## Prof. Edward Philip John PARROTT

Nationality British

### Education History

University of Cambridge (United Kingdom)
Physics and Chemical Engineering Ph. D.

University of Cambridge (United Kingdom)
Natural Sciences (Physical) M. Sci., M. A. (Cantab.)

### Work History

Research Assistant Professor (2012-present)
The Chinese University of Hong Kong, Hong Kong
Department of Electronic Engineering

Postdoctoral Fellow (2010-2012)
The Chinese University of Hong Kong, Hong Kong
Department of Electronic Engineering

Research Associate (2010)
University of Cambridge, United Kingdom
Department of Chemical Engineering and Biotechnology

### Research Interests

Terahertz spectroscopy of biological and chemical systems, materials characterisation by modelling terahertz dielectric response, probing kinetics of reactions. Hardware and software development of terahertz instruments.

### Biography

Edward Parrott studied Natural Sciences at Cambridge University before embarking upon a Ph.D. under the supervision of Prof. Lynn Gladden in the Chemical Engineering Department of Cambridge University, specialising in studying the applications of terahertz spectroscopy. His Ph.D. thesis included studies on a wide range of materials, from the catalytic activity of ordered carboncaeous materials (nanotubes) to the identification of structurally similar cocrystals. He joined Prof. MacPherson's group in 2010 as a postdoctoral fellow and in 2012 took up the position of Research Assistant Professor in the same department. He is involved in organising the day to day running of the research output of the group and is personally interested in the development of spectroscopic techniques that can be applied to the study of material systems.

### Publication List

Below are listed the publications of Prof. Parrott, including those published prior to joining the group (if any). Click on the years to show/hide the paper titles and select the paper titles for more information on each paper, including links to the publication websites.
1. Recent advances in terahertz technology for biomedical applications
Qiushuo Sun, Yuezhi He, Kai Liu, Shuting Fan, Edward P. J. Parrott, Emma Pickwell-MacPherson
Quantitative Imaging in Medicine and Surgery 7(3): 345-355 (2017) DOI: 10.21037/qims.2017.06.02 Link to article
Abstract
Terahertz instrumentation has improved significantly in recent years such that THz imaging systems have become more affordable and easier to use. THz systems can now be operated by non-THz experts greatly facilitating research into many potential applications. Due to the non-ionising nature of THz light and its high sensitivity to soft tissues, there is an increasing interest in biomedical applications including both in vivo and ex vivo studies. Additionally, research continues into understanding the origin of contrast and how to interpret terahertz biomedical images. This short review highlights some of the recent work in these areas and suggests some future research directions.
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2. Graphene Based Terahertz Light Modulator in Total Internal Reflection Geometry
Xudong Liu, Zefeng Chen, Edward P J Parrott, Benjamin S Y Ung, Jianbin Xu, Emma Pickwell-MacPherson
Abstract
Modulation of visible light has been easily achieved for decades, but modulation of terahertz (THz) light still remains a challenge. To address this issue, the Fresnel equations have been developed to describe a conductive interface in a total internal reflection geometry and reveal a new approach for modulation. To demonstrate this new mechanism, a broadband device achieving a modulation depth greater than 90% between 0.15 and 0.4 THz, and reaching a maximum of 99.3% at 0.24 THz has been designed. The modulation is achieved by applying a gate voltage between −0.1 and 2 V to a graphene layer in a total internal reflection geometry. Compared to conventional designs, the high modulation is realized without assistance from metamaterial structures, resonant cavities, or multistacked graphene layers. Thus, the design is efficient and easy-to-fabricate and can be easily retrofitted to most existing THz systems. This work opens up a new avenue of research as the device has verified the theory and demonstrates how it can be used to make practical devices, bringing a promising new paradigm for THz modulation, thin-film sensing, and noninvasive material characterization.
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3. Adaptive Sampling for Terahertz Time-Domain Spectroscopy and Imaging
Yuezhi He, Edward P. J. Parrott, Emma Pickwell-MacPherson
IEEE Transactions on Terahertz Science and Technology 7(2): 118-123 (2017) DOI: 10.1109/TTHZ.2016.2640663 Link to article
Abstract
We propose an adaptive sampling algorithm to improve the acquisition efficiency for terahertz time-domain spectroscopy (THz-TDS). Most THz-TDS measurements scan the delay line with constant speed and the data acquired have constant time steps. Our algorithm exploits the fact that the useful information within THz signals tends to cluster at certain positions: efficient sampling can be done by adaptively increasing the sample rate in regions containing more interesting features. The algorithm was implemented by programming a linear optical delay line. Depending on the experiment parameters, the sampling time of a pulse can be reduced by a factor of 2-3 with only slight degradation in accuracy, possible sources of error are discussed. We show how adaptive sampling algorithms can improve the acquisition time in applications where the main pulse is the primary concern.
