Overcoming Losses through Phase Locking in Nonlinear Quasi-Bound States in the Continuum Metasurfaces

The development of nonlinear flat-optic nanostructures, also known as optical metasurfaces, offers unprecedented enhancement of light-matter interactions without the need for phase matching, potentially revolutionizing future applications in integrated nonlinear optics. Quasi-bound states in the continuum (qBIC) have demonstrated the ability to achieve high-quality resonances with substantial field enhancement. However, most prior studies were limited to the first-order magnetic dipole qBIC resonances, where the enhanced electric field is concentrated in the regions outside rather than inside the high refractive index meta-atoms. In contrast, concentrating the electric field primarily within the high-index nonlinear resonators could significantly boost nonlinear light-matter interactions and enhance the conversion efficiency. Here, we demonstrate a qBIC-resonance-based metasurface designed to localize the electric field inside the high-index nonlinear meta-atoms and enhance conversion efficiency. The demonstrated high conversion efficiency is achieved even within the material's highly absorbing spectral range due to phase locking.