Journal Publications

1. Spatial Terahertz-Light Modulators for Single-Pixel Cameras [Online First]
Rayko Ivanov Stantchev, Emma Pickwell-MacPherson
IntechOpen (2021) DOI: 10.5772/intechopen.96691 Link to article
Abstract
Terahertz imaging looks set to become an integral part of future applications from semiconductor quality control to medical diagnosis. This will only become a reality when the technology is sufficiently cheap and capabilities adequate to compete with others. Single-pixel cameras use a spatial light modulator and a detector with no spatial-resolution in their imaging process. The spatial-modulator is key as it imparts a series of encoding masks on the beam and the detector measures the dot product of each mask and the object, thereby allowing computers to recover an image via post-processing. They are inherently slower than parallel-pixel imaging arrays although they are more robust and cheaper, hence are highly applicable to the terahertz regime. This chapter dedicates itself to terahertz single-pixel cameras; their current implementations, future directions and how they compare to other terahertz imaging techniques. We start by outlining the competing imaging techniques, then we discuss the theory behind single-pixel imaging; the main section shows the methods of spatially modulating a terahertz beam; and finally there is a discussion about the future limits of such cameras and the concluding remarks express the authors' vision for the future of single-pixel THz cameras.
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2. Gold nanoparticle enhanced detection of EGFR with a terahertz metamaterial biosensor
Kai Liu, Rui Zhang, Yao Liu, Xuequan Chen, Kaidi Li, Emma Pickwell-Macpherson
Biomed. Opt. Express 12(3): 1559-1567 (2021) DOI: 10.1364/BOE.418859 Link to article
Abstract
The epidermal growth factor receptor (EGFR) plays an important role in the proliferation of various cancers, and the expression level of EGFR in tumor tissues can provide a basis for the diagnosis and prognosis. Improving the detection technology of EGFR to achieve high sensitivity and faster speed will benefit the diagnosis of many types of tumors. Trace biological samples can be sensitively measured with terahertz (THz) metamaterial devices. Here, a bow-tie array THz metamaterial biosensor is presented and modified with gold nanoparticles (GNPs) and EGFR antibodies for specific EGFR detection. The effect of the size of the GNP on the sensitivity enhancement was also analyzed. Enhanced sensing for EGFR was achieved with the assistance of GNPs and EGFR antibodies. Importantly, the metamaterial functionalized by GNPs and antibodies with a bigger GNP diameter achieves a greater resonance frequency shift. The proposed metamaterial biosensor can also realize tiny-volume EGFR solution detection. Our proposed technique can therefore sense EGFR sensitively with high speed, and can potentially be applied to achieve quick and accurate detection of EGFR related tumors.
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3. Deep THz modulation at Fabry-Perot resonances using graphene in periodic microslits
Xudong Liu, Mingyang Jia, Shuting Fan, Rayko Ivanov Stantchev, Xuequan Chen, Emma Pickwell-Macpherson, Yiwen Sun
Opt. Express 29(4): 6199-6208 (2021) DOI: 10.1364/OE.413622 Link to article
Abstract
Potential applications of terahertz (THz) radiation are constantly being investigated for high-speed communication due to its large bandwidth. For example, frequency hopping communication technology would benefit from the large bandwidth. To attach the information to the carrier wave, THz modulators with deep and stable modulation at different frequencies are crucial, yet are still lacking. Here a THz modulator, designed by integrating a non-resonant field enhancement effect of periodic metal microslits to assist a Fabry-Perot resonance structure (MS-FP) is proposed and demonstrated. New equations are developed to describe the superior performance of the novel design. The {\textgreater}95{\&}{\#}x0025; modulation depth is achieved by a SiO2/Si gated graphene device at 14 Fabry-Perot resonant frequencies across 1.4 THz bandwidth, outperforming the recently reported 75{\&}{\#}x0025; modulation depth THz modulator with a similar Fabry-Perot structure.
