Bifunctional acoustic metamaterial lens designed with coordinate transformation (original) (raw)

Three-dimensional Ultrathin Planar Lenses by Acoustic Metamaterials

Scientific Reports, 2014

Acoustic lenses find applications in various areas ranging from ultrasound imaging to nondestructive testing. A compact-size and high-efficient planar acoustic lens is crucial to achieving miniaturization and integration, and should have deep implication for the acoustic field. However its realization remains challenging due to the trade-off between high refractive-index and impedance-mismatch. Here we have designed and experimentally realized the first ultrathin planar acoustic lens capable of steering the convergence of acoustic waves in three-dimensional space. A theoretical approach is developed to analytically describe the proposed metamaterial with hybrid labyrinthine units, which reveals the mechanism of coexistence of high refractive index and well-matched impedance. A hyperbolic gradient-index lens design is fabricated and characterized, which can enhance the acoustic energy by 15 dB at the focal point with very high transmission efficiency. Remarkably, the thickness of the lens is only approximately 1/6 of the operating wavelength. The lens can work within a certain frequency band for which the ratio between the bandwidth and the center frequency reaches 0.74. By tailoring the structure of the metamaterials, one can further reduce the thickness of the lens or even realize other acoustic functionalities, opening new opportunity for manipulation of low-frequency sounds with versatile potential.

Funneled focusing of planar acoustic waves utilizing the metamaterial properties of an acoustic lens

Metamaterial acoustic lenses are acoustic devices based on phononic crystal structures that take advantage of negative or near-zero indices of refraction. These unique properties arise due to either the antiparallel direction of the phase and group velocity or strongly anisotropic dispersion characteristics, usually above the first transmission band. In this study, we utilize an FDTD program to examine two phononic lenses that utilize anisotropic effects available in their second band to collimate and focus acoustic waves from a plane-wave source with a k00 wavevector. The phononic crystals consist of stainless steel rods arranged in a square lattice with water as the ambient material. Results show collimation and focusing in the second band for select frequencies, fc ± 0.005 𝑓𝑐.

Ultrasound beam steering with flattened acoustic metamaterial Luneburg lens

Applied Physics Letters

We report ultrasound beam steering based on 2D and 3D flattened acoustic metamaterial Luneburg lenses at 40 kHz. The effective properties of the lenses are obtained by using the quasi-conformal transformation (QCT) technique and solving the Laplace equation with Dirichlet and Neumann boundary conditions. A 2D lens and a 3D lens were designed and

Multifunction acoustic modulation by a multi-mode acoustic metamaterial architecture

Journal of Physics Communications

Exotic acoustical features, like acoustic transparency, ultrasonic beam focusing, acoustic band gap and super lensing capability using a single metamaterial architecture is unconventional and unprecedented in the literature, demonstrated herein. Conventional metamaterials can focus an ultrasonic beam at specific frequency which results into unwanted distortion of the output wave fields at neighboring sonic frequencies in the host medium. However, ultrasonic wave focusing by virtue of negative refraction and simultaneous transparency of the metamaterial at sonic frequencies are uncommon due to their frequency disparity. To circumvent this problem and to avoid the unwanted distortion of wave at sonic frequencies, metamaterial with an array of butterfly-shaped thin ring resonators are proposed to achieve the beam focusing at ultrasonic frequency (37.3 kHz) and keep the structure transparent to the sonic frequencies (<20 kHz). The butterfly metamaterial with local ring resonators or butterfly crystals (BC) were previously proposed to create wide band gaps (∼7 kHz) at ultrasonic frequencies above 20 kHz. However, in this study a unique sub-wavelength scale wave focusing capability of the butterfly metamaterial utilizing the negative refraction phenomenon is demonstrated, while keeping the metamaterial block transparent to the propagating wave at lower sonic frequencies below the previously reported bandgaps.

An acoustic beam shifter with enhanced transmission using perforated metamaterials

EPL (Europhysics Letters), 2015

We experimentally demonstrate an acoustic beam shifter with enhanced transmission based on subwavelength perforated metamaterials with a wide working frequency range from 2.8 to 4.6 kHz. An oblique perforation angle allows a flexible beam shifting distance and negative refraction for one side of incidence angles. While the beam shifting action is broadband due to the geometric nature of design, beam shifting with enhanced efficiency is found at the frequency with Fabry-Pérot (FP) resonance through a two-dimensional pressure field mapping. Such a method in combining extraordinary transmission and beam shifting with properly designed metamaterials, enables designing flexible and also transformation acoustic devices with high transmission efficiency in a general context. Introduction. -Inspired by the extraordinary optical transmission with plasmonic structures in electromagnetism, extraordinary transmission properties of acoustic waves through narrow slits and holes have been undergoing extensive investigations, through both numerical simulations and experiments . This counterintuitive acoustic phenomenon usually stems from resonances, e.g. Fabry-Pérot (FP), periodic-lattice and elastic Lambmode resonances. However, the narrow bandwidth and lossy properties arising from resonance may hinder further applications . There have been recent efforts in searching additional mechanisms of extraordinary transmission without relying heavily on resonances. In such a case, acoustic metamaterials have the advantage of their design flexibility and geometric tunability . For instance, a Brewster-like angle has been found for extraordinary acoustic transmission where the impedance at a particular incidence angle is matched to air . On the other hand, acoustic metamaterials are tightly linked to a series of devices in achieving exotic phenomena including cloaking and super resolution imaging , which (a)

Acoustic metamaterials for sound focusing and confinement

New Journal of Physics, 2007

We give a theoretical design for a locally resonant two-dimensional cylindrical structure involving a pair of C-shaped voids in an elastic medium which we term as double 'C' resonators (DCRs) and imbedded thin stiff bars, that displays the negative refraction effect in the low frequency regime. DCRs are responsible for a low frequency band gap which hybridizes with a tiny gap associated with the presence of the thin bars. Using an asymptotic analysis, typical working frequencies are given in closed form: DCRs behave as Helmholtz resonators modeled by masses connected to clamped walls by springs on either side, while thin bars behave as a periodic bi-atomic chain of masses connected by springs. The discrete models give an accurate description of the location and width of the stop band in the case of the DCR and the first two dispersion bands for the periodic thin bars. We then combine our asymptotic formulae for arrays of DCR and thin-bars to design a composite structure that displays a negative refraction effect and has a negative phase velocity in a frequency band, and thus behaves in many ways as a negative refractive acoustic medium (NRAM). Finite element computations show that at this frequency, a slab of such NRAM works as a phononic flat superlens whereas two corners of such NRAM sharing a vertex act as an open resonator and can be used to confine sound to a certain extent.

Three-dimensional acoustic lenses with axial symmetry

Applied Physics Letters, 2010

In this paper a technique to design three dimensional (3D) devices to focus acoustic waves composed of scattering elements is proposed. The devices are designed and optimized in two dimensions (2D) with the help of a genetic algorithm and the 2D multiple scattering formalism. The transition from 2D to 3D is made by applying a rotation operation to the optimized design, thus passing from a set of 2D circular scatters to their equivalent 3D concentric rings of circular section and finite dimensions, considerably improving its performance. The method has been applied to the design and theoretical characterization of a single-focus acoustic lens and a tunable lens capable of changing the focal length with frequency. A prototype lens was fabricated using aluminum rings clamped to a rigid frame, obtaining a good agreement between theory and experiment.