applications of terahertz wave technology in smart textiles...lead to unlock many new and...

Applications of Terahertz Wave Technology in Smart Textiles

Dongxiao Yang*Department of Information Science & Electronic Engineering, Zhejiang University, Hangzhou, China

Abstract

The terahertz radiation bridges the gap between microwave and infrared light, which consists ofelectromagnetic waves with frequencies ranging from 100 GHz to 1,000 GHz. There are approximatelyone-half of the total luminosity and most of the photons emitted since the Big Bang fall into the terahertzfrequency region. Terahertz spectroscopy and imaging are two important techniques for the applicationsto textiles, which are described in this chapter. Some terahertz spectroscopy experimental systems werepresented, such as time-domain spectroscopy-based terahertz pulsed system and backward-wave oscil-lator-based continuous-wave terahertz system. Several applications of the terahertz spectroscopy tech-nique were reviewed to textile identification and sensing, such as textile fibers, textile materials, and wooltextiles. Terahertz imaging of object behind textile barriers was demonstrated and the images weresegmented for target detection. Terahertz imaging applications to textiles were also reviewed, such asmeasuring textile water content, detecting target behind textile barriers, and testing compositesnondestructively.

Keywords

Terahertz wave; Textile; Composite; Material; Spectroscopy; Imaging; Identification; Sensing; Hiddenobject detecting; Nondestructive testing

Introduction

The microwave radiations are generally characterized by waves, because most microwave devices arecomparable in size to the wavelength of the radiation. In contrast, the optical radiations are generallycharacterized by beams or rays, because the dimensions of the optical devices are much larger than thewavelength [1]. Science and technology are well-developed in both microwave spectral region includingmillimeter wave region, and optical spectral region as well as infrared spectral region, which are based onelectronics and photonics, respectively. The terahertz radiation,whose frequencies ranging from 100 GHzto 1,000 GHz, bridges the gap between the microwave radiation and infrared radiation. Neither opticaltechnique nor microwave technique is directly applicable in the terahertz frequency range since opticalwavelengths and microwave wavelengths are too short and too long compared to terahertz field wave-lengths, respectively. The terahertz spectral region is the gap of these technologically well-developedspectral regions. This “terahertz gap” is a relatively unexplored band of the electromagnetic spectrum.With the development of sufficiently terahertz sources and detectors, this gap has been filling, which willlead to unlock many new and potentially revolutionary technologies and applications to many areas ofscience and technology.

*Email: [email protected]

Handbook of Smart TextilesDOI 10.1007/978-981-4451-68-0_41-2# Springer Science+Business Media Singapore 2015

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The principal application of terahertz techniques before 1990 was the research in astronomy, becauseastronomers can obtain a large amount of information of stars and intergalactic gases by detecting theterahertz radiations from outer space and need not set up terahertz source system. Terahertz techniqueswere used for material characterization by physicists early in the 1990s, such as determining the carrierconcentration and mobility of semiconductors. In the mid-1990s, some techniques for generationand detection of terahertz radiation had been accomplished with the help of different techniques,and corresponding terahertz sources and detectors had developed in the late 1990s.

Several commercial terahertz systems equipped with both source and detector were manufactured byelectronic engineers in the beginning of the 21st century, such as terahertz time-domain spectroscopybased on terahertz generation and detection techniques which was developed using ultrafast laser pulses inthe 1980s. The terahertz science and technology have been rapidly developed since 2000 with the help ofcommercial terahertz systems.The terahertz techniques have been used in many areas, such as spectrumanalysis, imaging and tomography, wireless communication with high data rate, high-temperaturesuperconductor characterization, protein folding measurement, molecular structure detection, cancerdetection and genetic analysis, concealed object detection and target detection behind barriers, nonde-structive testing and quality control, gas sensing and air pollution detection, and monitoring surroundingmedium. But, unfortunately, the terahertz techniques are seldom used for textiles. Terahertz spectroscopytechnique, terahertz imaging technique, and their applications to textiles were described in this chapter,which were developed by some research groups in the world.

Brief of Terahertz Wave Technology

Terahertz Wave SpectrumThe terahertz radiation bridges the gap between microwave and infrared light, which consists ofelectromagnetic waves with frequencies ranging from 0.1 to 10 THz (1 THz = 1012 Hz) [2], as shownin Fig. 1.

This terahertz wavelength interval is the wavelength region between the top edge of the microwave tothe boundary of the millimeter wave spectral region and the bottom edge of the optical spectrumcorresponding to the boundary of the far-infrared spectral region [3]. The term terahertz is synonymouswith the terms submillimeter and far infrared in the electromagnetic spectrum.

In order to get a better grasp of the terahertz frequency region, it is useful to mention that the frequencyof 1 THz corresponds to a wavelength of 0.3 mm, to an electric field period of 1 ps, to a wave number of33 cm�1 in common spectroscopic terms, to an equivalent blackbody temperature of 48 K, and to a photonenergy of 4.1 meV (small energy compared to the thermal energy of 26 meV at room temperature).

The terahertz frequency region has drawn much attention because of the unique physical phenomena ofthe terahertz radiation. Two of these unique properties are listed below in brief.

Applications of Terahertz Wave Technology in NatureThe radiation in terahertz frequency region is a useful tool for discovery in cosmic astronomy on externalplatforms out of the Earth atmosphere [4]. According to the results from the NASA Cosmic BackgroundExplorer Diffuse Infrared Background Experiment and the examination of the spectral energy distribu-tions in observable galaxies, the corresponding frequencies of approximately one-half of the totalluminosity and 98 % of the photons emitted since the Big Bang fall into the submillimeter wave andfar-infrared light which lie in the terahertz frequency region [5]. The composition and origin of the solarsystem, the evolution of matter in our galaxy, and the star formation history of galaxies over cosmictimescales could be better to understand and easier to discover with the help of the diagnostic spectral


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Fig.1

Terahertzfrequencyregion

andtheelectrom

agneticspectrum


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signatures of ions, atoms, and molecules in the terahertz frequency region [6]. The second figure in Ref.[5] shows the radiated power spectra for interstellar dust, light, and heavy molecules, as well as a 30 Kblackbody radiation curve and the 2.7 K cosmic background signature.

The relic radiations carry information about the cosmic space, galaxies, stars, and planet formation.Terahertz fields interact strongly with the polar substances and penetrate the nonpolar substances. There

are many distinct spectral peaks in the terahertz frequency region of the absorption spectra of many polarmolecules [3]. The terahertz radiation is efficiently absorbed in air, except for some narrow windows, asshown in the fourth figure in Ref. [4].

This unique signature of molecules in the terahertz frequency region is very important in monitoring thesurrounding medium, detecting air pollution, or gas sensing.

Applications of Terahertz Wave Technology in CommunicationThe demand for the ultrafast wireless communications is increasing due to more and more people usingwireless networks. Nevertheless, it may be quite difficult to keep up with the needs of users withoutincreasing the carrier frequencies for more spectral resources [7]. Since terahertz waves have hugebandwidth, terahertz wave technology has attracted a lot of attention in wireless communications forfuture with high data rates.

As shown in the fourth figure in Ref. [4], the Earth’s atmosphere is an absorber of terahertz radiation inthe water vapor strong absorption bands. Due to this limitation, the terahertz radiation is not suitable forlong-distance terahertz transmission on the Earth. Notwithstanding, the long-distance terahertz transmis-sion is less affected by adverse weather conditions like rain and fog in contrast to optical transmission[8]. Furthermore, terahertz waves are also suitable for high-altitude huge-data-capacity communicationswith high data rates, such as aircraft to satellite and between satellites. In spite of absorption inatmosphere, terahertz waves are suitable for short-distance huge-data-capacity communications withhigh data rates on the Earth [7, 9]. There are some terahertz transmission windows with low atmosphericlosses, such as the frequencies of 0.2–0.3 THz, as shown in the first figure and the fifth figure in Ref. [10].The latter figure shows the specific attenuation from 0.001 to 0.35 THz at sea level for dry air and watervapor with a density of 7.5 g/m3 [10].

A point-to-point terahertz wireless communication system with a carrier frequency of 0.2375 THz hasbeen established for transmitting data over 20 m at a data rate of 100 Gb/s [8]. This data rate is muchhigher than those of the commercial wireless communication systems, such as 0.3 Gb/s for the fourth-generation (4G) wireless network over 3 km and uplink data rate of 1 Gb/s and downlink data rate of10 Gb/s for the upcoming 5G wireless network. One of the applications for such a high-capacity terahertzwireless link will be the wireless link bridging a broad river in difficult-to-access terrain to provide high-speed internet access in remote and rural areas, as shown in the first figure in Ref. [8].

Because of considerable attenuation, terahertz waves are not very useful for long-distance communi-cations. However, due to this strong absorption, the transmission spectra of many materials can provideinformation about the physical properties of the materials. In addition, an important feature of terahertzradiation is the ability to penetrate into nonmetallic materials and distinguish between them [4].

