IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 19, NO. 1, JANUARY/FEBRUARY 2013

8600110

Real-Time Two-Dimensional Spatiotemporal
Terahertz Imaging Based on Noncollinear Free-SpaceElectrooptic Sampling and Application to Functional
Terahertz Imaging of Moving Object
Takeshi Yasui, Mukesh Jewariya, Takashi Yasuda, Markus Schirmer, Tsutomu Araki, and Emmanuel Abraham

Abstract—We review 2-D spatiotemporal (2D-ST) terahertz(THz) imaging based on noncollinear free-space electrooptic sampling and its application to THz reflection tomography, THz
spectral imaging, and THz spectral computed tomography (CT).
2D-ST THz imaging enables high-speed image acquisition at muchhigher rates than previously. Two-dimensional THz reflection tomography was effective for visualizing the internal structure of amoving paint film. A THz color scanner demonstrated the potential of rapid nondestructive inspection of moving pharmaceutical
tablets. A THz spectral CT system using real-time line projectionof a THz beam was effectively applied to a continuously rotating
object. 2D-ST THz imaging enables functional THz imaging ofmoving objects in various practical applications.
Index Terms—Electrooptic effects, imaging, nondestructive testing, spectroscopy, terahertz radiation.

I. INTRODUCTION
N RECENT years, pulsed terahertz (THz) radiation has attracted much attention as a new tool for nondestructive inspection, quality control, and material characterization [1], [2].
In particular, THz imaging with pulsed THz radiation is oneof the most innovative tools in this research field because it enables noncontact, nonionizing, and coherent imaging. Following

I

Manuscript received April 15, 2012; revised June 9, 2012; accepted July 21,
2012. Date of publication July 26, 2012; date of current version February 1,
2013. This work was supported by Collaborative Research Based on Industrial
Demand from the Japan Science and Technology Agency, Grants-in-Aid for
Scientific Research (21650111 and 23656265) from the Ministry of Education,
Culture, Sports, Science, and Technology of Japan, the Renovation Center ofInstruments for Science Education and Technology in Osaka University, andthe Tateishi Science and Technology Foundation, Japan.
T. Yasui and M. Jewariya are with the Institute of Technology and Science,
The University of Tokushima, Tokushima, Tokushima 770-8506, Japan (e-mail:yasui@me.tokushima-u.ac.jp; jewariya.mukesh@femto.me.tokushima-u.ac.
jp).
T. Yasuda was with the Graduate School of Engineering Science, Osaka
University, Toyonaka, Osaka 560-8531, Japan. He is now with Hamamatsu
Photonics K.K., Hamamatsu 435-8558, Japan (e-mail: yasuda@crl.hpk.co.jp).
M. Schirmer was with the Graduate School of Engineering Science,
Osaka University, Toyonaka, Osaka 560-8531, Japan. He is now with RWTH
Aachen University, 52062 Aachen, Germany (e-mail: markus.schirmer@rwth-aachen.de).
T. Araki is with the Graduate School of Engineering Science, Osaka
University, Toyonaka 560-8531, Japan (e-mail: araki@me.es.osaka-u.ac.jp).
E. Abraham is with the Laboratoire Ondes et Matière d’Aquitaine, Bordeaux
1 University, Talence Cedex 33405, France (e-mail: em.abraham@loma.
u-bordeaux1.fr).
Digital Object Identifier 10.1109/JSTQE.2012.2210393

THz transmission imaging [3], other unique imaging modalities
have been reported, including reflection tomography [4], spectral imaging [5], and computed tomography (CT) [6].
In THz reflection tomography, a cross-sectional image of thesample can be obtained because this technique employs timeof-flight measurement of THz echo pulses returning from theboundaries between different materials. One advantage of thistechnique over conventional methods using ultrasound and Xrays is the ability to function as a noncontact and nonionizing
probe. This technique has been used to visualize the internal
structure of floppy disks [4], pharmaceutical tablet coatings [7],automobile paint [8], [9], and skin cancer [10].
THz spectral imaging gives a “color” perspective view of asample in contrast to the conventional “monochrome” inspection achievable with X-rays and ultrasound. The combined useof THz color images with THz spectral fingerprints enables material characterization and spatial mapping, and this approachshows promise for screening of cancer tissue [5] and pharmaceutical tablet [7], detecting explosives and illicit drugs [11], [12],and art conservation [13].
THz CT is a ubiquitous technique providing 2-D cross sectionor 3-D volumetric images of objects in a transmission configuration. Reconstruction of them has been performed by analyzinga series of THz projection images at different projection angles
using the filtered back-projection (FBP) algorithm [14]. Thisimaging modality has been effectively applied to 3-D objects
constituted of low-density, opaque, soft materials, such as plastics [6], wood [15], bone [6], and pharmaceutical tablet [16], inwhich it is difficult to create good image contrast with X-ray CT
because X-rays are too penetrating.
The conventional THz imaging techniques described above
face several practical difficulties when implementing them inreal-world applications. One difficulty is the requirement formechanical time-delay scanning for the temporal waveform
measurement of the pulsed THz electric field. When time-delayscanning in THz imaging is performed using a stepping-motordriven translation stage, the image acquisition time is quite long.
This is because the sample has to be raster-scanned in the focal
spot of the THz beam whenever temporal waveform measurement of the THz pulse is completed. Consequently, this method
has so far only been applied to stationary objects. If this THz
imaging technique could be extended to moving objects, its
potential applications in industry would be greatly increased.

