1 CHAPTER 5 FABRICATION, COUPLING AND NONLINEAR OPTICS OF ULTRA- HIGH-Q MICRO-SPHERE AND CHIP-BASED TOROID MICROCAVITIES Tobias J. Kippenberg, Sean M. Spillane, Deniz K. Armani, Bumki Min, Lan Yang and Kerry J. Vahala Department of Applied Physics, California Institute of Technology Pasadena, CA 91125 Surface-tension-induced micro-cavities (STIMs) possess ultra-high-Q values (typically greater than 100 million) within micron-scale dimensions. Their “whispering-gallery” type optical modes are confined by continuous total internal reflection at the dielectric cavity interface. Silica micro-spheres are a well-known example of STIMs, which possess record Q values in excess of 10 9 . In this chapter we will discuss a novel type of ultra-high-Q (UHQ) microcavity fabricated on a microelectronic silicon chip, which allows a level of integration and control previously not available in the UHQ regime. The device merges concepts used in traditional surface-tension-induced microcavity fabrication with standard microfabrication techniques to combine UHQ with wafer-scale integration. We have studied the quality-factor (Q) and modal structure of these toroidally-shaped microcavities and have observed Q-factors exceeding 100 million, similar to values achieved in micro-spheres. Investigations of optical coupling to UHQ microcavities using tapered, optical fibers are also described and show that highly efficient coupling can be obtained, with efficiencies in excess of 99%. Ultra-high-Q combined with micron-scale modal volumes and the ability to efficiently transfer optical power both to and from the whispering gallery modes, leads to extremely high circulating intensities. These intensities can easily exceed the thresholds for all common nonlinear phenomena in silica. This has allowed us to observe a variety of nonlinear oscillations at threshold levels several orders lower than in any prior work and typically in the micro- Watt regime. In particular, stimulated and cascaded Raman scattering and parametric oscillation are studied both experimentally and theoretically. The ability to dope silica with rare earths to create on-chip silica micro-lasers in the important 1500 nm band is also investigated. Finally, we review the regime of strong modal coupling induced by backscatter effects within the resonator, and examine how backscattering modifies waveguide coupling properties including a change of the condition of “critical coupling”.