Real-time terahertz color scanner for moving
objects
Takeshi Yasui,1,* Ken-ichi Sawanaka,1 Atsushi Ihara,1 Emmanuel Abraham,2Mamoru Hashimoto,1 and Tsutomu Araki1
1

Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531,
Japan
2
Centre de Physique Moléculaire, Optique et Hertzienne, UMR5798, Université Bordeaux I,
351, Cours de la Libération , 33405 Talence, France*
Corresponding author: t-yasui@me.es.osaka-u.ac.jp

Abstract: Terahertz time-domain spectroscopic (THz-TDS) imaging is aninteresting new tool for nondestructive testing and other applications.
However, the current speed of image acquisition is relatively low, making it
difficult to use for moving objects. In this paper, we propose a real-time
THz-TDS line scanner based on electro-optical time-to-space conversion
and line focusing of a THz beam. The proposed system functions as a color
scanner in the terahertz spectral region with fast line-scanning and has beensuccessfully used to image objects, which are moved on a translation stage.
The achieved THz-TDS imaging rate is 23 200 pixels per second. This
proposed THz-TDS line scanner has the potential to become a powerful tool
for monitoring moving objects in various real-world applications.
©2008 Optical Society of America
OCIS codes: (110.6795) Terahertz imaging; (120.6200) Spectrometers and spectroscopic
instrumentation; (300.6495) Spectroscopy, teraherz

References and links
1.
2.
3.
4.
5.

6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.

D. E. Bray and D. McBride, Nondestructive Testing Techniques (John Wiley & Sons, 1992).
J. M. Chalmers and P. R. Griffiths, Handbook of vibrational spectroscopy (John Wiley & Sons, 2002).
D. Abbott and X.-C. Zhang, "T-ray imaging, sensing, and retection," Proc. IEEE 95, 1509-1513 (2007).
D. M. Mittleman, Sensing with THz radiation (Springer, 2003).
W. Withayachumnankul, G. M. Png, X. X. Yin, S. Atakaramians, I. Jones, H. Lin, B. S. Y. Ung, J.
Balakrishnan, B. W.-H. Ng, B. Ferguson, S. P. Mickan, B.M. Fischer, and D. Abbott, "T-ray sensing andimaging," Proc. IEEE 95, 1528-1558 (2007).
B. B. Hu and M. C. Nuss, “Imaging with terahertz waves,” Opt. Lett. 20, 1716-1718 (1995).
D. M. Mittleman, S. Hunsche, L. Boivin, and M. C. Nuss, “T-ray tomography,” Opt. Lett. 22, 904-906(1997).
B. Ferguson, S. Wang, D. Gray, D. Abbott, and X.-C. Zhang, “T-ray computed tomography,” Opt. Lett. 27,
1312-1314 (2002).
T. Löffler, T. Bauer, K. Siebert, H. Roskos, A. Fitzgerald, and S. Czasch, “Terahertz dark-field imaging of
biomedical tissue,” Opt. Express 9, 616-621 (2001).
S. Hunsche, M. Koch, I. Brener, and M. C. Nuss, “THz near-field imaging,” Opt. Commun. 150, 22-26(1998).
K. Kawase, Y. Ogawa, Y. Watanabe, and H. Inoue, “Non-destructive terahertz imaging of illicit drugs
using spectral fingerprints,” Opt. Express 11, 2549-2554 (2003).
Y. C. Shen, T. Lo, P. F. Taday, B. E. Cole, W. R. Tribe, and M. C. Kemp, “Detection and identification of
explosives using terahertz pulsed spectroscopic imaging,” Appl. Phys. Lett. 86, 241116 (2005).
X. X. Yin, B. W.-H. Ng, B. M. Fischer, B. Ferguson, and D. Abbott, "Support vector machine applicationsin Terahertz pulsed signals feature sets," IEEE Sensors J. 7, 1597 – 1608 (2007).
X. X. Yin, K. M. Kong, J. W. Lim, B. W.-H. Ng, B. Ferguson, S. P. Mickan, and D. Abbott, "Enhanced Tray signal classification using wavelet preprocessing," Med. Bio. Eng. Comp. 45, 611-616 (2007).
K. Fukunaga, Y. Ogawa, S. Hayashi and I. Hosako, “Terahertz spectroscopy for art conservation,” IEICEElectron. Express 4, 258-263 (2007).
T. Yasui, E. Saneyoshi, and T. Araki, “Asynchronous optical sampling terahertz time-domain spectroscopy
for ultrahigh spectral resolution and rapid data acquisition,” Appl. Phys. Lett. 87, 061101 (2005).

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Received 7 Dec 2007; revised 11 Jan 2008; accepted 11 Jan 2008; published 16 Jan 200821 January 2008 / Vol. 16, No. 2 / OPTICS EXPRESS 1208

17.

18.
19.
20.
21.
22.
23.

24.

25.
26.
27.

28.
29.

30.
31.
32.
33.
34.

