Nonlinear Properties of ITO-Based ENZ Materials and Metasurfaces Throughout the Short-Wave Infrared

Indium tin oxide (ITO) exhibits strong field enhancement and nonlinear optical response around its epsilon-near-zero (ENZ) wavelength, in which the real part of the permittivity switches from a positive to negative value. The ENZ wavelength of a particular film can be configured by adjusting free carrier concentrations through deposition parameters. We present here the detailed process parameters of a high temperature reactive sputter process which can produce ITO films with an ENZ wavelength from 1.1µm to well beyond 3µm. We then perform z-scan and second harmonic generation (SHG) measurements to determine the nonlinear response, and correlate this to the optical, electrical and structural properties of the films. Finally, we design and fabricate a vertical metamaterial which is a multilayer ITO-SiO₂device similar to a previously demonstrated NIR perfect absorber. We show that this structure greatly enhances SHG over a single ITO thin film.