1 Sonic crystal
1.1 Topological sonic crystal
1.1.1 Topological sonic crystal with preserved time-reversal symmetry
220.127.116.11 Acoustic topological insulator and robust one-way sound transport
Topological design of materials enables topological symmetries and facilitates unique backscattering-immune wave transport. In airborne acoustics, however, the intrinsic longitudinal nature of sound polarization makes the use of the conventional spin–orbital interaction mechanism impossible for achieving band inversion. The topological gauge flux is then typically introduced with a moving background in theoretical models. Its practical implementation is a serious challenge, though, due to inherent dynamic instabilities and noise. Here we realize the inversion of acoustic energy bands at a double Dirac cone and provide an experimental demonstration of an acoustic topological insulator. By manipulating the hopping interaction of neighbouring ’atoms’ in this new topological material, we successfully demonstrate the acoustic quantum spin Hall effect, characterized by robust pseudospin-dependent one-way edge sound transport. Our results are promising for the exploration of new routes for experimentally studying topological phenomena and related applications, for example, sound-noise reduction.
For details, please read Nature Physics (2016), doi:10.1038/nphys3867
18.104.22.168 Topological phononic states of underwater sound based on coupled ring resonators
We report a design of topological phononic states for underwater sound using arrays of acoustic coupled ring resonators. In each individual ring resonator, two degenerate acoustic modes, corresponding to clockwise and counter-clockwise propagation, are treated as opposite pseudospins. The gapless edge states arise in the bandgap resulting in protected pseudospin-dependent sound transportation, which is a phononic analogue of the quantum spin Hall effect. We also investigate the robustness of the topological sound state, suggesting that the observed pseudospin-dependent sound transportation remains unless the introduced defects facilitate coupling between the clockwise and counter-clockwise modes (in other words, the original mode degeneracy is broken). The topological engineering of sound transportation will certainly promise unique design for next generation of acoustic devices in sound guiding and switching, especially forunderwater acoustic devices.
For details, please read Appl. Phys. Lett. 108, 031904 (2016);
1.1.2 Topological sonic crystal with broken time-reversal symmetry
Recent explorations of topology in physical systems have led to a new paradigm of condensed matters characterized by topologically protected states and phase transition, for example, topologically protected photonic crystals enabled by magneto-optical effects. However, in other wave systems such as acoustics, topological states cannot be simply reproduced due to the absence of similar magnetics-related sound–matter interactions in naturally available materials. Here, we propose an acoustic topological structure by creating an effective gauge magnetic field for sound using circularly flowing air in the designed acoustic ring resonators. The created gauge magnetic field breaks the time-reversal symmetry, and therefore topological properties can be designed to be nontrivial with non-zero Chern numbers and thus to enable a topological sonic crystal, in which the topologically protected acoustic edge-state transport is observed, featuring robust one-way propagation characteristics against a variety of topological defects and impurities. Our results open a new venue to non-magnetic topological structures and promise a unique approach to effective manipulation of acoustic interfacial transport at will.
For details, please read New J. Phys. 17 (2015) 053016;
1.2 Sonic crystal with trivial topological properties
1.2.1 Negative refraction in sonic crystal
22.214.171.124 Negative refraction of sound
Acoustic negative refractions with backward-wave (BW) effects were both theoretically and experimentally established in the second band of a two-dimensional (2D) triangular sonic crystal (SC). Intense Bragg scatterings result in the extreme deformation of the second band equifrequency surface (EFS) into two classes: one around the K point and the other around the Γ point of the reduced Brillouin zone. The two classes can lead to BW negative refractions (BWNRs) but with reverse negative refraction dependences on frequencies and incident angles. Not only BWNR but BW positive refraction can be present at EFSs around the K point, so it is possible to enhance the resolution of acoustic waves with a subdiffraction limit regardless of refractions, which is no analogy in both left-handed material and SCs’ first band. These abundant characters make refractions in the second band distinguished.
For details, please read Phys. Rev. Lett. 96, 014301
126.96.36.199 Negative birefraction of sound
Optical birefringence and dichroism are classical and important effects originating from two independent polarizations of optical waves in anisotropic crystals1. Furthermore, the distinct dispersion relations of transverse electric and transverse magnetic polarized electromagnetic waves in photonic crystals can lead to birefringencemore easily. However, it is impossible for acoustic waves in the fluid to show such a birefringence because only the longitudinal mode exists. The emergence of an artificial sonic crystal (SC) has significantly broadened the range of acoustic materials in nature that can give rise to acoustic bandgaps and be used to control the propagation of acoustic waves. Recently, negative refraction has attracted a lot of attention and has been demonstrated in both left-handed materials and photonic crystals. Similar to left-handed materials and photonic crystals, negative refractions have also been found in SCs. Here we report, for the first time, the acoustic negative-birefraction phenomenon in a two-dimensional SC, even with the same frequency and the same ‘polarization’ state. By means of this feature, double focusing images of a point source have been realized. This birefraction concept may be extended to other periodic systems corresponding to other forms of waves, showing great impacts on both fundamental physics and device applications.
