12 ieee transactions on terahertz science … ieee transactions on terahertz science and technology,...

12 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 4, NO. 1, JANUARY 2014

Field Exposure and Dosimetry in theTHz Frequency Range

Thomas Kleine-Ostmann, Christian Jastrow, Kai Baaske, Bernd Heinen, Michael Schwerdtfeger, Uwe Kärst,Henning Hintzsche, Helga Stopper, Martin Koch, and Thorsten Schrader, Senior Member, IEEE

Abstract—With a growing number of applications utilizing THzradiation appearing on the market the question of health protec-tion against non-ionizing electromagnetic fields arises in this fre-quency range, as at lower frequencies before. To date, about 50independent empirical studies on living organisms, model systemsand cells have been performed to clarify bio-electromagnetic inter-action in the THz frequency range. Many of these studies find be-havioral effects or effects on the cellular level, even at non-thermalexposure levels, while others do not report effects other than ther-mally induced damage. We discuss the general challenges in per-forming reliable field exposure experiments in the THz frequencyrange and describe amethodology that was adopted in a large cam-paign searching for genotoxic effects of THz radiation in vitro.

Index Terms—Dosimetry of non-ionizing radiation, field expo-sure with sub-mm and Terahertz (THz) radiation, THz metrology,uncertainty analysis and traceability to the SI units.

I. INTRODUCTION AND BACKGROUND

D UE to the steadily increasing use of electromagneticwaves in the THz frequency range the question if human

exposure to this kind of radiation could be potentially dangerousbecomes more important. As in the frequency ranges below, it isstill not clear, whether non-ionizing radiation below the thermaldamage threshold could cause detrimental health effects. TheInternational Commission for Non-Ionizing Radiation Protec-tion (ICNIRP) limits the power flux density for general publicexposure for the frequency range between 2 and 300 GHz to 1mW cm . This safety limit is based on proven thermal effects,which have been examined extensively in the microwave regionso far, only, and includes a safety factor [1]. Above 300 GHzlimits for general public exposure do not exist and safety limitsapply to laser radiation, only. Depending on the type of laserradiation, the safety limits are in the range between 1 mW cm

Manuscript received May 06, 2013; revised August 29, 2013; acceptedNovember 17, 2013. Date of publication January 09, 2014; date of currentversion January 17, 2014. This work was supported by the German FederalOffice for Radiation Protection (Bundesamt für Strahlenschutz—BfS) underProject StSch 4533—Genotoxic Effects of Terahertz Radiation in vitro..T. Kleine-Ostmann, C. Jastrow, K. Baaske, and T. Schrader are with the

Physikalisch-Technische Bundesanstalt (PTB), 38116 Braunschweig, Germany.B. Heinen, M. Schwerdtfeger and M. Koch are with the Department of

Physics, Philipps-Universität Marburg, 35032 Marburg, Germany (e-mail:[email protected]).U. Kärst is with the Helmholtz Centre for Infection Research, 38124 Braun-

schweig, Germany (e-mail: [email protected]).H. Hintzsche and H. Stopper are with the Institut für Pharmakologie und

Toxikologie, Universität Würzburg, 97078 Würzburg, Germany (e-mail:[email protected]).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TTHZ.2013.2293115

and 100 mW cm [2]. An initial study by Berry et al. [3] showsthat human exposure due to THz imaging systems can usu-ally be assumed to be below 1 mW cm . However, possiblenon-thermal effects of non-ionizing radiation are still under dis-cussion [4]–[6]. In a theoretical study byAlexandrov et al., DNAbreathing dynamics that might open up DNA double strandsare discussed as possible cause for non-thermal effects [4]. Asseen as at much lower frequencies, many empirical studies onbio-electromagnetic interaction in the THz frequency rangefind effects on the cellular level or behavioral effects, even atnon-thermal exposure power densities [7]–[16]. One of the mostcomprehensive empirical studies on the interaction of biologicalmatterwithTHz radiation is theTHz-Bridge project [17].Koren-stein-Ilan et al. found an increased genomic instability in humanlymphocytes after exposure to 100 GHz radiation with powerdensities significantly below the safety limit of 1 mW cm(corresponding to 0.031 mW cm in the culture medium) [18].In order to verify and further investigate possible genotoxic

effects, the German Federal Office for Radiation Protection(Bundesamt für Strahlenschutz—BfS) initiated an experimentalcampaign in which two different types of skin cells (primarydermal fibroblasts (HDF) and a keratinocyte cell line (HaCaT))were exposed to continuous wave THz radiation at three distinctfrequencies of 106 GHz, 380 GHz, and 2.52 THz originatingfrom different sources. The cell monolayers, which are ad-herent at the bottom of the sample containers covered by culturemedium (Dulbecco’s Modified Eagle Medium—DMEM), wereexposed in a modified incubator from below with differentpower densities for 2 h and 8 h [19], [20]. To obtain validresults, the exposure parameters were monitored continuously,the exposure power densities were set traceable to the SI units[21], [22], sham exposures and positive controls were includedand a statistically significant number of cells were evaluatedin a blinded procedure. At 106 GHz field computations wereperformed that yielded an estimate of the specific absorptionrate (SAR) together with an estimate of the expected heatingof the cell medium. Temperature measurements confirmed thatheating is negligible at 1 mW cm ( 0.3 K) [19].Genomic damage on the chromosomal level was identified as

micronucleus formation [23] whereas DNA strand breaks andalkali-labile sites were quantified with the comet assay [24].Beyond the sham expositions with 0 mW cm the exposurepower densities were set with regard to the technical restric-tions of the exposure sources. Only at 106 GHz, exposure levelsof 1.96 mW cm significantly above the safety limit were fea-sible. Three independent exposure campaigns showed no indi-cation of genotoxic effects, neither as chromosomal damage nor

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KLEINE-OSTMANN et al.: FIELD EXPOSURE AND DOSIMETRY IN THE THZ FREQUENCY RANGE 13

