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282 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 1, NO. 1, SEPTEMBER 2011

Explosives Detection by TerahertzSpectroscopyA Bridge Too Far?

Michael C. Kemp, Member, IEEE

(Invited Paper)

AbstractTerahertz spectroscopy has been investigated as a

technique for concealed explosives detection since spectroscopicsignatures in common explosives were first identified almost 10years ago. This paper explores the progress towards practicaldevelopment of the technology and the physical basis of the

challenges involved in creating a deployable person-borne IEDdetection system. We conclude that, despite the theoretical poten-tial of terahertz to provide a safe, through-barrier spectroscopicdetection technique, this is unlikely to be possible in practice. Therelatively weak and broad explosives features tend to be maskedby the combined effects of atmospheric water vapor absorption,

barrier attenuation and scattering from both clothing and thetarget explosives. Imaging at the lower end of the terahertz fre-quency range, although not offering the same specificity, is a morepromising technique for security applications.

IndexTermsExplosives detection, security screening, stand-off

detection, terahertz imaging, terahertz spectroscopy.

I. INTRODUCTION

THE increased threats of criminal or terrorist action in re-

cent years have led to the search for new and improvedtechniques for detection of concealed weapons, contraband, ex-

plosives or other objects. A wide variety of different physical,

chemical and biological phenomena have been explored for sig-

natures of explosives materials which might lead to new detec-

tion techniques. Systems based on electromagnetic radiation be-

tween 30 GHz and 3 THz in the millimeter-wave and terahertz

regions of the electromagnetic spectrum may have particular ad-

vantages:

Radiation penetrates many common barrier materials en-

abling concealed objects to be seen;

Wavelengths are short enough to give adequate spatial res-

olution for imaging or localization of threat objects;

Radiation at these frequencies is non-ionizing and avoids

the issues associated with X-ray and other ionizing radia-

tion.

At terahertz frequencies above 500 GHz or so, some mate-

rials such as explosives have characteristic spectroscopic

signatures which can be used to identify them.

Manuscript received April 05, 2011; revised May 23, 2011; accepted June 01,2011. Date of current version August 31, 2011.

The author is with Iconal Technology Ltd., St. Johns Innovation Centre,Cambridge CB4 0WS, UK (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TTHZ.2011.2159647

The potential of people-safe, materials-specific, concealed

explosives detection is attractive for applications such as avi-

ation security, where the improved detection of non-metallic

threats has long been sought and which was brought into focus

with the failed attack by the so-called underpants bomber

on Christmas Day 2009. It could also be important for the

detection of suicide bombers in crowded places such as rail

stations or sports events.

It is almost 10 years since the terahertz spectroscopic featuresin a number of common explosives were first discovered [1].

A considerable amount of research and development has been,

and continues to be, carried out in university, government and

industrial laboratories around the world. However, after rapid

initial progress, development has stalled and the prospect of a

practical, terahertz spectroscopy-based deployable explosives

detection system for people screening seems as far away as ever.

This paper reviews the work carried out to date in terms of the

physical basis of the challenges which need to be overcome in

the development of a deployable system and analyzes whether

the signatures are strong enough for such developments to

succeed.Section II discusses the unique combination of characteris-

tics which makes the terahertz region of the spectrum attractive

and Section III reviews work on the spectroscopic signatures

of explosives and other materials which underpins the tech-

nique. Section IV explores the different factors and physical

effects encountered in real-world applications, and identifies

the challenges which need to be addressed in moving from

a laboratory spectrum measurement to a practical detection

system. Sections VX analyze each of these challenges in

detail and their combined effect is summarized in Section XI.

Section XII discusses terahertz imaging which may be better

aligned to the physical constraints and overall conclusions are

drawn in Section XIII.

II. SPECTROSCOPIC DETECTION REQUIREMENTS

The emergence of suicide terrorism as a threat has led to the

need for new techniques to detect explosives concealed under

a persons clothing or in carried bags. The requirements for a

spectroscopic detection system include the ability to:

detect the presence of small bulk quantities of explosive,

such as could be used to damage a structure like an aircraft

or cause loss of life and injury;

distinguish explosives from harmless materials which may

also be present;

detect explosives which are wrapped or inside a container;

2156-342X/$26.00 2011 IEEE


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KEMP: EXPLOSIVES DETECTION BY TERAHERTZ SPECTROSCOPYA BRIDGE TOO FAR? 283

detect explosives concealed under common clothing;

detect explosives concealed in handbags, backpacks, shop-

ping bags, etc.;

operate fast enough to screen human subjects and minimize

interruption to the normal flow of commerce;

be safe in use both for the operator, bystanders, and the

persons being screened.

