Overcoming Losses\nthrough Phase Locking in Nonlinear\nQuasi-Bound States in the Continuum Metasurfaces

The development of nonlinear flat-optic nanostructures,\nalso known\nas optical metasurfaces, offers unprecedented enhancement of light-matter\ninteractions without the need for phase matching, potentially revolutionizing\nfuture applications in integrated nonlinear optics. Quasi-bound states\nin the continuum (qBIC) have demonstrated the ability to achieve high-quality\nresonances with substantial field enhancement. However, most prior\nstudies were limited to the first-order magnetic dipole qBIC resonances,\nwhere the enhanced electric field is concentrated in the regions outside\nrather than inside the high refractive index meta-atoms. In contrast,\nconcentrating the electric field primarily within the high-index nonlinear\nresonators could significantly boost nonlinear light-matter interactions\nand enhance the conversion efficiency. Here, we demonstrate a qBIC-resonance-based\nmetasurface designed to localize the electric field inside the high-index\nnonlinear meta-atoms and enhance conversion efficiency. The demonstrated\nhigh conversion efficiency is achieved even within the material’s\nhighly absorbing spectral range due to phase locking.