Towards lab-on-chip ultrasensitive ethanol detection using photonic crystal waveguide operating in the mid-infrared

Ali Rostamian,Volker J Sorger,Ehsan Madadi-Kandjani,Hamed Dalir,Ray T Chen

doi:10.1515/nanoph-2020-0576

Abstract

Abstract Thanks to the unique molecular fingerprints in the mid-infrared spectral region, absorption spectroscopy in this regime has attracted widespread attention in recent years. Contrary to commercially available infrared spectrometers, which are limited by being bulky and cost-intensive, laboratory-on-chip infrared spectrometers can offer sensor advancements including raw sensing performance in addition to utilization such as enhanced portability. Several platforms have been proposed in the past for on-chip ethanol detection. However, selective sensing with high sensitivity at room temperature has remained a challenge. Here, we experimentally demonstrate an on-chip ethyl alcohol sensor based on a holey photonic crystal waveguide on silicon on insulator-based photonics sensing platform offering an enhanced photoabsorption thus improving sensitivity. This is achieved by designing and engineering an optical slow-light mode with a high group-index of n g = 73 and a strong localization of the modal power in analyte, enabled by the photonic crystal waveguide structure. This approach includes a codesign paradigm that uniquely features an increased effective path length traversed by the guided wave through the to-be-sensed gas analyte. This PIC-based lab-on-chip sensor is exemplary, spectrally designed to operate at the center wavelength of 3.4 μm to match the peak absorbance for ethanol. However, the slow-light enhancement concept is universal offering to cover a wide design-window and spectral ranges towards sensing a plurality of gas species. Using the holey photonic crystal waveguide, we demonstrate the capability of achieving parts per billion levels of gas detection precision. High sensitivity combined with tailorable spectral range along with a compact form-factor enables a new class of portable photonic sensor platforms when integrated with quantum cascade laser and detectors.

Highlights

Mid infrared absorption spectroscopy has attracted considerable attention thanks to the rovibrational signatures of compounds of interest in the solid, liquid, or gas phase in the spectral region spanning from 2 to 20 μm (500–5000 cm−1) – known as the molecular fingerprint region – and orders of magnitude larger absorption crosssections than the overtones in the near-IR [1]
Using the holey photonic crystal waveguide, we demonstrate the capability of achieving parts per billion levels of gas detection precision
The photonic crystal waveguides (PCW) devices are fabricated on an SOI wafer with a 500 nm thick silicon layer over a 3 μm thick buried oxide (BOX) layer, and the pattern of the passive waveguides is transferred to the silicon device layer using electron beam lithography and inductively coupled plasma etching

Summary

Introduction

Mid infrared (mid IR) absorption spectroscopy has attracted considerable attention thanks to the rovibrational signatures of compounds of interest in the solid, liquid, or gas phase in the spectral region spanning from 2 to 20 μm (500–5000 cm−1) – known as the molecular fingerprint region – and orders of magnitude larger absorption crosssections than the overtones in the near-IR [1]. Owing to the high index contrast between the silicon core and the oxide cladding, devices with small footprints can be realized on the SOI chips Both the PC-based microcavities and PCWs have been demonstrated in free-standing silicon membranes in SOI in the mid IR wavelengths [15, 16]. The principle of enhanced absorption is enabled by a slow-light mode and enhanced optical field intensities in the waveguide that combine to increase the effective path length traversed by the guided wave through the sensed gas Employing this structure, in this paper, we design and fabricate an on-chip ethyl alcohol sensor and use it to detect parts per billion (ppb) levels of ethanol vapor. A photonic crystal waveguide can enhance light–matter interaction through engineering both the group index and the filling factor, as discussed

Device description

Results and discussion

Summary