Non-electromagnetic techniques such as acoustic waves are

strongly reflected and attenuated by heavy clothing, and are

unable to identify specific materials. Induction metal detectors

and magnetometers only detect metallic objects, and nuclear

resonance techniques such as Nuclear Quadrupole Resonance

(NQR) and Magnetic Resonance Imaging (MRI) provide

specificity but require well shielded enclosures to avoid the

effects of interference. Spectroscopic techniques based on

neutrons or other energetic particles give rise to safety con-

cerns for people-screening. Electromagnetic techniques using

microwaves and millimeter-wave frequencies penetrate but

do not provide materials specificity; IR, Visible and UV laser

spectroscopy techniques are powerful but do not penetrateclothing; and X-ray spectroscopy techniques again raise safety

concerns [2], [3].

By contrast, terahertz has a unique position in the electromag-

netic spectrum in terms of its ability to penetrate clothing and

other barriers, its safety and the existence of spectroscopic fea-

tures. It is also a non-contact technique, can potentially be used

for stand-off detection, and requires neither sample preparation

nor consumables. This, then, is the context and motivation for

the considerable research effort which has been devoted to the

development of terahertz as a technique for people screening

over the last 10 years.

III. SPECTRA OF ENERGETIC MATERIALS AND EXPLOSIVES

The terahertz spectra of common energetic compounds

(RDX, PETN, HMX, TNT) and commercial explosives based

on these compounds (PE4, Semtex-H) were first reported in

2003 [1]. These results are shown in Fig. 1 and have sub-

sequently been independently validated and extended by a

number of groups [4][9] using time-domain terahertz spec-

troscopy and FTIR. More recent work has measured the spectra

of a large number of explosives [10] and has confirmed the

spectrum of TATP [11], important as a home-made explosive.

The spectra of most powdered organic crystalline energetic

compounds, both pure and in matrices such as plasticizer usedto make plastic explosive, exhibit clear features at room tem-

perature. The spectra are well behaved in the sense that they

are not strongly affected by matrix materials or environmental

temperature changes, and they obey Beers law such that ex-

plosives formed of two energetic compounds have a spectrum

which is the sum of the two.

Some early measurements [12] claimed features at frequen-

cies below 0.5 THz, but these have not been confirmed by other

groups. They may have been scattering artifacts caused by the

granular nature of many formulated explosives, which have par-

ticle sizes comparable to the terahertz wavelengths used. Mea-

surements up to 6 THz [8], [9] show additional spectral features

although, as we shall see, these frequencies are less useful in

practical applications.

Fig. 1. Terahertz transmission spectra of the raw explosive materials TNT,HMX, PETN and RDX together with the spectra of the compound explosivesPE4 and Semtex H [1].

Spectra of explosivesand other materials have been measured

at a range of temperatures down to 4 K [8]. Features become

sharper at low temperatures and shift in position.

Unlike infrared spectra which are mainly due to intramolec-

ular vibrations of bonds between individual atoms and groups

of atoms in a molecule, terahertz spectra in solids are caused

by long-range intermolecular forces between molecules in the

crystal lattice known as phonon modes. Theoretical modeling

using density functional theory to incorporate these long-range

forces has been successfully used to predict the terahertz spectra

of explosives such as HMX, PETN and RDX [13][15].

Recent work on formulated explosives and exploring the pos-sible effects of features such as solvent inclusions, different

manufacturing methods etc, has confirmed that the basic spec-

tral signatures are robust [16].

Not all explosives, however, have spectra with distinctive

features. Ammonium Nitrate and mixtures such as ANFO have

smooth spectra with no discernable features [17]. Amorphous

materials and liquids also have extremely broad spectra [18],

[19] due to the lack of long-range order. For example, the

spectra of crystalline and amorphous forms of sucrose and

other sugars were measured by Walther [19] and the strong

spectral peaks in the crystalline material were found to disap-

pear completely in the amorphous compound.In summary, many explosives based on solid crystalline or-

ganic energetic compounds have 34 relatively strong features

and several weaker ones in the frequency range 0.56 THz.

The peaks are always broad, typically around 100300 GHz in

width. The explosive RDX, which is a constituent of a number

of military and commercial plastic explosives such as C4,

PE4 and Semtex, has a very strong feature at approximately

0.8 THz. Most other explosives have their lowest frequency

features nearer 2 THz.

