Broadband giant nonlinear response using electrically tunable polaritonic metasurfaces.

Intersubband transitions in semiconductor heterostructures offer a way to achieve large and designable nonlinearities with dynamic modulation of intersubband energies through the Stark effect. One promising approach for incorporating these nonlinearities into free space optics is a nonlinear polaritonic metasurface, which derives resonant coupling between intersubband nonlinearities and optical modes in nanocavities. Recent work has shown efficient frequency mixing at low pumping intensities, with the ability to electrically tune the phase, amplitude, and spectral peak of it. However, the spectral tunability of intersubband nonlinearities is constrained by the static spectral response of nanocavities. To overcome this limitation, we present nonlinear polaritonic metasurfaces for a broadband giant nonlinear response. This is achieved by combining a Stark tunable nonlinear response from a quantum-engineered semi-conductor heterostructure with arrays of three nanocavities with different resonant wavelengths. We experimentally demonstrate broadband second harmonic generation (SHG) and a shift in the peak SHG efficiency within the range of 8.9-10.6 μm by applying bias voltage. This work will provide a promising route for achieving broadband and electrically tunable nonlinearities in metasurfaces.