Slow-Light Mid-IR Silicon Photonic Chips for NO₂ and CH₄ Gas Detection

Area of Science:

• Photonics
• Integrated Optics
• Gas Sensing

Background:

• Mid-infrared gas sensing is crucial for environmental monitoring and industrial safety.
• Silicon photonics offers a scalable platform for integrated optical devices.
• Photonic crystal waveguides (PCWs) enable enhanced light-matter interactions for sensitive detection.

Purpose of the Study:

• To demonstrate a compact, chip-scale mid-infrared gas sensor using a silicon-on-insulator (SOI) platform.
• To leverage slow-light propagation in a 2D photonic crystal waveguide (PCW) for enhanced sensitivity.
• To validate the sensor's performance for detecting nitrogen dioxide (NO2) and methane (CH4).

Main Methods:

• Fabrication of a 2D PCW on SOI with a hexagonal lattice and line defect for slow-light propagation (group index ~73).
• Operation at fundamental absorption wavelengths of NO2 (3.42 μm) and CH4 (3.40 μm).
• Experimental validation using mass flow controllers for gas mixtures and an InSb detector for signal acquisition.

Main Results:

• Achieved a linear response for NO2 (5-25 ppm, R²=0.9934) with a detection limit of 210 ppb, marking the first silicon photonic-based NO2 detection.
• Demonstrated multigas sensing capability with a 6.4% intensity decrease for 25 ppm CH4.
• Validated the sensor's compact 3 mm footprint and CMOS-compatible fabrication.

Conclusions:

• The SOI-PCW platform is a scalable, low-power solution for integrated mid-infrared gas sensing.
• The demonstrated sensor shows significant potential for real-time environmental monitoring and industrial safety applications.
• This technology advances chip-scale optical gas sensing capabilities.