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.

Plasma Ignition via High-Power Virtual Perfect Absorption

“…First, we have designed an aluminum (Al) resonant cavity, , incorporating two metallic concentrators that form a 6 mm gap and align parallel to the electric field, as depicted in Figure a. For optimal control and facilitation of plasma ignition, we have installed a glass tube within our aluminum cavity, strategically placed between the concentrators where the electric field peaks.…”

“…Recently, we proposed a method based on analyzing the temporal response of the cavity to harmonic excitation, enabling us to extract this Smatrix zero. 18 It is noteworthy that this method applies explicitly to overcoupled modes and only when they are sufficiently distanced from other resonances, which is the case in this study. In our single-port cavity experiment, we employed a vector network analyzer (VNA) to measure the S 11 across the relevant frequency band.…”

“…Determining the complex excitation frequency associated with VPA, also known as the S-matrix zero, , can be challenging in experimental resonant cavities. Recently, we proposed a method based on analyzing the temporal response of the cavity to harmonic excitation, enabling us to extract this S-matrix zero . It is noteworthy that this method applies explicitly to overcoupled modes and only when they are sufficiently distanced from other resonances, which is the case in this study.…”

“…This mechanism maintains a critical coupling in high-Q resonators, requiring a balance between directly reflected waves and leakage waves. , While VPA may be based on the temporal modulation of the resonant structure itself, , this balance is mainly achieved using a sinusoidal incident wave with an exponentially increasing amplitude. ,− The “virtual” in VPA indicates that it stores the incident wave’s electromagnetic energy, in contrast with conventional absorbers, which convert this energy to other forms. , VPA has been experimentally demonstrated in various systems, including elastodynamic waves, water waves, lumped elements, and microwave resonant cavities. , It holds great potential for efficient light storage and release in optical and quantum memories. Researchers have recently uncovered several novel effects that were previously hindered in the complex frequency plane, including virtual critical coupling, , unusual optical pulling force, virtual parity time symmetry, the non-Hermitian skin effect, and superlensing . We note that in the past decade, several researchers have explored the optimization of the interferential process at the resonator access by tailoring the incident signal. , These studies primarily hinged on time-reversal symmetry arguments.…”

“…1,2 VPA has been experimentally demonstrated in various systems, including elastodynamic waves, 15 water waves, 16 lumped elements, 17 and microwave resonant cavities. 18,19 It holds great potential for efficient light storage and release in optical and quantum memories. Researchers have recently uncovered several novel effects that were previously hindered in the complex frequency plane, including virtual critical coupling, 7,18 unusual optical pulling force, 20 virtual parity time symmetry, 21 the non-Hermitian skin effect, 22 and superlensing.…”