Mid-infrared nonlinear optics in etchless silicon resonators

The mid-infrared spectral region is one of the most important wavelength regimes for applications in gas sensing and spectroscopy. Due to the fact that a plethora of gas molecules have strong absorption peaks in the mid-IR, it is a particularly fruitful regime for high-sensitivity gas detection. This also makes it a difficult wavelength range to work in -- any contamination in a material is bound to absorb in the mid-IR. Nevertheless, new compact and robust wavelength sources are needed to enable the next generation of real time gas sensors. Optical frequency combs -- broadband coherent light sources consisting of equally spaced and sharp spectral lines -- are one of the most promising technologies for mid-IR spectroscopy. Although mid-IR frequency combs have been realized in a number of platforms, the realization of an on-chip integrated mid-IR comb source has proven illusive.;This dissertation shows the technological developments needed in order to realize the first on-chip integrated frequency comb source in the mid-infrared. We use silicon as our material of choice. Silicon is one of the most ideal materials for mid-IR nonlinear optics due to its high third order optical nonlinearity, high refractive index, and wide transparency window. However, silicon also has significant problems that have prevented realization of on-chip comb generation. Silicon's high linear losses in etched waveguides as well as silicon's high nonlinear losses had prevented the realization of a silicon micro-resonator comb at any wavelength. We show that we can overcome silicon's high linear losses by using a novel fabrication process known as "etchless" to fabricate silicon waveguides without reactive ion etching (RIE). We also show that we can integrate these waveguides with electronics to overcome the effects of silicon's high nonlinear losses. And, by engineering the profile of the etchless waveguides to achieve the dispersion needed for four wave mixing gain, we show mid-IR parametric comb generation in a silicon micro-ring resonator. We also characterize and examine the coherence of the generated frequency comb by measuring the RF noise of the comb using both off-chip and on-chip integrated electronics. In summary, we show the first on-chip integrated mid-IR optical frequency comb, and lay the groundwork for future mid-IR nonlinear optics in silicon resonators.