thz technology in security checks

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Chapter 9

THz Technology in Security Checks

The development of techniques for inspection of explosives and other hazardous

materials has become more and more attractive as concerns about public securityhave increased considerably in the past years. Among all explosive devices, land-mine is the most demanding target to be detected. Landmines were widely used inall kinds of battlefields and they are very difficult to eliminate once a conflict ends.The remaining landmines represent an enormous danger for the people, both mili-tary and civilian, that occupy the terrain affected by the presence of landmines. Asof today, more than 100 million mines remain active and undetected in many fieldsaround the world. Those mines claim more than 30,000 lives or injures each year.Although antilandmine technologies already exist and are being used in minefields,

most of these technologies tend to give high false-positive results due to the presenceof other objects present in the area.There are other types of improvised explosive devices or IED that have been

recently used in terrorism attacks. IEDs are more difficult to detect because thereis no standard way to fabricate them. Up to now, no standard method to detect IEDhas been developed. Despite the variety of formats of explosive devices, they haveone thing in common, which is that they contain explosives. Explosive compoundsare usually organic compounds that have nitryl bonds. Most explosives have verylow saturation vapor pressure, thus they are not easy for evaporating. On the one

hand, it is difficult to detect them by they vapor in the ambient air. On the otherhand, once an explosive contaminates a target, the residue will stay for a long time,which allows detecting the explosives by tracing the residue. Therefore, detection ofexplosive residue using their spectral features is very crucial in finding explosivesand is of great help to enforce public security.

Spectra of Explosives in THz Band

Being organic compounds, most of explosive molecules have their unique spectralfeatures in the THz band due to their rotation and collective vibration transitions.Those spectral features allow using THz wave spectroscopy to fingerprint explo-sives. The broadband THz radiation can be treated as sum of monochromatic waves,

201X.-C. Zhang, J. Xu, Introduction to THz Wave Photonics,DOI 10.1007/978-1-4419-0978-7_9, C Springer Science+Business Media, LLC 2010


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202 9 THz Technology in Security Checks

each monochromatic wave described as:

E0() = A()ei(tkz+0()), (1)

where k= /cis the wave vector, 0 is the original phase, tand zare temporaland spatial position of the wave, respectively. The electric field of the transmittedwave is:

ES() = t1 t2A()ei(tkz+0())eik(n1)d

= t1 t2 E0()ekdeik(n1)d, (2)

where t1and t2are the transmission coefficients of the THz field through both sur-faces of the target, and

n

=n

+iis the complex refractive index of the composition.

When the Fresnel loss is ignored, the effect of the target on the THz field is a decayof amplitude and a delay of the phase. The former is controlled by the imaginarypart of the refractive index and the later is related to the real part. The decay of THzfield can be measured through the absorption coefficient of the target = k. Theabsorption coefficient may have different format depending on the characteristicsof the samples. For instance, the absorption coefficient could be described basedon thickness for some homogenous samples, for other samples, it may be describedbased on mass or concentration. It can also be categorized as intensity absorption orelectric field absorption. In a laboratory environment, the absorption coefficient of

an explosive sample is practically based on the mass of the sample, which is:

() = ln I0()IS()

/m. (3)

Here, I0 is the original power of the THz wave, IS is the THz power measuredafter the transmission through the target, and m is mass of the explosive sam-ple. Absorption peak appears when there is transition in the explosive molecule.Figure 9.1 gives the absorption spectra of 4 explosives and their related com-

pounds from 2 to 21 THz. The spectra were measured using a Fourier transforminfrared (FTIR) spectrometer. Explosives and related compounds present rich spec-tral features in their THz spectra. Comparing to FTIR spectroscopy, THz wavetime-domain spectroscopy (TDS) provides better data in the low frequency regime.Figure9.2shows spectra of 10 different explosives and their related compoundsfrom 0.2 to 3 THz measured using THz TDS. Clear spectral features are detected forall those compounds in the lower THz band. Table9.1summarizes spectral featuresof the 15 most popular explosives and related compounds in the THz band. UsingTHz wave spectroscopy, it is possible to identify explosives through their spectralfeatures and, therefore, it is possible to detect explosive devices such as landminesand IEDs.

Explosives can be identified through THz wave absorption spectroscopy.However, in reality, a THz wave might not be able to transmit through the target.For instance, in passenger screening, THz waves are not able to penetrate through


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Spectra of Explosives in THz Band 203

Fig. 9.1 Spectra of explosives and related compounds from 2 to 21 THz measured by FTIR.

