NTT Technical Review
February 2012 Vol. 10 No. 2
Feature Articles:
Imaging and Sensing Technologies for Safety and Security
NTT R&D of Systems and Devices for Safety
Millimeter-wave Imaging for Detecting Surface Cracks on
Concrete Pole Covered with Bill-posting Prevention Sheet
Detection of Cracks in Concrete Structures from
Digital Camera Images
Remote Detection of Hazardous Gases in Full-scale Simulated Fire
by Using Terahertz Electromagnetic Waves
Development of Terahertz-wave Photomixer Module
Using a Uni-traveling-carrier Photodiode
Visualization of Pharmaceutical Drug Molecules by
Terahertz Chemical Imaging
Portable Sensor for Determining Benzene Concentration from
A irborne/liquid Samples with High Accuracy

Regular Articles
Acoustically Controlled Spin Precession Revealed by
Two-dimensional Imaging of Spin Transport in GaAs

Practical Field Information about
Telecommunication Technologies
Damage to Plastic Components Caused by Solvent Cracking

Information
NTT Receives Award from the Thomson Reuters 2011 Top 100
Global Innovators Program

External Awards
External Awards

Feature Articles: Imaging and Sensing Technologies for Safety and Security

NTT R&D of Systems andDevices for Safety
Kimihisa Aihara† and Naoya Kukutsu
Abstract

In this article, we introduce NTT’s research and development (R&D) of imaging and sensing technology for making the hidden visible in order to make our lives safe.

1. Introduction
Since the major earthquake that occurred in easternJapan last year (on March 11, 2011), expectations fortechnology that supports safety in our lives haveincreased. NTT has long been engaged in four broad
fields of research and development (R&D) related toimaging and sensing technology to support safety
from the viewpoints of meeting current needs andsowing seeds for the future (Fig. 1).
Representative of the former viewpoint is R&D offacility management for the maintenance and management of basic communication facilities, particularly aiming for greater accuracy and efficiency in theinspection and diagnosis of concrete structures suchas concrete poles, cable tunnels, and office buildings.
For the latter, there is R&D of new imaging andsensing technology that uses terahertz waves, which
occupy a frequency band that is currently undeveloped for basic communication technology. Lying
between radio waves and visible light in the electromagnetic spectrum, terahertz waves hold promise forcontributing to a safer and more secure society
through skillful use of the features of both radio andlight. They allow the implementation of new application fields that have previously been impossible, suchas remote gas sensing and spectroscopic analysis [1],[2]. Another example is technology for detecting
harmful substances based on NTT’s own μTAS
(micro-total analysis system), which uses ultraviolet† NTT Microsystem Integration Laboratories
Atsugi-shi, 243-0198 Japanregion spectroscopic analysis technology [3]. In
those R&D activities, repeated simulations based ontheory and measurements are used to understandelectromagnetic wave propagation or gas flow pathbehavior for application in device design and manufacture. Furthermore, the sensitivity of sensing andanalysis is improved by the addition of signal processing, with the result that these various systems arealmost ready for use.
In this article, we describe our efforts in the abovementioned areas.
2. Concrete structure crack
detection technology
NTT maintains a nationwide network of concrete
poles, cable tunnels, office buildings, and other concrete structures that support communication services.
Those facilities were mainly constructed during theperiod of rapid growth, so many of them are over 40
years old and are beginning to deteriorate. This situation has made facility management an important
issue.
The position of current concrete pole inspection
methods and current R&D of new inspection methods
is illustrated in Fig. 2. Here, we introduce image
analysis and millimeter-wave imaging techniques forcrack detection.
Surface cracking in concrete structures allows
moisture to enter and corrode the iron reinforcement
bars inside, reducing the strength of the structure. For
that reason, information about the presence of surface
cracks and their number, widths, and lengths isimportant for structural integrity evaluation.
NTT Technical Review

Feature Articles

Trends in needs

Trends in seeds

Concrete structure crack
detection technology

Remote toxic gas
detection technology

VOC gas sensing
technology

Terahertz chemical
imaging technology

Optical beat-signal
Optical fiber

Test tablets

A-typecrystals(1.6 THz)

B-typecrystals(1.2 THz)

PassivedryingCommerciallyavailable drugs

Microfluidic
tube

Concentration
cell

In

Outer waveguide

Extraction system

Out

Pump

Detection cell

Opticalfiber

UV light source
Personal computer

Spectrometer

Liquid sample moduleTHz-waves

DC cable
ModuleDegree of absorption
Low
High

Airborne detection system

Millimeter wave nearfield imaging system

Terahertz-wave continuouswave spectrometry system

Terahertz-wave pulse
spectrometry systemµTAS ultraviolet
spectrometry system

System integration

Signal processing technology (image processing, baseline correction)
Millimeter- & terahertzwave circuit and antenna
design technology

Broadband terahertz
device design andmanufacturing technology

UTC-PD device design
and manufacturing
technology

Electromagnetic wave propagation analysisµTAS device design
and manufacturing
technology
Path analysis

Simulation, measurement and evaluation environment
Theory construction and verification
Fundamental technology
UTC: uni-traveling carrier photodiode
VOC: volatile organic compoundFig. 1. Imaging and sensing technology supporting safety and security.

NTT EAST and NTT WEST together manage
about 11.85 million concrete poles. Because pole
inspections to detect cracking are currently done
visually by workers, there is a need to develop a more
efficient inspection method. Automatic crack detection by image analysis is a technique that could meetthis need. However, concrete poles have foot pegs,
belts, plates, and other items attached to them thathinder image analysis, so commercial software has ahigh rate of error in crack detection.
To solve that problem, NTT is developing automatic crack detection software that can reduce thedetection errors caused by such obstacles by applying
an obstacle removal algorithm and a neural network
for crack shape recognition. That R&D is describedin detail in the Feature Article “Detection of Cracks
in Concrete Structures from Digital Camera Images”
in this issue [4].
Some concrete poles have plastic sheeting attachedto them to prevent unwanted posters being pasted
Vol. 10 No. 2 Feb. 2012

there (bill-posting prevention sheets). Since thesheeting blocks the concrete surface from view, ultrasonic inspection equipment (pole tester) is used.
However, ultrasonic inspection has difficulty detecting fine cracks. Moreover, the surfaces of many concrete structures other than poles are often painted,
wallpapered, tiled, or covered with reinforcing material that makes visual inspection difficult. Techniques
for detecting cracks below such coverings include theuse of ultrasonic waves and x-rays, which can passthrough the coverings. Ultrasonic waves are used tomeasure cracks that are more than one millimeterwide, but that resolution is insufficient for early
detection of cracking in concrete poles, which
requires the detection of submillimeter cracks. Xrays have an extremely short wavelength that allows
high resolution, but x-ray inspection requires theobject to be inspected to be located between a transmitter and a receiver. Moreover, x rays are harmful tohumans, so x-ray equipment is difficult to use forFeature Articles

FutureNow
Crack detection

Visual(human eye)Integrity
diagnosis*2

Image processing(digital camera)
Impacteigenmode
analysisBill-posting
prevention sheet

Pole tester*1

Concrete polecrack scanning(this technology )Pole tester

Pole tester*1 Probes are positioned above and below the bill-posting prevention sheet, but this results in a long ultrasonic wave
propagation path (about 2 m), so the accuracy is low and it is difficult to detect fine cracks.
*2 Broken rebar diagnosis equipment that uses impact eigenmode analysis is currently being developed by NTT AccessNetwork Service Systems Laboratories.

