Subwavelength dielectric resonators for nonlinear nanophotonics

TLDR

Bound states in the continuum (BICs) are localized optical modes with theoretically infinite quality factors, but in practice are limited by size, absorption, disorder, and surface scattering, and have been demonstrated mainly in extended 2D/1D systems for applications such as lasing and sensing. Here, we demonstrate experimentally an isolated subwavelength nanoresonator hosting a quasi‑BIC resonance. We fabricate an AlGaAs nanoresonator on an engineered substrate and excite its quasi‑BIC mode with structured light. Using the resonator as a nonlinear nanoantenna, we achieve record‑high second‑harmonic generation efficiency, establishing a novel platform for resonant subwavelength photonics.

Abstract

Bound states in the continuum (BICs) represent localized modes with energies embedded in the continuous spectrum of radiating waves. BICs were discovered initially as a mathematical curiosity in quantum mechanics, and more recently were employed in photonics. Pure mathematical bound states have infinitely-large quality factors (Q factors) and zero resonant linewidth. In optics, BICs are physically limited by a finite size, material absorption, structural disorder, and surface scattering, and they manifest themselves as the resonant states with large Q factors, also known as supercavity modes or quasi-BICs. Optical BIC resonances have been demonstrated only in extended 2D and 1D systems and have been employed for distinct applications including lasing and sensing. Optical quasi-BIC modes in individual nanoresonators have been discovered recently but they were never observed in experiment. Here, we demonstrate experimentally an isolated subwavelength nanoresonator hosting a quasi-BIC resonance. We fabricate the resonator from AlGaAs material on an engineered substrate, and couple to the quasi-BIC mode using structured light. We employ the resonator as a nonlinear nanoantenna and demonstrate record-high efficiency of second-harmonic generation. Our study brings a novel platform to resonant subwavelength photonics.