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Generation and detection of terahertz pulsed radiation with photoconductive antennas and its

application to imaging

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2002 Meas. Sci. Technol. 13 1739

(http://iopscience.iop.org/0957-0233/13/11/310)

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INSTITUTE OF PHYSICS PUBLISHING MEASUREMENT SCIENCE AND TECHNOLOGY

Meas. Sci. Technol. 13 (2002) 1739–1745 PII: S0957-0233(02)35309-8

Generation and detection of terahertzpulsed radiation with photoconductiveantennas and its application to imagingMasahiko Tani1, Michael Herrmann and Kiyomi Sakai

Kansai Advanced Research Center, Communications Research Laboratory, 588-2 Iwaoka,Nishi-ku, Kobe 651-2492, Japan

E-mail: tani@rcsuper.osaka-u.ac.jp

Received 28 March 2002, in final form 23 April 2002, accepted forpublication 24 April 2002Published 3 October 2002Online at stacks.iop.org/MST/13/1739

AbstractWe describe the basic properties of photoconductive (PC) antennas asemitters and detectors of terahertz (THz) pulsed radiation. The efficiency ofPC antennas is discussed, taking into account the saturation effect due tofield screening by photo-generated carriers. We show that maximumemission efficiency under a constant pump power condition is achieved byadjusting the PC gap size so that the pump intensity (power divided by PCarea) is equal to the characteristic saturation intensity. We also discuss thebandwidth of PC antennas and conclude that PC antennas are capable ofgenerating and detecting ultra-broad THz radiation beyond 10 THz whenshort enough laser pulses are used. Our conclusion is demonstratedexperimentally by detecting ultra-broadband THz radiation, whose spectrumdistributed up to 40 THz, with a PC antenna. As an example of applicationsof THz pulsed radiation based on a PC emitter–detector system, results forthe imaging of objects in powders are presented and discussed.

Keywords: THz radiation, emission efficiency, THz time-domainspectroscopy, photoconductive antennas, THz imaging system

1. Introduction

After the pioneering research by Lee and co-workers [1, 2],Auston et al [3, 4] and Mourou et al [5], the technique forgenerating pulsed terahertz (THz) radiation using femtosecondlaser pulses has been studied intensively. It has now developedto a level at which we are able to use it for spectroscopyand sensing. The spectroscopic technique using THz pulsedradiation is called ‘THz time-domain spectroscopy (THz-TDS)’. The THz-TDS system consists of a THz emittertriggered by femtosecond laser pulses, beam optics (beamcollimation/focusing optics, a sample holder/cell, an opticaldelay line, etc), and a THz sampling detector probed withthe femtosecond laser pulses split from the pump pulses.By scanning the optical delay between the pump and probepulses, the trace of the signal (dc current) from the sampling

1 Present address: Research Center for Superconductor Photonics, OsakaUniversity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan.

detector gives us a THz signal waveform, which is analogousto the interferogram signal in Fourier transform spectroscopy.However, the most striking difference (and an advantageat the same time) from the conventional Fourier transformspectroscopy is that THz-TDS is able to measure the phaseof the radiation, in addition to the amplitude. Therefore,the measurement of the dispersion or refractive index of asample is straightforward without need to use the Kramers–Kronig relation or the Drude model (for conductive samples).Another advantage of THz-TDS over conventional far-infraredspectroscopy is that there is no need to cool the detector to acryogenic temperature to achieve a reasonable signal-to-noiseratio (SNR) because the peak intensity of the THz pulsedradiation is well above the thermal background-noise level. Adetailed description and demonstration of THz-TDS is givenby Hangyo et al [6] in this special issue.

In this paper, we describe the basic properties ofphotoconductive (PC) antennas and a new application of this

0957-0233/02/111739+07$30.00 © 2002 IOP Publishing Ltd Printed in the UK 1739

M Tani et al

THz pulsed radiation, that is, THz imaging with THz pulsedradiation. In the next section we describe the working principleand the basic properties of the THz PC antenna, which is one ofthe most popular THz emitters and detectors for THz-TDS andimaging applications. In the final section, we describe imagemeasurements of objects in granular or powder material as aunique example of imaging with THz pulsed radiation basedon the PC emitter–detector system.

