The Chinese University of Hong Kong | Faculty of Engineering | Department of Electronic Engineering | Division of Biomedical Engineering


Nationality British

Education History

Postgraduate (2002-2005)
University of Cambridge (United Kingdom)
Physics Ph. D.

Undergraduate (1997-2001)
University of Cambridge (United Kingdom)
Natural Sciences (Physical) M. Sci., M. A. (Cantab.)

Work History

Associate Professor (2015-present)
The Chinese University of Hong Kong, Hong Kong
Department of Electronic Engineering

Assistant Professor (2012-2015)
The Chinese University of Hong Kong, Hong Kong
Department of Electronic Engineering

Visiting Assistant Professor (2009-2012)
The Hong Kong University of Science and Technology, Hong Kong
Electronic and Computer Engineering Department

Assistant Professor (2006-2009)
The Chinese University of Hong Kong, Hong Kong
Department of Electronic Engineering

Medical Scientist (2005-2006)
Teraview, United Kingdom

Research Scientist (2002)
Teraview, United Kingdom

Research Scientist (2001-2002)
BT Labs, United Kingdom
Optics Group

Research Interests

Terahertz biomedical imaging and spectroscopy, terahertz metrology, novel terahertz devices. More details here.


Emma MacPherson (nee Pickwell) studied Natural Sciences at Cambridge University followed by an MSci in Physics where she specialised in Semiconductor Physics. After Cambridge, she worked on optical packet switching at BT labs, Martlesham, UK. In February 2002 she started working as a research assistant at TeraView Ltd, a company specialising in terahertz imaging which had spun out of Toshiba in 2001. She started her PhD with the Semiconductor Physics Group at Cambridge University and TeraView Ltd in October 2002. Her PhD thesis was entitled, "Biological applications of terahertz pulsed imaging and spectroscopy" - her PhD work focused on understanding contrast mechanisms in terahertz images of skin cancer. Having completed her thesis in 2005, she worked for TeraView Ltd as a Medical Scientist until moving to Hong Kong in 2006. Emma MacPherson set up a terahertz laboratory at the Department of Electronic Enginering, CUHK during her post between 2006 and 2009 as an assistant professor. She spent 3 years at HKUST as a visiting assistant professor (September 2009 -2012). She returned to the Department of Electronic Enginering, CUHK in September 2012. Prof MacPherson represents Hong Kong on the International Organising Commitee for the Infrared and Milimeter Wave and Terahertz Wave conference series.

