Coherent Perfect Absorption: Zero Reflection Without Linewidth Suppression

Abstract: Motivated by recent claims, we revisit how coherent perfect absorption (CPA) influences cavity and polaritonic linewidths. Using standard input output theory and measurements on single port bare microwave cavities and cavity magnon hybrids, we find that CPA drives the on resonance reflection to zero while the spectral width remains set by the total decay rate. Apparent narrowing observed near CPA is found to be a visual artifact that does not remain upon quantitative analysis. Extending the analysis to cavity magnomechanics, we show that logarithmic plots can exhibit apparent polaromechanical normal mode splitting, whereas linear scale spectra display no true splitting. These results clarify when CPA modifies amplitudes versus spectral poles, offer practical guidance for data presentation, and indicate that CPA alone is not a route to linewidth suppression or polaromechanical mode splitting in the linear, weak-probe regime.