Terahertz Spectroscopy Technique

Terahertz spectroscopy is an important technique in terahertz science and technology, which is useful forboth the fundamental research and the industrial application [11]. The former includes finding andunderstanding the mechanisms in physics, chemistry, and material science, and the latter includesimaging, sensing, material identification, safety-critical quality inspection, security scanning of concealed


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dangerous substances, contact-free testing, and nondestructive testing. Based on terahertz spectroscopytechnique, simultaneously acquiring insight image of objects and identifying materials give terahertztechnology a big advantage over other technologies. Terahertz radiation-related low-energy interactionscan be studied by these waves in many materials to complement the knowledge of material behaviorsobtained with far-infrared and Raman spectroscopy. One of the most attractive properties of terahertzwaves is the ability to penetrate many materials, which can be used to analyze content through many typesof packaging materials, such as paper, plastics, leather, and wood. Terahertz spectroscopy is a useful toolfor characterizing vibrational modes, such as rotational, torsional, phonon, intramolecular, andintermolecular. Since the terahertz response is linked to the collective behavior of molecules intheir environment and can distinguish polymorphism and chirality, terahertz spectroscopy has abilitiesdifferent from conventional far-infrared spectroscopy.

Time-Domain Spectroscopy-Based Terahertz Pulsed SystemTerahertz time-domain spectroscopy (THz-TDS) is a method for coherent generation and detection ofbroadband terahertz radiation using ultrafast laser pulse, as shown in Fig. 2.

A femtosecond pulse laser is used to generate ultrafast laser pulse beam at the wavelength around 1 mm.A beam splitter is used to split the laser beam of ultrafast pulse into a pump beam and a probe beam. Thelaser pump beam is used for generation of terahertz pulses. In order to generate terahertz pulses, a terahertzemitter is irradiated by the laser pump beam. The terahertz pulses are collimated and focused on thesample by a pair of 90� off-axis parabolic mirrors. A Si wafer is used to transmit the terahertz beam withlow transmission loss and to reflect the laser probe beam with high reflectivity. The terahertz pulsescarrying the sample information are collimated and focused on an electro-optic crystal (ZnTe) by anotherpair of 90� off-axis parabolic mirrors. Since the frequency in 1012 Hz of terahertz waves is much lowerthan that in 1015 Hz of the laser waves, the terahertz waves as a slow wave can modify the index ellipsoidof the electro-optic crystal for the laser wave transiently. The laser probe beam is used for detection ofterahertz pulses and linearly polarized by a polarizer. The linearly polarized laser probe beam reflected bythe Si wafer co-propagates with the terahertz pulse beam inside an electro-optic crystal where the diameterof the laser beam is smaller than that of the terahertz beam and is modulated by the refractive index changeinduced by the terahertz electric field as a slow wave. The phase change of the laser probe beam isconverted to an intensity change by a quarter-wave plate and a Wollaston prism and detected by a pair ofbalanced photodiodes. A delay stage is used to scan optical delay line for offsetting the laser probe beamand the terahertz pulse beam excited by the laser pump beam. This allows the terahertz temporal profile tobe iteratively sampled. This method for terahertz detection is called free-space electro-optic sampling.

Fig. 2 Schematic diagram of a terahertz time-domain spectroscopy


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Figure 3 is a photo of an experimental setup of a terahertz time-domain spectroscopy.A polarization-sensitive terahertz time-domain spectroscopy can be used to measure anisotropic

responses of materials and structures due to the applied factors, such as stress and magnetic fields[12]. A schematic diagram of a terahertz time-domain spectroscopic ellipsometry (THz-TDSE) isshown in the first figure in Ref. [13].

As shown in this figure, the terahertz time-domain system is similar as the terahertz time-domainsystem in Fig. 2. The differences between these two systems are that the terahertz time-domain spectro-scopic ellipsometry is based on the photoconducting effect with photoconductive antennas for thegeneration and detection of terahertz pulses. Furthermore, the terahertz pulse beam is reflected from thesample, and a polarizer and an analyzer are placed in front of the sample and the detection antenna,respectively [12, 13].

The measurement of terahertz pulse with terahertz time-domain spectroscopy allows to extract someinformation, such as frequency-dependent amplitude and phase, which can be related to the physicalproperties of materials and devices [14]. Typical frequency resolution and output power of terahertz time-domain spectroscopy are several ten GHz and several mW [14, 15], respectively. Some terahertz time-domain spectroscopies have reached relatively high-frequency resolution of around 2 GHz [16, 17].

Backward-Wave Oscillator-Based Continuous-Wave Terahertz SystemBackward-wave oscillator is a continuous-wave terahertz source [18] with highly monochromatic andpowerful, spectrally bright and polarized, and finely frequency-tunable properties, such as a high-frequency resolution of 0.3 GHz [19] and high output power of 15 mW. Backward-wave oscillator-based continuous-wave terahertz system can be used to detect polarized spectra of physical properties ofmaterials, such as real and imaginary refractive index, dielectric constant, and magnetic permeability.A backward-wave oscillator-based continuous-wave terahertz system is shown in Fig. 4a.

Monochromatic terahertz beam emitted from backward-wave oscillator with the frequency step of0.3 GHz is attenuated to proper power for sample by a terahertz attenuator. After transmission through theattenuator, the terahertz beam is collimated or focused on the sample by two high-density polyethylenelenses. The transmitted beam from the sample is focused on a highly sensitive pyroelectric detector fordetection by one or two high-density polyethylene lens. The response time of the pyroelectric detector isseveral milliseconds, which is a difficult-to-measure high-speed terahertz signal. One of the alternativeterahertz detectors for high-speed signals is the VDI terahertz detector, as shown in Fig. 4b. When there isno electrostatic discharge (ESD) protection, the response rate of VDI terahertz detector can reach as highas 30 GHz. However, operating without ESD protection circuit is very risky. The VDI terahertz detector iscomposed of zero-bias terahertz detector with ESD protection circuit, terahertz coupling waveguide

Fig. 3 Photo of a terahertz time-domain spectroscopy experimental setup


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antenna, and output amplifier. Typical response rate of VDI terahertz detector is 250 kHz. The responserate of VDI terahertz detector with ESD protection can be raised to GHz by improving the ESD protectioncircuit, waveguide, and amplifier, such as the response rate of 3 GHz for VDI WR3.4ZBD zero biaseddetector equipped with MJ-LNA-0005-0300 amplifier and WR3.4DH antenna.

Terahertz Spectroscopy for Textile Identification

Because of the low attenuation of most textile materials in the terahertz frequency region, terahertzspectroscopy has some advantages in examining textiles, such as easy measurement, accurate determi-nation, and effective identification. Since chemical additives in textile fibers, such as dyes, have multiplestrong absorption peaks in near- and mid-infrared frequency region, the measured responses of textilematerials by other spectroscopic techniques are obscure and disturbed in high levels [16]. Terahertzspectroscopy can be performed in real-time and in-line monitoring systems for textiles and can provide aneffective, reliable, rapid, contact-free, and nondestructive testing for the textile identification and com-bating fraud.

Wool Textile IdentificationAccording to different optical properties of the textile composed of wool of different animal origins, thetype of wool can be identified and the textile counterfeiting can be combated. In order to accuratelydetermine woolen textile composition, J. Molloy et al. [16, 20] have measured 15 wool fabric samples intwo fabric orientations: with the warp parallel to the terahertz wave polarization (designated pa) andperpendicular to it (designated pe) with terahertz time-domain spectroscopy, as shown in the first figure inRef. [16]. The second figure in this reference shows the transmission spectra of two samples in two fabricorientations.

Table 1 lists the wool fabrics, properties, and the distinct loss peaks and their frequencies and widths inthe transmission spectra below the frequency of 2 THz for 15 wool fabric samples of different yarns withdifferent compositions and structures [16].

Since the majority of the fabrics are dichroic, the transmission spectra are depended on the beampolarizations. The spectra of some woolen textile samples have different loss peaks with different peakfrequency and profile for the two orientations, as listed in Table 1. The differences of measured loss peaksare to be ascribed to diffraction and scattering caused by the weave pattern, because the fiber thickness andthe weave periodicity are both of the order of 50–1,000 mmwhich are comparable to terahertz wavelength.

According to the measurements, the terahertz transmission spectra have different spectral features fordifferent types of woolen textile, which may be used for the textile identification. Fabrics with similar

Fig. 4 Experimental setup for backward-wave oscillator-based continuous-wave terahertz system: (a) photo of continuous-wave terahertz system and (b) VDI detector equipped with antenna and amplifier


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Tab

le1

Woolfabrics,p

roperties,distinctloss

peak

frequencies,andpeak

widthsin

twofabricorientations

[16]

Fabric

Yarn

Com

positio

nWeave

Threads

(cm)

Peak(THz),inorientation

Peakwidth,F

WHM

(THz)

warp/weft

pepa

pepa

A1

8.0Nm

LW/LB

Herringbone

12/12

/1.37

�0.02

/0.35

�0.04

A2

9.0Nm

LW/NB

Herringbone

12/12

1.16

�0.02

1.33

�0.02

0.30

�0.03

0.42

�0.04

B1

3.6/2.0Nm

100%

WHerringbone

6/8

//

//

B2

3.6Nm

100%

WHerringbone

8/8

//

//

C1

3.8Nm

100%

MW

Plain

7/7

//

//

C2

3.6Nm

100%

WPlain

7/7

1.095�

0.005

1.035�

0.005

0.15

�0.02

0.15

�0.02

D3.6Nm

100%

WPlain

6/6

1.24

�0.02

0.845�

0.005

0.30

�0.03

0.13

�0.01

E3.6Nm

100%

WHerringbone

7/7

1.025�

0.005

0.88

�0.02

0.12

�0.02

0.15

�0.02

F1

3.6Nm

100%

W2/2twill

9/9

//

//

F2

Unknown

60%

WB

2/2twill

9/9

/1.35

�0.02

/0.30

�0.03

G1

13.5

Nm

50%

C–5

0%

LN

Knit

12/12

1.46

�0.02

1.315�

0.005

0.35

�0.04

0.30

�0.03

G2

6Nm

2-ply

68%

M–2

9%

C–3

%S

Knit

10/12

/1.89

�0.02

/0.35

�0.04

K1

14Nm

100%

CKnit

5/7

1.345�

0.005

1.280�

0.005

0.30

�0.03

0.30

�0.03

K2

8.5Nm

70%

M–3

0%

CKnit

7/9

1.56

�0.01

1.47

�0.01

0.35

�0.03

0.32

�0.03

L3.4Nm

70%

M–3

0%

CKnit

3/5

/1.135�

0.005

/0.27

�0.02

Note:Nm

=km

/kg,

LWlamb’swool,LBlycrablend,

NBnylonblend,

Wwool,MW

merinowool,WBwoolb

lend,C

cashmere,LNlin

en,M

merino,

Ssilk


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appearance and tactile texture have clearly distinct spectral features for different yarn compositions,which may be used for textile identification and fraud prevention.

