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Ultrahigh-Q silicon racetrack resonators

As a basic optical element, micro-resonator has been playing an important role in the field of silicon photonics. Ultra-high-Q and compact micro-resonators are recognized as the key for many functional devices for optical filtering, lasering, optical modulation/switching and all-optical photonics. Unfortunately, it is still challenging to realize compact silicon micro-resonators with Q>106 because silicon photonic waveguides fabricated with standard CMOS processes usually have a propagation loss of >1 dB/cm.

In order to solve the problem, Prof. Daoxin Dai's group from the College of Optical Science and Engineering at Zhejiang University, China, has proposed and demonstrated ultrahigh-Q silicon racetrack resonators with novel designs, which was published in Photonics Research, Vol. 8, Issue 5, 2020 (Long Zhang, Lanlan Jie, Ming Zhang, Yi Wang, Yiwei Xie, Yaocheng Shi, Daoxin Dai. Ultrahigh-Q silicon racetrack resonators[J]. Photonics Research, 2020, 8(5): 05000684).

In order to achieve ultra-high-Q resonator, the key is to realize ultra-low-loss optical waveguides. For silicon photonic waveguides, the propagation loss is mainly from the sidewall scattering, which can be reduced possibly by using a wide waveguide because the field intensity at the sidewalls can be minimized. However, the core width is usually limited by the singlemode condition. In this work, a uniform multimode silicon photonic waveguides, which is much wider than a regular singlemode silicon strip waveguide, was introduced for the first time and an ultra-high-Q silicon race-track resonator was proposed and realized.

In particular, the multimode waveguide bends (MWBs) in the race-track resonator are designed with modified-Euler curves, so that no inter-mode coupling happens when light propagates along the race-track resonator even when the effective bending radius for the MWBs is small. Furthermore, an asymmetric bent directional coupler, which is designed according to the phase-matching condition, is used to achieve the selective mode-coupling for the fundamental mode. In this way no higher-order mode is excited in the race-track, while the fundamental-mode is excited and propagates in the multimode race-track resonator with ultra-low loss and low inter-mode coupling. As a result, the resonance in the multimode silicon racetrack resonators is generated for the fundamental mode only. The present high-Q resonator is realized with a simple standard single-etching process provided by a multi-project-wafer foundry. The fabricated device, which has a measured intrinsic Q-factor as high as 2.3×106, is the smallest silicon resonators with a >106 Q-factor.

Prof. Dai believes that this work is of great significance to promote the development of high-performance silicon photonic devices, such as optical filters and nonlinear photonic devices. In addition, the design strategy can be easily applied to other optical waveguides based on silicon nitride, lithium niobite, etc., which is the next step in the near future. Furthermore, more applications of such high-performance photonic devices will be explored.

Proposed ultrahigh-Q silicon racetrack resonator.