ABSTRACT The sensitivity of transmission to the input wavefront is a hallmark feature of complex media and the basis for wavefront shaping techniques. Yet, intriguing special cases exist in which the output wavefront is “frozen” (agnostic to the input wavefront). This happens when special structure in the complex medium collapses the rank of its transmission matrix to unity. Here, an analogous, more universal phenomenon for differential scattering (including reflection) in reconfigurable complex media is demonstrated. Specifically, for a localized perturbation, the differential scattering matrix of any complex medium has rank one. One consequence is that the differential output signal is coherent irrespective of the input wavefront's coherence. Moreover, the thermal noise emitted into the frozen differential output mode has a structure that can be exploited for thermal noise management. Frozen differential scattering is experimentally evidenced in a rich‐scattering wireless link parametrized by a programmable meta‐atom. “Customized freezing” is achieved by optimizing the configuration of programmable meta‐atoms that parametrize the wireless link, as envisioned for 6G networks. Moreover, particular shapes of the frozen differential output mode are imposed, and a signal‐to‐thermal‐noise ratio is maximized. Potential applications include filtering and stabilization of differential wavefronts, as well as imaging, sensing, and communications in complex media.
Philipp del Hougne (Sun,) studied this question.