2. Photoconductive THz radiation source anddetector

2.1. PC antenna efficiency

The PC antenna [7–12] is one of the most commonly usedemitters and detectors for THz radiation as well as electro-optic (EO) crystals. In this paper, we limit our discussionto PC antennas due to restricted space. Readers who areinterested in the EO emitter and detector should refer to theliterature [13–15]. The structure of a PC antenna, which werefer to as a dipole-type antenna (or dipole antenna in short), isillustrated in figure 1. The strip line antenna is fabricated on aPC substrate with a gold or Au:Ge:Ni alloy for ohmic contactwith the GaAs substrate [16]. The antenna has a gap at thecentre of the antenna, which is biased with a dc voltage and isilluminated with femtosecond laser pulses. The typical size ofthe PC gapD is 5–10 µm, and those of the antenna width W andlength L are 10–20 and 30–50 µm, respectively, for a Hertziandipole-type antenna [7, 17]. The coplanar transmission lineis usually set to be long enough to avoid the reflection atthe line end (10 mm, for example). After excitation of thePC gap, photo-excited carriers are accelerated under the biasfield and create an ultra-short current pulse, which decayswith a time constant determined by the carrier lifetime inthe PC substrate. The transient current J (t) generates ultra-short electromagnetic pulsed radiation (THz radiation). Inthe Hertzian dipole approximation, the field amplitude of theradiation is proportional to the time derivative of the photocurrent J (t) in the far field:

ETHz ∝ ∂J (t)

∂t. (1)

The peak change of the photocurrent is proportional to theaveraged photocurrent J divided by the duty ratio of the currentpulse, which is approximated with the ratio of the lifetime ofthe photo carriers τc and the interval of the pump laser pulsesTint . Therefore

Epeak

THz ∝ �J ∼= JTint

τc

= GVb

Tint

τc

= σWδ

DVb

Tint

τc

= eµne

Wδ

DVb

Tint

τc

= eµτc

(1 − R)

hν

Pin

DWδ

Wδ

DVb

Tint

τc

= eµTint

(1 − R)

hν

Pin

D

Vb

D. (2)

Here, G is the time-averaged photoconductance of the PCgap, σ is the time-averaged conductivity, δ is the absorptiondepth of the pump light, ne is the averaged photo-carrierdensity, Vb is the bias voltage, µ is the mobility of the carriers,R is the reflectance of the PC substrate, hν is the photonenergy of the pump laser, Pin is the averaged pump laserpower, and D is the PC gap. It should be noted that, in the

Coplanar transmission line

LTG-GaAs Laser excitation

D

W

A L

Figure 1. Structure of a PC dipole antenna.

final expression of equation (2), the carrier lifetime τc hasdisappeared because of the cancellation with the conductivityterm σ = eµne = eµneτc/Tint , where ne is the peak carrierdensity.

From equation (2), we know that the emission efficiencyof a PC antenna is proportional to the carrier mobility but doesnot strongly depend on the carrier lifetime. This was confirmedby the comparison of the THz radiation power emittedfrom low-temperature-grown (LTG) GaAs-based antennas andthat from semi-insulating (SI) GaAs antennas, whose carrierlifetimes were subpicosecond and hundreds of picoseconds,respectively [17]. To increase the emission efficiency of aPC antenna, we should use PC material with a high carriermobility. However, to apply a bias voltage as high as possible(the efficiency linearly increases with the bias field Eb =Vb/D), we also need to use a high resistivity substrate. Usually,the requirement for high resistivity is most important for theemission efficiency, and the requirement for high mobility isnot easy to satisfy at the same time. Therefore, we have tochoose a material with a resistivity as high as possible, eventhough the mobility requirement is sacrificed. In this sense,LTG GaAs (annealed) has very good properties: it has anextraordinary high resistivity and a reasonably good mobility(100–300 cm2 V−1 s−1). A short carrier lifetime is not essentialfor the emission efficiency of PC antennas. However, a shortcarrier lifetime is preferable for reducing the detector noise inthe PC antenna, which originates from the thermal motion ofthe carriers.

