University of SheffieldDepartment of Physics and Astronomy

Problem Solving in Physics

T e r a h e r t z T e c h n o l o g ya n O v e r v i e w

Matthias PospiechTU Kaiserslautern

Germany

March 2003

Contents i

Contents

1 Basics 1

2 Sources for THz radiation 1Normal THz sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Broadband sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Narrowband sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3 Terahertz detectors 4

4 Spectroscopy and Imaging systems 5

5 Terahertz Imaging 5

6 Properties of THz radiation 6Water sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Safety / Medical implication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Theoretical prediction of water absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

7 Applications 7polar molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7material characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7quality control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7study of historical and archaeological work . . . . . . . . . . . . . . . . . . . . . . . . . . . 8biology (botany) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8biomedical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8security checks (Airport) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

8 Commercial Products 9

9 Conlusion 9

List of Figures

1 the Terahertz Gap (from Nature [1]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 relativistic electrons diffracted by a magnetic field (from Nature [3]) . . . . . . . . . 23 structure of Köhler’s quantum cascade terahertz laser (from Nature [2]) . . . . . . . 44 Illustration of a THz-TDS system (from Nature [2]) . . . . . . . . . . . . . . . . . . . 55 watercontent based image of a leaf (from IEEE Journal [5]) . . . . . . . . . . . . . . 86 terahertz image of men with hidden knife (from Spiegel Online[14] ) . . . . . . . . . 87 Teraview’s prototype »TPI-Scan« for medical imaging (from Article [9]) . . . . . . . . 9

This paper is available on http://www.matthiaspospiech.de/studium/artikel/

1

Abstract

New advances in different technologies havemade the previously unused terahertz fre-quency band accessible for imaging systems.Applications run from detecting weapons con-cealed underneath clothing (airports), prod-uct inspection (industry), spectroscopy (chem-istry, astronomy), material characterization(physics) to detection of cancer and caries.Most of these imply the use of terahertz imag-ing systems. This is such a new field that re-searchers around the world race to build thefirst practical system. This paper tries there-fore not only to provide a brief overview overthe imaging technology, but also over the wholerange of current systems and research in tera-hertz technology.

1 Basics

The electromagnetic range that is used is veryvast. At low frequencies end we have radiowaves up to millimetre waves, and at the otherend we have optical waves down to the far in-frared. Technologies have been developed forboth ends of the spectrum which we use in ev-eryday applications. But the terahertz region:

0.1− 10 THz 1 THz = 1012 Hz

with wavelength of 30µm to 3mm has remainedlargely underdeveloped, despite the identifi-cation of various applications, in particularTerahertz Imaging and others which will bediscussed later on.

Figure 1: the Terahertz Gap (from Nature [1])

It is possible to produce effectively radiationin the low frequency region (microwaves) withoscillating circuits based on high-speed tran-sistors and at high frequencies (visible spec-trum) with semiconductor lasers. But tran-sistors and other electric devices based on

electric transport have in principle a limit atabout 300 GHz, but are practically limited toabout 50 GHz, because devices above this areextremely inefficient [1], and the frequencyof semiconductor lasers can only be extendeddown to about 30 THz.Thus there is a region in between where bothtechnologies do not meet. This region is oftenreferred to as the terahertz gap.

2 Sources for THz radiation

Till this day the lack of high power, portable,room temperature THz sources is the mostsignificant limitation of THz systems.For this reason this is a very lively researchfield. Some very promising new approacheshave the potential to bring terahertz technol-ogy a step further to everyday applications.But these sources are still not fully developedtechnologies.So this article tries to provide an overviewover the currently used sources and these newapproaches.Further one in this section we divide betweenincoherent thermal sources, broadband pulsed(T-Ray) techniques, and narrowband continuous-wave methods.

Normal THz sourcesTHz radiation is naturally emitted by all bod-ies. The blackbody radiation in this spectralrange, below the far infrared, is comparativelyweak - lower than 1µW per cm−3. Sources likelight bulbs in the visible spectrum are there-fore unsuitable.