T. Yasui, Y. Kabetani, E. Saneyoshi, S. Yokoyama, and T. Araki, “Terahertz frequency comb by
multifrequency-heterodyning photoconductive detection for high-accuracy, high-resolution terahertz
spectroscopy,” Appl. Phys. Lett. 88, 241104 (2006).
A. Bartels, A. Thoma, C. Janke, T. Dekorsy, A. Dreyhaupt, S. Winnerl, and M. Helm, “High-resolution
THz spectrometer with kHz scan rates,” Opt. Express 14, 430-437 (2006).
Q. Wu, T. D. Hewitt, and X.-C. Zhang, “Two-dimensional electro-optic imaging of THz beams,” Appl.
Phys. Lett. 69, 1026-1028 (1996).
Z. Jiang and X.-C. Zhang, “Electro-optic measurement of THz field pulses with a chirped optical beam,”Appl. Phys. Lett. 72, 1945-1947 (1998).
B. Ferguson, S. Wang, D. Gray, D. Abbott, and X.-C. Zhang, "Identification of biological tissue using
chirped probe THz imaging," Microelectronics J. 33, 1043-1051 (2002).
Z. Jiang, F. G. Sun, and X.-C. Zhang, “Terahertz pulse measurement with an optical streak camera,” Opt.
Lett. 24, 1245-1247 (1999).
J. Shan, A. S. Weling, E. Knoesel, L. Bartels, M. Bonn, A. Nahata, G. A. Reider, and T. F. Heinz, “Singleshot measurement of terahertz electromagnetic pulses by use of electro-optic sampling,” Opt. Lett. 25, 426428 (2000).
K. Y. Kim, B. Yellampalle, A. J. Taylor, G. Rodriguez, and J. H. Glownia, “Single-shot terahertz pulse
characterization via two-dimensional electro-optic imaging with dual echelons,” Opt. Lett. 32, 1968-1970(2007).
F. G. Sun, Z. Jiang, and X.-C. Zhang, “Analysis of terahertz pulse measurement with a chirped probe
beam,” Appl. Phys. Lett. 73, 2233-2235 (1998).
T. Yasuda, T. Yasui, T. Araki, and E. Abraham, “Real-time two-dimensional terahertz tomography ofmoving objects,” Opt. Commun. 267, 128–136 (2006).
G. Gallot, J. Zhang, R. W. McGowan, T.-I. Jeon, and D. Grischkowsky, “Measurements of the THz
absorption and dispersion of ZnTe and their relevance to the electro-optic detection of THz radiation,”
Appl. Phys. Lett. 74, 3450-3452 (1998).
Z. Jiang, F. G. Sun, Q. Chen, and X.-C. Zhang, “Electro-optic sampling near zero optical transmission
point,” Appl. Phys. Lett. 74, 1191-1193 (1999).
F. Miyamaru, T. Yonera, M. Tani, and M. Hangyo, “Terahertz two-dimensional electrooptic sampling
using high speed complementary metal-oxide semiconductor camera,” Jpn. J. Appl. Phys. 43, L489–L491(2004).
A. Dobroiu, M. Yamashita, Y. Ohshima, Y. Morita, C. Otani, and K. Kawase, “Terahertz imaging system
based on a backward-wave oscillator,” Appl. Opt. 43, 5637–5646 (2004).
F. Miyamaru and M. Hangyo, “Finite size effect of transmission property for metal hole arrays insubterahertz region,” Appl. Phys. Lett. 84, 2742-2746 (2004).
F. Miyamaru, S. Hayashi, C. Otani, K. Kawase, Y. Ogawa, H. Yoshida, and E. Kato, “Terahertz surfacewave resonant sensor with a metal hole array,” Opt. Lett. 31, 1118-1120 (2006).
F. Miyamaru, M. W. Takeda, T. Suzuki, and C. Otani, “Highly sensitive surface plasmon terahertz imaging
with planar plasmonic crystals,” Opt. Express 15, 14804-14809 (2007).
Y. Watanabe, K. Kawase, T. Ikari, H. Ito, Y. Ishikawa, and H. Minamide, “Component spatial patternanalysis of chemicals using terahertz spectroscopic imaging,” Appl. Phys. Lett. 83, 800-802 (2003).

1. Introduction
In the fields of nondestructive testing, security screening, and biomedical applications, it isimportant not only to visualize the geometrical shapes of internal structures but also to
analyze and identify the chemical composition of test objects. Although both X-rays [1] andultrasonic waves [1] have been utilized in those applications, the hazardous ionizing effects ofthe former and the need for direct physical contact in the latter limit their utility. Furthermore,those imaging modalities only produce “monochrome” pictures of a sample, making it
difficult to identify its chemical components. Infrared spectroscopy is also frequently used tocharacterize semiconductors, medicines, and other materials on the basis of the rich spectral
fingerprints of molecular vibrational modes in the infrared region [2]. However, infrared
scattering severely limits the penetration depth in many samples. In the past decade, terahertz(THz) radiation, lying at the boundary between optical and electrical waves [3], has emerged
as a new mode for sensing and imaging [4]. Pulsed THz waves have attractive features such as
good penetration for dry, non-polar materials, low scattering, long free-space propagation in
dry air, low photon energy, good beam coherence, and broad spectral bandwidth [5]. Sincemany materials indicate spectral fingerprints in the THz region, the spectroscopic analysis in#90646 - $15.00 USD(C) 2008 OSA

