Phase-Matched Second-Harmonic Generation from Metasurfaces Inside Multipass Cells

We demonstrate a simple and scalable approach to increase conversion efficiencies of nonlinear metasurfaces by incorporating them into multipass cells and by letting the pump beam interact with the metasurfaces multiple times. We experimentally show that by metasurface design, the associated phase-matching criteria can be fulfilled. As a proof of principle, we achieve phase matching of second-harmonic generation (SHG) using a metasurface consisting of aluminum nanoparticles deposited on a glass substrate. The phase-matching condition is verified to be achieved by measuring the superlinear dependence of the detected SHG as a function of the number of passes. We measure an order of magnitude enhancement in the SHG signal when the incident pump traverses the metasurface up to nine passes. Results are found to agree well with a simple model developed to estimate the generated SHG signals. We also discuss strategies to further scale up the nonlinear signal generation. Our approach provides a clear pathway to enhance nonlinear optical responses of metasurface-based devices. The generic nature of our approach holds promise for diverse applications in nonlinear optics and photonics.