2.2. Saturation effect and optimization of photoconductivegap

From equation (2) it is expected that the emission efficiencyis inversely proportional to the PC gap D when the biasfield Eb = Vb/D and the pump power Pin are taken to beconstant. Therefore, it may be good to use a PC gap as smallas possible by focusing the pump laser beam very tightly onto the gap. This is true when the pump laser power is low.However, the efficiency saturates at higher pump intensities.The saturation behaviour of the THz field amplitude for thepump laser intensity F or the pump laser power Pin is givenby the following equation [18–20]:

Epeak

THz ∝(

F

F0 + F

)=

(Pin

P0 + Pin

). (3)

Here, F0 (saturation intensity) and P0 (saturation power)are constants, which characterize the saturation behaviour

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Generation and detection of THz pulsed radiation with photoconductive antennas

0 10 20 30 400.0

0.2

0.4

0.6

0.8

Dipole antennaBias 15V

F/(F+F0)

Pea

k am

plitu

de (

arb.

uni

t)

Pump power (mW)

Figure 2. Dependence of THz peak amplitude signal on the pumplaser power for a PC dipole antenna.

of a specific PC material and PC antenna, respectively.Although the above saturation relation was originally derivedfor large-aperture PC emitters, the validity was experimentallyconfirmed also for small gap PC emitters [17]. In figure 2,the intensity dependence of the peak signal for THz radiation,emitted from a 30 µm long PC dipole antenna and detectedwith another PC antenna, is represented by a fitted theoreticalcurve based on equation (3). (The saturation power P0 was setto 16 mW in the fit.) The PC gap was 5 µm in length and 10 µmin width, and the optical pump beam diameter focused on tothe gap was about 10 µm. The saturation behaviour originatesfrom the screening of the bias field by carriers excited andaccelerated in the PC gap. This saturation behaviour is alsounderstood as the depletion of the electrostatic energy storedin the PC gap, whose energy is partly released after thephotoexcitation as THz radiation energy. Because of thissaturation effect, the PC gap has to be optimized when theavailable pump laser power is limited. Suppose we focus thepump beam power Pin to a square-shaped PC gap (D × D).Then, the pump intensity on the PC gap is F = ηPin/D

2

(constant η depends on the geometrical condition). The powerof THz radiation is proportional to the PC gap area D × D

under a constant bias field (remember that THz radiation poweroriginates from the electrostatic energy stored in the volumeof the PC gap). Therefore, the THz radiation peak amplitudeis proportional to the following equation expressed by the gapD as the parameter:

Epeak

THz ∝ D

(F

F0 + F

)= D

(ηPin/D

2

F0 + ηPin/D2

). (4)

From this relation, we can see that, for a given total pumppower Pin, the gap D should be adjusted so that the pumpintensity is equal to the characteristic saturation intensity F0

to maximize the THz radiation:

F(Dm) = F0, Dm =√

ηPin/F0. (5)

To confirm our model, we measured the peak amplitudeof the THz radiation from large square-gap PC emitters withvarious sizes. The emitters were made with SI GaAs (0.6 mmthick) and pumped with a regenerative femtosecond laseramplifier, whose pulse width, centre wavelength and repetitionrate are 130 fs, 800 nm and 1 kHz, respectively. The pump laserbeam was expanded and collimated with a beam expander and

0 1 2 30.0

0.2

0.4

0.6

Measurement Theoretical f it

EO

sig

nal (

arb.

uni

t)

PC gap (cm)

Figure 3. Gap dependence of THz peak signal (obtained with anEO sampling measurement). The solid curve shows a theoreticalcurve fit.