For details, please read Nature Mater. 6, 744 (2007).
1.2.2 One way transmission of acoustic waves
Nonreciprocal wave propagation typically requires strong nonlinear materials to break time reversal symmetry. Here, we utilized a sonic-crystal-based acoustic diode that had broken spatial inversion symmetry and experimentally realized sound unidirectional transmission in this acoustic diode. These novel phenomena are attributed to different mode transitions as well as their associated different energy conversion efficiencies among different diffraction orders at two sides of the diode. This nonreciprocal sound transmission could be systematically controlled by simply mechanically rotating the square rods of the sonic crystal. Different from nonreciprocity due to the nonlinear acoustic effect and broken time reversal symmetry, this new model leads to a one-way effect with higher efficiency, broader bandwidth, and much less power consumption, showing promising applications in various sound devices.
For details, please read Phys. Rev. Lett. 106, 084301 (2011).
1.2.3 Dirac points in sonic crystal
188.8.131.52 Single Dirac cone
In this work, acoustic phase-reconstruction is studied and experimentally demonstrated in a triangular lattice two-dimensional phononic crystal (PnC) composed of steel rods in air. Owning to the fact that two bands of this triangular lattice PnC touch at the K/K′ point and thus give rise to a conical Dirac cone, acoustic wavestransmitting through this PnC can exhibit a pseudo-diffusion transportation feature, producing a reconstructed planar wavefront in the far field away from the interface of the PnC. Such phase reconstruction effect can be utilized in many applications, and here we demonstrate experimentally two important applications: an acoustic collimator and an acoustic cloak operating at a Dirac frequency of 41.3 kHz.
For details, please read Appl. Phys. Lett. 106, 151906 (2015);
184.108.40.206 Double Dirac cone
Artificial honeycomb lattices with Dirac cone dispersion provide a macroscopic platform to study the massless Dirac quasiparticles and their novel geometric phases. In this paper, a quadruple-degenerate state is achieved at the center of the Brillouin zone in a two-dimensional honeycomb lattice phononic crystal, which is a result of accidental degeneracy of two double-degenerate states. In the vicinity of the quadruple-degenerate state, the dispersion relation is linear. Such quadruple degeneracy is analyzed by rigorous representation theory of groups. Using method, a reduced Hamiltonian is obtained to describe the linear Dirac dispersion relations of this quadruple-degenerate state, which is well consistent with the simulation results. Near such accidental degeneracy, we observe some unique properties in wave propagating, such as defect-insensitive propagating character and the Talbot effect.
For details, please read Scientific Reports 4, Article number: 4613 (2014)
1.2.4 Acoustic cloaking in sonic crystal
Zero-refractive-index materials may lead to promising applications in various fields. Here, we design andfabricate a near Zero-Refractive-Index (ZRI) material using a phononic crystal (PC) composed of a square array of densely packed square iron rods in air. The dispersion relation exhibits a nearly flat band across the Brillouin zone at the reduced frequency f = 0.5443c/a, which is due to Fabry-Perot (FP) resonance. By using a retrieval method, we find that both the effective mass density and the reciprocal of the effective bulk modulus are close to zero at frequencies near the flat band. We also propose an equivalent tube network model to explain the mechanisms of the near ZRI effect. This FP-resonance-induced near ZRI material offers intriguing wave manipulation properties. We demonstrate both numerically and experimentally its ability to shield a scattering obstacle and guide acoustic waves through a bent structure.
For details, please read Appl. Phys. Lett. 104, 161904 (2014);
2 Acoustic metamaterials
2.1 One-way transmission of sound based on active acoustic metamaterials
An acoustic asymmetric transmission device exhibiting unidirectional transmission property for acoustic waves is extremely desirable in many practical scenarios. Such a unique property may be realized in various configurations utilizing acoustic Zeeman effects in moving media as well as frequency-conversion in passive nonlinear acoustic systems and in active acoustic systems. Here we demonstrate a new acoustic frequency conversion process in a time-varying system, consisting of a rotating blade and the surrounding air. The scattered acoustic waves from this time-varying system experience frequency shifts, which are linearly dependent on the blade’s rotating frequency. Such scattering mechanism can be well described theoretically by an acoustic linear time-varying perturbation theory. Combining such time-varying scattering effects with highly efficient acoustic filtering, we successfully develop a tunable acoustic unidirectional device with 20 dB power transmission contrast ratio between two counter propagation directions at audible frequencies.