Publication List

Below are listed the publications of Prof. MacPherson, 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. Broadband amplitude, frequency, and polarization splitter for terahertz frequencies using parallel-plate waveguide technology
    A. I. Hernandez-Serrano, Daniel M. Mittleman, Emma Pickwell-MacPherson
    Optics Letters 45(5): 1208-1211 (2020) DOI: 10.1364/ol.45.001208 Link to article
  2. Super Sub-Nyquist Single-Pixel Imaging by Total Variation Ascending Ordering of the Hadamard Basis
    Xiao Yu, Rayko Ivanov Stantchev, Fan Yang, Emma Pickwell-MacPherson
    Scientific Reports 10: 9338 (2020) DOI: 10.1038/s41598-020-66371-5 Link to article
    Single pixel imaging (SPI) captures images without array detectors or raster scanning. When combined with compressive sensing techniques it enables novel solutions for high-speed optical imaging and spectroscopy. However, when it comes to the real-time capture and analysis of a fast event, the challenge is the inherent trade-off between frame rate and image resolution. Due to the lack of sufficient sparsity and the intrinsic iterative process, conventional compressed sensing techniques have limited improvement in capturing natural scenes and displaying the images in real time. In this work, we demonstrate a novel alternative compressive imaging approach employing an efficient and easy-implementation sampling scheme based on reordering the deterministic Hadamard basis through their total variation. By this means, the number of measurements and acquisition are reduced significantly without needing complex minimization algorithms. We can recover a 128 × 128 image with a sampling ratio of 5% at the signal peak signal-to-noise ratio (PSNR) of 23.8 dB, achieving super sub-Nyquist sampling SPI. Compared to other widely used sampling e.g. standard Hadamard protocols and Gaussian matrix methods, this approach results in a significant improvement both in the compression ratio and image reconstruction quality, enabling SPI for high frame rate imaging or video applications.
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  1. In vivo estimation of water diffusivity in occluded human skin using terahertz reflection spectroscopy
    Qiushuo Sun, Rayko I. Stantchev, Jiarui Wang, Edward P.J. Parrott, Alan Cottenden, Tor-Wo Chiu, Anil T. Ahuja, Emma Pickwell-MacPherson
    Journal of Biophotonics 12(2): e201800145 (2019) DOI: 10.1002/jbio.201800145 Link to article
    Water diffusion and the concentration profile within the skin significantly affect the surrounding chemical absorption and molecular synthesis. Occluding the skin causes water to accumulate in the top layer of the skin (the stratum corneum) and also affects the water diffusivity. Scar treatments such as silicone gel and silicone sheets make use of occlusion to increase skin hydration. However with existing techniques, it is not possible to quantitatively measure the diffusivity of the water during occlusion: current methods determine water diffusivity by measuring the water evaporated through the skin and thus require the skin to breathe. In this work we use the high sensitivity of terahertz light to water to study how the water content in the stratum corneum changes upon occlusion. From our measurements, we can solve the diffusion equations in the stratum corneum to deduce the water concentration profile in occluded skin and subsequently to determine the diffusivity. To our knowledge this is the first work showing how the diffusivity of human skin can be measured during occlusion and we envisage this paper as being used as a guide for non-invasively determining the diffusivity of occluded human skin in vivo. This article is protected by copyright. All rights reserved.
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  2. Utilizing multilayer structures to enhance terahertz characterization of thin films ranging from aqueous solutions to histology slides
    Qiushuo Sun, Kai Liu, Xuequan Chen, Xudong Liu, A. I. Hernandez-Serrano, Emma Pickwell-MacPherson
    Optics Letters 44(9): 2149 (2019) DOI: 10.1364/OL.44.002149 Link to article
    We propose a multilayer geometry to characterize thin-film samples in reflection terahertz time domain spectroscopy. Theory indicates that this geometry has higher sensitivity compared to ordinary transmission or reflection geometries when characterizing both low- and high-absorption samples. Pure water and water{\&}{\#}x2013;ethanol mixtures are measured to verify the characterization accuracy of the proposed geometry and its capability to measure trace liquids. Paraffin-embedded oral cancer tissue is imaged to further show how the proposed geometry enhances the sensitivity for solid low-absorptive films.
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  3. A Sensitive and Versatile Thickness Determination Method Based on Non-Inflection Terahertz Property Fitting
    Xuequan Chen, Emma Pickwell-MacPherson
    Sensors 19: 4118 (2019) DOI: 10.3390/s19194118 Link to article
    The accuracy of thin-film characterization in terahertz spectroscopy is mainly set by the thickness uncertainty. Physical thickness measurement has limited accuracy for thin-film samples thinner than a few hundreds of micrometers and is sometimes even impossible. The temporal resolution of time-domain terahertz spectrometers is not sufficient to resolve such thin films. Previously reported numerical methods mainly only work for materials with low dispersion and absorption. Here, we propose a novel method for thickness determination by fitting a non-inflection offset exponential function to the material optical properties. Theoretical analysis predicts the best fitting to only be achieved when the correct thickness is given. Transmission measurements on a thin-film polymer, water, and a lactose pallet verify the theory and show the accurate thickness determination and property characterization on materials which are either achromatic or dispersive, transparent or absorptive, featureless or resonant. The measurements demonstrate the best versatility and sensitivity compared to the state-of-art. The method could be widely adapted to various types of research and industrial applications.