an iris to the size of the PC gap. The generated THz radiationwas focused with a pair of polyethylene lenses with a focaldiameter of 10 cm on to a 2 mm thick, (110)-cut, ZnTe crystal.The probe beam was combined with the THz beam by a Si-plate beamsplitter. The probe beam size on the EO crystalwas about 3 mm, which was comparable to the focused THzbeam size. The EO signal was detected with two balancedphotodetectors. To keep the THz beam diameter constant,irrespective of the size of the PC emitter, the EO crystal wasplaced at a slightly off-focused position, where the diffractioneffect is not so serious. The bias field (1 kV cm−1) in thePC gap and total pump pulse energy (1.5 µJ/pulse) were keptconstant for all measurements. Figure 3 shows the result. Aspredicted by equations (4) and (5), it shows a maximum at agap of D = 1 cm. In figure 3 a theoretical curve fit based onequation (4) is also shown by a solid curve, which reproduceswell the experimental data points except for the largest 3 cmPC emitter. The reason for the slight deviation for the largestPC emitter from the theoretical dependence is attributed to thetighter focus due to the widened THz beam diameter (smallerF number).

2.3. PC antenna bandwidth

For simplicity we assume that the momentum relaxation timeτm is much shorter than the pulse width of the optical pumplaser τL, which is in turn much shorter than the carrier lifetimeτc (τm � τL � τc). In this situation we can assume that thecarrier mobility µ is constant, and that the transient carrierdensity n(t) or carrier number N(t) is approximated by atime integral of the intensity profile of the pump laser I (t).Therefore, the THz waveform from a PC emitter is roughlyapproximated as

EPCTHz(t) ∝ ∂J (t)

∂t∝ ∂N(t)

∂t∝ I (t). (6)

Thus, the spectrum distribution is approximated by the Fouriertransform of the intensity profile of the pump laser:

EPCTHz(ω) ∝ I (ω). (7)

For the detector response of the PC antenna, similarconsiderations can be applied as in the case of the emitter.

For a more quantitative discussion of the spectralbandwidth of THz radiation emitted and detected by PC

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M Tani et al

0 10 20 30 40 50 60 700.0

0.2

0.4

0.6

0.8

1.0

12-µm ZnTe 0.1-mm GaSe

FF

T a

mpl

itude

(ar

b. u

nit)

Frequency (THz)

Figure 4. FFT spectra obtained with a PC dipole antenna for THzradiation generated from a 12 µm thick ZnTe crystal (solid curve)and 0.1 mm thick GaSe (dashed curve). The GaSe crystal wasoriented 45◦ to the pump beam to attain a phase matching betweenthe optical pump pulse and the THz pulse at about 25 THz.

antennas, we have to take several other factors into account,namely the effect of the antenna structure, the finite carrierscattering time, and the geometrical effect of the THz beamoptics. Jepsen et al [21] discussed the spectral bandwidthof the PC emitter–detector system including all the effectsmentioned above based on a Drude–Lorentz model. We donot repeat their discussion here. Instead, we point out that thebandwidth of the PC antenna is not limited by the slow carrierlifetime (∼0.5 ps for LTG GaAs) nor by the carrier momentumrelaxation time (about several femtoseconds in LTG GaAs),but it is limited by the laser pulse width (>10 fs). Recently,Duvillaret et al [22] carried out a model calculation includingthe momentum relaxation time and lifetime of photo-excitedcarriers for the THz generation and detection system based ona PC emitter–receiver system. Their conclusion derived fromthe model calculation is similar to ours: the laser pulse durationis the most crucial to enhance the broadband dynamics of thegeneration and detection of THz radiation with PC antennas.Therefore, contrary to conventional belief, the generation anddetection bandwidth of a PC antenna is extendable beyond10 THz if we use a short enough laser pulse as the pump orprobe.

