Developing Highly Nonlinear Materials and Nanostructures on the Largely Tunable ITO/TiN Platform

The engineering of the optical dispersion of thin-film transparent conductive indium tin oxide (ITO) and titanium nitride (TiN) has enabled novel nanostructured materials with largely tunable epsilon-near-zero (ENZ) behavior from the visible to the mid-infrared spectral range as well as optical nonlinearities dramatically enhanced by small refractive index values. Combined with their CMOS compatibility, these materials have been integrated into compact photonic devices and nanostructures that perform efficient nonlinear operations such as sub-picosecond all optical modulation, ultra-fast pulse shaping through frequency translation, and optical time reversal. However, the fundamental mechanisms governing their nonlinear responses remain unclear. In this talk, we use the Z-scan nonlinear optical characterization technique to systematically investigate the complex nonlinear susceptibility c⁽³⁾ and the intensity-dependent refractive index change D<i>n</i> of ITO samples grown by RF reactive magnetron sputtering followed by post-deposition annealing. Specifically, we study polycrystalline thin films with different microstructural and crystallographic orientations and we demonstrate that post-deposition conditions can modify the crystallographic texture of ITO and further enhance the nonlinear optical properties resulting in a 30X enhancement compared to commercial ITO thin films with similar thickness and optical dispersion. Finally, we discuss opportunities to further control and enhance the optical nonlinearity of ITO using ITO/TiN multilayer stacks with controlled volume fractions, providing large tunability of both ENZ wavelengths and nonlinear responses in the 680nm-1200nm spectral range.