Ultra-high-Q toroid microcavity on a chip

Abstract

The circulation of light within dielectric volumes enables storage of optical power near specific resonant frequencies and is important in a wide range of fields including cavity quantum electrodynamics^1,2, photonics^3,4, biosensing^5,6 and nonlinear optics^7,8,9. Optical trajectories occur near the interface of the volume with its surroundings, making their performance strongly dependent upon interface quality. With a nearly atomic-scale surface finish, surface-tension-induced microcavities such as liquid droplets or spheres^10,11,12,13 are superior to all other dielectric microresonant structures when comparing photon lifetime or, equivalently, cavity Q factor. Despite these advantageous properties, the physical characteristics of such systems are not easily controlled during fabrication. It is known that wafer-based processing¹⁴ of resonators can achieve parallel processing and control, as well as integration with other functions. However, such resonators-on-a-chip suffer from Q factors that are many orders of magnitude lower than for surface-tension-induced microcavities, making them unsuitable for ultra-high-Q experiments. Here we demonstrate a process for producing silica toroid-shaped microresonators-on-a-chip with Q factors in excess of 100 million using a combination of lithography, dry etching and a selective reflow process. Such a high Q value was previously attainable only by droplets or microspheres and represents an improvement of nearly four orders of magnitude over previous chip-based resonators.