IV. FROM SPECTRUM TO DETECTION SYSTEM

In order to use terahertz spectra as the basis for an effective

practical detection system, a number of considerations must be

taken into account. THz radiation at different frequencies needs


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Fig. 2. Typical remote threat detection system geometry, showing some of theenvironmental effects which need to be overcome.

to be generated by a source and directed at a target, from whichit

is reflected back and measured by a detector, as shown in Fig. 2.

However, this radiation has to propagate through the atmos-

phere, then through clothing and any wrapping or container, be-

fore reaching the target. It will then be reflected or scattered by

the possibly rough or irregularly shaped explosive before re-

turning through the same barriers to the detector. Consequently,

the target spectra must be sufficiently strong to be able to over-

come the signal attenuation and scattering. Furthermore, to be

useful as a real-world detection system, the spectroscopic fea-

tures of any target explosive must be distinct from those of

clothing and any benign objects which the person may be car-

rying, such as food, confectionary, or pharmaceuticals.

Moreover, a practical system requires sources and detectors

which are not too large, costly or inconvenient to use. It must

also respond quickly and be able to cope with the fact that the

target will be moving during the measurement. The followingsections address these issues to assess the potential of terahertz

as a practical spectroscopic detection technique.

V. SPECIFICITY

The first question we must address is whether the spectra of

explosives are sufficiently distinct from those of non-threat ma-

terials such as clothing or other benign substances that a person

may be carrying. Several studies have characterized common

barrier materials and potential confusion materials [20][23],

of which indicative results are shown in Fig. 3. While clothing

materials typically have smooth featureless spectra, confusion

materials, such as foodstuffs, confectionery, cosmetics and phar-maceuticals may have spectroscopic features in the THz range.

Fig. 3 also shows the effect of particle size on the observed

spectra. Granular sugar, which has particle sizes similar to the

measurement wavelengths, does not show the sharp spectral fea-

tures offinely powdered sugar. This is due to scattering and is

discussed further in Section IX.

Measured over a wide frequency range and with a sufficiently

high signal-to-noise ratio (SNR), the spectra of different sub-

stances are distinct. However, over a limited frequency range

and without the precision measurements of controlled labora-

tory conditions, it is possible to confuse substances. For ex-

ample, if a single feature around 1.8 THz is observed, from the

data in Figs. 2 and 3 can we tell whether it is the explosive HMX

or just sucrose? Many formulated explosives and benign objects

Fig. 3. Terahertz absorption measurements of common foodstuffs, examplesof potential confusion materials. Note the features due to sucrose in both thesugar and chocolate spectra [20].

are made up of mixtures of different compounds and this in-

creases the possibility of confusion.

The resolving power of a terahertz spectrometer depends on

the particular technology used. For example, pulsed, time do-

main systems may have a resolution between 5 and 50 GHz,

giving a resolving power of 40400 over a 2 THz range. Sys-

temsusinga bankof justa fewfixed frequency sources, as some-

times proposed, have a lower resolving power. However, the

main limit to performance will be from the relatively low SNRand high levels of clutter in the spectrum when measurements

are made in field conditions.

VI. REFLECTION SPECTRA AND GEOMETRY

Practical detection systemsneed to operate in reflection rather

than transmission geometry, due both to high absorption coeffi-

cients of the explosives and, for people screening applications,

absorption by the body. For the simplest case of a bare mate-

rial in air, the reflected power is given by ,

where is the frequency dependent refractiveindex of the mate-

rial. Measurements of the absorption coefficient and refractive

index of a number of explosives have been made by Lo [24].The calculated and measured reflection spectra of the explosive

Semtex H, a mixture of RDX and PETN, are shown in Fig. 4

[25].

In transmission mode, depending on sample thickness and

the strength of the peak, on- and off-peak measurements may

differ in intensity by 10 dB or more. In reflection, the con-

trast is unlikely to exceed 1 or 2 dB. This makes the measure-

ments much more susceptible to the effects of clutter, i.e., the

random changes in reflectance due to environmental effects such

as sample shape, multiple reflections, and scattering. If the am-

plitude and phase of the reflection spectrum is obtained, which

involves knowing the precise distance to the (flat) target, the ab-

sorption spectrum can be recovered [26]. Since the absorption

and reflection spectra are also linked by the KramersKronig


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Fig. 4. The measured (solid line) and calculated (dashed line) reflectancespectra of Semtex-H under normal atmospheric conditions, taken in real time.The calculated spectra were derived from transmission spectroscopy data [25].

(KK) relationship, in principle the reflection spectrum can be

inverted to obtain the absorption spectrum without knowledge

of the distance. This, however, involves approximations and can

lead to errors in the recovered spectrum [27]. Liu [28] used thisapproach, but noted the introduction of spurious features, as-

cribed to the surface structure of the material. Direct comparison

of measured reflection spectra with reference measurements is

probably a more robust approach for identification.

