Acoustic metamaterial design and applications

metamaterial;Acoustics;negative index

Zhang, Shu

关键词: metamaterial;    Acoustics;    negative index;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/16524/2_Zhang_Shu.pdf?sequence=3&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The explosion of interest in metamaterials is due to the dramatically increased manipulationability over light as well as sound waves. This material research was stimulated by theopportunity to develop an artificial media with negative refractive index and the application in superlens which allows super-resolution imaging. High-resolution acoustic imaging techniques are the essential tools for nondestructive testing and medical screening. However, the spatial resolution of the conventional acoustic imaging methods is restricted by the incident wavelengthof ultrasound. This is due to the quickly fading evanescent fields which carry the subwavelengthfeatures of objects. By focusing the propagating wave and recovering the evanescent field, a flat lens with negative-index can potentially overcome the diffraction limit. We present the first experimental demonstration of focusing ultrasound waves through a flat acoustic metamaterial lens composed of a planar network of subwavelength Helmholtz resonators. We observed a tight focus of half-wavelength in width at 60.5 KHz by imaging a point source. This result is inexcellent agreement with the numerical simulation by transmission line model in which wederived the effective mass density and compressibility. This metamaterial lens also displaysvariable focal length at different frequencies. Our experiment shows the promise of designing compact and light-weight ultrasound imaging elements.Moreover, the concept of metamaterial extends far beyond negative refraction, rather givingenormous choice of material parameters for different applications. One of the most interesting examples these years is the invisible cloak. Such a device is proposed to render the hidden object undetectable under the flow of light or sound, by guiding and controlling the wave path through an engineered space surrounding the object. However, the cloak designed by transformation optics usually calls for a highly anisotropic metamaterial, which make the experimental studiesremain challenging. We present here the first practical realization of a low-loss and broadband acoustic cloak for underwater ultrasound. This metamaterial cloak is constructed with a network of acoustic circuit elements, namely serial inductors and shunt capacitors. Our experiment clearly shows that the acoustic cloak can effectively bend the ultrasound waves around the hidden object, with reduced scattering and shadow. Due to the non-resonant nature of the buildingelements, this low loss (~6dB/m) cylindrical cloak exhibits excellent invisibility over a broadfrequency range from 52 to 64 kHz in the measurements. The low visibility of the cloaked objectfor underwater ultrasound shed a light on the fundamental understanding of manipulation,storage and control of acoustic waves. Furthermore, our experimental study indicates that this design approach should be scalable to different acoustic frequencies and offers the possibility for a variety of devices based on coordinate transformation.