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4. Tailoring Metamaterial Microstructures to Realize Broadband Polarization Modulation of Terahertz Waves
Chunrui Han, Edward P. J. Parrott, Emma Pickwell-MacPherson
IEEE Journal of Selected Topics in Quantum Electronics 23(4): 1-6 (2017) DOI: 10.1109/JSTQE.2016.2641581 Link to article
Abstract
We report ultrabroadband, easily tunable, and highly efficient metamaterial-based terahertz wave retarders that are able to convert linear polarization into elliptical and circular polarization states. The functional device consists of a metamaterial microstructure and a grating coupler patterned on each side of fused silica substrates. The dielectric response of the metamaterial microstructure and the angular dependent phase dispersion of the grating coupler allow tuning of the phase differences from -110° to 110° within the range of a few terahertz while keeping the magnitudes of the two orthogonally transmitted waves equal. In particular, a high degree of circular polarization ({\textgreater}0.99) can be achieved from 1.78 to 4.88 THz for a specific dielectric value of spacer material 2.8 and angle of incidence -13°. The experimental results in the accessible frequency range of 0.2-2.3 THz show good agreement with the numerical simulations. Our study opens new opportunities for manipulating the broadband polarization responses of terahertz waves. This facilitates the development of new functional devices based on metamaterials for terahertz imaging and spectroscopy.
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5. Exploiting a metal wire grating in total internal reflection geometry to achieve achromatic polarization conversion
Xudong Liu, Xuequan Chen, Edward P. J. Parrott, Emma Pickwell-MacPherson
Photonics Research 5(4): 299 (2017) DOI: 10.1364/PRJ.5.000299 Link to article
Abstract
We demonstrate how a metal wire grating can work as a 45° polarization converter, a quarter-wave retarder, and a half-wave retarder over a broadband terahertz range when set up in total internal reflection geometry. Classical electromagnetic theory is applied to understand the mechanism, and equations to calculate the polarization state of reflected light are derived. We use a metal grating with a period of 20 $\mu$m and width of 10 $\mu$m on a fused silica surface: linearly polarized terahertz light incident from fused silica with a supercritical incident angle of 52° is totally reflected by the metal grating and air. The polarization of the terahertz light is rotated by 45°, 90°, and circularly polarized by simply rotating the wire grating. The performance is achromatic over the measured range of 0.1–0.7 THz and comparable to commercial visible light wave retarders.
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1. Exploiting total internal reflection geometry for efficient optical modulation of terahertz light
Xudong Liu, Edward P. J. Parrott, Benjamin S.-Y. Ung, Emma Pickwell-MacPherson
APL Photonics 1(7): 076103 (2016) DOI: 10.1063/1.4963141 Link to article
Abstract
Efficient methods to modulate terahertz (THz) light are essential for realizing rapid THz imaging and communication applications. Here we report a novel THz modu- lator which utilizes the evanescent wave in a total internal reflection setup coupled with a conductive interface to enhance the attenuation efficiency of THz light. This approach makes it possible to achieve close to 100% modulation with a small interface conductivity of 12 mS. The frequency dependence of this technique is linked to the optical properties of the materials: a material with close to frequency indepen- dent conductivity that is also controllable will result in an achromatic modulation response, and the device performance can be optimized further by tuning the internal reflection angle. In this work, we focus on applying the technique in the terahertz frequency range. Using an LED array with a pump intensity of 475 mW/cm2 to produce carriers in a silicon wafer, we have achieved a modulation depth of up to 99.9% in a broad frequency range of 0.1 THz–0.8 THz. The required pumping power for the generation of the required free carriers is low because the sheet conductivity needed is far less than required for traditional transmission techniques. Consequently, the device can be modulated by an LED making it a very practical, lowcost, and scalable solution for THz modulation.
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2. Calibration method to improve the accuracy of THz imaging and spectroscopy in reflection geometry
Shuting Fan, Edward P J Parrott, Benjamin S Y Ung, Emma Pickwell-MacPherson
Photonics Res. 4(3): A29--A35 (2016) DOI: 10.1364/PRJ.4.000A29 Link to article
Abstract
We introduce a novel method to accurately extract the optical parameters in terahertz reflection imaging. Our method builds on standard self-referencing methods using the reflected signal from the bottom of the imaging window material to further compensate for time-dependent system fluctuations and position-dependent variation in the window thickness. Our proposed method not only improves the accuracy, but also simplifies the imaging procedure and reduces measurement times.