Figure 4 shows the fast Fourier transform (FFT) amplitudespectra measured with a PC dipole antenna (L = 30 µm,D = 5 µm, W = 10 µm) for THz radiation generated from EOemitters using 20 fs laser pulses, whose spectrum distributedfrom 670 to 920 nm (corresponding to a band at about 100 THz)with a centre wavelength at 800 nm. One of the spectra isobtained for a 12 µm thick ZnTe crystal emitter on a glasssubstrate. The crystal has to be very thin for generating theultra-broadband THz radiation because of the small coherencelength between the optical pulse and the high-frequency THzradiation. Another spectrum is for 0.1 mm thick GaSe witha 45◦ incident angle to the pump beam. Because of theuniaxial property of the z-cut GaSe crystal, THz radiation canbe angle phase matched to the optical pump pulse at a specificfrequency. The spectral peak at 25 THz can be explained bythe increased efficiency due to the phase matching at this angleof incidence. Both spectral distributions exceeded 30 THz,clearly demonstrating the ultra-broadband property of the PCsampling detector [23–25].

3. Imaging applications

3.1. Advantages and disadvantages of imaging with THzradiation

THz radiation can be transmitted through most non-metallicand non-polarizing materials, such as semiconductors, plastics,textiles, etc. Therefore, obtaining images of the inside ofthese materials with THz radiation is possible, which is usuallydifficult or impossible with optical light. Imaging with THzradiation has already been demonstrated for various materialsincluding leaves [26], semiconductors [27], floppy disks [28],IC packages [26], water marks on bank notes [29], biologicaltissues [30, 31], and teeth.

Since x-rays can penetrate any material, they are used forimaging the inside of general things, such as luggage or humanbodies. However, x-rays cannot easily obtain a clear image forobjects with low index materials. Therefore, imaging with THzradiation could be used as a complementary tool to x-raying oroptical imaging. One advantage THz radiation has over x-raysis that THz radiation is not ionizing radiation and would be con-sidered safe when the thermal specific absorption rate is wellbelow the safety standard (0.4 W kg−1 in a controlled situation,and 0.08 W kg−1 in a public exposure [32,33]). If imaging withTHz radiation is possible for medical use, its non-hazardousproperty is a great advantage over x-ray imaging. However,the water content of the human body prevents transmission-type imaging. Therefore, imaging with THz radiation wouldbe restricted to images of skin (in reflection geometry), hair,teeth [34], or dehydrated (dried) biological samples.

Compared to optical light, the THz radiation wavelength ismuch longer (about sub-millimetre range). This means that thespatial resolution of images with THz radiation is much worsethan that with optical light in principle. This is one of the majordisadvantages of THz imaging. On the other hand, the longwavelength means that THz radiation is less subject to scat-tering with fine structure of the imaging targets or backgroundmaterials. Granular materials or powders strongly scatter op-tical light when the effective diameter is comparable to theoptical wavelength (Mie scattering) and, therefore, they areopaque to optical light. In contrast, THz radiation is much lesssubject to scattering compared to the visible light because ofits longer wavelength, and thus can be relatively well trans-mitted through powders. In the next sub-section we describeour results for imaging of objects in powders [39]. When theeffective diameter of the powder or characteristic length ofmodulation in a medium is comparable to the characteristicwavelength of THz radiation, it is also subject to the Mie scat-tering. Readers who are interested in such scatterings of THzradiation can refer to the work of Pearce and Mittleman [36]for dielectric spheres and to Han et al [37] for biological tis-sues. For more general scattering problems, readers can referto the work by Kawato et al [38].

3.2. Imaging of objects in powders

Our imaging system is based on a standard PC antennaemitter–detector configuration of THz-TDS. A mode-lockedTi:sapphire laser oscillator (∼80 fs pulse width, ∼800 nmwavelength and 82 MHz repetition rate) was used as the lasersource. The generated THz beam from a biased PC antenna

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Generation and detection of THz pulsed radiation with photoconductive antennas

(a 30 µm long dipole antenna on an LTG GaAs substrate) wascollimated and focused on to an imaging target with a pairof off-axis paraboloidal mirrors, and the sample was scannedon the X–Y plane normal to the THz beam (Z-axis). Thetransmitted THz beam was then again collimated and focusedon to the PC detector, which was the same type of antenna asthe emitter. By scanning the optical delay in the optical beampath, we recorded a time-domain waveform at every imagingposition. The average pump laser beam power was 25 mW andthe raw spectrum of THz radiation showed a peak at around0.5 THz.

