An optical multilayer interference filter is made from two or more different dielectric materials layered in such
a way that it promotes constructive or destructive wave interference for a selected frequency in the direction
normal to the layers. Usually, each layer has the thickness of a quarter of wavelength at which the stop-band
is required. In this paper, a quarter-wavelength multilayer interference filter is realised for T-ray applications.
The dielectric materials used are high-resistivity silicon and free space, both of which have high transparency
to T-rays and flat all-pass responses over the frequencies of interest. The designed thickness of both materials
is in the order of a hundred microns, and thus allows the novelty of a retrofittable assembled structure. An
analysis of the affect of the number of layers on the spectral response is given for the first time. The THz-TDS
measurement of the fabricated structure is demonstrated to be in agreement with theory.
The ability to detect and measure a variety of gases under certain environmental conditions has significant
potential impact in many areas; from hazardous gas detection in the industrial domain to physical sciences
in academia. Gas sensing has long received attention with microwave and infrared spectroscopy. With many
molecular resonances occurring in the THz (T-ray) range leading to simple, unique spectral features, THz time-domain
spectroscopy (TDS) promises to be a potential tool for gas detection. This paper presents a preliminary
study on real-time gas recognition with THz-TDS. In particular, a simple method is proposed that involves
extracting line positions from gas species without a reference pulse and classifying them by means of the minimum
Euclidean distance using the Submillimeter, Millimeter, and Microwave Spectral Line Catalog.
In an open-air setting, one source of fluctuations in a T-ray (THz) pulsed signal is attributed to water vapor.
Fluctuations of this type are generally undesired, and so the water vapor is commonly removed in a closed
chamber. Yet, in some applications a closed chamber is not feasible. This paper presents a preliminary study
on a computational means to address the problem. Initially, the complex frequency response of water vapor is
modeled from spectral line data. Using a deconvolution technique, together with fine tuning of the line strength
at each frequency, the response is partially removed from a measured T-ray pulse, with minimal signal distortion.
Measurement precision is often required in the process of material parameter extraction. This fact is applicable
to terahertz time-domain spectroscopy (THz-TDS), which is able to determine the optical/dielectric constants of
material in the T-ray regime. Essentially, an ultrafast-pulsed THz-TDS system is composed of several mechanical,
optical, and electronic parts, each of which is limited in precision. In operation, the uncertainties of these
parts, along with the uncertainties introduced during the parameter extraction process, contribute to the overall
uncertainty appearing at the output, i.e. the uncertainty in the extracted optical constants. This paper analyzes
the sources of uncertainty and models error propagation through the process.
Terahertz (THz) imaging offers many attractive advantages over existing modalities especially in its ability
to obtain spectroscopic information. In particular, THz spectra are extremely sensitive to small changes of
the molecular structure and different isomeric and intermolecular configurations. With a comparatively longer
wavelength (0.3 mm at 1 THz), THz images suffer from the problem of low spatial resolution, as determined by
Rayleigh's criterion and proves to be a major limitation. This paper reviews the existing THz near-field methods
and recent developments for identifying potential areas of research.
Terahertz time-domain spectroscopy (THz-TDS) is able to extract optical or dielectric properties of materials,
whether in the solid, liquid, or gas phase, in the T-ray frequency region. Spectroscopy of a liquid or gas
often requires a receptacle to confine the sample. In order to allow T-rays to probe the sample effectively, the
receptacle must have T-ray transparent windows. However, even though windows are transparent to T-rays,
attenuation exists, because of multiple reflections at air-window and window-air interfaces, which accounts for
a major energy loss. Due to the recent emergence of T-ray technology, there has been very little work carried
out to-date on the reduction of reflection losses. This paper analyses the reduction of T-ray reflection loss by means of an antireflection coating. Because T-ray wavelengths are much larger than visible wavelengths, the
antireflection layer thickness for T-rays is much larger than the usual optical case. This creates an interesting
opportunity for retrofittable antireflection layers in T-ray systems. In the experiment, a coating material made
from polyethylene sheets is applied onto the surfaces of a silicon window. The coated window shows enhancement
of the transmittance within a range of frequencies.
