We apply the Pancharatnam-Berry phase approach to plasmonic metasurfaces loaded by highly nonlinear multi-quantum well substrates, establishing a platform to control the nonlinear wavefront at will based on giant localized nonlinear effects. We apply this approach to design flat nonlinear metasurfaces for efficient second-harmonic radiation, including beam steering, focusing, and polarization manipulation. Our findings open a new direction for nonlinear optics, in which phase matching issues are relaxed, and an unprecedented level of local wavefront control is achieved over thin devices with giant nonlinear responses.Artificially engineered metasurfaces have recently attracted a great deal of interest due to their ability to provide a large degree of control over the local amplitude, phase, and polarization of local fields, leading to many exciting advances in science and technology [1,2]. Conventional optical devices are based on the naturally weak interactions of light with matter, implying that volumetric effects dominate their optical response. Metasurfaces provide an elegant way to overcome these constraints, by manipulating the local field with suitably engineered inclusions that can enhance the local interaction with light, and pattern it in the desired way over

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A prominent nonlinear optical phenomenon that is extensively studied using nanostructured materials is second-harmonic generation (SHG) as it has applications in various fields. Achieving efficient SHG from a nanostructure requires a large second-order nonlinear susceptibility of the material system and large electromagnetic fields. For practical applications, the nanostructures should also have low losses, high damage thresholds, large bandwidths, wavelength scalability, dual mode operation in transmission and reflection, monolithic integrability, and ease of fabrication. While various approaches have demonstrated efficient SHG, to the best of our knowledge, none have demonstrated all these desired qualities simultaneously. Here, we present a hybrid approach for realizing efficient SHG in an ultrathin dielectricsemiconductor nonlinear device with all the above-mentioned desired properties. Our approach uses high quality factor leaky mode resonances in dielectric metasurfaces that are coupled to intersubband transitions of semiconductor quantum wells. Using our device, we demonstrate SHG at pump wavelengths ranging from 8.5 to 11 μm, with a maximum second-harmonic nonlinear conversion factor of 1.1 mW/W 2 and maximum second-harmonic conversion efficiency of 2.5 × 10 −5 at modest pump intensities of 10 kW/cm 2 . Our results open a new direction for designing low loss, broadband, and efficient ultrathin nonlinear optical devices.

The application of the Pancharatnam-Berry (PB)