Figure 5 shows the THz images of plastic objects takenfrom a small chess game contained in powders in a 10 mmthick plastic box. The imaged area is 20 × 20 mm2 for allimages. The images on the left show a plastic knight (about8 mm in width and thickness, and 13 mm in height) in babypowder (a kind of talc), and the images on the right show aplastic pawn (about 7 mm in diameter and 13 mm in height)in wheat flour. The typical grain size of the powders was∼20 µm for the talc and ∼50 µm for the wheat flour. Becauseof the versatility of the THz spectrum at each image position,spectrum information can be mapped out in many ways into atwo-dimensional image (display mode) [35]. In the upper rowof images, we applied the amplitude mode, where the peakamplitude in the THz waveform was mapped out on the X–Y

plane. In the lower row, we applied the maximum positionmode, where the time delay of the signal peak was mappedout. The objects appear bright in amplitude mode because thetransmittance of the plastic is greater than that of the powders.Because the refractive index of the plastic is also higher thanthat of the background powders, they are also bright in themaximum position mode.

Figure 6 shows images of a plastic cap of a bicyclevalve in wheat flour taken in amplitude mode (left) and inmaximum position mode (right). We can see that the contrastis particularly strong where the THz beam runs parallel to thewalls of the cap and thus crosses a long distance through theplastic material.

Because we measure the THz spectrum at each sampleposition, in principle, specifying the material of the imagedobject is possible if the background spectrum is constant andthe material shows a characteristic spectrum as its fingerprint.However, low index materials usually show a broad and flatspectrum over the THz frequency range. In addition, thethickness of the object is usually an unknown parameter whenwe try to detect and specify the objects in the powders (as inthe case of inspection for debris mixed in powders in the foodindustry). However, when it is assumed that the spectra of theobjects are relatively flat and the background power spectrumis constant, we can define a spectroscopic material parameterwhich does not depend on the thickness of the object:

Q = ln(r/r0)

ωc�t= �k

�n,

r

r0= exp(−�kωcd/c). (8)

Here r/r0 is an amplitude reduction caused by the transmissionthrough the object, ωc is the angular frequency of THz radiation(considered to be the frequency at the spectral peak, out ofconvenience), �t is the optical delay at the object position,�n is the difference in refractive indices of the powder andthe object, �k is the difference in extinction coefficients of

0 Amplitude max.

0 Time delay (ps) 5

ba

c d

Figure 5. Plastic objects from small chess game in powder. Theknight (a), (c) was put in talc, and the pawn (b), (d) in wheat flour.Both objects can easily be identified in the amplitude mode (a),(b) as well as in the maximum position mode (c), (d).

0 Amplitude max. 0 Time delay (ps) 6

ba

Figure 6. Cap of a bicycle valve in wheat flour imaged with THzradiation: amplitude mode (a) and maximum position mode (b).

the powder and the object, d is the object thickness, and c isthe vacuum velocity of light. The quantities with a subscript‘0’ refer to those for background (powder alone), and thosewithout a subscript ‘0’ refer to the quantities for objects tobe imaged. Because of the assumed low index contract forthe powder background (n0 ∼ 1.3–1.4 for wheat flour andtalc) and the object (n ∼ 1.6 for plastics), the effect of thereflection loss was ignored in equation (8).

With this material parameter Q, which can be estimatedfrom the transmission reduction and the time shift of the THzpulse, we can distinguish objects made of different materialsin powder.

Figure 7 shows a result of imaging using the Q parameterfor three plastic samples in wheat flour. The two objects atthe bottom and top right are small 2.8 and 1.0 mm thick plates

1743

M Tani et al

2.8 mm POM

1.0 mm ABS 1.0 mm POM

Wheat flour

Figure 7. Image of polymer pieces. The images were coloured withthe Q-values extracted from the transmission amplitude and phaseshift (see text). The ABS was coloured red, while the POM wascoloured green.

