Flat nonlinear optics with ultrathin highly-nonlinear metasurfaces

Extending the `flat optics' paradigm to the nonlinear optics faces important challenges, since, for any practical situation, we are required to simultaneously achieve sub-diffraction phase control and efficient frequency conversion in metasurfaces of sub-wavelength thickness. Here, we experimentally demonstrate giant nonlinear response and continuous phase control of the giant nonlinear response in metasurfaces based on plasmonic nanoresonators coupled to intersubband transitions in semiconductor multi-quantum wells. Over 0.075% of second-harmonic power conversion efficiency is achieved experimentally in a 400-nm-thick metasurface using 10 microns wavelength pump with 20 kW/cm2 intensity.