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this region has received interests as a new tool for material characterizations. Furthermore, theavailability of coherent optical imaging has made THz imaging an attractive non-contact, nonionizing method for a variety of applications. Since the first demonstration of THz
transmission imaging in 1995 [6], several other kinds of THz imaging techniques have beenreported: reflection tomography [7], computed tomography [8], dark-field imaging [9], nearfield imaging [10], and spectroscopic imaging. The last technique in particular permits
identification and spatial imaging of the chemical components in a sample on the basis of theTHz spectral fingerprints because this imaging modality effectively provides “color” picturesin the THz range in contrast to “monochrome” X-ray and ultrasound imaging. Furthermore, it
is straightforward to combine THz spectroscopic imaging with other imaging techniques.
Consequently, it has important applications in screening of illicit drugs [11] and explosives[12], detection of cancerous tissue [9] and RNA [13], classification of powders [14], and art
conservation [15]. Despite the advantages listed above, however, the technique of THz
spectroscopic imaging is hampered by the slow speed of image acquisition.
In a typical THz time-domain spectroscopy (THz-TDS) system, a mechanical translation
stage driven by a stepping motor is used for time-delay scanning between the THz pulse andthe optical probe pulse. When this system is extended to THz-TDS imaging by rasterscanning the sample through the THz beam focus, the overall image rate can be quite low. As
a result, the method has so far only been applied to stationary objects. If THz-TDS imaging
can be extended to moving objects in real time, such as industrial products on a conveyor belt,
the applications of THz-TDS imaging will be greatly increased. For more rapid sweeps acrossa range of time delays, a galvanometric scanner or piezoelectric device can be used instead ofa stepping motor. However, there is an inherent trade-off between scan range and rate, andthus between spectral resolution and measurement time in THz-TDS. A better alternative
would be to eliminate the need for time-delay scanning altogether, which would largely solve
the problem of the low imaging rate. Recently, new THz-TDS systems based on
asynchronous-optical-sampling [16] or multi-frequency-heterodyning [17] photoconductive
detection have been proposed to simultaneously achieve rapid data acquisition and highspectral resolution. In these methods, two asynchronous mode-locked lasers are used for thepump and probe lasers, and thus no mechanical translation stages for time delay scanning are
required, thereby overcoming the tradeoff between spectral resolution and measurement timein THz-TDS. Kilohertz rates of time delay have been attained using lasers with a 1-GHz
repetition rate and free-space electro-optic sampling (FSEOS) [18]. Unfortunately, manyscans have to be integrated together to achieve an adequate signal-to-noise ratio (SNR).
Furthermore, mechanical rastering of the sample position is still required.
In many applications, an object is typically moved unidirectionally at a constant speed bya translation stage. To construct a two-dimensional (2D) THz-TDS image of such an object, it
would be necessary to acquire and piece together consecutive one-dimensional (1D) THzTDS images. Real-time acquisition of 1D-THz-TDS images can be achieved if instead of amechanical stage we use alternative techniques for measuring the time delay and 1D sample
position. One effective stage-free configuration combines a single-shot measurement of thetemporal waveform with 1D transverse imaging, enabling real-time 2D spatiotemporal (2DST) imaging. In the THz region, there have been several reports on single-shot measurementsof the temporal waveform of a THz pulse based on 2D-FSEOS [19]. These include time-towavelength conversion using a chirped probe pulse [20, 21], use of a THz streak camera [22],time-to-space conversion based on non-collinear 2D-FSEOS [23], and an echelon method [24].
Although some of these techniques could be extended to 2D-ST THz imaging, non-collinear
2D-FSEOS should prove most suitable since it only requires a 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 [25] or by the
instrumental response [22]. We have previously proposed a 2D-ST THz imaging techniquebased on a combination of non-collinear 2D-FSEOS and line focusing of a THz beam onto asample, and demonstrated the method in a reflection geometry for real-time 2D THz#90646 - $15.00 USD(C) 2008 OSA

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tomography [26]. Such an imaging technique can also be applied to real-time 1D-THz-TDS
imaging in transmission geometry.
In the present paper, we propose to obtain real-time 1D-THz-TDS images of a moving
object, which is based on 2D-ST THz imaging by combining non-collinear 2D-FSEOS withline focusing of a THz beam at a sample in the transmission geometry. The proposed system
functions as a color-scanner machine in the THz region with fast line-scanning, namely THzTDS line scanner. After describing the experimental setup and performance, we present someexamples of THz-TDS images and movies collected for a moving object.
2. Experimental setup
Figure 1 shows the setup of our THz-TDS line scanner. A femtosecond Ti:sapphire
regenerative amplifier (Spectra-Physics Hurricane, pulse energy = 600 µJ, pulse duration =150 fs, central wavelength = 800 nm, repetition rate = 1 kHz, beam diameter = 10 mm) was
employed to generate and detect THz pulses. After diverging the pump light with a planoconcave lens (L1, f = -60 mm), an intense THz pulse is generated via optical rectification of
the pump light in a 1-mm-thick, 15-mm-square <110> ZnTe crystal (ZnTe1) manufactured by
Nikko Materials of Japan. The THz beam is emitted from the ZnTe1 in collinear with thepump beam, and its radiation angle is about 10°. The THz beam is then collimated by an offaxis parabolic mirror (OAP-M, f = 119 mm). A white polyethylene plate (IRF, thickness = 3mm) was used as a filter to block the residual laser light. In order to line focus and image orcollimate the THz beam, we used three THz lenses (CL1, L2, and CL2) manufactured by Paxof Japan. Figure 1 shows a top view and a side view of the THz beam optics. The first THz
cylindrical lens (CL1, f = 50 mm) focuses the THz beam onto the sample, resulting in a line ofillumination along the X-axis of the sample. The height of the THz focal line is 20 mm and its
width depends on the THz frequency, as discussed later. While passing through the sample,
the THz pulse experiences attenuation, delay, and/or distortion due to the frequencydependent absorption and refractive index. The THz beam is then incident on a 1-mm-thick,
25-mm-square, <110> ZnTe crystal (ZnTe2, manufactured by Nikko Materials of Japan) used
for non-collinear 2D-FSEOS, after it has passed through a THz plano-convex lens (L2, f = 100mm) and a second THz cylindrical lens (CL2, f = 100 mm). As a result of these optics, theTHz beam incident on ZnTe2 is collimated in the top view and imaged in the side view (seethe imaging ray indicated by the green broken line). Meanwhile, a probe beam is expanded toabout 30-mm diameter by a beam expander (BE), and is then polarized linearly by a Glanlaser polarizing prism (P, extinction ratio = 5 × 10-6, clear aperture = 20 mm diameter). A
mechanical stage is used to calibrate the time scale.
The THz and probe beams are incident on ZnTe2 at a crossed angle θ in air. This results in
non-collinear 2D-FSEOS for the time-to-space conversion of the pulsed THz electric field.
Because the principles of non-collinear 2D-FSEOS are given in detail elsewhere [23, 26], onlya brief description of the method is given here. Consider the THz electric field to be composedof a negative and a positive peak with a relative time delay of Δτ (indicated by the blue solidand broken line segments in the top view in Fig. 1). These two negative and positive
wavefronts overlap with the single wavefront of the probe beam (the red solid line segment in
Fig. 1) at different horizontal positions inside ZnTe2. In this way, the superposed THz electricfield at different times peaks at different spatial positions within the transverse profile of theprobe beam. This results in a time-to-space conversion factor determined by [23]Δτ = (Δh × tanθ) / c,(1)where Δh is the spatial displacement corresponding to the time delay Δτ and c is the speed oflight in vacuum. This temporal profile develops successively along the width of ZnTe2. The
resulting time window (tW) and step increment (Δt) are defined astW = (D × tanθ) / c,