1077-260X/$31.00 © 2012 IEEE

8600110IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 19, NO. 1, JANUARY/FEBRUARY 2013

One effective method to reduce the acquisition time of thetemporal waveform is to use a fast scanner for the mechanical time delay, such as galvanometric scanner, a piezoelectric
stage, or a rotating delay line [17]. However, there are several
drawbacks with these mechanical methods, such as mechanical vibrations, spot size variation of the laser light, and/or theissue of a tradeoff between the scanning range and scanning
rate. Asynchronous-optical-sampling THz time-domain spectroscopy (ASOPS-THz-TDS) seems to be a promising method
to accomplish rapid time-delay scanning because it involves
no mechanical movement [18]–[22]. Time-delay scanning at a
rate of 10 kHz has been achieved using a 1-GHz Ti:Sapphirelaser [19]. However, signal integration of numerous scans is required due to the fact that a lock-in amplifier cannot be used inthis method. Furthermore, mechanical rastering of the sample
position is still required because this method is based on the
point-by-point measurement.
Rapid image acquisition can be achieved if instead of a mechanical stage, we use an alternative technique for measuring
the time delay and sample position simultaneously. One effective method to realize a stage-free configuration is a combination of a single-shot measurement of the temporal waveform and its 1-D transverse imaging, enabling real-time twodimensional spatiotemporal (2D-ST) imaging. In THz region,
there are a few reports on single-shot temporal measurement ofTHz pulses requiring no mechanical stage for time delay, allof which are based on two-dimensional free-space electrooptic
sampling (2D-FSEOS) [23]. These include time-to-wavelength
conversion using a chirped probe pulse [24], use of a THz
streak camera [25], and time-to-space conversion based on noncollinear 2D-FSEOS [26]. Although some of these techniques
could be extended to 2D-ST THz imaging [27], noncollinear
2D-FSEOS should prove most suitable since it only requiresa slight and inexpensive modification of a regular 2D-FSEOS
system. More importantly, it has no limitations on time resolution caused by coupling between frequency- and time-domain
methods [28] or by the instrumental response [25]. We havepreviously proposed a 2D-ST THz imaging technique basedon a combination of noncollinear 2D-FSEOS and line focusingof a THz beam onto a sample [29]. This 2D-ST THz imaging
technique has achieved, for the first time, functional THz imaging of moving objects in different modalities, such as reflection tomography [29], spectral imaging [30], [31], and spectral
CT [32].
In this paper, we review recent work on three functional THz
imaging techniques, based on 2D-ST THz imaging, that are applicable to moving objects. After the principle of 2D-ST THz
imaging based on noncollinear 2D-FSEOS is described in Section II, this method is compared with other methods in Section
III. Then, real-time 2-D THz reflection tomography and its application to moving paint films are presented in Section IV. In
Section V, a THz spectral imaging system using real-time line
scanning of the THz beam and its application to moving pharmaceutical tablets are described. Section VI describes fast THz
CT with real-time line projection that is effectively applied tocross-sectional imaging of continuously rotating objects. The
paper is summarized in Section VII.

Fig. 1. Typical configuration of 2D-ST THz imaging. (CL1 and CL2: THz
cylindrical lenses; L1: THz planoconvex lens; L2: optical planoconvex lens; S:
sample; EO: electrooptic crystal; P: polarizer; A: analyzer.)

II. PRINCIPLE OF 2D-ST THZ IMAGING BASED ON
NONCOLLINEAR 2D-FSEOS
Fig. 1 shows a typical configuration for 2D-ST THz imaging
based on noncollinear 2D-FSEOS, using an intense THz pulse
generated by an amplified femtosecond laser pulse. A THz cylindrical lens (CL1) is used to produce a line-focused THz beamalong the Y-axis of the sample (see top and side views in Fig. 1).
The line-focused THz beam on the sample is imaged along the
Y-axis of an electrooptic (EO) crystal for 2D-FSEOS by usinga THz planoconcave lens (L1) and a cylindrical lens (CL2) (seeside view in Fig. 1).
On the other hand, as a probe beam, we use another beamthat crosses the EO crystal at an angle of θ with respect to
the THz beam. When the temporal width of the probe pulse ismuch shorter than that of the THz pulse, the wavefront of theTHz beam at different times overlaps with that of the probe
beam at different transverse positions. In this case, we use a
2D-FSEOS technique in the noncollinear configuration as asampling method. This noncollinear 2D-FSEOS method converts the time profile of the pulsed THz electric field to a spatial
polarization distribution of the probe beam [26]. After changing this spatial polarization distribution to a spatial intensity
distribution using crossed Nichol prisms (P and A), a 2D-ST
THz image is detected as a probe light image with an infrared
camera via an optical imaging lens (L2). The resulting image
is composed of the time profile of the THz pulse and the line
image of the sample, which are developed along the horizontal
and vertical coordinates of the camera, respectively. Since this
2D-ST THz image is obtained without any mechanical scanning, its acquisition rate can be increased up to the repetition
rate of the THz pulses in principle, for example, 1 kHz in ourcase.
Here, we assume that the pulsed THz electric field has anegative peak and a positive peak separated by a relative timedelay of Δτ (indicated by the solid and broken line segments inthe top view in Fig. 1). The resultant time-to-space conversion
factor is given byΔτ =(Δh · tan θ)c(1)

