Non-Hermitian photonic systems capable of perfectly absorbing incident radiation recently attracted much attention both because fundamentally they correspond to an exotic scattering phenomenon (real-valued scattering matrix zero) and because their extreme sensitivity holds great technological promise. The sharp reflection dip is a hallmark feature underlying many envisioned applications in precision sensing, secure communication, and wave filtering. However, a rigorous link between the underlying scattering anomaly and the system's sensitivity to a perturbation is still missing. Here, a theoretical description in complex scattering systems is developed which quantitatively explains the reflection dip's shape. It is furthermore demonstrated that coherent perfect absorption (CPA) is associated with a phase singularity and that the sign of the diverging time delay is related to the mismatch between excitation rate and intrinsic decay rate. The theoretical predictions are confirmed in experiments based on a 3D chaotic cavity excited by eight channels. Rather than relying on operation frequency and attenuation inside the system to be two free parameters, "on-demand" CPA is achieved at an arbitrary frequency by tweaking the chaotic cavity's scattering properties with programmable meta-atom inclusions. Finally, the optimal sensitivity of the CPA condition to minute perturbations of the system is proven theoretically and verified experimentally.

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In this paper, we apply a novel wavefront shaping technique within a cable network. By manipulating an array of crosstalk sources at different locations of the network, we demonstrate experimentally a strong enhancement of the intensity at a specific wire end where an impedance change occurs. The optimal wavefront for maximal focusing is determined noninvasively using the generalized Wigner-Smith operator. Our approach relies on two successive measurements of the scattering matrix measured at the injection ports, before and after the change. The optimal wavefront then closely corresponds to the first eigenstate of the generalized Wigner-Smith operator. Thus, a maximum intensity may be reached at a remote distance without cooperation of the target, opening up new perspectives in the context of electromagnetic aggressions or attacks (cybersecurity).

We determine the channel state information (CSI) of a multiple-input single-output (MISO) Rayleigh channel without prior communications between the transmitting antennas and the receiver. Our approach solely relies on an impedance modulation at the receiver and therefore circumvents the vexing need of a direct feedback between receiver and transmitters. We extract the wavefront to be transmitted in order to achieve optimal focusing on the receiver from a generalized Wigner-Smith operator (WSO). The latter is evaluated based on the scattering matrix of the transmit array measured in the two possible states of the receiver. Based on the CSI extracted via the WSO, any desired precoding technique can be implemented. Here, we show through in-situ time-domain measurements in a reverberation chamber that our approach closely reaches time-reversal performances in terms of focusing efficiency. We expect our approach to open up new perspectives for future wireless communications and wireless power transfer, including applications in the biomedical domain and electronic warfare.INDEX TERMS Time reversal communications, channel state information, Wigner-Smith operator, multiple-input single-output (MISO), reverberation chamber