This letter investigates a flow-free, pseudospin-based acoustic topological insulator. Zone folding, a strategy originated from photonic crystal, is used to form double Dirac cones in phononic crystal. The lattice symmetry of the phononic crystal is broken by tuning the size of the center "atom" of the unit cell in order to open the nontrivial topological gap. Robust sound one-way propagation is demonstrated both numerically and experimentally. This study provides a flexible approach for realizing acoustic topological insulators, which are promising for applications such as noise control and waveguide design.Recent discoveries in condensed matter physics have opened possibilities for topological physics which are characterized by either the quantum hall effect (QHE) [1,2] or quantum spin Hall effect (QSHE) [3,4]. Translating the concept of topological phases [5] to classical waves such as optic [6][7][8][9][10][11][12], acoustic [13][14][15][16][17][18][19][20][21][22][23][24][25], and elastic waves [26][27][28][29], is currently an active area of research. There are several barriers in realizing topological states in acoustics, such as the absence of polarization in longitudinal waves and the difficulty of breaking the time reversal symmetry in Hermitian systems. For acoustic topological Chern insulators, external fields such as circulating fluids have been used to break the time reversal symmetry [13][14][15][16][17][18][19][20]. The inevitable dynamic instability and flow-induced noise, however, could pose great challenges for experimental demonstration. On the other hand, the intrinsic spin-1/2 fermionic characteristic is the underpinning component of the QSHE for electrons. Direct analogy in acoustics is non-trivial, though, due to the spin-0 nature of acoustic waves. To address this issue, He [24] et al. and Mei [31] et al. independently provided a pseudospin approach for designing flow-free acoustic topological insulators based on accidentally formed double Dirac cones in phononic crystal [32,33]. However, the double Dirac cone can only be obtained at a fixed filling ratio, which is typically found by a trial-and-error approach. In addition, the accidental double Dirac cones either rely on a high impedance contrast [24] or composite materials [31]. While the former condition is very difficult to satisfy in acoustic media other than air, the latter adds substantial complexity to material fabrication. In photonic crystals, the "zone folding" mechanism was proposed to form double Dirac cones [9]. By expanding the unit cell of the lattice, the Brillouin zone will fold and the high symmetry points K and K in the original Brillouin Zone are mapped to Γ point of the new Brillouin zone, giving rise to double Dirac cones. The similar concept is introduced to acoustics to construct double Dirac cones as well as acoustic topological insulators for robust one-way propagation [23]. The structure proposed in acoustics, however, requires a refractive index higher than the background medium, which is difficult to realiz...

No abstract

Physical phenomena induced by nonlinear effects in an acoustic metamaterial with double-negativity based on two types of scatterers, side holes and membrane, located along a pipe are theoretically and experimentally studied. We find that the pass and forbidden bands related to double-negativity and single-negativity effects in the metamaterial vary with the input acoustic intensities because of the nonlinearities of both types of scatterers. The nonlinearities can disrupt the unique features associated with these characteristic frequency bands of the metamaterial; however, the nonlinear effects may also lead to applications as automatically triggered acoustic isolators, tunable acoustic metamaterials, and so on.

Over the past few years, the rapid development in the fields of condensed matter physics, electronic, and photonic systems have inspired the design and experimental demonstration of various acoustic topological insulators (TIs). Among these, the topologically protected one-way propagation is a phenomenon that is gaining increased attention. Pseudospin states, which is the analogue of the quantum spin Hall effect from electronic systems, has been proven to enable topological edge states in acoustics. Similarly, the valley Hall (VH) effect is also observed in acoustic systems and provides a pair of valley vortex states with opposite chirality. These valley vortex states can similarly form topologically protected edge states and, in turn, realize robust one-way propagation. However, the differences in the physics behind these acoustic systems give rise to distinct features such as different angle selections and immunization levels to various types of defects. This article conducts a comparison study between topological states in valley Hall phononic crystals and TIs that reveals the differences and similarities in several aspects. Both of them have topologically protected edge states and thus the robust one-way propagation. For the maximum transmission incident angle and defect immunization, however, VH topological waveguides and TI waveguides show different characteristics.