Liquid Refractive Index Measurements using a Terahertz Vector Network Analyser

Abstract

A lack of coherent sources has led the terahertz region of the electromagnetic spectrum to remain relatively unexplored as compared to its neighbouring frequencies. Over recent decades the development of these sources along with coherent detectors has led to the rapid development of terahertz technologies. To characterise and use materials within this region a database of material knowledge must be built up.

This thesis aims to outline various methods of material characterisation using a vector network analyser. It considers and overcomes issues to do with data collection using the vector network analyser within the frequency domain and secondly problems due to strong absorptions associated with water based samples within the terahertz region.

Two novel cuvette measurement methods are presented which use rotational stages to enable the vector network analyser to extract material data. A self-referencing technique rotating the test liquid and another self-referencing technique using a known refractive index reference liquid are shown. The latter is a fast and precise measurement technique which is as accurate as the known reference and could potentially be used for in-flow testing.

The use of paper at terahertz frequencies is explored with a particular interest in the use of paper for sensing. A range of metamaterial designs are simulated, fabricated and tested to develop a metamaterial device which can be used for liquid sensing. It was found that soaked filter paper in contact with the metamaterial surface is able to distinguish between different soaking liquids.

Serum samples of seven different tumour markers have been tested to determine if the metamaterial in contact with serum soaked filter paper could provide an indication of tumour marker concentration. The outcome of this feasibility study suggested possible relations between absorption peak position frequency and marker concentration for five of the seven tested markers.