microtoroid

A microtoroid is a micro-scale toroidal optical resonator that confines light by total internal reflection around its circular rim. Typically made of silica on a chip, microtoroids support whispering gallery modes with very high quality factors and small mode volumes, allowing light to circulate for many optical periods.

Fabrication involves creating a microdisk on a substrate and smoothing its rim with a CO2 laser or thermal reflow to form a toroidal shape. This process yields ultra-smooth surfaces and high Q values, often exceeding 10^7 and reaching up to 10^8 in optimized devices. External light is coupled into the resonator via a nearby tapered optical fiber or integrated waveguide that uses evanescent coupling.

Applications include optical filtering and lasing stabilization, nonlinear optics such as Kerr frequency comb generation, and sensing. In sensing, the resonance frequency shifts in response to changes in the surrounding refractive index, temperature, or pressure. In optomechanics, the circulating light interacts with mechanical modes of the toroid, enabling sensitive displacement measurements and cooling experiments.

Compared with planar resonators, microtoroids offer very high Q factors and small mode volumes, but pose fabrication challenges and sensitivity to environmental conditions due to their reliance on precise surface quality and coupling alignment.