(2)

and
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Fig. 1. Experimental setup. BS: beam splitter; OC: optical chopper; L1: plano-concave lens;
ZnTe1 and ZnTe2: zinc telluride crystals; OAP-M: off-axis parabolic mirror; IRF: infrared
cutoff filter; CL1 and CL2: THz cylindrical lenses; S: sample; L2: THz plano-convex lens; BE:beam expander; P: polarizer; A: analyzer; L3: plano-convex lens.

Δt = tW / N = (D × tanθ) / (cN),(3)

where D is the diameter of the probe beam and N is the number of pixels across the horizontal
coordinate of the 2D imaging device, which is described later. The corresponding frequency
resolution (Δν) and range (νR) in THz-TDS are
Δν = 1 / tW = c / (D × tanθ),(4)

νR = Δν (N/2) = (cN) / (2D × tanθ),

(5)

and
Therefore, the frequency resolution can be set by proper selection of D and θ. The resolution
is improved by increasing D and/or θ. The D value of the setup in Fig. 1 is limited by the clear
aperture of P (namely its 20 mm diameter). Furthermore, if the surface of ZnTe2 isperpendicular to the THz beam (as in conventional configurations for non-collinear 2DFSEOS [23, 26]), the tilted clear aperture of ZnTe2 in the probe beam will limit the D value inthe horizontal direction. Therefore, in the setup in Fig. 1, the surface of ZnTe2 is normal to theprobe beam. (We confirmed in advance that the differences in the FSEOS efficiencies anddistortions of the THz temporal waveform for the two configurations are negligible.) On theother hand, increase of the θ value reduces reflection of the p-polarized THz beam at the front
surface of ZnTe2, resulting in increase of THz signal. Given the refractive index of ZnTe
(approx. 3.2 below 2 THz [27]) and of air (namely 1), the Brewster’s angle at the crystal
surface is determined to be 73°. In the present setup where θ = 27°, D = 20 mm, and N = 232
pixels, the values of tw, Δt, Δν, and νR are 34 ps, 0.15 ps, 29 GHz, and 3.3 THz, respectively.

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Fig. 2. Time characteristics in the absence of a sample (with a measurement time of 100 ms).
(a) 2D-ST THz image and (b) temporal waveform of the pulsed THz electric field along line x
in panel (a).

On the other hand, the vertical direction of ZnTe2 is used for 1D transverse imaging of thesample along the THz focal line parallel to the X-axis. In this way, a 2D spatiotemporal (2DST) THz image is formed in ZnTe2, in which the temporal profile of the THz pulse and thesample’s 1D transverse image develop along the horizontal and vertical dimensions,
respectively. The procedure to align the ZnTe2 crystal is as follows. First, the crystal surface
is set to be perpendicular to the probe beam at an intersection of the THz beam and probebeam. And then, the crystal position is adjusted along the propagating direction of the probe
beam to maximize the THz signal while monitoring the 2D-ST image at real time, resulting in
attainment of long interaction length between the THz beam and probe beam in the crystal.
To convert that 2D-ST THz image into a 2D spatial distribution of the probe intensity,
FSEOS is performed near the zero optical transmission point [28] using a polarizer P andanother, crossed, Glan-laser polarizing prism (A, extinction ratio = 5 × 10-6, clear aperture =20 mm diameter). The 2D image of the probe beam is imaged with a demagnification of 0.25
by a lens (L3, f = 80 mm) to a high speed complementary metal-oxide semiconductor(CMOS) camera [Hamamatsu Photonics Intelligent Vision System, sensor area = 4.6 mm ×
4.6 mm, pixel size = 232 × 232 pixels, frame rate = 1000 frames per second (fps), exposure
time = 1 µs, digital output = 12 bit]. To improve the signal-to-noise ratio (SNR), we adopted a
dynamic subtraction technique by synchronizing the camera to the 1 kHz laser pulses [29]. To
this end, the pump beam is chopped using an optical chopper (OC) to block every other laser
pulse. We perform alternate acquisition of probe images with and without the presence of theTHz beam, and then subtract successive images, thereby suppressing the background noise.
Finally, a 1D-THz-TDS image of the THz amplitude of the sample along the focal line is
obtained by a fast Fourier transform (FFT) of the time dimension in the 2D-ST THz image.
3. Basic performance
3.1 Temporal and spectral characteristics
Figure 2(a) shows a 2D-ST THz image in the absence of a sample after integrating 50 images(corresponding measurement time = 100 ms). The green and red areas in the image indicate
positive and negative electric field, respectively. The time scale was calibrated using a known
time delay produced by a mechanical stage. The pulsed THz radiation is visible around 16 ps.
The fade-out profile of THz signal along vertical direction is mainly due to Gaussian spatial
distribution of THz electric field across the beam section. Figure 2 (b) shows a temporal
waveform of the THz electric field extracted along the white line x in the 2D-ST THz image.
The central peak corresponds to a 0.38-ps pulse duration.