YASUI et al.: REAL-TIME TWO-DIMENSIONAL SPATIOTEMPORAL TERAHERTZ IMAGING

Fig. 2. Relationship between crossing angle θ and time window ΔT withrespect to different probe beam diameters φ in noncollinear 2D-FSEOS.

where Δh is the spatial distribution of the probe light intensity
transcribed from the time delay Δτ , and c is the velocity oflight in vacuum. The temporal profile of the THz pulse can thenbe successively developed along the X-axis of the camera. The
measurable time window ΔT is defined asΔT =(φ · tan θ)c(2)where φ is the diameter of the probe beam. Fig. 2 shows themeasurable time window ΔT with respect to different angles
θ in the case of φ = 5 mm, used in the following 2-D THz
reflection tomography, and 25.4 mm, used in the following THz
spectral imaging and spectral CT. In this way, one can set ΔT
arbitrarily by selection of θ and φ.
III. COMPARISON WITH OTHER METHODS
Here, let us compare our method with other methods for 2DST THz imaging. Although all 2D-ST THz imaging methods
are based on 2D-FSEOS [23], they can be categorized into three
types based on single-shot measurement method of temporal
waveform; time-to-wavelength conversion based on a chirped
probe pulse [24], time-to-space conversion using streak camera [25], and time-to-space conversion based on noncollinear
2D-FSEOS [26]. In the first method, by combining a linearly
chirped optical probe pulse and a multichannel spectrometer,
the temporal waveform of a THz pulse can be converted into thewavelength spectrum of an optical probe pulse. Such a time-towavelength conversion method can be applied to 2D-ST THz
imaging [27]. However, the time resolution in this method islimited by a tradeoff inherent in combining frequency- and timedomain methods [28], though this limitation can be overcomeby the noncollinear cross correlation of EO-modulated opticalpulses, requiring a more complicated optical setup [33]. Furthermore, use of the spectrometer equipped with narrow entrance
slit makes it difficult to introduce the line-imaging configuration
into this method. The second method is the use of a long probe
pulse and a streak camera. In this method, the temporal profileof the EO modulated probe pulse is transformed into the spatial

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distribution of the probe light intensity through time-to-spaceconversion in a streak tube and is then detected with an infrared
camera. Although this method may be also extended to 2D-ST
THz imaging, the time resolution of this method is limited by the
response of the streak camera, typically from subpicosecond topicosecond. Furthermore, this single-shot measurement methodneeds expensive and complicated streak camera. The third is theuse of 2D-FSEOS with the THz beam and optical probe pulse
in a noncollinear geometry (noncollinear 2D-FSEOS). In thisgeometry, direct time-to-space conversion occurs through thespatial overlap of the two noncollinear beams in an EO crystal
and the resulting spatial distribution of the probe beam is detected with an infrared camera. This method is easily achievedby a slight modification of a regular 2D-FSEOS system without
any expensive apparatus or complicated setup, and most advantageously, it has no limitations on time resolution caused by the
intrinsic coupling between frequency- and time-domain methods [28] or the instrumental response [25]. Such simple configuration enables us to easily extend this method to functional
THz imaging such as reflection tomography, spectral imaging,or CT.
It will be also interesting to compare our 2D-ST THz imaging method with ASOPS-THz-TDS method. In ASOPS-THzTDS method, two mode-locked lasers with slightly mismatchedmode-locked frequencies are used for generation and detectionof the pulsed THz radiation. This asynchronous optical sampling enables us to linearly expand the time scale of picosecond
THz pulse to microsecond order. The resulting slow signal canbe captured rapidly on a standard oscilloscope without the needfor time-consuming, mechanical time-delay scanning. Although
the rate of time-delay scanning can reach kilohertz order, manyscans have to be integrated together to achieve an adequate
signal-to-noise ratio (SNR) because a lock-in amplifier cannot
be used in this method. Here, we compare 2D-ST THz imaging [30] with ASOPS-THz-TDS [22] from a viewpoint of a relation between SNR of temporal waveform and acquisition time.
Fig. 3(a) shows a comparison of SNR between them with respect to acquisition time required for single measurement. Bothmethods indicated comparable performance within the range ofacquisition time from 50 to 50 000 ms. However, to compare
the performance of them more fairy, we should consider difference of the data acquisition method between them because
ASOPS-THz-TDS captures a single temporal waveform of THz
pulse, whereas 2D-ST THz imaging enables parallel measurement of multiple temporal waveforms. Since 2D-ST THz imaging method can capture 232 temporal waveforms of the THz
pulse in parallel during this acquisition time [30], we modified
the horizontal scale in Fig. 3(a) to the acquisition time requiredfor 232 temporal waveforms, as shown in Fig. 3(b). It is obviousthat 2D-ST THz imaging surpasses ASOPS-THz-TDS in SNR
in the case of imaging measurement.
IV. TWO-DIMENSIONAL THZ REFLECTION TOMOGRAPHY
OF MOVING OBJECTS
Fig. 4 shows the experimental setup used for 2-D THz
reflection tomography. Amplified femtosecond pulse light8600110IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 19, NO. 1, JANUARY/FEBRUARY 2013

Fig. 5 (a) 2D-ST THz image of a flat Al mirror surface (θ = 28◦ , image size
= 9 ps × 5 mm, and CCD exposure time = 70 ms) and (b) temporal waveformof the THz electric field extracted from a line (A) in the 2D-ST THz image.

