Integrated Nonlinear Optics in Silicon Nitride Waveguides

Licentiate thesis, 2015

Integrated optics platforms offer the possibility to implement compact photonic devices for nonlinear optics applications. Modern nanofabrication facilities allow the fabrication of sub-micron-sized waveguide geometries that confine light to reach very high optical intensities. These intensities enable efficient nonlinear processes that are further enhanced by using materials with high nonlinear Kerr coefficients. Additionally, dispersion engineering by changing the waveguide dimensions allows for broadband operation. In this thesis, we explored silicon nitride as the core material of silica embedded waveguides. Silicon nitride does not show nonlinear loss constraints, which makes this material very suitable for high optical intensities. The material has a large transparency window, from the ultraviolet to the short-wave infrared, and it is completely compatible with CMOS fabrication standards. The potential of this platform for diverse linear and nonlinear optics applications has been demonstrated before. We studied two slightly distinct material platforms: stoichiometric silicon nitride, Si3N4, and non-stoichiometric silicon nitride, SixNy. The accessible Si3N4 material platform consisted of thin low-confinement waveguides with low propagation loss of 0.06 dB/cm and a moderate nonlinear coefficient of 285 (W*km)^-1. In a 1 m long waveguide the nonlinear performance was studied experimentally. The realized four-wave mixing (FWM) experiment showed a conversion efficiency of -26.1 dB and ultrafast all optical signal processing was demonstrated by wavelength conversion of high-speed data. The SixNy material was processed to realize thick high-confinement waveguides that show propagation loss of around 1 dB/cm and a nonlinear coefficient of 6100 (W*km)^-1. The material specific nonlinear Kerr coefficient was 1.4*10^18 m2/W, which is five times higher than Si3N4. With this material platform the fabrication of thick layers up to 700 nm in a single deposition step was demonstrated, a procedure not possible in Si3N4. The thick layers enable broadband dispersion engineering.