Underwater Analyte Sensing Using a Phononic Crystal Waveguide-Based Interferometric Acoustic Spectrometer

This work introduces a 2D PnC-based acoustic spectrometer capable of analyzing small solution volumes ( $25~\mu $ l) in aqueous environments with significative accuracy and reliability, thus addressing key limitations in current acoustic spectroscopic techniques. Optimally introducing rows of defects into the PnC structure enables guided acoustic modes to propagate at desired frequencies within the bandgap. We construct an acoustic interferometer to leverage the properties of acoustic cavities within these waveguides, which can configure and modulate wave propagation. Our approach involves harnessing the interference between acoustic waves in the two arms of a defects-based waveguide within a PnC, one arm containing an analyte cavity-holder. We demonstrate that the presence of an analyte (sucrose solutions at various concentrations) induces alterations in the acoustic properties of the cavity, leading to observable shifts in transmission characteristics of the propagating acoustic modes. We achieve exceptional spectral resolution through experimentation, facilitating highly sensitive acoustic sensing even with small analyte volumes ( $\lt 25~\mu $ l). We utilize finite element method simulations to validate our findings and predict spectral shifts resulting from modified acoustic interference. Additionally, we provide a phenomenological description using tight-binding models. Notably, our approach surpasses conventional PnC sensors like Mach-Zehnder interferometers by overcoming challenges associated with analyte uniformity.