Enhancing the sensitivity of photoacoustic spectrum system for liquid detection by coupling with acoustic metasurfaces

Photoacoustic spectroscopy (PAS) has been proven to have great potential in health monitoring applications, such as continuous blood glucose or oxygen monitoring. However, due to the significant mismatch between different acoustic media, 99.9% of acoustic energy cannot propagate across the gas–liquid interface, which greatly limits the liquid detection sensitivity of PAS. In this study, a Helmholtz-type acoustic metasurface (HAM) consisting of a perforated plate and a subwavelength cavity filled with air is proposed to enhance the acoustic transmission across the gas–liquid interface. The behavior of the air layer within the HAM is analogous to the transverse vibration of a membrane, which would drive the vibration of the water surface and provide an effective path for sound transmission from water to air. Experimental results confirmed that HAM can increase the power transmission coefficient of sound from water to air by two orders of magnitude while maintaining optical transparency for laser. Furthermore, the performance of the PAS system coupled with HAM is investigated. It is demonstrated that the liquid detection sensitivity of PAS systems can be enhanced remarkably, with a 409% magnification of the signal intensity. The working frequency of HAM can also be flexibly adjusted from 0.01 to 20 kHz through parameter configuration, enabling compatibility with the operating frequency of PAS systems. It is expected to be applied in the field of liquid ingredient analysis and health monitoring.