Virtual Critical Coupling

TL;DR

This paper introduces a novel method to achieve perfect excitation of high-Q, lossless electromagnetic resonators by tailoring the excitation wave's temporal profile, avoiding dissipation and enhancing energy coupling efficiency.

Contribution

It extends the concept of critical coupling to lossless resonators through temporal shaping of excitation waves, enabling unitary efficiency at high quality factors.

Findings

Achieves perfect excitation efficiency without loss in high-Q resonators.

Demonstrates the approach using coupled-mode theory and complex frequency excitations.

Applicable to quasi-bound states in the continuum regime.

Abstract

Electromagnetic resonators are a versatile platform to harvest, filter and trap electromagnetic energy, at the basis of many applications from microwaves to optics. Resonators with a large intrinsic quality factor (Q) are highly desirable since they can store a large amount of energy, leading to sharp filtering and low loss. Exciting high-Q cavities with monochromatic signals, however, suffers from poor excitation efficiency, i.e., most of the impinging energy is lost in the form of reflection, since high-Q resonators are weakly coupled to external radiation. Although critical coupling eliminates reflections in steady-state by matching the intrinsic and coupling decay rates, this approach requires the introduction of loss in the resonator, causing dissipation and lowering the overall Q-factor. Here, we extend the notion of critical coupling to high-Q lossless resonators based on tailoring the temporal profile of the excitation wave. Utilizing coupled-mode theory, we demonstrate an effect analogous to critical coupling by mimicking loss with non-monochromatic excitations at complex frequencies. Remarkably, we show that this approach enables unitary excitation efficiency in open systems, even in the limit of extreme quality factors in the regime of quasi-bound states in the continuum.