Fig. 2. NTT concrete pole inspection technology.

inspections conducted in the field.
To solve that problem, NTT has investigated imaging techniques that use millimeter waves (77-GHzband electromagnetic waves) to see what is hiddenfrom the unaided eye. That work has resulted in thedevelopment of a portable scanner that can observe
the condition of the pole surface lying beneath abill-posting prevention sheet or other such covering.
This R&D is explained in detail in the Feature Article“Millimeter-wave Imaging for Detecting Surface
Cracks on Concrete Pole Covered with Bill-posting
Prevention Sheet” [5]. It introduces technology for
detecting surface cracks in concrete structures causedby broken rebar, but NTT Access Network Service
Systems Laboratories has also been developing techniques that use impact mode analysis for diagnosing
rebar deterioration.
3. Basic technology for terahertz wave signals
Terahertz waves are electromagnetic waves in thefrequency range from 300 GHz to 3 THz (wave

length: from 100 μm to 1 mm). They have the ability
to pass through some materials and exhibit absorption
lines that are specific to the material. These characteristics can be used to identify materials. Another feature is that this radiation is not as harmful to thehuman body as x-rays are. If these features can beapplied to imaging and sensing, they will make it possible to obtain new data that cannot be gathered byconventional techniques that use x-rays, infrared
radiation, or microwaves, and we can expect many
applications. What has prevented industrial applications so far is that it is very difficult to generate sufficiently strong terahertz waves over a wide frequency
range by using electrical technology. The reason isthat no efficient means of generating terahertz-band
electromagnetic waves has been established because
these waves exceed the operating frequencies of transistors and other active devices [6].
NTT has applied many years of optical communication R&D to develop new technology that uses a
uni-traveling carrier photodiode (UTC-PD). The
UTC-PD, which features an extremely fast response,
NTT Technical Review

Feature Articles

Optical source unit
Data processing unit

Receiver for sensing

Fig. 3. Remote gas detection equipment.

was originally developed for use in communications
to convert an optical signal into an electrical signal.
The new UTC-PD-based terahertz-wave generator
uses a UTC-PD to convert an optical signal into anelectrical one, which is then converted to produce
terahertz waves [7]. This technique uses the stable,
broadband, ultrahigh-frequency characteristics ofphotonics to generate terahertz-band electromagnetic
waves for various applications. In this way, imaging
and sensing with this frequency band is made feasible. This R&D is explained in detail in the Feature
Article “Development of Terahertz-wave Photomixer
Module Using a Uni-travelling-carrier Photodiode”[8].
4. Remote detection of dangerous
gases with terahertz waves
At the site of fires or other disasters, a rapid
response is needed to prevent secondary damage.
There is thus a pressing demand for the developmentof systems that can detect carbon monoxide, hydrogen cyanide, and other dangerous gases from aremote location.
NTT is engaged in R&D for applying its terahertzwave generation technology to the remote detectionof dangerous gases (Fig. 3). Terahertz waves differ
from light in that they are not scattered by dust or
smoke and they can propagate through a fire without
any significant attenuation. Moreover, because terahertz waves resonate at a frequency of rotation andvibration of gas molecules, dangerous gases havespecific absorption spectra for this frequency band,and the absorption line patterns can be used to identify the types of gases present. These properties canbe used to detect toxic gases from a safe distance,even under conditions of extremely low visibility due
to smoke etc.
Vol. 10 No. 2 Feb. 2012

NTT has developed a prototype system for remotely detecting dangerous gases under a commission
from the National Institute of Information andCommunications Technology and in cooperation
with the University of Tokyo, the National Institute ofAdvanced Industrial Science and Technology, theSpectra Design Company, and others. This system
was tested and evaluated in a full-scale simulated fire
environment at the Fire Research and Test Laboratoryof Tokyo University of Science. The tests verified thesystem’s effectiveness for remotely detecting hydrogen cyanide gas as an example of a dangerous gas.
This R&D is described in detail in the Feature Article“Remote Detection of Hazardous Gases in Full-scale
Simulated Fire by Using Terahertz Electromagnetic
Waves” [9].
5. Terahertz spectroscopy
Terahertz spectroscopy is an analytical chemistry
technique that identifies substances by detectinghydrogen bonds and hydrate bonds and their interactions with other molecules [2] (Fig. 4).
By improving the latest terahertz-frequency components such as ultrashort-pulse lasers and photoconductive antennas, we are approaching realistic terahertz chemical imaging (TCI) and analysis technology. Because TCI can clarify the distribution ofhydrogen bonds and molecular networks, it is expected to be a powerful tool for biology, pharmacology,
and the life sciences. TCI technology can determine
the two-dimensional distribution of molecules on thebasis of molecular networks revealed by terahertzwave absorption spectra. As an application to actual
pharmaceutical molecules, two polymorphs offamotidine, a component of histamine receptor antagonist (H2 blocker) antacids, have been distinguishedin pills. Polymorphs are composed of the sameFeature Articles

Terahertz waves

Molecular clusters

Molecules

Hydrogen bonds etc.

Fig. 4. Concept of terahertz spectroscopy.

molecules, but differ in how these molecules arelinked. Large differences in the effectiveness of polymorphic drugs due to differences in chemical properties have been reported. New detection methods thus
have major significance for the development of evensafer pharmaceuticals.
This R&D is described in more detail in the Feature
Article “Visualization of Pharmaceutical Drug Molecules by Terahertz Chemical Imaging” [10].
6. VOC gas sensing by ultraviolet
absorption spectrometry
The Ministry of the Environment has establishedstandards for benzene, toluene, xylene, and otherhighly toxic, aromatic, volatile organic compounds(VOCs) as dangerous gases in our environment
besides combustion gases such as carbon monoxideand hydrogen cyanide to which terahertz waves havebeen applied for remote detection. The sources of
those toxic gases include gasoline stations, printing
shops, paint shops, and other small-scale facilities aswell as large-scale facilities such as chemical plants,
steelmaking plants, oil refineries, and power plants.
Such facilities may release even larger amounts ofdangerous materials into the atmosphere during anearthquake or fire. Those pollutants, together withradioactive pollution, are becoming a cause of serious
environmental pollution. Besides earthquakes andother disasters, the deterioration of gasoline station