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4. Exploiting complementary terahertz ellipsometry configurations to probe the hydration and cellular structure of skin in vivo
Xuequan Chen, Qiushuo Sun, Jiarui Wang, Hannah Lindley-Hatcher, Emma Pickwell‐MacPherson
Abstract
The noninvasive and water‐sensitive characteristics of terahertz (THz) light make it highly attractive for in vivo studies, especially for skin applications. However, THz instrumentation has not been developed sufficiently to fully explore all the potential applications arising: current systems cannot obtain uncorrelated reflections from multiple configurations to determine the complicated structure of living tissues. Herein, this bottleneck is overcome by implementing a novel ellipsometry configuration able to efficiently provide four complementary sets of spectral ratios, significantly enhancing characterization capabilities. An accurate model of the skin is established and validated. The anisotropy of the stratum corneum (SC) caused by its cellular structure is verified both theoretically and experimentally. The in vivo response of skin on the volar forearm to occlusion is observed by the dynamic changes in the SC and the epidermis. In addition, the THz dispersion and birefringence sensitively probe the level of hydration and the cellular inhomogeneity, producing results in good agreement with microscope images and the biological processes of the SC. The presented technique offers a brand‐new functionality in extracting insightful structural information from complex systems, significantly extending the versatility of THz spectroscopy.
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1. Real-time terahertz imaging with a single-pixel detector
Rayko Ivanov Stantchev, Xiao Yu, Thierry Blu, Emma Pickwell-macpherson
Nature Communications 11(2353): 1-8 (2020) DOI: 10.1038/s41467-020-16370-x Link to article
Abstract
Terahertz (THz) radiation is poised to have an essential role in many imaging applications, from industrial inspections to medical diagnosis. However, commercialization is prevented by impractical and expensive THz instrumentation. Single-pixel cameras have emerged as alternatives to multi-pixel cameras due to reduced costs and superior durability. Here, by optimizing the modulation geometry and post-processing algorithms, we demonstrate the acquisition of a THz-video (32 × 32 pixels at 6 frames-per-second), shown in real-time, using a single-pixel fiber-coupled photoconductive THz detector. A laser diode with a digital micromirror device shining visible light onto silicon acts as the spatial THz modulator. We mathematically account for the temporal response of the system, reduce noise with a lock-in free carrier-wave modulation and realize quick, noise-robust image undersampling. Since our modifications do not impose intricate manufacturing, require long post-processing, nor sacrifice the time-resolving capabilities of THz-spectrometers, their greatest asset, this work has the potential to serve as a foundation for all future single-pixel THz imaging systems.
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2. Additive manufacture of photonic components for the terahertz band
Enrique Castro-Camus, Martin Koch, Arturo I Hernandez-Serrano
Journal of Applied Physics 127(21): 210901 (2020) DOI: 10.1063/1.5140270 Link to article
Abstract
In this Perspective contribution, we present a brief review of the literature available on optical devices for terahertz frequencies, followed by an analysis of the challenges faced by this technology and its future potential to generate complex photonic systems, and in principle the possibilities of this technique for the production of components for the infrared and visible band.
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3. 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
Abstract
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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4. Classification for glucose and lactose Terahertz spectra based on SVM and DNN methods
Kaidi Li, Xuequan Chen, Rui Zhang, Emma MacPherson
IEEE Transactions on Terahertz Science and Technology 10(6): 617-623 (2020) DOI: 10.1109/TTHZ.2020.3013819 Link to article
Abstract
In recent decades, terahertz (THz) radiation has been widely applied in many chemical and biomedical areas. Due to its ability to resolve the absorption features of many compounds non-invasively, it is a promising technique for chemical recognition of substances such as drugs or explosives. A key challenge for THz technology is to be able to accurately classify spectral measurements acquired in unknown complicated environments, rather than those from ideal laboratory conditions. Support vector machine (SVM) and deep neural networks (DNN) are powerful and widely adopted approaches for complex classification with a high accuracy. In this paper, we explore and apply the SVM and DNN methods for classifying the frequency spectra of glucose and lactose. We measured 372 groups of independent signals under different conditions to provide a sufficient training set. The classification accuracies achieved were 99% for the SVM method and 89.6% for the DNN method. These high classification accuracies demonstrate great potential in chemical recognition.
