| Summary: | 碩士 === 國立中央大學 === 光電科學研究所 === 100 === A conventional optical lens cannot be used to break the diffraction limit, but an appropriately designed metamaterial superlens can do the job, and this fact leads to the realization of subwavelength imaging. Although a superlens can indeed form subwavelength image through reconstructing the information carried by the evanescent waves of the source, the image can only be found in the near field zone, which restricts the practicality of this component. After the idea of superlens has been proposed, scientists further developed other novel devices in order to resolve the disadvantage of near field imaging, hoping to transfer evanescent waves to propagating waves and reconstruct image in the far field zone, which will be more convenient for further manipulation by conventional optical devices. The hyperlens structure discussed in this thesis is a kind of design for fulfilling this purpose.
The basic structure of hyperlens consists of two kinds of materials having permittivities of different signs (usually they are metals and dielectrics), arranging alternatively as cylindrical multilayer structure. For the H-polarized waves, this structure can form a magnified image outside the hyperlens in the far field zone for a subwavelength object close to the inner surface of this device.
We calculate the light fields by using the transfer matrix method. Besides, we have developed a ray-tracing technique for predicting the location of the images according the geometric optics. Based on the results obtained by these two methods, we further discuss the change of imaging characteristics under the influence of changing the layer thickness. By defining the effective width of the single source image via calculating the standard deviation of the field strength distribution, we can quantify the influence of the side-lobes of the image field. We also explore the influence of the side-lobes to the two images of the two sources and the least-distinguishable distance between them.
According to our simulation results, oscillating behaviors are observed for imaging characteristics of the hyperlens when we vary the thickness of the layers. This oscillating behavior conflicts to our expectation that a thicker hyperlens is also a better one. This phenomenon provides us useful and important information for designing an ideal hyperlens.
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