TABLE IPUBLICATIONS BY THE AUTHORS DIRECTLY OR INDIRECTLY RELATED TO THE EXPERIMENTAL CAMPAIGNS

as strand breaks as indicated by the micronucleus test and thecomet assay [25]–[28].Earlier experiments at the Physikalisch-Technische Bunde-

sanstalt (PTB) with cells (a human–hamster hybrid cellline), which were exposed under similar conditions with powerdensities up to 4.3 mW cm , showed that THz radiation at106 GHz is a spindle-acting agent as indicated by the appear-ance of mitotic disturbances at the anaphase and telophase ofcell divisions [29]. Exposure of cells for 0.5 h or more to106 GHz radiation at power density levels of 0.43 mW cmand above induces statistically significant spindle disturbancesin the anaphase and telophase. Similar mitotic disturbanceshave been observed earlier already after exposure of cells to835 MHz radiation in a cell for 0.5 h at power densitylevels between 0.11 mW cm and 2.15 mW cm [31], [32].The effect is caused by the electric field component, only, as hasbeen found with exposure experiments in standing waves withspatially separated electric and magnetic fields [33]. This doesnot necessarily imply an enhanced risk for disease or injuryafter exposure to electromagnetic fields but it shows that thereare non-thermal interactions between cells and non-ionizing ra-diation that are potentially frequency-independent over a largefrequency range. Micronucleus induction was not observedunder exposure at similar experimental conditions [30].The micronucleus test was extended to find out why irra-

diation does not lead to micronucleus induction even thoughmitotic disturbances are found under similar conditions. In anextended set of experiments, cells were exposed for 24 h to apower density of 1.96 mW cm at 106 GHz. These exposuresdid not lead to increased micronucleus frequencies, neither inHaCaT and HDF cells, nor in cells in which mitotic distur-bances had been found. No increase in genomic damage in theform of micronucleus formation was observed as a consequenceof the irradiation [26]. A possible explanation could be that themitotic cells with disturbances die via apoptotic pathways orform aneuploid cells. Table I summarizes the publications so

far directly or indirectly related to the experimental campaigndiscussed here.Due to the large number of studies with often inconsistent

findings, a reliable methodology is needed to obtain reliable ex-posure results. In this paper we first discuss the general chal-lenges of field exposure experiments in the THz frequency rangein Section II. In Section III, we then describe the methodologyfor field exposure realized in the experimental campaign de-scribed in the introduction taking into account the general chal-lenges discussed before. Section IV describes the adjustment ofpower densities for field exposure traceable to the SI units andthe estimation of the associated uncertainties. The largest uncer-tainty in power density measurement is found to result from fieldnon-uniformity caused by standing waves (106 and 380 GHz) ornon-Gaussian shape and misalignment (2.52 THz). In Section Vthe required dosimetry for the determination of specific absorp-tion rates during field exposure is described. At 106 GHz, theestimate is based on numerical calculations whereas at higherfrequencies an accurate calculation fails due to the restrictedcomputational capabilities available. Here the estimate is basedon a simplified model. The paper is summarized and conclu-sions are drawn in Section VI.

II. GENERAL CHALLENGES

Field exposure experiments with living systems are chal-lenging since it is much more difficult to control all relevantconditions and parameters than in the case of experimentingwith other well-defined materials. While searching for po-tentially small effects it is difficult to exclude confoundingparameters in biological systems. Inter-disciplinary researchis needed. Despite thousands of studies on bio-electromag-netic interaction there is no clear picture whether non-thermaleffects exist since results are often contradictory [34]. Manystudies have severe shortcomings when analyzed in detail[35]. Shortcomings include mistakes in study planning such asfalse assumptions, negligence of biasing factors and too small


group sizes, case or sample numbers. Inadequate studies arelacking control groups or positive controls. Very often exposuredevices, applied fields and their intensity and characteristicsare not described sufficiently. The discussion of results is oftenone-sided or incomplete ignoring important references. Some-times the same data set is presented in different publicationswithout appropriate reference, preventing a realistic judgmentby the reader. This is the reason why radiation protectioncommissions such as the German Commission on RadiologicalProtection consider the results of a scientific publication a merescientific hint instead of a proof unless the results have beenverified by an independent laboratory [36].In addition to this, studies in the THz frequency range are

facing difficulties due to special efforts needed to operate stableTHz sources with sufficient output power and beam quality,to perform absolute field strength or power density measure-ments and to give realistic dosimetric estimates [7] in caseswhere exact dielectric properties of materials are often unavail-able and computational capabilities are limited. About 50 in-dependent empirical studies of different quality on living or-ganisms, model systems and cells have been performed so faron bio-electromagnetic interaction in the THz frequency rangewithout providing a clear picture [7], [8]. The authors come tothe conclusion that the aspects discussed below are of crucialimportance to perform in vitro experiments that lead to reliableresults.

A. Fields at the Sample Position

If a biological sample shall be exposed to THz radiation, itis crucial to know the exact field strength or power density inthe sample that is acting as physical agent. In order to quantifythe exposure strength, the power density at the location wherethe sample will be placed (empty field) might be suitable. How-ever, it must be taken into account that the sample (and its con-tainer) is electromagnetically coupled to the exposure field andthat it will change the effective field magnitude. Dosimetric cal-culations or estimates have to be performed to obtain the SAR,which is a measure for the strain imposed upon the biologicalsystem. Setting even the empty field power density can never beexact. However, to obtain meaningful results, this value has tobe set traceable to the SI units with known uncertainty in orderto be meaningful. Traceability to the SI units can be achievedby using a measurement instrument that has been calibrated byestablishing an unbroken measurement chain to the represen-tation of units in the National Metrology Institutes. An uncer-tainty analysis yields the overall uncertainty of the exposurefield acting on the sample and can serve to estimate the qualityand validity of the field exposure experiment.THz fields are often very nonhomogeneous due to the fact that

the wavelengths (and therefore also possible standing wave pat-terns) are in the order of the physical dimensions of the emittedTHz beam of most sources and of the sample containers. Thisleads to different exposure conditions within the sample con-tainer. In order to specify the exposure level for, e.g., cells, thesehave to be at fixed positions within the sample container. If theyare freely floating, different cells will encounter very differentexposure levels and results will not be meaningful.

B. Confounders

Results from exposure experiments can only produce mean-ingful results, if competing effects can be excluded. In the caseof THz field exposure that means shielding the samples fromall other relevant fields (e.g., from fields irradiated from mobilephones during the exposure procedure or from strong magneticfields during transport in a train). The relevant environmentalparameters such as temperature, pressure, humidity, and COcontent of the atmosphere but also the emission from the expo-sure source should be monitored permanently in order to be ableto identify and account for irregularities.When conducting terahertz exposure experiments, humidity

might contribute significantly to the absorption of terahertz ra-diation. In cell culture experiments, water might evaporate fromthe cell culture medium because the experiments are usuallyconducted at 37 C. Volume decrease leads to increased con-centrations of medium components, such as salts, amino acids,and vitamins. These concentration changes might trigger a va-riety of alterations within the cellular system. To avoid thesealterations, it is critical to keep the concentration of the compo-nents and therefore the medium volume constant. This can beachieved by working in a controlled humidity environment.

