Design of Slow-Light Subwavelength Grating Waveguides for Enhanced On-Chip Methane Sensing by Absorption Spectroscopy

Abstract

The performance of on-chip gas sensors using absorption spectroscopy are currently limited by the small overlap and reduced interaction length between the light and the analyte. Here, the use of slow-light in subwavelength grating (SWG) waveguide integrated on a silicon photonic chip is proposed to improve methane sensing by tunable diode laser absorption spectroscopy in the near-infrared. Such SWG waveguide increases the interaction by two means. Firstly, close to the photonic bandgap edge, a SWG waveguide no longer acts as a metamaterial with a homogeneous index, but rather as a 1D photonic crystal in which slow-light effect enhances the light-analyte interaction. Secondly, the subwavelength segmentation of the waveguide increases the modal overlap with the air. These two enhancement mechanisms results in a six-fold improvement of the interaction with respect to strip waveguides. In this paper, we discuss how to engineer the group index of SWG waveguides to exploit slow-light effect for the first time. Design guidelines for minimizing propagation loss and disorder effect are discussed considering limitations of typical fabrication processes. SWG waveguides could improve the sensitivity and the limit of detection of on-chip trace-gas sensors that provide a compact, fabrication-tolerant, inexpensive and selective sensing technology.

Towards Integrated Mid-Infrared Gas Sensors

“…Among various configurations, sensors based on dome-like gas cells [44,47,50,59], allow for planar integration of emitters and detectors, and could be used to further reduce their size. The main challenge is the relatively small pathlengths, l , which can be addressed by further device optimization, e.g., based on multi-pass designs [57,74,111], or use of photonic crystals [55,112] or optical cavities [56] for enhanced absorption. More recently, photoacoustic gas sensors, based on low-cost commercially available MEMS microphones have emerged as simple, compact, and highly reliable devices [15,50].…”

Compact Gas Sensor Using Silicon-on-Insulator Loop-Terminated Mach–Zehnder Interferometer

“…where S wg = dn e f f dn med (5) is the waveguide sensitivity. Note that a sensor with high sensitivity is not enough for good detection, as the FWHM of the sensor should also be minimized.…”

“…Next, we optimized the slot waveguide with a 100 nm slot gap width G slot such that we maximized the waveguide sensitivity S wg . S wg was determined at λ = 1.55 µm using the FDE solver and Equation (5). A change in the air cladding medium index ∆n med of 0.001 was introduced and the corresponding change in the mode effective index ∆n eff was calculated using the FDE solver from which the S wg value was determined with Equation (5).…”

“…Gas sensing is rapidly growing in importance due to its essential role in various fields and for a wide range of applications such as the detection of hazardous and toxic gases, industrial inspection, and environmental monitoring [1]. Several optical gas sensors have been developed over the years, owing to their high sensitivity, stability, wide dynamic range, fast response time, and multiplexing capabilities [1][2][3][4][5]. The demand for integrated on-chip photonics gas sensors is rising and is expected to increase further.…”

Slow Light in Subwavelength Grating Waveguides

“…Structural slow light can be used in many important applications like on-chip compact delay lines, all-optical switching and signal buffering for optical storage [11]. Furthermore, slow light strongly enhances light-matter interaction in transparent materials and is thus of great interest for non-linear optics [12], lasers [13] and sensing [14], [15].…”