Textile Fiber IdentificationThe terahertz spectral features of materials are mainly from both intramolecular and intermolecular forces.The material spectra in terahertz frequency region are sensitive to skeletal vibrations and weakintermolecular forces, such as hydrogen bond and van der Waals force. The remarkable loss peaks inthe spectra of the interstellar dust, light, heavy molecules, and gas molecules in Earth atmosphere areshown in the second figure in Ref. [5] and the fourth figure in Ref. [4]. Remarkable peaks in the materialspectra give the opportunity to fully investigate the internal vibration modes of textile fibers. In order toidentify the ramie fiber and bamboo fiber with similar chemical composition, C. Yan et al. [15] havesimulated and measured their terahertz spectra with terahertz time-domain spectroscopy.

Bamboo fiber is deemed to possess many advantages over other fibers, such as pleasant tactilesensations, ultraviolet protective ability, and antimicrobial activity. The bamboo fiber is easilycounterfeited by ramie fiber on the market due to the similar physicochemical properties, which increasesthe complexity of discrimination [15]. Fourier transform infrared (FTIR) spectroscopy has the potentialfor providing rich information on molecular activities, such as vibrational and torsional modes arisingfrom corresponding chemical bonds or functional groups. The FTIR spectra of ramie fiber and bamboofiber in two wave number regimes of 4,000–400 cm�1 corresponding to the frequency regime of12–120 THz and 200–30 cm�1 to 0.9–6.0 THz have almost the same features, such as the absorptionfeatures in terms of both attenuation and frequency, as shown in the third figure in Ref. [15].

Since the FTIR spectra of ramie fiber and bamboo fiber indicate that these two fibers consist of the samechemical constitution, it is a challenge to distinguish them from each other effectively by this approach.There are two remarkable peaks at the wave numbers of 175 and 125 cm�1 corresponding to thefrequencies of 5.25 and 3.75 THz of both ramie fiber and bamboo fiber, which are defined as vibrationscharacterizing the crystalline structure of monosaccharides in cellulosic fibers.

The rich spectral information in the wave number regime below 100 cm�1 corresponding to thefrequencies below 3 THz, which can be difficult to be obtained from the FTIR spectra, is useful todistinguish ramie fiber and bamboo fiber. Terahertz time-domain spectroscopy could provide thisimportant spectral information of ramie fiber and bamboo fiber and can be used to identify them withrelatively high resolution. The typical resolutions of Fourier transform infrared spectroscopy with thefrequency regime of 12–120 THz, terahertz time-domain spectroscopy with 0.1–3.0 THz, and backward-wave oscillator-based continuous-wave terahertz system with 0.1–1.0 THz are 120, 40, and 0.3 GHz,respectively. The sixth figure to the eighth figure in Ref. [15] show the terahertz absorption spectra oframie fiber and bamboo fiber measured with terahertz time-domain spectroscopy.

In contrast to the results obtained from FTIR spectroscopy, the distinct differences among terahertzabsorption spectra of ramie fiber and bamboo fiber can be observed in these three figures in Ref. [15]. Thisindicates that terahertz spectroscopy is an acceptable tool to distinguish ramie fibers from bamboo fibers.According to the reference spectra in the sixth figure and the seventh figure in Ref. [15], the terahertz waveattenuation of ramie fiber or bamboo fiber is small. It is obvious that the intensity of terahertz wave abovethe frequency of 2.5 THz is very low; the effective terahertz absorption spectra should be below thefrequency of 2.5 THz. The terahertz spectra of ramie fiber and bamboo fiber below the frequency of1.0 THz present almost the same features. According to these three figures in Ref. [15], the considerablefrequency range is 1.0–2.5 THz, where there are seven peaks and nine peaks in the terahertz absorptionspectra of ramie fiber and bamboo fiber, respectively. The absorption peaks of ramie fiber are unambig-uously observed at the frequencies of 1.09, 1.35, 1.5, 1.75, 1.94, 2.13, and 2.4 THz, especially theprominent spectral features at 1.94 and 2.4 THz. The absorption peaks of bamboo fiber are at the


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frequencies of 1.18, 1.31, 1.47, 1.62, 1.75, 1.92, 2.19, 2.35, and 2.5 THz. The strongest absorption ofbamboo fibers is located at 1.92 THz. A gradual rise of the absorption baseline with increasing frequencyresults from nonresonant wave scattered at the samples. The ramie fiber and bamboo fiber can be readilydistinguished according to the peak number and relative absorption level of their absorption spectra.

C. Yan et al. [15] have also simulated the ground-state structures and harmonic vibrational frequenciesof the ramie fiber and bamboo fiber samples based on the density function theory. The experimentalabsorption peaks at 1.92 THz for bamboo fiber and 1.94 THz for ramie fiber may be attributed to thespectral feature of glucose at 1.95 THz obtained by the simulation, which originates from the torsionaldeformation of hexatomic ring of b-D-glucose. The spectral features observed at 2.19 THz for bamboofiber and 2.13 THz for ramie fiber in experimental spectra are associated with the skeletal vibration. Thespectral features at low frequencies in experimental spectra of ramie fiber and bamboo fiber may beattributed to the intermolecular hydrogen function.

Spectra of Clothing MaterialsTerahertz radiation can easily penetrate through many clothing materials, such as typical household andmilitary fabrics, and is nonionizing due to its relatively low frequencies. Terahertz active imaging is safefor humans compared to the ultraviolet radiation and X-ray and is suitable for security imaging,inspection, and nondestructive identification of contraband concealed beneath clothing, such as ceramicknife, plastic explosive, and illicit drug.

Several common clothing materials were measured by I. Dunayevskiy et al. [21]. Three independentsystems, which are backward-wave oscillator-based continuous-wave terahertz system, Fourier transforminfrared spectroscopy, and far-infrared gas laser, were used to measure the terahertz transmission spectraof 14 fabrics. The samples measured were the materials of heavy cotton shirt, denim shorts, knit sweater,wool jacket, polyester blouse, polyester-blend pants, rayon blouse, nylon jacket, suede shorts, silk blouse,Cordura (baggage material), canvas, Kevlar, and velvet, as listed in Table 2 [21].

All of the materials are regular clothing materials except ballistic Kevlar. The measured terahertztransmission spectra of 14 fabrics are shown in the first figure and the second figure in Ref. [21].

According to these two figures, the majority of the materials exhibit a single-layer transmission around90 % in the frequency range from 130 to 170 GHz measured with backward-wave oscillator-basedterahertz system. Nevertheless, the single-layer transmission measurements of Cordura and ballisticKevlar are lower than 80 % in this frequency range. Most of the materials except Kevlar, Cordura, andknit materials have a transmission of higher than 50 % in the frequency range from 300 to 500 GHzmeasured with Fourier transform infrared spectroscopy. The transmission measurement results of silk,polyester, and nylon are higher than 50 % in the frequency range from 300 to 1,200 GHz. Nylon andpolyester are the better transmission fabrics among these materials. The far-infrared gas laser used in themeasurement irradiates terahertz waves at five frequencies of 566, 694, 715, 763, and 965 GHz, as shownin these two figures in Ref. [21].

The reflection spectra of knit, wool, rayon, nylon, Cordura, and velvet have beenmeasured with Fouriertransform infrared spectroscopy, as shown in the third figure in Ref. [21].

The reflectivities of the materials measured are low in the frequency region of 0.3–1.2 THz. Even ifnylon and Cordura have higher reflectivity, the reflectivity around 10 % was measured for nylon andCordura due to the very low level of the reflected signal.

J. E. Bjarnason et al. [22] have studied the terahertz wave transmission through cloth samples fromeight types of fabrics used for garment and baggage, as listed in Table 3.

The transmission spectra through eight samples and the microphotographs of six samples are shown inthe second figure and the third figure in Ref. [22], respectively. At the low-frequency end, all samplesapproach near-unity transmission in normalized terahertz transmission spectra, as shown on the left side


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of the second figure in Ref. [22]. As the frequency increases, each transmission decreases and is highlydependent on sample. The higher rate of increasing opacity occurs in the leather and wool samples, andthe lower rate occurs in the nylon, rayon, and silk. There is a 9 dB discrepancy between wool and rayon atthe frequency of 1 THz due to the different features of the samples in thickness and frequency-dependentabsorption. The transmission spectra from 38 to 100 THz of the samples have been also measured withFTIR spectroscopy by J. E. Bjarnason et al. [22], as shown on the right side of the second figure in Ref.[22].