Loss of contrast in reflection spectroscopy can also be caused

by barrier materials, since these reflect some of the incident ra-

diation and add a background to the spectrum. They can also

change the shape of the reflection spectrum. As discussed in the

paper by Yamamoto [4], the formula relating reflectivity to re-

fractive index becomes more complex when the target is cov-

ered in a material with a refractive index other than unity, or

even varying with frequency. Fig. 5 shows how the shape of the

reflection spectrum varies with the refractive index of the cov-

ering material. Since in a practical application the nature of any

covering material will not be known, interpreting the reflection

spectra can be difficult, a situation exacerbated if the radiation

is polarized and non-normal reflection angles are encountered.

Reflection spectroscopy is also dependent on the surface and

its orientation. If the target is not flat or is rough, then the re-

turned signal can be very weak. Rough surfaces provide a re-

flected signal in all directions due to scattering, but this is typ-

ically 2030 dB weaker than specular reflection [28]. Further

issues occur if the target is uneven. Different parts of the beam

have different path lengths, causing interference effects, partic-

ularly if phase-sensitive coherent detection is used [29].Detection systems operating at any stand-off distance need

to consider specular and diffuse reflection. If the target is flat,

smooth and perfectly aligned, then the incident beam will be

reflected back to the detector. However if the target is tilted,

then no signal will be returned. Rough or curved surfaces will

return part of the signal, through so-called diffuse reflection,

but this signal will typically be 1020 dB weaker than specular

reflection.

VII. ATMOSPHERIC ABSORPTION

Terahertz radiation is attenuated as it passes through the at-

mosphere, principally due to absorption from water vapor [30].

As shown in Fig. 6, this attenuation increases with frequency,

Fig. 5. Calculated reflection spectra of the RDX-based explosive C-4 and theirdependence on the refractive index, , of covering materials: (a) no covering

, (b) covering with materials with (solid line),( dotted line) and ( broken line), and (c) (thin solid line),

(broken line), (dotted line) and (thick solid line) [4].

as do the number and depth of water lines. Since the water

vapor content of air varies with temperature and relative hu-

midity (R.H.), the attenuation varies signifi

cantly according tothe climate conditions. This needs to be taken into account when

interpreting experimental data taken in different laboratories.

The challenge for spectroscopy is in removing the water lines,

so that any underlying spectrum from the target can be seen. In

laboratory experiments this is relatively straightforward: a ref-

erence spectrum can be taken using a mirror at the same dis-

tance as the target, which is then used to normalize the spec-

trum. Many of the laboratory experiments in the terahertz field

are also carried out using a system purged with dry nitrogen

to avoid these issues. More sophisticated approaches have also

been adopted. Withayachumnankul [31] developed a line re-

moval method based on a spectral line library and modeled

line-widths and line-shapes. Lines are successively removedusing a minimum fluctuation-energy criterion. This was found

to work successfully in high signalnoise conditions, but de-

grades at higher terahertz frequencies where the SNR is lower

and the water vapor lines become stronger and more closely

spaced.

The overall attenuation of the terahertz beam limits the

stand-off distance which can be used. For example, at 2 THz (at

20 C with 40% R.H.) the baseline attenuation is approximately

0.3 dB/m, giving a 20 dB loss over a 10 m return path.

VIII. BARRIER MATERIALS

In practical explosives detection, target explosive material is

likely to be concealed behind several layers of barrier material.


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Fig. 6. (a) Atmospheric attenuation at sea level [30], calculated using currentlyaccepted models at sea level. Rain 4 mm/hr, fog 100 m visibility, standardatmosphere ( 7.5 g/m water vapor, g/m water vapor).(b)Atmosphericattenuation measured at 293K, 27%R.H. usinga terahertztime

domain spectrometer.

This may include plastic, cardboard, or other containers for the

explosives, as well as cloth or sticky tape holding it to the body

and several layers of clothing. These can have several effects:

attenuation of both the transmitted and reflected terahertz signal

from the target material; multiple reflections from layered mate-

rials; scattering from inhomogeneous materials such as woven

cloth; and, possibly, confusing spectral features from the barrier

materials themselves. The etalon effects caused by multiple re-

flections in and between barrier layers are a well known source

of artifacts in terahertz spectroscopy.

Several studies have measured the absorption of clothing andother materials [20], [23]. Fig. 7 shows some typical results.

