Cavitiesundercoupled

Cavitiesundercoupled is a term used to describe a regime of resonant cavities—such as optical microresonators, microwave cavities, or acoustic cavities—in which the coupling to an external channel is weaker than the inherent losses of the cavity. In this regime, energy exchange with the external port is limited, and most energy decays through intrinsic loss mechanisms such as absorption, scattering, or radiation.

The regime is defined by the external coupling rate κ_ext being smaller than the intrinsic loss rate κ_int. The total energy decay rate of the cavity is κ = κ_int + κ_ext, and the loaded quality factor is Q_L = ω0/κ, where ω0 is the resonance frequency. In undercoupled cavities, κ_ext ≪ κ_int, so Q_L is dominated by Q_int, and the cavity tends to store energy with relatively little leakage into the external channel. As a result, the presence of the external port perturbs the cavity only modestly, and the coupling-induced alteration of the resonance is modest.

In a typical transmission measurement using a bus waveguide, undercoupled cavities exhibit a shallow dip in the through-port transmission at resonance, with limited power extraction via the external channel. The depth of the dip and the existence of any drop port depend on the specific configuration, but the undercoupled regime generally yields high intrinsic energy storage and a resonance that is less sensitive to external loading.

Applications and considerations include situations where high internal quality and energy storage are prioritized over efficient readout, such as sensing, nonlinear optics, or experiments requiring minimal perturbation of the cavity by the measurement port. The coupling regime can be tuned by adjusting the physical gap, overlap, or mode matching between the cavity and the external channel.