Observation of acoustic valley vortex states and valley-chirality locked beam splitting

Abstract

We report an experimental observation of the classical version of valley polarized states in a two-dimensional hexagonal sonic crystal, where the inversion-symmetry breaking of scatterers induces an omnidirectional frequency gap. The acoustic valley states, which carry specific linear momenta and orbital angular momenta, were selectively excited by external Gaussian beams and conveniently confirmed by the pressure distribution outside the crystal, according to the criterion of momentum conservation. The vortex nature of such intriguing crystal states was directly characterized by scanning the phase profile inside the crystal. In addition, we observed a peculiar beam splitting phenomenon, in which the separated beams are constructed by different valleys and locked to the opposite vortex chirality. The exceptional sound transport, encoded with valley-chirality locked information, may serve as the basis of designing conceptually novel acoustic devices with unconventional functions.

Snowflake phononic topological insulator at the nanoscale

“…The first examples of topologically protected mechanical wave transport have just emerged during the past three years. So far, experimental implementations exist on the centimeter scale, both for the case of time-reversal symmetry broken by external driving [1], such as in coupled gyroscopes, as well as for the case without driving [2][3][4][5][6], such as in coupled pendula. Moreover, a multitude of different implementations have been envisioned theoretically [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24].…”

Artificial gauge fields in materials and engineered systems

“…In these systems a spin-orbit coupling can be engineered by encoding an effective spin in two acoustic modes brought close to degeneracy. The experimental systems studied so far consist in discrete lattices of simple elementary mechanical systems, such as pendula [191,469] or gyroscopes [192], as well as acoustic crystals, made of a continuous fluid flowing in an engineered lattice geometry [206,470,471,472,473].…”

Observation of valleylike edge states of sound at a momentum away from the high-symmetry points

“…At the high symmetric points of the Brillouin zones, the phononic graphene-like lattices possess the Dirac cones with two-fold degeneracies (58) and the double Dirac cones with four-fold degeneracies (59)(60)(61). When the hexagonal lattice undergoes an inversion symmetry breaking, the band structures near these Dirac degenerate cones will inverse, accompanied with the topological phase transitions and the topologically protected one-way transports (62)(63)(64)(65)(66)(67). The Dirac conical dispersion, attributing to the unavoidable band crossing, is protected by the point group symmetry (68).…”