Coherent Perfect Absorption: Zero Reflection Without Linewidth Suppression

TL;DR

The paper revisits coherent perfect absorption (CPA) as a potential route to linewidth suppression in linear, weak-probe regimes. It combines standard input-output theory with experiments on a 1-port bare cavity and a cavity–magnonics hybrid to show that CPA yields a zero in the on-resonance reflection at $\omega_p=\omega_a$ (or $\omega_p^{CPA}$ for hybrids) when $\kappa_{\mathrm{ext}}=\kappa_{\mathrm{int}}$ and, crucially, that the spectral poles remain at $\tilde{\omega}=\omega-i\kappa/2$ with width $\kappa=\kappa_{\mathrm{int}}+\kappa_{\mathrm{ext}}$ (or the corresponding hybrid analog). Logarithmic plots can visually mimic linewidth narrowing or polaromechanical splitting, but linear-scale spectra reveal no true splitting. The results provide practical guidelines for data presentation and establish that CPA does not reduce linewidth or produce splitting in the linear, weak-probe regime, even in cavity magnomechanics.

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.

Figures (14)

• Figure 1: Schematic of the single-port reflection measurement of a bare cavity. The external coupling is tuned by adjusting the adapter-pin insertion depth.
• Figure 2: Reflection spectra of a single-port bare microwave cavity for several pin insertion length $\delta l$ (equivalently, external dissipation rate $\kappa_{\mathrm{ext}}$): (a) magnitude in logarithmic and (b) magnitude in linear units. The arrow marks coherent perfect absorption (CPA), realized at critical coupling $\kappa_{\mathrm{ext}}=\kappa_{\mathrm{int}}$, where the on-resonance dip ideally vanishes. The linewidth scale is set by the total decay rate $\kappa_a=\kappa_{\mathrm{int}}+\kappa_{\mathrm{ext}}$.
• Figure 3: Fitted dissipation rates of the bare microwave cavity as a function of pin insertion length $\delta l$ (equivalently, external dissipation rate $\kappa_{\mathrm{ext}}$). Blue: $\kappa_{\mathrm{int}}$ (nearly constant); orange: $\kappa_{\mathrm{ext}}$ (tunable); green: $\kappa_{a}=\kappa_{\mathrm{int}}+\kappa_{\mathrm{ext}}$. The rates $\kappa_{\mathrm{ext}}$, $\kappa_{\mathrm{int}}$, and $\kappa_{a}$ are extracted by fitting the spectra to Eq. \ref{['eq:S11-power']}. Red circles show the FWHM from Lorentzian fits to the measured data (equal to $\kappa_{a}$). The vertical dashed line indicates CPA (critical coupling).
• Figure 4: Schematic of the cavity–magnomechanics experiment. A weak VNA probe monitors the reflection $S_{11}$, while an independent microwave source (MWS) is power-combined onto the same feedline to resonantly drive the upper-polariton mode.
• Figure 5: Reflection spectra of a single-port cavity magnonics (Fig. \ref{['fig4']}) for several pin insertion length $\delta l$ (equivalently, external dissipation rate $\kappa_{\mathrm{ext}}$) , shown in (a) logarithmic and (b) linear units. A two-polariton modes with deepest dip in the lower mode is bolded. The vertical dashed line marks CPA of the lower polariton (critical coupling, $\kappa_{\mathrm{ext}}=\kappa_{\mathrm{int}}$), where the on-resonance dip ideally vanishes.