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3. Freeze-thaw hysteresis effects in terahertz imaging of biomedical tissues
Yuezhi He, Benjamin S.-Y. Ung, Edward P J Parrott, Anil T Ahuja, Emma Pickwell-MacPherson
Biomed. Opt. Express 7(11): 4711 (2016) DOI: 10.1364/BOE.7.004711 Link to article
Abstract
There have recently been several studies published involving terahertz (THz) imaging of frozen biomedical samples. In this paper, we investigate the effects of the freezethaw cycle on THz properties of porcine muscle and fat samples. For ordinary freezing, there was a significant change in the THz properties after thawing for muscle tissue but not for fat tissue. However, if snap-freezing was combined with fast-thawing instead of ordinary freezing and ordinary thawing, then the freeze thaw hysteresis was removed.
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4. Vanadium dioxide devices for terahertz wave modulation : a study of wire grid structures
Edward P J Parrott, Chunrui Han, Fei Yan, Georges Humbert, Annie Bessaudou, Aurelian Crunteanu, Emma Pickwell-MacPherson
Nanotechnology 27(20): 205206 (2016) DOI: 10.1088/0957-4484/27/20/205206 Link to article
Abstract
Vandium dioxide (VO2) shows promise as the basis for a terahertz wave modulator due to its phase transition properties. Its insulator–metal-transition (IMT) can be induced either through temperature changes, optically or electronically. Recently, a metal-VO2 wire grid structure was proposed which was able to increase the modulation depth (MD) from 0.65 to 0.9, suggesting that these simple metallic structures could greatly increase the difference in terahertz transmission for the insulating and metallic states of VO2 based structures. In this paper, we have found that the increase in MD decreases with increasing VO2 conductivity in the metallic state, resulting in a maximum modulation depth of approximately 0.95 for wire grid structures that preserves a high transmission in the insulating state. Surprisingly, we find that deposition of VO2 on top of metallic structures results in reduced performance. However, we find that devices based upon VO2 alone can achieve unexpectedly high performance. In this work we present a device with a switchable wire-grid polariser effect over a broadband frequency range (from 0.3 to 2 THz). To our knowledge this is the first such broadband metamaterial based solely on VO2. The ability to switch on a metamaterial property like this to produce a polarisation effect is very useful for future terahertz optical devices such as rotators and waveplates.
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5. In vivo terahertz reflection imaging of human scars during and after the healing process
Shuting Fan, Benjamin S Y Ung, Edward P J Parrott, Vincent P Wallace, Emma Pickwell-MacPherson
J. Biophotonics 9: 1-9 (2016) DOI: 10.1002/jbio.201600171 Link to article
Abstract
We use terahertz imaging to measure four human skin scars in vivo. Clear contrast between the refractive index of the scar and surrounding tissue was observed for all of the scars, despite some being difficult to see with the naked eye. Additionally, we monitored the healing process of a hypertrophic scar. We found that the contrast in the absorption coefficient became less prominent after a few months post-injury, but that the contrast in the refractive index, was still significant even months post-injury. Our results demonstrate the capability of terahertz imaging to quantitatively measure subtle changes in skin properties and this may be useful for improving scar treat- ment and management.
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1. Gelatin embedding: a novel way to preserve biological samples for terahertz imaging and spectroscopy
Shuting Fan, Benjamin Ung, Edward P J Parrott, Emma Pickwell-MacPherson
Phys. Med. Biol. 60: 2703-2713 (2015) DOI: 10.1088/0031-9155/60/7/2703 Link to article
Abstract
Sample dehydration has traditionally been a challenging problem in ex vivo terahertz biomedical experiments as water content changes significantly affect the terahertz properties and can diminish important contrast features. In this paper, we propose a novel method to prevent sample dehydration using gelatin embedding. By looking at terahertz image data and calculating the optical properties of the gelatin-embedded sample, we find that our method successfully preserves the sample for at least 35 h, both for imaging and spectroscopy. Our novel preservation method demonstrates for the first time the capability to simultaneously maintain sample structural integrity and prevent dehydration at room temperature. This is particularly relevant for terahertz studies of freshly excised tissues but could be beneficial for other imaging and spectroscopy techniques.
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2. Terahertz Time-Domain and Low-Frequency Raman Spectroscopy of Organic Materials.
