Gradient nonlinear metasurfaces for continuous phase control

Gradient metasurfaces, or ultrathin optical components with engineered transverse impedance gradients along the surface have recently been demonstrated to provide control of the phase of scattered fields over sub-wavelength scale, enabling a broad range of linear optical components. More recently, sub-wavelength-thin nonlinear metasurfaces with tailored nonlinear responses have provided new degrees of freedom in metamaterial design, with potential applications such as super-resolution imaging, frequency conversion with greatly relaxed phase-matching constraints, and in all-optical switching and memories at nanoscale. However for all practical purposes, the level of nonlinear responses enabled by traditional materials, such as metals and dielectric crystals, is too weak, and nonlinear metasurfaces with orders of magnitude larger nonlinear susceptibilities are necessary to realistically enable a new paradigm in nonlinear optics, based on efficient frequency mixing in sub-wavelength films with relaxed phase-matching conditions and local in-place phase and amplitude control of the nonlinear response. We have recently reported polaritonic nonlinear metasurfaces with record-high nonlinear optical responses for second-harmonic generation based on coupling of electromagnetic modes in plasmonic nanoresonators with quantum-engineered intersubband nonlinearities in multi-quantum-well (MQW) semiconductor heterostructures. In this work, we experimentally demonstrate continuous 360-degrees phase control of the nonlinear response for second-harmonic nonlinear optical process at the individual nanoresonator level with practical level of nonlinear optical responses using a generalization of the Pancharatnam-Berry (PB) phase approach to our MQW-based structures. Extending the ‘flat optics’ paradigm established with linear gradient metasurface, our result unveil a ‘flat nonlinear optics’ paradigm based on efficient frequency mixing and a complete control of the output wavefront with subwavelngth resolution.