Impact of epsilon-near-zero multilayer composition on simultaneous difference frequency and second harmonic generation

We experimentally analyze the simultaneous processes of difference frequency generation (DFG) and second harmonic (SH) generation in multilayer structures exhibiting an effective epsilon-near-zero (ENZ) response and ENZ properties in one of their layers. The structures consist of subwavelength-thin tri-layer periods of 75% ITO, 12.5% Al2O3, and 12.5% BaTiO3, with the total thickness kept near 120 nm. The number of periods and ITO layer thickness (3-30 nm) vary between samples, allowing to tune the effective ENZ wavelength over 1000 nm. We demonstrate that the level of DFG and SH enhancement can be increased by over two orders of magnitude with multilayer composition, with the highest enhancement in samples having 12-15 nm thick ITO layers. The peak enhancement wavelength follows the effective ENZ wavelength, while the relative enhancement levels of DFG and SH depend on sample composition. Our findings are supported by COMSOL simulations, TEM analysis, and ellipsometry data.