The interaction of electromagnetic radiation with matter has led to a large number of interesting applications. The propagation of electromagnetic waves within materials is described by Maxwell’s equations. However, the fundamental understanding of the causes of the response of the material, defined by constitutive relations for its complex, frequency-dependent dielectric constant, can only be achieved through the study of processes occurring at the molecular scale. The fluctuation-dissipation theorem relates the frequency-dependent dielectric constant of a material to equilibrium fluctuations in its dipole moment. This fact can be used to determine dielectric properties from equilibrium molecular dynamics simulations for frequencies covering the microwave region of the electromagnetic spectrum (300 MHz – 300 GHz). In this work, the ability of current force fields to predict dielectric spectra of one component systems and mixtures is examined, showing accurate results when compared with experimental data for the systems under consideration. Additionally, the influence of temperature on the dielectric spectra is analysed, yielding equally satisfactory results. In the particular case of ethanol/water mixtures, the estimation of dielectric spectra at intermediate concentrations using molecular dynamics simulations outperforms the traditional use of mixing rules. The simulations of these systems reveal the importance of collaborative processes between groups of molecules, such as hydrogen bond networks, in the overall dielectric response. The reduction of the contribution of these processes as temperature increases confirms the weakening of these networks at high temperatures. The predicted dielectric properties are used in a heating model to estimate temperature profiles in microwave heating processes. Unexpected results are obtained which reveal the need for accurate determination of the electric field distribution within the workload in order to obtain representative heating profiles. In contrast, penetration depths are accurately determined from dielectric properties generated through molecular simulations.

An important class of problems within radiation (e.g. neutrons, photons) transport are those in which the radiation migrates through a medium which has a random or stochastic composition. The uncertain medium composition introduces an uncertainty in the radiation angular/scalar flux, current, reaction rates and other quantities of interest. Stochastic media play an important role within radiation transport and have numerous applications such as radiation shielding, nuclear criticality assessment, as well as radiative transfer in clouds, stellar atmospheres and plasma physics. Stochastic radiation transport problems reduce to treating the adsorption, scatter and other macroscopic cross-section data as spatially correlated random fields. Several methods for the treatment of these uncertain-ties have been proposed however they are limited in their scope and computational efficiency. Multiphase and spatially random media are often characterised by non-Gaussian random fields which are much more challenging to model than Gaussian random fields. This thesis aims to investigate, develop and implement mathematically rigorous computational algorithms that are more efficient than the current methods for solving radiation transport with multiphase and spatially random media. In particular this thesis applies iso-probabilistic (e.g. Nataf) transforms to transform Gaussian random fields into non-Gaussian random fields. This approach enables the use of optimal spectral stochastic representations, such as the Karhunen-Loève and generalized polynomial chaos methods, to be used to simulate non-Gaussian random fields. This thesis also describes the verification of these iso-probabilistic spectral stochastic projection methods against standard radiation transport in random media benchmarks, such as the widely used Adams-Levermore-Pomraning benchmark. This thesis is the first time the general Nataf method has been applied to model radiation transport through multiphase and spatially random media.

The unique properties of terahertz (THz) radiation make it useful for application in the field of imaging. The use of THz time-domain spectroscopy (THz-TDS) systems for the identification and characterisation of various materials is becoming more widespread, with this technology being applied in industry, security and scientific research. The common method for generating THz in commercial TDS systems is through the use of photoconductive antennas (PCA's). These are semiconductor-based devices capable of generating several W of power. PCA's require an external bias to generate a strong electric field across a small electrode gap. However, the high fields generated cause electromigration of the electrode metals and can result in damage to the antenna. This thesis investigates the lateral photo-Dember (LPD) effect; a method of generating THz radiation from a semiconductor without the need for an applied electrical bias. The LPD effect relies on the difference in mobilities between electrons and holes to create opposing dipoles parallel to the semiconductor surface. The selective suppression of dipoles formed underneath a metal mask due to reflection then introduces the asymmetry required for observable THz radiation. In this thesis the generation mechanism behind the LPD effect is investigated using 1D and 2D models, as well as the effect of the presence of a metal mask to suppress dipoles. The output from LPD emitters is enhanced through multiplexing and two different designs are investigated and fully characterised. The novel double-metal multiplexed emitters are then used to demonstrate THz beam shaping and focusing through control of the optical pump beam and a lensless THz-TDS system is proposed.