At frequencies below 1 THz many materials are relatively

transparent, but the absorption increases significantly with

frequency. Bjarnason [23] proposes that many materials have

an absorption coefficient proportional to the square of the

frequency, so that the attenuation varies as . Fig. 7

shows that at 2 THz many clothing materials absorb 20 dB or

more per layer.

Other materials such as glass and liquid water are also very

strongly absorbing. Furthermore, metal is, naturally, a reflector

and even thin layers such as metalized plastics are more-or-less

completely opaque.

The strong absorption at higher frequencies by clothing is a

significant problem and one which has largely been missed or

Fig. 7. Attenuation due to clothing materials [20]. Note that the absorption canbe tens of dB and even 100 dB or more for several layers of clothing at 2 THz

and above.

ignored in previous publications. To take a plausible example,

assume the explosive is wrapped in thin card 0.5 mm thick held

in position by a denim vest and concealed under a sweatshirt.

At 1 THz each layer will attenuate the signal by approximately

5 dB, giving a total attenuation of the reflected signal as it passes

in both directions of 30 dB. At 2 THz, each of these layers will

attenuate the signal by approximately 20 dB, giving a total atten-

uation of 120 dB. This level of attenuation exceeds the dynamic

range of most terahertz spectroscopy systems. The overall dy-

namic range, however, is not the key parameter in this appli-

cation. Since the target explosive is covered by the clothing or

other barrier, some of the incident radiation will be reflected orscattered from the barrier. Therefore, the signal reflected from

the target has to be detected in the presence of the signal re-

flected from the barrier. The reflectivity of the target and barrier

are similar in order of magnitude, both reflect a few percent of

the incident radiation. In our example, at 2 THz, this means that

we would have to detect the signal from the target in the pres-

ence of a signal from the barrier which is 120 dB stronger! Such

a small signal will be completely lost in the clutter from irreg-

ularities in the barrier.

A pulsed or radar based terahertz system can provide some

degree of separation of the target signal from the barrier signal

based on its range. The barrier will generally be in contact withthe target, however, so that the two signals overlap in the time

domain and are unable to be separated effectively.

Since, as noted above, operation at 2 THz is required in order

to see the features in a number of explosives, barrier attenuation

represents a very serious difficulty for this application.

IX. SCATTERING

Since formulated explosives are often made of granular ma-

terial or crystallites with particle sizes comparable to terahertz

wavelengths [16], scattering is another important phenomenon.

The effect of scattering from rough surfaces on terahertz reflec-

tion spectra has been the subject of a number of recent theoret-

ical and experimental studies [32][35]. Scattering causes four

effects, each of which degrades the reflection spectrum:


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Fig. 8. Illustration of the artifacts caused by surface roughness in terahertz re-flection spectroscopy. (a) Computed reflection spectra from a random surfacewith different degrees of roughness [32]. (b) Measured spectra from roughenedmetal surfaces (created by evaporating copper on abrasive paper of differentgrades) showing the high frequency roll-off [37].

Although it can be detected in all directions, scattered

power in so-called diffuse reflection is typically2030 dB weaker than specular reflection from a smooth

surface [28], [36].

Reflected power decreases with frequency as the phase dif-

ference between signal contributions from different parts

of the surface increases. At normal incidence, this acts

on average as a low pass filter with a Gaussian roll-off,

the cut-off frequency decreasing as the surface becomes

rougher [37].

If the incident wave is at an oblique angle, the shape of the

average reflection spectrum changes to one with a broad

peak, whose frequency depends on the incident angle (and

the surface roughness) [33]. The above effects are those obtained by averaging over a

number of surfaces. Each particularinstance will be subject

to random fluctuations as shown in Fig. 8 [32].

The combined impact of these effects makes the interpretation

of reflection spectra from rough surfaces very difficult. If the

surface roughness and measurement geometry are known, and

if enough samples can be averaged to smooth out the random

fluctuations, then the effect of rough surface scattering can be

removed from the spectrum [36]. However, in practical detec-

tion tasks, the geometry and roughness are not known a priori,

and averaging incurs a penalty of increased detection time and

loss of spatial resolution.

Scattering by barrier materials is also an issue. Typical

clothing has fi bers, weave patterns and layer thickness all

on the same length scale as the terahertz wavelengths being

measured. This can lead to scattering from surface reflections,

scattering during transmission and to multiple reflection or

etalon effects between layers. Scattering in terahertz transmis-

sion spectroscopy is well known to affect observed spectra

unless particle sizes are small compared with the wavelength

[39], [40]. The periodic nature of clothing weaves can also be

expected to cause diffraction effects. These can be seen in some

of the studies on terahertz transmission of clothing [22]. Any

of these frequency dependent fluctuations in barrier materials

will, of course, imprint themselves on the observed reflection

spectra of the target material.