Table 2 Sample properties of 14 fabrics [21]

Sample description Material composition Thickness (mm)

Heavy cotton shirt 100 % cotton 1.50

Denim shorts 100 % cotton 0.90

Knit sweater 75 % cotton, 25 % acrylic 2.40

Wool jacket 100 % wool 0.65

Polyester blouse 100 % polyester 0.20

Polyester-blend pants 70 % polyester, 25 % rayon, 5 % spandex 1.00

Rayon blouse 100 % rayon 0.35

Nylon jacket 100 % nylon 0.12

Suede shorts 100 % pig suede 0.90

Silk blouse 100 % silk 0.30

Cordura (baggage material) Nylon with rubberized backing 0.81

Canvas 100 % cotton 0.65

Kevlar 100 % ballistic Kevlar 0.65

Velvet Velvet 1.20

Table 3 Sample properties of eight fabrics [22]

Fabric Morphology Origin Chemical basisThickness(mm)

Density(kg/m3)

�3 dBfrequency(THz)

Attenuation at1 THz (dB)

Wool Nap Animal Protein (a-keratin) 2.2 214 0.35 11.0

Linen Woven Plant (flax) Structuralpolysaccharide(cellulose)

1.1 509 0.35 8.0

Leather Compoundpolymer

Animal(mammal)

Fibrous protein(keratin)

0.75 813 0.40 10.0

Denim Woven Plant(cotton)

Structuralpolysaccharide(cellulose)

0.96 490 0.50 6.5

Naugahyde Homogeneouspolymer

Synthetic Polyvinyl chloride 0.65 800 0.70 5.5

Silk Woven Animal(insect)

Polypeptide (fibroin) 0.36 256 1.0 3.0

Nylon Knit Synthetic Polyamide 0.19 379 1.0 3.0

Rayon Woven Synthetic Structuralpolysaccharide(cellulose)

0.15 733 >1.0 2.5


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Transmission spectra and reflection spectra of fabrics are different for the same materials with differentspinning and weaving structures due to wave scattering and diffraction, such as shown in the first figureand the second figure in Ref. [16], respectively. É. Hérault et al. [23] have measured the influences ofterahertz wave diffraction by the yarn network in common cloths with terahertz time-domain spectros-copy. The structure of most fabrics is an almost periodic network of interlaced yarns or threads with theperiodicity around the terahertz wavelength, where the scattering and diffraction can be induced. Thelinen threads have a mean diameter of 375 mm (�30 %) that corresponds to the frequency of 0.8 THz, andthe stitches are almost rectangular with an average size around 785 mm� 585 mm. The second figure andthe third figure in Ref. [23] show the comparison between experimental (dots or triangles) and simulation(line) diffraction of linen threads along two orientations.

The linen fabric is described as a perfect two-dimensional periodic structure with the cell size of 800 mm� 645 mm and the thread diameter of 375 mm in simulation.

The second figure in Ref. [23] shows experimental (dots or triangles) and simulation (line) first-orderdiffraction angle of linen threads irradiated by terahertz wave with the polarization field oriented along thex-axis and y-axis, respectively. The measured result coincides with the simulated result. The normalizedterahertz wave diffraction fields along two orientations obtained from measurement and simulation atthree frequencies of 0.6, 0.75, and 1 THz in different detection angles are shown in the third figure in Ref.[23], respectively. The measured and simulated relations between the normalize terahertz wave diffractionfield and the detection angle are shown in this figure, which is shifted vertically by one at each frequencyin each orientation for the sake of legibility. É. Hérault et al. [23] have also observed the terahertzdiffraction results of other fabrics, such as fleece, denim, and T-shirts, with different thread size andweaving tightness. The strength distribution of diffraction differs for each fabric and is concentrated atsome specific angles. The additional diffracted signal could drastically disturb the signal detected or theimage captured. Away to overcome the diffraction disturbance would be to perform experimental systemwith the terahertz wave having short coherent length or incoherent terahertz wave.

Besides diffraction effect in measuring spectra of clothing materials, isotropic scattering problemshould be considered. The interaction of terahertz radiation with irregularly structured materials is anessential part of the design of terahertz sensing and imaging systems. Scattering within materials andstructures, such as fibers in clothing and granules in powder, may produce false signatures in spectra orimages. J. R. Fletcher et al. [24] have studied propagation of terahertz radiation through random structuresand proposed a model describing the terahertz wave propagation through inhomogeneous materials. Theoptical properties of a randomly structured layer are analyzed with the help of a phase distributionfunction. The measured and simulated transmission losses of Harris tweed, fleece, and shirt are shown inthe tenth figure in Ref. [24], respectively.

According to this figure, the agreement between the experimental data and the simulation predictions issatisfactory. This analytical approach can deliver predictions instantaneously and could be of great valuefor real surveillance systems operating at terahertz frequencies.

Terahertz Spectroscopy for Textile SensingMetamaterials are periodic artificial fabricated structures whose element is composed of a metallicresonator held together in a dielectric. Metamaterials have the ability to control the propagation ofelectromagnetic waves; in other words, they can tailor electromagnetic responses. Textiles are flexible,easily manufactured, and with similar structures to metamaterials. Moreover, the fabric periodicities are inthe order of the terahertz wavelength. Textile metamaterials with metallic fibers allow strong coupling toterahertz radiation and tailor the wave responses which can be used to realize many applications, such asperfect absorbers and strain sensors. M. Ghebrebrhan et al. [25] have simulated and measured a textile


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metamaterial created by adding metal fibers directly into the polymer yarns. The microscope images andcomputational cells of woven and knitted fabrics are shown in the first figure in Ref. [25].

The yarn for producing the samples of woven fabrics and knitted fabrics consists of an iCon fiber withthe resistance of 130O/m twisted with a 72-filament polypropylene fiber at a rate of 0.5 twists per inch, asshown in this figure. Since the spacing of the iCon fibers is different in the warp direction corresponding tovertically oriented and weft direction corresponding to horizontally oriented, the spectra should bepolarization dependent. The periodicities of the woven fabric sample in the warp and the weft directionsare approximately 1.894 and 0.446 mm, respectively, as shown in this figure. As indicated in wool textileidentification by J. Molloy et al. [16, 20] that the spectra of woolen textile samples have differentfrequencies of loss peaks for different polarizations due to the fabric orientations, fabrics with conductingfibers may increase this polarization dependence. Furthermore, the iCon conducting fibers within thefabric samples establish the periodic metallic resonators in the fabric metamaterials with the element of thesplit ring resonator. Strong response of resonant peaks can be induced by the textile metamaterials. Theresonant frequencies can be tuned by varying the geometric parameters of the fabric samples. Theinductance L of the resonator element increases with the crimp height due to the increasing area of theloop, and the capacitance C of the resonator element increases with decreasing the period of the weftfibers. The resonant frequency of (LC)�1/2 can be directly controlled by the weft period. The longer theweft period is, the higher the resonant frequency becomes [25]. The tuning properties of the resonantfrequencies of the textile metamaterial samples can be used to measure the strain in real time.

The response of fabrics can be attributed to resonances created by the iCon fibers. The periods of thesefabric samples are roughly 0.5–1.5 mmwith the polypropylene fiber diameter around 0.4 mm. The propertiesof the textile metamaterial samples have been measured by M. Ghebrebrhan et al., as shown in the fourthfigure in Ref. [25].

As shown in this figure, the transmission spectra of a linearly polarized beam are measured for differentpolarization angles from 0� to 90� with the increment of 15� with respect to direction B in the first figure inRef. [25]. Strong oscillations can be observed from the transmission spectrum of the woven fabric sampleirradiated by the terahertz waves polarized along the warp fibers, while much weaker oscillations for theorthogonal polarization, as shown in the left part of the fourth figure in Ref. [25]. The transmission spectraof the knitted fabric sample have beenmeasured, as shown in the right part of the fourth figure in Ref. [25].The oscillation properties of the woven fabric metamaterial sample depend more heavily on the polari-zation of the incident terahertz waves than those of the knitted fabric metamaterial sample.

H. Tao et al. [26] have fabricated and characterized metamaterial structures sprayed directly on thepremade silk films with micro-fabricated stencils by a shadowmask evaporation technique. The responsesof the metamaterial silk composites were measured with terahertz time-domain spectroscopy. Therefractive index of the silk films was measured to be n = 1.91 + i0.12 with the spectroscopy. Threesamples with different metamaterial element structures of the split ring resonators were measured atnormal incidence of terahertz beam. The size of metamaterial elements of sample 1 is 50 mm� 50 mm, andsample 2 has the same size of metamaterial element as sample 3 which is 100 mm � 100 mm. Thetransmission spectra of 80-mm-thick pure silk film and these three samples are shown in the second figurein Ref. [26].

The black solid lines and the red dash lines show the experimentally measured and simulatedtransmission spectra, respectively. The resonant frequencies of the samples are 0.85, 0.7, and 0.4 THz,respectively.

According to the features of the resonant frequencies, such as the transmission spectra in the fourth figurein Ref. [25] and the second figure in Ref. [26], the textile metamaterials can be used to sense the properties ofmaterials and structures. J. Li et al. have demonstrated a fourfold symmetric flexible metamaterial which canbe used for dual-axis strain sensing, as shown in the second figure and the third figure in Ref. [27].


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The metamaterial elements composed of 200-nm-thick gold layer with the structure shown in thesefigures are encapsulated between the 100 mm- and 10-mm-thick polymer substrate and superstrate. Theresonant features of this flexible metamaterial are sensitive to strain along the polarization orientation ofthe incident terahertz waves and insensitive to strain perpendicular to the polarization orientation, asshown in the second figure in Ref. [27]. It can be used to measure large dual-axis strain of flexiblematerials and structures, such as textiles, as shown in the second figure and the third figure in Ref. [27].