For details, please read Scientific Reports 5, Article number: 10880 (2015)
2.2 Acoustic rainbow trapping based on space-coiling metamaterials
We numerically realize the acoustic rainbow trapping effect by tapping an air waveguide with space-coiling metamaterials. Due to the high refractive-index of the space-coiling metamaterials, our device is more compact compared to the reported trapped-rainbow devices. A numerical model utilizing effective parameters is also calculated, whose results are consistent well with the direct numerical simulation of space-coiling structure. Moreover, such device with the capability of dropping different frequency components of a broadband incident temporal acoustic signal into different channels can function as an acoustic wavelength division de-multiplexer. These results may have potential applications in acoustic device design such as an acoustic filter and an artificial cochlea.
For details, please read Scientific Reports 4, Article number: 7038 (2014)
2.3 Omnidirectional acoustic absorber
We present a design for a two-dimensional omnidirectional acoustic absorber that can achieve 98.6%absorption of acoustic waves in water, forming an effective acoustic black hole. This artificial black holeconsists of an absorptive core coated with layers of periodically distributed polymer cylinders embedded in water. Effective medium theory describes the response of the coating layers to the acoustic waves. Thepolymer parameters can be adjusted, allowing practical fabrication of the absorber. Since the proposedstructure does not rely on resonances, it is applicable to broad bandwidths. The design might be extended to a variety of applications.
For details, please read AIP Advances 3, 102122 (2013);
2.4 Acoustic cloaking
A type of acoustic carpet cloak has been theoretically designed and numerically implemented in air using steel/air composites. By using the effective medium theory, the effective density and bulk modulus of the composite material are designed to agree with the spatially variant parameters calculated from the coordinate transformation approach. Great cloaking performance is achieved as an object is well hidden under a sound reflective surface in a wide frequency range. It has also been shown that sound can be effectively manipulated using the proposed composite materials because of its low complexity.
For details, please read Phys. Lett. A 376, 493 (2012).
3 One-dimensional acoustic structure
3.1 Extraordinary acoustic transmission
3.1.1 Extraordinary transmission of sound in an acoustic grating
Recently, there has been an increased interest in studying extraordinary optical transmission (EOT) through subwavelength aperture arrays perforated in a metallic film. In this Letter, we report that the transmission of an incident acoustic wave through a one-dimensional acoustic grating can also be drastically enhanced. This extraordinary acoustic transmission (EAT) has been investigated both theoretically and experimentally, showing that the coupling between the diffractive wave and the wave-guide mode plays an important role in EAT. This phenomenon can have potential applications in acoustics and also might provide a better understanding of EOT in optical subwavelength systems.
For details, please read Phys. Rev. Lett. 99, 174301
3.1.2 Extraordinary transmission of sound in an acoustic grating with Helmholtz resonance cavities
We investigate both experimentally and numerically a complex structure, where 'face-to-face' Helmholtz resonance cavities (HRCs) are introduced to construct a one-dimensional acoustic grating. In this system, pairs of HRCs can intensely couple with each other in two forms: a bonding state and an anti-bonding state, analogous to the character of hydrogen molecule with two atoms due to the interference of wave functions of sound among the acoustic local-resonating structures. The bonding state is a 'bright' state that interferes with the Fabry–Pèrot resonance mode, thereby causing this state to break up into two modes as the splitting of the extraordinary acoustic transmission peak. On the contrary, the anti-bonding state is a 'dark' state in which the resonance mode remains entirely localized within the HRCs, and has no contribution to the acoustic transmission.
For details, please read CHIN. PHYS. LETT. Vol. 33, No. 4 (2016) 044302
3.2 Acoustic collimation
We demonstrate both theoretically and experimentally the physical mechanism that underlies extraordinary acoustic transmission and collimation of sound through a one-dimensional decorated plate. A microscopic theory considers the total field as the sum of the scattered waves by every periodically aligned groove on the plate, which divides the total field into far-field radiative cylindrical waves and acoustic surface evanescent waves (ASEWs). Different from the well-known acoustic surface waves like Rayleigh waves and Lamb waves, ASEW is closely analogous to a surface plasmon polariton in the optical case. By mapping the total field, the experiments well confirm the theoretical calculations with ASEWs excited. The establishment of the concept of ASEW provides a new route for the integration of subwavelength acoustic devices with a structured solid surface.
For details, please read Phys. Rev. Lett. 104, 164301
3.3 Acoustic Rabi splitting
We design a structure to realize Rabi splitting and Rabi oscillation in acoustics. We develop rigorous analytical models to analyze the splitting effect from the aspect of phase matching, and from the aspect of mode coupling using a coupled mode model. In this model, we discover that the splitting effect is caused by the coupling of the Fabry–Perot fundamental mode with the resonant mode of an artificial acoustic 'atom'. We then extract the coupling strength and analyze the impact of structural parameters on it. In addition, we demonstrate Rabi oscillation in the time domain. Such quantum phenomena in the classical regime may have potential applications in the design of novel ultrasonic devices.
For details, please read New Journal of Physics 16 (2014) 043006