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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.

On‐Demand Coherent Perfect Absorption in Complex Scattering Systems: Time Delay Divergence and Enhanced Sensitivity to Perturbations