Experimental demonstration of virtual critical coupling to a single-mode microwave cavity

Abstract

We present an experimental realization of virtual critical coupling in microwave, i.e., virtual perfect absorption of an incident wave by a resonant cavity, through transient time modulation of its amplitude. The design of a waveform matched to the ignition process of a plasma, characterized in a simplified way by two operating modes over time (plasma off/plasma on), motivates this first step in practical realization of virtual critical coupling in microwaves. We propose a time domain method for extracting necessary parameters for realization of virtual critical coupling, especially the complex frequency called zero of the S-matrix. To this end, we start from the experimental characterization of a single-mode and single-access microwave cavity including metal protrusions for future plasma ignition. Then, the method relies on the analysis of harmonic response of the overcoupled cavity during three time periods: the transient under excitation, the steady state under excitation, and the transient after excitation cutoff. Finally, an experimental demonstration of virtual critical coupling is performed.