Edward P J Parrott, J Axel Zeitler
Appl. Spectrosc. 69(1): 1-25 (2015) DOI: 10.1366/14-07707 Link to article
Abstract
With the ongoing proliferation of terahertz time-domain instrumentation from semiconductor physics into applied spectroscopy over the past decade, measurements at terahertz frequencies (1 THz ≡ 10(12) Hz ≡ 33 cm(-1)) have attracted a sustained growing interest, in particular the investigating hydrogen-bonding interactions in organic materials. More recently, the availability of Raman spectrometers that are readily able to measure in the equivalent spectral region very close to the elastic scattering background has also grown significantly. This development has led to a renewed interest in performing spectroscopy at the interface between dielectric relaxation phenomena and vibrational spectroscopy. In this review, we briefly outline the underlying technology, the physical phenomena governing the light-matter interaction at terahertz frequencies, recent examples of spectroscopic studies, and the current state of the art in assigning spectral features to vibrational modes based on computational techniques.
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3. Low-cost and broadband terahertz antireflection coatings based on DMSO-doped PEDOT / PSS
Fei Yan, Edward P J Parrott, Xu Dong Liu, Emma Pickwell-MacPherson
Opt. Lett. 40(12): 2886-2889 (2015) DOI: 10.1364/OL.40.002886 Link to article
Abstract
We report the potential application of 6{\{}%{\}} dimethylsulfoxide (DMSO)-doped poly (3, 4-ethylenedioxythiophene)/poly (4-styrenesulfonate) (PEDOT/PSS) as a low cost and broadband terahertz (THz) antireflection coating based on the impedance matching effect. The reflected pulses from the quartz and silicon substrates are observed to change with the thickness of the PEDOT/PSS layer. Theoretical analysis based on an equivalent transmission line circuit model and FDTD computational simulations have been used to understand the experimental results. Excellent impedance matching is achieved by a ∼39-nm-thick 6{\{}%{\}} DMSO-doped PEDOT/PSS layer on quartz, and a ∼101-nm-thick 6{\{}%{\}} DMSO-doped PEDOT/PSS layer on silicon due to the almost-frequency-independent conductivity of the thin film between 0.3 and 2.5 THz. In the critical conditions, the normalized main pulse transmission remains as high as 74{\{}%{\}} and 64{\{}%{\}}, for the quartz and silicon substrates, respectively, significantly higher than the existing state of the art THz antireflection coatings.
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4. Solvent Doping of PEDOT/PSS: Effect on Terahertz Optoelectronic Properties and Utilization in Terahertz Devices
Fei Yan, Edward P J Parrott, Benjamin S.-Y. Ung, Emma Pickwell-MacPherson
J. Phys. Chem. C 119(12): 6813-6818 (2015) DOI: 10.1021/acs.jpcc.5b00465 Link to article
Abstract
Poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) is a conducting polymer and is a promising material for use in optoelectronic devices. Adding dopants to PEDOT/PSS significantly affects its optoelectronic properties: in this article we use terahertz time domain spectroscopy (THz-TDS) to probe the effects of dopants dimethyl sulfoxide (DMSO) and ethylene glycol. The carrier density, mobility, and conductivity are calculated from the THz measurements by fitting the dielectric permittivity to the Drude−Smith model. This gives us an insight into the conductivity enhancement mechanisms, and we find evidence to suggest that both carrier delocalization and charge screening play a role, although the relative importance of these two mechanisms depends upon both dopant polarity and concentration. To demonstrate an application of this finding, we design and fabricate broadband terahertz neutral density filters based upon 6{\{}%{\}} DMSO doped PEDOT/PSS thin films of varying thickness and demonstrate optical densities between 0.14 and 0.53 from 0.5 to 2.2 THz with a comparable frequency variation to commercially available optical frequency ND filters.
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1. Direct evidence to support the restriction of intramolecular rotation hypothesis for the mechanism of aggregation-induced emission: temperature resolved terahertz spectra of tetraphenylethene
Edward P J Parrott, Nicholas Y Tan, Rongrong Hu, J Axel Zeitler, Ben Zhong Tang, Emma Pickwell-MacPherson
Mater. Horiz. 1(2): 251-258 (2014) DOI: 10.1039/c3mh00078h Link to article
Abstract
In contrast to the traditional fluorescent dyes that exhibit a decrease in fluorescence upon aggregation, Aggregation- Induced Emission (AIE) molecules are a family of fluorophors which exhibit increased fluorescence upon aggregation. Consequently, AIE molecules represent an interesting new material with potential applications in fluorescent chemo/biosensors, light emitting devices and medical diagnostics. Numerous mechanisms have been proposed to explain this phenomenon, including E–Z isomerization, and restriction of intramolecular rotations (RIR). However, there has not been any direct experimental evidence to support either one of these hypotheses. Here we use terahertz time-domain-spectroscopy (THz-TDS) and solid-state computational simulations of an AIE molecule to link the increase in intensity of intramolecular rotation and rocking modes to the measured fluorescence and reveal direct evidence supporting the RIR hypothesis. This is the first time that terahertz spectroscopy has been used to directly probe such molecular motions in AIE materials and in doing so we have found conclusive evidence to fully explain the AIE mechanism.
