Active Nonlinear Metasurfaces for Broadband Second-Harmonic Generation

Abstract

Nonlinear response from optical metasurfaces have opened a new direction in research, with interesting applications such as super-resolution imaging and efficient frequency conversion in subwavelength thin-film with greatly-relaxed phase-matching constraints [1,2]. Recently, nonlinear metasurfaces with record second-order nonlinearities up to 4-5 orders of magnitude larger than in traditional bulk nonlinear materials based on coupling of electromagnetic modes with intersubband nonlinearities in multiple-quantum-well structure have been demonstrated [2,3]. However, efficient SHG from the metasurface is only possible near the intersubband resonant frequency due to the resonant characteristics of the intersubband nonlinearities, and thus the operating wavelength for the SHG (SHG bandwidth) is very limited. In this work, we theoretically study active nonlinear metasurfaces for broadband SHG based on Stark-tunable intersubband nonlinearities in which the intersubband transition energies in the MQW are modulated from bias voltages applied on the MQW layer. We designed plasmonic resonators to achieve the maximum SHG conversion efficiencies at each bias voltages. The maximum SHG conversion efficiencies and the spectral response of SH power with input pump wavelength range from λFF=7 to 13μm were calculated for the bias voltages from -4V to 4V with 1V step. From the theoretical calculations, we obtained conversion efficiencies over 0.1% for the bias voltages and 5 times broader SHG operation wavelength compared to the passive device (0V). The active nonlinear metasurfaces proposed in this work may have significant practical impact on variety of applications, including broadband frequency up- and down-conversion, spectroscopy and imaging systems requiring broadband operation.