Theoretical modeling of Helmholtz-type acoustic metasurface for enhanced photoacoustic detection in liquids

Abstract

Impedance mismatch at the aqueous-gaseous interface poses a fundamental challenge for photoacoustic detection in liquid media, limiting applications in health monitoring, such as continuous blood glucose monitoring. Our previous work demonstrated a Helmholtz-type acoustic metasurface (HAM), consisting of a perforated plate with a subwavelength air layer, could substantially enhance acoustic transmission from water to air, thereby improving the liquid detection sensitivity of photoacoustic spectroscopy. However, the traditional theory of Helmholtz resonators proves unsuitable for analyzing HAMs due to the extremely narrow air layer, where near-field effects dominate. Accordingly, this work presents an analytical model for the transmission coefficient of HAMs. The modal expansion method is utilized to account for evanescent wave components in the vicinity of both the inlet and outlet of the orifice. Thermal viscosity loss along the walls of the orifice and air layer is also incorporated via equivalent fluid theory. Numerical and experimental results validate the effectiveness of the proposed model in predicting the transmission coefficient of the HAM at the water–air interface. Furthermore, an analytical model is derived to evaluate the signal amplification factor of photoacoustic cells coupled with HAMs, which can provide theoretical guidance for the optimization and design of such systems.