Dual-band valley topological refraction materials for bulk elastic waves manipulation

Valley topological refraction material (TRM) of elastic wave and its direction signal transmittance with robustness against structural disturbance is crucial for various applications such as communication, navigation and detection devices for medical and industrial purpose. However, the existing TRM is limited by the single topological band, which obviously hinders the exploration of the multi-band topological ultrasonic devices to guarantee its efficiency and stability. Here, we demonstrate the dual-band valley TRM for elastic waves, in which the valley edge states and their robustness are investigated. Furthermore, the topological refractions of the valley edge states from the interface into the free space at zigzag termination in two frequency bands are explored, where the mode conversion of the bulk elastic wave occurs at interface termination. Interestingly, the full mode conversion from longitudinal to transverse waves is revealed in the low frequency band while the non-full mode conversion between the longitudinal and transverse takes place in the high frequency band. The researched dual-band elastic TRM could facilitate the potential applications in multi-band and multi-directional devices.