microcavities

Microcavities are optical resonators that confine light to small volumes and sustain discrete resonant modes for relatively long times. They are characterized by a resonance frequency or wavelength, a quality factor Q that measures energy storage relative to loss per cycle, and a mode volume V that quantifies the spatial extent of the confined field. The ratio Q/V governs the strength of light–matter interactions inside the cavity and, together with the Purcell factor, describes how emission from nearby atoms or quantum emitters may be enhanced or suppressed.

Common geometries include Fabry-Pérot microcavities formed by two mirrors, such as distributed Bragg reflectors, which trap light between parallel surfaces; whispering-gallery-mode WGMs in microdisks, microrings, or microspheres where light travels around the periphery by total internal reflection; and photonic-crystal cavities created by breaking symmetry in a periodic dielectric lattice to form a localized mode. Other approaches use plasmonic or hybrid metal–dielectric structures to achieve extreme confinement at the cost of higher losses.

Fabrication typically involves semiconductor epitaxy to form stacked mirror structures, followed by lithography and etching; silica or glass microresonators are produced by melting, polishing, or laser machining. Applications span low-threshold microlasers, narrow-linewidth lasers, high-sensitivity optical sensors, nonlinear optics and frequency conversion, and cavity quantum electrodynamics experiments where single emitters couple to a single cavity mode. Microcavities also enable integrated photonic devices, optical frequency comb generation, and optomechanical sensing where light confined in the cavity interacts with mechanical motion.