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5. Evaluation of in vivo THz sensing for assessing human skin hydration
Hannah Lindley-Hatcher, A I Hernandez-Serrano, Jiarui Wang, Juan Cebrian, Joseph Hardwicke, Emma Pickwell-MacPherson
Journal of Physics: Photonics 3(1): 14001 (2020) DOI: 10.1088/2515-7647/abcb71 Link to article
Abstract
Terahertz (THz) in vivo reflection imaging can be used to assess the water content of the surface of the skin. This study presents the results of treating 20 subjects with aqueous, anhydrous and water-oil emulsion samples and observing the changes induced in the skin using THz sensing. These regions were also measured with a corneometer, the present gold standard for skin hydration assessment within the cosmetics industry. We find that THz sensing is effective at observing the presence of oil and water on the surface of the skin, these results can be verified with the measurements of capacitance taken by the corneometer. The THz measurements reveal a distinction between the responses of subjects with initially dry or well hydrated skin, this observation is particularly noticeable with the oil-based samples. Additionally, moderate correlation was found between the THz reflected amplitude and capacitance of untreated skin with a correlation coefficient of r = −0.66, suggesting THz sensing has promising potential for assessing skin hydration.
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6. 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
Abstract
In this Letter, we report a broadband frequency/polarization demultiplexer based on parallel-plate waveguides (PPWGs) for terahertz (THz) frequencies. The fabrication and experimental validation of this polarization sensitive demultiplexer is demonstrated for the range from 0.2 to 1 THz. Upgrading the demultiplexer by adding a second demultiplexer stage, a fifty-fifty amplitude splitter is also demonstrated in the same frequency range. The multiplexer is based on a stainless-steel traveling-wave antenna, exhibiting strong mechanical robustness. This unique device exhibits three splitting mechanisms in the same device: amplitude, polarization, and frequency splitting. This is a significant improvement for the next generation of THz passive components for communication purposes.
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7. Evaluation of transdermal drug delivery usingterahertz pulsed imaging
Jiarui Wang, Hannah Hatcher, Kai Liu, Emma Pickwell-MacPherson
Biomedical Optics Express 11(8): 4484-4490 (2020) DOI: 10.1364/boe.394436 Link to article
Abstract
Transdermal drug delivery (TDD) is widely used for painless dosing due to its minimally invasive nature compared to hypodermic needle injection and its avoidance of the gastrointestinal tract. However, the stratum corneum obstructs the permeation of drugs into skin. Microneedle and nanoneedle patches are ways to enhance this permeation. In this work, terahertz (THz) imaging is utilized to compare the efficacy of different TDD methods including topical application and via a needle patch. Our work shows the feasibility and potential of using THz imaging to quantify and evaluate different transdermal application methods.
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1. A Robust Protocol for In Vivo THz Skin Measurements
Hannah Lindley-Hatcher, A I Hernandez-Serrano, Qiushuo Sun, Jiarui Wang, Juan Cebrian, Laurent Blasco, Emma Pickwell-MacPherson
Journal of Infrared, Millimeter, and Terahertz Waves 40 (2019) DOI: 10.1007/s10762-019-00619-8 Link to article
Abstract
This work presents an experimental setup to control the way in which pressure interferes with the repeatability of in vivo THz skin measurements. By integrating a pressure sensor circuit into our THz system, it is possible to identify which measurements were taken within a previously specified pressure range. The live response of the pressure sensor helps to acquire data within the desired pressure leading to greater consistency of data between measurements. Additionally, a protocol is proposed to help achieve repeatable results and to remove the effects of the natural variation of the skin through the course of the day. This technique has been shown to be able to quantify the changes induced in the skin following the application of a moisturising skin product and shows the measured result to be significantly different from natural skin variation. This research therefore prepares the way for further studies on the effectiveness of different skin products using in vivo THz measurements.