Fig. 3. Comparison of 2D-ST THz imaging with ASOPS-THz-TDS regarding
a relation between SNR and acquisition time required for (a) single measurementand (b) 232 temporal waveforms.

Fig. 4. Experimental setup for 2-D THz reflection tomography. (BS: beam
splitter; ZnTe1 and ZnTe2: zinc telluride crystals; L1: THz planoconvex lens;
IRF: infrared-cut filter; CL: THz cylindrical lens; L2: THz biconvex lens; P:
polarizer; A: analyzer; L3: optical planoconvex lens; CCD: charge-coupleddevice camera.)(1 mJ/pulse, 150 fs, 800 nm, 1 kHz) was separated into pumplight for THz generation and probe light for THz detection. An
intense THz pulse was generated via optical rectification of thepump light in a ZnTe crystal (ZnTe1; 110 orientation, 10 mm
× 10 mm × 4 mm thick). A THz planoconvex lens (L1, f =50 mm) was used to collimate the generated THz beam. The
collimated THz beam was then delivered to the optical sys-

tem for 2D-ST THz imaging, using a setup slightly modified
from that in Fig. 1 for a reflection configuration. A THz cylindrical lens (CL, f = 50 mm) along the Y-axis of the sample
was used to line-focus the THz beam onto the sample. The reflected THz beam from the sample was imaged on another ZnTe
crystal for noncollinear 2D-FSEOS (ZnTe2; 110 orientation,
10 mm × 10 mm × 1 mm thick) by CL and a THz biconvex
lens (L2, f = 100 mm). The probe beam, with a diameter of
5 mm, was incident on the crystal ZnTe2 noncollinearly with
the THz beam at a crossing angle θ. A 2D-ST THz image wasobtained using crossed Nichol prisms (P and A) near the zero
optical transmission point on the EO crystal [34]. Finally, the 2D spatial distribution of the probe light intensity, corresponding
to the 2D-ST THz image, was imaged via a planoconvex lens(L3, f = 150 mm) onto a 12-bit thermoelectric-cooled chargecoupled-device (CCD) camera (640 × 480 pixels, 10 frames/s).
The resulting 2D-ST THz image reflected a YZ cross-sectional
image of the sample because the horizontal coordinate of the
2D-ST THz image corresponded to the time delay of the THz
echo pulse, and hence the depth profile of the sample.
A single-frame 2D-ST THz image of a flat aluminum mirror
surface is shown in Fig. 5(a) (image size = 9 ps × 5 mm, θ= 28◦ ). The white and black areas in the image indicate positive and negative THz electric field, respectively. The verticalwhite line around 4.1 ps in Fig. 5(a) indicates the flat surfaceof the mirror without any internal structure. Fig. 5(b) shows the
temporal waveform of the pulsed THz electric field extracted
from a line profile of the 2D-ST THz image [see line (A) in
Fig. 5(a)], in which a pulse duration of 0.5 ps and SNR of 80
were achieved.
THz paintmeters for automobile paint are an interesting application of THz reflection tomography [8], [9], [29]. We adapted
our system for this application and evaluated its performance asa real-time THz paintmeter. A film of white alkyd paint (grouprefractive index in THz region ng = 2.14) was applied to half ofthe surface of an Al substrate and allowed to dry (see upper partof Fig. 6). While the sample was continuously moved parallelto the Y-axis in Figs. 4 and 6 (moving speed = 5 mm/s), 2-D
cross-sectional images of the moving paint film were successively captured at a frame rate of 10 frames/s. The lower part of
Fig. 6 shows the cross-sectional images of the sample at threeYASUI et al.: REAL-TIME TWO-DIMENSIONAL SPATIOTEMPORAL TERAHERTZ IMAGING

Fig. 6. Two-dimensional THz tomography at three different illuminating positions of a moving paint film covering half of a substrate (θ = 20◦ , image
size = 6 ps × 5 mm): (a) unpainted area, (b) boundary between unpainted andpainted areas, and (c) painted area.

different illuminating positions (image size = 6 ps × 5 mm, θ =20◦ ). The unpainted area in Fig. 6(a) shows only one THz echo
signal due to the absence of internal structures. At the boundary
between the unpainted and painted areas in Fig. 6(b), although a
mixture of one and two echoes should be observed, insufficienttransverse resolution blurred the edge at the boundary between
the two areas. In the case of the painted area in Fig. 6(c), two
THz echo signals were observed at 2.3 and 4.5 ps from the air–paint boundary and the paint–substrate boundary. The thicknessof the paint film d can be obtained using the time separation Δτ
between the two echoes and the group refractive index ng of thepaint film as follows:d=(cΔτ ).
(2ng )(3)

