High-Q quasi-bound states in the continuum in dimer dielectric resonators

Bound states in the continuum (BICs) offer an exceptional platform for wave trapping. Quasi-BICs in an isolated dielectric resonator with high quality factors (Q-factors) can be realized through leveraging avoided crossing between two modes due to strong coupling arising from destructive interference. In this work, we present a general strategy for constructing quasi-BICs in dimer dielectric resonators, achieving Q-factors that are enhanced by two to tens of times compared to quasi-BICs in a single dielectric resonator. Starting from a single dielectric structure supporting quasi-BICs, we optimize the Q-factors of supercavity modes by introducing an air gap within the single structure and systematically varying several structural dimensions of the dimer configuration. Multipole decomposition indicates that the enhancement in Q-factors originates from the suppression of multiple radiation channels. Moreover, we demonstrate that the design approach is universal via investigating high-Q quasi-BICs in both one-dimensional (1D) dimer rectangular nanowires (NWs) for transverse electric (TE) and transverse magnetic (TM) polarizations and three-dimensional (3D) dimer cuboids. Additionally, we explore the potential for further increasing the Q-factors of quasi-BICs using a trimer-resonator system. This work provides a rational framework for designing ultrahigh-Q resonances based on dimer or trimer dielectric resonators, with promising applications for enhancing light-matter interactions.