Broadband sourcesFollowing reference [2] most techniques areproviding broadband pulsed THz sources basedon the excitation of different materials with ul-trashort laser pulses.These are photocarrieer acceleration in photo-conducting antennas, second order non-lineareffects in electro optic crystals, plasma oscilla-tions, and electronic non linear transmissionlines.Unfortunately most of the different technolo-gies have very low conversion efficiencies (nano

2 2 Sources for THz radiation

to micro watts compared to about 1 W powerfrom the optical source).In the photoconduction approach a photocon-ductor (GaAs, InP) is illuminated with ultrafast laser pulses (with photon energy greaterthan the Bandgap of the material) to createelectron hole pairs. An electric field of about10V/cm is generated in the semiconductor byapplying a DC voltage. Then the free carriersaccelerate in the static bias field and form ashort photocurrent. Because of the accelera-tion this moving electrons radiate electromag-netic waves.According to [2] these photoconductive emit-ters are capable of relatively large averageTHz powers of 40 µW and bandwidths as highas 4 THz (see also references mentioned in theindicated article).This source operates with comparatively lowpower, but the beam is stable and coherentwith well known temporal characteristics.Hence it is used for spectroscopy with highspectral resolution and excellent signal-to-noise ratio and for imaging technologies.Another approach is the Optical rectification.Here again we use ultrafast laser pulses, butwe make use of the non-linear properties ofmaterials. This means that the optical beamitself is the origin of our terahertz radiation.These non-linear effects arise when one illumi-nates a crystal with high intensities.The required lower energy photons are achievedthrough a process called down conversion.This means that one incoming beam splits intotwo outgoing frequencies of lower frequencies.With the condition of conservation of energy(ωin = ωout1 + ωout2) the output frequency is nat-urally not defined. We get therefore a rangeof frequencies. (for more information see forexample [15])According to [2] research in this field has fo-cused in the past on materials like GaAs, ZnTe,and organic crystals.This process provides terahertz radiation onlywith very low efficiency, but has the advantageof very high bandwidths. Frequencies up to41 THz have been accomplished by one group(see therefore reference 23 in [2]).

First high power broadband THz sourceUntil recently there was no high power broad-band THz source, but in [3] Carr et. al. de-scribe a new method using energy recoveredlinacs1 (ERL) at the Jefferson Laboratories2 toaccelerate electrons to relativistic energies.As before this method begins with pulsed laserbeams, but uses them for photoemission tocreate short bunches of electrons (∼ 500 fs) infree space. The energy recovered linac bringsthese bunches of electrons to relativistic ener-gies (∼ 40MeV).As long as the electrons travel in a straightline they do not emit light. After passing throughthe linac system the electron beam is diagonalaccelerated by a magnetic field with a bendingradius of 1m , which produces the THz radia-tion as synchrotron radiation.

Figure 2: relativistic electrons diffracted by a mag-netic field (from Nature [3])

This process produces coherent THz radia-tion with an average power of nearly 20 Watts,which is several orders of magnitudes higherthan any other source.

Narrowband sourcesThe techniques under development range fromupconversion of radio frequency sources to dif-ferent kinds of lasers, including gas lasers,free electron lasers, and in particular quan-tum cascade lasers.One technique to generate (low power) contin-uous wave THz radiation is through upconver-sion of lower frequency microwave oscillators.Frequencies up to 2.7 THz have been demon-strated3