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Fig. 3. Spectral characteristics in the absence of a sample (with a measurement time of 100ms). 1D-THz-TDS images of (a) the amplitude and (b) the phase. Line scans along x of (c) the
amplitude and (d) the phase.

An FFT of the time dimension in the 2D-ST THz image results in 1D-THz-TDS images of
the amplitude and phase shown in Figs. 3(a) and (b), respectively, for which the frequency
resolution is 29 GHz. Figures 3(c) and (d) show respectively the amplitude and phase spectraof the THz radiation extracted along line x. Note that the tail of the THz spectra almost
reaches 3 THz.
In conventional real-time 2D THz imaging using a CCD or CMOS camera, the small
modulations in the probe light induced by the THz electric field are detected despite
significant leakage of probe light due to intrinsic birefringence and/or multiple scattering in
the ZnTe2 crystal. For detection of a weak signal within the strong background light, a lock-in
detection scheme is used. Miyamaru et al. have proposed a fast lock-in imaging techniqueusing dynamic subtraction by synchronizing a high-speed CMOS camera to the 1 kHz laser
pulses, and have improved the SNR for collinear 2D-FSEOS equipped with a mechanicaltime-delay stage [29]. On the other hand, for non-collinear 2D-FSEOS, high-speed single-shotmeasurements of the THz temporal waveform can be achieved because no mechanical timedelay stage is needed. This capability avoids reductions in the SNR due to pulse-to-pulse
fluctuations of the amplified femtosecond pulse intensity. To evaluate the effectiveness of
combining use of dynamic subtraction detection and single-shot measurements, weinvestigated the relationship between the SNR and the image integration. Here, the SNR is
defined as a ratio of a peak-to-peak value of bipolar, pulsed signal in the presence of THz
beam to a standard deviation of noise signal in the absence of THz beam with respect to thetemporal waveform of THz electric field. Figure 4 shows the result. Even with no image
integration (measurement time = 2 ms, frame rate = 500 fps), a SNR of about 70 wasachieved. A power law relationship between measurement time and SNR was found in theform Y = αXβ, where Y is the SNR, X is the measurement time, α = 47.7 and β = 0.469 with aregression coefficient of 0.999. When the system noise is limited by shot noise, the SNR is
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Fig. 4. Relationship between signal-to-noise ratio and measurement time or number of images
integrated. The red solid line is a fit to the data in the form Y = αXβ .

be proportional to the square root of the number of images integrated (Y ∝ X0.5). Therefore,
the noise level of the present system is close to the shot-noise limit.
3.2 Imaging characteristics
When investigating the imaging performance of the system, we have to consider thetransverse resolution (δline) and depth of focus (DOFline) of the line focusing lens CL1 with ahorizontal numerical aperture (NA) of 0.20, as well as those (δimage and DOFimage) of theimaging lenses L2 and CL2 with a vertical NA of 0.22 (see Fig. 1). Assuming that theintensity profile of the THz beam is Gaussian, the theoretical diameter of the focal spot at
half-maximum (δ) and the depth of focus (DOF) are [30]δ=

λf 2ln2 cf 2ln2=,d0 π
νd 0 π(6)and(d ) π = ν (dmin ) π ,DOF = min
2

λ

2

ln2

c

ln2

(7)

where λ is the wavelength, f is the focal length of the lens, d0 is the diameter of the collimatedbeam at half-height, and ν is the frequency. These equations indicate that the values of δ and
DOF depend on the THz frequency. Figures 5(a) and (b) plot δline, δimage, DOFline, andDOFimage, respectively, as a function of frequency. The width of the THz focal line onto asample is determined by δline whereas the transverse resolution along the X-direction of asample [vertical axes in Figs. 3(a) and 3(b)] depends on δimage. The DOFline value determines
the tolerance to the thickness and axial positioning of the sample. We must also compare theZnTe2 thickness to the DOFimage. The DOFimage value in the setup of Fig. 1 is greater than theZnTe2 thickness of 1 mm within a 2-THz spectral range, so that there is no degradation of theimaging characteristics. Another factor that could degrade the transverse resolution is thespherical and/or chromatic aberrations of L2 and CL2. Furthermore, the tilt of the ZnTe2
surface relative to the THz beam could deteriorate the imaging characteristics. Therefore theactual transverse resolution of the THz-TDS line scanner is a convolution of those effects.