Transmission spectra and diffused reflection spectra are compared with calculation results

Fig. 9.2 Absorption spectra of explosives and related compounds measured using THz TDS(0.23 THz)


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Table 9.1 Absorption peaks of explosives and related components in THz band

Explosives andrelated compounds Absorption peaks (THz)

TNT 1.66, 2.20, 3.69, 4.71, 5.58, 8.16, 9.15, 9.75, 10.65, 13.89, 15.09, 19.17RDX 0.82, 1.05, 1.50, 1.96, 2.20, 3.08, 6.73, 10.34, 12.33, 13.86, 14.54, 17.74,

18.12, 20.13HMX 1.78, 2.51, 2.82, 6.06, 11.10, 11.97, 13.56, 14.52, 18.18, 18.51PETN 2.0, 2.84Tetryl 5.97, 10.11, 11.28, 14.67, 16.14, 18.362-amino-4, 6-DNT 0.96, 1.43, 1.87, 3.96, 5.07, 6.27, 8.49, 9.87, 10.77, 12.15, 13.44, 16.684-amino-2, 6-DNT 0.52, 1.24, 2.64, 3.96, 5.04, 5.82, 7.53, 9.30, 10.20, 11.13, 13.86, 14.97,

17.704-nitrotoluene 1.20, 1.37, 1.86, 6.75, 8.85, 10.83, 14.04, 15.66, 18.511,3,5-TNB 4.17, 4.62, 10.05, 11.19, 13.80, 15.75, 19.05

1,3-DNB 0.94, 1.19, 2.37, 10.56, 12.18, 15.33, 17.131,4-DNB 3.24, 3.96, 5.55, 10.38, 12.45, 13.29, 15.21, 15.542,4-DNT 0.45, 0.66, 1.08, 2.52, 5.01, 8.88, 10.56, 11.58, 12.81, 14.34, 15.81, 19.052,6-DNT 1.10, 1.35, 1.56, 2.50, 5.61, 6.75, 9.78, 11.43, 13.32, 13.89, 15.39, 17.253,5-dinitro aniline 0.96, 1.20, 3.18, 4.62, 5.04, 5.91, 7.44, 10.62, 10.98, 14.46, 16.41, 18.182-nitro diphenyl

anine2.19, 2.58, 2.88, 3.45, 5.13, 6.18, 7.56, 10.08, 12.33, 13.05, 15.00, 15.60,

16.29, 17.34, 18.51, 19.32

the body of the passenger. In such a case, reflection geometry is the only feasible

choice. THz wave reflection spectroscopy analyzes the spectral feature of the spec-ular reflection or the diffused reflection coming from a sample. Specular reflectionis used when the target has a smooth surface and diffuse reflection is used for sam-ples that have a rough surface. The reflectance of an object is described by Fresnelformula as:

r//=n cos i cos tn cos i+ cos t

r=cos i

n cos t

cos i + n cos t

, (4)

where r//and r are the reflectance ofpand spolarization wave, nis the reflec-tive index of the material, iand tare the incident angle and transmission angle,respectively. When the sample absorbs a THz wave, its reflective index is a com-plex value. Thus, the reflectance will have both amplitude and phase elements. Fornormal incidence, Equation (4) is simplified to:

r//=

r=

n2 + 2 1

(n+ 1)2 + 2+i

2

(n + 1)2 + 2 (5)

Thus, spectral features of the target are reflected in both amplitude and phasechanges in the reflection spectrum. Figure9.3shows the reflection spectrum of RDX


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Fig. 9.3 Spectra of RDXmeasured by THz TDS intransmission and reflectiongeometries. Thedashed linesindict locations of absorptionpeaks

with its absorption spectrum as comparison. Both spectra show the same spectralfeatures.

If the target contains fine structures (for example powder) THz waves scatter oncethey penetrate into the target. Partial of the backscattered beam could be collectedand analyzed. If certain approximations are satisfied, the problem can be solvedas a pure diffuse spectroscopy analysis. These approximations are: (i) first of all,

specular reflection from the surface of the target is ignored; (ii) secondly, the depthof the sample can be considered infinite compared to the penetration depth; and(iii) finally, the sample is homogenous and the illumination of the target is uniform.Under these conditions, the spectrum can be described using the Kubelka-Munkmethod [1] as:

F(R) =(1R)2

2R, (6)

where R= RSig/RRefis defined as the diffuse factor of the target, Rsig denotesthe reflectance of the target and RRefdenotes the reflectance of a reference sample.The reference has similar physical characters as the target but it does not containany spectral features. For instance, polyethylene powder can be used as a referencewhen THz wave spectroscopy is used in measuring explosive powder samples.

