Terahertz evanescent field imaging and sensing for biological application

Freer, Suzanna (2022). Terahertz evanescent field imaging and sensing for biological application. University of Birmingham. Ph.D.

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Abstract

The terahertz regime sits between the infrared and microwave frequencies. Termed the ‘terahertz gap’, terahertz generation and detection technology remains underdeveloped compared to its bounding frequency bands. Since the first terahertz image in the early 1990s, terahertz has experienced an explosion of interest and development of technology, for biological and medical applications in particular. The regime is rich with spectral fingerprints of biological molecules, with vibrational, rotational and librational modes with terahertz eigenfrequencies. It is non-ionising, in contrast to medical imaging technologies such as x-rays. Meanwhile, it is highly sensitive to water content, a property which can be exploited given that 70% of the human body is composed of water. These properties make terahertz a highly attractive frequency regime for imaging. There is, however, a drawback: a mismatch between terahertz wavelengths and the dimensions of biological analytes such as cells. Hence, efforts have been focused on enhancing interactions with samples. This can be achieved through evanescent fields.

This thesis presents the exploitation of the large water absorption experienced by terahertz radiation for biological sensing and quantitative imaging. The work sets out to improve upon standard time domain spectroscopy (TDS) through evanescent field subwavelength detection. The work involves characterisation of the TDS measurement system using beam profiling techniques, unveiling previously neglected nuances in the terahertz beam. Attention is turned to the extraction of quantitative information from images, through the development of several algorithms, to reliably retrieve the dielectric properties of complex multilayer samples (heterotopic ossification bone slices). The implementation of the algorithms stems from the desire to overcome widely known and problematic artifacts in extracted dielectric properties. Finally, evanescent fields are exploited for enhanced sensing and imaging beyond standard TDS capabilities. This involved design, fabrication and characterisation
of structures supporting evanescent surface waves, demonstrating great potential as highly sensitive liquid sensing platforms for biological applications. Highly efficient and broadband field coupling to a surface wave structure is presented, alongside investigations of evanescent
field mechanisms

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):