Piezoelectric tuning forks employed as photodetectors for hydrogen sensing

A study on the H2 spectral properties and a comparison between two different piezoelectric resonators employed as infrared detectors for hydrogen sensing are reported. A quartz tuning fork (QTF) and a lithium niobate tuning fork (LiNTF) are implemented in the same light-induced thermoelastic spectroscopy experimental set-up, employing i) a laser diode to target the hydrogen absorption feature at 4712.90 cm-1, characterized by low interference from other contaminants; ii) a multi-pass cell with a 10.4 m pathlength, to enhance the interaction between light and H2 molecules. Both resonators demonstrate a linear response with respect to the hydrogen concentration and a minimum detection limit (MDL) of 0.50 % and of 1.50 % at 0.1 s of integration time with the QTF and the LiNTF, respectively. The long-term stability analysis highlights a bias instability for the QTF, mainly addressed to the inhomogeneities at the edge of the active area. Conversely, the uniform surface of the LiNTF returns a highly stable detection, allowing an MDL as low as 0.1 % at 64 s of integration time. This first demonstration of a LiNTF as photodetector paves the way to the realization of fully integrated sensors based on lithium-niobate-on-insulator platforms.