Abstract
This letter presents the design and experimental demonstration of a gradient metasurface guiding spoof surface acoustic waves (SSAWs) in the manner of a Luneburg lens for sound. By correlating the propagation characteristics of SSAWs with the effective surface acoustic impedance, a straightforward concentric surface structure design is proposed to realize the required refractive index distribution. The results from both simulation and measurement show that grazing incident sound is converted into SSAWs propagating along the metasurface and focusing on the edge of the opposite side of the lens, which may find applications in direction detection and acoustic sensing.
Highlights
The possibility of adjusting the effective refractive index of SSAWs through engineering the surface building blocks have inspired a new direction towards subwavelength sound manipulation and gradient index (GRIN) design for acoustic waves
The Luneburg lens was first proposed in geometrical optics33 more than fifty years ago, in which the parallel rays of incident plane waves from one side are gradually bent to focus on the opposite edge of the circular or spherical lens
We propose a surface-type acoustic Luneburg lens by modulating SSAWs with a gradient metasurface
Summary
Spoof surface acoustic waves (SSAWs) are a type of evanescent waves propagating over structured (e.g., perforated or grooved) surfaces and exhibiting compressed wavelength and structure-governed dispersion. They have attracted increasing attention in the past decade and have been extensively investigated for various associated effects and functionalities such as extraordinary acoustic transmission, beam collimation, directional or anisotropic wave guiding, and subwavelength focusing and imaging. Since the wave field of such airborne surface mode can be readily measured in a non-invasive manner, structures supporting SSAWs appear as an ideal testbed for the direct observation of many intriguing physical phenomena. In parallel, the possibility of adjusting the effective refractive index of SSAWs through engineering the surface building blocks have inspired a new direction towards subwavelength sound manipulation and gradient index (GRIN) design for acoustic waves. Spoof surface acoustic waves (SSAWs) are a type of evanescent waves propagating over structured (e.g., perforated or grooved) surfaces and exhibiting compressed wavelength and structure-governed dispersion.. Spoof surface acoustic waves (SSAWs) are a type of evanescent waves propagating over structured (e.g., perforated or grooved) surfaces and exhibiting compressed wavelength and structure-governed dispersion.1–3 They have attracted increasing attention in the past decade and have been extensively investigated for various associated effects and functionalities such as extraordinary acoustic transmission, beam collimation, directional or anisotropic wave guiding, and subwavelength focusing and imaging.. The possibility of adjusting the effective refractive index of SSAWs through engineering the surface building blocks have inspired a new direction towards subwavelength sound manipulation and gradient index (GRIN) design for acoustic waves.
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