Another common source are gas lasers. The

1linear accelerator2Jefferson Laboratory Newport News, Virginia,

http://www.jlab.org/3see reference 26 in [2] for more information

3

gases used are mainly methanol and hydrogencyanide. In this method a C02 laser pumps alow-pressure gas cavity with one of the gasesmentioned, which lasers at the gas moleculesemission lines.These frequencies are not continuous tuneableand require large cavities and high (kilowatts)power supplies with only output power of themagnitude of milli Watts.Extremely high power THz emission havebeen demonstrated using free electron laserswith energy recovering linacs.Free electron lasers use a beam of electronbunches of high velocity propagating througha vacuum. This bunches of electrons pass aspatial varying magnetic field, which causesthe electrons to oscillate and emit photons.Mirrors confine the electrons to the electronbeam line to form the gain medium for thelaser.But such systems are huge in costs and size,and require often a dedicated facility. Howeverthey can generate continuous or pulsed waveswith a brightness several orders better thanany other source.Another highly awaited source for THz radi-ation are semiconductor lasers. In the pastthese lasers have revolutionist applications inindustry because of their small size, low costs,and high efficiency. Such a compact system forTHz radiation is still missing.The first terahertz laser was demonstratedover 20 years ago. This laser was based onlightly doped germanium at cryogenic tem-peratures under crossed electric and magneticfields4. These lasers are tuneable through theapplied magnetic field or external stress. Butthey have many inherent limitations such aslow efficiency, low output power, and the needfor cryogenic cooling.Another approach to build semiconductor lasersin the terahertz region is based on the newertechnique of a Quantum Cascade Laser. Theselasers were first demonstrated in 1994.A Quantum Cascade Laser consists of periodiclayers of two semiconductor materials, whichform a series of coupled quantum wells andbarriers with a repeating structure. The wells

4see reference 35 in [2] for more information

and barriers are usually nanometer thick lay-ers of GaAs between potential barriers of Al-GaAs. Quantum confinement within the wellscauses the conduction bands to split into anumber of distinct sub bands5. By transitionof electrons from a higher state to a lowerstate in the well light is emitted. As the differ-ence between the energy levels is determinedby the thickness of the layers, the producedfrequency can be chosen by design of the lay-ers.Unfortunately these lasers operate only atvery low temperatures. The energy spacingbetween the intersubbands is about ∆E ∼0.004 eV. Compared to room temperature kBT ∼0.025eV is this very small. This means thatif kBT is not lower than ∆E electrons are ex-ited into higher subbandstates. In a quantumcascade laser the subbands couple from thelower state to the higher state of the follow-ing unit. If the electrons are now all in upperstates they cannot jump anymore through thestairs of the well structure and cannot be usedfor the laser anymore.An interesting and very descriptive explana-tion of quantum cascade lasers can be foundon the homepage6 of the bell laboratories.

First Terahertz Quantum Cascade laser(Köhler et. al.)In [4] Rüdiger Köhler and Alessandro Tredicucciof the Scuola Normale Superiore in Pisa andcolleagues in Turin and Cambridge give areport on a Terahertz Laser build up with aQuantum Cascade structure. This article (2ndMay 2002) attracted much attention in physicsand science related magazines [1, 11, 12, 13]As Köhler mentions in his letter in Nature[4] electroluminescence devices have been re-ported by several groups, but laser action hadnot been possible in THz wavelength before.Whereas they developed a laser, which emits asingle mode at 4.4 THz and shows high outputpowers of more than 2 mW up to 50 K.In [1] Sirtori C. summarises the problems thatarise with a laser in the Terahertz region:"Like other semiconductor lasers, Köhler’s

5see book [15] for more details6http://www.bell-labs.com/org/

physicalsciences/projects/qcl/qcl.html

4 3 Terahertz detectors

device is composed of an active region andan optical waveguide that confines the radia-tion. But these two fundamental elements ofa laser face complete different demands in theterahertz region. The main problems are theintrinsic optical losses induced by the free elec-trons in the material, the large size of the opti-cal mode - imposed by the wavelength and theshort lifetime of the excited state of the lasertransition, in which the electrons accumulateto create the population inversions necessaryfor lasing."This laser consists of 104 repetitions of thebasic unit which comprises seven GaAs quan-tum wells (shown in figure 3) separated byAl0.15Ga0.85As barriers with the active regionconsisting of three closely coupled quantumwells. The total structure consists thus of atotal of over 700 quantum wells which weregrown by molecular beam epitaxie (MBE).

Figure 3: structure of Köhler’s quantum cascadeterahertz laser (from Nature [2])

More details about the layout and their ap-proaches to overcome the problems with semi-conductor lasers in this region (as mentioned)are available in Köhler’s article in Nature [4].These results are very promising althoughthis laser operates only with cryogenic cooling.If they achieve to improve the design and canoptimise fabrication, so that this semiconduc-tor lasers works at liquid nitrogen tempera-tures, this would be a very significant improve-ment in terahertz technology.

