Extremely efficient coupling of silica toroid microresonator to silicon chip with photonic crystal waveguide

Compact and low-loss coupling to whispering gallery mode (WGM) microresonators is important for a wide range of applications from fundamental physics to sensing and lasing, including a microresonator frequency comb. High-Q microresonators have been fabricated with silica and crystalline WGMs but highly efficient, robust, and compact coupling remained a challenge [1]. Although coupling to a silicon (Si) chip will allow a variety of applications, the major challenge is the large refractive index mismatch between high-Q microresonators (e.g., silica; nsilica = 1.44) and the commonly used photonic platforms (e.g., Si; nSi = 3.48) [2]. The large refractive index difference leads to a large phase mismatch. Here we found and demonstrated experimentally that extremely efficient coupling between a WGM and a Si chip is possible by using a 2D photonic crystal (PhC) waveguide (WG).