Determination of the origin of birefringence in wood at terahertz frequencies.

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.