Optical cooling is a well-established class of methods for reducing the temperature of a wide variety of atomic, mechanical and condensed matter systems. In this thesis, we demonstrate that it may be possible to cool the fluctuations of the Josephson phase in a cuprate single crystal by the application of coherent terahertz radiation. Such cooling strengthens the superconducting state as measured by switching current distribution, and may lead to an enhancement of the transition temperature Tc We develop a parametric cooling scheme for bilayer cuprates in which intense terahertz driving produces a modulation in the dielectric material properties. This modulates the coupling between interlayer and intralayer Josephson plasmons which we exploit to upconvert thermal fluctuations of the Josephson phase and reduce their effective temperature. We quantify this as the suppression of the fluctuations in both position and momentum quadratures, and additionally via simulated switching current measurements. We predict a reduction in temperature of 30% given realistic material parameters, and identify the cuprates YBCO and TBCCO-2201 as candidate materials in which this effect may be observed experimentally. With a view to developing cavity cooling schemes, we study the coupling between the Josephson modes and the electromagnetic modes of an external cavity. Following a review of the literature, we examine in detail a specific microcavity array geometry which is expected to provide a large coupling through minimisation of the mode volume. Numerical calculations using finite element methods and an effective dielectric model suggest that strong coupling with g / ωp ~ 0.2 can be achieved using this geometry.

Theoretical and experimental results are reported for the dispersion relations found for electromagnetic wave propagation along a coaxial line partially filled with an annular plasma column. Using the slow wave approximation it had been predicted by Napoli and Swartz and Pinder and Foulds that such a structure should support a backward wave and that this backward wave would be virtually unaffected if the inner conductor is withdrawn. The exact analysis which is here reported shows that whilst the backward wave is present without the inner conductor it is no longer present when the inner conductor is inserted. The absence of the backward wave in the coaxial line was also shown by the experiment that was performed. The experiment was performed with a novel standing wave detector which was developed for this purpose. For the theoretical aspect of this work a technique for solving certain types of differential equations was developed. The theory when applied to a multi-layered model of a plasma also failed to show the presence of the backward wave for the coaxial line arrangement.

Ionising radiation is often useful to our society, and has been implemented for medicine, industry, energy generation and defence. However, nuclear and radiation accidents have the capacity to have a negative impact upon humans and may have a long-lasting legacy due to challenges associated with remediation and the slow decay of radionuclides. It is therefore a priority to ensure that there is adequate emergency preparedness, to prevent and manage any accidental release of ionising radiation to the communities that surround nuclear installations (NI). The emergency planning process includes desktop studies and exercises which are designed to examine the impact of hypothetical scenarios in real-time. However, greater spatiotemporal realism is required to understand the scale of a hypothetical radiation exposure to specific populations in space and time, to anticipate how the behaviour of the population will affect the outcome of an emergency, and to determine the strategy required for its management. This thesis presents a new modelling framework for radiation protection, called RADPOP. This framework combines spatiotemporal aggregate population density subgroup estimates with radionuclide plume dispersal modelling and agent-based modelling, to begin to understand how changes in spatiotemporal population density can influence the likelihood of exposure. Whilst sophisticated estimates of meteorological and atmospheric dispersal exist, there limited resources for the production of equivalent and contemporary high-resolution spatiotemporal population statistics. There is also no existing modelling framework for radiation protection and emergency preparedness, which implements spatiotemporal population estimates to understand the subsequent movement of an aggregate population, as it seeks shelter during a radiation emergency. This thesis investigates these challenges with focus upon the female population subgroup, as a group which has been identified in the literature as having greater vulnerability during evacuation.

