Matériaux, résonances et interfaces (MRI)

Résumé: Achieving low-latency time-reversal of broadband radiofrequency signals is crucial for reliable communications in dynamic, uncontrolled environments. However, existing approaches are either digitally assisted—making broadband extension challenging—or limited to amplitude modulation. In this work, we report the very first, to our knowledge, experimental realization of a fully analog, phase-preserving time-reversal architecture for optically carried radiofrequency signals. The method exploits the exceptional coherence properties of rare-earth ion-doped materials and leverages the well-established photon echo mechanism, widely used in quantum technologies. While our demonstration is conducted with a modest bandwidth, we identify the fundamental cause of this limitation and propose solutions for future scalability.

Résumé: Understanding the rheological behavior of rapid granular flows is crucial for understanding various geological processes, such as fast fault slip and rapid motion of landslides. In this study, we conducted rotary shear experiments on different granular materials, spanning a range of shear velocities from slow to rapid and under varying normal stresses, to investigate the evolution of mechanical behavior under different flow conditions. The experimental results showed that steady-state shear resistance varied with normal stress and material composition at shear velocities below 1 m/s. A consistent velocity-dependent trend was observed. The steady-state shear resistance of the sample experienced a transition from velocity-strengthening behavior at low shear velocities (below 0.1 m/s) to velocity-weakening behavior at higher shear velocities (above 0.1 m/s). Interestingly, at shear velocities exceeding 1 m/s, the steady-state shear resistance became independent of normal stress and material composition, converging to a similar steady-state value for both crushable and uncrushable materials. Although normal stress and mineral composition had a limited influence on steady-state shear resistance at high shear rates, they significantly affected the weakening rate (the transition from peak strength to steady-state shear resistance), which was strongly correlated with the material's crushing ability, as characterized by the Weibull modulus. These findings provide insights into the mechanisms governing the hypermobility of mega-landslides and the rapid dynamics of geological flows.