facilities with age and other such situations present
the danger of toxins leaking onto the ground surface
or into groundwater. The methods of detecting suchdangerous substances have generally been analyticalones that combine gas chromatography and mass
spectroscopy. Such methods are performed in laboratories and require a large amount of equipment. They
also rely on manual collection of gas samples. All ofthese factors hinder immediate and continuous measurement and increase costs, making them unrealistic
for constant monitoring of the environment.
For these reasons, NTT has developed a compact
and highly sensitive gas sensor system, which uses
NTT’s own μTAS together with ultraviolet spectroscopic technology. Even though the system’s structure is simple, its sensitivity to minute quantities ofgas is 1000 to 10,000 times greater than previous
methods and it can perform continuous monitoring.
Recently, we have also achieved a detection sensitivity of 1 ppb or better for benzene in water by adding a liquid sample module based on newly developed
bubbling and pervaporation methods [3] (Fig. 5).
This R&D is described in detail in the Feature Article“Portable Sensor for Determining Benzene Concentration from Airborne/liquid Samples with HighAccuracy” [11].

NTT Technical Review

Feature Articles

Microfluidic
tube

Concentration
cell

In
PassivedryingOut

Pump

Detection cell

Extraction system

Opticalfiber

Liquid sample module
SpectrometerUV light source
Personal computer
Airborne detection system

Fig. 5. Equipment for detecting benzene in water.

7. Conclusion
We have presented an overview of the work on
imaging and sensing technology for safety that isunderway in NTT. Efforts to improve the practicality
and performance of the technology introduced here
are continuing. Our objective is to achieve a safe andsecure society by timely response to needs bothwithin and outside NTT and by creating new technology.
References
[1]

[2]

[3]

[4]

N. Shimizu, H. -J. Song, Y. Kado, T. Furuta, A. Wakatsuki, and Y.
Muramoto, “Gas Detection Using Terahertz Waves,” NTT Technical
Review, Vol. 7, No. 3, 2009.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2009
03sf7.html
K. Ajito, Y. Ueno, T. Haga, and N. Kukutsu, “Terahertz Spectroscopy
Technology for Molecular Networks,” NTT Technical Review, Vol. 7,
No. 3, 2009.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2009
03sf6.html
S. Camou, A. Shimizu, T. Horiuchi, and T. Haga, “Ppb-Level Detection of Benzene Diluted in Water with Portable Device Based on
Bubbling Extraction and UV Spectroscopy,” Sens. and Act. B, Vol. 32,
No 2, pp. 601–607, 2008.
S. Kaneko, S. Oka, and N. Matsumiya, “Detection of Cracks in Concrete Structures from Digital Camera Images,” NTT Technical
Review, Vol. 10, No. 2, 2012.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2012
02fa3.html

Vol. 10 No. 2 Feb. 2012[5]

H. Togo, T. Kojima, S. Mochizuki, and N. Kukutsu, “Millimeter-wave
Imaging for Detecting Surface Cracks on Concrete Pole Covered with
Bill-posting Prevention Sheet,” NTT Technical Review, Vol. 10, No.
2, 2012.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2012
02fa2.html[6] M. Tonouchi, “Cutting-edge Terahertz Technology,” Nature Photonics, Vol. 1, No. 2, pp. 97–105, 2007.
[7] H. Ito, T. Nagatsuma, A. Hirata, T. Minotani, A. Sasaki, Y. Hirota, and
T. Ishibashi, “High-power Photonic Millimetre Wave Generation at
100 GHz Using Matching-circuit-integrated Uni-traveling-carrier
Photodiodes,” Proc. of IEE Optoelectronics, Vol. 150, No. 2, pp.
138–142, 2003.
[8] A. Wakatsuki, Y. Muramoto, and T. Ishibashi, “Development of Terahertz-wave Photomixer Module Using a Uni-travelling-carrier Photodiode,” NTT Technical Review, Vol. 10, No. 2, 2012.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2012
02fa5.html[9] N. Shimizu, N. Kukutsu, A. Wakatsuki, and Y. Muramoto, “Remote
Detection of Hazardous Gases in Full-scale Simulated Fire by Using
Terahertz Electromagnetic Waves,” NTT Technical Review, Vol. 10,
No. 2, 2012.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2012
02fa4.html[10] K. Ajito, Y. Ueno, and H. -J. Song, “Visualization of Pharmaceutical
Drug Molecules by Terahertz Chemical Imaging,” NTT Technical
Review, Vol. 10, No. 2, 2012.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2012
02fa6.html[11] S. Camou, E. Tamechika, and T. Horiuchi, “Portable Sensor for Determining Benzene Concentration from Airborne/liquid Samples withHigh Accuracy,” NTT Technical Review, Vol. 10, No. 2, 2012.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2012
02fa7.html

Feature Articles

Kimihisa Aihara

Director, Smart Devices Laboratory, NTT
Microsystem Integration Laboratories.
He received the B.E. degree in electronics from
Keio University, Kanagawa, in 1984. He joined
Nippon Telegraph and Telephone Public Corporation (now NTT) in 1984. He is currently
responsible for ubiquitous communications
appliance technologies at NTT Microsystem
Integration Laboratories. He is leading R&D
projects on sub-terahertz-wave wireless communications, ultralow-power network appliancesand electrical near-field communication forhuman area networks. He is a member of IEEEand the Institute of Electronics, Information andCommunication Engineers (IEICE).

Naoya Kukutsu

Senior Research Engineer, Supervisor, Group
Leader, Smart Devices Laboratory, NTT Microsystem Integration Laboratories.
He received the B.E., M.E., and D.E. degrees inelectrical engineering from Hokkaido University
in 1986, 1988, and 1991, respectively. His D.E.
dissertation described research for a time-domain
electromagnetic wave numerical analysis method. In 1991, he joined NTT Applied Electronics
Laboratories in Musashino. His current research
involves millimeter-wave and terahertz-wave
transmission, as well as imaging systems. He is amember of the IEEE MTT and COM Societies
and IEICE.

NTT Technical Review

Feature Articles: Imaging and Sensing Technologies for Safety and Security

Millimeter-wave Imaging for
Detecting Surface Cracks on
Concrete Pole Covered with
Bill-posting Prevention Sheet
Hiroyoshi Togo†, Takafumi Kojima, Shoji Mochizuki,
and Naoya Kukutsu
Abstract

In this article, we introduce millimeter-wave imaging technology for detecting submillimeter-wide
cracks under a bill-posting prevention sheet on a concrete telegraph pole.