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Fig. 5. Theoretical curves of (a) the transverse resolution and (b) the depth of focus for the line
focusing and imaging systems of THz beam. δline and δimage are transverse resolution for theTHz line-focusing system (CL1) and the THz imaging system (L2 and CL2) while DOFline and
DOFimage are depth of focus for the line-focusing system and the imaging system, respectively.
The actual transverse resolution, measured by a knife-edge test, is also indicated forcomparison.

To measure the actual transverse resolution, we performed a knife-edge test. A 400-µmthick blade was placed at the sample position to block the upper half of the THz focal line. We
measured 2D-ST THz images with and without the presence of the blade, and obtained thecorresponding 1D-THz-TDS amplitude images by Fourier transforming the time dimension in
the 2D-ST images. To cancel the effect of the uneven amplitude profile along the X-direction,
we calculated a ratio of the 1D-THz-TDS image with the blade to that without the blade.
Figure 6 (a) shows the resulting ratio image of the THz amplitude as a function of frequency.
Figure 6(b) compares knife-edge profiles for various THz frequencies within the spectral
range of the present system, which are extracted from amplitude-ratio profiles along verticalimage line in Fig. 6(a). Movie of Fig. 6 shows the change in the knife-edge profile as the THz
frequency is varied. In this movie, the left panel shows the ratio image of the THz amplitude
while the right panel shows the knife-edge profile along the sweeping red line in the ratio
image. It can be seen that the slope of the knife-edge profile becomes steeper as the THz
frequency increases.
Assuming a Gaussian distribution of THz power across the focal spot, the knife-edge
profile can be expressed in terms of an error function (erf) as [30]I(x) = I0⎛
1⎡x − x c ⎞⎤⎟⎥ ,⎢1+ erf ⎜2 ln2
⎝
2⎣
d ⎠⎦

(8)

where I0 is the total intensity of the beam, x is the knife’s position, xc is its position as it
crosses the center of the beam, and d is the beam’s focused diameter at half-maximum. The
square of the knife-edge profile of the THz amplitude ratio equals the corresponding power
ratio. We fitted it to Eq. 8 and computed the transverse resolution at each THz frequency. The
results are plotted in Fig. 5(a) for comparison with the theoretical values; this figure indicatesthat the actual transverse resolution attains the diffraction limit of the imaging system.
However, the experimental plots are partially better than the theoretical diffraction-limit
curve. We consider two reasons for this result. One is an underestimation of THz beam
diameter (d0) in the THz imaging optics. Although we estimated the THz beam diameter fromradiation angle of THz beam form ZnTe1 and focal length of OAP-M, the actual beam
diameter might be larger than it. In this case, there is a possibility that theoretical diffraction
limit is worse than the actual transverse resolution. Other reason is error of the fitting basedon Eq. 8 in analysis of the knife-edge test.
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Received 7 Dec 2007; revised 11 Jan 2008; accepted 11 Jan 2008; published 16 Jan 200821 January 2008 / Vol. 16, No. 2 / OPTICS EXPRESS 1216

Fig. 6. (3.7 MB) (a) 1D-THz-TDS image of the amplitude ratio around a knife edge. (b) Knifeedge profiles of the THz amplitude ratio at various frequencies.

4. Measurement of a moving object
4.1 Test sample
To test the THz-TDS imaging system, we used as a test sample a metal hole array (MHA)
whose spectral characteristics can be designed arbitrarily. It is a metal plate having 2D
periodic arrays of circular holes of sub-wavelength diameter. Due to resonant coupling of theincident THz wave with surface plasmon polaritons excited on its surface, the MHA exhibits
characteristic optical transmission spectrum at a specific pass-band frequency determined by
the hole diameter and spacing. Such MHAs not only serve as band-pass filters with hightransmittance in the sub-THz and THz region [31] but can also be used as sensitive THz
sensing devices [32, 33]. We used a four-segment MHA (4seg-MHA) made of stainless steel(SUS304) with a thickness of 300 µm manufactured by the Tokyo Process Service Companyof Japan. The four segments each have different pass-bands (0.2, 0.4, 0.8, and 1.6 THz) andare distributed spatially as shown in Fig. 7(a). Specifications of the hole sizes and spacings arelisted in Table 1.
To characterize the 4seg-MHA, we set up a THz-TDS imaging system to raster scan the
MHA sample through the THz beam focus. This system was composed of a Ti:sapphire laser
oscillator (Spectra-Physics MaiTai, pulse energy = 12 nJ, pulse duration = 100 fs, central
wavelength = 800 nm, repetition rate = 80 MHz), two dipole-shaped low-temperature-grown
GaAs photoconductive antennas for THz generation and detection, four off-axis parabolic
mirrors (OAP-Ms), a mechanical time-delay stage (with a speed of 22 seconds per scan), anda lock-in amplifier (with a time constant of 10 ms). After being generated at thephotoconductive antenna and collimated by the first OAP-M (f = 101.6 mm), the THz beam is
focused onto the MHA sample by the second OAP-M (f = 50.8 mm). The diameter of the THz
beam focus is calculated to be 4.2 mm at 0.2 THz, 2.1 mm at 0.4 THz, 1.0 mm at 0.8 THz,and 0.52 mm at 1.6 THz, which are the frequencies used for the THz-TDS images describedlater. The THz beam passing through the sample is re-collimated by the third OAP-M (f =50.8 mm) and is then focused onto another photoconductive antenna for THz detection using
the fourth OAP-M (f = 101.6 mm). For the purpose of THz-TDS imaging measurements, thesample position is raster scanned by a 2D moving stage. The temporal waveform of a 0.35-ps
THz pulse was obtained at a SNR of 1100. Also the THz spectrum over a 2 THz range wasmeasured with a resolution of 21 GHz.

