Acoustic metamaterials can manipulate and control sound waves in ways that are not possible in conventional materials. Metamaterials with zero, or even negative, refractive index for sound offer new possibilities for acoustic imaging and for the control of sound at subwavelength scales. The combination of transformation acoustics theory and highly anisotropic acoustic metamaterials enables precise control over the deformation of sound fields, which can be used, for example, to hide or cloak objects from incident acoustic energy. Active acoustic metamaterials use external control to create effective material properties that are not possible with passive structures and have led to the development of dynamically reconfigurable, loss-compensating and parity–time-symmetric materials for sound manipulation. Challenges remain, including the development of efficient techniques for fabricating large-scale metamaterial structures and converting laboratory experiments into useful devices. In this Review, we outline the designs and properties of materials with unusual acoustic parameters (for example, negative refractive index), discuss examples of extreme manipulation of sound and, finally, provide an overview of future directions in the field. Acoustic metamaterials can be used manipulate sound waves with a high degree of control. Their applications include acoustic imaging and cloaking. This Review outlines the designs and properties of these materials, discussing transformation acoustics theory, anisotropic materials and active acoustic metamaterials.

Controlling sound with acoustic metamaterials

Dynamics of Phononic Materials and Structures: Historical Origins, Recent Progress, and Future Outlook

This review provides a perspective on the recent developments in the transmission of light through subwavelength apertures in metal films. The main focus is on the phenomenon of extraordinary optical transmission in periodic hole arrays, discovered over a decade ago. It is shown that surface electromagnetic modes play a key role in the emergence of the resonant transmission. These modes are also shown to be at the root of both the enhanced transmission and beaming of light found in single apertures surrounded by periodic corrugations. This review describes both the theoretical and experimental aspects of the subject. For clarity, the physical mechanisms operating in the different structures considered are analyzed within a common theoretical framework. Several applications based on the transmission properties of subwavelength apertures are also addressed.

http://digital.csic.es/bitstream/10261/47904/1/Light%20passing.pdf

Light passing through subwavelength apertures

Within a time span of 15 years, acoustic metamaterials have emerged from academic curiosity to become an active field driven by scientific discoveries and diverse application potentials. This review traces the development of acoustic metamaterials from the initial findings of mass density and bulk modulus frequency dispersions in locally resonant structures to the diverse functionalities afforded by the perspective of negative constitutive parameter values, and their implications for acoustic wave behaviors. We survey the more recent developments, which include compact phase manipulation structures, superabsorption, and actively controllable metamaterials as well as the new directions on acoustic wave transport in moving fluid, elastic, and mechanical metamaterials, graphene-inspired metamaterials, and structures whose characteristics are best delineated by non-Hermitian Hamiltonians. Many of the novel acoustic metamaterial structures have transcended the original definition of metamaterials as arising from the collective manifestations of constituent resonating units, but they continue to extend wave manipulation functionalities beyond those found in nature.

/pdf/acoustic-metamaterials-from-local-resonances-to-broad-4s4297ihw6.pdf

Acoustic metamaterials: From local resonances to broad horizons.

This article reviews acoustic microfiuidics: the use of acoustic fields, principally ultrasonics, for application in microfiuidics. Although acoustics is a classical field, its promising, and indeed perplexing, capabilities in powerfully manipulating both fluids and particles within those fluids on the microscale to nanoscale has revived interest in it. The bewildering state of the literature and ample jargon from decades of research is reorganized and presented in the context of models derived from first principles. This hopefully will make the area accessible for researchers with experience in materials science, fluid mechanics, or dynamics. The abundance of interesting phenomena arising from nonlinear interactions in ultrasound that easily appear at these small scales is considered, especially in surface acoustic wave devices that are simple to fabricate with planar lithography techniques common in microfluidics, along with the many applications in microfluidics and nanofluidics that appear through the literature.

/pdf/microscale-acoustofluidics-microfluidics-driven-via-5bet39j13p.pdf

Microscale acoustofluidics: Microfluidics driven via acoustics and ultrasonics

Extraordinary lateral beaming of sound from a square-lattice phononic crystal

The thermoelectric power of charge-density-wave conductors Rb0.3MoO3 and Rb0.15K0.15MoO3

Acoustic lens: A thin plate with quasi-periodic array of holes

Higher-order topological phases have raised widespread interest in recent years with the occurrence of the topological boundary states of dimension two or more less than that of the system bulk. The higher-order topological states have been verified in gapped phases, in a wide variety of systems, such as photonic and acoustic systems, and recently also observed in gapless semimetal phase, such as Weyl and Dirac phases, in systems alike. The higher-order topology is signaled by the hinge states emerging in the common band gaps of the bulk states and the surface states. In this Letter, we report our first prediction and observation of a new type of hinge states, the bound hinge states in the continuum (BHICs) bulk band, in a higher-order Weyl semimetal implemented in phononic crystal. In contrast to the hinge state in gap, which is characterized by the bulk polarization, the BHIC is identified by the nontrivial surface polarization. The finding of the topological BHICs broadens our insight to the topological states, and may stimulate similar researches in other systems such as electronic, photonic, and cold atoms systems. Our Letter may pave the way toward high-Q acoustic devices in application.

Acoustic Higher-Order Weyl Semimetal with Bound Hinge States in the Continuum.

We show that by utilizing an anisotropic metamaterials ring with near zero azimuthal component of the density tensor, multiple separated sound sources can overlap each other and appear like only one source. The powers emitted from the different sources within the metamaterials ring can be combined together. Analytical and numerical results show that such ideal combination performance is independent of the positions and the amount of the sources and even obstacles appearing inside the metamaterials ring cannot alter it, exhibiting great robustness. The proposed scheme is potentially useful for applications such as omnidirectional sound sources, high-resolution ultrasonography, and advanced signal modulation.

Manzhu Ke

Papers

Extraordinary lateral beaming of sound from a square-lattice phononic crystal

The thermoelectric power of charge-density-wave conductors Rb0.3MoO3 and Rb0.15K0.15MoO3

Acoustic lens: A thin plate with quasi-periodic array of holes

Acoustic Higher-Order Weyl Semimetal with Bound Hinge States in the Continuum.

Realizing robust overlapped effect of multiple sound sources via anisotropic zero density metamaterials