1. Introduction
NTT expends much effort in the long-term maintenance and management of the communication network to provide safe and secure communication services to customers throughout the country. Concrete
telegraph poles, which are perhaps the most familiarof those facilities to most of us, are susceptible tophysical damage, so a high degree of structural integrity is required. Concrete poles and other structures
degrade over time; therefore, integrity maintenancerequires periodic inspections so that degraded structures can be repaired. Since NTT owns more than ten
million poles, such inspection is an every-day job fora large number of workers.
Concrete poles carry cables, transformers, andother equipment that produces an unbalanced load on
the structure over many years. The resulting bending
stress produces fatigue cracking on the pole surface.
When such cracking penetrates as far as the rebar
inside the pole, rainwater may enter and cause corrosion that can lead to the rebar breaking. The steel
rebar, which is strong in tension, is vital to balance
the concrete, which is strong in compression but
weak in tension, so broken rebar reduces the integrity† NTT Microsystem Integration Laboratories
Atsugi-shi, 243-0198 Japan

of a concrete pole. Considering this degradation
mechanism, we focused on the initial stage of degradation, which is the appearance of cracks on the concrete surface. NTT has experimentally verified that
submillimeter-wide surface cracks have a high probability of reaching the rebar, so early discovery of theinitial degradation is considered to be the best
approach to structural integrity maintenance. Therefore, careful visual inspection of the pole surface isperformed.
NTT Access Network Service Systems Laboratories is developing a new technique to replace humanvisual inspection by using software to automaticallydetect cracks from images acquired by a digital camera. More details of that technique are given in the
Feature Article “Detection of Cracks in Concrete
Structures from Digital Camera Images,” in this issue[1].
To prevent unauthorized posting of advertisementson concrete poles, 2-mm-thick sheets of textured
polyvinyl chloride (bill-posting prevention sheets)are bonded to the poles with acrylic resin in theregion within reach by hand (Fig. 1). Exposing thecovered surface for a visual inspection would requiresignificant work to strip off the sheet and then replace
it with a new one after the inspection. Since thatwould not be economically feasible, we conducted
research and development (R&D) of a nondestructive
NTT Technical Review

Feature Articles

5 mm

Fig. 1. Bill-posting prevention sheet on a concrete pole.

inspection technique that allows observation of thecondition of the pole surface without the need toremove the bill-posting prevention sheet.
2. Nondestructive inspection technology
Methods for seeing through the covering material
to observe the condition of the surface beneath it areclassified by the physical phenomena that are utilized. In particular, methods that use x-rays, ultrasonic waves, or microwaves have been applied invarious fields.
X-rays are a form of ionizing radiation that havehigh transmittance for materials other than water andheavy metals and allow imaging with micrometerorder spatial resolution. They are applied for medical
purposes such as 3D imaging of internal organs andbones, but since they are harmful to humans, exposure levels must be carefully controlled: this would bea safety problem for workers using x-rays to inspectmultiple concrete poles every day. Another problem
is that practical x-ray imaging equipment is based ontransmission, so it should really have a separate transmitter and receiver; however, for work sites withlimited work space, reflection-based imaging equipment can be used.
Ultrasonic waves are elastic waves *1 that havealmost no effect on the human body and have hightransmittance and millimeter-order spatial resolution*1	 Elastic wave: Wave of deformation in an elastic body.

Vol. 10 No. 2 Feb. 2012

for reflection-based imaging. They are therefore used
for medical three-dimensional imaging of fetusesetc., but the procedure requires the application ofgrease between the ultrasonic wave transceiver andthe object to be observed for efficient sound wave
transmission. The expense of applying and removing
grease would be a problem of the inspection of a huge
number of poles.
Microwaves are electromagnetic waves that havehigh transmittance and centimeter-order spatial resolution inside concrete, so they are used to inspectrebar. However, this technique does not provide thesubmillimeter-order spatial resolution required forcrack detection, so imaging performance remains a
problem.
3. Millimeter-wave imaging technology
To overcome the problems of safety, economy, andimaging performance, we took up the difficult R&D
challenge of using millimeter waves, which are electromagnetic waves of higher frequencies than
micrometer waves, to develop improved imaging
technology. The millimeter-wave band includes frequencies from 30 GHz to 300 GHz, but the frequencies specified by the Radio Law for radar use (imaging is also one use of radar) range only from 76 GHz
to 77 GHz (the band allocated to millimeter-wave
radar). Considering the near future, the allocation of
millimeter-wave radar band frequencies for inspection purposes is being studied. The main problems ofconcern in developing millimeter-wave radar technology for detecting cracks in concrete poles underbill-posting prevention sheets are 1) the transmittanceof the plastic sheet, 2) submillimeter spatial resolution, and 3) the ease of equipment operation.
In the band allocated for millimeter-wave radar, therelative permittivity of polyvinyl chloride is about 3
and the dielectric tangent is about 0.01, so if we
assume a planar wave incident on a 4-mm-thick infinite plane, the attenuation due to reflection andpropagation loss is low, about -10 dB, so transmittance is not a significant problem for equipment
design. There is concern, however, that the texture of
the bill-posting prevention sheet may have a greater
effect on attenuation than the sheet’s material properties because of wave scattering at the surface.
Another factor is that the width of the cracks to be
detected is less than 1/10 the wavelength of the band
allocated to millimeter-wave radar, so considering theradar cross-section area, the back scattering will beless than -30 dB relative to the power irradiating theFeature Articles

Radius of far-field wave dispersion

Dispersion of scattered wave
Radius of near-field wave dispersion

Crack

Inside the concrete

01

Scattered wave intensity (arb. units)

Fig. 2. Dispersion of wave-scattering from a crack.

surface. On the basis of the above estimates, we areinvestigating an equipment design in which the irradiation power of the millimeter-wave radiation is set
to -50 dB to pass through the bill-posting prevention
sheet to reach the crack and then pass back throughthe sheet again as back-scattering.
Because the minimum beam diameter obtainable
by focusing with a quasi-optical system using a lensis limited by diffraction, the beam width is limited toapproximately the wavelength. The spatial resolution
is therefore about half the wavelength, which meansthat it is difficult to detect submillimeter-wide cracks
using the approximately 4-mm wavelength of theband allocated to millimeter-wave radar. For that reason, we have been trying to improve the spatial resolution by detecting near-field scattering. The concept,
as shown in Fig. 2, is that the spatial resolution can be
improved because the waves scattered from the crack,
which can be regarded as a minute point in a crosssection that includes the crack width direction and thecrack depth direction, are spherical waves*2. The dispersion of the scattered waves is thus small in the nearfield. Detecting near-field scattering with submillimeter-order spatial resolution requires the use of anantenna whose aperture is equivalent to the width of
the scattered waves at the detection height. We are

therefore investigating aperture design at the antenna
placement height with consideration given to operability.
Inspecting all of the concrete telegraph poles in theentire country for structural integrity requires highly
efficient work. The shortest imaging time could beachieved by covering the entire bill-posting prevention sheet with the antenna for detecting near-field
scattering that is needed in the detection of submillimeter-wide cracks, but it is difficult to arrange antenna elements at submillimeter intervals in the crack
width direction. Even if the elements were arranged
at millimeter intervals, since the bill-posting prevention sheet is about 1.5 m long in the vertical direction,
the number of antenna and millimeter-wave transmitter and receiver components would be huge and theequipment cost would be uneconomical. The equipment would also be large, which would reduce its
portability. To maintain economy and portability, we
considered a method in which ten antennas arearranged in a one-dimensional array in the crack
length direction and the array is scanned in the crack
width direction. Spatial resolution in the crack width
direction is maintained by detecting scattered waves
at submillimeter intervals. The equipment can perform multiple scans in the crack length direction inorder to image the entire surface.

