Continuous Wave Second Harmonic Generation from an Etchless Lithium Niobate Resonant Metasurface

Abstract: Nonlinear metasurfaces provide a route to compact frequency conversion by replacing phase matching and long interaction lengths with resonantly enhanced light matter interaction in subwavelength structures. Extending this capability to continuous wave (CW) operation is particularly important for applications requiring narrow linewidth, stable frequency, and stationary optical fields, but remains extremely challenging. Here, we demonstrate CW second-harmonic generation on a transmission mode, etchless thin film lithium niobate platform enabled by a patterned silicon rich nitride metasurface. This hybrid design combines guided mode resonance coupling, low optical loss, and CMOS compatible processing while keeping most of the optical mode confined in the unpatterned lithium niobate, yielding a measured quality factor of ~2300. Clearly resolved SHG is achieved under sub-kW/cm^2 CW pumping, with a normalized conversion efficiency of 0.156 % cm^2/GW in the low power regime. Interestingly, our work reveals that CW resonant SHG in metasurfaces can exhibit pronounced transient dynamics, including power dependent resonance evolution, overshoot, and nonideal scaling. These findings establish etchless LN SRN metasurfaces as a promising platform for compact CW nonlinear photonics, and show that resonance dynamics are central to the operation and evaluation of CW driven nonlinear metasurfaces.