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  4. Terahertz Microfluidic Metamaterial Biosensor for Sensitive Detection of Small-Volume Liquid Samples
    Rui Zhang, Qingming Chen, Kai Liu, Zefeng Chen, Kaidi Li, Xuming Zhang, Jianbin Xu, Emma Pickwell-MacPherson
    IEEE Transactions on Terahertz Science and Technology 9(2): 209-214 (2019) DOI: 10.1109/TTHZ.2019.2898390 Link to article
    Metamaterial (MM) assisted terahertz (THz) label-free biosensing has promising applications. However, the sensitive THz detection of highly absorptive liquid samples remains challenging. Here, we present a novel multi-microfluidic-channel MM biosensor for highly sensitive THz sensing of small-volume liquid samples. The multichannels are set mostly in the strong electric field enhancement area of the MM, which significantly decreases the liquid's amount and enhances the interaction between the sensing targets and the THz wave (thus increasing the sensitivity). The sensing results of isopropyl-alcohol-water mixtures and bovine serum albumin solutions based on the bow-tie array MM with multichannels demonstrate the effectiveness of the proposed design and the great potential in THz biosensing. This design has the advantages of being highly sensitive, label-free, cost-effective, easy to operate, and only needing a tiny liquid volume. Thus, our device provides a robust route for MM-assisted THz label-free biosensing of liquid-based substances.
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  5. In vivo terahertz imaging to evaluate scar treatment strategies: silicone gel sheeting
    Jiarui Wang, Qiushuo Sun, Rayko I. Stantchev, Tor-Wo Chiu, Anil T. Ahuja, Emma Pickwell-MacPherson
    Biomedical Optics Express 10(7): 3584-3590 (2019) DOI: 10.1364/BOE.10.003584 Link to article
    Silicone gel sheeting (SGS) is widely used for scar treatment; however, studies showing its interaction with skin and efficacy of scar treatment are still lacking. THz light is non-ionizing and highly sensitive to changes in water content and thus skin hydration. In this work, we use in-vivo THz imaging to monitor how SGS affects the THz response of human skin during occlusion, and the associated THz reflectivity and refractive index changes are presented. We find that SGS effectively hydrates the skin beneath it, with minimal lateral effects beyond the sheeting. Our work demonstrates that THz imaging is able to detect the subtle hydration changes on the surface of human skin caused by SGS, and it has the potential to be used to evaluate different scar treatment strategies.
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  6. Design and fabrication of 3-D printed conductive polymer structures for THz polarization control
    A. I. Hernandez-Serrano, Qiushuo Sun, Elizabeth G. Bishop, Elliott R. Griffiths, Christopher P. Purssell, Simon J. Leigh, J. Lloyd-Hughes, Emma Pickwell-MacPherson
    Optics Express 27(8): 11635 (2019) DOI: 10.1364/OE.27.011635 Link to article
    In this paper, we numerically and experimentally demonstrate the inverse polarization effect in three-dimensional (3-D) printed polarizers for the frequency range of 0.5 - 2.7 THz. The polarizers simply consist of 3-D printed strip lines of conductive polylactic acid (CPLA, Proto-Pasta) and do not require a substrate or any further metallic deposition. The experimental and numerical results show that the proposed structure acts as a broadband polarizer between the range of 0.3 THz to 2.7 THz, in which the inverse polarization effect is clearly seen for frequencies above 0.5 THz. In the inverse polarization effect, the transmission of the transverse electric (TE) component exceeds that of the TM component, in contrast to the behavior of a typical wire-grid polarizer. We show how the performance of the polarizers depends on the spacing and thickness of the CPLA structure; extinction ratios higher than 20 dB are achieved. This is the first report using CPLA to fabricate THz polarizers, demonstrating the potential of using conductive polymers to design THz components efficiently and robustly.
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  1. Robust and accurate terahertz time-domain spectroscopic ellipsometry
    Xuequan Chen, Edward P. J. Parrott, Zhe Huang, Hau-Ping Chan, Emma Pickwell-MacPherson
    Photonics Research 6(8): 768 (2018) DOI: 10.1364/PRJ.6.000768 Link to article
    In this work, we show how fiber-based terahertz systems can be robustly configured for accurate terahertz ellipsometry. To this end, we explain how our algorithms can be successfully applied to achieve accurate spectroscopic ellipsometry with a high tolerance on the imperfect polarizer extinction ratio and pulse shift errors. Highly accurate characterization of transparent, absorptive, and conductive samples comprehensively demonstrates the versatility of our algorithms. The improved accuracy we achieve is a fundamental breakthrough for reflection-based measurements and overcomes the hurdle of phase uncertainty.
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  2. Graphitic carbon nitride nanosheet wrapped mesoporous titanium dioxide for enhanced photoelectrocatalytic water splitting
    Lin Jing, Wee Jun Ong, Rui Zhang, Emma Pickwell-MacPherson, Jimmy C. Yu
    Catalysis Today 315: 103-109 (2018) DOI: 10.1016/j.cattod.2018.04.007 Link to article
    We report a new strategy to fabricate a core-shell TiO2@g-C3N4 composite for photoelectrochemical water splitting. The heterojunction structure is prepared by chemically wrapping exfoliated thin layer g-C3N4 nanosheets on the surface of anatase TiO2 particles. The TiO2@g-C3N4 sample demonstrates high visible-light photoactivity towards water splitting, resulting in an increase in photocurrent density by a factor of 2.5 times compared to the bare TiO2. This is ascribed to the inhibition of electron-hole pair recombination due to the synergistic effect between TiO2 and g-C3N4, which enhances the charge transfer and separation. The prolonged lifetime of the charges is confirmed by using the transient absorption spectroscopic measurements.
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  3. Graphene controlled Brewster angle device for ultra broadband terahertz modulation