*2 Spherical wave: Wave motion that emanates from a single point
in a three-dimensional isotropic medium or that converges to asingle point.



NTT Technical Review

Feature Articles

Millimeter-wave transmitter

Antenna

16-antenna array

Fig. 3. CP Scan equipment.

Fig. 4. Schematic illustration of CP Scan in use.

4. Concrete pole inspection equipment

cracks ranging in width from 0.3 mm to 0.15 mm.
Although this preliminary experiment was suitablefor confirming the method’s principle, the attachedbill-posting prevention sheet was not bonded to thepole. The scanning rate was 120 mm/s with data
being acquired at intervals of 0.2 mm in the scanning
direction. That rate will allow one concrete pole to beinspected in about three minutes. A millimeter-wave
image was obtained after three scans (Fig. 6).
Although the equipment must be calibrated, theimaging only requires the equipment to be kept
tightly against the pole during the scan, so the operation is simple.
To allow a single person to perform the inspection,
as shown in Fig. 5, the power supply and personal
computer parts of the system must be small and light,
and image processing is essential for automatic crack
detection from the acquired images. Therefore, NTT
Access System Laboratories is currently developing
an algorithm suitable for millimeter-wave image
analysis.
These experiments used the Crack Scan automatic
crack detection software that was subject to hard disclosure commissioning to Airec Engineering Corporation in 2008 for evaluation of the detection characteristics. The red solid lines in the bottom row ofFig. 6 indicate cracks detected by the Crack Scan
automatic detection algorithm, confirming that 0.15mm-wide cracks can be detected. These results open
up the prospect of developing portable equipment

On the basis of the design policy described above,
we constructed portable equipment (CP Scan) for
detecting cracks in concrete poles under bill-posting
prevention sheets (Fig. 3). Sixteen intensity-phasedetection modules having tapered slot antennas arearranged in a one-dimensional array. A signal fromthe module is input to the baseband circuit at submillimeter scanning intervals in sync with a signal froman encoder attached to the wheels of the unit: in thisway, the condition of the concrete pole surface isimaged. The case containing the module, encoder,and baseband circuit weighs less than 4.1 kg and canbe held in one hand for scanning. The entire system,
including the power supply and personal computer,weighs less than 10 kg and can be carried by a single
person. The 16 modules can image an 8-cm-wide
path in a single scan, and one or two workers caninspect a pole in about three minutes by making fiveor six scans to completely image the entire region of
the bill-posting prevention sheet (Fig. 4).
5. Feasibility of submillimeter-wide
crack detection
To test the practicality of the CP Scan, a bill-posting
prevention sheet was attached to a concrete pole that
had been removed for renovation (Fig. 5) and the CP
Scan was used to inspect this area. The pole had
Vol. 10 No. 2 Feb. 2012Feature Articles

Concrete pole scanner (sensor)

Scan
Bill-posting
prevention sheet(tape remaining)

Concrete pole test specimen

Fig. 5. Experimental setup.

Visible image (without sheet)

0.3 mm

0.2 mm

0.1–0.15 mm

Millimeter-wave image(with sheet)

Signalstrength

Automatic crack
detection results

Fig. 6. Results of crack detection in test specimen.

that can detect cracks that are at least 0.15 mm widewith a scanning time of about three minutes perpole.
6. Future plans
In future work, we will test the system’s practicality
on actual poles with bill-posting prevention sheets
bonded to them and evaluate the detection performance with our automatic crack detection software
for millimeter-wave images of concrete poles. We
will continue conducting R&D that will have wide
applicability with highest priority on ease of operation and higher inspection efficiency while achieving
NTT Technical Review

Feature Articles

practical detection performance suitable for equipment that can be used in actual concrete pole inspection work.

S. Kaneko, S. Oka, and N. Matsumiya, “Detection of Cracks in Con-crete Structures from Digital Camera Images,” NTT Technical
Review, Vol. 10, No. 2, 2012.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2012
02fa3.html
S. Oka, S. Mochizuki, H. Togo, and N. Kukutsu, “Inspection of Concrete Structures Using Millimeter-wave Imaging Technology,” NTT
Technical Review, Vol. 7, No. 3, 2009.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2009
03sf4.html

Hiroyoshi Togo

Shoji Mochizuki[2]

References[1]

Senior Research Engineer, NTT Microsystem
Integration Laboratories.
He received the M.Sc. and Ph.D. degrees inapplied physics and electronic engineering fromthe University of Tsukuba, Ibaraki, in 1996 and2010, respectively. He joined NTT Musashino
Opto-electronics Laboratories in 1996. From1996 to 2001, he was engaged in the development of a thermo-capillary waveguide-based
optical switch and from 2001 to 2002, he endeavored to commercialize it for NTT Electronics
Inc. Since 2002, he has been researching ultrawideband impulse radio systems using photonic
techniques and millimeter-wave tomography
with electro-optic probing. He is a member of theIEEE Photonics, Antenna and Propagation, andMicrowave Theory and Techniques Society andthe Institute of Electronics, Information andCommunication Engineers (IEICE). He received
the 2006 Asia Pacific Microwave Photonics Conference AP-MWP Award and the 2010 European
Conference on Antennas and Propagation Award.

Takafumi Kojima

Research Scientist, NTT Microsystem Integration Laboratories.
He received the B.S., M.S., and Ph.D. degrees
in physics from Osaka Prefecture University in
2005, 2007, and 2010, respectively. During
2007–2010, he was engaged in R&D of terahertz
superconductor-insulator-superconductor mixers
at the National Astronomical Observatory ofJapan, Tokyo. He joined NTT Microsystem Integration Laboratories in 2010 and is currently
studying radar signal processing for millimeter
imaging. He received the 27th JSAP (Japan Society of Applied Physics) Incentive Award for
Excellent Presentation in 2009, the 14th Superconductivity Science and the Technology Award
from the Society of Non-Traditional Technology
in 2011. He is a member of IEEE, JSAP, and
IEICE.

Vol. 10 No. 2 Feb. 2012

Research Scientist, NTT Microsystem Integration Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in electrical and electronic engineering from
Chuo University, Tokyo, in 2001, 2003, and2005, respectively. He joined NTT Microsystem
Integration Laboratories in 2005 and is currently
working on R&D of millimeter-wave and microwave imaging. He received the 2004 International Union of Radio Science (URSI) Commission B Young Scientist Award. He is a member ofIEEE and IEICE.