X. SYSTEM ASPECTS

Generation and detection at terahertz frequencies is not

straightforward. The maximum frequency of an electronic

device is inversely proportional to the transit time, i.e., the time

taken for a charge carrier to travel across the device. Fabrication

limits and electrical breakdown mechanisms mean that it ishard to make devices operate above a few hundred gigahertz

and the power of electronic sources falls as or . The

alternative method is to use approaches from optics, but the

low photon energy at terahertz frequencies compared

with thermal effects (kT) limits the performance of optical

techniquesconsequently, this region of the spectrum has been

known as the terahertz gap. A wide variety of technologies,

both optical and electronic, have been considered to provide

terahertz sources and detectors as described in the reviews by

Chamberlain [41] and Siegel [42].

The most mature technology, and the one used in most com-

mercial terahertz imagingand spectroscopy systems, is the time-domain system which produces broadband pulses of electro-

magnetic radiation with a spectrum from below 100 GHz to

4 THz and above. The total power produced is typically in the

100 nW1 W range. Coherent detection of the incident THz

radiation in a photoconductive antenna circuit with respect to a

reference pulse measures both the amplitude and phase of the

detected THz wave. An optical delay line is used to match the

distance between the reference and detected waves to maintain

coherence. This provides an engineering challenge if the dis-

tance to the target can vary by more than a few centimeters.

Pulsed terahertz systems can achieve a SNR of 6080 dB at fre-

quencies up to 1 THz although this rolls off at higher frequen-cies. The Fourier transform of the detected time domain pulse is

divided by that of the source to obtain the frequency spectrum.

The pulses contain information on both the spectral and the spa-

tial structure of the target, encoded onto a single waveform. In

some cases this can be an advantage, as it may be possible to re-

move the effect of reflection from the front surface of clothing

by time-gating the signal. However, in general, if the structure

of the target is more complex, then it is not possible to decon-

volve the two effects and artifacts can be introduced into the

spectrum [29].

Another approach is to use photomixing to generate THz

waves from the difference frequency between two near-IR diode

lasers [43], [44]. Tuning one of the lasers enables the THz dif-

ference frequency to be swept across a range of up to 2 THz.


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Coherent homodyne detection is possible using a second pho-

tomixer as a detector [45]. These systems have the advantage

of using inexpensive, compact diode lasers rather than the more

costly femtosecond pulsed lasers of the time domain systems.

At low frequencies they have similar performance, but above

1 THz the power available falls off more rapidly.

Other sources can be used which provide more power.

THz-wave optical parametric oscillators (OPOs) have an output

power up to a few milliwatts and are tunable over a range of

0.73 THz [46]. Banks offixed-frequency sources could also

be used for spectroscopy, including terahertz quantum cascade

lasers (QCLs), electronic multiplier sources and vacuum tube

sources such as backward-wave oscillators and travelling wave

tubes. QCLs can produce power of a few hundred milliwatts

and frequencies down to 1 THz [47], although low frequency,

high power sources require low temperatures. A number of

sources would also be needed to cover the frequency range with

sufficient resolution to differentiate between materials. Detec-

tors for these sources include heterodyne detectors based on

Schottky diode mixers, and broadband power detector devicessuch as pyroelectric detectors and bolometers, typically cooled

to liquid helium temperatures to increase sensitivity. Power

detector systems do not have the sensitivity advantage of the

coherent detection used in time domain and photomixer-based

systems, so despite the increased source power available,

the overall performance of these combinations is fairly sim-

ilar and again a SNR in the range 6080 dB represents the

state-of-the-art at frequencies of 1 THz and above.

An alternative approach, to avoid the issues of atmospheric

water vapor absorption in stand-off detection, is to propagate

the optical pulse to the target and generate terahertz waves

locally. This is known as air photonics, and is achieved byusing a so-called four-wave-mixing approach between a NIR

frequency fundamental pulse and a second beam at its second

harmonic focused down so as to break air down into a plasma.

Mixing by a third order optical process generates terahertz

waves. Detection can be achieved by a similar technique

[48][50]. This is an ingenious approach, although it should be

noted that other barrier attenuation effects remain and, unlike

all the other techniques discussed here, it directs potentially

harmful laser radiation at the target, rather than just terahertz

waves.

XI. SPECTROSCOPIC DETECTIONSUMMARY

A number of groups have carried out experiments and de-

veloped proof-of-principle prototypes aimed at concealed ex-

plosives detection. Early work addressing certain aspects of the

problem was quite successful. Pulsed terahertz systems have

been used to demonstrate proof-of-principle stand-off explo-

sives detection in laboratory conditions. Using specular reflec-

tion from smooth flat samples, features in RDX-based explo-

sives have been detected through several layers of thin clothing

in real time at a distance of 1 m [25]. The atmospheric water

vapor spectrum was removed using a reference measurement.