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Received 7 Dec 2007; revised 11 Jan 2008; accepted 11 Jan 2008; published 16 Jan 200821 January 2008 / Vol. 16, No. 2 / OPTICS EXPRESS 1217

Fig. 7. (a) Photograph of the 4seg-MHA and schematic of the measurement area for (b) the
raster-scanning THz-TDS imaging system and (c) the THz-TDS line scanner.
Table 1. Specifications of the four segments of the 4seg-MHA shown in Fig. 7(a).

Pass-band frequency (THz)Hole diameter (mm)Hole spacing* (mm)

0.2
0.4
0.8
1.6

0.75
0.38
0.19
0.1

1.5
0.75
0.38
0.19

*Hole spacing is the center-to-center spacing.

The amplitude spectra of THz signal passing through each segment in the 4seg-MHA are
shown in Fig. 8. The measured peaks coincide with their specified pass-band frequencies. The
substantial peak widths may be caused by dispersion of the incident angle of the focused THz
beam onto the sample and/or by an insufficient number of holes within the THz focus spot forresonant coupling with the surface plasmon polaritons. Next we measured THz-TDS imagesof the 4seg-MHA using the same system. To this end, the sample position was raster scanned
within a 12.8 mm × 12.8 mm area using a 2D stage moving at 320 µm/step [see Fig. 7(b)],resulting in a THz-TDS image composed of 41 × 41 pixels. Since the measurement time forthe temporal waveform is 22 seconds per pixel, it takes about 10 hours to obtain the image
with 41 by 41 pixels. Figure 9 shows the resulting THz-TDS images at 0.189, 0.399, 0.798,
and 1.596 THz. Crosstalk between MHA segments in the latter three THz-TDS images is dueto the high-frequency tails in the transmission spectra of the MHA areas in Fig. 8.
Nevertheless, the image of each MHA area could be easily separated from each other by
principal component analysis [34].
4.2 Real-time THz-TDS imaging of a moving object
To test the performance of the proposed method, we measured real-time THz-TDS images ofa moving 4seg-MHA. Because the THz beam is line-focused onto the sample, the MHA wascontinuously moved across it at a rate of 1 mm/s using a translation stage [see Fig. 7(c)]. Wecollected 500 2D-ST THz images per second for 20 seconds, and integrated each group of 50
images to end up with 200 2D-ST THz images. The acquisition speed of the vertical line
image was 10 lines per second. The SNR in this case is estimated from Fig. 4 to be 403 in theabsence of a sample. Next, 1D-THz-TDS amplitude images were obtained by Fouriertransforming the time dimension in the 2D-ST images. Finally, we obtain successive THzTDS images at an increment of frequency resolution of 29 GHz by rearranging a set of 200
1D-THz-TDS images with regard to THz frequency. The resulting THz-TDS images andmovies of the 4seg-MHA at 0.204, 0.407, 0.815, and 1.600 THz are shown in Fig. 10(a) andits movie with an image size of 20 mm × 20 mm, corresponding to 200 × 232 pixels.
Crosstalk between MHA segments is again seen. By comparison with the THz-TDS images#90646 - $15.00 USD(C) 2008 OSA

Received 7 Dec 2007; revised 11 Jan 2008; accepted 11 Jan 2008; published 16 Jan 200821 January 2008 / Vol. 16, No. 2 / OPTICS EXPRESS 1218

Fig. 8. Transmission spectra of each segment of the 4seg-MHA of Fig. 7(a) measured using araster-scanning THz-TDS imaging system.

Fig. 9. THz-TDS images of the 4seg-MHA at four THz frequencies measured using a rasterscanning THz-TDS imaging system. Each image consists of 41 by 41 pixels.

obtained by the raster-scanning system (see Fig. 9), we conclude that the proposed THz-TDS
line scanner measured the moving 4seg-MHA correctly. It is important to note that this movie
is not a fast-forwarding movie but a “real-speed” movie of the measurement. Thus the present
system can generate vertical line images of a moving sample consecutively in the orderdepending on the movement at all THz frequencies. Here, we consider a possibility that
integration of 50 2D-ST THz images blurs the THz-TDS images in Fig. 10(a). In thisdemonstration, 10 000 2D-ST THz images were measured for the sample movement of 20
mm (2 µm/image). Hence, integration of 50 images is corresponding to averaged image forthe sample movement of 100 µm. Since the sample movement of 100 µm is smaller than thewidth of THz focal line within the spectral range of 2 THz [see δline curve in Fig. 5(a)], one
can neglect the reduction of spatial resolution caused by the image integration of the moving
object. In this way, the proposed system can be applied for moving object. The fade-out
profile of THz signal along vertical direction is due to the Gaussian spatial distribution of THz
amplitude across the beam section, which is caused by the beam profile of the pump light.
Effect of such profile will be improved by two methods. One is to change the Gaussian beamprofile of the pump light to a top-hat profile by use of the beam shaping techniques. Anotheris use of ratio of the 1D-THz-TDS amplitude image with the 4seg-MHA to that without the
4seg-MHA.
To evaluate the limit on the speed of the sample, we increased the motion of the 4segMHA to 10 mm/s. We acquired 1000 2D-ST THz images for 2 seconds at 500 fps andintegrated groups of 5 images to maintain a sufficient number of vertical image lines (10line/mm) in the THz-TDS images. The SNR in this case, with a 100 line/s acquisition speed,
is estimated from Fig. 4 to be 132 in the absence of a sample. THz-TDS images acquired in#90646 - $15.00 USD(C) 2008 OSA

