New silicon-based micro-electro-mechanical systems for photo-acoustic trace-gas detection

The achievable sensitivity level of photo-acoustic trace-gas sensors essentially depends on the performances of the acoustic transducer. In this work, the mechanical response of different silicon-based micro-electro-mechanical systems (MEMS) is characterized, aiming at investigating both their mechanical properties, namely the resonance frequency and the quality factor, and the minimum detection limit (MDL) achievable when they are exploited as an acoustic-to-voltage transducer in a trace-gas photoacoustic setup. For this purpose, a 4.56 µm Continuous-Wave (CW) quantum cascade laser (QCL) is used to excite a strong N₂O roto-vibrational transition with a line strength of 2.14 × 10⁻¹⁹ cm/molecule, and the detection of MEMS oscillations is performed via an interferometric readout. As a general trend, the minimum detection limit decreases when the resonance frequency investigated increases, achieving a value of 15 parts per billion with a 3 dB cut-off lock-in bandwidth equal to 100 mHz, around 10 kHz.