In principle, longer distances are possible. Zhong [26]

demonstrated that a parallel collimated terahertz beam can

travel up to 30 m in air and then be focused down, with a

remote optical system close to the target, to collect specular

Fig. 9. Results from proof-of-principle detection experiments. (a) Reflectancespectrum of smooth RDX based sample behind cotton clothing, and (b) the firstderivative of the reflectance [25]. (c) Diffuse reflection measurements of RDX,polyethylene and flour samples and (d) RDX spectra taken behind a single bar-rier layer [51].

reflection spectra, again of RDX-based explosive. Liu [28],

[51] measured diffuse reflection spectra of RDX explosive,

both bare and covered with a single thin barrier layer, with

the signal collected over a large solid angle. Results, after

averaging several measurements, are shown in Fig. 9.


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These experiments show that real-time point spectroscopic

measurements can be used for detection in idealized conditions

at distances of a few meters at frequencies up to about 1.5 THz.

On thevery strong 0.8 THz RDX peak specular reflection allows

detection through three or four layers of thin cloth, while the

weaker diffuse reflection was only detected though a single thin

layer.

Moving from these initial experiments towards more realistic

scenarios has proved challenging. As discussed in the previous

sections, there has been continued incremental progress in ter-

ahertz sources and detectors, automatic removal of water vapor

absorption effects, and in the understanding of the effects of

scattering. However, the author is not aware of reports of any

work in the last five years which have been able to demonstrate

detection in more realistic conditions, such as diffuse reflection

from explosives concealed under thicker barriers and detection

based on features at 2 THz or above. In order to do this, the fol-

lowing problems need to be addressed:

The limited region of the terahertz spectrum that is ac-

cessible due to rapidly increasing absorption by clothing

and other barriers at higher frequencies. This reduces the

number of features in the spectra of explosives that can

be used for detection, reducing the specificity of the tech-

nique.

Spectra in the terahertz region are relatively broad, making

the distinction between target and benign materials diffi-

cult, particularly when both may be complex mixtures of

different substances.

The need to work in reflection geometry, where reflection

spectra have much lower contrast than in absorption.

Real targets are unlikely to be flat, smooth and aligned

to give strong specular reflections. Practical systems willneed to work with glint from curved surfaces or the weak

and variable diffuse reflection caused by scattering from

rough surfaces such as granular or powdered explosives.

Water vapor in the atmosphere causes strong absorption

lines which need to be removed in order to interpret the

spectra.

Scattering, etalon and diffraction effects from clothing

which has fi ber diameters, thickness or weave patterns

with similar length scales to the terahertz wavelengths

being measured.

The need for frequency agile, room temperature, efficient

terahertz sources and sensitive detectors.A deployable system needs to address all of the above issues

simultaneously, and do so in conditions where there is limited

control or knowledge of the geometry of the target or the barrier

materials.

Our view is that while each of the effects we have described

can individually be mitigated by developments in source and

detector power or signal processing algorithms, their combina-

tion will always mask the rather weak spectroscopic signatures

which exist in the usable part of the spectrum. Unfortunately

therefore, it seems unlikely that terahertz spectroscopic detec-

tion will be able to provide a practical method for detecting con-

cealed explosives and that this quest may just be a bridge toofar.

While people screening for concealed explosives may not

be achievable, there may be other applications where terahertz

spectroscopy of explosives does have a role. The low power

and energy of terahertz photons, unlikely to cause an unwanted

fire or detonation, may be useful for identification of small

visible quantities of explosives in first responder applications

[52], [53]. Transmission terahertz spectroscopy has been devel-

oped as a tool for detecting small quantities of explosives and

(especially) drugs of abuse in mail [54]. The fact that terahertz

spectroscopy is effective at distinguishing between different

polymorphs of explosives may also have forensic and detona-

tion science applications in the laboratory [55], [56]. Finally,

vapor-phase terahertz spectroscopy has been considered as an

alternative to other analytical techniques such as ion-mobility

spectrometry and mass spectroscopy in trace detection systems,

where trace particles of explosives are collected by swabbing

and then evaporated by heating to produce a vapor for analysis.

A few vapor-phase measurements of explosives have been

published [65], [66]

XII. IMAGING SYSTEMS

Imaging applications of terahertz in security appear a more

promising alternative. As previously discussed, at frequencies

below 1 THz, penetration through clothing is relatively good.

