Nonlinear frequency conversion at the nanoscale is important for many applications in free space and integrated photonics. In epsilon-near-zero (ENZ) materials, second-harmonic generation (SHG) is significantly enhanced but the oblique incidence is required to address nonlinearity. To circumvent this constraint, we design a hybrid metasurface consisting of plasmonic nanostructures on an ENZ nanofilm generating strongly enhanced SHG at normal incidence in transmission. We show that the Au meta-atoms on an indium-tin-oxide (ITO) layer provide an approximately 104-fold experimentally measured SHG enhancement at normal incidence at the fundamental wavelength near the ENZ condition of ITO. This giant enhancement stems from reshaping the vectorial properties of the incident light near the Au nanostructures and its increased coupling to the ENZ film. The proposed hybrid ENZ metasurface offers a promising platform for developing ultracompact and efficient nonlinear optical sources at the nanoscale.
Keywords: Epsilon-near-zero materials; Nonlinear metasurfaces; Plasmonics; Second-harmonic generation.