High- Q Exterior Whispering-Gallery Modes in a Double-Layer Crystalline Microdisk Resonator

Exterior whispering-gallery modes (WGMs), whose mode energy is mainly confined outside the microcavity, can achieve large mode overlapping with the ambient environment, as well as a strong electric field and gradient force at the surface. Here, we demonstrate highly localized WGMs in the nanoair gap of a double-layer crystalline microdisk. The geometry is based on a horizontal slot-waveguide structure of two vertically stacked crystalline microdisks made of lithium niobate thin films. The slot WGM possesses a high quality factor in excess of 10^{5} without metallic loss. The absorption and scattering loss is reduced by use of the crystalline nanofilm at sub-nm rms surface roughness. The demonstrated configuration can be highly favored in various applications including optical sensing, nonlinear optics, and optomechanics.