    Zefeng Chen, Xuequan Chen, Li Tao, Kun Chen, Mingzhu Long, Xudong Liu, Keyou Yan, Rayko I. Stantchev, Emma Pickwell-MacPherson, Jian-Bin Xu
    Nature Communications 9(1): 4909 (2018) DOI: 10.1038/s41467-018-07367-8 Link to article
    Terahertz modulators with high tunability of both intensity and phase are essential for effective control of electromagnetic properties. Due to the underlying physics behind existing approaches there is still a lack of broadband devices able to achieve deep modulation. Here, we demonstrate the effect of tunable Brewster angle controlled by graphene, and develop a highly-tunable solid-state graphene/quartz modulator based on this mechanism. The Brewster angle of the device can be tuned by varying the conductivity of the graphene through an electrical gate. In this way, we achieve near perfect intensity modulation with spectrally flat modulation depth of 99.3 to 99.9 percent and phase tunability of up to 140 degree in the frequency range from 0.5 to 1.6 THz. Different from using electromagnetic resonance effects (for example, metamaterials), this principle ensures that our device can operate in ultra-broadband. Thus it is an effective principle for terahertz modulation.
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  4. Towards a Rapid Terahertz Liquid Crystal Phase Shifter: Terahertz In-Plane and Terahertz Out-Plane (TIP-TOP) Switching
    Benjamin S.-Y. Ung, Xudong Liu, Edward P. J. Parrott, Abhishek Kumar Srivastava, Hongkyu Park, Vladimir G. Chigrinov, Emma Pickwell-MacPherson
    IEEE Transactions on Terahertz Science and Technology 8(2): 209-214 (2018) DOI: 10.1109/TTHZ.2018.2790708 Link to article
    Terahertz (THz) phase shifters are an essential component needed to realize many potential applications. Liquid crystals (LC) are commonly used at optical frequencies, yet to achieve an equivalent phase shift at THz frequencies the LC layer needs to be orders of magnitude thicker. Consequently, the time for the LC to relax back to its initial state is prohibitively slow. In this paper, we show for the first time how a thick, nematic phase LC cell can be switched actively in both directions, thereby achieving fast phase shifting of THz light. We call this THz in-plane and THz out-plane (TIP-TOP) switching. To achieve this, we have designed and fabricated a novel electrode structure, able to switch to and from both in- and out-plane orientations (TIP-TOP). The performance of the fabricated device provides an actively controllable phase delay with an ON-OFF cycle switching time of approximately 0.5 s: almost 100 times faster than the usual cycle time which exceeds 40 s. Furthermore, the analysis of the director distributions allows us to understand the causes of the asymmetric switching times. The TIP-TOP cell presents the capability to work as a low insertion loss, fast THz phase shifter and could be scaled up to realize a phased array device.
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  5. Invited Article: An active terahertz polarization converter employing vanadium dioxide and a metal wire grating in total internal reflection geometry
    Xudong Liu, Xuequan Chen, Edward P. J. Parrott, Chunrui Han, Georges Humbert, Aurelian Crunteanu, Emma Pickwell-MacPherson
    APL Photonics 3(5): 051604 (2018) DOI: 10.1063/1.5010940 Link to article
    Active broadband terahertz (THz) polarization manipulation devices are challenging to realize, but also of great demand in broadband terahertz systems. Vanadium dioxide (VO2) shows a promising phase transition for active control of THz waves and provides broadband polarization characteristics when integrated within grating-type structures. We creatively combine a VO2-based grating structure with a total internal reflection (TIR) geometry providing a novel interaction mechanism between the electromagnetic waves and the device, to realize a powerful active broadband THz polarization-controlling device. The device is based on a Si-substrate coated with a VO2 layer and a metal grating structure on top, attached to a prism for generating the TIR condition on the Si-VO2-grating interface. The grating is connected to electrodes for electrically switching the VO2 between its insulating and conducting phases. By properly selecting the incident angle of the THz waves, the grating direction, and the incident polariz...
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  6. Highly Sensitive Terahertz Thin-Film Total Internal Reflection Spectroscopy Reveals in Situ Photoinduced Structural Changes in Methylammonium Lead Halide Perovskites
    Qiushuo Sun, Xudong Liu, Jie Cao, Rayko I. Stantchev, Yang Zhou, Xuequan Chen, Edward P. J. Parrott, James Lloyd-Hughes, Ni Zhao, Emma Pickwell-MacPherson
    The Journal of Physical Chemistry C 122(30): 17552-17558 (2018) DOI: 10.1021/acs.jpcc.8b05695 Link to article
    Terahertz (THz) thin-film total internal reflection (TF-TIR) spectroscopy is shown to have an enhanced sensitivity to the vibrational properties of thin films in comparison with standard THz transmission spectroscopy. This increased sensitivity was used to track photoinduced modifications to the structure of thin films of methylammonium (MA) lead halide, MAPbI3–xBrx (x = 0, 0.5, 1, and 3). Initially, illumination strengthened the phonon modes around 2 THz, associated with Pb–I stretch modes coupled to the MA ions, whereas the 1 THz twist modes of the inorganic octahedra did not alter in strength. Under longer term illumination, the 1 THz phonon modes of encapsulated films slowly reduced in strength, whereas in films exposed to moisture and oxygen, these phonons weaken more rapidly and blue-shift in frequency. The rapid monitoring of environmentally induced changes to the vibrational modes afforded by TF-TIR spectroscopy offers applications in the characterization and quality control of the perovskite thin...
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  7. In vivo THz imaging of human skin: Accounting for occlusion effects
    Qiushuo Sun, Edward P.J. Parrott, Yuezhi He, Emma Pickwell-MacPherson
    Journal of Biophotonics 11(2): e201700111 (2018) DOI: 10.1002/jbio.201700111 Link to article