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2. Highly Efficient Ultra‐Broadband Terahertz Modulation Using Bidirectional Switching of Liquid Crystals
Xuequan Chen, Kaidi Li, Rui Zhang, Swadesh Kumar Gupta, Abhishek Kumar Srivastava, Emma Pickwell‐MacPherson
Abstract
Accurately manipulating field strength and polarization state are essential in various terahertz applications. Such manipulations are based on the efficient modulation of the amplitude and phase of electromagnetic waves. However, there is a lack of such terahertz modulators with sufficient efficiency and bandwidth. Herein, the Brewster–critical angle is exploited for modulation by using a nematic liquid crystal. Unlike liquid crystal phase shifters that only give a narrowband phase delay via a one‐directional switch, the presented device modulates both the amplitude and phase across an ultra‐broadbandwidth via a bidirectional active switch. An average intensity modulation depth over 99.6% is achieved for 0.2–1.6 THz. Furthermore, highly accurate polarization conversion between linear and circular states is also realized for 0.4–1.8 THz, with the average degree of linear and circular polarizations as high as 0.994 and 0.998, respectively. The superior accuracy, bandwidth, and active control achieved provide great potential for multifunctional terahertz modulation.
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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
Abstract
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. 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
Abstract
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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5. 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
Abstract
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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6. 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
Abstract
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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7. Genetic Algorithm Based Optimization for Terahertz Time - Domain Adaptive Sampling
Kaidi Li, Xuequan Chen, Shuaiqi Shen, Rui Zhang, Emma Pickwell-macpherson
IEEE Transactions on Terahertz Science and Technology 9(6): 675-683 (2019) DOI: 10.1109/TTHZ.2019.2935635 Link to article
Abstract
We propose a genetic algorithm (GA) based method to improve the sampling efficiency in THz time domain spectroscopy (THz-TDS). For a typical time domain THz signal, most information are contained in a short region of the pulse which needs to be densely sampled, while the other regions fluctuating around zero can be represented by fewer points. Based on this clustering feature of the THz signal, we can use much fewer sampling points and optimize the distribution by using a GA to achieve an accurate scanning in less time. Both reflection and transmission measurements were conducted to experimentally verify the performance. The measurement results show that the sampling time can be greatly reduced while maintaining very high accuracy both in the time-domain and frequency-domain compared with a high-resolution step scan. This method significantly improves the measurement efficiency. It can be easily adapted to most THz-TDS systems equipped with a mechanical delay stage for fast detection and THz imaging.
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8. Scalable interdigitated photoconductive emitters for the electrical modulation of terahertz beams with arbitrary linear polarization
C D W Mosley, M Staniforth, A I.Hernandez Serrano, E Pickwell-Macpherson, J Lloyd-Hughes
Abstract
A multi-element interdigitated photoconductive emitter for broadband THz polarization rotation is proposed and experimentally verified. The device consists of separate pixels for the emission of horizontally and vertically polarized THz radiation. The broadband (0.3-5.0 THz) nature of the device is demonstrated, and the polarization angle of the generated far-field THz radiation is shown to be readily controlled by varying the relative bias voltage applied to the horizontally and vertically emitting pixels. The device is scalable in design, and with its simple method of polarization rotation it allows the modulation of the generated THz polarization at rates significantly faster than those achievable in ellipsometry systems based on mechanically rotating components.