The actual dimensions of this paint film are given on the upperhorizontal axis in Fig. 6(c). From this experiment, we confirmedthat this paint film had an uneven thickness of 162 ± 21 μm[(mean) ± (standard deviation)] along the Y-axis of the sample.
Since the paint thickness around the center of the painted area
was determined to be 175 μm by a contact-type thickness meter(precision = 3%), both methods were reasonably consistent witheach other. The thickness resolution dm in in the present system
was determined from the duration of the THz pulse (Δt) and thegroup refractive index of the sample, ng , as follows:
dm in = (cΔt)/(2ng )= (3 × 108 × 0.5 × 10−12 )/(2 × 2.14) = 35 μm. (4)One interesting application of real-time 2-D reflection tomography will be in the field of art conservation as well as
car painting. In this field, it is required to know history of therestoration of an old painting without destructive sampling. Recently, THz reflection tomography has attracted attention in thisfield because it nondestructively provides the cross-sectional
image of the overlapping paint layer based on the optical thickness [13]. However, existing THz tomography system used forthe art conservation needs mechanical scanning of time delay
and raster scanning of sample position, resulting in long acquisition time for 2-D cross-sectional image, typically several to8600110

Fig. 7. Experimental setup for THz spectral imaging. (ZnTe1 and ZnTe2: zinctelluride crystals; CL1 and CL2: THz cylindrical lenses; L1: THz planoconvex
lens; P: polarizer; A: analyzer; L2: optical planoconvex lens.)

several tens minute. On the other hand, our system can provide
the 2-D cross-sectional image in real time and even 3-D volumeof its internal structure rapidly. This will lead to the efficient
work in the restoration of the old painting.
V. THZ SPECTRAL IMAGING OF MOVING OBJECTS
Fig. 7 illustrates the experimental setup used for THz spectral imaging. Amplified femtosecond pulsed light (550 μJ/pulse,
150 fs, 800 nm, 1 kHz) was used to generate an intense THz pulse
via optical rectification in a ZnTe crystal (ZnTe1; 110 orientation, 25 mm × 25 mm × 1 mm thick). The generated THz pulse
passing through the sample and the probe light from the samelaser were made to cross in another ZnTe crystal (ZnTe2; 110
orientation, 25 mm × 25 mm × 1 mm thick) for 2D-ST THz
imaging. The resulting 2-D image of the probe light, corresponding to the 2D-ST THz image, was imaged with a lens (L2) onto ahigh-speed 12-bit CMOS camera (232 × 232 pixels, maximum
frame rate = 1000 frames/s) synchronized with the 1-kHz laser
pulse. The CMOS camera acquired a 2D-ST THz image of thesample in a dynamic subtraction mode at a frame rate of 500 Hz[35]. Fourier transformation of the horizontal axis in the 2D-ST
THz image resulted in a THz spectral line image of the sample
along the THz focal line. To obtain a 2-D spectral image of thesample, the sample was continuously moved across the THz focal line by a mechanical stage while acquiring a series of line images consecutively (acquisition interval = 100 μm), and the individual line images were then pieced together. Since construction of a 2-D color image by successive stacking of real-timecolor line image is similar to the processing performed in color
scanners, we call this system a THz color scanner [30], [31].
Without any sample, we acquired a THz spectral line image of the incident THz power at an image acquisition time of
100 ms, as shown in Fig. 8(a). The horizontal and vertical axesin this image, respectively, show the frequency scale and thespatial scale (frequency window = 2.96 THz, frequency resolution = 25.5 GHz, spatial window = 20 mm). We calibrated
the frequency scale of this image using a metal hole array serving as a frequency reference [30]. The power spectrum of thepulsed THz radiation in Fig. 8(b) was extracted along line A in

8600110IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 19, NO. 1, JANUARY/FEBRUARY 2013

Fig. 8. Spectral characteristics of THz profile in the absence of a sample. (a)THz spectral line images of power and (b) power spectrum along line A (image
acquisition rate = 10 line/s).

Fig. 10. THz spectral images of transmittance in four tablet samples. Each
image consists of 232 × 600 pixels (image acquisition time = 60 s).

Fig. 9. (a) Photograph of tablet samples and sample holder concealed in apaper envelope. (b) Absorption spectra of D-glucose, D-maltose, and lactose
tablets.

