Subwavelength Racetrack Resonators With Enhanced Critically Coupled Tolerance for On-Chip Sensing

Lijun Huang,Xianwu Mi,Ni Zhou,Dajiang He,Hai Yan,La Xiang

doi:10.1109/access.2021.3056797

Abstract

We propose a coupled-tolerant subwavelength photonic racetrack resonator (PRR) on the silicon-on-insulator (SOI) platform, in which a subwavelength grating (SWG) PRR is coupled to a symmetrical SWG bus waveguide. The material equivalent refractive index can be manipulated by the SWG characteristic and thus an additional fabrication degree of freedom can be achieved. The resonant characteristics can be improved significantly by controlling the pillar size and increasing the interaction length between SWG racetrack and bus waveguide (RBW). The simulation results show that the PRR has a high-quality factor and large extinction ratio around 1550nm. Two groups of SWG and strip waveguide (SW) PRRs are fabricated. The fabrication tolerance is tested when fabrication error is changed from 0 to 20 nm and the quality factor is more than 7000 in de-ionized water. The testing results show that the tolerance in SWG-PRR has a 2.0-fold improvement than that in the SW-PRR. Then a fabricated SWG-PRR is employed to monitor different concentration glycerol injected on the chip. An optimal quality factor of 9500 around 1550 nm and bulk sensitivity of 412± 0.5 nm/RIU are achieved. Therefore, the structure is a potential candidate for label-free sensing in medical diagnosis and environmental monitoring.

Highlights

Due to a low power, small size and high sensitivity, optical microring resonators are fundamental building blocks in a broad range of applications in photonics, including optical filters [1], [2], lasers [3], [4], amplifiers [5], multiplexers [6], [7], reflectors [8], modulators [9], [10] and sensors [11]–[18]
In this article, compared with the strip waveguide (SW) ring resonator and racetrack resonator (RTR) [25], [41], we propose a subwavelength grating (SWG) photonic racetrack resonator (PRR), in which the phase-matching condition is enhanced by manipulating the material equivalent refractive index
The device is exposed by the e-beam lithography (EBL), and an inductively coupled plasma reactive ion etching (RIE) process with HBr and Cl2 is employed to transfer the pattern to silicon layer, in which an acceptable roughness over the silicon slits roughness and nearly vertical side-walls is accomplished [52], [53]

Summary

Introduction

Due to a low power, small size and high sensitivity, optical microring resonators are fundamental building blocks in a broad range of applications in photonics, including optical filters [1], [2], lasers [3], [4], amplifiers [5], multiplexers [6], [7], reflectors [8], modulators [9], [10] and sensors [11]–[18]. On the siliconon-insulator (SOI) platform, SWG waveguide consists of periodic silicon pillars with a low index material (such as SiO2 [30], SU-8 polymer [31], air [32] or sensing solution [33]–[37]). The light propagation is included around the top and side of the waveguide, and the space between the silicon pillars, so the SWG structure can increase the interaction

Methods

Results

Conclusion