Controlling the Phase of Titanium Dioxide for Nanophotonic Devices

Abstract

In this thesis we investigate using different phases of titanium dioxide (TiO2) for nanophotonic devices. We build on our previous work with amorphous and polycrystalline anatase TiO2 thin films and nanophotonic devices to growth and develop rutile TiO2 for nanophotonics. Rutile TiO2 has a higher index of refraction than anatase or amorphous films, a property that is beneficial for photonic applications requiring high index contrast, such as waveguiding. We optimize the growth of rutile TiO2 on an m-sapphire substrate by systematically varying deposition parameters such as temperature, pressure and oxygen concentration. We observe epitaxial growth of rutile TiO2, verified by x-ray diffraction rocking curve and phi-scans of multiple crystalline planes. We characterize rutile films in terms of material and optical properties. We select deposition conditions that maximize the epitaxial quality of our film to fabricate nanostructures. We design and fabricate photonic devices using different phases of TiO2. We fabricate nanopillars of different diameters and measure their reflection using hyperspectral dark-field imaging. We observe that pillars of different diameters reflect different wavelengths. We also observe that the numerical aperture used to collect reflected light affects the wavelength of light. We conclude that anatase TiO2 is well suited for engineering wavelength-selective reflective metasurfaces. We investigate using amorphous TiO2 for a multi-layer metal/dielectric stack broadband absorber. We simplify previous designs that use multi-layer metal dielectric stacks to obtain near-perfect broadband absorption. We investigate configurations with N = 1, 2 and 3 layers of Cr/TiO2. We optimize the dimensions using a genetic algorithm and obtain absorption values of 84.6%, 97.5% and 98.9%, respectively for the 0.4 to 1.6 um wavelength region. This approach improves the viability and scalability for fabrication. We demonstrate experimentally the absorptive properties of a one-layer device over the visible wavelength region and compare it to a gold substrate. We observe robustness against fabrication tolerances, achieving absorptive behavior for a range of dimensions.