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2. High extinction ratio and low transmission loss thin-film terahertz polarizer with a tunable bilayer metal wire-grid structure
Zhe Huang, Edward P J Parrott, Hongkyu Park, Hau Ping Chan, Emma Pickwell-MacPherson
Opt. Lett. 39(4): 793-796 (2014) DOI: 10.1364/OL.39.000793 Link to article
Abstract
A thin-film terahertz polarizer is proposed and realized via a tunable bilayer metal wire-grid structure to achieve high extinction ratios and good transmission. The polarizer is fabricated on top of a thin silica layer by standard micro-fabrication techniques to eliminate the multireflection effects. The tunable alignment of the bilayer alumi- num-wire grid structure enables tailoring of the extinction ratio and transmission characteristics. Using terahertz time-domain spectroscopy (THz-TDS), a fabricated polarizer is characterized, with extinction ratios greater than 50 dB and transmission losses below 1 dB reported in the 0.2–1.1 THz frequency range. These characteristics can be improved by further tuning the polarizer parameters such as the pitch, metal film thickness, and lateral displacement.
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1. Advances in Polarizer Technology for Terahertz Frequency Applications
Fei Yan, Calvin Yu, Hongkyu Park, Edward P J Parrott, Emma Pickwell-MacPherson
J. Infrared Milli. Terahz Waves 34(9): 489-499 (2013) DOI: 10.1007/s10762-013-0005-4 Link to article
Abstract
As investigations into potential applications of terahertz technology grow, there is an increasing need for improved terahertz optical components such as polarizers. To determine the optical properties of a sample accurately, the polarization properties of the light must also be known accurately. Many terahertz emitters will have both horizontal and vertical polarization components and often assumptions are made about device characteris- tics without measuring them-even the position of excitation beam on the photoconductive emitter can affect the resulting terahertz electric field and so the exact optical properties of a given device will vary depending on how they are configured. Polarizers operating at terahertz frequencies can be used to characterize the electric field accurately or remove unwanted components as long as the polarizer is of sufficiently high performance. In this paper we review the key properties of polarizers and look at recent advances in their design and development at terahertz frequencies.
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2. Robust Thin-Film Wire-Grid THz Polarizer Fabricated Via a Low-Cost Approach
Zhe Huang, Hongkyu Park, Edward P J Parrott, Hau Ping Chan, Emma Pickwell-MacPherson
IEEE Photon. Techn. Lett. 25(1): 81-84 (2013) DOI: 10.1109/LPT.2012.2228184 Link to article
Abstract
A robust thin-film wire-grid terahertz (THz) polarizer was fabricated via a low-cost, mass-producible man- ufacturing approach. This polarizer is built on a very thin silica layer structurally supported by a silicon substrate. In addition, the metal grating is protected by a polymer thin film, which eliminates the multireflection effect and enhances the robustness of the polarizer for easy packaging. The polarizer can be easily mounted onto a Newport rotation holder for immediate use. A THz time-domain spectrometer is used to characterize its performance, and an excellent agreement is found between the FDTD-simulated results and the experimental results. The polarizer offered 20–40 dB and 0.8 dB of extinction ratio and transmission loss over a frequency range of 0.2–2.0 THz, respectively.
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1. Probing biological systems with terahertz spectroscopy
Emma Pickwell-MacPherson, Yiwen Sun, Edward P J Parrott
Proc. SPIE 8496: 84960R--84960R--5 (2012) DOI: 10.1117/12.928185 Link to article
2. Tailoring liquid crystals to become fast and efficient terahertz devices
E Pickwell-MacPherson, E P J Parrott, H Park, F Fan, V G Chigrinov
Proc. SPIE 8475: 84750D--84750D--6 (2012) DOI: 10.1117/12.928191 Link to article
3. Evaluating liquid crystal properties for use in terahertz devices
Hongkyu Park, Edward P J Parrott, Fan Fan, Meehyun Lim, Haewook Han, Vladimir G Chigrinov, Emma Pickwell-MacPherson
Opt. Express 20(11): 11899-11905 (2012) DOI: 10.1364/OE.20.011899 Link to article
4. Accurate photoconductive antenna characterization using a thin film polarizer
H Park, E P J Parrott, Z Huang, H P Chan, E Pickwell-MacPherson
Appl. Phys. Lett. 101(12): 121108 (2012) DOI: 10.1063/1.4753795 Link to article
1. Terahertz pulsed imaging in vivo: measurements and processing methods
Edward P J Parrott, Stanley M Y Sy, Thierry Blu, Vincent P Wallace, Emma Pickwell-MacPherson
J. Biomed. Opt. 16(10): 106010 (2011) DOI: 10.1117/1.3642002 Link to article
Abstract
This paper presents a number of data processing algorithms developed to improve the accuracy of results derived from datasets acquired by a recently designed terahertz handheld probe. These techniques include a baseline subtraction algorithm and a number of algorithms to extract the sample impulse response: double Gaussian inverse filtering, frequency-wavelet domain deconvolution, and sparse deconvolution. In vivo measurements of human skin are used as examples, and a comparison is made of the terahertz impulse response from a number of different skin positions. The algorithms presented enables both the spectroscopic and time domain properties of samples measured in reflection geometry to be better determined compared to previous calculation methods.
