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A systemized view of superluminal wave propagation
(2010-01-01)
Withayachumnankul, Withawat
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Fischer, Bernd M.
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Ferguson, Bradley
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Davis, Bruce R.
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Abbott, Derek
This paper reviews earlier studies on superluminal wave propagation in anomalously dispersive media that have been carried out in the electronic, microwave, and optical regimes. Those studies are relevant to observation of modulated Gaussian pulses transmitted through various media at speeds apparently faster than $c$ without distortion. This paper also presents the condition for superluminal propagation that is established based on the magnitudephase relation of a causal and minimum-phase filter. Since the condition is modeled on the basis of filter theory, it is applicable to all types of media. A terahertz experiment with a periodic bandgap structure is also included to illustrate superluminal propagation. © 2006 IEEE.
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Measurement of linearity in THz-TDS
(2009-12-31)
Withayachumnankul, Withawat
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Ung, Benjamin S.Y.
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Fischer, Bernd M.
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Abbott, Derek
This article presents an approach to the measurement of the amplitude linearity in terahertz time-domain spectroscopy (THz-TDS) systems. The approach exploits a single wafer of high-purity float-zone silicon to produce multiple Fabry-Pérot reflections, which are stepwise attenuated and delayed. Comparison between the theoretical and experimental results can indicate a deviation in linearity. © 2009 IEEE.
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Optimization of material thickness for THz-TDS
(2008-12-31)
Withayachumnankul, Withawat
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Fischer, Bernd M.
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Abbott, Derek
How thick should a sample be for a transmission-mode THz-TDS measurement? Should a sample be as thick as possible? The answer is 'no'. Although greater thickness allows T-rays to interact more with bulk material, the SNR rolls off with thickness due to signal attenuation. So, should a sample be extremely thin? Again, the answer is 'no'. A sample that is too thin renders itself nearly invisible to T-rays, in such a way that the system can hardly sense the difference between the sample and a free space path. Hence, where is the optimal boundary between 'too thick' and 'too thin'? The analytical expression to find the optimum thickness is revealed in this paper. This optimality results in the minimal uncertainty of measured optical constants. The derived model for optimal thickness is supported by the results from experiments performed with polyvinyl chloride (PVC) and other materials. ©IEEE.
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Numerical removal of water vapour effects from terahertz time-domain spectroscopy measurements
(2008-09-08)
Withayachumnankul, Withawat
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Fischer, Bernd M.
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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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Material thickness optimization for transmission-mode terahertz time-domain spectroscopy
(2008-05-12)
Withayachumnankul, Withawat
;
Fischer, Bernd M.
;
Abbott, Derek
The thickness of a sample material for a transmission-mode terahertz time-domain spectroscopy (THz-TDS) measurement is the subject of interest in this paper. A sample that is too thick or too thin can raise the problem of measurement uncertainty. Although greater thickness allows the terahertz radiation - or T-rays - to interact more with bulk material, the SNR rolls off with thickness due to signal attenuation. A sample that is too thin renders itself nearly invisible to T-rays, in such a way that the system can hardly sense the difference between the sample and a free space path. The optimal trade-off is analyzed and revealed in this paper, where our approach is to find the optimal thickness that results in the minimal uncertainty of measured optical constants. The derived model for optimal thickness is supported by the results from experiments performed with polyvinyl chloride (PVC), high-density polyethylene (HDPE), and lactose samples. © 2008 Optical Society of America.
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Quarter-wavelength multilayer interference filter for terahertz waves
(2008-05-01)
Withayachumnankul, Withawat
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Fischer, Bernd M.
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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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Uncertainty in terahertz time-domain spectroscopy measurement
(2008-01-01)
Withayachumnankul, Withawat
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Fischer, Bernd M.
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Lin, Hungyen
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Abbott, Derek
Measurements of optical constants at terahertz - or T-ray - frequencies have been performed extensively using terahertz time-domain spectroscopy (THz-TDS). Spectrometers, together with physical models explaining the interaction between a sample and T-ray radiation, are progressively being developed. Nevertheless, measurement errors in the optical constants, so far, have not been systematically analyzed. This situation calls for a comprehensive analysis of measurement uncertainty in THz-TDS systems. The sources of error existing in a terahertz spectrometer and throughout the parameter estimation process are identified. The analysis herein quantifies the impact of each source on the output optical constants. The resulting analytical model is evaluated against experimental THz-TDS data. © 2008 Optical Society of America.

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