Dynamic polarization control of strongly coupled quasi-guided modes enables high-efficiency THG

Enhancing the third-harmonic generation (THG) efficiency of near-infrared light is critical for advancing nonlinear photonic devices. This advancement enables the established advantages of infrared laser technology to be efficiently extended to cutting-edge 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 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 <i>Q</i>-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 the theoretical <i>Q</i>-factor by two orders of magnitude over conventional guided-mode resonances, as confirmed by eigenmode analysis. Leveraging the guided-mode resonance (GMR) of the upper-band TM-like mode, a theoretical <i>Q</i>-factor of 10¹² is achieved, as verified by eigenmode analysis. Analysis of the third-harmonic conversion efficiency via the frequency perturbation method suggests the potential to reach a maximum value on the order of 10^-2, when only local field enhancement is considered.