Photoacoustic Gas Sensing Using a Novel Fluidic Microphone Based on Thermal MEMS

Photoacoustic spectroscopy (PAS) is a powerful technique for selective gas detection; however, its performance in non-resonant configurations is fundamentally constrained by the poor low-frequency response of conventional acoustic detectors. Commercial MEMS microphones, although compact and cost-effective, exhibit limited infrasound sensitivity, which restricts the development of truly miniaturised and broadband PAS systems. To address this limitation, we present a novel MEMS fluidic microphone (f-mic) that operates on a thermal sensing principle and is explicitly optimised for the infrasound regime. The sensor demonstrates a constant sensitivity of 32 µV/Pa for frequencies below 20 Hz. A detailed analytical model incorporating frequency-dependent effects was developed to identify and investigate the critical design parameters that influence system performance. Based on these insights, a miniaturised photoacoustic cell was fabricated, ensuring efficient optical coupling and f-mic integration. Experimental validation using a CO2-targeted laser system demonstrates a linear response up to 5000 ppm, a sensitivity of 6 nV/ppm, and a theoretical detection limit of 300 ppb over 100 seconds, resulting in an NNEA of 6e-6 W cm^-1 Hz^-0.5. Long-term measurements indicate good stability, with minor drift primarily due to gas leakage and chopper fluctuations. These results establish the f-mic as a robust, scalable solution for non-resonant PAS, effectively overcoming a significant bottleneck in compact gas sensing technologies.