Development and Applications of Terahertz Near-Field Microscopes for Surface Plasmon Imaging

摘要

The confined nature of surface plasmons (SPs) often imposes challenges on their experimental detection and makes specific near-field probes necessary. While various SP detection methods have been developed in the optical domain, only a few examples of SP imaging have been reported in the terahertz range. In this thesis, specific problems of current terahertz near-field detection systems have been addressed which has led to the development of two new SP imaging methods. In the first method, SP imaging is demonstrated using the integrated subwavelength aperture near-field probe. The photoconductive antenna inside the probe is sensitive to the SP electric-field despite the orthogonal spatial orientation between the antenna and the SP polarisation. This enables SP imaging directly on a metallic surface employing a photoconductive antenna. This unexpected sensitivity has been applied to SP imaging in two examples: first, the SP propagation has been imaged on a resonant THz bow-tie antenna and second, the SP excitation by a strongly focused terahertz beam directly on the metallic probe surface has been investigated. The second method presents an electro-optic micro-resonator for SP imaging. A micro-resonator structure has the potential to provide a better sensitivity and spatial resolution, as well as a lower level of invasiveness compared to bulk crystals, which are commonly used in terahertz near-field systems. The micro-resonator design is explained in detail and the impact of the micro-resonator geometry on the probe performance is discussed. This micro-resonator has then been fabricated and embedded into an electro-optic detection system. This detection system has been fully characterised with the focus on two functional units which are essential for its performance: a tapered parallel plate waveguide for broadband terahertz transmission and the balanced detector for noise reduction. The overall performance of the detection system has been evaluated for its use as a terahertz near-field microscope.