    In vivo terahertz (THz) imaging of human skin needs to be done in reflection geometry due to the high attenuation of THz light by water in the skin. To aid the measurement procedure, there is typically an imaging window onto which the patient places the area of interest. The window enables better pulse alignment and helps keep the patient correctly positioned during the measurement. In this paper, we demonstrate how the occlusion caused by the skin contact with the imaging window during the measurement affects the THz response. By studying both rapid point measurements and imaging over an area of a human volar forearm, we find that even 5 seconds of occlusion affects the THz response. As the occlusion time increases, the skin surface water content increases, resulting in the reduction of the amplitude of the reflected THz pulse, especially in the first 3 minutes. Furthermore, it was found that the refractive index of the volar forearm increased by 10% to 15% after 20 minutes of occlusion. In this work, we examine and propose a model for the occlusion effects due to the quartz window with a view to compensating for its influence.
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  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
    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. Determination of terahertz permittivity of dehydrated biological samples
    Yuezhi He, Kai Liu, Corinna Au, Qiushuo Sun, Edward P.J. Parrott, Emma Pickwell-MacPherson
    Physics in Medicine and Biology 62(23): 8882-8893 (2017) DOI: 10.1088/1361-6560/aa8ebe Link to article
    Abstract A key step to transform terahertz imaging to a practical medical imaging modality lies in the understanding the interactions between terahertz (THz) waves and biological tissues. Most of the models in the literature use the permittivity of liquid water to simulate the THz-tissue interactions but they often neglect the contributions from biological background such as proteins and lipids as dehydrated biological samples are experimentally difficult to prepare. In this work, we present a method to prepare thin and flat dehydrated samples which can be easily handled and measured in a transmission setup. Our results will also provide fundamental parameters for modelling THz-tissue interactions.
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  3. 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
    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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  4. Composite multiscale entropy analysis of reflective terahertz signals for biological tissues
    Rui Zhang, Yuezhi He, Kai Liu, Liangliang Zhang, Shijing Zhang, Emma Pickwell-MacPherson, Yuejin Zhao, Cunlin Zhang
    Optics Express 25(20): 23669 (2017) DOI: 10.1364/OE.25.023669 Link to article
    We demonstrate a composite multiscale entropy (CMSE) method of terahertz (THz) signal complexity analysis to distinguish different biological tissues. The THz signals reflected from fresh porcine skin and muscle tissues were measured and analyzed. The statistically significant difference and separation of the two tissues based on several parameters were analyzed and compared for THz spectroscopy and imaging, which verified the better performance of the CMSE method and further enhancement of the contrast among THz signals that interact with different tissues. This process provides a better analysis and discrimination method for THz spectroscopy and imaging in biomedical applications.
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  5. 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
    Advanced Optical Materials 5(3) (2017) DOI: 10.1002/adom.201600697 Link to article
    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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  6. 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
    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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  7. 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
    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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  1. 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
    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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  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
    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. 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
    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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  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
    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. 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
    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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  1. 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
    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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  2. Label-free detection and characterization of the binding of hemagglutinin protein and broadly neutralizing monoclonal antibodies using terahertz spectroscopy
    Yiwen Sun, Junlan Zhong, Cunlin Zhang, Jian Zuo, Emma Pickwell-MacPherson
    J. Biomed. Opt. 20(3): 37006 (2015) DOI: 10.1117/1.JBO.20.3.037006 Link to article
  3. 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
    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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  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
    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. 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
    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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  2. 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
    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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  3. The growth of biomedical terahertz research
    Shuting Fan, Yuezhi He, Benjamin S Ung, Emma Pickwell-MacPherson
    J. Phys. D. Appl. Phys. 47(37): 374009 (2014) DOI: 10.1088/0022-3727/47/37/374009 Link to article
