'Over the THz Horizon' thermal infrared technologies and applications
Creators: name: Sun, Jingye
Publisher: name: Imperial College London
Year: 2017

description: Terahertz (THz) is loosely defined by the frequency range from 0.1 to 10 THz, and the `over the THz horizon' thermal infrared from 10 to 100 THz is considered as an extension of the THz spectrum. This broad portion within the electromagnetic spectrum sees many applications due to its unique radiation characteristics. This thesis investigates the technologies and applications for the THz and the `over the THz horizon' thermal infrared, and develops the proposed low cost thermal infrared `THz Torch' spectrometer for material identification. In this thesis, the general background of the THz and extended THz spectrum, as well as the limiting factors for practical free-space THz applications and material characterizations from THz and infrared spectroscopy will be discussed in Chapter 1. State-of-the-art extended THz spectrum technologies, THz and infrared spectroscopy, atmospheric attenuations modelings and its applications will be reviewed in Chapter 2. In addition, the thermal infrared `THz Torch' wireless secure communications system will also be reviewed. Chapter 3 will report on a research application-led study for predicting atmospheric attenuation, and tries to bridge the knowledge gap between applied engineering and atmospheric sciences. As a useful comparative baseline, Chapter 3 focuses specifically on atmospheric attenuation under pristine conditions, over the extended terahertz spectrum. Three well-known simulation software packages (`HITRAN on the Web', MODTRAN\textsuperscript{\textregistered}4 and LBLRTM) will be compared and contrasted. Techniques used for modelling atmospheric attenuation have been applied to investigate the resilience of (ultra-)wide fractional bandwidth applications to the effects of molecular absorption. Finally, with molecular emission included, carrier-to-noise ratio fade margins can be calculated for the effects of line broadening due to changes in macroscopic atmospheric conditions with sub-1 THz ultra-narrow fractional bandwidth applications. Outdoors can be far from pristine, with additional atmospheric contributions only briefly introduced here; further discussion is beyond the scope of this study, but relevant references have been cited. A comprehensive analytical review of methods for calculating the normalized power spectra, used to extract the effective complex dielectric properties of a sample will be undertaken in Chapter 4. Three generic power response models (zero-order, power propagation and electric field propagation) will be derived; these models act as a consolidated mathematical framework for the whole Chapter. With our unified engineering approach, the voltage-wave propagation, transmission line and telegrapher's equation transmission line models will be then independently derived; the first two giving the same mathematical solutions, while the third generates the same numerical results, as the exact electric field propagation model. Mathematically traceable simulation results from the various models will be compared and contrasted using an arbitrarily chosen dataset (window glass) from 1 to 100 THz. We will show how to extract the approximate effective complex dielectric properties using time-gated time-domain spectroscopy and also the exact values with our graphical techniques from the first-order reflectance and transmittance in Chapter 5. Our approach is then taken further by considering all the Fabry-Perot reflections with frequency- and space-domain spectroscopy. With scalar reflection-transmission mode infrared spectroscopy, the threshold conditions between the solution space that gives the single (exact) solution for the complex refractive index and that which gives multiple mathematical solutions will be modelled. By knowing threshold conditions, it is possible to gain a much deeper insight, in terms of sample constraints and metrology techniques that can be adopted, to determine the single solution. Finally, we propose a simple additional measurement/simulation step to resolve the ambiguity within the multiple solution space. Here, sample thickness is arbitrary and no initial guesses are required. The result from this work allows for the exact extraction of complex dielectric properties using simpler and lower cost scalar reflection-transmission mode spectroscopy. A thermal infrared spectrometer based on the `THz Torch' concept will be introduced in Chapter 6. Both transmission and reflection modes of operation will be demonstrated within the extended THz range. Some preliminary experimental results, including the normalized power responses of common materials (e.g., fused silica), and paper and plastic banknotes will also be shown. In addition, a detailed power link budget analysis for the thermal infrared spectrometer will be discussed. Finally, several statistical techniques will be compared and contrasted to implement the material identification function of our spectrometer. The conclusions and further work relating to this thesis will be summarised in the last Chapter.