Compact dielectric cavities based on frozen bound states in the continuum

Dielectric microcavities are used widely today for confining the light to its wavelength scale, which is important for fundamental physics studies of light-matter interactions such as cavity quantum electrodynamics (QED) and cavity polaritons, as well as various applications including ultrafast lasers and single-photon light sources [1]. They have been implemented in various platforms such as microrings, microdisks, micropilars, photonic crystals (PhCs), etc. Usually, it is desirable to reduce the mode volume while keeping the quality-factor (Q-factor) as high as possible for an optical cavity to enhance the light-matter interaction. Recently, a particular type of optical mode with an infinite Q-factor has been reported in a PhC slab, which is referred to as bound state in the continuum (BIC) [2]. A BIC is a special solution of a wave equation, which is discrete and bounded while it lies inside a continuum of unbounded states [2].