In recent years, the structural design and application of phoxonic crystals have received extensive attention. Its main feature is the simultaneous manipulation and modulation of acoustic waves and optical waves, receiving the localization of photons and phonons in the same structure. Traditional phononic or photonic crystal sensors detect the acoustic or optical properties of the object under test in a single channel, while the phoxonic crystal sensor can sense the optical signal and the acoustic signal at the same time, realize the acousto-optic dual-channel detection of the object to be measured and improve the sensing accuracy of the object under test to a certain extent. In this study, we design a phoxonic crystal sensor with a heterogeneous cavity that can simultaneously measure the refractive index and sound velocity of the liquid and achieved high sensitivity sensing for three types of liquids. Therefore, it can be applied to the field of measurement of related physical quantities of liquids and may have application value in biochemical sensing and water quality monitoring, etc.

Our main calculation method is the finite element method, which is combined with COMSOL Multiphysics 6.0 software. Firstly, the designed structural model is discretized into a certain number of finite small element ensembles. According to the elastic wave propagation equation or Maxwell’s equation of electromagnetic waves, the relationship between the element junction force and the node displacement is created by combining the variational principle, and then the finite element equation is established according to the equilibrium condition of the junction force. Boundary conditions are introduced at the structure boundary and the system of linear equations is solved to obtain the band structures of phonon and photons. To calculate the light transmittance, we apply the scattering boundary condition to all the outer boundaries of the structure, setting port 1 at the left boundary of the air slit region to apply excitation and port 2 at the right boundary of the air slit region to receive excitation. The optical transmission spectrum is obtained by calculating the energy ratio of the output and input of the phoxonic crystal under light excitation. When calculating the acoustic transmission losses, we apply the support to all the outer boundaries of the structure, and we apply the boundary load to the leftmost internal boundary of the structure. By calculating the ratio of the output energy to the input energy of the phoxonic crystal under external force, the acoustic transmission loss is obtained.

The designed phoxonic crystal sensor has high sensitivity (Q) and figure of merit (FOM) in terms of optics. By adding 1-propanol, sodium chloride, and glucose solutions to the air slit, the transmission spectra of the sensing structure to the three solutions at different mass fractions are calculated, and it is found that the resonance wavelength varies linearly with the change of refractive index (Fig. 6). The results show that the structure achieves high optical sensitivity sensing for the three solutions, with the sensitivity (Q) reaching 822.88, 825.00, and 821.89 nm/RIU respectively and the merit factor reaching 1782.15, 1790.89, and 1980.34 RIU^-1 respectively (Tables 2-4). In terms of acoustics, it has a high sensing sensitivity (Q), and the structure has double characteristic peaks, which improves the stability and accuracy of acoustic sensing performance. By adding 1-propanol, sodium chloride, and glucose solutions to the air slit, the transmission loss of the sensing structure to the three solutions at different mass fractions is calculated (Fig. 8). The analysis shows that the resonant frequency of the acoustic cavity mode is basically linear with the change of sound velocity, and the frequency difference between the two resonance peaks at the same sound velocity is basically constant (Fig. 9). The results show that the structure achieves high acoustic sensitivity sensing for the three solutions, with the sensitivity (Q) reaching 3.289, 2.974, and 3.038 MHz/(m·s^-1) respectively (Tables 5-7). Therefore, the designed phoxonic crystal sensor can sense the optical signal and the acoustic signal at the same time, improve the sensing sensitivity of the acousto-optic signal, realize the acousto-optic dual-channel detection of the DUT, and establish a platform for multi-physical sensing of liquids.

We design a two-dimensional phoxonic crystal liquid sensing structure with a heterostructure cavity and air slit, and the acousto-optic sensing characteristics of the structure for different solutions are calculated and discussed. The results show that the sensing structure can realize vertical sensing with different liquid acousto-optic characteristics in the same structure. With a heterostructure cavity, the acousto-optic energy is well localized in the cavity area to enhance the interaction between phonons, photons and the solution, so as to improve the acousto-optic sensing sensitivity of the solution. In terms of optical sensing, the optical sensing sensitivity of the structure for 1-propanol, sodium chloride and glucose solutions reaches 822.88, 825.00, and 821.99 nm/RIU, respectively. In terms of acoustic sensing, there are double characteristic peaks in the structure, and the frequency shift of the double peaks shows a sound linear change along with liquid mass fraction, which improves the stability and accuracy of acoustic sensing performance. The acoustic sensing sensitivity of the structure for 1-propanol, sodium chloride, and glucose solutions reached 3.289, 2.974, and 3.038 MHz/(m·s^-1), respectively. The structure provides a platform for highly sensitive measurement and sensing of multiphysical quantities of liquids.