Abstract-Terahertz Analysis of Phthalocyanine Pigments

A. D. Squires, R. A. Lewis,

https://link.springer.com/article/10.1007/s10762-019-00599-9

In situ, non-invasive and non-destructive analysis of important artworks and cultural pieces is largely important in art conservation science. Terahertz time-domain spectroscopy and imaging delivers these requirements but lacks a materials database with fundamental understanding of salient pigments, binders and substrates. In this study, the most important synthetic pigments, the copper phthalocyanines, are investigated through terahertz time-domain spectroscopy. The terahertz spectrum reveals a series of characteristic modes in a 0.1–3-THz range across 14 pigment samples. Identification and distinction of copper phthalocyanines’ α, β and 𝜖 crystal polymorphs is demonstrated. This uniqueness within the terahertz regime is extended to two halogenated variants and a metal-free form. This invites the use of THz spectroscopy in investigation of contemporary artworks, post 1935, containing these pigments and promotes applications such as identifying fraudulent works of art.

Abstract-3D Printing Metallised Plastics as Terahertz Reflectors

J. A. Colla, R. E. M. Vickers, M. Nancarrow, R. A. Lewis,

https://link.springer.com/article/10.1007%2Fs10762-019-00596-y

3D printing of new metallised plastics is investigated as a means to realise rapid creation of reflective optics for the terahertz regime. The suitability of three commercially available candidate materials was tested across a range of systems which span from 0.2 to 10 THz. Simple reflective planes were printed and characterised by spectroscopy and microscopy. Samples were polished which is shown to give a dramatic improvement to the reflectivity for these materials. The results indicate that metallised plastics have potential uses in low frequency rapid prototyping of reflective optics.

Abstract-Feasibility and Characterization of Common and Exotic Filaments for Use in 3D Printed Terahertz Devices

A. D. Squires, R. A. Lewis,

https://link.springer.com/article/10.1007%2Fs10762-018-0498-y

Recent years have seen an influx of applications utilizing 3D printed devices in the terahertz regime. The simplest, and perhaps most versatile, modality allowing this is Fused Deposition Modelling. In this work, a holistic analysis of the terahertz optical, mechanical and printing properties of 17 common and exotic 3D printer filaments used in Fused Deposition Modelling is performed. High impact polystyrene is found to be the best filament, with a useable frequency range of 0.1–1.3 THz, while remaining easily printed. Nylon, polylactic acid and polyvinyl alcohol give the least desirable terahertz response, satisfactory only below 0.5 THz. Interestingly, most modified filaments aimed at increasing mechanical properties and ease of printing do so without compromising the useable terahertz optical window.

Abstract-Electrical Versus Optical: Comparing Methods for Detecting Terahertz Radiation Using Neon Lamps

L. L. Slocombe, R. A. Lewis,

https://link.springer.com/article/10.1007%2Fs10762-018-0495-1


Terahertz radiation impinging on a lit neon tube causes additional ionization of the encapsulated gas. As a result, the electrical current flowing between the electrodes increases and the glow discharge in the tube brightens. These dual phenomena suggest two distinct modes of terahertz sensing. The electrical mode simply involves measuring the electrical current. The optical mode involves monitoring the brightness of the weakly ionized plasma glow discharge. Here, we directly compare the two detection modes under identical experimental conditions. We measure 0.1-THz radiation modulated at frequencies in the range 0.1–10 kHz, for lamp currents in the range 1–10 mA. We find that electrical detection provides a superior signal-to-noise ratio while optical detection has a faster response. Either method serves as the basis of a compact, robust, and inexpensive room-temperature detector of terahertz radiation.

Abstract-Invited Review Terahertz Transmission, Scattering, Reflection, and Absorption—the Interaction of THz Radiation with Soils

R. A. Lewis

https://link.springer.com/article/10.1007%2Fs10762-017-0384-z

Terahertz radiation has been proposed as a useful tool in the study of soils and related materials from such diverse perspectives as detection of non-metallic landmines to improving soil fertility by agricultural charcoals produced by pyrolysis of organic material. The main barrier to such applications is that soils are rather opaque at terahertz frequencies. In this article, the main findings to date on the interaction of terahertz radiation with soils are reviewed, organized around the four phenomena of terahertz: transmission, scattering, reflection, and absorption. Terahertz transmission through soils is generally low and decreases with frequency. Terahertz scattering is evident in many THz-soil interactions, as the wavelength of the radiation is of the order of the particle size. Terahertz reflection is important to communications as these develop from the GHz into the THz band. Terahertz absorption on diluted soil samples has been demonstrated to be effective in identifying soil constituents, such as aromatic compounds, and soil contaminants, such as pesticides.

