Phys Rev Lett. 2014 ;112(14):144301. García-Chocano VM1, Christensen J2, Sánchez-Dehesa J1.
1Wave Phenomena Group, Department of Electronics Engineering, Universitat Politècnica de València, Camino de Vera s.n. (Edificio 7F), ES-46022 Valencia, Spain.and
2Department of Photonics Engineering, Technical University of Denmark, Orsteds Plads, Building 343, DK-2800 Kongens Lyngby, Denmark.
This Letter reports the design, fabrication, and experimental characterization of hyperbolic materials showing negative refraction and energy funneling of airborne sound. Negative refraction is demonstrated using a stack of five holey Plexiglas plates where their thicknesses, layer separation, hole diameters, and lattice periodicity have been determined to show hyperbolic dispersion around 40 kHz. The resulting hyperbolic material shows a flat band profile in the equifrequency contour allowing the gathering of acoustic energy in a broad range of incident angles and its funneling through the material. Our demonstrations foresee interesting developments based on both phenomena. Acoustic imaging with subwavelength resolution and spot-size converters that harvest and squeeze sound waves irradiating from many directions into a collimated beam are just two possible applications among many.
Demonstration of negative refraction and energy funneling based on hyperbolic acoustics materials was not reported yet. In comparison with previous predictions this work shows that such phenomena can be obtained using structures with dimensions of the order of the impinging sound. This article shows that a very simple structure consisting of a multilayer stack of holey plates is able to manipulate the sound propagation in an ingenious manner. An interesting phenomena related to the hyperbolic band profile is the squeezing and collimation of the beam, which travels ballistically through the sample material. The phenomena here described are non-resonant, consequently the bandwidth is not strongly limited.
Figure Legend: “Experimental set up employed in the characterization of the hyperbolic material sample, which consist of 9 holey plates in this photo”.