Quasi-Bound States in the Continuum-Induced Second Harmonic Generation Enhancement in High-$Q$ Triple Dielectric Nanoresonators

TL;DR

This paper presents a method to create ultra-high-Q resonances in triple dielectric nanoresonators using destructive interference, significantly enhancing second harmonic generation efficiency for advanced photonic applications.

Contribution

The authors introduce a general design approach for achieving Q-factors over 10,000 in subwavelength dielectric resonators through mode interference and structural tuning.

Findings

Achieved Q-factors >10,000 in triple dielectric nanoresonators.

Demonstrated 6.6% second harmonic generation efficiency.

Enhanced mode overlap with azimuthally polarized light.

Abstract

High-$Q$ optical nanocavities are fundamental to modern optics and photonics, enabling enhanced light-matter interactions. Previous studies have demonstrated that high-$Q$ supercavity modes can be constructed within a single dielectric resonator by leveraging quasi-bound states in the continuum. However, their $Q$-factors are limited to few tens or hundreds when such a resonator is subwavelength scale. Here, we propose a general recipe for achieving high-$Q$ resonances with $Q>10,000$ in triple subwavelength dielectric resonators. This is realized through destructive interference between two resonant modes, optimized by structural tuning. Multipole analysis confirms that destructive interference across radiation channels suppresses loss, forming the ultrahigh-$Q$ states. These resonances can be efficiently excited by azimuthally polarized light due to improved mode overlap. As a key application, we demonstrate efficient second harmonic generation under this excitation, achieving a conversion efficiency of $6.6\%$ at an incident intensity of 0.78 GW/cm$^2$. Our results may find exciting applications in developing ultracompact photonic devices with superior performance.