C. Human Bias

Results from bio-electromagnetic studies might suffer fromhuman bias. On the project level, human bias might lead tofinding effects where there are none since positive findingsgain more attention, are easier to publish and foster follow-upprojects. The scientific community should take care that pos-itive and negative results are valued in the same way. Whenevaluating samples, human bias might lead to results that matchthe expectation of the involved persons. To prevent this, ablinded procedure should be implemented. This means thatneither the person performing the exposure nor the personevaluating the effect knows about the applied exposure level.In THz exposure experiments, a fully blinded procedure can bevery difficult to realize since THz sources are often not fullyautomated and require an operator that sets the exposure level.However, a blinded sample evaluation will still prevent thathuman bias affects the results from the biological end points.

D. Significance of Results

In order to produce significant results, the detectability of thebiological end points has to be verified. This can be done withpositive controls with physical or chemical agents that yield apositive test result. In addition to this, sham expositions withzero field intensity but otherwise identical conditions for thesample as during exposure can be used to exclude the most sig-nificant confounding parameters.When evaluating biological samples in distinct end points, a

sufficient number of cells have to be evaluated in order to gainstatistically significant results. In addition to this, it is not suf-ficient to perform an exposure at a certain power density onlyonce, since biological samples might differ. One or more replicacan account for these effects. Evenmore reliability of results canbe obtained when performing more than one independent ex-perimental campaign, repeating the exposure experiments after


a certain time. When evaluating the findings, a statistical sig-nificance test such as the Mann–Whitney-U-test [37] should beapplied to quantify the reliability of the result.

III. FIELD EXPOSURE METHODOLOGY

In order to expose cells in liquid cell medium to THz radi-ation of defined intensity, the cells are adherent to the bottomof a sample container that is sufficiently transparent in the THzfrequency range. The thin monolayer of cells is exposed frombelow while it is covered with cell medium. Since cell mediumis water-based, it is strongly absorbing THz radiation and usingadherent cells is the only way to achieve a reasonable deposi-tion of power into the cells during exposure.Undisturbed cell growth requires certain environmental con-

ditions during exposure such as a temperature close to 37 C, acertain humidity and a CO content of the surrounding atmos-phere close to 5%. Therefore, the exposure experiments are per-formed in an incubator, which has been modified to host thesample container. To allow for exposure experiments at dif-ferent frequencies, different THz sources can be coupled intothe incubator and directed onto the sample from below.At the bottom of the sample container, where the cell mono-

layer is located, only a circular area with a diameter ofcm is evaluated with regard to the biological end points. To

expose this area in a most homogenous way, Gaussian beams ofTHz radiation with larger beam diameter (full width half max-imum cm in the case of 106 GHz and 2.52 THz and

cm in the case of 380 GHz due to technical reasons) aredirected onto this circular area from below. The power densityof a Gaussian beam with diameter and maximum intensitydepending on the radius can be written as [38]

(1)

The fraction of power that hits the evaluated area with di-ameter , can be determined by integration according to

(2)

In the case of cm, 22.1% of the overall beam powerare incident on the evaluated area, whereas in the case ofcm, only 10.5% hit the evaluated cells. Withcm and being the average power density to be set for

the evaluated cells, the required overall beam power for the ex-posure experiment can be found as

(3)

For cm, an overall beam power of 5.11 mW per 1mW cm is needed, whereas in the case of cm 10.77mWper 1mW cm average power density in the exposure zoneis needed.

A. Sample Container

The cells are cultivated in DMEM in a custom-built samplecontainer (ibidi GmbH, Martinsried, Germany) as shown in

Fig. 1. Sample container for in vitro field exposure experiments in the THz fre-quency range. (a) 3D computer model (CST Microwave Studio™) for specificabsorption rate calculation in a cell container filled with culture medium and (b)photograph (only dish without lid).

Fig. 1. The bottom of the containers consists of m thickfoils which are sufficiently transparent in the frequency range ofinterest (due to their low dielectric loss angle ofat THz frequencies). The foil, on which cell monolayers aregrown, has a diameter of 3 cm, whereas the whole dish has adiameter of 5 cm. Including the lid, the sample container has aheight of 10.5 mm. It has been found in numerical simulationsthat the diameter of the foil area has to be at least 3 cm, sincethe rim of the sample containers causes standing wave patternsthat lead to increased nonhomogeneity of the exposure field.At 2.52 THz, where the transmission of plastics is minimal

for the frequencies examined here, the transparency of samplecontainers from different batches used within the exposure cam-paigns has been found to be above 90%. Since water adsorptionof many polymers limits their usability, the effect of storingDMEM solution in the sample container on the transparencyof the bottom foil has been checked. After four days of storingDMEM, no noticeable effect on the transparency of the foil hasbeen found.

B. Modified Incubator

In order to be able to irradiate the sample containers frombelow at different THz frequencies at defined environmentalconditions as needed for undisturbed cell growth, an incubator(NuAire NU-5100) has been modified. As can be seen in Fig. 2,a hole in the side allows for coupling in a Gaussian THz beamthat is directed onto the sample container from below via a flatmetallic mirror. The sample container is mounted on an sup-port made from Rohacell™, which has a low dielectric constantat radio frequencies. The inner walls of the incubator are cov-ered with absorber foil to prevent standing waves (metal coatedpolyester foil with sheet resistance between 50 and 80 , notcommercially available). The Gaussian beam is coupled into theincubator in such a way, that it has the required beam diam-eter of cm or cm at the location of the samplecontainer. In order to seal the incubator atmosphere from thesurrounding air, a window made of a 25 m thin polymer foil(Lumox, Saint-Ouen-l’Aumône, France) has been placed intothe opening. The transparency of the window has been checkedand is larger than 90%.


Fig. 2. (a) Modified incubator. The sample container is exposed to THz radi-ation from below with a Gaussian beam. The position of the focus (minimumbeamwaist) depends on the source. (b) Frequencymultiplier cascade with corru-gated horn antenna, (c) backward-wave oscillator, and (d) far-infrared gas laserused as exposure sources.

During exposure, the sample containers were kept at definedatmospheric conditions of 37 C and 5% CO as required bythe cells. All relevant experimental parameters such as temper-ature, humidity and CO content within the incubator and RFpower of the source were monitored continuously during theexposures (Almemo Datalogger, Ahlborn Mess- und Regelung-stechnik GmbH, Holzkirchen, Germany) in 5 min intervals.Humidity was logged during all exposure scenarios includingsham-exposure and was found to be unaffected by the radia-tion. Thus, cells were permanently maintained under constanthumidity conditions. The humidity inside the incubator rangedbetween 6% and 41%. At the end of the exposure experiments,the volume of the cell culture medium was measured when themedium was removed to proceed with the cells for the differentendpoints. The volume was not reduced significantly duringthe exposure, which confirms appropriate humidity values.

