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    Survey of terahertz metamaterial devices
    (2008-12-01)
    Withayachumnankul, Withawat
    ;
    Abbott, Derek
    Metamaterials have arisen in an attempt to engineer the electromagnetic properties of natural substances. It has been acknowledged that the emergence of metamaterials has implications to nearly all branches of science and engineering exploiting the electromagnetic radiation. This paper reviews seminal work of metamaterials from the vision to the realisation of subwavelength elements that contribute to varieties of electric and magnetic responses. Emphasis is given to the significance and opportunity of this new class of material augmenting terahertz technology. Although now there remain major milestones that scientists and engineers need to conquer, the future of this cutting-edge material technology is very bright. © 2008 SPIE.
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    Numerical removal of water vapour effects from terahertz time-domain spectroscopy measurements
    (2008-09-08)
    Withayachumnankul, Withawat
    ;
    Fischer, Bernd M.
    ;
    Abbott, Derek
    The use of T-rays, or terahertz radiation, to identify substances by their spectroscopic fingerprints is a rapidly moving field. The dominant approach is presently terahertz time-domain spectroscopy. However, a key problem is that ambient water vapour is ubiquitous and the consequent water absorption distorts the T-ray pulses. Water molecules in the gas phase selectively absorb incident T-rays at discrete frequencies corresponding to their molecular rotational transitions. When T-rays propagate through an atmosphere, this results in prominent resonances spread over the T-ray spectrum; furthermore, in the time domain, fluctuations after the main pulse are observed in the T-ray signal. These effects are generally undesired, since they may mask critical spectroscopic data. So, ambient water vapour is commonly removed from the T-ray path by using a closed chamber during the measurement. Yet, in some applications, a closed chamber is not always feasible. This situation, therefore, motivates the need for an optional alternative method for reducing these unwanted artefacts. This paper represents a study on a computational means that is a step towards addressing the problem arising from water vapour absorption over a moderate propagation distance. Initially, the complex frequency response of water vapour is modelled from a spectroscopic catalogue. Using a deconvolution technique, together with fine tuning of the strength of each resonance, parts of the water vapour response are removed from a measured T-ray signal, with minimal signal distortion, thus providing experimental validation of the technique. © 2008 The Royal Society.
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    Quarter-wavelength multilayer interference filter for terahertz waves
    (2008-05-01)
    Withayachumnankul, Withawat
    ;
    Fischer, Bernd M.
    ;
    Abbott, Derek
    This work presents a multilayer interference filter, suitable for operation with terahertz waves (T-rays). An analysis of the effect of the number of layers on the spectral response is given, with full measurement data including time-resolved signals, transmittances, and phase spectra. The silicon-air structure with a submillimeter thickness shows a stop-band between 0.2 and 0.5 THz, and the attenuation inside the stop-band increases in proportion to the number of layers in the structure. The measurement of the fabricated structure is in agreement with a characteristic matrix analysis. © 2008 Elsevier B.V. All rights reserved.
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    Direct fabry-pérot effect removal
    (2006-06-01)
    Withayachumnankul, Withawat
    ;
    Ferguson, Bradley
    ;
    Rainsford, Tamath
    ;
    Mickan, Samuel
    ;
    Abbott, Derek
    This study indicates that the removal of reflections from T-ray signals can be carried out in the frequency domain without prior knowledge of material parameters or sample thickness. By fitting polynomials to the logarithm and the argument of the sample's transfer function, the Fabry-Pérot reflection term is canceled out, leading to disappearance of the reflections in spatial domain. The method successfully removes the reflections for optically thick samples under the condition of noise or amplitude fluctuations. The application to optically thin samples is possible when the samples are subjected to broadband terahertz measurements. The Fabry-Pérot free signal, when used as input to the parameter estimation method, results in correct material parameters with low variance. © World Scientific Publishing Company.
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    T-ray relevant frequencies for osteosarcoma classification
    (2006-04-17)
    Withayachumnankul, W.
    ;
    Ferguson, B.
    ;
    Rainsford, T.
    ;
    Findlay, D.
    ;
    Mickan, S. P.
    We investigate the classification of the T-ray response of normal human bone cells and human osteosarcoma cells, grown in culture. Given the magnitude and phase responses within a reliable spectral range as features for input vectors, a trained support vector machine can correctly classify the two cell types to some extent. Performance of the support vector machine is deteriorated by the curse of dimensionality, resulting from the comparatively large number of features in the input vectors. Feature subset selection methods are used to select only an optimal number of relevant features for inputs. As a result, an improvement in generalization performance is attainable, and the selected frequencies can be used for further describing different mechanisms of the cells, responding to T-rays. We demonstrate a consistent classification accuracy of 89.6%, while the only one fifth of the original features are retained in the data set.
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    T-rays in biomedicine and security
    (2005-12-01)
    Rainsford, T.
    ;
    Png, G. M.
    ;
    Withayachumnankul, W.
    ;
    Ferguson, B.
    ;
    Mickan, S. P.
    With a number of commercial systems becoming increasingly available, and improved generation and detection techniques T-ray systems are increasingly finding new applications. One of the main technical challenges is to make systems low cost, compact and easy to use. In this paper we suggest some novel solutions for parameter extraction, image enhancement, improved signal-to-noise ratio and classification schemes. These tools are useful in a number of biomedical and security applications. © 2005 IEEE.