Liquid Sensor Based on Interaction between Decoupled Waveguides and a Cavity with Transverse Offset in a Phononic Crystal

A liquid sensor employing a cavity in the form of a point defect with a transverse offset along the normal bisector of a barrier at the center of a linear waveguide in a two-dimensional phononic crystal, which gives rise to two decoupled waveguides, is proposed. The phononic crystal consists of cylindrical steel rods with 2.0 mm radius in water, arranged with 4.2 mm lattice constant in the square lattice. Linear waveguides are formed by removing a single row from the phononic crystal, whereas the point defect is formed by substituting a single cylindrical steel rod by a polyethylene tubing comprising the analyte of interest. The cavity acts as a cross-bridge between the waveguides through the interaction of the linear defect mode in the input waveguide with the point defect mode, which in turn interacts with the output waveguide mode. Finite-element method simulations reveal that at frequencies around 200 kHz, a sharp peak with a quality factor of the order of 1000 occurs in the transmission spectrum of the system, where resonant transmission occurs. In case of determining the ratio of methanol in ethanol as an instance, it is found that the peak frequency exhibits a quadratic shift with the molar ratio of methanol. On the other hand, the transmission value decreases exponentially with increasing methanol ratio at the frequency of 196.19 kHz, which is the peak frequency for pure ethanol. The proposed sensing scheme can be utilized in many applications such as the identification of fake beverages and in high-throughput concentration measurements in the industry.