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10.1117/2.1201105.003651
Terahertz imaging for detectionor diagnosisIwao Hosako and Naoki Oda
A camera that operates at video frame rate offers a solution to stand-off
imaging for search and rescue in fire disasters and label-free biomaterial
detection.
Terahertz (THz) imaging is a powerful technique that exploits a
nonionizing (i.e., safe) portion of the electromagnetic spectrum
located between microwaves and the IR. Directed toward an
appropriate sample, such as wood or ceramics, THz light passes
through and is detected by a camera. Because of its nondestruc-
tive nature, THz imaging is applicable to many areas. For exam-
ple, it has advantages over x-rays for diagnosis of skin cancer1
and can detect sketches underlying paintings.2 The technique is
also potentially advantageous in conditions of hot, black smoke,
where conventional imaging is unsuitable. Even ordinary smoke
is difficult to see through at visible wavelengths, and hot, black
smoke saturates and scatters IR radiation. THz wavelengths
are absorbed by smoke less strongly and transmitted more
readily through it, compared with the IR. However, although
well-placed for (bio)materials analysis, THz imaging was until
recently considered ill-adapted for hot conditions,3 limiting its
utility in search and rescue missions.
We report a THz camera consisting of a quantum-cascade
laser light source4, 5 and a microbolometer focal-plane array (an
IR detector common in thermal cameras). The camera is based
on one we previously designed,3 but with improved power and
sensitivity. We describe two applications of our imaging system:
stand-off imaging for search and rescue in a fire disaster, and
label-free biomaterial detection.
The camera requires high-intensity illumination. A blackbody
is normally used as a reference source to calibrate imaging in
the mid-IR (thermal IR) region. Yet its relatively weak radiative
power makes it unsuitable for THz calibration and imaging (see
Figure 1). Here, strong THz sources, such as quantum cascade
lasers, are indispensable. The average power output of our liq-
uid nitrogen-cooled laser system—which operates for several
hours—is several microwatts, sufficient for the improved
Figure 1. Top: Blackbody spectra at various temperatures. The smaller
spectral radiance in the terahertz (THz) range, relative to the mid-
IR, is insufficient for imaging. Bottom: Photograph of stand-off THz-
imaging experiments. LN2: Liquid nitrogen. QCL: Quantum-cascade
laser. FPA: Focal-plane array.
Continued on next page
10.1117/2.1201105.003651 Page 2/3
Figure 2. Demonstration of the label-free biomaterial detection system
at a 2009 exhibition in Kyoto. A 1200K blackbody was used as a light
source because venue regulations prohibited the use of liquid nitrogen.
Inset: Schematic of the detection principles. PVDF: Polyvinylidene di-
fluoride.
microbolometer array (320�240 elements). Thus, the quantum
cascade laser acts as an external source.
We improved that camera’s sensitivity by changing the sheet
(electrical) resistance of an absorption (light-gathering) layer,
developing THz-transparent materials, and adding long-pass
metal mesh filters to block IR radiation.4, 5 We included frame-
and pixel-integration functions to increase the signal-to-noise
ratio (SNR). However, simple frame integration did not work as
expected because the microbolometer generates low-frequency
noise. Consequently, we incorporated lock-in imaging, which
reduces low-frequency noise and increases SNR as a function of
the square root of the frame-integration number.
With these technical improvements in place, we performed
stand-off (�5m distance) THz-imaging experiments under sim-
ulated fire conditions (see Figure 1). We obtained images in hot,
black smoke (with an SNR of�140 vs.�6000 with no smoke) that
blocked visible light and and induced signal saturation in the
long-wavelength IR (LWIR) region. These results clearly show
an advantage of THz compared with LWIR or visible imaging.6
Our camera is particularly adapted to search and rescue
missions due to its 60Hz (real-time) image-acquisition rate.
In pharmaceutical drug discovery, molecular interactions are
commonly tracked using chemical ‘labels’ that are costly and
prone to error. We applied our THz camera to label-free detec-
tion of small-molecule reactions with proteins. The THz waves
are readily absorbed and enable sensing of very small changes
Figure 3. Expected future applications of the THz-imaging system.
�: Optical wavelength.
in biomaterials (see Figure 2). The detection sensitivity of a
label-free biotin-streptavidin reaction (routinely used in biotech-
nology), for example, was nearly the same as that of conven-
tional methods, and the THz system has the advantage of high
throughput and low cost. We note that for real life-science appli-
cations, our detection system requires a very large biomaterial
spectral database in the THz range.
In summary, we have used a THz camera to acquire images
through hot, black smoke and to detect a biomolecular reaction.
In the future, we plan to improve the performance of the imaging
system. In terms of the light source, we expect soon to be produc-
ing milliwatt-class laser output, which represents much higher
power than the several microwatts currently possible. The sensi-
tivity of the microbolometer array can be enhanced one order of
magnitude by optimizing the bolometer structure to THz waves
and using better materials to fabricate the device. Ultimately, as
suggested in Figure 3, we expect such an imaging system to be
useful in a range of applications from preventive health care to
quality control, surgery, and nondestructive evaluation.
Author Information
Iwao Hosako
National Institute of Information and
Communications Technology
Koganei, Japan
Continued on next page
10.1117/2.1201105.003651 Page 3/3
Iwao Hosako is a leader of the THz project. His research in-
terests include semiconductor emitters, detectors, and optical
thin films. He serves on several research committees for THz
technologies.
Naoki Oda
Guidance and Electro-Optics Division
NEC Corporation
Fuchu, Japan
References
1. J. Gudde, M. Rohleder, T. Meier, S. W. Koch, and U. Hofer, Time-resolved investi-gation of coherently controlled electric currents at a metal surface, Science 318, pp. 1287–1291, 2007. doi:10.1126/science.11467642. J. B. Jackson, M. Mourou, J. F. Whitaker, I. N. Duling III, S. L. Williamson, M.Menuc, and G. A. Mourou, Terahertz imaging for non-destructive evaluation of muralpaintings, Opt. Commun. 281, pp. 527–532, 2008. doi:10.1016/j.optcom.2007.10.0493. N. Sekine and I. Hosako, Design and analysis of high-performanceterahertz quantum cascade lasers, Phys. Status Solidi C 6, pp. 1428–1431, 2009.doi:10.1002/pssc.2008815044. N. Oda, H. Yoneyama, T. Sasaki, M. Sano, S. Kurashina, I. Hosako, N. Sekine,T. Sudou, and T. Irie, Detection of terahertz radiation from quantum cascade laser usingvanadium oxide microbolometer focal plane arrays, Proc. SPIE 6940, p. 69402Y, 2008.doi:10.1117/12.7816305. N. Oda, Uncooled bolometer-type terahertz focal plane array and camera for real-timeimaging, C. R. Phys. 11, pp. 496–509, 2010. doi:10.1016/j.crhy.2010.05.0016. N. Oda, M. Sano, K. Sonoda, H. Yoneyama, S. Kurashina, M. Miyoshi, T. Sasaki,et al., Development of terahertz focal plane arrays and handy camera. Paper accepted atSPIE Defense, Security, and Sensing in Orlando, FL, 25–29 April 2011.
c 2011 SPIE
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