Fundamental Principles of Operation and Notes on Fabrication of Photonic Microresonators

Abstract

Light confining microresonators based on evanescent wave propagation and whispering gallery (WG) modes have received much attention in the past decades, due to their conceptual similarity with their standing wave counterparts, improvements in fabrication technology, and their versatility in realizing various functions in telecommunications, sensing, measurement, and instrumentation. In this chapter the general concepts, design principles, and practical realizations of optical microresonators are briefly introduced. Using a simple but generic model, important design parameters such as the Q-factor, the finesse, the free spectral range, the intensity buildup, and the effects of loss are derived in general terms from basic principles. The discussion on cavity design is completed by reviewing several intrinsic properties of available material systems, such as the refractive index contrast, which is essential for field confinement, which limits the resonator geometrical size, contributes to material loss, and influences the nonlinear response. Finally, fabrication techniques of microring and WG resonators are also outlined, from the surface tension-mediated processes in silica microspheres and microtoroids to the wafer-based technologies such as deep ultraviolet (DUV), electron beam, and nanoimprint lithography. Some notable examples of fabricated resonators are discussed and compared.