Mid-infrared trace gas detection plays a significant role in many sectors such as energy systems, transportation, environmental monitoring, agriculture, safety, and security. With quantum cascade laser (QCL) and interband cascade laser (ICL) used as the light source, it is promising to develop high-resolution mid-infrared laser-based spectrometers with a portable size and low power consumption. Photoacoustic spectroscopy (PAS) and photothermal spectroscopy (PTS) are two highly sensitive methods for chemical analysis by detecting the absorption-induced acoustic wave and refractive index change, respectively. However, there is still room for improvement compared to the most sensitive spectroscopic techniques such as cavity ring-down spectroscopy (CRDS) or noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy (NICE-OHMS).

In this talk I will discuss recent advances in the development of ultra-sensitive PAS enhanced by tuning fork and optical cavity, as well as PTS enhanced by hollow-core fiber and phase-sensitive interferometry. PAS signal is proportional to the overall incident laser power (W). The most recent innovation of PAS with a double opto-acoustic resonance enables ultra-sensitive and wide-dynamic-range gas detection [1]. The merging of a high-Q-factor acoustic resonator (i.e., quartz tuning fork) with a high-finesse optical resonator leverages on a double standing wave effect. By using a mW-level QCL, the doubly resonant QEPAS sensor demonstrates ppt-level CO detection in the mid-infrared. In comparison, PTS signal is proportional to the light power density (W/m²), which can be readily achieved in a hollow-core fiber [2]. By taking advantage of mid-infrared fiber technology, I will present our recent innovation of mid-infrared-pump near-infrared-probe PTS for trace gas sensing [3-5]. The variation of refractive index caused by the pump-laser can be sensitively detected by agile interferometric methods such as the Mach-Zehnder interferometer, heterodyne interferometer, Fabry-Pérot interferometer, and fiber mode interferometer.

Fast and accurate monitoring of pollutant gases in the environment is critical to safeguard public health. Among different sensing solutions, quartz enhanced photoacoustic spectroscopy (QEPAS) is a highly sensitive optical technique, implementing quartz tuning forks (QTFs) to convert sound waves, produced by gas molecules when modulated light is absorbed, into an electric signal. The slow signal acquisition speed depends on the long scan time of the gas absorption feature, requiring few minutes. Furthermore, the real-time monitoring of the QTF resonance frequency (f₀) and quality factor (Q) cannot be carried out during the laser tuning range scan. In this work, the beat frequency-QEPAS (BF-QEPAS) approach was employed to both overcome these limitations and detect NO, using an interband cascaded laser emitting at a central wavelength of 5.263 μm and a 12.4 kHz custom QTF. In BF-QEPAS, a staircase ramp with a rising time of ~1s and a sinewave detuned with respect to f₀ allow exciting the QTF with an acoustic pulse. Considering the typical BF-QEPAS signal, i) the gas concentration is retrieved from the value of the first peak, ii) f₀ is measured from the time difference between the detectable peaks, and iii) Q is determined by the decay time, evaluated with an exponential fit of the detectable peaks. We achieved a minimum detection limit and a normalized noise equivalent absorption of 180 ppb at 5 ms of the lock-in time constant and 2.5·10^-9 cm^-1WHz^-1/2, respectively. Furthermore, the BF-QEPAS signal allows determining the f₀ with an accuracy of 0.1 Hz and the Q with a relative error of ~1%.