3 Terahertz detectors

The detection of THz radiation necessitatesvery sensitive methods, as sources come withlow output power and the thermal backgroundradiation is comparatively high.For broadband detection direct detectors arebased on thermal absorption commonly used.These systems require cooling to reduce thethermal background. Systems common usedare helium cooled silicon, germanium, andInSb bolometers.Bolometers measure the incidented electro-magnetic radiation through absorption andthe resulting heating. The heating in turn ismeasured through the change in resistance.Extremely sensitive bolometers are based onthe change of state of a superconductor suchas niobium.If high spectral resolution is required, hetero-dyne sensors are used. In these systems thefrequency of interest is produced by a local os-cillator and heterodyned with the external sig-nal. The downshifted signal is then amplifiedand measured.For pulsed THz detection in THz Time Do-main Spectroscopy systems (see section 4) co-herent detectors are required. The electro opticeffect in birefringent crystals is used for themeasurement of the THz beam.The electric field of the THz radiation modu-lates the birefringence of the sensor crystal.This in turn modulates then the polarisationof the optical beam passing through the crys-tal. This modulation can be measured to findthe amplitude and phase of the applied elec-tric field.Very thin crystals and very short laser pulsesallow the detection of radiation in the THz re-gion.Photoconductive antennas are widely used forpulsed detection - the structure is identicalto the photoconductive antennas earlier dis-cussed (see page 2), but here the appearingcurrent is measured.

5

4 Spectroscopy and Imaging systems

It should be mentioned here that one can ma-nipulate THz beams with mirrors and lensesas with light in the visible spectrum, but dif-ferent materials have to be used. These areopaque for our eyes, but transparent for THzradiation.Two used materials are high resistivity sili-con and high-density polyethylene. The formerhas no absorption or chromatic dispersion overthe whole THz range, but has a high refrac-tion index (≈ 3.42) and thus relatively largeFresnel losses. The latter has lower Fresnellosses, but has some small absorption above 1THz, and a resonance at ≈ 2.2 THz [7].According to [2] Fourier Transform Spectros-copy (FTS) is the most common technique forstudying molecular resonances. With this tech-nology one can characterise materials fromTHz to infrared frequencies.The sample is placed in an optical interferome-ter and illuminated with a broadband thermalsource. Of interest is the path length differ-ence of one of the interferometer arms. Theinterference signal is then detected with anhelium cooled bolometer. The Fourier transfor-mation yields to the spectral power density ofthe sample. The main disadvantage of FTS isits limited spectral resolution.If a much higher spectral resolution is requiredone can use a narrowband system with a tune-able THz source or detector.This technique is used in passive systems formonitoring thermal emission lines of molecules,particularly in astronomy applications [2].Another more recent technology is the THzTime Domain Spectroscopy (THz TDS). Thismethod of measurement is also termed Tera-hertz Pulsed Imaging (TPI) and as the namesuggests the dominant method of imaging.In this system the pulsed optical beam createdby an ultrafast laser is splitted into a probebeam and a pump beam. The pump beam is in-cidented on the THz emitter (the THz source)to generate picosecond terahertz pulses. Theterahertz radiation is collimated and then fo-cused on the sample with parabolic mirrors.After transmission through the target the

beam is collimated and refocused on the THzdetector.The detectors used for TDS are described insection 3. These detectors can measure theelectric field coherently.The optical probe beam is used to gate the de-tector and to measure the THz electric field in-stantaneously. At this the optical delay stage(computer controlled) is used to measure thetransmitted terahertz pulse profile at a dis-crete number of time points to provide tempo-ral information.Figure 4 illustrates such a system.

Figure 4: Illustration of a THz-TDS system (fromNature [2])

Following Ferguson B., Zhang Xi-Cheng [2]has this technology disadvantages comparedwith Fourier transform spectroscopy describedbefore. They say that the spectral resolutionwas worse than with narrowband techniquesand its spectral range much less than of FTS,but it had some other advantages that madeit so important: The transmitted electric fieldis measured coherently. Therefore high timeresolved phase information can be received.

5 Terahertz Imaging

Terahertz Imaging or also called T-Ray Imag-ing is based on time domain spectroscopy (TDS).The possibility to detect the radiation coher-ently makes it possible to record not only theintensity, but also the time resolved amplitudeand phase of the electric field. In turn thisleads to the possibility of obtaining a spectrumby Fourier transformation of the time domainsignal.

