microresonator

A microresonator is a compact optical or microwave resonator that stores electromagnetic energy in a small volume for many cycles, enabling narrow spectral features and strong nonlinear interactions. In optical versions, energy is confined by total internal reflection in circular or annular structures such as rings, disks, or toroids, supporting whispering-gallery modes with high quality factors. Nonlinear processes in microresonators, notably the Kerr effect, can convert a continuous-wave input into a comb of equally spaced frequencies.

Common optical microresonator geometries include ring resonators, disk resonators, and toroidal or spherical microresonators, often fabricated in materials such as silicon nitride, silicon dioxide (silica), or calcium fluoride. On-chip implementations frequently use silicon photonics platforms, where light couples into the resonator via evanescent coupling from a bus waveguide or a fiber taper. Other dielectric resonators are used at microwave and RF frequencies.

Key performance parameters are the quality factor (Q), which measures energy storage relative to losses, and the free spectral range (FSR), which is the spacing between resonant modes and is inversely proportional to the resonator circumference. The effective refractive index and mode volume also influence confinement and nonlinear efficiency.

Applications span frequency comb generation, ultra-stable lasers, high-resolution filtering, and sensing of refractive index, mass, temperature, or bioaffinity through shifts in resonance. Challenges include minimizing scattering losses, achieving precise coupling, and stabilizing resonances against thermal drift.