Matthew Reid

Ab initiomodel of optical properties of two-temperature warm dense matter

We present a model to describe thermophysical and optical properties of two-temperature systems consisted of heated electrons and cold ions in a solid lattice that occur during ultrafast heating experiments. Our model is based on ab initio simulations within the framework of density functional theory. The optical properties are obtained by evaluating the Kubo-Greenwood formula. By applying the material parameters of our ab initio model to a two-temperature model we are able to describe the temperature relaxation process of femtosecond-laser-heated gold and its optical properties within the same theoretical framework. Recent time-resolved measurements of optical properties of ultrafast heated gold revealed the dynamics of the interaction between femtosecond laser pulses and solid state matter. Different scenarios obtained from simulations of our study are compared with experimental data [Chen, Holst, Kirkwood, Sametoglu, Reid, Tsui, Recoules, and Ng, Phys. Rev. Lett. 110, 135001 (2013)].

Origin of Birefringence in Wood at Terahertz Frequencies

In the wood products industry, terahertz (THz) radiation is showing promise for new types of sensing and imaging applications, which depend on the ability of THz radiation to probe the gross fiber structure of wood. The sensitivity to this gross fiber structure is a result of the strong birefringence of wood at THz frequencies. Wood is a complex structure, and the large birefringence observed may be due to either intrinsic or form birefringence. In this paper, the origin of birefringence in wood at THz frequencies is examined in detail. Although the source of birefringence varies according to species, the trend shows contributions from both types of birefringence. This has implications for application development in the wood products industry, where the possibility of probing both the gross physical structure of wood and wood products, and the intrinsic properties of wood such as crystallinity and microfibril angle, simultaneously may allow for nondestructive noncontact strength testing of wood and composite wood products.

Prediction of oven-dry density of wood by time-domain terahertz spectroscopy

Abstract Wood is relatively transparent to terahertz (THz) radiation with wavelengths in the submillimeter range. This radiation has a high potential for sensing and imaging wood with a good spatial resolution. THz is especially sensitive to moisture content, fiber alignment, and density – all of which are critical in the manufacturing of wood products. In this work, a systematic study was undertaken on 46 very different wood species by means of THz time-domain spectroscopy with density determination in focus. The dielectric response of wood was modeled based on the Maxwell-Garnett effective medium theory. The dielectric function of the cell wall material was found to be extremely consistent over this large number of species with very different properties. This renders possible to determine wood density by THz time-domain spectroscopy. A strong correlation between the measured and predicted densities has been observed for all the samples investigated.

Birefringence of wood at terahertz frequencies

Fibre content of solid wood plays an important role in the wood products industry in terms of value. Additionally, fibre structure in composite wood products such as Oriented Strand Board (OSB) and paper products plays an important role in terms of strength properties. The effect of moisture content on wood properties is important in the manufacturing process and final product performance, and therefore its effect on the birefringence is of considerable interest. Since solid wood exhibits strong birefringence at terahertz frequencies, there may be potential applications of terahertz spectroscopy to fibre content and structure sensing. There are two potential sources for this strong birefringence: (i) form birefringence resulting from the porous structure of solid wood and (ii) intrinsic birefringence resulting from the dielectric properties of the material itself. In this report, the variability of birefringence within and between species, the dependence of the birefringence on moisture content and the relative contributions from form and intrinsic birefringence are examined. In order to clarify the role of these contributions to the measured birefringence, polarized terahertz reflection spectroscopy is examined and compared to the results obtained in a transmission geometry. Comparison of the birefringence measured in transmission and reflection geometries suggests that form birefringence may dominate.

Multiple scattering in low-energy electron holography

The theory of the low-energy electron point source (LEEPS) microscope is reformulated in matrix form to readily account for multiple scattering. An algorithm is developed for the storage of the structure matrix and an iterative method is used to solve the matrix equation for the structure factor. Examples of small and large clusters of atoms are given to compare single and multiple scattering. A Kirchhoff-Helmholtz transform is used for the reconstruction. We find that in some cases the multiple scattering is too strong and reconstruction is not possible. We give examples which show that, even when multiple scattering is important, one can still obtain reconstructions that reveal the atomic structure both along and lateral to the optical axis. We also compare our results with those found in LEED.