6 6 Properties of THz radiation

In article [8] from the Center of Medical Imag-ing Research (CoMIR) is reported that Fouriertransformation methods have been used to ac-cess the spectral information from the timedomain signal, but these are not capable of ad-dressing temporal information of frequencycomponents.Alternatively wavelet methods are used for thebroadband THz pulses. These provide addi-tional information about the temporal informa-tion of spectral content.Wavelet methods are based on different math-ematics, compared to Fourier transformation.Their original purpose lies in the field of imagecompression. More information about signalprocessing algorithms used in this field can befound in [9], which seems to be from CoMIR aswell7.Images are obtained with THz-TDS by spatialscanning of the object for imaging. This is per-formed by using raster scanning of either theterahertz beam or the sample itself.An imaging system in which the beam is re-flected off of the sample, rather than trans-mitted can be used for tomographic imaging.This means that the regions of reflection inthe sample are measured. This is possiblebecause the arrival time of the reflected THzwaves can be determined with an accuracy ofa few femtoseconds. This is less than the pulseduration, so the positions of reflecting surfacescan be determined with an accuracy of few mi-crometers, but under the condition that succes-sive reflections are well separated in time [6].The applications of tomographic imaging arelater discussed under applications on page 8.D.M. Mittleman says in [7] about THz-TDSimaging: "Because THz-TDS does not requireany cryogenics or shielding for the detector,it has the potential to be the first THz imag-ing system that is portable, compact and reli-able enough for practical applications in »real-world« environments". (As described in section3, the detectors measure an optical beam andthus do not need any kind of cooling).

7This paper seems not to be officially published

6 Properties of THz radiation

THz radiation has some properties that opena wide range of applications, particularly forimaging. On the other have materials veryinteresting properties in this frequency range:

Compared to microwaves, THz waves havemore energy so they can penetrate deeperand make sharper images due to their shorterwavelength. Additionally it is expected thatterahertz frequency radiation should be scat-tered less than visible and near infrared fre-quencies, as the amount of Rayleigh scatteringdecreases with the fourth power of the wave-length.If one will illuminate materials with THz radi-ation they will show these characteristics:Polar liquids absorb strongly in the terahertzband. An example of such a liquid is water.Metals are opaque to terahertz radiation, whereasnon-metals such as plastics, paper-products,and non-polar substances are transparent. Di-electrics in constrast have characteristic ab-sorption features peculiar to each material.

Water sensitivityAccording to [5] the minimum detectable wa-ter concentration is given by

n · x ∼ 1016cm−2

n is the density of water molecules and x isthe length of the path traversed by the tera-hertz beam in the material.

Safety / Medical implicationImplications on living tissue are not expectedas T-Rays are non-ionising in contrast to x-rays or ultraviolet (UV) light. Moreover, Ter-ahertz signals are strongly absorbed by water,so that terahertz radiation cannot go throughliving tissue because of the high percentage ofwater in it.The article [9] nevertheless warns that it shouldbe taken into consideration that there is alsothe possibility of thermomechanical and ther-mochemical effects for the pulsed radiationused in terahertz pulsed imaging (TPI)/(TDS).A project8 that works on this issue is called

8http://www.frascati.enea.it/thz-bridge/

7

»THz-Bridge«.

Theoretical prediction of water absorp-tionOn one homepage of CoMIR9 [10] the scien-tists say that preliminary measurements ontissue penetration have showed that predic-tions of attenuation based on water contentalone have been incorrect.This group wants to develop a theoretical modelfor the absorption of terahertz radiation andcompare the results with experimental mea-surements.So their work will show how safe the use ofTHz imaging is for living tissue.