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9. 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
Abstract
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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1. 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
Abstract
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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2. 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
Abstract
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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3. 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
Abstract
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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4. 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
Abstract
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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5. THz in vivo measurements: the effects of pressure on skin reflectivity
Jiarui Wang, Rayko I. Stantchev, Qiushuo Sun, Tor-Wo Chiu, Anil T. Ahuja, Emma Pickwell MacPherson
Biomedical Optics Express 9(12): 6467 (2018) DOI: 10.1364/BOE.9.006467 Link to article
Abstract
Terahertz (THz) light is non-ionizing and highly sensitive to subtle changes in water concentration which can be indicative of disease. The short THz penetration depth in bio-samples restricts in vivo measurements to be in a reflection geometry and the sample is often placed onto an imaging window. Upon contacting the imaging window, occlusion and compression of the skin affect the THz response. If not appropriately controlled, this could cause misleading results. In this work, we investigate and quantify how the applied pressure affects the THz response of skin and employ a stratified model to help understand the mechanisms at play. This work will enable future THz studies to have a more rigorous experimental protocol, which in turn will facilitate research in various potential biomedical applications under investigation.
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6. Three dimensional chiral plasmon rulers based on silver nanorod trimers
Chunrui Han, Lechen Yang, Piao Ye, Edward P. J. Parrott, Emma Pickwell-Macpherson, Wing Yim Tam
Optics Express 26(8): 10315 (2018) DOI: 10.1364/OE.26.010315 Link to article
Abstract
The symmetry dependences of plasmon excitation modes are studied in 3D silver nanorod trimers. The degenerate plasmon modes split into chiral modes by breaking the inversion and mirror symmetry of the nanorod trimer through translation and/or rotation of the middle rod. With a translation operation, successive evolution of the circular dichroism (CD) spectrum can be achieved through gradual breaking of the inversion symmetry. An additional rotation operation produces even dramatic spectral changes due to breaking a quasi-mirror symmetry resulted from the same angular distance of the middle rod to the top and bottom rods. Especially, pairs of new chiral modes can be excited due to the contact of the middle rod with the top-bottom rod pair. The spectral changes in the simulations, which are also demonstrated experimentally, envision the 3D chiral nanorod trimer system as plasmon ruler for spatial configuration retrieval and dynamic bio-process analysis at the single molecule level.
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7. Graphene-loaded metal wire grating for deep and broadband THz modulation in total internal reflection geometry
Yiwen Sun, Riccardo Degl'Innocenti, David A. Ritchie, Harvey E. Beere, Long Xiao, Michael Ruggiero, J. Axel Zeitler, Rayko I. Stantchev, Danni Chen, Zhengchun Peng, Emma MacPherson, Xudong Liu
Photonics Research 6(12): 1151 (2018) DOI: 10.1364/PRJ.6.001151 Link to article
Abstract
We employed a metallic wire grating loaded with graphene and operating in total internal reflection (TIR) geometry to realize deep and broadband THz modulation. The non-resonant field enhancement effect of the evanescent wave in TIR geometry and in the subwavelength wire grating was combined to demonstrate a 77% modulation depth (MD) in the frequency range of 0.2 to 1.4 THz. This MD, achieved electrically with a SiO2/Si gated graphene device, was 4.5 times higher than that of the device without a metal grating in transmission geometry. By optimizing the parameters of the metallic wire grating, the required sheet conductivity of graphene for deep modulation was lowered to 0.87mS. This work has potential applications in THz communication and real-time THz imaging.
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8. 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
Abstract
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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9. 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 : acs.jpcc.8b05695 (2018) DOI: 10.1021/acs.jpcc.8b05695 Link to article
Abstract
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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10. 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
Abstract
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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1. A Robust Baseline and Reference Modification and Acquisition Algorithm for Accurate THz Imaging
Xuequan Chen, Edward P.J. Parrott, Benjamin S.Y. Ung, Emma Pickwell-Macpherson
IEEE Transactions on Terahertz Science and Technology 7(5): 493-501 (2017) DOI: 10.1109/TTHZ.2017.2722981 Link to article
Abstract
IEEE This paper reports an accurate baseline and reference modification and acquisition algorithm for terahertz (THz) reflection measurements. The algorithm solves the spatial phase variation problem, which is a major accuracy limitation of THz reflection imaging. It also overcomes the sampling error problem without taking multiple measurements by utilizing two-dimensional data fitting. The algorithm records the spatial information and the laser power status as a database to precisely modify the baseline and reference in a measurement, providing a simple, efficient, and accurate real-time baseline and reference acquisition method. The experimental results show that the algorithm significantly improves the accuracy of the extracted optical properties of the sample with only half of the time required in a traditional THz imaging by avoiding additional baseline and reference measurements. The strong robustness of the algorithm is also proved by the experiment, giving a stable performance against laser power fluctuations.