The diffused reflection spectra of 4 explosives and their related compounds arecompared with their transmission spectra in Fig.9.1. The diffused reflection spectrashow similar spectral features as the absorption spectra. For certain weak absorptionpeaks, diffused reflection spectra show even a higher sensitivity. The modificationof the frequency band caused by the target can be used as the effective absorptionstrength of the target, which can be modeled as the product of the absorption coeffi-cient and the effective interaction distance lEff. In the transmission spectrum, theeffective absorption strength can be simply described as the product of the absorp-tion coefficient of the target and its thickness. In diffuse reflection spectroscopy the


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Fig. 9.4 The effectiveabsorption strength as afunction of the absorptioncoefficient in transmissionand diffuse reflectionspectroscopy

scattering of the material significantly increases the effective interaction distanceof the THz waves inside the target. Additionally, the interaction length is differentdepending on the scattering paths. Therefore, the effective absorption needs to becalculated using a statistic method as [2]:

lEff= (1+ 2s/)1/2, (7)

wheres is the scattering factor of the target, which is determined by the size of the

fine structures and other properties of the target. Figure9.4compares the effectiveabsorption strength in transmission and diffused reflection spectrum as a function ofthe absorption coefficient . The figure shows that the diffused reflection spectrumshows a higher sensitivity of weaker absorption features.

The spectral features of explosives and related compounds are the result of thecollective rotation or vibration modes of the molecules. The resonant structure ofmolecules can be calculated using density function methods once the structureof the molecule is determined. Figure9.5shows the molecular structure of 2,4-DNT, which is produced from the degradation of TNT, the most popular explosive.

Fig. 9.5 Molecular structureof 2, 4-DNT


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Therefore, it is considered as a hint for the presence of TNT. A molecule of 2,4-DNThas a benzene ring as the main frame, a methyl connecting with the first carbon atomC1, and two nitryls connecting to the second (C2) and the forth (C4) carbon atoms.Due to the asymmetry of the structure, the benzene ring is distorted in the 2,4-

DNT molecule. The methyl acts as an electron donor and the nitryles act as electronacceptors. The interaction between methyl and the two nitryles prolongs C1-C2 andC1-C6 bonds, while C2-C3, C3-C4, C4-C5 and C5-C6 are compressed shorter thannormal. This distortion causes the bond angle C2 C1 C6 to be smaller thanother bond angles. The forth nitryle, the benzene ring and the carbon atom in themethyl lie in the same plane. The second nitryle is pushed out of this plane 30 because of conflicts between its oxygen atom and the hydrogen atom in the methyl.Based on the previous analysis, its vibration transitions at ground state can be calcu-lated using density function theory. Table9.2gives the calculated parameters of the

2,4-DNT molecule, including bond lengths, bond angles, the plane angle of the sec-ond nitryle, polarizations of the molecule as well as its energy structure. Figure9.6acompares the theoretically predicted resonant features with the absorption spectrummeasured using FTIR spectrometer. Most of the calculated features match the exper-imental result well, except for the absorption peak that appears at 2.52 THz in theexperimental data, which is not present in the calculations. This peak is assumed tocome from lattice vibration of crystalline 2,4-DNT. The calculation only consideredsingle molecule, thus, lattice vibration modes are not considered. This assumptionis confirmed by further experimental results. Figure9.6bcompares absorption spec-

tra of a solid 2,4-DNT sample and the toluene solution sample. The absorption peaklocated at 2.52 THz disappears in the solution sample, where no crystalline structureexists. Table9.3 compares calculation results and experimental data. The calculationdoes not only predict the location of those resonances but also provides the strength

Table 9.2 Parameters of2,4-DNT molecular structurecalculated using densityfunction

Parameters Calculation result

r(C1-C2) 1.406 r(C2-C3) 1.390 r(C3-C4) 1.384 r(C4-C5) 1.390 r(C5-C6) 1.388 r(C1-C6) 1.403 r(C1-C7) 1.506 (C2 C1 C6) 116.0(C1 C2 C3) 123.0(C2 C3 C4) 118.2(C3 C4 C5) 121.5(C4 C5 C6) 118.7

