Theoretical design of coupled high contrast grating (CHCG) waveguides to enhance CO2 light-absorption for gas sensing applications

Abstract

In this work, we present a theoretical study on using high contrast grating (HCG) designs to enhance light–gas interaction in the mid-infrared range. The optical behavior of a single layer HCG was studied under the presence of CO2 gas. Through optimizing the structure parameters, we could confine an intense electric field over the grating layer. Consequently, about 200 times of light-absorption enhancement was observed. To further improve the performance, a coupled HCG (CHCG) was proposed to introduce another vertical photonic confinement mechanism. We found that CHCG can restrict much intense light energy in the structure leading to over 600 times of light-absorption enhancement. However, it is noticed that a significant part of the concentrated electric field was still trapped in the high index areas, where the gas cannot interact. To address this issue, a modified CHCG with a thin substrate thickness was proposed. Through the optimization (T=1.149μm), we were able to redistribute most of the light energy into the void space of the CHCG layer which resulted in close to 1400 times of improvement. This work clearly demonstrates that using HCG for enhancing light–gas interaction is a promising approach to make on-chip gas sensing devices. Furthermore, it can also be integrated into other photonic components, e.g., fibers for advanced sensing system development.