Degang Zhao

A simple interlayer-coupling mechanism drives the transition from type-I to type-II higher-order semimetal phases in phononic crystals. Here, the authors realize a type-II higher-order Weyl phononic crystal supporting coexisting Fermi arc surface states and hinge states. Boundary engineering through unit-cell rotation enables selective activation of hinge states, opening new opportunities for programmable topological wave manipulation.

The linear enhancement of mechanical compliance through zeolite filling in compact loudspeakers has been theoretically and experimentally confirmed. However, utilization of this enhancement to elucidate the correlation between porous structure and acoustic properties remains elusive. This research optimizes gas diffusion ability in zeolite to enhance the acoustics performance of compact loudspeakers. Based on the mechanical compliance linear enhancement theory in zeolite-filled loudspeakers, an intrinsic parameter α pore ref is introduced to measure adsorption site accessibility. By using a hydrothermal method and etching process, ZSM-5 with varying mesostructures, including normal stacking plates, b-oriented, and radial channel plate porous configurations, are successfully synthesized. The optimal (labeled by Z5-BR) sample combines the advantages of b-oriented and radial porous structures in favor of mass transfer. At 0.3 cm³ Z5-BR filling, the loudspeaker achieves 70 Hz resonance frequency shift Δf ₀, 1.05 mH·cm⁻³ mechanical compliance enhancement slope β , and 35.79 mH·m⁻² α pore ref , leading to a maximum 3 dB sound pressure level increase at a sound frequency of 106 Hz. This study quantitatively links the refine microstructure of porous zeolite to acoustic behavior, offering a comprehensive framework for loudspeaker material estimation and igniting the fire to design the composition and structure of acoustic materials at hundreds of Hz through microscopic diffusion-adsorption processes.

Exceptional point (EP) is referred to degeneracies in a non-Hermitian system where two or more eigenvalues and their corresponding eigenvectors coalesce. Recently there have been significantly increased interests in harnessing EPs to enhance responsivities and achieve ultrasensitive detections in optics, electronics and acoustics, although there are few similar studies focused on using surface acoustic wave (SAW) sensing technologies, probably due to its great technical challenges. Herein, we proposed a scheme for accessing EPs in an on-chip architecture consisted of coupled-SAW-resonators system, forming a passive parity-time (PT) symmetric system. We demonstrated that by tuning additional losses in one of resonators and regulating the system in the proximity of the EP, the sensor exhibited significantly enhanced responses. As an example, we present an EP-based SAW gas sensor, which showed a much-improved sensitivity compared to that of a conventional delay-line SAW sensor. The fundamental mechanisms behind this excellent sensing performance have been elucidated.

Pseudomagnetic fields are promising for manipulating classical waves, including optic, acoustic, and elastic waves. One fascinating phenomenon is the presence of Landau energy levels and the involved quantum Hall effect, which are usually considered to exist only in quantum systems. Unlike using synthetic Landau gauge fields, as in existing works, here we successfully create a symmetric gauge field to achieve a large-area uniform pseudomagnetic field in a two-dimensional phononic crystal by introducing linear variations in the single-cell parameters of the scatterer along both the x and y directions. Discrete Landau levels are generated near the Dirac cone region, and the unidirectional propagation of acoustic waves along the domain wall, i.e., quantum Hall transport, is observed in experiment. The proposed structure offers greater flexibility by allowing free choice of the effective vector potential. Our study expands the application perspective of artificial acoustic devices that rely on a pseudomagnetic field.

Topological phases in photonic systems have garnered significant attention, often relying on precise structural design for generating non-trivial topological phases. However, this dependency on fixed structures limits their adaptability. This study systematically explores incident angle-induced topological phase transitions in a one-dimensional photonic crystal (PC). Both TE and TM polarized modes undergo topological phase transitions at the same critical transition angles. Additionally, the TM-polarized mode undergoes a unique topological phase transition at the Brewster angle. When these two kinds of transition angles coincide, even if the band structure of the TM-polarized mode undergoes an open-close-reopen process, the topological properties of the corresponding bandgap remain unchanged. Based on theoretical analysis, we design the composite PCs comprising two interfaced PCs having common bandgaps but different topological properties. By tuning the incident angle, we theoretically and experimentally achieve TE-TM splitting of topological interface states in the visible region, which may have potential applications in optical communications, optical switching, photonic integrated circuits, and so on.

Acoustic vortex states with spiral phase dislocation that can carry orbital angular momentum (OAM) have aroused much research interest in recent years. The mainstream methods of generating acoustic vortexes are based on the Huygens-Fresnel principle to modulate the wave front to create spatial spiral phase dislocation. In this work, we realize acoustic vortexes in a waveguide with a chiral gradient sawtooth metasurface. The physical mechanism of our method is to lift the degenerate dipole eigenmodes through the scattering effect of the chiral surface structure, and then their superposition yields both +1 and −1 order vortexes. Compared with the existing methods of acoustic vortex generation, our design has many merits. For instance, it is easy to manufacture and control, the working frequency is relatively broadband, and the sign of vortex order can be readily flipped. Both the full-wave simulations and experimental measurements validate the existence of the acoustic vortexes. The torque effect of the acoustic vortexes is also successfully performed by rotating a foam disk as a practical application. Our work opens a new route for generating acoustic vortex and could have potential significance in areas including microfluidics, acoustic tweezers, and ultrasonic communication.

The concept of “rainbow trapping” has generated considerable interest in wave propagation and energy harvesting, offering new possibilities for diverse and efficient acoustic wave operations. In this work, we investigate a dual-channel topological rainbow trapping device implemented within an underwater two-dimensional phononic crystal based on synthetic dimension. The topological edge states with different frequencies are separated and trapped at different spatial locations. Acoustic waves propagate simultaneously along two boundaries due to the degeneracy of the edge states. In particular, the propagation of a dual-channel topological rainbow is also realized by using a bend design. This work contributes to the advancement of multi-channel devices in synthetic space and provides a reference for the design of highly efficient underwater acoustic devices.

Exceptional points (EPs) refer to degeneracies in non-Hermitian systems where two or more eigenvalues and their corresponding eigenvectors coalesce. Recently, there has been growing interest in harnessing EPs to enhance the responsivity of sensors. Significant improvements in the sensitivity of sensors in optics and electronics have been developed. In this work, we present a novel ultrasensitive surface acoustic wave (SAW) gas sensor based on EP. We demonstrate its ability to significantly respond to trace amount of hydrogen sulfide (H2S) gas by tuning additional loss to approach the EP, thereby enhancing the responsivity compared to the conventional delay line gas sensors. In addition to high sensitivity, our sensor is robust to temperature variation and exclusive to H2S gas. We propose an innovative method for designing a new generation of ultrasensitive gas sensor.