All-Dielectric Metawaveguide Ring Resonators with Deeply Sub-Diffractive Mode Volumes

Abstract

Whispering gallery mode (WGM) resonators provide an essential platform for various optical applications but are typically limited to mode volumes V ≈2πR(λ₀/2n)² where R is the bend radius and n is the refractive index. Here, the theory, simulation, and experimental realization of WGM resonators capable of achieving deeply sub-diffractive mode volumes are presented, V << 2πR(λ₀/2n)², while preserving high Q factors. Rather than relying on plasmonics to reduce the mode volume, the work relies on all-dielectric metamaterial waveguides that support localized field enhancements within the high index medium. Combined with the excitation of standing wave rather than traveling wave WGM resonances, it is shown how the mode volume of a silicon microring resonator can be reduced by factors >10 – 100 depending upon nanostructure dimensions and choice of cladding. The analysis further suggests a lower bound for the sub-diffractive all-dielectric mode volume, V_min’, which scales in proportion to the mode order m times the refractive index raised to the seventh power, i.e.: V_min’ ≈mn⁻⁷. Experimentally, these sub-diffractive WGM devices are shown to support standing wave resonances while maintaining high intrinsic quality factors ≈10⁴–10⁵. These metawaveguide ring resonators present a promising WGM platform for interfacing wavelength-scale optics with sub-wavelength matter.