C. Frequency Multiplier as Exposure Source

At 106 GHz, the cell culture dishes are irradiated with aGaussian beam originating from a frequency multiplier cascadewith horn antenna as shown in Fig. 2(b). A continuous wavesignal at approximately 17.67 GHz from a frequency synthe-sizer (Agilent E8257D) is sextupled in a Schottky multiplier.The wave is then fed into a round corrugated horn antenna viaa variable attenuator that allows for adjustment of the radiatedpower between zero and approximately 155 mW. The beam isthen collimated to a beam width of 2 cm at the location of thesample container using an off-center elliptic metallic mirror(shown in Fig. 3) with major semi-axis cm andminor semi-axis cm. The mirror produces a weaklyfocused Gaussian beam with a focus that lies between incu-bator window and flat metallic mirror inside the incubator. Thedistance between incubator and frequency multiplier sourcehas been adjusted in such a way that the beam diameter is

cm at the location of sample container.

Fig. 3. Beam scan of the 106 GHz source with elliptic mirror at the distance tothe source where the beam diameter is 2 cm. The inset shows the mounted tipof a dielectric fiber used for the raster scanning.

Fig. 4. Results of a beam scan at 106 GHz at a position where the beam hasa diameter of 2 cm. (a) 3-D representation. (b) 2-D representation. (c) Lateralcross section with Gaussian fit.

At frequencies in the order of several THz beam profiles caneasily be measured by shifting a sharp knife edge through thebeam. However, this method reaches its limitations at lower fre-quencies due to diffraction effects and is not feasible, when cou-pling to the detector aperture is affected. Instead, the tip of acircular dielectric waveguide (polyethylene, 2 mm) is used toscan the intensity distribution by coupling a small amount ofpower into an Erickson Power Meter PM4™ [22]. Fig. 4 showsthe measurement results for a beam scan behind the focus loca-tion where the beam diameter is 2 cm.

D. Backward-Wave Oscillator as Exposure Source

At 380 GHz the output of a backward-wave oscillator (Car-cinotron, Thomson-CSF, Frankfurt, Germany) was used as ex-posure source [39], [40]. As can be seen in Fig. 2(c), the output


Fig. 5. Results of a beam scan at 380 GHz at a position where the beam hasa diameter of 3 cm. (a) 3-D representation. (b) 2-D representation. (c) Lateralcross section with Gaussian fit.

signal is radiated by a corrugated horn antenna combined withan integrated polyethylene lens. A flat metallic mirror directsthe collimated radiation in a lateral direction. Depending onthe anode voltage, the output frequency is variable over sev-eral 10 GHz and set to a fixed value of 380 GHz. The outputpower depends on the anode current andwas adjusted to its max-imum value of approximately 25 mW to set a stable operationpoint. The converging beam reaches its focus too close to thecarcinotron, so that a beam diameter of 2 cm cannot be real-ized within the incubator where the sample container is placed.The incubator was placed in such a way that the beam diam-eter is set to 3 cm instead, with the focus lying between incu-bator window and flat metallic mirror inside the incubator. Fig. 5shows the measurement results for a beam scan at the positionwhere cm.

E. Far-Infrared Gas Laser as Exposure Source

As radiation source at 2.520 THz, a self-built far-infraredmethanol gas laser pumped by a CO laser [41] is coupled intothe incubator as shown in Fig. 6. A maximum stable outputpower of 5.3 mW has been reached after the quartz windowfilter that prevents the CO laser emission to enter the incubator.HDPE plates of different thickness have been used to set theexact power density for the exposure experiment. The divergentoutput beam of the far-infrared laser is guided by metallic mir-rors and transformed to a nearly collimated Gaussian beam witha diameter of 2 cm in the exposure zone. Fig. 7 shows resultsof the beam characterization using the knife-edge method. Thecollimated beam used for field exposure has been coupled into aGolay cell using a parabolic mirror. Then a sharp edge has been

Fig. 6. Schematic of the far-infrared laser source pumped by a CO laser. AGolay cell is used to monitor the output power of the exposure source.

Fig. 7. Beamwidth measurement with the knife-edge method. The beamwidthis determined by a direct fit with the integral of the Gaussian function.

moved through the beam. By fitting the measured Golay cellvoltage values as a function of the edge position directly withthe integral of the Gaussian function, the beam width (FWHM)has been determined at different axial positions. By extrapola-tion, the position where the beam diameter is 2 cm has beenfound and the incubator position has been adjusted accordingly.

IV. TRACEABLE ADJUSTMENT OF POWER DENSITIES

To quantify the uncertainty of the exposure fields seen by thebiological sample, the overall beam power has to be measuredtraceable to the SI units with known measurement uncertainty.

A. Traceable Measurement and Adjustment of ExposurePower Density at 106 GHz and 380 GHz

At 106 GHz and 380 GHz, the traceable measurementof the overall beam power is done with a Thomas KeatingPower Meter™ [42], [22]. Fig. 8(a) shows a photograph ofthe mount used to place the measurement head at the sample


Fig. 8. (a) Photograph of the large area detector as positioned into the beamunder the Brewster angle inside the incubator. (b) Operation principle with in-dication of the rectangular voltage amplitude applied to the thin film forsubstitution and the resulting voltage reading as well as the voltagemeasured during operation in a THz beam.

container position in the incubator, whereas Fig. 8(b) depictsthe operation principle. The power meter consists of a largephoto-acoustic detector with a sensitive area greater than themaximum beam diameter of 3 cm. It is placed in the THz beamunder the Brewster angle of 55.5 in order to prevent reflectionlosses and standing waves. The detector consists of a closedgas cell formed by two closely spaced parallel windows madeof polymethylpentene (TPX, Mitsui Chemicals, Tokyo, Japan).A thin metal film in the gap absorbs part of the chopped THzbeam passing through the cell and heats the gas. The resultingpressure modulation is detected with an acoustic detector. Forsubstitution, a rectangular voltage can be applied to thefilm leading to a signal at the output of the detector. Theresponsivity

(4)

where is the thin film resistance, can be used to determinethe overall beam power traceable to the SI units, if all quan-tities are determined with known uncertainty using calibratedinstruments. When using the power meter in the THz beam, theoverall beam power can be determined from the detectorvoltage by

(5)

where is the absorption of the thin film and is the transmis-sion of the window.Using the procedure described in the Guide to the expression

of uncertainty in measurement (GUM) [43], and considering thetwo equations (4) and (5) modeling the measurement process,adjustment of the overall beam power for exposure atthe position of the sample container inside the incubator can bedescribed by the model equation

