  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.