Spatial dispersion is the effect where media respond not only to a signal at one particular point, but to signals in an area around that point. While temporal dispersion is a well studied topic, spatial dispersion is relatively unexplored. This thesis investigates the behaviour of electromagnetic waves in spatially dispersive, inhomogeneous media. In particular, two types of inhomogeneity are considered: media formed from two homogeneous regions with a common interface, and those with a periodic structure. For a material made of two homogeneous regions joined together we establish a set of boundary conditions to describe the behaviour of waves at this interface. These boundary conditions are additional to the standard ones provided by Maxwell’s equations. The conditions found are shown to reduce to those established previously by Pekar in the case of a boundary between a spatially dispersive region and a purely temporally dispersive region. The polarisation is also found for a spatially dispersive medium with periodic structure. Numeric solutions are found and non-divergent modes are identified. Analytic solutions are also found for small magnitudes of the inhomogeneity. Most interestingly these results show that, for certain conditions, there exist coupled mode solutions. This is an unusual phenomena which arises as a result of the spatial dispersion in the system.

Third generation synchrotron facilities such as the Diamond Light Source (DLS) have a wide range of experiments performed for a wide range of science fields. The DLS operates at energies up to 150 keV which introduces great challenges to radiation detector technology. This work focuses on the requirements that the detector technology faces for X-ray Absorption Fine Structure (XAFS) and powder diffraction experiments in I12 and I15 beam lines, respectively. A segmented HPGe demonstrator detector with in-built charge sensitive CUBE preamplifiers and a Schottky e- collection CdTe Medipix3RX detector systems were investigated to understand the underlying mechanisms that limit spectroscopic, imaging performances and stability and to find ways to overcome or minimise those limitations. The energy resolution and stability of the Ge demonstrator detector was found to have the required characteristics for XAFS measurements. Charge sharing was identified as a limiting factor to the resolution which is going to be addressed in the future development of a full detector system as well as reductions in electronic noise and cross-talk effects. The stability study of the Schottky CdTe Medipix3RX detector showed that polarization is highly dependent on temperature, irradiation duration and incoming flux. A new pixel behaviour called tri-phase (3-P) pixel was identified and a novel method for determining optimum operational conditions was developed. The use of the 3-P pixels as a criterion for depolarization resulted in a stable performance of the detector. Furthermore, the detector was applied in powder diffraction measurement at the I15 beam line and resulted in the detector diffraction pattern matching the simulated data. CdTe Medipix3RX and HEXITEC spectroscopic imaging detectors were applied in identification and discrimination of transitional metals for security application and K-edge subtraction for medical applications. The results showed that both detectors have potential to be applied in fields outside synchrotron radiation.

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Some problems which arise in the analysis and design of multilayer filters are discussed in this thesis. The filters consist of sequences of parallel-sided media which reflect and transmit electromagnetic radiation. The cases considered are those appropriate to the optical region of the spectrum although the analysis is quite general. In the optical region, the refractive index of a thin film is generally measured by the Abeles method, which entails determining the angle of incidence at which the film and the bare substrate have the same value of Rp. The presence of a small amount of absorption can produce errors in measurements of this kind. Two ways of estimating the magnitude of this error are given. Considerable broadening of the reflectance band of a multilayer may be obtained by 'staggering' the layer thicknesses in such a way that they form either an arithmetic or geometric progression. Results are shown for fifteen, twenty-five and thirty-five layers. The presence of the narrow band transmission peaks exhibited by the symmetric filters is explained, and the advantages of the use of this type of filter as an interference filter arediscussed. A closed form expression for the matrix product of staggered layers is obtained for the case when the difference in thickness is small. A 'least squares' method of filter design is introduced. This method may be used either to design a filter automatically if no initial design is available, or to 'refine' an existing design. The method is applied to the design of antireflection coatings, beam splitters, low-and high-pass filters and broad-band high reflectance coatings. In addition, one or two well-known filter designs are used to test the method.