In the wood products industry, terahertz (THz) radiation is an emerging technology that is showing promise for new types of monitoring. These applications are dependent on the ability of THz radiation to probe the gross fibre structure of wood. The sensitivity to this gross fibre structure results from strong birefringence at THz frequencies. Prior to this thesis work, the nature of the birefringence exhibited in wood was an open question. Based on the physical structure of wood, the large birefringence observed can be due to intrinsic birefringence, resulting from the dielectric properties of the wood material present in the cell walls, or form birefringence, resulting from the repetitive array of cylindrical cells present in the wood structure. This thesis determines for the first time the origin of birefringence at THz frequencies in wood. This question is important to potential applications in the wood products industries because if the birefringence has a large contribution from form birefringence, then it is the gross fibre structure of wood products that is being probed by THz radiation, which opens the door to applications in non-contact, non-destructing strength testing for example. This thesis outlines a number of systematic studies that will address the relative contributions of form and intrinsic birefringence to the large birefringence seen in wood at THz frequencies, and demonstrate, for the first time, what the source of this large birefringence is in several different species of wood. --P. ii.
In the wood products industry, terahertz (THz) radiation is showing promise as a new remote sensing technology with many applications. The purpose of this project was to probe the interaction of terahertz radiation with wood to determine the extent of these possibilities. Three avenues of study were undertaken: 1. Probe the cylindrical structure of wood in a reflection geometry, 2. Study the birefringence of different species of oven dry wood, 3. Study the extinction coefficient of different species of oven dry wood. These three studies increased our understanding of the interaction between THz radiation and wood. Prior to this study, the extinction mechanism in THz transmission through wood was an open question. Three mechanisms were identified: absorption by the vibrational and rotational modes of the wood and water molecules scattering by the dielectric cylindrical wood cells and, diffraction by the earlywood-latewood pattern. This project determined that absorption by the wood and water molecules is the dominant extinction mechanism in transmission through wood, contributing to ~90% of the signal attenuation. --P. i.
There is an ever-growing demand in the wood products industry for cost-effective and compact equipment to ensure wood quality measures and specifications are met. Recently, terahertz (THz) technology has begun to pave the way for new, non-destructive imaging techniques rather than the standard use of X-ray or microwave technology. A prototype THz Michelson interferometer operating at 0.102 THz was developed to measure the indices of refraction and absorption coefficients of maple, hemlock and birch. These measurements were compared to the Picometrix system, which uses conventional THz time-domain spectroscopy and has been implemented in an industrial setting for the real-time measurement of the density of OSB flakes. However, the cost and complexity of the Picometrix system prohibits its use for many industrial applications. The interferometer is cost-effective, compact and provides accurate data at low THz frequencies, thus providing an excellent alternative direction for new THz technology in the wood products industry. --Leaf ii.
The aim of this thesis is to study pollutants that are emitted from combustion systems using Terahertz time domain spectroscopy (THz-TDS). In particular, this thesis will focus on Nitric Oxide (NO), which is present in flue gases and contributes to air pollution. It is also interesting from a Physics perspective because it contains an unpaired electron in the valence shell, which leads to a complicated energy level structure, and interesting Physics. In this thesis, we aim to answer the question: “what is the detection limit of NO, using THz-TDS, and is it suitable for industrial emissions monitoring?” It is therefore necessary to look at detection limits, which in turn requires conducting THz spectroscopy at different gas pressures. ...