This study investigates binary and multiple classes of classification via support vector machines (SVMs). A couple of groups of two dimensional features are extracted via frequency orientation components, which result in the effective classification of Terahertz (T-ray) pulses for discrimination of RNA data and various powder samples. For each classification task, a pair of extracted feature vectors from the terahertz signals corresponding to each class is viewed as two coordinates and plotted in the same coordinate system. The current classification method extracts specific features from the Fourier spectrum, without applying an extra feature extractor. This method shows that SVMs can employ conventional feature extraction methods for a T-ray classification task. Moreover, we discuss the challenges faced by this method. A pairwise classification method is applied for the multi-class classification of powder samples. Plots of learning vectors assist in understanding the classification task, which exhibit improved clustering, clear learning margins, and least support vectors. This paper highlights the ability to use a small number of features (2D features) for classification via analyzing the frequency spectrum, which greatly reduces the computation complexity in achieving the preferred classification performance.
Liquid spectroscopy allows analysis of chemical composition and provides a better understanding of the solvation
dynamics of various types of liquids. Although it has been shown that liquid spectroscopy using T-rays is feasible,
liquid water absorption is still considered to be one of the most challenging problems facing THz imaging and
spectroscopy in biomedical applications. The absorption coefficient for liquid water shows a very high THz
absorption, 200 cm<sup>-1</sup> at 1 THz. This paper describes a promising novel liquid double-modulated differential
time-domain spectroscopy (Double-modulated DTDS) technique to extract the optical parameters with a dual-thickness
measurement. The described technique improves on the previous work, by replacing the required
sample dithering technique with a rotating spinning wheel resulting in an improved noise performance up to two
orders of magnitude.
Terahertz transmission through freshly excised biological tissue is limited by the tissue's high water content.
Tissue fixation methods that remove water, such as fixation in Formalin, destroy the structural information
of proteins hence are not suitable for THz applications. Dehydration is one possible method for revealing the
tissue's underlying molecular structure and components. In this study, we measured the THz responses over time
of dehydrating fresh, necrotic and lyophilized rat tissue. Our results show that as expected, THz absorption
increases dramatically with drying and tissue freshness can be maintained through lyophilization. Dehydrated
biological tissue with retained molecular structure can be useful for future laser-based THz wave molecular
Terahertz spectroscopy, which investigates the electromagnetic spectrum of samples between 0.1 and 10 THz, allows not only for exploration of molecular structures but also of molecular dynamics. One difficulty in performing THz spectroscopy is that the data can be noisy and difficult to interpret. Ab initio molecular modelling has recently become more and more useful in the prediction of, for example, molecular structures, dynamic states and isomeric forms. Since the structure of biomolecules is closely related to their functionality there are broad ranging applications in biomedicine, for example in DNA sensing. An a priori knowledge of the expected THz spectra allows for improved experimentation. There is a growing and recognised need for THz spectroscopic databases to be created and made available along with classifiers that are able to effectively
detect a specific substance. We show, for a specific example, the 9-cis and all-trans retinal isomers, how ab initio molecular orbital calculations and quantum chemical modelling programs, such as Gamess, can aid in this endeavour.
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.
This study investigates the application of one dimensional discrete wavelet transforms in the classification of T-ray pulsed signals. The Fast Fourier Transform (FFT) is used as a feature extraction tool and a Mahalanobis distance classifier is employed for classification. In this work, soft threshold wavelet shrinkage de-noising plays an important part in de-noising and reconstructing T-ray pulsed signals. In addition, Mallat's pyramid algorithm and a local modulus maxima method to reconstruct T-ray signals are investigated. Particularly the local modulus maxima method is analyzed and comparisons are made before and after reconstruction of signals. The results demonstrate that these two methods are especially effective in analyzing and reconstructing T-ray pulsed responses. Moreover, to test wavelet de-noising effectiveness, the accuracy of the classiffication is calculated and results are displayed in the form of scatter-plots. Results show that soft threshold wavelet shrinkage de-noising improves the classification accuracy and successfully generates visually pleasing scatter plots at selected three frequency components.
The reflection characteristic of terahertz radiation (T-rays) in stratified media is being explored through the use of computer models. When T-rays are reflected off a sample, the measured T-ray signal contains coherent spectroscopic information about the sample. In the time domain, this spectroscopic information becomes the time response of the sample-a useful method for determining layer thickness and the number of interfaces in the sample. In order to confidently determine thickness and interfaces, the propagation characteristic of T-rays in a stratified medium needs to be understood. Internal reflections, interference, and water absorption within the layers can significantly alter the T-ray signal. This paper reports on a study of T-ray propagation in tissue layers inside the head, in reflection mode. Simulated results are presented and discussed.