7 Applications

We use x-ray scanners to examine luggage.Hospitals are equipped with ultrasound scan-ners and MRI (magneto resonance imaging)machines. In industry x-rays are used for pack-age inspection and materials can be scannedfor defects with microwaves or ultrasound.But although these technologies are very suc-cessful they all have shortcomings.In some cases these old techniques can be re-placed by THz based methods, but there arealso applications, which are unavailable withother frequencies, but possible with THz dueto the characteristics of the materials as de-scribed before.Hence, many possible applications rely eitheron the extreme sensitivity of water or the abil-ity to propagate through common packagingmaterials or both.The list of possible applications is quite exten-sive. Therefore, the examples of applicationsshowed here are only a cutout and not a com-plete list:

polar moleculesWith THz radiation one has the ability to de-tect and identify most polar molecules in thegas phase. This application requires THz ra-diation with broad bandwidth and relies onthe fact that, as in the mid-infrared, manymolecules have characteristic "fingerprint"absorption spectra in the terahertz region. [5]

9Center of Medical Imaging Research

For these reason it was astronomy in the past,which required research in the THz field be-cause of the huge amount of information avail-able in space through THz radiation. [2]

material characterizationFor physics this new technology is very inter-esting because they can use it for characteri-zation of materials like semiconductors andlightweight molecules. The radiation can beused to determine the carrier concentrationand mobility of semiconductors10, and in thesuperconductor research it can be used to de-termine the parameters of superconductingmaterials. (See the article [5] from Mittlemanfor a description of the physics behind mate-rial characterisation with THz radiation).

quality controlTHz imaging systems are ideal for imagingdry dielectric substances including paper, card-board, thin wood, most plastics and ceramics,because these materials are relatively non-absorbing in this frequency range. And addi-tionally, these materials cannot be analysedwith optical frequencies, because they areopaque for them or with x-rays, which haveno absorption to provide contrast for imaging.Therefore spatially resolved transmission mea-surements can be used in the area of qualitycontrol of packaged goods, if the packagingconsists of one of the transparent materialslisted above.In many industries x-ray imaging is currentlyused for such tasks. T-ray imaging is thus a de-sirable alternative because of the health andsafety issues involved with ionising radiation.The sensitivity to water is important here aswell. To measure the water content of mois-ture sensitive products exist various methods,but these are not applicable if the content isin a cardboard or a plastic package. Obviously,THz radiation is predestined to be used here.D.M.Mittleman reports in his papers [5, 7]about various experiments in the field of qual-ity control applications.

10see references 48-50 in [2] for more information

8 7 Applications

study of historical and archaeologicalworkTerahertz imaging methods work with ex-tremely low powers, about 2 to 3 magnitudesof order lower than the blackbody radiation inthis frequency range. The radiation is there-fore non invasive to historical work, neither topaintings or paper in common nor to any kindof stone or metal. Thus this technique could beuseful in history, archaeology etc.In [7] D.M.Mittleman discusses the possibilityof using THz imaging for the investigation ofunderdrawings beneath paintings. This couldbe an excellent complimentary technology tothe mid infrared and x-ray based imaging sys-tems currently used for such studies.

biology (botany)This extreme sensitivity to water content is ofgreat interest here as a method of measuringthe water content of leaves on living plants.Currently there is no accepted non-destructiveprocedure for measuring the leaf water statusof a transpiring plant. [5] (see figure 5 as anexample)

Figure 5: watercontent based image of a leaf (fromIEEE Journal [5])

The cellular structure can be studied withTHz as well. But a fundamental limit here isthe resolution of current systems [2].

biomedicalIn the area of biomedical diagnostics we canmake use of THz tomographie. Although thereis a limited penetration depth of the radiationdue to the strong water absorption, whichexcludes the use of THz radiation in mostbiomedical research areas, it can be used toexamine tissue near the surface, in particularskin and teeth. On the other hand, the sensi-

tive to water enables the investigation of tis-sue hydration.This opens a range of applications includinganalysis of burn depth and severity, and detec-tion of skin cancer and caries.A reliable non-invasive probe of burn depthwould be of great value to physicians, who cur-rently have no such technology. In [7] simpleexperiments are explained which test the useof T-Rays for this application.The detection of skin cancer works very wellalthough from the medical part not very wellunderstood yet [14]. It seems that the cancercells have a distinct different proportion of wa-ter compared to healthy cells, so that simplythe higher absorption due to the water is mea-sured. Breast cancer detection could be an ap-plication as well, because of the lower watercontent of the tissue there.The detection of caries is another possible ap-plication. But there are still problems with theimaging technology which have to be solvedbefore this is ready for application.An overview over the imaging techniques andtheir problems with tooth imaging as one ex-ample can be found in [9].

security checks (Airport)At airports or other security critical placesdangerous non-metallic substances like ce-ramic knifes or plastic explosives now can bedetected with terahertz beams. This is possi-

Figure 6: terahertz image of men with hidden knife(from Spiegel Online[14] )

ble because T-Rays get through clothes, but

9

cannot get through the upper skin (becauseof the water content). Figure 6 illustratesvery clear how effective this imaging methodworks11. The British company Qinetiq hastested such a system on airports already [14].A picture12 of their camera can be seen ontheir homepage. This picture is similar to theone shown here.

