Nonlinear optics in titanium dioxide: from bulk to integrated optical devices

In this thesis, we explore titanium dioxide (TiO2) for ultrafast, on-chip nonlinear optics by studying it in bulk, thin films, and in integrated nonlinear optical devices. TiO2's large nonlinear index of refraction (30 times that of silica) and low two-photon absorption can enable all-optical switching, logic, and wavelength conversion across wavelengths spanning the telecommunications octave (800--1600 nm). In addition, its high linear index of refraction can enhance optical confinement down to nano-scale dimensions and facilitate the tight waveguide bends necessary for dense on-chip integration. Throughout this thesis, we develop TiO2 as a novel on-chip nonlinear optics platform.;We begin by studying nonlinear refraction and multiphoton absorption in bulk rutile TiO2 using the Z-scan technique. We quantify nonlinear refraction and mixed two- and three-photon absorption near TiO2's half-bandgap (800 nm). This data shows that we can avoid parasitic two-photon absorption by operating at wavelengths longer than 813 nm.;Planar integrated photonic devices require high-quality thin films that can act as low-loss planar waveguides. We deposit thin films of TiO2 on oxidized silicon wafers using reactive sputtering of titanium metal in an oxygen environment. Our optimized amorphous and polycrystalline anatase TiO2 thin films have high refractive indices from 2.4--2.8 at visible wavelengths and planar waveguiding losses as low as 0.4 dB/cm (826 nm).;Using electron-beam lithography, we structure these films to fabricate waveguides with sub-micron features. We quantify propagation losses at wavelengths from 633 to 1550 nm and observe losses as low as 4 dB/cm. Next, we demonstrate devices including bends, directional couplers, and racetrack resonators that form the basic building-blocks for more advanced photonic devices.;Finally, we observe the first nonlinear optics in integrated TiO 2 waveguides. By measuring the spectral broadening of femtosecond pulses in TiO2, we quantify the nonlinear index of anatase TiO2 around 1565 and 794 nm. In addition, we explore stimulated Raman scattering and third-harmonic generation in our waveguides. With this first demonstration of an integrated nonlinear optical device in TiO2, we conclude that TiO2 is a viable and promising material for all-optical applications.