Fig. 8(a). Several absorption lines of atmospheric water vapor
were observed (see arrows) because the THz beam path was
not purged with nitrogen gas. To estimate the dynamic rangeof the THz signal, we compared the power spectrum with one
measured in the absence of the THz beam (noise spectrum), as
shown by the gray line in Fig. 8(b). The SNR of the THz signal
was greatly enhanced due to the dynamic subtraction mode of
the CMOS camera operating at a frame rate of 500 Hz.
Quality control of pharmaceutical tablets is one promising
application of THz spectral imaging because many medicines
show sharp spectral fingerprints in the THz region resulting
from their crystal structures. Although many researches havebeen performed extensively in this field, achieved acquisition
time in existing THz-TDS systems is no more than a few tens
ms at a single-point measurement [7]. This data acquisition rateis still insufficient to perform THz spectral imaging of moving objects. As a result, the scope of this application does notinclude total inspection in a manufacturing process. Here, we
demonstrate the potential of our system for rapid nondestructiveclassification of pharmaceutical tablets moving on a translation
stage. In this demonstration, we used three different kinds ofsugar tablets (D-glucose, D-maltose, and lactose powders mixed
with polyethylene powder; diameter = 10 mm, thickness =1 mm) and one nonsugar tablet (polyethylene powder; diameter= 10 mm, thickness = 1 mm) as samples, as shown in Fig. 9(a).
THz spectral fingerprints of the sugar tablets were measured
beforehand with a commercial THz-TDS system (Otsuka Elec-tronics Co., Ltd., TR-1000). Fig. 9(b) compares the absorption
spectra among the three sugar tablets. Because characteristic
spectral fingerprints appeared in these spectra, it should be possible to classify these sugar tablets based on their THz spectral
fingerprints even though they have a similar appearance.
We acquired 116 consecutive spectral images of the THz
transmittance of the sample in a range from 0 to 2.98 THz, witha frequency resolution of 25.5 GHz. Fig. 10 shows THz powertransmittance images of the samples (image area = 60 mm ×
20 mm, pixel size = 600 pixels × 232 pixels, image acquisition
time = 60 s) at four different frequencies appearing in the THz
spectral fingerprints: 0.511 THz for the lactose, 1.073 THz for
the D-maltose, 1.405 THz for the D-glucose and lactose, and1.609 THz for the D-maltose [see Fig. 9(b)]. The black color inthese images corresponds to a low transmittance and therefore
to a high absorption. Low-transmittance images appeared at
0.511 and 1.405 THz for the first tablet, at 1.073 and 1.609 THz
for the second tablet, and at 1.405 THz for the fourth tablet,
respectively. The third tablet did not show low-transmittanceimages at any frequencies. From these results, we determinedthat the first, second, third, and fourth samples were the lactose,
D-maltose, reference, and D-glucose tablets, respectively.
VI. THZ SPECTRAL CT FOR CONTINUOUSLYROTATING OBJECTSTo reconstruct 2-D spectral cross section of an object basedon CT algorithm, one has to acquire 3-D information composedof one spatial dimension (line image), one color dimension(spectral data), and one angular dimension (projection angle).
However, since existing THz CT system is based on both pointby-point and angle-by-angle measurements of the time-domain

YASUI et al.: REAL-TIME TWO-DIMENSIONAL SPATIOTEMPORAL TERAHERTZ IMAGING

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Fig. 11. Experimental setup. (THz-CL1 and THz-CL2: THz cylindrical lenses,
THz-L: THz planoconvex lens, P: polarizer, A: analyzer, L: optical planoconvex
lens.)

spectrometer, it needs to perform a serial mechanical scanningof time-delay, sample position, and projection angle. The resulting long acquisition time is a strong limitation in practicaluse. On the other hand, 2D-ST THz imaging provides 2-D information of time delay and sample position simultaneously andhence THz color line image in real time without the need formechanical scanning (see Section V). Therefore, if the 2D-ST
THz imaging is applied for THz CT, only continuous rotation ofan object is required for 2-D spectral cross section. This leads
to large decrease of acquisition time in THz spectral CT.
The experimental setup of our THz spectral CT system isshown in Fig. 11, in which we modified two parts of the THzcolor scanner system (see Fig. 7) for THz CT. The first modification is in the optical system for generating an intense THz
pulse (not shown in Fig. 11). To greatly increase the THz pulse
energy and hence decrease the image acquisition time, we used
Cherenkov radiation from a LiNbO3 crystal excited by a pulsefront-tilted pump beam [36], [37]. We further modified the line
focusing of the THz beam across the cross section of the sample
by using a THz cylindrical lens (CL1). These modifications enabled us to achieve real-time line projection of the THz beam on
the sample. Then, the THz beam passing through the sample wasused for 2D-ST THz imaging. The resulting 2D-ST THz image
had a temporal range of 37 ps and a spatial range of 15 mm.
Fourier transformation of the time axis in the 2D-ST THz image gave spectral line projection data of the sample (spectral
resolution = 27 GHz).
In the following demonstrations, we rotated the sample
through a full turn by continuous rotation for 6 s. During this rotation, 600 line-projection datasets of a 2D-ST THz image werecollected at a rate of 100 Hz, resulting in an angular sampling
resolution of only 0.6◦ . In the traditional THz-CT, only 18 (angular step of 10◦ ) or 36 (angular step of 5◦ ) projections angles
are selected owing to the long acquisition time of the pointby-point and angle-by-angle scanning measurement (more than
1 h). Therefore, a significant advantage of the proposed system over conventional systems is the ability to perform fast andprecise CT imaging. Finally, the spectral cross section of thesample across the THz focal line was reconstructed by analyz-

Fig. 12. Four metallic bars (2-mm diameter). (a) Optical photograph, (b)sinogram of spectral amplitude at 0.6 THz, and (c) spectral cross section at
0.6 THz.