(This figure is in colour only in the electronic version)

of polyoxymethylene (POM). The top left is a 10 mm thickplate of acrylnitrile–butadiene–styrole (ABS) copolymer. Infigure 7, the Q values are scaled in colours ranging from red togreen: the POM plates are green, while the ABS plate is red.The Q-mode image shows that THz imaging can distinguishdifferent materials irrespective of the thickness of the objects.

3.3. Recent development in THz imaging

The main disadvantage of using the imaging system based onsample scanning is the long acquisition time. To overcome thisproblem a real-time THz imaging system using an EO samplingtechnique and an optical CCD camera has been proposed byWu et al [40]. In this system, a THz field pattern incident onthe EO crystal plate (ZnTe crystal is usually used) is imprintedas the change in the polarization of the readout beam throughthe EO effect. The CCD camera detects the THz field patternas the intensity of the readout beam transmitted through apolarization analyser (a linear polarizer). Readers who areinterested in this subject are advised to refer to [40, 41].

To obtain a three-dimensional (3D) image (the depthinformation is added to the two-dimensional image) with aTHz radiation pulse, Mittleman et al [28] employed a time-of-flight technique. This technique is capable of resolvingthe 3D refractive index profiles of objects with micrometreorder depth resolution, although it is only applicable to objectsconsisting of well separated and defined layers of differentindices. Recently, a novel 3D THz imaging technique (called‘T-ray computed tomography’) was demonstrated by a groupat Rensselaer Polytechnic Institute in the US [42]. Theyreconstructed 3D THz images of objects consisting of lowindex materials by measuring THz transmission images atseveral rotation angles of objects using the chirped-pulse EOsampling technique [43], which reduced the data acquisitiontime considerably. Because the diffraction and reflection

effects were neglected in the image reconstruction algorithm(the filtered back-projection algorithm), their method is notapplicable to complex and dispersive objects. However, theirresults showed the possibility of THz pulsed radiation forapplications in the non-destructive, 3D inspection of objects,such as mails, packages, plastic products, etc.

To increase the spatial resolution of the THz image,efforts have been made by several research groups using thenear-field technique [44–49]. For example, Yang et al [44]used GaAs probe chips to detect a local microwave field at100 GHz by the EO sampling technique and achieved an 8 µmspatial resolution. Chen et al [45] used a dynamical aperturetechnique, where photo-generated carriers modulated the THzbeam transmission of the area defined by the optical gate beamfocus. They achieved a spatial resolution better than 50 µm.Mitrofanov et al [49] demonstrated an 8 µm spatial resolutionfor a broad frequency range (200 GHz–2.5 THz) using a smallaperture on a PC detector.

4. Conclusions

The emission efficiency and the optimization of the PC gaphave been discussed considering the saturation effect for thepump intensity. We have indicated that, under a constant pumppower, the maximum emission efficiency is achieved at a PCgap size, for which the focused pump beam intensity becomesthe characteristic saturation intensity. We have also discussedand concluded that the PC antenna response is mainly limitedby the gating optical pulse width, and not critically dependenton other parameters, such as carrier lifetime or the momentumrelaxation time. The advantages of the THz spectroscopicsystem based on THz pulsed radiation (THz-TDS) overconventional far-infrared spectroscopy (far-infrared Fouriertransform spectroscopy), such as phase sensitive detection,have been pointed out. As a unique example of applicationof the pulsed THz generation and detection system based onPC antennas, THz imaging for objects in powders has beenpresented. Our demonstration was only a small part of manypossible applications of THz pulsed radiation. Among them,a challenging but important application is THz imaging andsensing for biomedical materials and objects. To this end, itis essential to develop more powerful THz radiation sourceswith a good stability.

Acknowledgments

This study was partly supported by the Industrial TechnologyResearch Grant Programme in 2001 of the New Energy andIndustrial Technology Development Organization (NEDO) ofJapan.

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