Sensitivity Enhancement in Si Nanophotonic Waveguides Used for Refractive Index Sensing

Yaocheng Shi,Daoxin Dai,Ke Ma

doi:10.3390/s16030324

Abstract

A comparative study is given for the sensitivity of several typical Si nanophotonic waveguides, including SOI (silicon-on-insulator) nanowires, nanoslot waveguides, suspended Si nanowires, and nanofibers. The cases for gas sensing (ncl ~ 1.0) and liquid sensing (ncl ~ 1.33) are considered. When using SOI nanowires (with a SiO2 buffer layer), the sensitivity for liquid sensing (S ~ 0.55) is higher than that for gas sensing (S ~ 0.35) due to lower asymmetry in the vertical direction. By using SOI nanoslot waveguides, suspended Si nanowires, and Si nanofibers, one could achieve a higher sensitivity compared to sensing with a free-space beam (S = 1.0). The sensitivity for gas sensing is higher than that for liquid sensing due to the higher index-contrast. The waveguide sensitivity of an optimized suspended Si nanowire for gas sensing is as high as 1.5, which is much higher than that of a SOI nanoslot waveguide. Furthermore, the optimal design has very large tolerance to the core width variation due to the fabrication error (∆w ~ ±50 nm). In contrast, a Si nanofiber could also give a very high sensitivity (e.g., ~1.43) while the fabrication tolerance is very small (i.e., ∆w < ±5 nm). The comparative study shows that suspended Si nanowire is a good choice to achieve ultra-high waveguide sensitivity.

Highlights

Optical waveguide sensors are paving the way for realizing low-cost, highly sensitive, ultra-compact optical sensors, which are desired for many applications such as biological, environmental and chemical detections [1,2,3,4,5,6,7,8,9]
The principle of optical waveguide sensors is based on the perturbation of the field of a guided mode caused by optical absorptions, fluorescence or refractive index changes of the measured sample [8]
People have developed various integrated optical sensors based on different structures and mechanisms, e.g., Mach–Zehnder interferometers (MZI) [1], and high-Q

Summary

Introduction

Optical waveguide sensors are paving the way for realizing low-cost, highly sensitive, ultra-compact optical sensors, which are desired for many applications such as biological, environmental and chemical detections [1,2,3,4,5,6,7,8,9]. The analyzed medium, the sensitivity SWG of a guided mode in an optical waveguide is usually assumed to be smaller or much smaller than that of a free-space beam (S = 1) [15] This is true when using a conventional strip or rib waveguide with a large core size and a low index-contrast because the evanescent fields are not very strong. In 2004, a nanoslot waveguide was introduced as a novel guided-wave configuration [17], in which there is a field enhancement in the low-index slot region due to the boundary condition of the perpendicular electrical component This makes it very attractive to achieve high sensitivity for optical sensing [18,19,20]. Our simulation shows that an enhanced sensitivity of about 1.5 could be achieved by using suspended Si nanowires

Analysis and Discussion

SOI Nanowires

SOI in Figure

Suspended

Nanofibers

Conclusions