  4. Automatic online detection of atrial fibrillation based on symbolic dynamics and Shannon entropy
    Xiaolin Zhou, Hongxia Ding, Benjamin Ung, Emma Pickwell-MacPherson, Yuanting Zhang
    Biomed. Eng. Online 13(1): 18 (2014) DOI: 10.1186/1475-925X-13-18 Link to article
  5. Use of Finite Difference Time Domain Simulations and Debye Theory for Modelling the Terahertz Reflection Response of Normal and Tumour Breast Tissue
    Anthony J Fitzgerald, Emma Pickwell-MacPherson, Vincent P Wallace
    PLoS One 9(7): e99291 (2014) DOI: 10.1371/journal.pone.0099291 Link to article
  1. 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
    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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  2. Guest Editorial: Terahertz imaging and spectroscopy for biology and biomedicine
    Emma Pickwell-MacPherson, Gian Piero Gallerano, Gun-Sik Park, Henning Hintzsche, Gerald Joseph Wilmink
    IEEE J. Biomed. Heal. Informatics 17(4): 765-767 (2013) DOI: 10.1109/JBHI.2013.2257333 Link to article
  3. 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
    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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  1. 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
  2. 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
  3. 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
  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
  5. The potential of terahertz imaging for cancer diagnosis: A review of investigations to date
    Calvin Yu, Shuting Fan, Yiwen Sun, Emma Pickwell-MacPherson
    Quantitative Imaging in Medicine and Surgery 2(1): 33-45 (2012) DOI: 10.3978/j.issn.2223-4292.2012.01.04 Link to article
    The terahertz region lies between the microwave and infrared regions of the electromagnetic spectrum such that it is strongly attenuated by water and very sensitive to water content. Terahertz radiation has very low photon energy and thus it does not pose any ionization hazard for biological tissues. Because of these characteristic properties, there has been an increasing interest in terahertz imaging and spectroscopy for biological applications within the last few years and more and more terahertz spectra are being reported, including spectroscopic studies of cancer. The presence of cancer often causes increased blood supply to affected tissues and a local increase in tissue water content may be observed: this acts as a natural contrast mechanism for terahertz imaging of cancer. Furthermore the structural changes that occur in affected tissues have also been shown to contribute to terahertz image contrast. This paper introduces terahertz technology and provides a short review of recent advances in terahertz imaging and spectroscopy techniques. In particular investigations relating to the potential of terahertz imaging and spectroscopy for cancer diagnosis will be highlighted.
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  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
    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. Total variation deconvolution for terahertz pulsed imaging
    Yang Chen, Yiwen Sun, Emma Pickwell-MacPherson
    Inverse Probl. Sci. Eng. 19(2): 223-232 (2011) DOI: 10.1080/17415977.2010.550045 Link to article
  3. A promising diagnostic method: Terahertz pulsed imaging and spectroscopy
    Yiwen Sun, Stanley M Y Sy, Y X Wang, Anil T Ahuja, Yuanting Zhang, Emma Pickwell-MacPherson
    World J. Radiol. 3(3): 55 (2011) DOI: 10.4329/wjr.v3.i3.55 Link to article
  4. 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
  5. Terahertz pulsed imaging of freshly excised human colonic tissues
    Caroline B Reid, Anthony J Fitzgerald, George Reese, Robert Goldin, Emma Pickwell-MacPherson, Adam P Gibson, Vincent P Wallace
    Phys. Med. Biol. 56: 4333-4353 (2011) DOI: 10.1088/0031-9155/56/14/008 Link to article
  6. The effects of pre-ejection period on post-exercise systolic blood pressure estimation using the pulse arrival time technique
    Mico Yee Man Wong, Emma Pickwell-MacPherson, Yuan Ting Zhang, Jack C Y Cheng
    Eur. J. Appl. Physiol. 111(1): 135-144 (2011) DOI: 10.1007/s00421-010-1626-0 Link to article
  1. Improving extraction of impulse response functions using stationary wavelet shrinkage in terahertz reflection imaging
    Yang Chen, Yiwen Sun, Emma Pickwell-MacPherson
    Fluct. Noise Lett. (2010) DOI: 10.1142/S0219477510000307 Link to article
  2. Accuracy and resolution of THz reflection spectroscopy for medical imaging.
    Caroline B Reid, Emma Pickwell-MacPherson, Jan G Laufer, Adam P Gibson, Jeremy C Hebden, Vincent P Wallace
    Phys. Med. Biol. 55(16): 4825-4838 (2010) DOI: 10.1088/0031-9155/55/16/013 Link to article
    The use of THz radiation as a potential tool for medical imaging is of increasing interest. In this paper three methods of analysis of THz spectroscopic information for diagnosis of tissue pathologies at THz frequencies are presented. The frequency-dependent absorption coefficients, refractive indices and Debye relaxation times of pure water and pure lipids were measured and used as prior knowledge in the different theoretical methods for the determination of concentration. Three concentration analysis methods were investigated: (a) linear spectral decomposition, (b) spectrally averaged dielectric coefficient method and (c) the Debye relaxation coefficient method. These methods were validated on water and lipid emulsions by determining the concentrations of phantom chromophores and comparing to the known composition. The accuracy and resolution of each method were determined to assess the potential of each method as a tool for medical diagnosis at THz frequencies.
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  3. Practical Considerations for in Vivo THz Imaging
    Emma Pickwell-MacPherson
    Terahertz Sci. Technol. 3(4): 163-171 (2010)
  4. Contactless and continuous monitoring of heart rate based on photoplethysmography on a mattress
    M Y M Wong, E Pickwell-MacPherson, Y T Zhang
    Physiol. Meas. 31(7): 1065-1074 (2010) DOI: 10.1088/0967-3334/31/7/014 Link to article