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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. 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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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. 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
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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6. 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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7. 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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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
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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2. 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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3. 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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4. 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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1. 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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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. 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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4. 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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5. 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
1. 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
2. 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
3. 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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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. 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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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. 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. 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. 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. 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
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. Observing the temperature dependent transition of the GP2 peptide using terahertz spectroscopy.
Yiwen Sun, Zexuan Zhu, Siping Chen, Jega Balakrishnan, Derek Abbott, Anil T Ahuja, Emma Pickwell-Macpherson
PLoS One 7(11): e50306 (2012) DOI: 10.1371/journal.pone.0050306 Link to article
Abstract
The GP2 peptide is derived from the Human Epidermal growth factor Receptor 2 (HER2/nue), a marker protein for breast cancer present in saliva. In this paper we study the temperature dependent behavior of hydrated GP2 at terahertz frequencies and find that the peptide undergoes a dynamic transition between 200 and 220 K. By fitting suitable molecular models to the frequency response we determine the molecular processes involved above and below the transition temperature (T(D)). In particular, we show that below T(D) the dynamic transition is dominated by a simple harmonic vibration with a slow and temperature dependent relaxation time constant and that above T(D), the dynamic behavior is governed by two oscillators, one of which has a fast and temperature independent relaxation time constant and the other of which is a heavily damped oscillator with a slow and temperature dependent time constant. Furthermore a red shifting of the characteristic frequency of the damped oscillator was observed, confirming the presence of a non-harmonic vibration potential. Our measurements and modeling of GP2 highlight the unique capabilities of THz spectroscopy for protein characterization.
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5. 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. 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
2. 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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3. 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
4. Investigating antibody interactions with a polar liquid using terahertz pulsed spectroscopy.
Yiwen Sun, Yuan-Ting Zhang, Emma Pickwell-Macpherson
Biophys. J. 100(1): 225-231 (2011) DOI: 10.1016/j.bpj.2010.11.020 Link to article
Abstract
In this article, we use terahertz spectroscopy to study the dielectric properties of the peroxidase-conjugated affinity purified goat anti-cat immunoglobulin G and the fluorescein-conjugated affinity purified goat anti-cat immunoglobulin G when they interact with polar liquids. The influence of protein concentration, as well as presence of glycerol as a cosolvent, is determined by estimation of the effective hydration shell radius of the protein in solution. The dielectric spectra in this study are measured over the frequency range 0.1-1.3 THz and it is found that the dielectric properties are dependent on the type of the charges in the hydrogen-bonded antibodies' networks. Our results indicate that the terahertz dielectric properties of polar liquids are strongly affected by the presence of the antibody and suggest that the dielectric spectrum is particularly powerful in the study of structural and conformational properties of proteins. Therefore, terahertz spectroscopy is a very sensitive approach to investigate structural features of biological systems.
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5. 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
6. 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
1. 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
2. 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
3. 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
Abstract
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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4. 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
Abstract
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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5. 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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6. Practical Considerations for in Vivo THz Imaging
Emma Pickwell-MacPherson
Terahertz Sci. Technol. 3(4): 163-171 (2010)