Naoya Kukutsu

Senior Research Engineer, Supervisor, Group
Leader, Smart Devices Laboratory, NTT Microsystem Integration Laboratories.
He received the B.E., M.E., and D.E. degrees inelectrical engineering from Hokkaido University
in 1986, 1988, and 1991, respectively. His D.E.
dissertation described research for a time-domain
electromagnetic wave numerical analysis method. In 1991, he joined NTT Applied Electronics
Laboratories in Musashino. His current research
involves millimeter-wave and terahertz-wave
transmission, as well as imaging systems. He is amember of the IEEE MTT and COM Societies
and IEICE.

Feature Articles: Imaging and Sensing Technologies for Safety and Security

Detection of Cracks in Concrete
Structures from Digital Camera
ImagesSuguru Kaneko†, Soichi Oka, and Naoki Matsumiya
Abstract

In this article, we introduce an image processing technique for detecting fine cracks in concrete polesby examining remotely acquired photographs.

1. Introduction
NTT has many concrete structures as communication infrastructure facilities and it performs periodic
inspections of them for safety maintenance and management. Those structures include a particularly large
number of concrete poles, so efficient inspection isessential.
The on-site inspection for cracks in the concretesurface of a concrete pole is meticulous because
cracks allow rainwater to enter and corrode the rebar
inside, which reduces the strength of the structures.
Current crack inspection methods mainly involve
human visual inspection, but it is difficult for workers
to maintain concentration over a long period of work,and even skilled inspectors can overlook fine cracks.
In particular, when inspecting concrete poles forcracks, workers must use binoculars rather than the
naked eye and continuously focus on locations thatcan be ten meters high or higher. This is fatiguing,which also makes it difficult for the workers to maintain concentration. Furthermore, the number ofinspectors is decreasing year by year and this has
increased the workload per worker.
These circumstances necessitate increased efficiency in crack inspection through automation of onsite inspections. From the viewpoint of ease of handling and operation of equipment, remote photography of structures with digital cameras and automatic† NTT Access Network Service Systems Laboratories
Tsukuba-shi, 305-0805 Japan

crack detection from the obtained images by software
is an effective approach. In this article, we describe
the most recent image processing algorithms thathave overcome technical problems for practical use.
2. Difficulty of crack detection by
image processing techniques
Algorithms for automatic detection of cracks fromimages of concrete surfaces have already been proposed, but the existing methods have problems interms of practicality. In efforts to detect cracks by
image processing, cracks are generally defined usingedge features, regardless of the algorithm used.
Cracks are represented as sharp changes in gray-scalevalues between edges and adjoining pixels that form
an outline and appear in images as darkened lines.
Thus, edges are the unique feature that indicatescracks. These algorithms are effective for images that
include only concrete surfaces, but they do not work
well when the image also includes extraneous obstacles. The reason for the poor performance in practiceis that images of actual on-site concrete structuresoften include obstacles such as windows and sashes
in the case of buildings and cables or foot pegs in thecase of concrete poles. The edges formed by the outlines of such obstacles can be mistaken for cracks. An
example of results produced by commercial crack
detection software is presented in Fig. 1. The pixels
colored red indicate areas judged by the software tobe cracks, and we can see that the background, belts,
plates, and other edges were erroneously detected as
NTT Technical Review

Feature Articles

L100
Background

Belt

Gray areas are extracted as thecharacteristic color of concrete.

+b
Detection error–a+aChroma–b

Foot peg

Cable’s color is completely black.
Crack

L0
* CIE is a color coordinate system defined by the Commission
Internationale de l'Eclairage.

Belt

Fig. 2. CIE Lab color system*.
Plate

Fig. 1. Results from commercial crack detection software.

cracks. To overcome this problem, it is necessary toeliminate such obstacles from the image before performing the crack detection processing. However, it
is not easy to obtain a highly accurate solution withcurrent pattern recognition techniques because of theinfinite variety in obstacle shapes. Accordingly,
extracting only the cracks from images that include
such obstacles has been very difficult.

10

3. Obstacle removal algorithm

Fig. 3. Color classification results.

Because various obstacles exist at the site of actual
facilities, as mentioned above, it is not easy to eliminate obstacles by defining their shapes and creating
an algorithm to remove them directly. Therefore, with
our algorithm, we chose to take the opposite approachof indirect obstacle removal. The indirect approach
involves extracting the pole’s concrete surface andinverting the results. We explain the main points of
the algorithm below.
First, we extract from the input image pixels thatseem to represent concrete surface. Because the concrete surface is generally gray, we can use a color
system such as the one shown in Fig. 2 to represent
color chrominance numerically and extract the graypixels, which have low chrominance values. In thecolor classification results (Fig. 3), the pixels that arenearly gray are made white and pixels that are notclose to gray are made black. This image has a green

forest as the background, so color classification canbe used to identify the rough outline of the concretepole, but if the image contains a gray structure, theconcrete pole can be identified by the amount of texture. It was therefore necessary to supplement thecolor information with texture information as
explained below in order to extract concrete pole surfaces from various types of images with greater certainty.
The concrete surface, which consists of cement andfine grains of sand, has a characteristically rougher
texture pattern than other materials usually found inimages. We know the pole’s roughness pattern andcan thus distinguish concrete pole from other grayobjects. This surface roughness pattern is referred toas texture in image analysis terminology, and it isVol. 10 No. 2 Feb. 2012Feature Articles

16

12

15

8

11

14

7

10

13

6

9

5

A

1

2

3

4

Fig. 4. Area for texture calculations.

Concrete surface
Non-concrete surface
1.00

Fig. 6. Obstacle removal results.

9

Sk

0.90

0.80

0.70

1 2 3 4 5 6 7

8 9 10 11 12 13 14 15 16kFig. 5. Comparison of texture for concrete andnon-concrete surfaces.

used in judging materials. In our algorithm, we obtaina numerical value for the texture feature S of the concrete surface by calculating the homogeneity of 16
pixels in a pattern around a particular pixel, such as
the light blue pixel formation (d1 to d16) around thered pixel A in Fig. 4. The specific method used to
calculate the texture feature is described in Ref. [1].
Calculating the homogeneity for the 16 pixels produces 16 texture values, Sk (k=1, 2, …, 16). The relation between d and S is plotted in Fig. 5, where thesolid line represents the calculated results for theconcrete surface and the broken line represents thecalculated results for non-concrete surfaces. As we
can see from the figure, the shape of the graph differs
greatly for concrete and non-concrete surfaces. It istherefore possible to determine whether or not a pixel
is part of a concrete surface by comparing its graphwith stored reference graphs for concrete surface texture features.
In the final step, the conjunction of the color classification results and texture classification results istaken. If either result does not indicate a concrete
pixel, then that pixel is judged to be not a concrete
pixel and obstacle removal is completed. Obstacle
removal results are shown in Fig. 6, where the transparent red areas show the obstacle pixels. We can seethat the background, bands, plates, and foot pegs, etc.
have been removed.
4. Crack detection algorithm
After obstacle removal, cracks are extracted fromthe concrete surface areas of the image. This algorithm applies a method that emulates human neuralcircuitry with a neural network.
The areas of concrete surface in the image after
obstacle removal are partitioned into small rectangles,
and a search for cracks is performed. The crack edge
features are extracted, and cracks are detected by theneural network. With this algorithm, we emulate the
NTT Technical Review

Feature Articles

Fig. 7. Crack image (left) and extracted edge feature (right).