    This paper reports a novel contactless monitoring method to record photoplethysmogram (PPG) on a mattress for the continuous measurement of heart rate (HR). PPGs were obtained from subjects' fingers and backs with and without making a direct contact between the PPG sensor and their skin when they rested in a supine position on the mattress. Electrocardiograms (ECGs) were measured from the subjects' limbs for reference. Clear PPG waveforms were obtained from the subjects' backs. Beat-to-beat HR derived from contactless PPG measurement was comparable to those measured from contact PPG and ECG measurements. Thus we found that contactless PPG could be captured from the subjects' backs and it was sufficient to provide accurate HR measurements. This contactless monitoring of PPG has the potential to reduce obstruction in sleep and provide clinical evaluation in sleep study.
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  5. Frequency-Wavelet Domain Deconvolution for terahertz reflection imaging and spectroscopy.
    Yang Chen, Shengyang Huang, Emma Pickwell-MacPherson
    Opt. Express 18(2): 1177-1190 (2010) Link to article
    In terahertz reflection imaging, a deconvolution process is often employed to extract the impulse function of the sample of interest. A band-pass filter such as a double Gaussian filter is typically incorporated into the inverse filtering to suppress the noise, but this can result in over-smoothing due to the loss of useful information. In this paper, with a view to improving the calculation of terahertz impulse response functions for systems with a low signal to noise ratio, we propose a hybrid Frequency-Wavelet Domain Deconvolution (FWDD) for terahertz reflection imaging. Our approach works well; it retrieves more accurate impulse response functions than existing approaches and these impulse functions can then also be used to better extract the terahertz spectroscopic properties of the sample.
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  6. Terahertz pulsed imaging of knee cartilage
    Wai-Chi Kan, Win-Sze Lee, Wing-Hoi Cheung, Vincent P Wallace, Emma Pickwell-MacPherson
    Biomed. Opt. Express 1(3): 967 (2010) DOI: 10.1364/BOE.1.000967 Link to article
  1. Effects of formalin fixing on the terahertz properties of biological tissues
    Yiwen Sun, Bernd M Fischer, Emma Pickwell-MacPherson
    J. Biomed. Opt. 14(6): 64017 (2009) Link to article
    Wedemonstrate how the terahertz properties of porcine adipose tissue andskeletal muscle are affected by formalin fixing. Terahertz radiation issensitive to covalently cross-linked proteins and can be used toprobe unique spectroscopic signatures. We study in detail the changesarising from different fixation times and see that formalin fixingreduces the refractive index and the absorption coefficient of thesamples in the terahertz regime. These fundamental properties affect thetime-domain terahertz response of the samples and determine the levelof image contrast that can be achieved. {\{}{\textcopyright}{\}}2009 Society of Photo-Optical Instrumentation Engineers
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  2. Terahertz pulsed spectroscopy of freshly excised human breast cancer.
    Philip C Ashworth, Emma Pickwell-MacPherson, Elena Provenzano, Sarah E Pinder, Anand D Purushotham, Michael Pepper, Vincent P Wallace
    Opt. Express 17(15): 12444-12454 (2009) DOI: 10.1364/OE.17.012444 Link to article
    The complex refractive indices of freshly excised healthy breast tissue and breast cancers collected from 20 patients were measured in the range of 0.15 - 2.0 THz using a portable terahertz pulsed transmission spectrometer. Histology was performed to classify the tissue samples as healthy adipose tissue, healthy fibrous breast tissue, or breast cancers. The average complex refractive index was determined for each group and it was found that samples containing cancer had a higher refractive index and absorption coefficient. The terahertz properties of the tissues were also used to simulate the impulse response functions expected when imaging breast tissue in a reflection geometry as in terahertz pulsed imaging (TPI). Our results indicate that both TPS and TPI can be used to distinguish between healthy adipose breast tissue, healthy fibrous breast tissue and breast cancer due to the differences in the fundamental optical properties.
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  3. Terahertz pulsed imaging--a potential medical imaging modality?
    Emma Pickwell-MacPherson, Vincent P Wallace
    Photodiagn. Photodyn. 6(2): 128-134 (2009) DOI: 10.1016/j.pdpdt.2009.07.002 Link to article
    Terahertz imaging has progressed significantly over the last decade and there is now a significant body of research in its application to biomedical problems with the possibility of developing it into viable medical imaging modality in the future. The motivation being to fill some of the shortfalls in existing medical imaging technologies especially in detecting early stage cancers. We review the main developments in terahertz imaging to-date and highlight the most promising current areas of biomedical terahertz research. Additionally, we provide an overview of the principles behind terahertz imaging along with illustrated examples to aid understanding for those new to the technology. Our aim is to increase awareness of the existence and potential of the technology and inspire solutions to the remaining challenges in developing terahertz imaging into a novel medical imaging modality.
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  4. The acute effects of running on blood pressure estimation using pulse transit time in normotensive subjects
    Mico Yee-Man Wong, Emma Pickwell-MacPherson, Yuan-Ting Zhang
    Eur. J. Appl. Physiol. 107(2): 169-175 (2009) DOI: 10.1007/s00421-009-1112-8 Link to article