Comment on: Curling rock dynamics — The motion of a curling rock: inertial vs. noninertial reference frames

We examine the approach used and the results presented in a recent publication (Can. J. Phys. 76, 295 (1998)) in which ( i) a noninertial reference frame is used to examine the motion of a curling rock, and ( ii ) the lateral motion of a curling rock is attributed to left–right asymmetry in the force acting on the rock. We point out the important differences between describing the motion in an inertial frame as opposed to a noninertial frame. We show that a force exhibiting left–right asymmetry in an inertial frame cannotexplain the lateral motion of a curling rock. We also examine, as was apparently done in the recent publication, an effective force that has left–right asymmetry in a noninertial, rotating frame . We show that such a force is not left–right asymmetricin an inertial frame, and that any lateral motion of a curling rock attributed to theeffectiveforce in the noninertial frame is actually due to a re l force, in an inertial frame, which has a net nonzero component transverse to the velocity of the center of mass. We inquire as to the physical basis for the transverse component of this real force. We also examine the motion of a rotating cylinder sliding over a smooth surface for which there is no melting: we show that the motion is easily analyzed in an inertial frame and that there is little to be gained by considering a rotating frame. We relate the results for this simple case to the more involved problem of the motion of a curling rock: we find that the motion of curling rocks is best studied in i ertial frames. Perhaps most importantly, we show that the approach taken and the results presented in the recent publication lead to predicted motionsof curling rocks that arein disagreement with observed motions of real curling rocks. PACS Nos.: 46.00, 01.80+b Résumé: Nous examinons l’approche utilisée et les résultats obtenus dans une récente publication (Can. J. Phys. 76, 295 (1998)) dans laquelle ( i) un référentiel non-inertiel est utilisé pour étudier le mouvement d’une pierre de curling et ( ii ) la dérive latérale de la pierre est attribuée à une asymétrie gauche-droite dans la force agissant sur la pierre. Nous soulignons les différences importantes dans la description du mouvement dans un référentiel inertiel et dans Received February 4, 1999. Accepted December 1, 1999. M.R.A. Shegelski 1 and M. Reid. Department of Physics, University of Northern British Columbia, 3333 University Way, Prince George, BC V2N 4Z9, Canada. 1 Corresponding author: Telephone:(250) 960–6663; FAX: (250) 960–5545; e-mail: mras@unbc.ca Can. J. Phys. 77: 903–922 (1999)

Enhanced Terahertz Emission from Porous InP

Terahertz and second-harmonic emission from bulk and porous InP samples is investigated under irradation with femtosecond Ti:Sapphire laser pulses. The emission from the porous samples is enhanced relative to the bulk samples.

Evaluation of Compatibility between Beetle-Killed Lodgepole Pine (Pinus Contorta var. Latifolia) Wood with Portland Cement

The compatibility of wood from mountain pine beetle (Dendroctonus ponderosa) killed lodgepole pine (Pinus contorta var. latifolia) with Portland cement was investigated based on time-since-death as a quantitative estimator, and the presence of blue-stained sapwood, brown rot, or white rot as qualitative indicators. The exothermic behavior of cement hydration, maximum heat rate, time to reach this maximum, and total heat released within a 3.5–24 h interval were used for defining a new wood-cement compatibility index (CX). CX was developed and accounted for large discrepancies in assessing wood-cement compatibility compared to the previous methods. Using CX, no significant differences were found between fresh or beetle-killed wood with respect to the suitability for cement; except for the white rot samples which reached or exceeded the levels of incompatibility. An outstanding physicochemical behavior was also found for blue-stained sapwood and cement, producing significantly higher compatibility indices.

Extreme nonlinear carrier dynamics induced by intense quasi-half-cycle THz pulses in n-doped InGaAs thin film

We report extreme nonlinear carrier dynamics in highly n-doped In0.53Ga0.47As thin film using terahertz time-domain spectroscopy. We attribute our observations to the dramatic reduction of terahertz photoconductivity induced by strong intervalley scattering effects.

Terahertz Spectroscopy and Brewster Angle Reflection Imaging of Acoustic Tiles

A Brewster angle reflection imaging apparatus is demonstrated which is capable of detecting hidden water-filled voids in a rubber tile sample. This imaging application simulates a real-world hull inspection problem for Royal Canadian Navy Victoria-class submarines. The tile samples represent a challenging imaging application due to their large refractive index and absorption coefficient. With a rubber transmission window at approximately 80 GHz, terahertz (THz) sensing methods have shown promise for probing these structures in the laboratory. Operating at Brewster’s angle allows for the typically strong front surface reflection to be minimized while also conveniently making the method insensitive to air-filled voids. Using a broadband THz time-domain waveform imaging system (THz-TDS), we demonstrate satisfactory imaging and detection of water-filled voids without complicated signal processing. Optical properties of the tile samples at low THz frequencies are also reported.