Enhancement of Optical Nonlinearity in Epsilon Near Zero Thin Bilayers

Epsilon Near Zero (ENZ) materials are highly desirable candidates for nonlinear interactions due to their strong enhancement and localization of electric fields within the materials. This ENZ mode is characterized by a large resonance at the wavelength in which the real permittivity of the material approaches zero. Most metals display ENZ behavior in the ultraviolet regime, making it difficult to probe this resonance for nonlinear interactions. Additionally, this ENZ mode is classified as a bound mode due to its dispersion existing to the right of the light line. Thus, optically accessing this mode requires excitation from oblique angles and oftentimes specific geometries such as the Kretschmann configuration. 

           Here, we present Indium Tin Oxide (ITO) as a suitable candidate for probing the ENZ resonance. ITO’s material parameters are highly tunable during fabrication, allowing us to carefully select the ENZ wavelength in the near infrared regime and explore the nonlinear enhancement through processes such as Second Harmonic Generation (SHG). We conduct power law, angle-dependent, and wavelength-dependent SHG and THG measurements on ITO thin films, showing a strong nonlinear response in Indium Tin Oxide's SHG response near the ENZ resonance. Then, we repeat our measurements on an ITO-Au bilayer thin film and demonstrate SHG enhancement of 2 orders of magnitude compared to the single ITO thin film case at near zero incident angles. We present this ITO-Au bilayer thin film to help enable more efficient development and design of photonic materials and nonlinear devices in optoelectronics.