Sub-wavelength Resolution in Ultrasonic waves by hyperbolic metamaterials

Abstract

The research in this dissertation aims at realizing sub-wavelength resolution by newly designed elastic metamaterial lens. Generally, elastic wave based imaging technology has been considered as highly useful imaging method, but its resolution is relatively low compared to other imaging method. The most critical resolution issue in elastic wave based imaging is diffraction limit which forbids resolution over certain limit in general media. Thus, the following question had been risen – how to overcome diffraction limit?. In recent years, a big progress in metamaterial has realized hyperlens which allow resolution over diffraction limit. However, although proposition of hyperlens made a big break-through, researches on hyperlens were mainly focused on electromagnetic waves. Researches on hyperlens for elastic waves have been rare, and the performance of the previously designed elastic hyperlens was still limited. Motivated by this, a new elastic hyperlens that exhibits much improved performance than the previously proposed one is proposed in this work. The study of new elastic hyperlens performed in this work not only deals proposition of the new hyperlens but also covers physical, numerical, experimental and analytic approaches. After reviewing background physics related to diffraction limit and hyperlens, the way to break the previous hyperlens limitation is presented. New elastic metamaterial that satisfies pre-considered design requirements is engineered, and new elastic hyperlens is designed from the metamaterial. To verify the proposed elastic metamaterial and hyperlens, finite element analysis is formulated and numerical wave simulation is performed. For confirmation of performance improvement, numerical simulations with sub-wavelength sources are conducted for both the proposed hyperlens and the previous one, and the results are directly compared. In the experimental approach, the proposed hyperlens is realized in aluminum plate and sub-wavelength resolution of the hyperlens is experimentally shown. To achieve desired sub-wavelength sources, a new elastic wave transducer is developed. During experiments, actuation pulse tailoring and measured signal calibration is introduced to facilitate the experiment. Finally, analytic approach for the elastic metamaterial and hyperlens is performed. Equivalent mass-spring system for the designed metamaterial is constructed, and wave dispersion equation is analytically formulated. From the constructed analytic mass-spring system, design guidelines for further improvement of the proposed elastic hyperlens are shown.