    Pulse transit time (PTT) is a potential parameter for cuffless blood pressure (BP) estimation. Since exercise induces changes in arterial properties that can influence the relationship between BP and PTT, we investigate whether PTT can be used to estimate BP after successive bouts of exercise. PTT-foot, PTT-peak (time intervals from the peak of electrocardiogram R-wave to the foot and peak of photoplethysmogram, respectively) and BP of 41 normotensive subjects (aged 25 +/- 4 years) were measured in the first test. A repeatability test was then conducted on 14 subjects after 6 months. Each test included two periods of running on the treadmill at 10 and 8 km/h (with a rest in between). In both tests, systolic BP (SBP) was closely correlated with PTT-foot and PTT-peak. For each subject, the best fit linear relationships between SBP and PTTs were determined over all phases of each test. The differences between the linear fits and measured data were greater after the second period of running for all subjects in both tests. This implied that the relationships started to change after the second period of running. When SBP in the repeatability test was predicted using the linear regression coefficients from the first test, the linear fit after the first period of exercise was still better than after the second. The repeated observations in both tests suggest that PTT is a potential parameter for cuffless BP estimation after one period of exercise, but would need re-calibration (relationship between BP and PTTs) for measurements after successive phases of exercise.
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  5. Effects of formalin fixing on the terahertz properties of biological tissues
    Yiwen Sun, Bernd M. Fischer, Emma Pickwell-MacPherson
    Journal of Biomedical Optics 14(6): 064017 (2009) DOI: 10.1117/1.3268439 Link to article
    We demonstrate how the terahertz properties of porcine adipose tissue and skeletal muscle are affected by formalin fixing. Terahertz radiation is sensitive to covalently cross-linked proteins and can be used to probe unique spectroscopic signatures. We study in detail the changes arising from different fixation times and see that formalin fixing reduces the refractive index and the absorption coefficient of the samples in the terahertz regime. These fundamental properties affect the time-domain terahertz response of the samples and determine the level of image contrast that can be achieved.
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  6. Improved sample characterization in terahertz reflection imaging and spectroscopy.
    Shengyang Huang, Philip C Ashworth, Kanis W Kan, Yang Chen, Vincent P Wallace, Yuan-Ting Zhang, Emma Pickwell-MacPherson
    Opt. Express 17(5): 3848-3854 (2009) DOI: 10.1364/OE.17.003848 Link to article
    For imaging applications involving biological subjects, the strong attenuation of terahertz radiation by water means that terahertz pulsed imaging is most likely to be successfully implemented in a reflection geometry. Many terahertz reflection geometry systems have a window onto which the sample is placed - this window may introduce unwanted reflections which interfere with the reflection of interest from the sample. In this paper we derive a new approach to account for the effects of these reflections and illustrate its success with improved calculations of sample optical properties.
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  1. Recent developments of terahertz technology in biomedicine
    Emma Pickwell-MacPherson, Shengyang Huang, Kanis W Kan, Yiwen Sun, Yuan-Ting Zhang
    J. Innov. Opt. Health Sci. 1(1): 29-44 (2008) DOI: 10.1142/S1793545808000042 Link to article
  1. A comparison of terahertz pulsed imaging with transmission microradiography for depth measurement of enamel demineralisation in vitro.
    Emma Pickwell, Vincent P Wallace, Bryan E Cole, Sophia Ali, Christopher Longbottom, Richard J M Lynch, Michael Pepper
    Caries Res. 41(1): 49-55 (2007) DOI: 10.1159/000096105 Link to article
    Terahertz pulsed imaging (TPI) is a relatively new, non-ionising and non-destructive imaging technique for studying hard tissues which does not require tooth section preparation, unlike transmission microradiography (TMR). If TPI can measure the depths of caries/demineralisation lesions accurately the same tooth samples could be reused and remeasured during in vitro and in situ studies on de- and/or re-mineralisation. The aim of this study was to compare TPI and TMR for measuring the depths of a range of artificially induced bovine enamel demineralised lesions in vitro. Bovine slabs with artificial caries, induced to different levels of demineralisation by two different but standard demineralisation techniques ('acid gel' and 'carbopol') were measured by TPI and TMR and the readings compared. The set of TPI/TMR measurements obtained on the gel-demineralised slabs showed an extremely high coefficient of determination (r(2) = 0.995). Detailed analysis of the results and theoretical considerations (involving the relationship between refractive index profiling and mineral loss profile) are used to explain the findings and show that for acid gel lesions TPI is measuring demineralisation in the range of 47{\{}%{\}} of that of TMR depth plus an intercept of 16 microm, with further calculations allowing the TMR depths to be determined to within 5{\{}%{\}} using TPI.
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  1. Terahertz pulsed spectroscopy of human basal cell carcinoma
    V P Wallace, A J Fitzgerald, E Pickwell, R J Pye, P F Taday, N Flanagan, T Ha
    Appl. Spectrosc. 60: 1127-1133 (2006) DOI: 10.1366/000370206778664635 Link to article
  2. Biomedical applications of terahertz technology
    E Pickwell, V P Wallace
    J. Phys. D Appl. Phys. 39(17): R301--R310 (2006) DOI: 10.1088/0022-3727/39/17/R01 Link to article
  1. Simulating the response of terahertz radiation to basal cell carcinoma using ex vivo spectroscopy measurements
    E Pickwell, A J Fitzgerald, B E Cole, P F Taday, R J Pye, T Ha, M Pepper, V P Wallace
    J. Biomed. Opt. 10: 64021 (2005)
  1. In vivo study of human skin using pulsed terahertz radiation
    E Pickwell, B E Cole, A J Fitzgerald, M Pepper, V P Wallace
    Phys. Med. Biol. 49(9): 1595-1607 (2004) DOI: 10.1088/0031-9155/49/9/001 Link to article
  2. Simulation of terahertz pulse propagation in biological systems
    E Pickwell, B E Cole, A J Fitzgerald, V P Wallace, M Pepper
    Appl. Phys. Lett. 84(12): 2190 (2004) DOI: 10.1063/1.1688448 Link to article