a, Optical set-up for metasurface characterization at 45° incidence angle. LP and SP are long- and short-pulse filters, respectively. HWP is a half-wave plate for FF polarization control. Directions of S- and P-polarizations and orientation of the metasurface are indicated. Inset, SEM image of the metasurface with x and y axes shown. b, Measured SH peak power output as a function of FF peak power squared (bottom axis) or peak intensity squared (top axis) at FF wavenumber 1,240 cm−1 for different input/output polarization combinations (SSS and so on, see key). c, Simulated absorption spectrum of the metasurface at 45° incidence for different input light polarizations.