1: Crack candidate pixelymechanism with which we see objects with our eyes,
and implement a crack line detection function thatworks in exactly the same way as the human eye andbrain do. An original crack image and the extractededge feature are shown in Fig. 7. The crack image is
300 × 300 pixels and the edge filter is 26 × 18 pixels.
Next, the neural network extracts the crack shapefrom the obtained edge feature. As shown in Fig. 8,
the problem is represented as a two-dimensional
array of neurons that is the same size as the edge
image, and the neural network searches for edges that
conform to the crack shape. The specific method ofusing the neural network to construct the expressionof behavior is described in Ref. [2]. In the crack
detection results (Fig. 9), the neurons (green pixels)
are seen to correspond to the crack.
5. Concluding remarks

0: Non-crack candidate pixel

NTT Group companies are going to apply the crack
detection technique described here to concrete pole
inspection, and the technique is expected to play auseful role in the maintenance and management ofsafe facilities. Furthermore, NTT manages many concrete structures that require periodic inspection
besides concrete poles, so we would like to improve
this technique to enable it to meet the crack inspectionneeds of other structures.

x

References
Fig. 8. Problem representation using a two-dimensional
array of neurons.

[1]
[2]

Ed. by M. Takagi, “Image Analysis Handbook,” New Edition, University of Tokyo Press, 2004.
S. Oka, S. Mochizuki, H. Togo, and N. Kukutsu, “A Neural Network
Algorithm for Detecting Invisible Concrete Surface Cracks in Nearfield Millimeter-wave Images,” Proc. of IEEE International Conference on Systems, Man, and Cybernetics (SMC2009), San Antonio,
TX, USA, 2009

Fig. 9. Crack detection results.

Vol. 10 No. 2 Feb. 2012Feature Articles

Suguru Kaneko

Engineer, Civil System Project, NTT AccessNetwork Service Systems Laboratories.
He received the B.E. degree in quantum andelectronic engineering from Tsukuba University,
Tsukuba, in 2002 and the M.E. degree in physicalelectronics from Tokyo Institute of Technology,
Tokyo, in 2003. He joined NTT EAST in 2003
and moved to NTT Access Network Service
Systems Laboratories in 2008. He is currently
engaged in research on maintenance technology
for civil infrastructure facilities.

Naoki Matsumiya

Senior Chief Engineer, NTT Access Network
Service Systems Laboratories.
He received the B.E. degree in environmental
information and the M.E. degree in civil engineering from Nagoya University, Aichi, in 1985
and 1987, respectively. He joined NTT in 1987
and worked in the Access Network Service Systems Laboratories from 1988 to 1992. He is currently engaged in R&D of management andmaintenance technology for civil infrastructure
facilities.

Soichi Oka

Senior Research Engineer, NTT Access Network Service Systems Laboratories.
He received the B.E. degree in environmental
information and the M.E. and Ph.D. degrees inmedia and governance from Keio University,
Kanagawa, in 1996, 1998, and 2001, respectively. During 2002–2003, he was engaged in R&Dof evanescent microwave imaging at Wright Patterson Air Force Research Laboratories and the
University of Cincinnati in Ohio, USA. Sincejoining NTT in 2004, he has been engaged in
R&D of nondestructive inspection systems.

NTT Technical Review

Feature Articles: Imaging and Sensing Technologies for Safety and Security

Remote Detection of Hazardous Gases
in Full-scale Simulated Fire by Using
Terahertz Electromagnetic Waves
Naofumi Shimizu†, Naoya Kukutsu, Atsushi Wakatsuki,
and Yoshifumi Muramoto
Abstract

This article describes a prototype spectroscopic sensing system that we have developed for detecting
hazardous gases at the scene of a fire from a distance. First, the system’s terahertz signal generator andreceiver are briefly described. Then, sub-terahertz-band absorption spectra of smoke generated fromheated nylon fabric are explained. Finally, a performance test of the developed sensing system carriedout in a full-scale simulated fire is presented.

1. Introduction
Our research and development (R&D) activities for
detecting hazardous gases from a distance with terahertz electromagnetic waves were previously reported in NTT Technical Review in 2009 [1]. The researchbegan in 2006 and continued under commission fromthe National Institute of Information and Communication Technology (NICT). At the end of 2010, thelast year of the commission, we succeeded in detecting hazardous gases from a distance in a full-scale
simulated fire by using a prototype of the terahertz
spectroscopic sensing system [2]. In the following
sections, we first outline the basic concept of theremote spectroscopic sensing system. We thendescribe the terahertz signal generator and receiverused in the system. Finally, we present the results ofsome experiments done with the system: small-scale
experiments using a gas cell and large-scale experiments with a full-scale simulated fire.
2. Sensing system
The operation of the remote gas sensing system is† NTT Microsystem Integration Laboratories
Atsugi-shi, 243-0198 Japan

illustrated schematically in Fig. 1. Carbon monoxide,hydrogen cyanide, and other hazardous gases generated by fires have characteristic absorption spectra in
the terahertz band. The remote spectroscopic sensing
system determines whether or not a hazardous gas ispresent around the target by sending terahertz electromagnetic waves to the target and analyzing the spectrum of the waves returned from the target. The system requires a terahertz transmitter and terahertz
receiver with a wide operating frequency range. To
meet the requirement, we used a photomixing technique to generate the terahertz electromagnetic
waves. The optical signal generator comprises threelaser sources that emit light waves in the 1.5-µm band
(Fig. 2). One of the lasers is a tunable-wavelength
laser source (TLS); the other two are fixed-wavelength laser sources (FLSs). The signal generated by
the TLS is first split and the resulting signals are
mixed with the outputs of both FLS1 and FLS2 togenerate optical beat signals. Those two optical beatsignals are converted into terahertz electromagnetic
waves by a uni-traveling carrier photodiode (UTCPD) module [3]. The FLS wavelengths are set so that
there is a 350-MHz frequency difference between
them. As a result, the frequency difference of the twogenerated terahertz electromagnetic waves is 350
MHz and is independent of the wavelength of the
NTT Technical Review

Feature Articles

Fire
Absorption characteristicsof hazardous gas
f3
Frequencyf2
f1

Terahertz waves
Absorption

Remote spectroscopic
sensing system
CO: 1.2%

Fig. 1. Operating principle of remote spectroscopic sensing system.

