Intersubband polaritonic metasurfaces for flat nonlinear optics with giant second- and third-order nonlinear response

Quantum-engineered intersubband transitions in n-doped multiple-quantum-well semiconductor heterostructures allow one to produce semiconductor films with a nonlinear optical response many orders of magnitude that of bulk nonlinear materials. This nonlinear response can be further enhanced by processing semiconductor heterostructures as metasurfaces in which intersubband transitions are coupled to optical modes of the metasurface nanoresonators. As a result, one can fabricate optically-thin metasurface films that display second- and third-order nonlinear susceptibility values 4-7 orders of magnitude higher than that of traditional nonlinear optical crystals. Using these films, we experimentally achieve efficient (0.1-1%) frequency mixing at moderate pumping intensities (10-100 kW/cm) without phase-matching constraints associated with bulk nonlinear optical crystals. I will discuss the design principles of these metasurfaces and their capabilities, including electrical control of the phase and amplitude of the nonlinear optical response at the individual nanoresonator level, all-optical reflectivity control, and their practical applications for broadly-tunable continuous-wave terahertz generation with tens of microwatts of power output in the difficult to access 4-12 THz spectral range.