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2. Terahertz spectroscopy: Its future role in medical diagnoses
Edward Philip John Parrott, Yiwen Sun, Emma Pickwell-MacPherson
J. Mol. Struct. 1006(1-3): 66-76 (2011) DOI: 10.1016/j.molstruc.2011.05.048 Link to article
1. A study into the effect of subtle structural details and disorder on the terahertz spectrum of crystalline benzoic acid
Ruoyu Li, J Axel Zeitler, Daniele Tomerini, Edward P J Parrott, Lynn F Gladden, Graeme M Day
Phys. Chem. Chem. Phys. 12: 5329-5340 (2010) DOI: 10.1039/b926536h Link to article
Abstract
The phonon modes of crystalline benzoic acid have been investigated using terahertz time-domain spectroscopy, rigid molecule atom–atom model potential and plane-wave density functional theory lattice dynamics calculations. The simulation results show good agreement with the measured terahertz spectra and an assignment of the terahertz absorption features of benzoic acid is made with the help of both computational methods. Focussing on the strongest interactions in the crystal, we describe each vibration in terms of distortions of the benzoic acid hydrogen bonded dimers that are present in the crystal structure. The terahertz spectrum is also shown to be highly sensitive to the location of the carboxylic acid hydrogen atoms in the cyclic hydrogen-bonded dimers and we have systematically explored the influence of the observed disorder in the hydrogen atom positions on the lattice dynamics.
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2. Tuning the acid/base properties of nanocarbons by functionalization via amination.
Rosa Arrigo, Michael Ha, Sabine Wrabetz, Raoul Blume, Martin Lerch, James Mcgregor, Edward P J Parrott, J Axel Zeitler, Lynn F Gladden, Axel Knop-gericke, Robert Schlo
J. Am. Chem. Soc. 132(28): 9616-9630 (2010) DOI: 10.1021/ja910169v Link to article
Abstract
The surface chemical properties and the electronic properties of vapor grown carbon nanofibers (VGCNFs) have been modified by treatment of the oxidized CNFs with NH(3). The effect of treatment temperature on the types of nitrogen functionalities introduced was evaluated by synchrotron based X-ray photoelectron spectroscopy (XPS), while the impact of the preparation methods on the surface acid-base properties was investigated by potentiometric titration, microcalorimetry, and zeta potential measurements. The impact of the N-functionalization on the electronic properties was measured by THz-Time Domain spectroscopy. The samples functionalized via amination are characterized by the coexistence of acidic and basic O and N sites. The population of O and N species is temperature dependent. In particular, at 873 K nitrogen is stabilized in substitutional positions within the graphitic structure, as heterocyclic-like moieties. The surface presents heterogeneously distributed and energetically different basic sites. A small amount of strong basic sites gives rise to a differential heat of CO(2) adsorption of 150 kJ mol(-1). However, when functionalization is carried out at 473 K, nitrogen moieties with basic character are introduced and the maximum heat of adsorption is significantly lower, at approximately 90 kJ mol(-1). In the latter sample, energetically different basic sites coexist with acidic oxygen groups introduced during the oxidative step. Under these conditions, a bifunctional acidic and basic surface is obtained with high hydrophilic character. N-functionalization carried out at higher temperature changes the electronic properties of the CNFs as evaluated by THz-TDS. The functionalization procedure presented in this work allows high versatility and flexibility in tailoring the surface chemistry of nanocarbon material to specific needs. This work shows the potential of the N-containing nanocarbon materials obtained via amination in catalysis as well as electronic device materials.
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3. Quantitative water content measurements in food wafers using terahertz radiation
P Parasoglou, E P J Parrott, J A Zeitler, J Rasburn, H Powell, L F Gladden, M L Johns
Terahertz Sci. {\{}…{\ 3(4): 172-182 (2010) DOI: 10.11906/TST.172-182.2010.12.17 Link to article
Abstract
A quantitative, non-invasive method of measuring the moisture content ({\{}{\textless}{\}} 25 wt {\{}%{\}}) of solid food wafers is presented. The method utilizes terahertz (THz) radiation and exploits its high sensitivity to small amounts of water. A linear relationship between transmitted THz signal intensity and moisture content is observed in the range 0.2-0.6 THz. Higher frequencies are affected by scattering due to the size of the pores in the food wafer. A robust quantitative measurement protocol, which requires simple calibration, is implemented based on a direct interpretation of the THz time domain signal.