(C5 C6 C1) 122.5(O12-N11-C2-C1) 29.9(O16-N15-C4-C3) 0.8|t| 5.246 DebyeE 680.8 Hartree


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Fig. 9.6 (a) Comparing of absorption spectrum of 2,4-DNT in THz band with calculated resultusing density function. The absorption peak located at 2.52 THz does not appear in calculated

result. (b) Absorption spectrum of 2,4-DNT sample in solid state and toluene solution. Theabsorption peak at 2.52 THz only appears in solid state sample

Table 9.3 Comparison of the absorption peaks and strength of 2,4-DNT between the experimentalresults and calculation results

Experimental result Calculation result

Frequency(THz) Strength

Frequency(THz) Strength Cause of absorption

1.08 Phonon or intermolecular transition2.52 Phonon or inter molecular transition5.01 Strong 4.92 Strong 2,4 C-NO2in-plane bending vibration8.88 Strong 8.61 Strong Benzene ring out-of-plane bending vibration

10.56 Strong 10.53 Strong Benzene ring in-plane bending vibration11.58 Weak 11.82 Weak Distortion of methyl and benzene ring

out-of-plane bending vibration12.81 Weak 13.08 Weak C-CH3out-of-plane swing14.34 Strong 14.58 Weak 4 C-N out-of-plane swing15.81 Middle 15.96 Middle 4 C-N in plane bending and benzene ring

distortion

19.05 Strong 19.38 Strong Benzene ring distortion

information of those absorption structures. Calculation results of more explosiveand related compounds are presented in Fig.9.1.

Remote Sensing with THz Wave

One of the most attractive advantages of using THz waves to inspect hazardousmaterials is that the propagation of THz waves allows the operator to stay at a dis-tance from the target. The propagation properties of THz wave in the ambient air areessential factors in the study of the remote sensing capabilities of the THz waves.


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Remote Sensing with THz Wave 209

Fig. 9.7 Waveform and spectrum of THz pulse evolute with propagation distance in free space

Figure9.7shows the waveforms (a) and the spectra (b) of THz pulses after propa-gating through air for different distances. The amplitude of the THz field decreasesfor long propagation distances while the absorption due to water vapor becomesmore severe. THz waveform is still detectable after 100 m of propagation, and it canstill provide spectral measurement except the windows that are close to water vaporabsorptions. Figure9.8shows the propagation windows for THz waves below 1.6THz. There are 7 major windows in that region that can be used in remote sensingapplications.

Fig. 9.8 THz propagationwindows in air


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Fig. 9.9 Concept of using THz ABCD technique in remote sensing applications

Although THz waves, especially those components away from water vaporabsorption lines, are able to propagate in air for more than a hundred meters, itshigh attenuation is still a hurdle that limits the distance of THz wave remote sens-ing. An alternative way is to generate and then detect THz waves in ambient airadjacent to the target. Since an optical beam has much lower attenuation in air(except dusty, smoky or foggy conditions) than THz waves, using air generationand detection promises longer sensing distance than directly send THz waves tothe target. Figure9.9shows the concept of using THz wave air breakdown coher-

ent detection (ABCD) technique in remote sensing. An ultrafast laser pulse andits double frequency pulses are focused next to the target, where they generateTHz wave through the breakdown of air. THz waves coming from the target aredetected locally using air as the sensor. Because a long propagation distance forTHz waves has been avoided, the full spectral band and full dynamic range ofthe THz system can be used. Figure9.10shows a THz waveform generated anddetected at 17 m away from the light source. Remote generation and detection ofTHz waves in air can also be performed using THz radiation enhanced-emission-of-fluorescence (REEF) technique, as presented in Fig.9.11.Comparing to ABCD

Fig. 9.10 THz waveform ofTHz pulses generated anddetected in air 17 m awayfrom the light source


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Remote Sensing with THz Wave 211

Fig. 9.11 Concept of using THz REEF technique in remote sensing applications

method, the REEF method detects fluorescence rather than second harmonic gener-ation. Figure9.12compares THz waveforms detected using EO sampling methodand REEF method. Using THz-TDS powered with REEF detection technique, itis possible to fingerprint explosives through their spectral features obtained locallybesides the target.

