been made about whether or not the transport anisotropy is affected by the interference effects that lead to Anderson localization. We find that the transport anisotropy is significantly reduced as the mobility edge is approached---a behavior similar to the one predicted recently for matter waves in infinite anisotropic 3D media.

cond-mat.dis-nn↗