A simple method to extract the far-infrared dielectric parameters of a homogeneous material from terahertz signals is explored in this paper. Provided with a reference, sample-probing terahertz signal and a known sample thickness, the method can determine the underlying complex refractive index of the sample within a few iterations based on the technique of fixed-point iteration. The iterative process is guaranteed to converge and gives the correct parameters when the material thickness exceeds 200 μm at a frequency of 0.1 THz or 20 μm at a frequency of 1.0 THz.
Terahertz wavelengths can pass through dry, non-polar, non-metallic materials that are opaque at visible wavelengths. Moreover they can be manipulated using millimeter wave and quasi-optical techniques to form an image. Sensing in this band potentially provides advantages in a number of areas of interest for security and defense, such as screening of personnel for hidden objects, and the detection of chemical and biological agents. This paper reviews recent research into THz applications by groups across Europe, the US, Australasia, and the UK. Several private companies are developing smaller and cheaper reliable devices allowing for commercialisation of these applications. While there are a number of challenges to be overcome there is little doubt that THz technologies will play a major role in the near future for advancement of security, public health and defense.
Velocity of Detonation (VoD) is an important measured characteristic parameter of explosive materials. When new explosives are developed, their VoD must be determined. Devices used to measure VoD are always destroyed in the process, however replacing these devices represents a considerable cost in the characterisation of new explosives. This paper reports the design and performance of three low-cost implementations of a point-to-point VoD measurement system, two using optical fibre and a third using piezoelectric polymers (PolyVinyliDine Flouride, PVDF). The devices were designed for short charges used under controlled laboratory conditions and were tested using the common explosive 'Composition B'. These new devices are a fraction of the cost of currently available VoD sensors and show promise in achieving comparable accuracy. Their future development will dramatically reduce the cost of testing and aid the characterisation of new explosives.
Terahertz imaging is presently in its exploratory stage. Although plots of time versus terahertz amplitude, and frequency versus terahertz magnitude are some of the most common ways of analyzing terahertz data, no standard rendering technique has been established. While existing methods are indispensable, improvements to how terahertz data is rendered and analyzed should be explored so that new techniques can complement existing ones and/or provide a means of displaying new information that existing methods cannot. This paper reports on one solution to terahertz imaging: an implementation of a new form of phase contrast imaging, which is based on a well-established technique for optical microscopy. This will provide us with a further way of interpreting information from terahertz imaging systems.
Vibrations from a target provide a difficult to mask target signature. Vibrometry shows potential for long-range target identification whilst a fibre implementation may lead to a smaller, more compact system when compared with an equivalent solid-state source solution. A prototype fibre LDV system and electronic demodulation scheme using low-cost telecommunications components is described and tested. The aim of the system is to remove the velocity component of a target signature for target identification purposes. Signal processing methods and signature measurements are described which demonstrate the utility of the system for target recognition.
Pulsed THz (T-ray) spectroscopy is sensitive, non-invasive tool for studying materials from physics to biology, but transmission measurements of liquid samples, especially water, have been limited by noise. This paper shows that the accuracy of T-ray material parameter measurements of liquid samples can be greatly increased, especially for highly-absorbing liquids, by using a rapid modulation of the liquid in the T-ray beam path, coupled with a novel implementation of mean and amplitude detection to T-ray spectroscopy. The experiments are supported by calculations quantifying the sources of uncertainty. Liquid transmission T-ray studies are valuable for understanding solvation dynamics of salts, exploring long-range structure in mixtures and probing biomolecules in suspension.
This paper describes a study conducted into the limit on spectral resolution due to the dynamic range of a T-ray spectrometer. The pulsed nature of terahertz time-domain spectroscopy (THz-TDS) sets a fundamental limit on its spectral resolution. The spectral resolution of THz-TDS can be improved by increasing the duration of the temporal measurement, but is limited by the dynamic range of the system in the time-domain. This paper presents calculations and experimental results relating the temporal dynamic range of a THz-TDS system to its spectral resolution. We discuss three typical pulsed terahertz sources in terms of their dynamic range and hence achievable spectral resolution.
The development of terahertz radiation (T-rays) is spurring new applications in spectroscopy and imaging. To maximize the use of T-rays in more applications, a high average terahertz power is needed. Rather than using fast diodes or laser sources, this paper will show that a synchrotron can generate high average power T-rays. This is achieved by creating an electron bunch in the synchrotron ring with high intensity in the terahertz frequency region via Thomson scattering.
