Enhanced third-harmonic generation empowered by doubly degenerate quasi-bound states in the continuum

TL;DR

This paper demonstrates enhanced third-harmonic generation in silicon metasurfaces using doubly degenerate quasi-bound states in the continuum, achieving high efficiency and polarization independence for advanced nanophotonic applications.

Contribution

It introduces a novel approach leveraging doubly degenerate quasi-BICs governed by $C_{4v}$ symmetry to enhance THG in silicon metasurfaces, with tunable Q-factors and polarization independence.

Findings

Maximum THG efficiency of 1.03×10⁻⁵ at 5.85 GW/cm² pump intensity

Polarization-independent THG confirmed across different polarization directions

Effective control of Q-factors and field confinement through geometric tailoring

Abstract

Recent advancements in nonlinear nanophotonics are driven by the exploration of sharp resonances within high-index dielectric metasurfaces. In this work, we leverage doubly degenerate quasi-bound states in the continuum (quasi-BICs) to demonstrate robust enhancement of third-harmonic generation (THG) in silicon metasurfaces. These quasi-BICs are governed by $C_{4v}$ symmetry and therefore can be equally excited with the pump light regardless of polarization. By tailoring the geometric parameters, we effectively control $Q$-factors and field confinement of quasi-BICs, and thus regulate their resonantly enhanced THG process. A maximum THG conversion efficiency up to $1.03\times10^{-5}$ is recorded under a pump intensity of 5.85 GW/cm$^{2}$. Polarization-independent THG profile is further confirmed by mapping its signal across the polarization directions. This work establishes foundational strategies for the ultracompact design of robust and high-efficiency photon upconversion systems.