Anisotropic and Negative Acoustic Index Metamaterials
Microstructured materials are used in material science and engineering to attain desired material properties. Acoustic metamaterials are a rapidly growing area in this field of engineered materials that use deep subwavelength microstructures to attain exotic acoustic properties unavailable in nature. These properties, such as negative acoustic index, allow unprecedented capabilities such as sub-diffraction limit resolution, which have the potential to greatly improve existing technologies like high intensity focused ultrasound, sound shielding, and ultrasonic imaging, as well as create the entirely new ones like acoustic cloaking and energy trapping. Two acoustic material properties were pursued in this dissertation, anisotropic density and negative acoustic index.
Anisotropic density in a fluid acoustic metamaterial was pursued for use in a 2D acoustic hyperlens. Originally developed to image sub-diffraction limit electromagnetic information in the far-field, the theory behind the hyperlens imaging method in acoustics is presented and the performance is shown to be directly related to the density anisotropy magnitude. An acoustic metamaterial composed of alternating deep subwavelength layers of air and brass is then shown theoretically and numerically to attain the necessary density anisotropy, with a three order of magnitude difference in component amplitudes. Experiments using a 2D cylindrical acoustic hyperlens constructed from this metamaterial confirm a significant portion of the evanescent components are converted into propagating components at the lens exit, resulting in far-field acoustic imaging of information far below the diffraction limit.
Development of a negative acoustic index fluid metamaterial was motivated by the reversed wave phenomena leading to a perfect lens. Negative bulk modulus and density are shown in simulations to result from the combination of Helmholtz and rod-spring resonators, which creates a band in the power ultrasound frequency range with negative real acoustic index components. Following a several design changes to accommodate the available experimental facilities, samples of this metamaterial were tested and indeed found to attain a negative real component of acoustic index. However, intrinsic loss and design restrictions led to a small real to imaginary index component ratio, severely limiting the application prospects of this metamaterial. Work on active acoustic metamaterials to overcome these and other problems follows, with simulations indicating piezoelectric elements in water may be used as gain elements to amplify damped waves or as direct replacements for passive resonators that both combat loss and broaden the bandwidth of anomalous properties.