Abstract-Terahertz Spectroscopy of 2,4-Dinitrotoluene over a Wide Temperature Range (7–245 K)

Lucia M. Lepodise , Joseph Horvat *, and R. A. Lewis

Institute for Superconducting and Electronic Materials and School of Physics, University of Wollongong, Wollongong, New South Wales 2522, Australia

J. Phys. Chem. A, Article ASAP

DOI: 10.1021/jp5052134

Publication Date (Web): December 16, 2014

Copyright © 2014 American Chemical Society

*E-mail: jhorvat@uow.edu.au. Tel: 61-2-4221 8073.

http://pubs.acs.org/doi/abs/10.1021/jp5052134

Section:

Optical, Electron, and Mass Spectroscopy and Other Related Properties

Previous THz spectroscopy of the TNT explosive precursor, 2,4-dinitrotoluene (DNT), has been restricted to room temperature (apart from one set of data at 11 K). Here, for the first time, we investigate the spectrum as the temperature is systematically varied, from 7 to 245 K. Many new features appear in the spectrum on cooling below room temperature. As well as the five absorption lines observed previously, we observe five additional lines. In addition, a new room-temperature line at 8.52 THz (281 cm–1) is observed. Six of the lines red-shift with temperature and four of them blue-shift. The blue shift is explained by interplay between intramolecular and intermolecular hydrogen bonds. The variation in line width and line intensity with temperature is not systematic, although a conspicuous decrease in line intensity with temperature is observed in all cases. Modeling with hybrid PBE0 and TPSSh functionals helps identify absorption modes.

Abstract-Complementary terahertz absorption and inelastic neutron study of the dynamic anisotropy contribution to zone-center spin waves in a canted antiferromagnet NdFeO3

Evan Constable, D. L. Cortie, Joseph Horvat, R. A. Lewis, Zhenxiang Cheng, Guochu Deng, Shixun Cao, Shujuan Yuan, and Guohong Ma
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.054413

We employ a combination of pulsed- and continuous-wave polarized terahertz spectroscopy techniques to probe temperature-dependent spin waves in the antiferromagnet NdFeO3. Our optical data span 1.6–467 K and reveal a conspicuous spin reorientation between 110 and 170 K, during which the lower-energy mode softens completely. Complementary inelastic neutron scattering reveals that the frequencies of the optically excited spin waves are consistent with a temperature-variable spin gap in the low-energy spin-wave dispersion of NdFeO3. The result links the temperature dependence of the spin waves to a dynamic in-plane anisotropy. The magnetic anisotropy is calculated based on the results of the optical measurements. The change observed in the anisotropy energy along the a and c crystal axes suggests that the spin reorientation evident in NdFeO3 is driven by temperature-dependent in-plane anisotropy.

DOI: http://dx.doi.org/10.1103/PhysRevB.90.054413

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Abstract-The importance of scattering, surface potential, and vanguard counter-potential in terahertz emission from gallium arsenide

D. L. Cortie and R. A. Lewis

Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, New South Wales 2522, Australia

It is well established that under excitation by short (<1 ps), above-band-gap optical pulses, semiconductor surfaces may emit terahertz-frequency electromagnetic radiation via photocarrier diffusion (the dominant mechanism in InAs) or photocarrier drift (dominant in GaAs). Our three-dimensional ensemble Monte Carlo simulations allow multiple physical parameters to vary over wide ranges and provide unique direct insight into the factors controlling terahertz emission. We find for GaAs (in contrast to InAs), scattering and the surface potential are key factors. We further delineate in GaAs (as in InAs) the role of a vanguard counter-potential. The effects of varying dielectric constant, band-gap, and effective mass are similar in both emitter types.

© 2012 American Institute of Physics