Ultrahigh Sensitive LITES Sensor Based on a Trilayer Ultrathin Perfect Absorber Coated T-Head Quartz Tuning Fork

In this paper, an ultrahigh sensitive light-induced thermoelastic spectroscopy (LITES) sensor based on a trilayer ultrathin perfect absorber coated T-head quartz tuning fork (QTF) is reported for the first time. The absorber is designed according to the streamlined theoretical framework based on admittance matching principles. The incorporation of Cr as the primary absorber and the dual-layer anti-reflection coating of SiO₂ and Si in the tri-layer (Si-SiO₂-Cr) ultra-thin perfect absorber enables high absorption across a wide spectral range. It results in a 93% absorbance of the 1530 nm laser by the QTF. This feature enhances the QTF's photothermal-elastic-electric conversion efficiency and detection sensitivity. The self-designed T-head QTF with a low resonant frequency of 8.78 kHz is beneficial to increase the energy accumulation time. The signal-to-noise ratio of the T-head QTF increases by 3.35-fold after coating the tri-layer absorber, and it is 6.90 times greater than that of the standard QTF. To achieve ultra-high sensitive acetylene (C₂H₂) detection, a 40 m fiber-coupled multi-pass cell and an erbium-doped fiber amplifier are integrated into the sensing system to augment the target gas's absorption of laser and boost laser power. Finally, a minimum detecting limit (MDL) of 209.59 ppt is obtained, indicating the ultrahigh detection sensitivity and the best MDL among all the reported C₂H₂-LITES sensors. This work paves an effective new way to enhance the detection sensitivity of the LITES sensor, and can provide broader absorption enhancement of the QTF to detect additional gases with optimized coating architecture.