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Valley pseudo-spin and its associated interface wave transport in sonic crystals has attracted increasing attention from researchers for the potential manipulation of acoustic waves. The topological interface state, projected from a specific valley, is valley-locked, and, thus, renders robust reflection immunity against defects. In this work, we report on the experimental observation of the different parity generations of interface states at two distinct zigzag interfaces. By designing a "C"-shaped domain wall, we experimentally demonstrated the parity generation and selective excitation of interface valley-locked states. Benefiting from different parities of the interface states, one-way valley parity conversion was verified in sonic crystals without breaking the time reversal symmetry. Our findings contribute to the applications in noise control, acoustic communication, and logic processing for topological functional devices with unidirectional responses. [ABSTRACT FROM AUTHOR]

Acoustic metamaterials and metasurfaces have been explored in the past few years to realize a wide range of extreme responses for sound waves. As one remarkable phenomenon, extreme anisotropy and hyperbolic sound propagation are particularly challenging to realize compared to electromagnetic waves because of the scalar nature of airborne acoustics. In parallel, moiré superlattices and the rapidly expanding domain of twistronics have shown that large anisotropy combined with tailored geometrical rotations can enable tantalizing emerging phenomena, such as tailored phase transitions in metamaterials. Connecting these areas of research, here, we explore the realization of acoustic hyperbolic metasurfaces and their combination to drive topological phase transitions from hyperbolic to elliptic sound propagation. The transition point occurring at a specific rotation angle between two acoustic metasurfaces supports highly directional canalization of sound, opening exciting opportunities for twisted acoustics metasurfaces for robust surface wave guiding and steering. [ABSTRACT FROM AUTHOR]

Breaking the diffraction limit to achieve super-resolution focusing is a long-sought goal in the field of acoustic imaging and detection. Here, we demonstrated super-resolution focusing of high-frequency ultrasound (5 MHz) based on an acoustic meta-lens with four centrosymmetric dart-like sharp-edge apertures. For the diffracted ultrasound carrying high spatial frequencies, super-resolution focusing is generated in the far field with mainlobe size breaking the diffraction limit (<1.22λ). Different from previous far-field super-resolution focusing, where the sidelobe intensity is close to or even larger than the mainlobe intensity, the sidelobe intensity in our case is weak. We further conducted fine defect detection (∼200 μm) by utilizing our designed far-field super-resolution focusing to verify its nondestructive testing performance. [ABSTRACT FROM AUTHOR]

The topological sound transport, as an interesting phenomenon discovered in sonic crystals, has drawn tremendous interest in recent years. Here, in resonant acoustic systems, we demonstrate the existence of band inversion by slightly changing the lengths of tube resonators, which unveils the acoustic valley Hall phase transition characterized by the inverted valley Chern number. However, when the valley topological insulator is largely distorted, we can obtain flat-band-like edge states in the bandgaps with topological protection still existing. Those edge states can propagate along zigzag delay-lines with the backscatterings suppressed to a large amount. Our work provides a prototype of topological-insulator-based acoustic devices with the frequency-selecting functionality. [ABSTRACT FROM AUTHOR]

Topological manipulation of sound has recently been a hot spot in acoustics due to the fascinating property of defect immune transport. To the best of our knowledge, the studies on one-dimensional (1D) topological acoustic systems hitherto mainly focus on the case of the Su-Schrieffer-Heeger model. Here, we show that topologically protected bound states may also exist in 1D periodically modulated acoustic waveguide systems, viz., 1D Floquet topological insulators. The results show that tuning the coupling strength in a waveguide lattice could trigger topological phase transition, which gives rise to topologically protected interface states as we put together two waveguide lattices featured with different topological phases or winding numbers. However, for the combined lattice, input at the waveguides other than the interfacial ones will excite bulk states. We have further verified the robustness of interface bound states against the variation of coupling strengths between the two distinct waveguide lattices. This work extends the scope of topological acoustics and may promote potential applications for acoustic devices with topological functionalities. [ABSTRACT FROM AUTHOR]