S. Wietzke et al. have determined the dielectric spectra and the glass transition features of polymerswith the terahertz time-domain spectroscopy, as shown in the fifth figure and the seventh figure in Ref.[28].

At room temperature, refractive index and extinction coefficient spectra of the nonpolar polymers ofhigh-density polyethylene (HDPE) with the thickness at room temperature of 1,952 mm, cyclic olefincopolymer (COC) with 2,003 mm, linear low-density polyethylene (LLDPE) with 1,930 mm, low-densitypolyethylene (LDPE) with 3,981 mm, polypropylene (PP) with 2,062 mm, polymethylpentene (PMP) with953 mm, and polytetrafluorethylene (PTFE) with 518 mm have been measured with the terahertz time-domain spectroscopy, as shown in the upper left part and the lower left part of the fifth figure in Ref. [28],respectively. Similar spectra of the polar polymers of polyamide 6 (PA6) with 501 mm, polyoxymethylene(POM) with 965 mm, polyvinyl chloride (PVC) with 1,467 mm, polycarbonate (PC) with 517 mm,polymethyl methacrylate (PMMA) with 573 mm, and polyvinylidene fluoride (PVDF) with 511 mmhave been also measured with the terahertz time-domain spectroscopy, as shown in the upper right partand the lower right part of the fifth figure in Ref. [28], respectively.

The glass transition temperatures Tg of the polymers of polypentylene heptanoate (PPH) with 462 mm,PA6, POM, PC, PVDF, PMMA, HDPE, and LLDPE have measured to be 220, 286, 203, 422, 222, 367,231, and 218 K with the terahertz time-domain spectroscopy, respectively. The illustration for the conceptof the free volume of the glass transition and the temperature-dependent refractive indexes of thepolymers of PA6, POM, PPH, PC, PVDF, PMMA, HDPE, and LLDPE are shown in the seventh figurein Ref. [28].

H. Suzuki, et al. [29] have measured the terahertz spectra of nylon-6 in various solid states, and detecteda phase transition (known as the “Brill transition”) and a glass transition, in addition to a new anomaly inthe amorphous phase. The measured absorption spectra of three kinds of nylon-6 samples are shown in thefirst figure in Ref. [29].

There are two broad peaks at 3 and 9 THz for spectrum of sample a in amorphous phase. Two of thebroad peaks at 3 and 9 THz for spectrum of sample b containing the g form are sharper than those ofsample a. There are other three sharp peaks at 13.5, 15.5, and 17.5 THz corresponding to the formation ofa crystalline phase in the spectrum of sample b. The peaks for the spectrum of sample c containing thea-form are sharper than those of sample a and sample b. Furthermore, there are two additional peaks at 2.0and 6.5 THz in the spectrum of sample c.

The Brill transition of the a-form with TB � 160 �C was investigated by monitoring the temperaturevariation heating from 26 �C to 202 �C of the terahertz spectra for sample c, as shown in the second figurein Ref. [29].

According to this figure, there are remarkable changes of both the absorbance peak and its second-derivative intensity for sample c at 6.6 THz, in which the absorbance decreases and its second-derivativeintensity increases with the temperature increasing, respectively. According to the third figure in Ref. [29],there is an inflection point at the temperature of 160 �C for sample c at the frequency of 6.6 THz. Thisinflection point is caused by the Brill transition of the a-form. This temperature is called Brill transitiontemperature. For sample a, three inflection points can be observed at the temperatures of 60 �C, 110 �C,and 190 �C at the frequency of 6.6 THz. The inflection points at 60 �C and 190 �C correspond to the glasstransition (Tg � 54 �C) and crystallization to the pseudohexagonal form, respectively.


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Terahertz Imaging Technique

Due to the abilities to penetrate many materials and to characterize intramolecular and intermolecularvibrational modes, simultaneously acquiring insight image with terahertz imaging system and identifyingmaterials with terahertz spectroscopy give terahertz technology a big advantage over other technologies.There are many applications of the terahertz imaging technique, such as biological and biomedicalimaging in life science and medical diagnosis, nondestructive and contact-free testing in industry,safety-critical quality inspection of food products, and security scanning of concealed dangerous sub-stances in public life.

Terahertz ImagingThe terahertz imaging techniques were a natural development of terahertz spectroscopy techniques. Manyterahertz imaging systems need external terahertz source, which are active imaging systems. Conven-tional terahertz spectroscopies can generate highly precise spectroscopic information at a single point of asample. In a terahertz imaging system based on a terahertz time-domain spectroscopy, as shown in Fig. 2,the object is placed in the focal plane of the terahertz beam and is moved step by step in the x-y plane forimage acquisition. The terahertz intensity can be recorded at each point by the scanning. Both spatialdistribution of the object and the material spectra can be obtained with this terahertz imaging system.Nevertheless, the two-dimensional terahertz image is obtained pixel by pixel with long duration of timefor completion. Backward-wave oscillator-based terahertz continuous-wave imaging system can bedemonstrated by the terahertz continuous-wave spectroscopy with the similar scanningmethod. However,the material spectra cannot be obtained with this terahertz imaging system.

In order to reduce the time of image acquisition, two-dimensional terahertz field distribution isconverted into a two-dimensional optical intensity by an electro-optic crystal, and a CCD camera isused to record two-dimensional optical intensity. M. Yamashita et al. have established a terahertzspectroscopic imaging system based on the two-dimensional electro-optic sampling technique, asshown in the first figure in Ref. [30].

This terahertz spectroscopic imaging system has the ability to shorten the data acquisition timedramatically in comparison with the scanning type of the imaging system based on the terahertz time-domain spectroscopy. There are some differences between the terahertz spectroscopic imaging system inthis figure and the terahertz time-domain spectroscopy in Fig. 2. The laser pump beam is collimated andbroadened to the diameter around the electrode gap of the photoconductive antenna by a pair of opticallenses. A high-voltage-biased large-aperture photoconductive antenna with the electrode gap of 25 mm isused as the terahertz emitter. The terahertz pulse beam irradiates the sample area to be imaged and isfocused on an electro-optic crystal by a pair of polyethylene lenses. The diameter of terahertz pulse beamon the electro-optic crystal approximates the diameter of the polarized laser probe beam which iscollimated and broadened by another pair of optical lenses. The polarized laser probe beamco-propagates with the terahertz pulse beam with the sample spatial distribution information inside theelectro-optic crystal and is modulated by the refractive index change induced by the terahertz electric fieldwith spatial distribution of the sample. An analyzer that is perpendicular to the polarizer is placed behindthe electro-optic crystal to form the optical intensity with the same spatial distribution as the sample. Thisspatial distribution of the optical intensity is recorded by a CCD camera (384 � 288 pixels, 20 frames/s).Terahertz images in the frequency domain with the frequency resolution of 22.5 GHz are obtained at eachpixel in the terahertz time-domain images [30].

The spatial resolution of a conventional imaging system is diffraction limited to the scale of half awavelength. The resolution can be improved with near-field approach, in which the resolution is no longerdetermined by the wavelength but by the aperture size [31]. However, the terahertz transmission energy


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through the aperture of size d decreases by d3 due to much of the radiation reflected at the aperture plane.Significantly deteriorating the signal-to-noise ratio of the image for small apertures is the major limitationto aperture-based methods [32]. In order to break the diffraction limit and to enhance the spatial resolutionof terahertz imaging system, H. T. Chen et al. [33] have demonstrated the application of scanning near-field microscopy techniques for terahertz imaging with aperture-based technique, tip-based technique,and focused beammethod. A sharp metallic tip is placed above the structure, as shown in the first figure inRef. [33].

The incident terahertz beam is focused onto the surface of the sample to the diffraction limit around theterahertz wavelength l. The dipole moment is induced by the terahertz radiation in the tip and in the regionbelow the sample surface and leads to scattering and absorption of part of the terahertz radiation. Theterahertz transmission is detected. A spatial resolution of 150 nm has been achieved, which corresponds toabout l/1,000. The terahertz imaging and its spatial resolution can be achieved with the scanning tip intwo dimensions.

J. H. Son [34] has reviewed the principle and applications of terahertz molecular imaging. S. J. Ohet al. [35] have achieved dramatically enhanced sensitivity compared with that of conventional terahertzimaging. The sample was located at the focus of the terahertz pulse beam from a reflection-mode terahertztime-domain spectroscopic system and was irradiated by a continuous-wave infrared laser beam with thewavelength of 800 nm, as shown in the second figure and the fourth figure in Ref. [35].

The sample temperature rose due to the hyperthermia effect induced by the surface plasmon resonanceon the nanoparticle surface of gold nano-rods under infrared laser irradiation. The terahertz reflectionsignal increased with the sample temperature. Peak reflection changes of the terahertz signals from livecancer cells with and without nanoparticles of gold nano-rods under continuous-wave infrared laserirradiation are shown in the fourth figure in Ref. [35]. In the differential mode, the terahertz signal from thecancer cells with nanoparticles of gold nano-rods was 30 times higher than that from the cancer cellswithout nanoparticles of gold nano-rods [35].

Even though the terahertz radiation is nonionizing, it is important to consider the problems of theterahertz radiation on human body in ethical, legal, and other issues. With the help of the terahertzradiation from natural sources, such as humans, passive imaging systems can be developed. ThruVisioncompany has produced commercially available passive terahertz imaging systemwhich captures naturallyoccurring terahertz radiation and processes it to create images that can reveal objects beneath a person’sclothing or in a bag, such as T4000 and T5000, as shown in the third figure and the fourth figure inRef. [36].