8 Commercial Products

Many of the systems in use for research arelaboratory based and occupy an area of a fewsquare meters, but more compact and portablesystems are under development.An example is the product from TeraViewLtd., Cambridge, UK (www.teraview.co.uk )which is a prototype reflection system for usein dermatology. They launched their productin July of 2002. Teraview is a start up spunout of Toshiba’s Cambridge research labor-atory.

Figure 7: Teraview’s prototype »TPI-Scan« for medi-cal imaging (from Article [9])

Another commercially available system isthe T -Ray 2000 by Picometerix13, Ann Arbor,Michigan USA.

11Unfortunately is on this webpage no original source forthis image specified. The authenticity of the image cantherefore be querified.

12available on: www.qinetiq.com/technologies/sensors/

13www.picometrix.com/t-ray/

According to [9] there is another system un-der development by the Zomega TechnologyCorporation. An announcement can be foundon a homepage14 of the Army Research Office(USA).

9 Conlusion

The applications mentioned here show thatTHz imaging is desired by many differentparts of industry and research, so that it canbe expected that much efford will go into thisfield.But despite the number of potential applica-tions for THz imaging no technology is yet theideal way, though the recent advances couldlead to practicable and compact systems.Hence, reasearch in this field is going to bevery lifely and interesting in the future.

14www.arpo.army.mil/arowash/rt/sttr/st992abs.htm

10 References

References

[1] Sirtori C., Bridge for the terahertz gap,Nature 417, 132-133 (2002)

[2] Ferguson B., Zhang Xi-Cheng, Materials for terahertz scinence and technology,Nature 1, 26-33 (2002)

[3] Carr et. al., High power terahertz radiation from relativistic electrons,Nature 420, 153-156 (2002)

[4] Köhler R. et. al., Terahertz semiconductor heterostructure laser,Nature 417, 156-159 (2002)

[5] Mittleman D.M. et. al., T-Ray Imaging,IEEE Journal of selected topics in quantum electronics, Vol 2, No. 3 (Sep 1996)

[6] Mittleman D.M. et. al., T-Ray Tomography,Optics Letters, Vol 22, No. 12 (15 Jun 1997)

[7] Mittleman D.M. et. al., Recent advances in terahertz imaging,Applied Physics B: Laser and Optics (1999)

[8] James H., Fitzgerald A., Löffler T., Seibert K., Berry E., Boyle R,Potential Medical applications of THz Imaging,http://www.comp.leeds.ac.uk/jwh/pubs/miua2001.pdf (09 Jan 2003)

[9] Berry E., Boyle R, James H., Fitzgerald A.,Time frequency analysis in terahertz pulsed imaging,http://www.vcl.uh.edu/$\sim$pavlidis/Restricted/Chapter5.pdf (09 Jan 2003)

[10] CoMIR : Center of Medical Imaging Research,The interaction of terahertz tadiation with biological tissue,http://www.comp.leeds.ac.uk/comir/research/terahertz/GRN39678.html (20 Nov2002)

[11] physicsweb.org, Semiconductor plug terahertz gap,http://physicsweb.org/article/news/6/5/5 (20 Nov 2002)

[12] www.nature.com, Filling the terahertz gap,http://www.nature.com/physics/highlights/6885-3.html (22 Nov 2002)

[13] www.nature.com, Laser vision plugs gap,http://www.nature.com/nsu/020506/020506-5.html (22 Nov 2002)

[14] Spiegel Online, Splitternackt auf dem Monitor,http://www.spiegel.de/spiegel0,1518,druck-223301,00.html (20 Nov 2002)

[15] Dr. M. Fox, Optical Properties of Solids, New York: Oxford University Press (2001)