We demonstrate how the terahertz properties of porcine adipose tissue and skeletal muscle are affected by formalin fixing. Terahertz radiation is sensitive to covalently cross-linked proteins and can be used to probe unique spectroscopic signatures. We study in detail the changes arising from different fixation times and see that formalin fixing reduces the refractive index and the absorption coefficient of the samples in the terahertz regime. These fundamental properties affect the time-domain terahertz response of the samples and determine the level of image contrast that can be achieved.
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.
Terahertz radiation or T-rays, show promise in quality control of food products. As T-rays are inherently sensitive to water, they are very suitable for moisture detection. This proves to be a valuable asset in detecting the moisture content of dried food, a critical area for some products. As T-rays are transparent to plastics, food
additives can also be probed through the packaging, providing checks against a manufacturer's claims, such as the presence of certain substances in foods.
KEYWORDS: Terahertz radiation, Signal to noise ratio, Wavelets, Spectroscopy, Statistical modeling, System identification, Absorption, Signal processing, Signal attenuation, Classification systems
This work compares classification results of lactose, mandelic acid and dl-mandelic acid, obtained on the basis of their
respective THz transients. The performance of three different pre-processing algorithms applied to the time-domain
signatures obtained using a THz-transient spectrometer are contrasted by evaluating the classifier performance. A range
of amplitudes of zero-mean white Gaussian noise are used to artificially degrade the signal-to-noise ratio of the time-domain
signatures to generate the data sets that are presented to the classifier for both learning and validation purposes.
This gradual degradation of interferograms by increasing the noise level is equivalent to performing measurements
assuming a reduced integration time. Three signal processing algorithms were adopted for the evaluation of the complex
insertion loss function of the samples under study; a) standard evaluation by ratioing the sample with the background
spectra, b) a subspace identification algorithm and c) a novel wavelet-packet identification procedure. Within class and
between class dispersion metrics are adopted for the three data sets. A discrimination metric evaluates how well the three
classes can be distinguished within the frequency range 0.1 - 1.0 THz using the above algorithms.
In the recent years, it has been shown that terahertz (or T-ray) spectroscopy is a versatile tool for biosensing and
safety applications. This is due to the fact that the THz-spectra of many biomolecules show very characteristic,
distinct spectroscopic features. Furthermore, most non-metallic packaging materials are nearly transparent in
this frequency range (0.1 - 6 THz, 3 cm-1 - 200 cm-1), so that it is possible to non-invasively identify even
sealed substances like pharmaceuticals, illicit drugs or explosives by their spectroscopic signatures. This opens a
significant potential for a wide range of applications from quality control of pharmaceutical substances via safety
applications through to biomedical applications.
The individual spectroscopic features below approximately 5 THz that spurred the increased world wide
interest in T-ray spectroscopy are mainly due to intermolecular rather than intramolecular vibrations in the
polycrystalline samples. The spectra of more complex biomolecules, like proteins and nucleotides, typically
show less or even no sharp features, due to the lack of long- range intermolecular order. Furthermore, due to the
typically significantly smaller sample amount, the signal to noise ratio is strongly increased. Water shows a strong
absorption in this frequency range, which all together makes real biomedical applications of T-ray spectroscopy
rather difficult. Yet, by combining a careful sample preparation, novel experimental techniques and an advanced
signal processing of the experimental data we can still clearly distinguish between even complex biomolecules
and therefore demonstrate the potential the technique holds for biomedical applications.
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) radiation has many far reaching applications - of specific interest is that many non-metallic
and non-polar substances are transparent in the THz frequency range. This provides many practical uses
for security purposes, where it is possible to detect and determine various substances that may be hidden or
undetectable via conventional methods such as X-rays. In addition to this property, terahertz radiation can
either be used in reflection or transmission modes.
This paper will look into the use of transmission techniques to detect various substances using a terahertz
system. Common materials used in bags and suitcases such as nylon, polycarbonate (PC), and polyethylene
(PE) are tested for transparency. These materials then sandwich various illicit substances, and are scanned
by the terahertz system to obtain spectral data, simulating the probing of a suitcase. The sample materials
are then subtracted from the obtained data, which is then compared with previously obtained data of known
substances, and an examination of features in the sample is carried out to determine if a particular substance
is present in the sample.
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.
In this paper, we will investigate microwire fibers for low-loss terahertz transmission. Microwires, air-clad wire
waveguides with diameter smaller than the operating wavelength (a few μm), have an enhanced evanescent
field and tight wave confinement resulting in a low loss waveguide structure for the terahertz (T-ray) frequency
regime. Based on our experimental data for the bulk material absorption of four glasses (F2, SF6, SF57 and
Bismuth) and a polymer (PMMA), we calculate the normalized field distribution, power fraction outside the
wire and effective loss. It will be shown that regardless of material, the effective loss of all microwires converges
to the same order < 0.01 cm-1.
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.
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.
In this paper we compare the value of different molecular modeling techniques for the prediction of vibrational
modes, especially in the mid- and far-infrared region. There is a wide range of different levels of theory available
for molecular modelling - the choice depending on the kind of system to be investigated. For our calculations
we use different theoretical approaches such as Hartree-Fock and Density functional theory. We also compare
the performances of two available electronic structure programs-Gamess-US and Gaussian03. As examples,
we use two different retinoids - all-trans retinal and all-trans retinoic acid - derivatives of Vitamin A.
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-1 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.
The terahertz (or T-ray) spectra of many small molecules of biological relevance show very characteristic, specific
features that are sensitive to small changes of the molecular structure and even isomerization. On the other hand,
most packaging materials like plastics, paper or even clothing are transparent for T-rays. Therefore, it is possible
to differentiate and identify different substances by their spectral fingerprints, even through their packaging. This
supports the potential of this technique in a wide range of applications from safety and security applications, via
biosensing, through to pharmaceutical quality control. However, most of the molecular vibrations that give rise to the characteristic features in the T-ray spectra are
phonon-like intermolecular vibrations of weakly bound crystalline compounds. This can be easily demonstrated
by comparing the spectra of different crystals of the same molecule. Whereas this sensitivity on the intermolecular
structure can be used to probe the crystalline structure and detect phase transitions, it is a hurdle when it comes
to identify samples that lack such a well defined intermolecular structure. Yet, we have recently shown that
a comparison of the absolute absorption values can still be used to differentiate between complex biomolecules
such as RNA.
In this paper we will demonstrate, based on a wide range of spectra, the potential of T-ray spectroscopy for
biosensing and will show examples where this technique can be used to probe the crystalline configuration and
probe phase transitions and will discuss the feasibility of using this technique for biosensing.
In the recent years, there has been an increased interest in the exploitation of the far-infrared spectral region for applications based on chemical recognition. The fact that on the one hand many packaging materials are transparent for THz radiation and on the other hand the THz-spectra of many pharmaceuticals, illicit drugs, and explosives show very specific fingerprints show the potential that THz spectroscopy holds for identification of concealed substances by comparing the spectral signatures with the entries in a database. Yet, due to the lack of appropriate techniques the far-infrared region had for a long time be relatively unexplored, and therefore a detailed study of the far-infrared spectra and the character of the molecular vibrations that give rise to the characteristic spectral signatures can help to evaluate the applicability of THz spectroscopy and imaging for quality control, chemical recognition and biomedical applications.
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