The penetration capability of THz wave through several kinds of materials hasbeen discussed in Chapter 8. THz waves have been used in NDE applications and itspenetration capability has also lead using them in security inspection applications.Figure9.13shows a 0.094 THz wave image of a person [3]. A 1.2 m square shape

Fig. 9.12 THz waveformsdetected using THz REEFmethods comparing to thosedetected using EO samplingmethod


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Fig. 9.13 THz wave (0.094 THz) image of a person.Leftis a photo of the person as comparisonand therightis his THz wave image (Courtesy of QinetiQ)

antenna placed at 7 m away from the person is used to take this image. THz waveimage can unveil the knife hidden in the folding news paper and a pistol under cloth.THz wave imaging can be used in reflection geometry and, in some cases: transmis-sion geometry is also possible, for example in package inspections. Figure9.14shows an experimental setup of a transmission THz wave imaging system operationin raster scanning configuration. Figure 9.15shows THz wave image of a brief-case taken by 0.2 THz wave. The briefcase contenting items are clearly shown inits THz wave image, which included knife, CD, video and audiocassettes and pens,

Fig. 9.14 Concept of THz wave transmission imaging


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THz Wave Stand-Off Detection 213

Fig. 9.15 THz wave image (0.2 THz) of a briefcase

etc. (Sub) THZ wave imaging system has already been tested in many airports forpassenger screening, and the result is very permissing. It is very possible that this

technology will be used in real security inspection in a near future.When considering landmine detection, THz wave penetration into the soil is a

critical factor. Since water highly absorbs THz waves, THz wave imaging can onlybe used in the detection of mines that are buried under dry soil, i.e. in the desert.According to Chapter 8, the penetration depth of THz wave in materials such assand and rock is in the range of few centimeters. As a result, THz wave imaging isable to see a target buried under a few cm of soil. There are basically two kinds oflandmines, which are the antipersonnel mines and the antitank mines. The formerare usually buried within a very shallow depth, usually less than a few cm. The later

can be buried as deep as ten centimeters or deeper. Thus THz wave image is appro-priate to be used in detection of antipersonnel mines under dry soil. Figure 9.16shows THz wave images of a metal target buried under dry sand at different depths.The result indicates that THz wave image can see metal target under sand up to2 cm. Using THz wave inspection does not only see those hidden targets but also beable to identify those targets through their spectral features. Figure9.17shows THzwave transmission spectra of RDX sample hidden under paper, polyethylene film,polyester film and leather. The absorption peak at 0.82 THz can clearly be observedeven with different covers.

THz Wave Stand-Off Detection

THz wave spectroscopy can fingerprint hazard materials, such as explosives, andit can be used in remote sensing condition as well as to detect targets under cover.


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Fig. 9.16 Pulsed THz wave images of metal block buried under dry sand. The depths of metal:(a) 5 mm, (b) 10 mm, and (c) 20 mm

Fig. 9.17 THz TDS signal of a RDX sample under different covers


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THz Wave Stand-Off Detection 215

Fig. 9.18 THz TDS signal ofa RDX sample when thespectrometer locating atdifferent distances from thetarget. The reference spectrawere taken usingpolyethylene as the referencesample

These features allow using THz wave in stand-off sensing, which allows the opera-tor to stay out of the hazardous zone and interrogate the suspect target. Figure 9.18shows THz wave spectroscopy of a RDX sample when the sample is located at dif-ferent distances from the THz wave spectrometer. The experimental result showsthat THz wave spectrometer can identify RDX even at 30 m away by catching theabsorption peak at 0.82 THz. The reflection spectroscopy geometry is the primary

choice in stand-off detection because the source and detector stay close to eachother into a single unit. Therefore, the measured results will be analyzed usingEquation (5) rather than Equation (3). In Equation (5), the reflectance has a realpart and imaginary part. If


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a b

Fig. 9.19 Phase and amplitude spectra of RDX in THz-TDS

where absorption reduces the amplitude, it is the phase that changes, presenting theabsorption feature in the reflection spectroscopy. The amplitude spectrum, however,reflects the refractive index variation adjacent to the absorption peak.