The applications of passive terahertz imaging systems are security, anti-terrorism, and law enforce-ment. The passive terahertz imaging systems can detect concealed weapons and solid or liquid explosivesat airports, public transport facilities, military installations, and other high-security sites. A. Svetlitzaet al. [37] have presented a low-cost measurement setup for terahertz applications based on a blackbodysource.

Terahertz TomographyTerahertz tomography can be used to capture the internal inspection of any object transparent ortranslucent to terahertz waves. A series of reflected broadband terahertz pulses at refractive indexdiscontinuities inside the object can be collected with time delay. The internal discontinuities of theobject to a refractive index profile in the propagation direction could be mapped. A full three-dimensionalmap of the object can be obtained by raster scanning. If the received signals are line integrals along thedirect paths, the Fourier-slice theorem can be applied [32].


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Terahertz computed tomography (THz-CT) has the same principle as X-ray CT. A CT scan emits aplane wave and records a series of waves that are transmitted through or reflected from a target intwo-dimensional plane, which is repeated at different angles around the object.

M. Jewariya et al. have demonstrated fast three-dimensional transmission terahertz computed tomog-raphy using real-time line projection of intense terahertz beam, as shown in the first figure in Ref. [38].

As shown in this figure, the sample is irradiated by the line-focused terahertz pulses along the y-axisand is scanned along the x-axis as well as rotated to capture the three-dimensional image. The three-dimensional reconstruction can be performed with a standard reconstruction algorithm [38, 39].

If the object inhomogeneities are comparable in size to the propagation wavelength, the diffractionsalong the path must be considered in terahertz tomography. Terahertz diffraction tomography (THz-DT) isused in this situation. The terahertz diffraction tomography is used to determine the spatial distribution ofa sample’s refractive index by the measurement of the diffracted terahertz field. In diffraction tomography,a terahertz probe beam interacts with a sample to capture the three-dimensional image of the sample withthe waves scattered and diffracted inside the sample. This is the main difference with computedtomography which generally uses the amplitude signal transmitted through the sample. Terahertzdiffraction tomography is a useful tool for capturing the complex samples with fine structures in whichthe diffraction effects dominate the measurements. S. Wang et al. have established terahertz diffractiontomography by using a femtosecond laser, generating terahertz radiation by optical rectification in a ZnTecrystal and detecting the signal with a CCD camera, as shown in the 11th figure in Ref. [40].

There are some differences between the terahertz diffraction tomography system in this figure and theterahertz time-domain spectroscopy in Fig. 2. The laser pump beam irradiated on the emitter of a ZnTecrystal is broadened by an optical lens. The terahertz pulse beam from the emitter is collimated andbroadened by a 90� off-axis parabolic mirrors and irradiates on the sample area to be image. A tin-dopedindium dioxide (ITO) slab is used to transmit the laser probe beam with low transmission loss and toreflect the terahertz beam with high reflectivity. The diameter of terahertz pulse beam on the electro-opticcrystal of another ZnTe approximates the diameter of the polarized laser probe beam which is collimatedand broadened by another optical lens. The polarized laser probe beam co-propagates with the scatteredterahertz pulse beam with the scattered sample spatial distribution information inside the electro-opticcrystal is modulated by the refractive index change induced by the terahertz electric field spatialdistribution. An analyzer that is perpendicular to the polarizer is placed behind the electro-optic crystalto form the optical intensity with the same spatial distribution as the sample. The third optical lens is usedto focus the laser probe beam whose optical intensity has the same spatial distribution as the sample ontothe CCD camera. The object can be rotated along the y-axis and moved along the x- or z-axes to scanthree-dimensional terahertz image with the internal structure [40]. The application of terahertz diffractiontomography is able to provide the refractive index distribution inside the object. Terahertz diffractiontomography often provides poor reconstructed images due to the problem of reconstruction algorithmsand signal interpretation. Nevertheless, the image acquisition speed of terahertz diffraction tomography isrelatively faster than terahertz computed tomography.

Terahertz Imaging of Textiles

Textile ImagingA. Redo-Sanchez et al. have reviewed terahertz technology readiness assessment and applications, suchas to inspect defects in the thread of a textiles sample that cannot be seen optically, as shown as in the fifthfigure in Ref. [41].


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It is obvious from this figure that the invisible defects in the thread of textiles sample can be inspectedwith terahertz imaging technique.

Terahertz technology, including the terahertz imaging and terahertz spectroscopy, could become acomplement to X-ray technique for the noninvasive investigation of relics, such as ancient mummies.X-ray technique is a useful tool to obtain the image with the internal features of the mummified body. Theterahertz imaging and terahertz spectroscopy are noncontact, noninvasive, and nondestructive manners,which can be used to probe the layers of bandages constituting the mummy wrappings and to obtain theimage and the material composition. Furthermore, terahertz radiation is safe for the operators and theobjects measured. Due to the terahertz wave reflection by any discontinuity inside the materials, terahertzimaging can indicate the sequence of fabric layers and the eventual objects placed in between thebandages. Terahertz imaging can be used to probe the fabric layers surrounding Egyptian mummies.Terahertz waves can penetrate into nonmetallic materials and its reflection depends on the refractive indexof materials at the interface, such as the interface textiles and the air and the interface between textiles.

K. Fukunaga et al. have measured the Kharushere mummy which lived during Egypt’s 22nd dynasty(circa 945–712 BC) with terahertz time-domain reflection imaging, as shown as in the first figure in Ref.[42]. The upper left part of the second figure in Ref. [42] shows the terahertz image of the bandage layersand the mummy shroud penetrated of area 1 in the first figure in Ref. [42]. It is evident that the informationof the bandage layers under the mummy shroud, such as the sequence and the number, can be obtainedfrom the terahertz cross-sectional image scanned along the line of a-a’, as shown in the lower left part ofthe second figure in Ref. [42].

The terahertz time-domain waveform along the middle point A of the line a-a’ is shown in the right partof the second figure in Ref. [42]. According to this waveform, the first two peaks due to the reflectionsfrom the diagonal strap circling the mummy from the left shoulder to the right hip indicate that thediagonal strap consists of two fabric layers, and the two packets of four reflection peaks of the waveformare reflected from eight layers of the shroud. These two packets with four reflection peaks each are similarand indicate that the first four bandage layers are followed by another set of four layers with a gap inbetween. It is difficult to obtain the information of these eight bandage layers with the CT-scanning image,as shown in the right part of the first figure in Ref. [42].

According to the terahertz image in the c-c line, more than 10 layers of fabrics are observed in area 2 inthe first figure in Ref. [42], as shown in the third figure in Ref. [42].

The terahertz images along the b-b’ line and c-c’ line of area 2 in the first figure in Ref. [42] indicate theintense reflections coming from the layers underneath the surface, such as reflected under the surfacearound 20 mm, as shown in the third figure in Ref. [42].

Textile Water ContentThere several techniques of directly measuring the percentage of water incorporated in materials andstructures, such as the Karl Fischer titration, chemometric method, and thermogravimetric analysis. Butall of the above methods do not provide any spatially resolved information [43]. Furthermore, the samplesfor these destructive and time-consuming measurements should be specially prepared. The terahertzimaging and spectroscopy techniques are ideal tools for a nondestructive, contactless determination of thewater content due to the sensitivity of terahertz radiation to water and the ability to penetrate many objects.C. Joerdens et al. [43] have used terahertz time-domain spectroscopy to study the sorption of water intopolyamide and wood plastic composite. A model for the dielectric properties depending on the watercontent was developed and experimentally verified [43]. The dielectric properties of polyamide 6 andwood plastic composite samples are measured with a terahertz time-domain spectroscopy, as shown in thefifth figure and the sixth figure in Ref. [43], respectively.


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The absorption coefficient of each sample linearly increases with water content, as shown in the rightparts both the figures. The refractive index increases with water content with a slightly nonlinear behavior,as shown in the left parts both the figures. The measured values coincide with the simulation results.C. Joerdens et al. [43] have also measured the sample image of wood plastic composite based on theterahertz time-domain spectroscopy. One-half of a piece of wood plastic composite filled with 60 wt% ofwood fibers was immersed in distilled water for 4 days, as shown in the left part of the seventh figure inRef. [43]. The water incorporated in wood plastic composite can be obviously seen in the terahertz imageof the sample, as shown in the right part of the seventh figure in Ref. [43].

H. B. Zhang et al. [44] have proposed a quantitative method to obtain water content of thin materialswith terahertz imaging. The fourth figure in Ref. [44] shows the visible image of a dry sample of cottoncloth [44]. The terahertz images of this piece of cotton cloth with different water contents and thecorresponding water content images have been obtained and processed, as shown in the sixth figureand the tenth figure in Ref. [44], respectively. Both the water content and its spatial distribution can bemeasured with the terahertz time-domain spectroscopy.

Terahertz Imaging Behind Textile BarriersD. Cleary [45] has reported brainstorming the way to an imaging revolution and a news focus on terahertzwave with the ability to see through clothes and paper in Science magazine, as shown in the news focusfigure in Ref. [45].

Similar image of a metallic object hidden behind a paper, as shown in Fig. 5, can be captured by asimple terahertz continuous-wave imaging system based on backward-wave oscillator shown in Fig. 6.

