Temperature tuning of defect state induced by the periodic cavities between the phononic crystals

The defect state in periodic structures usually leads to a pass band in the forbidden bandgap and energy localization at the position of geometric change. In this paper, we have introduced periodic cavities into a symmetric structure composing of two separated phononic crystals to construct the defect state in the forbidden band and realize the temperature tuning of the defect state. The proposed structure is composed of two mirror-symmetric phononic crystals, with triangular steel arrays embedded in water. These two phononic crystals have opposite rotation angles possessing different topological properties, and the relative position of them leads to a forbidden band for the underwater ultrasonic waves. Interestingly, the periodic cavities between the boundaries of the phononic crystals result in a peak in the forbidden bandgap, which behaves as energy localization inside the cavities. The simulated results reflect the spectral and spatial characteristics of the defect state of this structure and exhibit the temperature tuning of the mentioned defect state. The designed structure provides a valid platform to filter the ultrasonic waves in fluid via controlling temperature, and the defect state manipulations benefit the smart structures of wave propagation, such as acoustic switches and underwater sound waveguides.