Backward Phase-Matched Second-Harmonic Generation from Stacked Metasurfaces

We demonstrate phase-matched second-harmonic generation (SHG) from three-dimensional metamaterials consisting of stacked metasurfaces. To achieve phase matching, we utilize a novel mechanism based on engineering the metasurfaces at interacting wavelengths, facilitating SHG in unconventional backward direction. By stacking up to five metasurfaces, obtain expected factor 25 enhancement efficiency. Our results motivate further investigations higher conversion efficiencies also with more complex wavefronts.