Mid-IR parts-per-billion level nitric oxide detection using solid-state laser intracavity photothermal sensor (SLIPS)

Abstract

Highly sensitive, laser-based sensors often utilize extended gas interaction paths (e.g., multi-pass cells) that impact their size, weight, and susceptibility to misalignment, limiting their use to laboratory applications. This study reports the first demonstration of photothermal nitric oxide (NO) detection in the mid-infrared range at 5.26 µm utilizing a unique and miniature Solid-state Laser Intracavity Photothermal Sensor (SLIPS) with an ultra-short laser-gas interaction path length of 1 mm. The gas sample detection is realized inside the resonator of a monolithic diode-pumped solid-state laser (DPSSL) emitting at 1064 nm. Modulated mid-infrared radiation absorbed by NO inside the laser cavity causes local changes in photothermally-induced gas refractive index (RI). This effect results in the DPSSL frequency shifts proportional to gas concentration, which are being detected. A minimum detection limit of 50 ppbv (parts per billion of volume) and a noise equivalent absorption (NEA) coefficient of 7.9 × 10−7 cm−1 for 10 s integration time have been achieved in an ultra-compact sensing volume (4 µl) and ultra-short interaction length of the sensor as a result of its superb RI sensitivity of 1.8 × 10−11. Furthermore, the SLIPS is not limited in gas excitation laser wavelength because only a near-infrared detector is needed for spectroscopic signal readout.