ABSTRACT
Due to the potential engineering needs, the passive tunable metasurfaces with a high performance equivalent to the active phased array is worthy of research. Here, a passive ultrathin metasurface unit composed of a piezoelectric composite structure (PCS) connected to an external capacitor, which can modulate the phase of the transmitted acoustic waves at a deep subwavelength scale only by controlling the external capacitor but without changing the structure, is proposed. Then, a tunable acoustic metasurface composed of 20 identical PCSs is introduced to realize three acoustic functions, beam steering, beam focusing, and tweezer-like beam generating, just by changing the external capacitors. The phase-control abilities of the PCS unit and three functions of the designed metasurface are proved both numerically and experimentally. This study provides the possibility to design ultrathin tunable acoustic metasurfaces with the ability of precise control and passive materials.
ABSTRACT
The topological states in quantum Hall insulators and quantum spin Hall insulators that emerge helical are considered nondissipative. However, in crystalline systems without spin-orbit couplings, the existing higher-order topological states are considered not helical, and the energy suffers from dissipation during propagation. In this work, by introducing the intrinsic pseudospin degree of freedom, we theoretically and experimentally present the existence of the helical higher-order topological states in the C_{6}-symmetric topological crystalline insulators based on the acoustic samples. Crucially, rather than considering the global interaction of the large bulk, we further intuitively reveal the impacts of the geometries of the crystal on the generation mechanisms and natural behaviors of these states based on the simple equivalent models. These results provide a versatile way for guiding the design of the desired topological materials.
