Subwavelength confinement in an all-dielectric slot waveguide with bound states in the continuum for ultrahigh-Q microresonators

Bound state in the continuum (BIC) has been considered a promising strategy to reduce fabrication complexity and radiative loss. Despite the successful demonstration of many BIC-based configurations, their mode confinement is comparable to conventional dielectric waveguides. Even though plasmonics with strong light localization have been introduced into BICs, the intrinsic ohmic loss is still a facing challenge to be conquered. Herein, we propose a new approach, to our knowledge for the first time, that integrates subwavelength confinement and optical BIC in all dielectric slot waveguides. A nanogap with low refractive index is utilized to separate the top waveguide and high-index substrate, leading to strong mode localization and energy confinement (∼ λ