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4. Atomic charge distribution in sodosilicate glasses from terahertz time-domain spectroscopy
Edward P J Parrott, J Axel Zeitler, Guilhem Simon, Bernard Hehlen, Lynn F Gladden, Sergei N Taraskin, Stephen R Elliott
Phys. Rev. B 82(14): 140203 (2010) DOI: 10.1103/PhysRevB.82.140203 Link to article
Abstract
Terahertz time-domain spectroscopy has been used to extract the light-to-vibration coupling coefficient for sodosilicate glasses, from which it was possible to calculate the variance in the distribution of uncorrelated charges. It was found that increasing the sodium content of the glasses increased the standard deviation of the uncorrelated charge distribution in a linear fashion, and was almost an order of magnitude higher when compared to the charge distribution for pure silica reported previously, in agreement with previously published simulation data.
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5. Active coke: Carbonaceous materials as catalysts for alkane dehydrogenation
James McGregor, Zhenyu Huang, Edward P J Parrott, J Axel Zeitler, K Lien Nguyen, Jeremy M Rawson, Albert Carley, Thomas W Hansen, Jean-Philippe Tessonnier, Dang Sheng Su
J. Catal. 269(2): 329-339 (2010) DOI: 10.1016/j.jcat.2009.11.016 Link to article
Abstract
The catalytic dehydrogenation (DH) and oxidative dehydrogenation (ODH) of light alkanes are of significant industrial importance. In this work both carbonaceous material deposited on VOx/Al2O3 catalysts during reaction and unsupported carbon nanofibres (CNFs) are shown to be active for the dehydrogenation of butane in the absence of gas-phase oxygen. Their activity in these reactions is shown to be dependent upon their structure, with different reaction temperatures yielding structurally different coke deposits. Terahertz time-domain spectroscopy (THz-TDS), among other techniques, has been applied to the characterisation of these deposits – the first time this technique has been employed in coke studies. TEM and other techniques show that coke encapsulates the catalyst, preventing access to VOx sites, without a loss of activity. Studies on CNFs confirm that carbonaceous materials act as catalysts in this reaction. Carbon-based catalysts represent an important new class of potential catalysts for DH and ODH reactions.
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1. Terahertz pulsed spectroscopic imaging using optimized binary masks
Y C Shen, L Gan, M Stringer, A Burnett, K Tych, H Shen, J E Cunningham, E P J Parrott, J A Zeitler, L F Gladden, E H Linfield, A G Davies
Appl. Phys. Lett. 95(23): 231112 (2009) DOI: 10.1063/1.3271030 Link to article
Abstract
We report the development of a terahertz pulsed spectroscopicimagingsystem based on the concept of compressive sensing. A single-point terahertz detector, together with a set of 40 optimized two-dimensional binary masks, was used to measure the terahertz waveforms transmitted through a sample. Terahertz time- and frequency-domain images of the sample comprising 20x20 pixels were subsequently reconstructed. We demonstrate that both the spatial distribution and the spectral characteristics of a sample can be obtained by this means. Compared with conventional terahertz pulsed imaging, no raster scanning of the object is required, and ten times fewer terahertz spectra need be taken. It is therefore ideal for real-time imaging applications.
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2. Testing the sensitivity of terahertz spectroscopy to changes in molecular and supramolecular structure: A study of structurally similar cocrystals
Edward P J Parrott, J Axel Zeitler, Tomislav Friscić, Michael Pepper, William Jones, Graeme M Day, Lynn F Gladden
Cryst. Growth Des. 9(3): 1452-1460 (2009) DOI: 10.1021/cg8008893 Link to article
Abstract
Terahertz time-domain-spectroscopy (THz-TDS) has emerged as a versatile spectroscopic technique, and an alternative to powder X-ray diffraction in the characterization of molecular crystals. We tested the ability of terahertz spectroscopy to distinguish between chiral and racemic hydrogen-bonded cocrystals that are similar in molecular and supramolecular structure. Terahertz spectroscopy readily distinguished between the isostructural cocrystals of theophylline with chiral and racemic forms of malic acid which are almost identical in molecular structure and supramolecular architecture. Similarly, the cocrystals of theophylline with chiral and racemic forms of tartaric acid, which are similar at the molecular level but dissimilar in crystal packing, were distinguished unequivocally. The investigation of the same cocrystals using X-ray powder diffraction and Raman spectroscopy suggested that THz-TDS is comparable in sensitivity to diffraction methods and more sensitive than Raman spectroscopy to changes in cocrystal architecture. The differences in spectra acquired by THz-TDS could be further enhanced by cooling the samples to 109 K.