(6)

where , and describe contributions due tothe adjustment process, whereas , and describe con-tributions due to the measurement process. Since for none of

these contributions except for a correction to the ad-justed value can be applied, their value is set to 1 whereas theiruncertainty contributes to the overall uncertainty of the adjustedpower value. According to the law of error propagation as man-ifested in the GUM, the overall uncertainty of is cal-culated as geometrical sum of the different measurement un-certainty contributions weighted by their sensitivity coefficients(partial derivatives of with regard to the contributingquantity). Table II shows the measurement uncertainty budgetaccording to the GUM requirements for the adjustment of anoverall beam power of 10.1 mW (corresponding to a power den-sity of 1.96 mW cm ) at 106 GHz. To determine the respon-sivity, the value for has been measured with a calibratedmultimeter and the value of to achieve a fixed value of 5 mVat the detector has been determined with a calibrated oscillo-scope. The attributed measurement uncertainties are taken fromthe calibration certificates and consequently have a normal dis-tribution. and are taken from the product data sheet. Theiruncertainties have been estimated taking into account the usualuncertainties in THz spectroscopy [44]–[46]. In the beam powermeasurement, three additional uncertainty contributions haveto be taken into account. Since the responsivity is determinedwith a rectangular voltage and chopping of the THz beam witha chopper blade leads to a non-rectangular modulation of thesignal, there is an error contribution due to different form fac-tors , which has been investigated and estimated to lead to astandard uncertainty of 2.5%. The chopper frequency is fluctu-ating, which influences the accuracy of the measurement result.After investigating this effect accounted for by , the stan-dard uncertainty has been found to be in the order of 2.5%, also.Despite the fact that the detector is positioned into the beamunder the Brewster angle, there is spurious reflectivity of thedetector left that leads to standing waves in front of the de-tector. When moving the detector along the axis of the mea-sured beam, the fluctuation of the displayed value is .Hence, assuming a rectangular probability distribution, a stan-dard uncertainty of has to be assigned to that isaccounting for interference.For the power density adjustment, additional contributions

have to be taken into account. The source has a limited stability.Measuring the output power over the designated exposure inter-vals and determining the standard deviation of the measurementas a measure for the uncertainty contribution of accountsfor the limited stability. As in the case of , the contribu-tion of this uncertainty to the overall adjustment uncertainty isnegligible. Much more important is the effect of limited homo-geneity in the exposure zone due to standing waves in the ex-posure plane caused by reflections from the brim of the hole inthe Rohacell™ support that carries the sample container. Fig. 9shows measurements of the beam profiles at 106 and 380 GHzin the exposure zone inside the incubator using the method de-scribed in Sections III-C and III-D. From Fig. 9(a) a deviationof can be obtained, leading to a standard uncertainty of

for the factor that describes this effect. Finally,is introduced to account for the fact, that throughout the

measurement campaign the software provided by the manufac-turer of the power meter has been used instead of the proceduredescribed below to determine the THz beam power. It has been


TABLE IIUNCERTAINTY BUDGET FOR ADJUSTMENT OF BEAM POWER WITH THOMAS KEATING POWER METER™

Fig. 9. Beam profile measurements at (a) 106 GHz and at (b) 380 GHz. Incontrast to the measurements shown in Figs. 4 and 5, the beam scan has beenobtained inside the incubator with the optical arrangement for exposure. Theelectric field is polarized in -direction.

found that the real value traceable to the SI units is constantlybelow the reading displayed by the commercial software by afactor of 0.9865. However, statistics show that for this transfer,a standard uncertainty of 1.86% has to be assumed.

B. Traceable Measurement and Adjustment of ExposurePower Density at 2.52 THz

At 2.52 THz the sensitivity of the Thomas Keating PowerMeter™ decreases and many of the uncertainties listed inTable II need to be increased significantly. Furthermore,the beam can be focused much more at 2.52 THz, so thata large area detector is not necessary. Since a commercialcalibration service based on detector radiometry is available

at Physikalisch-Technische Bundesanstalt (PTB), the GermanNational Metrology Institute, in Berlin [47], [48], a cali-brated pyroelectric detector [21] with an aperture diameter of11.8 mm has been used to adjust the exposure power densityat 2.52 THz. The pyroelectric detector (Laser Probe Polytec™Rk-5700) is based on a cavity absorber in a radiation trapdesign with built-in chopper. It has been calibrated using aquantum cascade laser as stable radiation source at 2.52 THz,whose emission power has been determined by focusing thebeam into a cryo-radiometer. Its calibration factor (ratio oftrue power value to displayed value) was determined with astandard uncertainty of 7.3%. In the calibration process, thelargest uncertainty contribution originates from the uncertaintyof the absorption properties of the cryo-radiometer cavity. Thecalibration factor of the detector depends on the polarization.A calibration factor of 1.184 for horizontal polarization and of1.070 for vertical polarization was determined. These resultsare in good agreement with results obtained by source-basedradiometry using a calculable black-body radiation source [48].In this case the adjustment process for exposure can be de-

scribed using the model equation

(7)

The required beam power is adjusted with thereading of the Golay cell depicted in Fig. 6. Itmeasures only a small part of the available THz power directedinto the detector via a beam splitter. Before the adjustment, theratio between the power reaching the exposure zoneand the power detected by the Golay cell (reading )are determined in a calibration. is measured usingthe Polytec™ power meter. The uncertainty analysis for theadjustment of the THz beam power leads to the budget shownin Table III. describes the deviation of the beam profileinside the incubator from the ideal Gaussian shape. In contrastto the lower frequencies, standing waves in the lateral direction


TABLE IIIUNCERTAINTY BUDGET FOR ADJUSTMENT OF BEAM POWER WITH LASER PROBE POLYTEC™ POWER METER

TABLE IVADJUSTED POWER DENSITIES

can be neglected in the exposure zone. Instead, proper adjust-ment of the incubator position with regard to the beam becomesdifficult. To account for uncertainties of the beam guiding andfluctuations, an average of 0.85 of the designated beam powerhitting the exposure zone is estimated. A rectangular distribu-tion with a half width of 0.15 can be assumed. and

are measured values whose uncertainties representthe stability of the Golay cell reading as determined by thestandard deviation of a series of measurements.represents the stability of the far-infrared laser whose standarduncertainty has been determined as standard deviation of a se-ries of measurements to 3.61%. represents a measuredvalue corrected by the calibration factor whose uncertaintyis given in the calibration certificate of the Polytec™ powermeter. Finally, represents the error when positioningthe pyroelectric detector. It has been found that the real mea-surement value is on average 1.05 of the displayed value due tonon-perfect positioning and that this factor fluctuates betweenrepeated measurements. A rectangular distribution with a halfwidth of 0.05 can be assumed.