ing a spectral line projection data series, that is, a sinogram,with the FBP algorithm [14].
We investigated a simple sample consisting of four metallic bars (2-mm diameter) illuminated by a THz focal line, todemonstrate the rapid cross-sectional reconstruction, as shownin Fig. 12(a). Fig. 12(b) shows the sinogram of the spectral
amplitude at 0.6 THz for this sample, where the horizontal
and vertical coordinates, respectively, give the rotation angle(600 points from 0◦ to 360◦ ) and the THz projection-line image(232 points from 0 to 15 mm). The white color indicates a decrease in the spectral amplitude. An isolated distribution of thefour metallic bars generated four overlapping sinusoidal waveforms in the sinogram with different phases of the rotation angle. Using the standard FBP algorithm [14], we reconstructed across-sectional image corresponding to the portion of the object
illuminated by the THz line, as shown in Fig. 12(c). Since whiteand black colors indicated weak and strong signals in the THz
spectral amplitude, respectively, the white regions in Fig. 12(c)visualize some structures where the signal is decreased. Although no spectroscopic information could be obtained fromthis metallic sample, the positions and shapes of the four metallic bars were clearly visualized, demonstrating the ability of thesystem to properly reconstruct cross-sectional images of simple
samples in only 6 s.
The second sample was a foam cylinder (16-mm diameter) in
which an off-axis cylindrical hole (4-mm diameter) was drilled,
as shown in Fig. 13(a). To demonstrate THz spectral CT, the hole
was filled with lactose powder mixed with polyethylene powder(30% lactose and 70% polyethylene). As shown in Fig. 9(b), lactose exhibits a THz spectral fingerprint at 0.511 THz. Fig. 13(b)and (c) represents the sinogram of the spectral amplitude at
0.53 THz and the corresponding reconstructed spectral crosssection of the sample, respectively. The area of the foam cylinder was not visible owing to the low refractive index change

8600110IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 19, NO. 1, JANUARY/FEBRUARY 2013

Fig. 13. Lactose powder filled in one off-axis cylindrical hole formed in foam
cylinder. (a) Schematic drawing, (b) sinogram of spectral amplitude at 0.53 THz,and (c) spectral cross section at 0.53 THz.

from air to foam and the low absorption coefficient of the foam(Rohacell, α = 0.1 cm−1 , refractive index = 1.15). In Fig. 13(b)and (c), the position and the size of the cylindrical hole are revealed due to the different transmission of the THz radiation inthis part of the sample. The lower transmission was due to bothrefraction losses and lactose absorption at 0.53 THz.
Distortions of the cross-sectional THz images were observedin Figs. 12(c) and 13(c). To explain it, we have to take into
account several phenomena. First, naturally, diffraction effects
are important since the millimeter size object is close to theimaging wavelength. These effects uniformly blurred the final
reconstruction of the THz cross section. However, they cannot
explain why the distortion is mainly elliptical concerning theimaging of cylindrical metal bars [see Fig. 12(c)] or lactose
powder [see Fig. 13(c)]. Another reason is the position of thecylinder (metallic bar or lactose powder), at the edge of thereconstruction area. For instance, in Fig. 13(b) (sinogram), we
can observe that the position of the lactose cylinder is near theedge of the observing area for the angles 0◦ , 180◦ , and 360◦ .
Consequently, the boundary of the 16-mm diameter foam holder
is not directly involved here. During the application of the FBP
algorithm, this induces a distortion since the algorithm requiresa slightly larger visualization of the cylinder (the cylinder itself
and a few mm around) in order to properly determine its actual
shape.
VII. CONCLUSION
We reviewed three techniques for rapid, functional THz imaging based on 2D-ST THz imaging. It is well known that real-timemovie of a moving object can be measured at the fixed timedelay [38]. However, THz imaging without time-delay scanningmerely gives monochromatic shadowgraph without any spectral

information (or depth information in the case of THz tomography) in the same manner as X-ray or ultrasound methods. It
should be emphasized in this review article that incomparable
high-speed data acquisition achieved by 2D-ST THz imaging
opens the door to THz spectral imaging and tomographic imaging of moving objects.
First, using 2-D THz reflection tomography, the temporal evolution of 2-D cross-sectional images was clearly visualized as
the illuminating position of the THz beam moved across a paintsample. Interesting applications of this real-time tomography
will be in the field of art conservation as well as car painting.
Second, we demonstrated the ability to distinguish and spatiallyimage three different sugar tablets by using a THz color scanner
employing real-time line scanning of a THz beam. This demonstration showed the potential of the THz color scanner in total
inspection for rapid quality control of pharmaceutical tablets
moving on a conveyor belt. Third, we showed the potential ofreal-time THz line projection for THz spectral CT of continuously rotating objects. Owing to the 10-ms acquisition time of
the 2D-ST THz image, line projection data were recorded in realtime in association with the continuous rotation of the sample.
To the best of our knowledge, this is the first time that thesefunctional THz imaging techniques have been applied to moving objects. These rapid, functional THz imaging techniques
based on 2D-ST THz imaging will be a powerful tool for sensing, nondestructive inspection, and material characterization ofmoving objects in practical applications.
ACKNOWLEDGMENT
The authors would like to acknowledge Dr. K. Kitagishi and
Dr. Y. Izutani of Otsuka Electronics Co. Ltd., Japan, for preparation of the sugar tablets and for conducting THz-TDS measurements. They also thank K. Sawanaka, A. Ihara, M. Minami,
M. Fujio, and Y. Ohgi of the Graduate School of Engineering
Science, Osaka University for their experimental support in partof this work.
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Takeshi Yasui received the Ph.D. degree in mechanical engineering from the University of Tokushima,
Tokushima, Japan, in 1997.
From 1997 to 1999, he was a Postdoctoral Research Fellow in the National Research Laboratoryof Metrology. He was with the Graduate School ofEngineering Science, Osaka University, from 1999 to2010, and was briefly with the University of Bordeaux
I in 2007 and the University of Littoral Côte d’Opalein 2010 as an Invited Professor. He is currently a Professor at the Institute of Technology and Science, The
University of Tokushima, and an Invited Professor in the Graduate School ofEngineering Science, Osaka University, Toyonaka, Japan. His research interests
include terahertz instrumentation and metrology, laser stabilization, nonlinear
optical microscopy, and ultrafast time-resolved measurement.
Dr. Yasui is a member of the Optical Society of America, the Japan Societyof Applied Physics, the Optical Society of Japan, the Laser Society of Japan,
the Japan Society of Medical Electronics and Biological Engineering, the Japan
Society of Mechanical Engineers, and the Institute of Electronics, Information
and Communication Engineers. He received the Award for the Most Promising
Young Scientist from the Optical Society of Japan in 1998, the Sakamoto Award
from the Japan Society of Medical Electronics and Biological Engineering in
2006, the Optics Paper Award from the Japan Society of Applied Physics, andthe Funai Award from the Japan Society of Mechanical Engineers in 2009.