Hidden objects are identifiable as anomalies which may be sus-

picious by their shape, size or position and which should lead

to further inspection. Systems are already in use in airports for

short range imaging at millimeter-wave frequencies. The use of

higher frequencies enables image resolution to be maintained at

stand-off distances without increasing system size. Several pro-

totypes and systems, both active radar and passive imagers, are

under development in the submillimeter-wave frequency range

(300600 GHz) which offers a good balance between resolu-

tion, clothing penetration and availability of source and detector

technology [59][63]. See also thepaper by Cooper in this issue,

which focuses on imaging [64].

In addition to broadband and single frequency imaging

systems, it may be possible to make use of the differences in

frequency dependence of absorption, emissivity and reflectivity

of different materials to help distinguish them, despite the

fact that they do not allow full materials identification. In a

laboratory experiment, Kemp [65] showed how multi-spectral

imaging can improve the interpretation of terahertz cross sec-

tional images (see Fig. 10). The images, from a pulsed terahertzimagers show a B-scan depth slice through a number of test

objects covered with cloth. The black and white image is a

broad-band image of the object. The multi-spectral image sep-

arates the image into three frequency bands (0.50.7, 0.71.0,

and 1.03.0 THz) which are colored red, green, and blue,

respectively, and then recombined into a false-color image.

This image reveals differences between the test objects due to

the way they absorb high frequencies, as well as the texture in

the cloth largely due to scattering.

Grossman [66] developed a rotating frequency-selective

filter wheel which could be used in front of a broad-band mi-

crobolometer imager for multispectral imaging in the frequencyrange 300900 GHz. Other work in this area is in progress.


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Fig. 10. Broadband and multispectral terahertz cross-sectional images of anumber of objects hidden beneath two layers of cloth [65].

Multispectral imaging need not be restricted to millimeter-

wave and terahertz frequencies: infrared thermal imaging can

be used to infer the presence of hidden objects through the waythey block heat conduction to the surface. Infrared and vis-

ible wavelength imaging can help distinguish the outline of the

body from the background and provide surface detail to fa-

cilitate interpretation. The fusion of visible, infrared and mil-

limeter-wave images has been explored since the earliest days

of millimeter-wave imaging for security and is used in a number

of commercial products. Fig. 11 shows an example of image

fusion [60].

XIII. CONCLUSION

The initial promise of terahertz spectroscopy for concealed

explosives detection has led to a considerable number of re-search projects to develop and determine its potential as a prac-

tical threat detection technique. We have reviewed this work and

highlighted the problems which need to be overcome in the de-

velopment of a deployable system. Our view is that the relative

weakness of spectral features in reflection, the limited frequency

range available due to absorption by clothing, fluctuations and

distortions in the spectra due to scattering from rough surfaces,

attenuation by water vapor in the atmosphere, and other effects,

all combine to make it very difficult for terahertz spectroscopy

to be developed as a practical technique for this application.

Whilst it impossible to say that it will never be done, poten-

tial system developers should develop performance models to

explore the capabilities and limits of their proposed approach,

taking into account all of the factors identified in this paper.

Fig. 11. Image fusion example from the Safe VISITOR project [60].

At frequencies up to 1 THz where clothing is still relatively

transparent, concealed object imaging systems are being de-

veloped. Since the terahertz properties of materials vary with

frequency in different ways, multispectral imaging, although it

does not identify substances, may be a valuable technique for

distinguishing between objects, in the same way that color pho-

tography reveals details not visible in a black-and-white picture.

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[63] K. B. Cooper, R. J. Dengler, N. Llombart, A. Talukder, A. V. Panan-gadan, C. S. Peay, I. Mehdi, and P. H. Siegel, Fast, high-resolutionterahertz radar imaging at 25 meters, in Proc. SPIE, 2010, vol. 7671,p. 76710Y.

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Michael C. Kemp (M08) read Natural Sciences atDowning College, Universityof Cambridge,UK, andreceived the Ph.D. degree in radio astronomy fromthe University of Cambridge, Cavendish Laboratory,in 1979.

He is founder and Director of Iconal TechnologyLtd. which carries out research and provides consul-tancy in new and emerging technologies for security.He is known internationally as an expert on stand-offdetection of explosives and works as an adviser to anumber of government programs in the UK, EU, and

USA. He has worked in a number offields, including speech recognition, imageprocessing, medical imaging, and the application of millimeter wave, terahertzand other imaging technologies to security screening.

Dr. Kemp is a Chartered Engineer, a Member of SPIE and a Member of theInstitution of Engineering and Technology.

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