C. Adjusted Power Densities

For all power densities adjusted within the exposure cam-paigns, similar uncertainty budgets can be set up. Table IV lists

the outcome of the uncertainty analysis made for all frequen-cies and exposure power densities for which results have beenreported in [27] and [28]. Despite all efforts, the uncertaintiesare in the order of 1/3 of the adjusted power densities. In case oflow power densities, the relative uncertainty tends to be largerdue to the difficulties to realize a known attenuation in the THzfrequency range. For the far-infrared laser, the uncertainties de-pend on the stability of operation that can be achieved at thedistinct intensity needed to adjust the required power densitylevel. It should be noted, that the uncertainties of the adjustedpower densities are generally large in the THz frequency range.This should be taken into account when evaluating the outcomeof exposure studies.

V. DOSIMETRIC CALCULATIONS

Safety limits should be based on basic restrictions that allowto quantify the exposure level threshold for which detrimentalhealth effects are proven. Although ICNIRP [1] uses the powerdensity for basic restrictions in the frequency range between 2and 300 GHz, it is still not clear whether this is appropriate forthe THz frequency range. Above 100 GHz, the basic restric-tions are merely extrapolated from proven thermal effects ofmicrowaves [49]. The authors consider the specific absorptionrate to be an appropriate measure for potential effects of elec-tromagnetic radiation on biological systems, especially if theyare thermal or thermally induced. The SAR value takes into ac-count the exact sample geometry and its dielectric properties.The SAR is defined as absorbed high frequency power per massunit due to dielectric losses in a defined volume. As a result ofthe high losses of the cell medium and the small wavelength ofthe THz radiation compared to the size of the cell container, thespecific absorption rates in the sample are subject to strong vari-ation on a sub-mm scale. This implies that the volume elementand its mass have to be small for the SAR value to be mean-ingful. It is noted that the locally defined specific absorptionrate should only be interpreted as initial boundary condition foran evolving temperature field of the whole sample. Beyond 10GHz, SAR distributions cannot be accessed by measurementsusing standard probes [50], [51]. Therefore, the exact distribu-tion of the SAR values is only accessible by numerical compu-tations. Due to the fact that the SAR takes into account the exactgeometry of the exposed system and that potential effects might


be thermally induced, we consider obtaining SAR values to be anecessary extension of the ICNIRP concept of specifying powerdensities in this frequency range.

A. Numerical Calculation of Specific Absorption Rates andTemperature Increase at 106 GHz

Dosimetric calculations to quantify the power introducedinto the cell layer have been performed by the finite-integrationtechnique in the time-domain using the simulation softwareCST Microwave Studio™ [52] on a double quad-core 3.2 GHzprocessor machine with 64 GByte RAM. The calculations arestrongly limited by computational resources to approximately140 million voxels due to the small vacuum wavelength ofthe radiation of approximately 3 mm at 106 GHz compared tothe size of the cell container. As a consequence, calculationsusing the model shown in Fig. 1(a) which consists of 35 millionvoxels, require a processing time of approximately two weeks.The excitation had to be simplified assuming a planar wave ofgiven power flux density applied from below due to programrestrictions. However, compared to the real excitation witha Gaussian beam having an equivalent power density in theexposure zone (see Section III), more energy will be coupledinto the sample containers off-center so that the calculated SARvalues should represent upper limits for the real case. How-ever, the SAR values in the center should not be affected verymuch by this effect due to the high dielectric losses of the cellmedium. As can be seen from the temperature measurementsdiscussed in Section V-C, the overall error is negligible.Realistic calculation of the SAR in the cell layer critically

depends on the proper assumption of material parameters forthe simulation. Cell layer and DMEM culture medium are as-sumed to have similar properties and have been modeled with

and using the dielectric propertiesmeasured for the culture medium using THz time-domain spec-troscopy for liquids [53]. Table V shows dielectric and thermalparameters assumed for the SAR calculations and subsequentthermal simulations. The values for dish, bottom foil and lidof the sample container are modeled using the dielectric prop-erties specified by the manufacturer at 100 GHz. Density ,heat conductivity and heat capacity are also manufacturerspecifications. For lateral surfaces, open boundary conditionshave been chosen, since the sample container is surroundedby the heat-insulating Rohacell™ foam material. In contrastto this, top and bottom of the sample container are modeledwith open or isothermic boundary conditions corresponding toa good thermal insulation or good heat transfer by convectionof the surrounding air, respectively. As expected, the calcu-lated SAR distributions do not depend on the choice of theseboundary conditions, since the SAR value represents the powerabsorption, only. In contrast to this, the calculated temperatureincrease is significantly higher in case of open boundary condi-tions everywhere.First calculations have been performed for exposure power

densities of 10 mW cm and 2 mW cm at 100 GHz [19].Using averaging over mass elements of 0.001 g, a maximumSAR of 66.3 W/kg for 10 mW cm and 13.34 W/kg for2 mW cm has been found in a plane 100 m above the foil.Within the numerical accuracy the maximum SAR value scales

TABLE VDIELECTRIC AND THERMAL MATERIAL PROPERTIES FOR SIMULATIONS

Fig. 10. Calculated SAR distribution in the sample container. (a) Plane 20 mabove the bottom foil representing the cell layer position. (b) cross-section. (c)Plot of SAR along the propagation direction in the center. The bottom of thesample container is at 0.1 mm and the surface of the DMEM liquid at 2.8 mm.

linearly with the applied power density. Further calculationshave shown that a mass element of 0.001 g is still too large formeaningful results and that using point SAR is more realistic.Furthermore, a plane 20 m above the foil has been consideredto be more representative for SAR estimation of the cell layer,whose exact thickness cannot be measured (we estimate athickness between 10 m and 50 m). Fig. 10 shows the resultsof a SAR calculation for a power density of 1 mW cm at afrequency of 106 GHz using point-SAR, which we considerto be more realistic then the earlier results presented in [19].Shown are cuts in the cross-section and in the plane 20 mabove the foil together with a plot of the SAR along the prop-agation direction. The effects of standing waves orthogonal tothe propagation direction are clearly visible. The maximumSAR value is found to be 27 W/kg at the location of the cellswhich is consistent with the earlier results taking into accountthe missing special averaging and the evaluation closer to thebottom of the sample container.Fig. 11 shows the results of the thermal simulation of the

sample container under irradiation of a wave with 2 mW cm


Fig. 11. Calculated heat distribution at 106 GHz in the cell container for a planewave with 2 mW cm from below. The cut plane is a cross section through thecenter. Isothermic boundary conditions have been chosen for top and bottomsurfaces of the sample container.

travelling in the -direction and at a surrounding incubator tem-perature of 36 C (for certain experiments [29] the incubatortemperature has been lowered to compensate for the expectedtemperature increase and to keep the temperature in a biolog-ically acceptable range). Isothermic boundary conditions havebeen assumed for the top and bottom surfaces of the sample con-tainer since we assume good convection in the ventilated incu-bator atmosphere. The overall temperature increase is limited to0.2 K in the sample.

