A high-sensitivity liquid concentration-sensing structure based on a phoxonic crystal slot nanobeam

A high-sensitivity liquid concentration-sensing structure based on a phoxonic crystal slot nanobeam with gradient cavities is presented and its sensing properties are investigated using the finite element method. The proposed sensing structure, which can be made from either isotropic or anisotropic materials, can have excellent sensing properties that are designed via geometric optimization. We investigate the influences of various solution concentrations on electromagnetic and elastic wave transmission spectra. The results demonstrate that the introduction of gradient cavities can enable the system to avoid lattice mutation and reduce energy loss, thereby concentrating light and sound energy in the slot and holes, enhancing interactions between the electromagnetic and elastic waves, and improving sensitivity. It is worth noting that the sensing characteristics are related to the electric field distribution in the light sensor. That is, the sensitivity is better when more electric energy is distributed in the liquid-filled slot and holes. The sensitivity can reach 238.1 nm/RIU. The acoustic sensing properties are related to the solid-liquid interaction. This is especially true for the modal sensing characteristics, where the acoustic energy is concentrated on the solid-liquid interface. Therefore, greater interaction strength implies better sensitivity. The acoustic sensitivity can reach 3167 kHz/ms(-1). The proposed structure provides acoustic and optical cross-checks for different types of solutions. This helps us to improve sensing accuracy and reduce sensing uncertainty.