Pulsed THz (T-ray) spectroscopy is an increasingly wide-spread tool for studying materials from physics to biology. Liquid transmission T-ray studies are valuable for understanding solvation dynamics of salts, exploring long-range structure in mixtures and probing biomolecules in suspension. In this paper the uncertainty in parameter estimation of liquid samples is shown to be dependent on the thickness change of the sample, and on the noise of the T-ray spectrometer. For many important liquids, such as water, with high THz absorption, we show that measurement uncertainty can be greatly decreased using a dynamically-modulated liquid sample, using differential T-ray time-domain spectroscopy (DTDS). Preliminary experiments support these calculations.
We present three T-ray (terahertz wave) tomographic imaging modalities: T-ray computed tomography, T-ray diffraction tomography, and tomographic imaging with a Fresnel binary lens. Each of these techniques uses pulses of broadband terahertz radiation to obtain 3-dimensional images of targets with wide potential application. We present images demonstrating the performance of each technique and discuss their relative advantages.
The study of enzymatic protein molecules using terahertz time-domain spectroscopy (THz-TDS) has the potential to reveal molecular activity in real time without the use of labelling. Molecular hydration, or bound water, is a critical parameter in enzyme activity and THz-TDS measurements. For the first time we experimentally measure the terahertz-frequency response of nano-sized particles of protein and their level of molecular hydration. These measurements are valuable in understanding the terahertz response of biological systems and in studying the interaction between bound water and proteins.
A new bioaffinity sensor based on pulsed terahertz (THz) spectroscopy is able to sensitively detect the presence of ultra-thin bound biomolecular layers. The protein avidin and lipid biotin are noted for their very high binding affinity, and the ease for which they can be attached to residues with importance in many biosensing applications. We demonstrate the sensitivity of the pulsed THz spectrometer to thin avidin layers and to avidin amplified with micron-sized agarose beads. The experimental results can be simply modelled by considering transmission of the THz radiation at the thin film interfaces. We detect less than 10.3 ng/cm<sup>2</sup> avidin, giving the THz system a detection capability of sub-thin solid films better than ellipsometry and reflectometry techniques.
Characterizing the optical and dielectric properties of thin films in the GHz to THz range is critical for the development of new technologies in integrated circuitry, photonics systems and micro-opto-electro-mechanical systems (MOEMS). Terahertz differential time-domain spectroscopy (DTDS) is a new technique that uses pulsed terahertz (THz) radiation to detect phase changes of less than 0.6 femtoseconds (fs) and absorption changes corresponding to several molecular monolayers. This paper shows how DTDS can be combined with double modulation in the pump-probe system to improve sensitivity by an order of magnitude. The technique is experimentally verified using 1 μm thick samples of silicon dioxide on silicon.
T-ray imaging and spectroscopy both exploit the terahertz (THz) region of the spectrum. This gives rise to very promising industrial and biomedical applications, where non-invasive and sensitive identification of a substance is achievable, through a material's distinct absorption features in the THz band. Present T-ray systems are limited by low output power, and the race is now on to find more efficient THz emitters. We discuss the feasibility of a novel high-power gallium nitride emitter for terahertz generation. This paper details the advantages of such an emitter, primarily by virtue of its high-voltage capability, and evaluates the benefits of sapphire and silicon carbide substrates. The far-infrared transmission spectra for thin samples of GaN, sapphire and SiC are reported. A high-power THz emitter, that operates at room temperature and is potentially low-cost will open up a host of new possibilities and applications. The central result in this paper demonstrates that sapphire is the better choice over SiC, for the GaN supporting substrate, as we show that it has superior THz transmission characteristics.
T-ray systems o er an exciting range of capabilities for chemical and biological diagnostics using the emerging technology of terahertz pulse imaging.1,2 We report results from the rst Australian T-ray program and discuss how MOEMS techniques can be applied to decrease the system size.3 A small portable T-Ray system will cost less and is needed, for example, in endoscopic applications.4
The rapidly developing field of terahertz (T-ray) imagin promises to provide a non-invasive method of identifying the composition of various objects. Biomedical diagnostics, semiconductor device diagnostics, trace gas analysis and moisture analysis for agriculture are among the growing number of important T-ray applications. We present results using an electro-optical sampling method and discuss how this can be simplified by sign a micro antenna array realized by MEMS technology. The challenges and advantages of both approaches are compared.
We evaluate the efficiency of a new optoelectronic quenched avalanche sensor, with the potential of enabling high resolution imagin through turbid media with femtosecond lasers. Our target application is for imaging cancer in the human breast.