Terahertz evanescent field imaging and sensing for biological application

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 mechanism

Loss Characteristics of TeraHertz Surface Waves on Laser Micromachined Textured Metals

For the application of geometrically-induced THz surface wave technology for communication and sensing, a critical analysis of the propagation characteristics (i.e. dispersion and attenuation) for different textured surfaces should be studied and benchmarked. For the broadband characterisation of archetypal textured surfaces (e.g. corrugated plane, two-dimensional array of blind holes and bed of nails) supporting THz transverse magnetic (i.e., p-polarized) surface waves, we employ time-domain spectroscopy and edge-diffraction coupling methods. Measurements of laser micromachined prototypes demonstrate strong frequency-dependent dispersion and the large impact that surface roughness of the order of few μm has on the path loss, increasing it by a factor ranging from 1.6 to 4.3 compared to smooth textured surfaces. Together with numerical modelling, we disentangle all loss mechanisms (namely, ohmic, scattering, propagation divergence and phase mismatch) and highlight the challenge of loss estimation due to surface roughness in highly confined THz surface waves

Revealing the underlying mechanisms behind TE extraordinary THz transmission

Transmission through seemingly opaque surfaces, so-called extraordinary transmission, provides an exciting plat- form for strong light–matter interaction, spectroscopy, optical trapping, and color filtering. Much ofthe effort has been devoted to understanding and exploiting TM extraordinary transmission, while TE anomalous extraordinary transmission has been largely omitted in the literature. This is regrettable from a practical point ofview since the stronger dependence ofthe TE anomalous extraordinary transmission on the array’s substrate provides additional design parameters for exploitation. To provide high-performance and cost-effective applications based on TE anomalous extraordinary transmission, a complete physical insight about the underlying mechanisms ofthe phe- nomenon must be first laid down. To this end, resorting to a combined methodology including quasi-optical terahertz (THz) time-domain measurements, full-wave simulations, and method ofmoments analysis, subwave- length slit arrays under s-polarized illumination are studied here, filling the void in the current literature. We believe this work unequivocally reveals the leaky-wave role of the grounded-dielectric slab mode mediating in TE anomalous extraordinary transmission and provides the necessary framework to design practical high-performance THz components and systemEngineering and Physical Sciences Research Council (EP/L015331/1, EP/S018395/1, 2137478); Russian Foundation for Basic Research (18-29-20066); Ministerio de Ciencia, Innovación y Universidades (RTI2018-094475-B-I00); Royal Society (RSG/R1/180040); University of Birmingham (Birmingham Fellowship); Ministerio de Ciencia, Innovación y Universidades (TEC2017-84724-P)