Research on Millimeter-Wave and Submillimeter-Wave Scanning Near-Field Microscopy

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The objective of this research project is to demonstrate the new type of scanning near-field microscopy, i.e. scanning near-field anisotropy microscopy (SNAM) and to quantify images obtained from the scanning near-field microscope system using a metal slit-type probe which we have proposed and developed, in order to visualize physical parameters of objects such as refractive index and refractive index anisotropy, in the millimeter-wave and submillimeter-wave regions. The results obtained in the term of the project follow.In order to quantify images more precisely in the millimeter- and submillimeter-wave regions, we have proposed, designed, and made a new type of sample base to reduce needless signal fluctuations originated from surface modes. We have successfully demonstrated the effectiveness of this new hardware and found that the signal fluctuation reduces to less than 10% of the previous one. This result means that the microscope system we have developed have a potential to give a limiting refractive index sensitivity of better than Δn/n〜0.1. Furthermore, in order to visualize physical parameters of objects, we have proposed an equivalent circuit to describe the interaction between the probe and object, and succeeded to demonstrate the validity of the circuit by computer simulation using a finite element method and by experiments in the millimeter-wave region.We have tried to visualize anisotropy of objects by including an anisotropy model derived from two-dimensional refractive idex ellipsoid in the image reconstruction process using a block-gradient method, which is a kind of iterative reconstruction technique. An isotropic metal patch, a wire gird with large anisotropy, and a LiNbO_3 crystal which is a typical anisotropic dielectric, were experimentally tested in the millimeter-wave region and the images were reconstructed. We have found that anisotropy of these objects are clearly imaged with a resolution of less than 1/10 of the radiation wavelength used.