K. B. Cooper et al. [46] have used NASA Jet Propulsion Laboratory’s (JPL’s) 675 GHz imaging radarwith the ability to penetrate cloths to obtain the terahertz images of person-borne concealed objects. Thisterahertz imaging radar is an effective tool for rapidly searching targets from a long standoff range withhigh three-dimensional spatial resolution. Several images obtained by this terahertz imaging radar areshown in the eighth figure and the ninth figure in Ref. [46].

The terahertz radar images in the eighth figure in Ref. [46] were obtained with the frame rate of5 s/frame. The upper left part of this figure shows the image of a back surface of a person who wore ajacket and stood in 25-m distance from the radar. The curves in the upper right part of this figure show theterahertz radar spectra around two selected points on the image in the upper left part of this figure,respectively. The images in the lower left part and lower right part of this figure show terahertz radarimagery revealing the hidden metallic handgun and mock bomb belt containing ceramic shrapnel andexplosive stimulant. The ninth figure in Ref. [46] shows six images with 66 � 58 (azimuth � eleva-tion)= 3,828 pixels of a 1 Hz frame rate video captured with the terahertz imaging radar. The person worea cotton Polartec jacket and concealed three 1-in PVC pipes. There is no evidence of three 1-in PVC pipesconcealed by a cotton Polartec jacket if the target is not in the terahertz field of view, as shown in the

Fig. 5 Terahertz image of object hidden behind paper: (a) visible image of object and (b) terahertz image of hidden object


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upper left part of this figure. The pipes can be seen when the subject moves into the radar field of view, asshown as in the upper right part of this figure. The image in the left-center part of this figure is the terahertzradar image for the back of the person without hidden objects detected. The person turned to face theterahertz radar again and moved his right arm to unzip his jacket. The right hand and the edge of the PVCpipes can be seen from the terahertz image, as shown in the right-center part of this figure. Both theterahertz image and optical image of PVC pipes without jacket for concealing can be seen, as shown in thelower left part and the lower right part of this figure, respectively.

J. C. Chen et al. have constructed a terahertz sensing system based on an interferometer, as shown in thefirst figure in Ref. [47].

Terahertz wave at the frequency of 0.6 THzwas used to penetrate a barrier and to measure the vibrationsbehind the barrier. Awhite cotton T-shirt, a black wool vest, and a blue plastic recycling bin were used asbarriers. A speaker behind a barrier was driven with only a single sine wave at the frequency of 100 Hz.The terahertz systemwas able to record the signal of 100 Hz tone with good fidelity, as shown in the fourthfigure in Ref. [47].

Target Detection Technique of Terahertz Textile ImagingThere are terahertz image speckles due to the interference between different randomly wavelets scatteredfrom the surface and the interface of the clothing layer. The fifth figure in Ref. [48] shows the speckles ofseveral terahertz images through a layer of clothing using a broadband time-domain spectroscopy systemwith the frequency range from 200 GHz to 1 THz and a narrow terahertz wave with the frequencyspectrum of 430 � 3 GHz, respectively.

The speckles can be suppressed in the terahertz broadband image based on terahertz time-domainspectroscopy. Since the coherence length of the terahertz wave generated by terahertz time-domainspectroscopy is much shorter than that generated by backward-wave oscillator, the wavelets which travelpaths of different length cannot interfere with one another.

In order to distinguish the target from the background, it is necessary to reduce the disturbance causedby irregular interference fringes. The disturbance of fringes can be reduced by measuring techniques, suchas modulating the bias voltage of the supply for backward-wave oscillator. The frequency versus biasvoltage of the backward-wave oscillator is shown in Fig. 7. The bias voltage of the backward-waveoscillator in Fig. 6 was modulated by the triangular voltage wave with the amplitude of several tenvoltages at the frequency of 2.5 kHz. There is a fluctuation within 5 GHz due to the modulation at theterahertz frequency of 338 GHz from the backward-wave oscillator. This frequency fluctuation of the

Fig. 6 Imaging system based on backward-wave oscillator


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backward-wave oscillator can reduce the fringes in terahertz images, as shown in Fig. 8. However, thisfrequency of the backward-wave oscillator causes a fluctuation of terahertz irradiation power.

According to Fig. 8b, there are interference fringes due to the coherence of the terahertz wave generatedby the backward-wave oscillator. The fringes are obviously decreased through the frequency fluctuationof the terahertz wave induced by the modulation of the bias voltage for the backward-wave oscillator.

With the help of segmentation algorithms, the disturbance of fringes can also be drastically reduced.Fuzzy C-means is an efficient algorithm for data clustering. The clustering algorithm based on the fuzzylocal information C-means (FLICM), with improved the membership function therein according to theproperties of terahertz images, was used for the target detection from terahertz images, as shown in Fig. 9.

Fig. 7 Frequency versus bias voltage of the backward-wave oscillator in Fig. 6

Fig. 8 Visible and terahertz images of word “THz”written with conducting glue with different modulations: (a) visible image,(b) without modulation, (c) modulation with 10 V, and (d) modulation with 20 V


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Target Detection Behind Textile BarriersA simple experiment with a backward-wave oscillator-based terahertz continuous-wave imaging systemis helpful in order to obtain a brief view on capturing image of an object hidden behind textile barriers anddetecting the target from the image, as shown in Fig. 10a. Each sample was prepared in sandwichstructure, which a surgical blade fixed on a paper is covered by a fabric, as shown in Fig. 10b, c.

The first sample is covered by 100 % cotton in the experiment, as shown in the left image in Fig. 11a.The terahertz image and the segmented image are shown in the center and right images in Fig. 11a,respectively. The fabrics of 100 % flax, blending fabric of 70 % cotton and 30 % flax, polyester, andpolyurethane, were used to prepare other four samples. The corresponding images are shown inFig. 11b–e, respectively.

Terahertz imaging technique makes it possible to acquire images of dangerous substances concealedunderneath clothing. X. L. Shen et al. have automatically detected and segmented concealed objects, asshown in the eighth figure and the 11th figure in Ref. [49].

Terahertz Nondestructive Testing for CompositesBecause both the terahertz spectroscopy technique and the terahertz imaging can detect the propertiesinside many objects in a noncontact and noninvasive manner, terahertz nondestructive testing based onterahertz spectroscopy technique and terahertz imaging technique is a useful technique for structures andmaterials, especially for composites. Besides, both the spectra and the images of objects can be obtainedwith one time-domain spectroscopy-based terahertz pulsed system.

Fig. 9 Segmentation of terahertz image of a blade hidden behind paper with improved FLICM algorithm: (a) visible image,(b) hidden terahertz image, and (c) after segmentation

Fig. 10 Backward-wave oscillator-based terahertz imaging system and sample with sandwich structure: (a) experimen-talsetup, (b) a surgical blade fixed on paper, and (c) covered by a fabric


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The heat from jet engines, which may cause damage to the aircraft structures, should be considered indesigning and maintaining aircraft. C. Stoik et al. [50] have used terahertz time-domain spectroscopy tomeasure the material properties of the aircraft composites in order to determine the changes of theproperties and to find various damages of the composites according to terahertz images and terahertzspectra. Five composite samples with different features were used by C. Stoik et al., as shown in the thirdfigure in Ref. [50].

Sample 1 is for thickness calibration in the measurement; sample 2 was burned at 440 �C for 4 min;sample 3 is another burn sample which was burned at 430 �C for 6 min in an area and at 425 �C for 20 minin another area; and samples 4 and 5 are mechanical stress sample and hidden defect sample, respectively.Terahertz images for one burned area on composite sample 2 and two burned areas on composite sample3 are shown in the sixth figure in Ref. [50].

Terahertz images for the interesting areas on composite sample 4 and 5 are shown in the eighth figureand the ninth figure in Ref. [50], respectively.

It is obvious that the image properties of the burned areas, the area with mechanical stress, and the areawith hidden defect are significantly different from other areas on the samples.

N. Palka et al. [51] have detected the internal structure of an ultrahigh-molecular-weight polyethylenecomposite material with a terahertz time-domain spectroscopy. Due to the very hard and resistant properties,this composite can used to manufacture personal armors. This multilayer composite structure was investi-gated and its properties were measured, as shown in the eighth figure and the ninth figure in Ref. [51].

A terahertz image of the structure indicates the regions with defects, as shown in the darker regions inthe left part of the eighth figure in Ref. [51]. The terahertz time-domain pulse signal along the z-axis atpoint A in the defect regions was measured, as shown in the right part of the eighth figure in Ref. [51]. The

Fig. 11 Optical image of samples, terahertz images, and terahertz images after segmentation of target hidden behind fivefabrics: (a) dark blue 100 % cotton with the thickness of 0.3 mm, (b) white 100 % flax with the thickness of 0.3 mm, (c) orangeblending fabric of 70 % cotton and 30 % flax with the thickness of 0.4mm, (d) yellow polyester with the thickness of 0.3mm,and (e) brown polyurethane with the thickness of 0.7 mm


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first pulse and the third pulse in the waveform at point A are the reflected signals from the front surface andthe back surface of the composite. The distance between the pulses corresponds to the compositethickness of 3.3 mm. The second pulse corresponds to the defect position along the z-axis.

The terahertz images of the horizontal and vertical B scans along the lines marked in left part of theeighth figure in Ref. [51] are shown in the images in the ninth figure in Ref. [51], respectively. The detailedthree-dimensional terahertz images of the 3.3-mm-thick composite sample with all 74 layers and somedefects inside the composite are clearly seen from the ninth figure in Ref. [51].

