Lithium niobate tuning forks as piezoelectric transducers in photoacoustic spectroscopy

In the last few decades, quartz has been the chosen material to fabricate piezoelectric resonators such as MEMS, racetrack resonators and tuning forks (TFs), which have been widely employed for a variety of sensing applications in several different fields, spanning from environmental monitoring to oil and gas industry. However, throughout the last decade, lithium niobate (LiN) started being employed in the integrated photonics field to build acousto-optical, electro-optical and nonlinear optical devices. In particular, 128° y-cut LiN has a higher density (4.64 g/cm3), Young modulus (145 GPa), and its piezoelectric coefficients are overall one order of magnitude greater compared to z-cut quartz. In this work, a custom lithium niobate tuning fork (LiNTF) is employed for the first time as a piezoelectric transducer in a photoacoustic spectroscopy-based apparatus devoted to gas sensing. The LiNTF was obtained from a 128° y-cut LN wafer and exhibits a resonance frequency f0 = 39196.6 Hz and a quality factor Q = 5900 at atmospheric pressure. For this proof of concept, a water vapor absorption line falling at 7181.14 cm-1 (1.392 μm) was targeted, achieving a signal to noise ratio (SNR) of 400 for a standard air sample having a 1.2% concentration of water vapor at atmospheric pressure and 100 ms of lock-in integration time. An Allan – Werle analysis showed a one order of magnitude improvement in the SNR when increasing the integration time up to 20 s. These preliminary results mark a first step towards the realization of LiNTF-based devices integrated on LiN platforms for gas sensing applications.