a, Linear absorption spectrum of a metasurface in which plasmonic resonances were not well-overlapped spectrally with intersubband transitions of the MQW structure for fundamental and SH frequencies. b, SH peak power (left axis) or intensity (right axis) as a function of FF peak power squared (bottom axis) or peak intensity squared (top axis) for different input/output polarization combinations (yyy and so on, see key) at FF pump wavenumber 1/λpump = 1,310 cm−1 and SH wavenumber 2,620 cm−1 both in resonance with the plasmonic absorption peaks. SH response was close to the noise limit of our setup. Inset, SEM image of the metasurface. c, SH peak power output as a function of FF peak power squared (bottom axis) or peak intensity squared (top axis) for different input/output polarization combinations at FF wavenumber 1,240 cm−1 and SHG wavenumber of 2,480 cm−1, both away from plasmonic resonances of the metasurface but in resonance with intersubband transitions in the MQW structure.