7. Terahertz spectroscopy of liver cirrhosis: investigating the origin of contrast.
Stanley Sy, Shengyang Huang, Yi-Xiang J Wang, Jun Yu, Anil T Ahuja, Yuan-Ting Zhang, Emma Pickwell-Macpherson
Phys. Med. Biol. 55(24): 7587-7596 (2010) DOI: 10.1088/0031-9155/55/24/013 Link to article
Abstract
We have previously demonstrated that terahertz pulsed imaging is able to distinguish between rat tissues from different healthy organs. In this paper we report our measurements of healthy and cirrhotic liver tissues using terahertz reflection spectroscopy. The water content of the fresh tissue samples was also measured in order to investigate the correlations between the terahertz properties, water content, structural changes and cirrhosis. Finally, the samples were fixed in formalin to determine whether water was the sole source of image contrast in this study. We found that the cirrhotic tissue had a higher water content and absorption coefficient than the normal tissue and that even after formalin fixing there were significant differences between the normal and cirrhotic tissues' terahertz properties. Our results show that terahertz pulsed imaging can distinguish between healthy and diseased tissue due to differences in absorption originating from both water content and tissue structure.
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8. 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
Abstract
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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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
Abstract
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. Tissue characterization using terahertz pulsed imaging in reflection geometry.
S Y Huang, Y X J Wang, D K W Yeung, A T Ahuja, Y-T Zhang, E Pickwell-Macpherson
Phys. Med. Biol. 54(1): 149-160 (2009) DOI: 10.1088/0031-9155/54/1/010 Link to article
Abstract
Terahertz pulsed imaging (TPI) is a non-ionizing and non-destructive imaging technique that has been recently used to study a wide range of biological materials. The severe attenuation of terahertz radiation in samples with high water content means that biological samples need to be very thin if they are to be measured in transmission geometry. To overcome this limitation, samples could be measured in reflection geometry and this is the most feasible way in which TPI could be performed in a clinical setting. In this study, we therefore used TPI in reflection geometry to characterize the terahertz properties of several organ samples freshly harvested from laboratory rats. We observed differences in the measured time domain responses and determined the frequency-dependent optical properties to characterize the samples further. We found statistically significant differences between the tissue types. These results show that TPI has the potential to accurately differentiate between tissue types non-invasively.
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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
Abstract
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. 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
Abstract
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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5. 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
Abstract
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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6. 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
Abstract
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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1. Relations Between the Timing of the Second Heart Sound and Aortic Blood Pressure
Xin-Yu Zhang, E MacPherson, Yuan-Ting Zhang
IEEE Trans. Biomed. Eng. 55(4): 1291-1297 (2008) DOI: 10.1109/TBME.2007.912422 Link to article
Abstract
The second heart sound (S2) is triggered by an aortic valve closure as a result of the ventricular-arterial interaction of the cardiovascular system. The objective of this paper is to investigate the timing of S2 in response to the changes in hemodynamic parameters and its relation to aortic blood pressure (BP). An improved model of the left ventricular-arterial interaction was proposed based on the combination of the newly established pressure source model of the ventricle and the nonlinear pressure-dependent compliance model of the arterial system. The time delay from the onset of left ventricular pressure rise to the onset of S2 (RS2) was used to measure the timing of S2. The results revealed that RS2 bears a strong negative correlation with both systolic blood pressure and diastolic blood pressure under the effect of changing peripheral resistance, heart rate, and contractility. The results were further validated by a series of measurements of 16 normal subjects submitted to dynamic exercise. This study helps understand the relationship between the timing of S2 and aortic BP under various physiological and pathological conditions.
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2. 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
3. Three-dimensional imaging of optically opaque materials using nonionizing terahertz radiation.
Vincent P Wallace, Emma Macpherson, J Axel Zeitler, Caroline Reid
J. Opt. Soc. Am. A 25(12): 3120-3133 (2008) Link to article
Abstract
Terahertz electromagnetic radiation has already been shown to have a wide number of uses. We consider specific applications of terahertz time-domain imaging that are inherently three-dimensional. This paper highlights the ability of terahertz radiation to reveal subsurface information as we exploit the fact that the radiation can penetrate optically opaque materials such as clothing, cardboard, plastics, and to some extent biological tissue. Using interactive science publishing tools, we concentrate on full three-dimensional terahertz data from three specific areas of application, namely, security, pharmaceutical, and biomedical.
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