B. Estimation of Specific Absorption Rates at 380 GHz andAbove

Due to the limited computational resources, the sample con-tainer model cannot be simulated at 380 GHz and 2.52 THz. Toget an idea which SAR values to expect at higher frequencies, amuch simpler model consisting of a 2 mm 2 mm block cut outof the sample container has been simulated. Fig. 12(a) showsthe simulated volume consisting of DMEM and bottom foil.Perpendicular to the propagation direction, periodic boundaryconditions have been chosen taking into account that the lat-eral dimensions of the sample container are much larger thanthe wavelength. Discretization led to 0.17 million voxels at 106GHz, 4.63 million voxels at 380 GHz and 68.5 million voxelsat 1 THz. For the dielectric properties of the sample containerthe values for 100 GHz from Table IV have been used, as alarger variation with frequency is not expected. For DMEM,measured values have been used. Fig. 12(b) shows the results ofthe point-SAR calculation along the axis of propagation in thecenter of the model. As in the case of the more detailed model,the SAR value reaches 27 W/kg at 106 GHz, whereas it reaches43 W/kg at 380 GHz and 58 W/kg at 1.0 THz. In all three cases,the maximum power deposition is in the first 50 m of the cellmedium where the cell layer is located. However, due to thehigher absorption of the medium at higher frequencies, the SARvalue decreases faster at 380 GHz and 1.0 THz.

C. Temperature Measurements for Verification

In order to verify the validity of the numerical simulations,temperature measurements are made in sample containers filledwith DMEM under exposure using a metal-free and fiber-cou-pled electro-optic thermometer Fotemp4 (OptoCon, Dresden,Germany) and the probe TK5/2 (0.55 mm thickness) also usedin [32]. Fig. 13 shows the results of four measurements over ex-posure time at exposure levels of 12 mW cm , 5 mW cm andtwice of 1mW cm . Although the incubator temperature fluctu-ates in the order of 0.3 K leading to different temperatures at the

Fig. 12. (a) Simplified model of DMEM on top of the bottom foil. (b) SARvalues simulated along the propagation direction indicated with blue line andarrow in (a).

Fig. 13. Temperature measurements inside sample containers filled withDMEM measured under exposure with a fiber-optic thermometer. Exposurebegins at .

start of exposure at , it is clearly visible that an exposureof 1 mW cm leads to a temperature increase of approximately0.2 K. Within the available accuracy, this is consistent with thecalculations and earlier estimates given in [27]–[30].

VI. CONCLUSION

We have presented a comprehensive approach for field ex-posure and dosimetry at THz frequencies that has been used


in a large experimental study funded by the German FederalOffice for Radiation Protection. The paper lists the main hur-dles that have to be taken to obtain meaningful results. Thecontrol of exposure conditions is of utmost importance if thelevel of exposure is to be quantified and interpreted. Only ifthe exposure field is well characterized and the exposure levelsare adjusted traceable to the SI units, a specification of abso-lute intensities is possible. Traceability includes the analysisof uncertainties. Without the specification of uncertainties, theexposure levels are incomplete and cannot be interpreted. Weidentify the most relevant uncertainties in power density mea-surement resulting from standing waves (106 and 380 GHz)or non-Gaussian shape, misalignment and detector calibrationuncertainty (2.52 THz). We have presented our approach fortraceability and have shown numerical calculations that allowedquantification of the heat deposition into the cell layer. Integraltemperature measurements in the sample container have shownthat the specific absorption rate calculations are realistic.

ACKNOWLEDGMENT

This work is dedicated to J. Wehland from Helmholtz Centrefor Infection Research who passed away prematurely. Theauthors thank A. Enders from the Institute for ElectromagneticCompatibility at Technische Universität Braunschweig andE. Schmid from the Radiobiological Institute at University ofMunich for fruitful discussions and scientific advice. Further-more, the authors thank A. Steiger, R. Müller and C. Montefrom PTB Berlin for detector calibration and for a veryhelpful discussion about traceability. They also acknowledgeR. Dickhoff from PTB Braunschweig for his programmingwork and measurement of beam profiles and M. K. Shakfafrom Philipps-Universität Marburg for his assistance during theexposure campaigns.

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Thomas Kleine-Ostmann was born in Lemgo, Ger-many, in 1975. He received the M.Sc. degree in Elec-trical Engineering from the Virginia Polytechnic In-stitute and State University, Blacksburg, VA, USA, in1999, the Dipl.-Ing. degree in Radio Frequency Engi-neering from Technische Universität Braunschweig,Braunschweig, Germany, in 2001, and the Dr.-Ing.degree from Technische Universität Braunschweig,Germany, in 2005.He was a research assistant with the Ultrafast Op-

tics Group, Joint Institute of the National Institute ofStandards and Technology and the University of Colorado, Boulder, CO, USA,and with the Semiconductor Group, Physikalisch-Technische Bundesanstalt,Braunschweig, before he started working on the Ph.D. degree in the field ofTHz spectroscopy at Technische Universität Braunschweig. Since 2006, hehas been with the Electromagnetic Fields Group, Physikalisch-TechnischeBundesanstal, Braunschweig, Germany, working as a permanent scientist.Currently, he is working on realization and transfer of the electromagnetic fieldstrength, electromagnetic compatibility, antenna measuring techniques andTHz metrology. In 2007, he became head of the Electromagnetic Fields Group,and in 2012, of the Electromagnetic Fields and Antenna Measuring TechniquesGroup.Dr. Kleine-Ostmann is a member of the VDE and the URSI. He received the

Kaiser-Friedrich Research Award in 2003 for his work on a continuous-waveTHz imaging system.

