Abstract-Exploring the Reliability of DFT Calculations of the Infrared and Terahertz Spectra of Sodium Peroxodisulfateghggg

John Kendrick, Andrew D. Burnett

https://link.springer.com/article/10.1007/s10762-019-00643-8

A number of DFT programs with various combinations of pseudo-potentials and van der Waals’ dispersive corrections have been used to optimize the structure of sodium peroxodisulfate, Na2(SO4)2, and to calculate the infrared, attenuated total reflectance and terahertz absorption spectra of the powdered crystal. Comparison of the results from the different methods highlights the problems of calculating the absorption spectrum reliably. In particular the low frequency phonon modes are especially sensitive to the choice of grids to represent the wavefunction or the charge distribution, k-point integration grid and the energy cutoff. A comparison is made between the Maxwell-Garnett (MG) and Bruggeman effective medium methods used to account for the effect of crystal shape on the predicted spectrum. Possible scattering of light by air inclusions in the sample and by larger particles of Na2(SO4)2 is also considered using the Mie method. The results of the calculations are compared with experimental measurements of the transmission and attenuated total reflection spectra.

Abstract-Free-space terahertz radiation from a LT-GaAs-on-quartz large-area photoconductive emitter

Free-space terahertz radiation from a LT-GaAs-on-quartz large-area photoconductive emitter

David R. Bacon, Andrew D. Burnett, Matthew Swithenbank, Christopher Russell, Lianhe Li, Christopher D. Wood, John Cunningham, Edmund H. Linfield, A. Giles Davies, Paul Dean, and Joshua R. Freeman

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-23-26986

We report on large-area photoconductive terahertz (THz) emitters with a low-temperature-grown GaAs (LT-GaAs) active layer fabricated on quartz substrates using a lift-off transfer process. These devices are compared to the same LT-GaAs emitters when fabricated on the growth substrate. We find that the transferred devices show higher optical-to-THz conversion efficiencies and significantly larger breakdown fields, which we attribute to reduced parasitic current in the substrate. Through these improvements, we demonstrate a factor of ~8 increase in emitted THz field strength at the maximum operating voltage. In addition we find improved performance when these devices are used for photoconductive detection, which we explain through a combination of reduced parasitic substrate currents and reduced space-charge build-up in the device.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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Abstract-PDielec: The calculation of infrared and terahertz absorption for powdered crystals

John Kendrick, Andrew D. Burnett,
http://onlinelibrary.wiley.com/doi/10.1002/jcc.24344/full

The Python package PDielec is described, which calculates the infrared absorption characteristics of a crystalline material supported in a non-absorbing medium. PDielec post processes solid-state quantum mechanical and molecular mechanical calculations of the phonons and dielectric response of the crystalline material. Using an effective medium method, the package calculates the internal electric field arising from different particle morphologies and calculates the resulting shift in absorption frequency and intensity arising from the coupling between a phonon and the internal field. The theory of the approach is described, followed by a description of the implementation within PDielec. Finally, a section providing several examples of its application is given. 
© 2016 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.