Flat nonlinear optics: Efficient frequency conversion in ultrathin nonlinear metasurfaces

We report highly-nonlinear ultrathin metasurfaces based on coupling of plasmonic nanoresonators with intersubband nonlinearities. Nonlinear susceptibilities over 1×10
⁶
pm/V and conversion efficiencies ~0.1% were measured for second harmonic generation in mid-infrared for 10 kW/cm
²
input intensity. Complete phase control of local nonlinear response is demonstrated.