Torunds

Torunds are a class of toroidal optical resonators used in integrated photonics. The term is derived from torus and round, reflecting their doughnut-shaped geometry. Torunds are typically fabricated from dielectric materials such as silicon nitride or silicon dioxide and shaped into a smooth toroidal surface through lithographic patterning and thermal reflow or other polishing techniques.

Light is confined around the circumference by total internal reflection, supporting whispering-gallery modes that can achieve small optical mode volumes and high quality factors. This confinement enables strong light–matter interactions and sensitive optical responses. Diameters of torunds generally range from tens to hundreds of micrometers, with precise control over surface smoothness and boundary conditions to maximize performance. Variants of torunds include solid torunds, which have continuous material throughout the cross-section, and hollow or shell torunds, where the optical field is more strongly confined to the periphery.

Key properties commonly cited for torunds are high quality factors (Q factors often spanning a wide range depending on material quality and fabrication), low optical loss, and tunable resonance frequencies through thermal, mechanical, or electro-optic means. Their performance can be influenced by surface roughness, material absorption, and device packaging. Torunds are studied for applications such as narrow-linewidth lasers, dense wavelength-division multiplexing filtering, refractive-index sensing, nonlinear optical processes, and microresonator-based frequency combs.

See also microtoroid, whispering-gallery mode, optical resonator, and dielectric microresonator.