Ultra-Highly Sensitive Hydrogen Chloride Detection Based on Quartz-Enhanced Photothermal Spectroscopy

Ultra-highly sensitive HCl-LITES sensor based on a low-frequency quartz tuning fork and a fiber-coupled multi-pass cell

In this paper, an ultra-highly sensitive light-induced thermoelastic spectroscopy (LITES) based hydrogen chloride (HCl) sensor, exploiting a custom low-frequency quartz tuning fork (QTF) and a fiber-coupled multi-pass cell (MPC) with optical length of 40 m, was demonstrated. A low resonant frequency of 2.89 kHz of QTF is advantageous to produce a long energy accumulation time in LITES. Furthermore, the use of an MPC with the fiber-coupled structure not only avoids the difficulty in optical alignment but also enhances the system robustness. A distributed feedback (DFB) diode laser emitting at 1.74 µm was used as the excitation source. Under the same operating conditions, the using of low-frequency QTF provided a ~2 times signal improvement compared to that achieved using a standard 32 kHz QTF. At an integration time of 200 ms, a minimum detection limit (MDL) of 148 ppb was achieved. The reported sensor also shows an excellent linear response to HCl gas concentration in the investigated range.

Sensitive vapor detection with hollow thin film arrays

In this manuscript, we explored the performance of a hollow thin film array (HTFA) for the detection of HCl vapor based on fluorescence quenching. The HTFA structure was fabricated by manually stacking layers of an active thin film and a supporting film, alternately, with a hollow structure in each supporting film. The total penetration depth of vapor molecules in the HTFA sample is 2n times increased, where n is the layer number of the active thin film. We tested the sensing performance of the HTFA sample using fluorescence emission and laser emission in a Fabry-Pérot (FP) microcavity. In the fluorescence sensing, the sensing efficiency increases with the vapor concentration, and can be as high as 80% with a vapor concentration of 400 ppm. While in the laser sensing, the efficiency can achieve 100% with an external pump intensity three times of the lasing threshold at a vapor concentration of 85 ppm. The HTFA sample is not only suitable for vapor detection but also suitable for molecule detection in liquid.

Ultra-Highly Sensitive Ammonia Detection Based on Light-Induced Thermoelastic Spectroscopy

This invited paper demonstrated an ultra-highly sensitive ammonia (NH₃) sensor based on the light-induced thermoelastic spectroscopy (LITES) technique for the first time. A quartz tuning fork (QTF) with a resonance frequency of 32.768 kHz was employed as a detector. A fiber-coupled, continuous wave (CW), distributed feedback (DFB) diode laser emitting at 1530.33 nm was chosen as the excitation source. Wavelength modulation spectroscopy (WMS) and second-harmonic (2f) detection techniques were applied to reduce the background noise. In a one scan period, a 2f signal of the two absorption lines located at 6534.6 cm⁻¹ and 6533.4 cm⁻¹ were acquired simultaneously. The 2f signal amplitude at the two absorption lines was proved to be proportional to the concentration, respectively, by changing the concentration of NH₃ in the analyte. The calculated R-square values of the linear fit are equal to ~0.99. The wavelength modulation depth was optimized to be 13.38 mA, and a minimum detection limit (MDL) of ~5.85 ppm was achieved for the reported NH₃ sensor.