Christian Jastrow was born in Ostercappeln, Ger-many, in 1981. He has received the Diploma degree inElectrical Engineering from the Technische Univer-sität Braunschweig, Germany, in 2008. Currently, heis pursuing his Ph.D. as a research assistant with theElectromagnetic Fields Group at Physikalisch-Tech-nische Bundesanstalt in Braunschweig.During his diploma thesis, he has already begun

working on a 300 GHz transmission system, whichwas characterized and used for first data transmissionby him. Now he is primarily concerned with field ex-

posure experiments dealing with possible non-thermal effects of THz radiation.Furthermore, he is involved in channel and propagation measurements as wellas high data rate demonstration experiments at 300 GHz.Mr. Jastrow is a member of the VDE.

Kai Baaske was born in Uslar, Germany in 1976.In 2005 he received the Dipl.-Ing. and in 2011the Dr.-Ing. degree in electrical engineering fromthe Technical University Braunschweig, Germany,specializing in the field of THz spectroscopy andCW THz sources.Since 2009, he is with the Electromagnetic Fields

Group at Physikalisch-Technische Bundesanstalt inBraunschweig, Germany. Currently he is working asa permanent scientist in the field of electromagneticcompatibility and high frequency metrology in the

Electromagnetic Fields and Antenna Measuring Techniques Group.

Bernd Heinen was born in Jever, Germany in1984. He received the Diploma in InformationTechnology from the University of Applied SciencesOldenburg/Ostfriesland/Wilhelmshaven (OOW),Wilhelmshaven, Germany in 2008. Currently, he isworking toward the Ph.D. degree from the Tech-nische Universität Braunschweig.His research interests are the high power operation

of vertical-external-cavity surface-emitting lasersand their output power limiting factors.

Michael Schwerdtfeger was born in Salzgitter,Germany, in 1980. He studied electrical engineeringin Braunschweig, Germany, and Oulu, Finland, andreceived the Dipl.-Ing. degree from the TechnischeUniversität Braunschweig in 2009. Currently heis working towards the Ph.D. degree as a researchassistant with the Faculty of Physics and MaterialSciences Center at the Philipps-Universität Marburg,Germany.His current research interests include the develop-

ment of THz spectrometers for industrial and securityapplications and material analysis in the THz frequency range.

Uwe Kärstwas born in Bielefeld, Germany, in 1955.He received the Diploma in Microbiology in 1982and his Ph.D. degree in Biology in 1985, both fromthe University of Göttingen, Germany.In 1985, he started working as a Postdoctorate at

the Gesellschaft fuer Biotechnologische Forschung(GBF) in Braunschweig, Germany (now HelmholtzCentre for Infection Research, HZI), focusing on theproduction and characterization of proteins and en-zymes for biotechnological applications. Since 1988,he has been working as a senior scientist at the HZI

in the fields of enzymology and cell biology, and established proteomics as anew field of activity at the HZI.Dr. Kärst is a member of the ASM, VAAM and DGPF.


Henning Hintzsche was born in Halle an der Saale,Germany, in 1980. He studied pharmacy at theUniversity of Würzburg, Germany, and workedfor Bayer HealthCare, Caracas, Venezuela andWaisenhaus-Apotheke, Halle an der Saale, Germany,before receiving his State Exam and Approbation asa pharmacist in 2007. He received the Ph.D. degreein 2011 working on putative genotoxic effects ofterahertz radiation.In 2011, he was a visiting scientist at the Jawa-

harlal Nehru University, NewDelhi, India. Currently,he is a scientist at the Institute of Pharmacology and Toxicology of the Univer-sity of Würzburg, Germany. His research interests include biological effectsof non-ionizing radiation, hyperthermia in toxicology, and genotoxicity testing.Amongst other prizes he received the Young Scientist Award of the MicrowaveApplication Society of India in 2011 and the Young Scientist Award of the Ger-many Environmental Mutagens Society 2012.

Helga Stopper was born in Karlsruhe, Germany, in1960. She studied biology in Regensburg, Germanyand Boulder, Colorado and received her diploma in1984. Her Ph.D. work was in the area of biotech-nology at the University of Würzburg, Germany,where she received her degree in 1987.In 1988, she joined the Department of Toxi-

cology of the Medical Faculty of the University ofWürzburg, where she finished her Habilitation in1995, and became Professor of Analytical Toxi-cology in 2000. Since 2009, she has been the acting

chair of the Department of Toxicology. Her main research interest is genetictoxicology and carcinogenesis, investigating the mutagenicity of variousmedical and environmental agents.Dr. Stopper received the Young Scientist Award of the European Environ-

mental Mutagen Society in 1996, the Bernd Tersteegen Award in 2001, andthePro Meritis Scientiae et Litterarum“ of the Bavarian Ministery of Science,Research, and Arts in 2002. She is a member of the German Society of Toxi-cology (GT /DGPT) and of the German Mutation Research Society GUM, ofwhich she was president between 2003 and 2009, and is currently a member ofthe steering committee as past president.

Martin Koch was born in Marburg, Germany in1963. He received the Diploma and Ph.D. degreefrom the University of Marburg in 1991 and 1995,respectively.From 1995 to 1996 he was a post-doctorate at

Bell Labs/Lucent Technologies, Homdel, NJ, USA.From 1996 to 1998, he worked in the photonics andoptoelectronics group at the University of Munich.From 1998 to 2008 he was associate professor atthe Technical University of braunschweig. In 2003,he did a three-month sabbatical at the University

of California in Santa Barbara, CA, USA. Since 2009, he is full professor ofphysics at the Philipps University Marburg, Germany. His research interestsare terahertz systems and their applications, semiconductor disk lasers andultrafast spectroscopy on semiconductors.In 2003, Dr. Koch was awarded the Kaiser-Friedrich Research Prize.

Thorsten Schrader (M’04–SM’11) was born inBraunschweig, Germany, in 1967. He receivedthe Dipl.-Ing. and Dr.-Ing. degree in electricalengineering from the Technische Universität Braun-schweig, Germany, in 1992 and 1997, respectively.

In 1998, he worked for EMC Test SystemsLP, Austin, TX, USA (now ETS-Lindgren, CedarPark, TX, USA). In 1999, he joined the WorkingGroup ”High frequency Measurement Techniques”of Physikalisch-Technische Bundesanstalt (PTB),Braunschweig, Germany. During 2000, he was

a member of the Presidential Staff Office at PTB. In 2004, he became theHead of the Working Group ”Electromagnetic Fields and ElectromagneticCompatibility”. Since 2005, Dir. and Prof. Dr. Thorsten Schrader is the Headof the Department ”High frequency and Fields” and from 2006 until 2011,he has also been responsible for the Working Group ”Antenna MeasuringTechniques”. His current interest is the metrology for RF quantities in the mm-and sub-mm-wave range.Dr. Schrader is a member of VDE/VDE-GMA, VDI.

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