Maximizing intensity in TiO2 waveguides for nonlinear optics

Titanium dioxide (TiO2) represents an attractive candidate for nonlinear optical devices due to its large refractive index and large Kerr nonlinearity. These properties can strongly enhance confinement and nonlinear interactions. TiO2 also possesses high transparency, exhibiting no linear absorption within the entire visible spectrum and no two-photon absorption at wavelengths above 800 nm. Considering these qualities, TiO2 is capable of outperforming most other widely transparent materials, such as fused silica, silicon nitride, and diamond. Using electron beam lithography and a liftoff procedure followed by reactive ion etching, we structure both amorphous TiO2 as well as polycrystalline anatase thin films to create photonic devices that exploit this material’s properties in order to study nonlinear optics (Bradley et al. Opt Express 20:23821–23831, 2012; Evans et al. Opt Express, submitted) Nonlinear optics benefit from prolonged interactions, necessitating large intensities along extended waveguide lengths. For this reason, waveguide losses need to be minimized. We study the effects of mask materials and annealing procedures on waveguide propagation losses. We also study a variety of taper structures and optimize the insertion losses of these waveguides. For ultrafast pulses, dispersion becomes an important parameter since strong dispersion can elongate a pulse and lower the large peak intensities needed for nonlinear optics. Within nano-scale structures, this parameter can be tailored without difficulty by changing the waveguide geometry. We present a finite-element analysis that demonstrates the geometries necessary to obtain negligible or anomalous dispersion and thus maintain large pulse intensities. These techniques can readily be applied to other novel photonic material platforms.