Mukesh Jewariya received the M.Sc. degree inphysics from Indian Institute of Technology, Roorkee, India, in 2003, the M.Eng. degree in laser technology from Devi Ahilya Vishwavidyalaya, Indore,
India, in 2006, and the D.Sc. degree in physics from
Kyoto University, Kyoto, Japan, in 2010.
From 2011 to 2012, he was a Specially Appointed
Researcher in the Renovation Center of Instruments
for Science Education and Technology, Osaka University. He is currently an Assistant Professor at theInstitute of Technology and Science, The University of Tokushima, Tokushima, Japan. His research interest includes generation of high-power terahertz pulse, terahertz imaging, and terahertz frequency
metrology.
Dr. Jewariya is a member of the Japan Society of Applied Physics, thePhysical Society of Japan, and the Optical Society of America. He is also Life
Member of Indian Laser Association and Indian Physics Association.

Takashi Yasuda received the B.S. and M.S. degrees in mechanical engineeringfrom Osaka University, Toyonaka, Japan, in 2001 and 2003, respectively.
He has been with the Hamamatsu Photonics K.K., Hamamatsu, Japan, since2003. His research interests include terahertz imaging and spectroscopy.
Mr. Yasuda is a member of the Japan Society of Applied Physics.

8600110IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 19, NO. 1, JANUARY/FEBRUARY 2013Markus Schirmer received the M.S. degree in physics from RWTH Aachen
University, Aachen, Germany in 2011.
In 2009, he worked on the terahertz imaging as an exchange student in the
Graduate School of Engineering Science, Osaka University, Toyonaka, Japan.
He was employed as a Research Assistant at RWTH Aachen University, Germany, in 2011 after his graduation.
Mr. Schirmer is a member of the Deutsche Physikalische Gesellschaft.

Tsutomu Araki received the Ph.D. degree in applied physics from Osaka University, Suita, Japan, in
1977, and the Ph.D. degree in medical sciences fromthe University of Tokushima, Tokushima, Japan, in
1986.
From 1978 to 1979, he was a Postdoctoral Fellow in the University of Wisconsin. He was with theFaculty of Medicine of the University of Tokushimafrom 1979 to 1987 and the Faculty of Engineeringof the University of Tokushima from 1987 to 1996.
He is currently a Professor in the Graduate School ofEngineering Science, Osaka University, Toyonaka, Japan. His research interests
include development of a new biomedical and biophysical measurement systemusing an ultrafast pulse laser.
Dr. Araki is a member of the Society for Applied Spectroscopy, the OpticalSociety of America, the Japan Society of Mechanical Engineers, and the Japan
Society of Applied Physics. He received the Sakamoto Award from the Japan
Society of Medical Electronics and Biological Engineering, the Achievement
Award from the Bioengineering Division in the Japan Society of MechanicalEngineers in 2006, the Achievement Award of Education and Research fromthe Osaka University in 2008, and the Funai Award from the Japan Society of
Mechanical Engineers in 2009.

Emmanuel Abraham received the Ph.D. degree inphysics from Bordeaux 1 University, Talence Cedex,
France, in 1997.
In 1997–1998, he joined as a Postdoctoral Research Fellow the National Research Laboratory ofMetrology, AIST, Tsukuba, Japan, and the FemtoSecond Technology project to study industrial applications of femtosecond lasers, including ultrafast optical Kerr gate. Since 1998, he has been an AssistantProfessor at Bordeaux 1 University, working at theLaboratoire Ondes et Matière d’Aquitaine. He wasinvited in Osaka University as an Invited Researcher in 2007 and Universityof Tokushima as a Short-Term Invited Fellow Researcher of Japan Society for
the Promotion of Science in 2010. His research interests include femtosecond
time-resolved spectroscopy and nonlinear optics (ultrasfast electron transfer inorganic molecules, energy transfer, elementary chemical reactions such as photoisomerization, etc.), applications of femtosecond infrared lasers to medical
imaging (optical echography, propagation of optical pulses in turbid media).
More recently, he has investigated the potential of terahertz radiation for the
analysis of objects related to cultural heritage such as paintings, sculptures,ceramics, etc., by using pulsed and continuous millimeter-wave sources for 2-D
imaging and 3-D terahertz computed tomography.