A high transmission broadband gradient index lens using elastic shell acoustic metamaterial elements

Abstract

The use of cylindrical elastic shells as elements in acoustic metamaterial devices is demonstrated through simulations and underwater measurements of a cylindrical-to-plane wave lens. Transformation acoustics (TA) of a circular region to a square dictates that the effective density in the lens remain constant and equal to that of water. Piecewise approximation to the desired effective compressibility is achieved using a square array with elements based on the elastic shell metamaterial concept developed in [30]. The size of the elements are chosen based on availability of shells, minimizing fabrication difficulties. The tested device is neutrally buoyant comprising 48 elements of nine different types of commercial shells made from aluminum, brass, copper, and polymers. Simulations indicate a broadband range in which the device acts as a cylindrical to plane wave lens. The experimental findings confirm the broadband quadropolar response from approximately 20 to 40 kHz, with positive gain of the radiation pattern in the four plane wave directions.

Looking Into Surface Plasmon Polaritons Guided by the Acoustic Metamaterials

“…Aiming to have a deeper insight into the properties of the structure under investigation the geometric parameters along with the effective permittivity can be tailored according to the elastic features of the scatterers. These might include either the medium preference [5,6], or an some external factors, i.e., either electric field [7] or temperature [8].…”

Gradient index phononic crystals and metamaterials

“…For instance, the effective index obviously depends on the filling ratio of scatterers, namely the size of scatterers, which is initially proposed for gradient index control of acoustic waves [17][18][19]; instead, the variation in the lattice spacing while keeping the size of scatterers is also achievable [20,21]; for triangular shape of scatterers, rotating the angles of the triangular shape can also affect the effective acoustic velocity [22]; for lead-rubber pillared metamaterial plate, by changing the height of lead layer in pillars, the effective mass density is tuned, resulting in a change in effective phase velocity following a given law [23];for phononic crystal plates, the effective velocity of antisymmetric Lamb wave is directly related with the thickness of plates, which can also further affect the effective velocity of symmetric mode [24][25][26]. In addition to the geometric parameters, the effective refractive index can also be tuned in relation to the elastic properties of the scatterers, for instance the choice of materials [18,27], or to an external stimulus such as electric field[28], or temperature [29].The attained effective refractive index of the GRIN device can be smaller or larger than that of the background medium, which corresponds to phase advance [20] or delay approaches in wave propagation. Nevertheless, the phase delay approach is mostly employed as higher refractive index 3 enables to design more advanced functionalities and reduce the entire width of the devices.Comparing to the wavelength, the thinner is the device, the larger will be the highest required index.If the width of the whole device is downscaled to the subwavelength regime, the GRIN device becomes a metasurface [30,31].…”

“…GRIN devices can also be designed with negative index of refraction at frequencies laying in the first negative slope of the acoustic band structure [21]. However, these devices would be narrow band and it may limit potential applications.GRIN phononic crystals and metamaterials can be applied to various types of waves in a long frequency limit, such as surface water waves [35,36] for a few Hz, air-borne sound waves [19,[37][38][39][40][41][42][43][44][45][46] for 10 3 Hz-10 5 Hz, water-borne acoustic waves [20,27,[47][48][49] for 10 4 Hz-10 6 Hz, Rayleigh waves[50-54] for 10Hz-10 8 Hz, Lamb waves [23-25, 28, 55-67] for 10 3 -10 8 Hz, among others, with functionalities like focusing [18,19], waveguiding [65,68], mirage[69], beam splitting or deflection [36,57,58], cloaking [50,59] or energy harvesting [66,67,70]. It is in the domain of…”

“…for phononic crystal plates, the effective velocity of antisymmetric Lamb wave is directly related with the thickness of plates, which can also further affect the effective velocity of symmetric mode [24][25][26]. In addition to the geometric parameters, the effective refractive index can also be tuned in relation to the elastic properties of the scatterers, for instance the choice of materials [18,27], or to an external stimulus such as electric field[28], or temperature [29].…”

“…For instance, the effective index obviously depends on the filling ratio of scatterers, namely the size of scatterers, which is initially proposed for gradient index control of acoustic waves [17][18][19]; instead, the variation in the lattice spacing while keeping the size of scatterers is also achievable [20,21]; for triangular shape of scatterers, rotating the angles of the triangular shape can also affect the effective acoustic velocity [22]; for lead-rubber pillared metamaterial plate, by changing the height of lead layer in pillars, the effective mass density is tuned, resulting in a change in effective phase velocity following a given law [23]; for phononic crystal plates, the effective velocity of antisymmetric Lamb wave is directly related with the thickness of plates, which can also further affect the effective velocity of symmetric mode [24][25][26]. In addition to the geometric parameters, the effective refractive index can also be tuned in relation to the elastic properties of the scatterers, for instance the choice of materials [18,27], or to an external stimulus such as electric field [28], or temperature [29].…”