High-Q Microresonators as Lasing Elements for Silicon Photonics

Although the concept of constructing active optical waveguides in crystalline silicon has existed for over twenty years, it is only in the past few years that silicon photonics has been given serious attention as a displacing technology. Fueled by the predicted saturation of “Moore’s Law” within the next decade, universities and industries from all over the world are exploring the possibilities of creating truly integrated silicon opto-electronic devices in a cost effective manner. Some of the most promising silicon photonics technologies are chip-to-chip and intra-chip optical interconnects. Now that compact high-speed modulators in silicon have been achieved, the limiting factor in the widespread adoption of optical interconnects is the lack of practical on-chip optical sources. These sources are critical for the generation of the many wavelengths of light necessary for high-speed communication between the logical elements between and within microprocessors. Unfortunately, crystalline silicon is widely known as a poor emitter because of its indirect bandgap. This thesis focuses on the many challenges in generating silicon-based laser sources. As most CMOS compatible gain materials possess at most 1 dB/cm of gain, much of our work has been devoted to minimizing the optical losses in silicon optical microresonators. Silicon microdisk resonators fabricated from silicon-on-insulator wafers were employed to study and minimize the different sources of scattering and absorption present in high-index contrast Si microcavities. These microdisks supported whispering-gallery modes with quality factors as high as 5 × 10, close to the bulk limit of lightly doped silicon wafers. An external silica fiber taper probe was developed to test the microcavities in a rapid wafer-scale manner. Analytic theory and numerical simulation aided in the optimization of the cavity design and interpretation of experimental results. After successfully developing surface chemistry treatments xi and passivation layers, erbium-doped glasses were deposited over undercut microdisks and planar microrings. Single-mode laser oscillation was observed and carefully characterized for heavily oxidized silicon microdisks. Dropped power thresholds of 690 nW, corresponding to 170 nW of absorbed power, were measured from gain-spectra and Light in–Light out curves. In addition, quantum efficiencies for these lasers were as high as 24%, indicating that this technology may be ready for further development into real-world devices.