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3. Extracting accurate optical parameters from glasses using terahertz time-domain spectroscopy
E P J Parrott, J A Zeitler, L F Gladden, S N Taraskin, S R Elliott
J. Non-Cryst. Solids 355(37-42): 1824-1827 (2009) DOI: 10.1016/j.jnoncrysol.2009.04.071 Link to article
Abstract
erahertz time-domain spectroscopy (THz-TDS) can be used to characterize the optical properties of glasses in a frequency regime that is barely accessible by other techniques. It can be used to measure the electric field of a pulse of broadband terahertz radiation with frequency components between 0.1 and 4.5 THz propagating through the material of interest. Data processing is required to transform the information from the raw time-domain waveform data into the absorption spectrum. While most crystalline materials exhibit a number of distinct spectral features, amorphous materials show an almost featureless frequency response in the terahertz range and very accurate measurements of the absorption coefficient are required to characterize the materials. The accuracy of existing data processing techniques is limited as a result of truncating the time-domain data to remove multiple reflections of the terahertz pulse within the sample material and by neglecting scattering losses at the surface of the sample material. Using the experimental THz-TDS data of As2S3 we present a new approach to extract accurate optical parameters from THz-TDS experimental data.
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4. Understanding the Dielectric Properties of Heat-Treated Carbon Nanofibers at Terahertz Frequencies: a New Perspective on the Catalytic Activity of Structured Carbonaceous Materials
Edward P J Parrott, J Axel Zeitler, James McGregor, Shu-Pei Oei, Husnu Emrah Unalan, Swee-Ching Tan, William I Milne, Jean-Philippe Tessonnier, Robert Schlögl, Lynn F Gladden
J. Phys. Chem. C 113: 10554-10559 (2009) DOI: 10.1021/jp811226d Link to article
Abstract
Terahertz time domain spectroscopy (THz-TDS) has been used to study the electrical and optical properties of a series of carbon nanofibers (CNFs) that have undergone different heat treatments. The high-temperature heat-treated (HHT) sample displayed increased absorption and real refractive indices across the range 0.3−3.5 THz when compared to the low-temperature heat-treated (LHT) and pyrolitically stripped (PS) samples. The experimental results were fitted by using a Drude−Lorentz model and an effective medium approximation to yield the electrical parameters of the sample such as the plasma frequency, phonon mode frequency, and oscillator strength. These parameters were used to rationalize the differences as being due to an increase in graphitic order in the HHT sample when compared to the LHT sample and to an even greater extent the PS sample. HHT, LHT, and PS CNFs can be used as catalysts for the oxidative dehydrogenation of ethylbenzene to styrene. They exhibit different catalytic yield and selectivity which can be correlated with their dielectric properties at terahertz frequencies. The results suggest that THz-TDS is a useful tool for characterizing the graphiticity of CNFs in terms of electon density and mobility which, in turn, correlate with the catalytic performance of these materials.
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5. Accurate determination of optical coefficients from chemical samples using terahertz time-domain spectroscopy and effective medium theory
Edward P J Parrott, J Axel Zeitler, Lynn F Gladden
Opt. Lett. 34(23): 3722-3724 (2009) DOI: 10.1364/OL.34.003722 Link to article
Abstract
A technique for calculating the optical constants of powdered materials that have been pressed into pellets along with a diluent such as polyethylene, as is commonly used in the spectroscopy community, is introduced. The simple Beer–Lambert law typically used to calculate the optical constants has the inherent weakness that it treats the absorbing medium as a single nonporous solid, as opposed to a dielectric material embedded within a medium. This leads to a systematic underestimation of both the absorption coefficient and the refractive index, especially at low filling factors. Effective medium theory provides a way to calculate more accurate optical constants and produces similar optical constants across a wide range of filling factors, including crucially at the low filling factors commonly used experimentally. The technique can also be extended to estimate the porosity of the samples and provide a true, preparation-independent value for the optical properties of the sample material.
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6. The Use of Terahertz Spectroscopy as a Sensitive Probe in Discriminating the Electronic Properties of Structurally Similar Multi-Walled Carbon Nanotubes
Edward P J Parrott, J Axel Zeitler, James McGregor, Shu-Pei Oei, Husnu Emrah Unalan, William I Milne, Jean-Philippe Tessonnier, Dang Sheng Su, Robert Schlögl, Lynn F Gladden