Received 7 Dec 2007; revised 11 Jan 2008; accepted 11 Jan 2008; published 16 Jan 200821 January 2008 / Vol. 16, No. 2 / OPTICS EXPRESS 1219

Fig. 10. THz-TDS images of the 4seg-MHA at four frequencies measured by real-time THzTDS line scanner with the sample moving at (a) (1.5 MB) 1 mm/s and (b) (1.3 MB) 10 mm/s.
Each image consists of 200 by 232 pixels.

the aforementioned manner and a movie of the 4seg-MHA at 0.204, 0.407, 0.815, and 1.600
THz are shown in Fig. 10(b) and its movie with an image size of 20 mm × 20 mm,
corresponding to 200 × 232 pixels. Even at this sample speed of 10 mm/s, the THz-TDS
images at 0.204, 0.407, and 0.815 THz still show the spectral characteristics of the 4segMHA, despite the decreased SNR. On the other hand, in the THz-TDS image at 1.600 THz,
the signal for the 1.6-THz MHA segment has been quite low. This is mainly due to thedecreased SNR and to the limited dynamic range of the signal at higher THz frequencies.
Improvement of the SNR and/or increase in the THz spectral bandwidth would permit THzTDS imaging of fast-moving objects at higher THz frequencies.
Finally, we compare the presented THz-TDS line scanner with the conventional rasterscanning THz-TDS imaging system in regards to image pixel rate. Figure 11 compares
successive THz-TDS images obtained by the raster-scanning system and the line scanner. In
the raster-scanning system, with a measurement time for the temporal waveform of 22
seconds per pixel, it requires 36 982 sec to acquire an image with 41 by 41 pixels [see Fig.
11(a)]. We here have to consider difference of frequency resolution between the two systems
for fair comparison of the pixel rate because measurement time is dependent on frequency
resolution in the raster-scanning system. In Fig. 11, frequency resolution of the rasterscanning system (= 21 GHz) was 1.38 times better than that of the line scanner (= 29 GHz). If
the frequency resolution of the raster scanning system is set to be 29 GHz, the measurement
time of the 4seg-MHA is estimated to be 26 780 sec. The resulting pixel rate is thus only
0.063 pixel/s. For the line scanner with a sample speed of 1 mm/s, an image with 200 by 232
pixels can be obtained in 20 seconds [see Fig. 11(b)]. The resulting pixel rate is as high as2320 pixel/s, which is 36 825 times higher than that of the raster-scanning system.
Furthermore, when the sample speed is increased to 10 mm/s, the line scanner attains a pixel
rate of 23 200 pixel/s [see Fig. 11(c)]. On the other hand, the SNR of the raster-scanningsystem is about 10 times better than that of the line scanner for a sample speed of 10 mm/s
because a lock-in amplifier is used in the raster-scanning system. To achieve a SNR similar tothat of the raster-scanning system, the line scanner would require a measurement time of 163 s
per THz-TDS image (see Fig. 4), corresponding to a rate of 285 pixel/s. We conclude that theline scanner is superior to the raster-scanning system with regard to the pixel rate.

#90646 - $15.00 USD(C) 2008 OSA

Received 7 Dec 2007; revised 11 Jan 2008; accepted 11 Jan 2008; published 16 Jan 200821 January 2008 / Vol. 16, No. 2 / OPTICS EXPRESS 1220

Fig. 11. Successive THz-TDS images of the 4seg-MHA measured by (a) (1.1 MB) rasterscanning THz-TDS imaging system, (b) (1.0 MB) real-time THz-TDS line scanner (sample
speed = 1 mm/s), and (c) (1.5 MB) real-time THz-TDS line scanner (sample speed = 10 mm/s).

5. Conclusion
We have developed a real-time THz-TDS line scanner that can be used to image moving
objects. The proposed method is based on 2D-ST THz imaging by combining non-collinear
2D-FSEOS with line focusing of a THz beam onto a sample. 2D-ST images with a size of 20
mm by 34 ps were obtained at a high SNR by adopting a 500-Hz lock-in imaging technique aswell as image integration. FFT calculation of the time dimension in the 2D-ST THz images
gives 1D-THz-TDS images of size 20 mm by 3 THz at a frequency resolution of 29 GHz. The
spatial resolution of the proposed THz-TDS scanner depends on the THz frequency but is near
the diffraction limit of the imaging optics. We applied the technique to successfully image amoving 4seg-MHA. The pixel rate of the THz-TDS line scanner was as high as 2320 pixel/s
and 23 200 pixel/s for sample speeds of 1 mm/s and 10 mm/s, respectively. These are muchhigher pixel rates than those of a raster-scanning THz-TDS imaging system. To the best of ourknowledge, this is the first time that THz-TDS imaging of a moving object has been achieved.
Consequently, this method has the potential to become a powerful tool for monitoring moving
objects in real-world applications.
Acknowledgements
This work was supported by Grants-in-Aid for Scientific Research Nos. 18686008 and
18650121 from the Ministry of Education, Culture, Sports, Science, and Technology of Japan,
the Sumitomo Foundation, and a Sasakawa Scientific Research Grant.

#90646 - $15.00 USD(C) 2008 OSA

Received 7 Dec 2007; revised 11 Jan 2008; accepted 11 Jan 2008; published 16 Jan 200821 January 2008 / Vol. 16, No. 2 / OPTICS EXPRESS 1221