Efficient Third-Harmonic Generation via Strong Coupling of Quasi-Guided Modes

TL;DR

This paper demonstrates a bilayer waveguide with a nanocube array that achieves strong mode coupling, leading to a significant increase in third-harmonic generation efficiency through enhanced Q-factors and dynamic control.

Contribution

It introduces a novel hybrid waveguide structure with strong mode coupling that dramatically boosts THG efficiency and allows for tunable resonant properties.

Findings

Achieved a maximum Q-factor of 10^12 in simulations.

Demonstrated THG efficiency of approximately 1%.

Observed clear avoided crossing indicating strong coupling.

Abstract

Improving the conversion efficiency is critical for advancing nonlinear photonic devices, enabling applications in ultrafast optics, quantum light sources, and high-resolution imaging. Herein, we present a bilayer waveguide integrated with a periodic square nanocube array to enhance third-harmonic generation (THG) efficiency. This structure achieves strong coupling between TE- and TM-polarized quasi-guided mode (QGM) resonances, thereby enabling simultaneous dynamic control of both resonant Q-factor and coupling strength through incident polarization angle. The distinct avoided crossing observed in both reflection spectra and eigenfrequency diagrams indicates the strong coupling regime. This mechanism, facilitated by the inter-mode energy exchange within the hybridized system, results in an enhancement of Q-factor by two orders of magnitude when compared to conventional guided-mode resonances. Simulation results verify an unprecedented maximum Q-factor of 10^12 for the upper-branch mode. Leveraging strong-coupling-induced field enhancement, the system attains THG conversion efficiency of order 10^-2, demonstrating unprecedented nonlinear performance.