Summary

In this chapter, terahertz spectroscopy technique, terahertz imaging technique, and their applications totextiles were presented. First, a brief description of terahertz wave technology was given, including theproperties of terahertz wave spectrum, applications of terahertz wave techniques to cosmic astronomy andwireless communication with high data rate. Then two important terahertz spectroscopy techniques,terahertz time-domain spectroscopy-based terahertz pulsed system and backward-wave oscillator-basedcontinuous-wave terahertz system, were described. And then several applications of terahertz spectros-copy technique to textiles were reviewed, such as wool textile identification, textile fiber identification,and textile sensing. On the other hand, terahertz imaging technique was also described, including terahertzimaging based on two-dimensional electro-optic sampling technique, terahertz imaging based on near-field microscopy technique, terahertz computed tomography, and terahertz diffraction tomography. In thelast section, several applications of terahertz imaging technique to textiles were reviewed, such asinspection of textile material for defects in the threads, detection of textile water content, imaging ofobject hidden behind textile barriers, target detection of textile imaging, and nondestructive testing forcomposites. With the help of the abilities of terahertz wave techniques to penetrate many materials and tocharacterize intramolecular and intermolecular vibrational modes, simultaneously acquiring insightimage with terahertz imaging system and identifying materials with terahertz spectroscopy give terahertztechnology a big advantage over other technologies.

This work is supported by the National Natural Science Foundation of China under Grant No.60971059. The author gratefully acknowledges all the authors of the cited references in this chapter fortheir research achievements, figures and tables. Without their information, this chapter would not havebeen possible.

References

1. Grischkowsky DR, Mittleman D (2003) Introduction. In: Mittleman D (ed) Sensing with terahertzradiation. Springer, Berlin, pp 1–7

2. Rao L, Yang DX, Zhang L et al (2012) Design and experimental verification of terahertz widebandfilter based on double-layered metal hole arrays. Appl Optics 51:912–916

3. DragomanD DM (2004) Terahertz fields and applications. Prog Quantum Electron 28:1–664. Sizov F, Rogalski A (2010) THz detectors. Prog Quantum Electron 34:278–3475. Siegel PH (2002) Terahertz technology. IEEE Trans Microwave Theory Tech 50:910–9286. Kulesa C (2011) Terahertz spectroscopy for astronomy: from comets to cosmology. IEEE Trans

Terahertz SciTechnol 1:232–2407. Song H, Nagatsuma T (2011) Present and future of terahertz communications. IEEE Trans Terahertz

Sci Technol 1:256–263


of 26

8. Koenig S, Lopez-Diaz D, Antes J et al (2013) Wireless sub-THz communication system with highdata rate. Nat Photonics 7:977–981

9. Pawar AY, Sonawane DD, Erande KB et al (2013) Terahertz technology and its applications. DrugInvent Today 5:157–163

10. International Telecommunications Union (2013) Attenuation by atmospheric gases. Radiocommu-nication sector of ITU, Recommendation ITU-R P.676-10

11. Haddad J, Bousquet B, Canioni L et al (2013) Review in terahertz spectral analysis. Trends AnalChem 44:98–105

12. Nagashima T, Tani M, Hangyo M (2013) Polarization-sensitive THz-TDS and its application toanisotropy sensing. J Infrared Millimeter Terahertz Waves 34:740–775

13. Nagashima T, Hangyo M (2001) Measurement of complex optical constants of a highly doped Siwafer using terahertz ellipsometry. Appl Phys Lett 79:3917–3919

14. Amenabar I, Lopez F, Mendikute A (2013) In introductory review to THz non-destructive testing ofcomposite mater. J Infrared Millimeter Terahertz Waves 34:152–169

15. Yan C, Yang B, Yu Z (2013) Terahertz time domain spectroscopy for the identification of twocellulosic fibers with similar chemical composition. Anal Lett 46:946–958

16. Molloy J, Naftaly M (2014) Wool textile identification by terahertz spectroscopy. J Text Inst105:794–798

17. Xia S, Yang DX, Li T et al (2014) Role of surface plasmon resonant modes in anomalous terahertztransmission through double-layer metal loop arrays. Opt Lett 39:1270–1273

18. Gorshunov B, Volkov A, Spektor I et al (2005) Terahertz BWO-spectroscopy. Int J Infrared Milli-meter Waves 26:1217–1240

19. Rao L, Yang DX, Hong Z (2012) Guiding terahertz wave within a line defect of photonic crystal slab.Microw Opt Technol Lett 54:2856–2858

20. Naftaly M, Molloy JF, Lanskii GV et al (2013) Terahertz time-domain spectroscopy for textileidentification. Appl Optics 52:4433–4437

21. Dunayevskiy I, Bortnik B, Geary K et al (2007) Millimeter- and submillimeter-wave characterizationof various fabrics. Appl Optics 46:6161–6165

22. Bjarnason JE, Chan TLJ, Lee AWM et al (2004) Millimeter-wave, terahertz, and mid-infraredtransmission through common clothing. Appl Phys Lett 85:519–521

23. Hérault É, Hofman M, Garet F et al (2013) Observation of terahertz beam diffraction by fabrics. OptLett 38:2708–2710

24. Fletcher JR, Swift GP, Dai DC et al (2007) Propagation of terahertz radiation through randomstructures: an alternative theoretical approach and experimental validation. J Appl Phys 101:013102

25. Ghebrebrhan M, Aranda FJ, Ziegler DP et al (2014) Tunable millimeter and sub-millimeter spectralresponse of textile metamaterial via resonant states. Opt Express 22:2853–2859

26. Tao H, Amsden JJ, Strikwerda AC et al (2010) Metamaterial silk composites at terahertz frequencies.Adv Mater 22:3527–3531

27. Li J, Shah CM, Withayachumnankul W et al (2013) Flexible terahertz metamaterials for dual-axisstrain sensing. Opt Lett 38:2104–2106

28. Wietzke S, Jansen C, Reuter M et al (2011) Terahertz spectroscopy on polymers: a review ofmorphological studies. J Mol Struct 1006:41–51

29. Suzuki H, Ishii S, Sato H et al (2013) Brill transition of nylon-6 characterized by low-frequencyvibration through terahertz absorption spectroscopy. Chem Phys Lett 21:36–39

30. Yamashita M, Usami M, Fukushima K et al (2005) Component spatial pattern analysis of chemicalsby use of two-dimensional electro-optic terahertz imaging. Appl Optics 44:5198–5201


of 26

31. Knoll B, Keilmann F (1999) Near-field probing of vibrational absorption for chemical microscopy.Nature 399:134–137

32. Withayachumnankul W, Png GM, Yin X (2007) T-ray sensing and imaging. Proc IEEE95:1528–1558

33. Chen HT, Kersting R, Cho GC (2003) Terahertz imaging with nanometer resolution. Appl Phys Lett83:3009–3011

34. Son JH (2013) Principle and applications of terahertz molecular imaging. Nanotechnology24:214001

35. Oh SJ, Kang JY, Maeng I et al (2009) Nanoparticle-enabled terahertz imaging for cancer diagnosis.Opt Express 17:3469–3475

36. Bogue R (2009) Terahertz imaging: a report on progress. Sens Rev 29:6–1237. Svetlitza A, Slavenko AM, Blank T et al (2014) THz measurements and calibration based on a

blackbody source. IEEE Trans Terahertz Sci Technol 4:347–35938. Jewariya M, Abraham E, Kitaguchi T et al (2013) Fast three-dimensional terahertz computed

tomography using real-time line projection of intense terahertz pulse. Opt Express 21:2423–243339. Guillet JP, Recur B, Frederique L et al (2014) Review of terahertz tomography techniques. J Infrared

Millim Terahertz Waves 35:382–41140. Wang S, Zhang XC (2004) Pulsed terahertz tomography. J Phys D Appl Phys 37:R1–R3641. Redo-Sanchez A, Laman N, Schulkin B et al (2013) Review of terahertz technology readiness

assessment and applications. J Infrared Millim Terahertz Waves 34:500–51842. Fukunaga K, Cortes E, Cosentin A et al (2011) Investigating the use of terahertz pulsed time domain

reflection imaging for the study of fabric layers of an Egyptian mummy. J Eur Opt Soc-Rapid Publ6:11040

43. Joerdens C,Wietzke S, Scheller M et al (2010) Investigation of the water absorption in polyamide andwood plastic composite by terahertz time-domain spectroscopy. Polym Test 29:209–215

44. Zhang HB, Mitobe K, Yoshimura N (2008) Application of terahertz imaging to water contentmeasurement. Jpn J Appl Phys 47:8065–8070

45. Cleary D (2002) Sensing: brainstorming their way to an imaging revolution. Science 297:761–76346. Cooper KB, Dengler RJ, Llombart N et al (2011) THz imaging radar for standoff personnel screening.

IEEE Trans Terahertz Sci Technol 1:169–18247. Chen JC, Kaushik S (2007) Terahertz interferometer that senses vibrations behind barriers. IEEE

Photonics Technol Lett 19:486–48848. Chan WL, Deibel J, Mittleman DM (2007) Imaging with terahertz radiation. Rep Prog Phys

70:1325–137949. Shen XL, Dietlein CR, Grossman E et al (2008) Detection and segmentation of concealed objects in

terahertz images. IEEE Trans Image Process 17:2465–247550. Stoik C, Bohn M, Blackshire J (2010) Nondestructive evaluation of aircraft composites using

reflective terahertz time domain spectroscopy. NDT&E Int 43:106–11551. Palka N, Miedzinska D (2014) Detailed non-destructive evaluation of UHMWPE composites in the

terahertz range. Opt Quant Electron 46:515–525


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applications of terahertz wave technology in smart textiles...lead to unlock many new and...

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