When the phase shift is used to evaluate spectrum in the stand-off detection,the dynamic range of the measurement can be described as D= Max/Fluc,whereMaxis the phase shift at absorption peak, and Flucis the fluctuation ofthe phase shift of the measurement. The major sources of phase fluctuation (noise)include disturbance in air, fluctuation of water vapor concentration, timing jitterbetween laser pulses, as well as vibration of the optic components. Figure9.20shows phase shift of a monochromatic wave resulting from the measurement noise.The phase fluctuation of the THz wave is affected by the dynamic range of thesystem according to:

1D0

t. (9)

HereD0denotes the dynamic range of the THz system and tis the temporal res-

olution in the recording of the THz waveform. As a result, the detectable range ofa THz wave stand-off detection system can be estimated according to the dynamic

Fig. 9.20 Phase fluctuationin THz wave stand-offdetection


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THz Wave Spectroscopic Imaging 217

Fig. 9.21 Dynamic range ofTHz wave spectrum inmeasuring RDX samplingfrom different stand-offdistances

range of the system, the beam emission and collection geometry, and the specificdetails of the target. Figure9.21provides the detectable range of a THz TDS systemwith a dynamic range of 15, which is used in the detection of RDX samples withdifferent sizes. A collection aperture of 150 mm is used in the calculation.

THz Wave Spectroscopic Imaging

The combination of THz wave imaging technology with its spectroscopic capabil-ities results in THz wave spectroscopic imaging. THz wave spectroscopic imagedoes not only see the profile of the target but also is able to identify its compositionaccording to the spectral features. The example introduced in Chapter 8, where thespectral response of THz wave is used in detecting the defect in the carbon fibercomposite sample. Following the unique spectral features of explosives, it is possi-ble to identify a target containing explosives using THz wave spectroscopic imaging.Figure9.22shows spectroscopic images of three samples made by lactose, sugar,

and RDX, respectively [4]. Those samples cannot be distinguished without usingspectroscopic information. However, using their spectral features, each item can bedistinguished from others.

THz wave spectroscopic imaging can be implemented with broadband pulsedTHz wave, but it can also be implemented with several CW THz sources operat-ing at selected wavelengths. Figure9.23shows a concept of using a serial of THzwave transceivers with multiwavelengths to provide THz wave spectroscopic imag-ing. Some of the wavelengths are located away from all absorption peaks in orderto obtain a baseline. Other wavelengths are located on the absorption features ofinterest. Thus, by comparing images taken at different wavelengths, it is possibleto distinguish the composition of the material. The simplest system uses just twowavelengths. One of the wavelengths is located away from the absorption peak andthe other one located on the absorption peak or close to it. Figure9.24shows THzwave images of three targets made of polyethylene, TNT, and RDX acquired at


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Fig. 9.22 THz wavespectroscopic images oftargets made of lactose, sugarand RDX. (a) layout ofsamples. From theleftto theright, lactose, sugar andRDX. (b) THz wave imagesof the three samples bymeasuring peak amplitude ofTHz pulses. (c,d, ande) arespectroscopic images of thosethree targets respectively(Courtesy of TeraView)

Fig. 9.23 THz wave spectroscopic imaging using cw THz wave transceivers


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References 219

Fig. 9.24 (a) 0.6 THz wave images of three targets made of polyethylene, TNT, and RDX.(b) Identification of RDX from the other samples using spectroscopic imaging technique

0.2 and 0.6 THz frequencies. Figure9.24ashows the images acquired at 0.6 THz.Although 0.6 THz does not overlap to the absorption peak of RDX (at 0.82 THz),the RDX sample still shows a higher absorption than the other samples at 0.6 THz.However, using 0.6 THz itself is not sufficient to highlight RDX. Suppressing theimage acquired at 0.6 THz with the image acquired at 0.2 THz (Fig. 9.24b), theRDX sample is highlighted in the retrieved image while the other two samples arenot presented because they do not have absorption feature close to 0.6 THz.

References1. F. M. Mirabella, Modern Techniques in Applied Molecular Spectroscopy. Wiley, New York

(1998).2. M. Milosevic, and S. L. Berets, A review of FT-IR diffuse reflection sampling considerations,

Appl. Spectr. Rev.37, 347 (2002).3. D. Clery, Brainstorming their way to an imaging revolution,Science,297, 761 (2002).4. Y. C. Shen, T. Lo, P. F. Taday, B. E. Cole, W. R. Tribe, and M. C. Kemp, Detection and

identification of explosives using terahertz pulsed spectroscopic imaging, Appl. Phys. Lett.86, 241116 (2005).

thz technology in security checks

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