Study on on-chip high-nonlinear optical microresonator with high-Q factor

Abstract

As revolutionary researches have been underway to recreate state-of-the-art optical technologies (e.g., frequency comb spectroscopy, optical clock) into a miniaturized system integrated on a single chip, interests in on-chip optical microresonators with high quality (Q) factor have been massively increased. Many attempts have been recently made to introduce new materials that strongly interact with light into the resonators. However, unlike conventional materials with reliable microfabrication protocols, it is exceedingly difficult to establish an optimized process for each newly introduced material, and often the processing itself has been considered challenging. Therefore, in the resonators photon lifetime is significantly reduced so that the feasible Q-factor has been far lower than the material limit. In this thesis, to overcome these limitations, we investigate the working principle of light-guiding structures from the viewpoint of transformation optics and propose a universal method to realize on-chip high-Q resonators for any material capable of film deposition. To substantiate the validity of the proposed method experimentally, we prepare and analyze the resonators using new materials including As$_2$S$_3$, one of the chalcogenide glasses featured in notably high nonlinearity. A flip-chip evanescent coupling configuration is also applied for accessing the resonators efficiently. The Q-factor of the As$_2$S$_3$ resonator was measured to 1.44x10$^7$, which is ~30-fold improved compared to the previous and further close to the performance of the chalcogenide fibers. Also, using the high-nonlinear resonator, stimulated Brillouin scattering laser is successfully demonstrated. Benefitting from the boosted Q-factor, it is confirmed that the laser has ~100 times reduced threshold pump power than the former record.