LD current

Tunable-wavelength
source (TLS)

Beat signal for transmitter

Fixed-wavelength
source (FLS1)

Beat signal for receiver

Fixed-wavelength
ource (FLS2)

PhotodiodeDetector

FM-AM converter

AM: amplitude modulation
FM: frequency modulation

Microwave mixer

Reference signal

Fig. 2. Optical signal generator.

TLS. When we use the two terahertz electromagnetic
waves as sensing waves and as the local oscillator
signal of the heterodyne receiver, the frequency of theheterodyne receiver’s intermediate frequency signal
can be made constant at 350 MHz. This means that
we do not have to widen the bandwidth of the intermediate frequency circuit needlessly, even if wechange the frequency of the terahertz electromagnetic
Vol. 10 No. 2 Feb. 2012

waves very rapidly. Consequently, we can keep thereceiver’s noise level very low.
Furthermore, small proportions of the two FLS
outputs are extracted and coupled, and their beat signal is obtained. This beat signal is mixed with thereference signal and their frequency error is extracted.
This error is fed back as the driving condition of
FLS2. Thus, the frequency difference of the FLSs isFeature Articles

THz wave forsensing
Half-mirror

SIS
mixer
Detector
Low-temperatureamplifierLocal
oscillator
signalADC

Band-passfilterCryostat

Personalcomputer
ADC: analog-to-digital converter

Fig. 3. Receiver block diagram.

locked. In this study, the FLS wavelengths were set tonear 1548.959 nm, and the TLS wavelength was
swept between 1547.3631 nm and 1544.971 nm inless than 1 s. The corresponding frequency sweep
was from 200 GHz to 500 GHz. Details of the UTCPD modules used in this study are given in another
Feature Article in this issue [3].
A block diagram of the receiver is shown in Fig. 3.
The mixer that we used for heterodyne detection was
the superconductor-insulator-superconductor tunneling (SIS) mixer developed by the National Institute ofAdvanced Industrial Science and Technology [4].
The mixer and the low-temperature amplifier that
amplifies the intermediate frequency signal output
are inside a mechanical cooler and maintained at thetemperature of liquid helium. The terahertz signal
used for sensing is combined with the local oscillator
signal by a coupler placed just in front of the receiverand then fed to the SIS mixer. The received sensing
waves are converted to an intermediate frequency
signal by the SIS mixer. After boosting by the lowtemperature amplifier, the out-of-band noise isremoved and the signal is converted to a voltage signal that is proportional to the signal strength. The
voltage signal is acquired by a personal computer via
an analog-to-digital converter connected to the detector’s output. The signal is combined with the wavelength data to obtain the spectrum. An overall view ofa remote spectroscopic sensing system that comprises
the terahertz signal generator, receiver, and optics isshown in Ref. [5]. We performed all of the experiments described below in electromagnetically shielded areas.
3. Transmission spectrum of smoke
Smoke from heated nylon fabric was supplied via aFig. 4. Gas cell filled with smoke.

heated pipe to a gas cell that was 1 m long and 10 cm
in diameter. A photograph of the cell filled with thegenerated smoke is shown in Fig. 4. The smoke wasso dense that it was impossible to see the wall on theother side of the cell. The smoke consisted of a large
number of suspended particles with diameters of several micrometers. The wavelength of the terahertz
waves used for sensing was about 1 mm, which is twoor three orders of magnitude larger than the particles.
Accordingly, the sensing waves were not scattered by
the particles. That is, such a dense smoke is transparent to the sensing waves. Therefore, we could observe
the characteristic absorption spectra of the gases inthe smoke when we measured the smoke’s absorption
spectra with the developed terahertz electromagnetic
waves generator and receiver. The results are shownin Fig. 5. At 240 s and 480 s after the nylon fabricbegan to be heated, absorption peaks were observed
at 265 GHz, 355 GHz, and 444 GHz. Those peakfrequencies agree well with the absorption frequencies of hydrogen cyanide molecules. A detailed
NTT Technical Review

Feature Articles

1.2

Transmittance

1.0
0.8
0.6
0.4
240 s
0.2

480 s
Calculated: dry air + HCN (1000 ppm)

0.0
200

250

300

350

400

450

500

Frequency (GHz)

Fig. 5. Absorption spectra of smoke.

Exhaust hood
Testing room (2.4 m x 2.4 m x 3.6 m)
Opening
SensingsystemCorner reflector

5m
8.6 m

Fig. 6. Setup in the fire test hall.

analysis clarified that the spectrum included theabsorption peaks of hydrogen cyanide, acetonitrile,and water [6]. Nylon is an organic polymer that contains amide bonds. Therefore, the observed spectrasuggest that the nylon fabric burned incompletely inthis experiment.
4. Gas detection in a full-scale fire
Next, we present the results of remote gas sensing
experiments conducted in a full-scale simulated fire.
The setup in the fire test hall at the Center for Fire
Science and Technology at the Tokyo University ofVol. 10 No. 2 Feb. 2012

Science is illustrated in Fig. 6. We put urethane
blocks in a testing room and ignited them. The testing
room imitates a small room and its inside dimensions
are 2.4 m wide, 3.6 m deep, and 2.4 m high. The testing room filled with smoke and gases generated by
the fire within a minute after ignition. The remote
spectroscopic sensing system was placed 5 m fromthe testing room and transmitted terahertz waves
toward the room’s opening. In this experiment, theterahertz waves were reflected by a metal cornerreflector placed on the opposite side of the testing
room facing the sensing system. Some of the returned
terahertz waves were received by the remoteFeature Articles

HCN
265 GHzHCN
355 GHzHCN
444 GHz

160 s

Transmittance

130 s
100 s

2

70 s
1
0
200

40 s
300

400

500

Frequency (GHz)

Fig. 8. A bsorption spectra measured with the remote
spectroscopic sensing system.
Fig. 7. Opening of the testing room seen from the sensing
system.

spectroscopic sensing system.
The room’s opening as seen from behind the remote
spectroscopic sensing system is shown in Fig. 7. The
measured absorption spectra of the terahertz waves
are shown in Fig. 8. Absorption peaks were clearlyobserved at 265 GHz, 355 GHz, and 444 GHz
between 70 s and 160 s after ignition. Those frequencies are consistent with the absorption frequencies ofhydrogen cyanide molecules. The average value ofthe hydrogen cyanide concentration estimated fromthe measured absorption intensity at a time between
100 s and 250 s was 890 ppm. In this experiment, thegas inside the testing room was sampled over the time
period between 90 s to 300 s. Chemical analysis of
the sampled gas revealed a hydrogen cyanide concentration of 640 ppm, which is consistent with the value
obtained by the remote spectroscopic sensing system.
These results show that the terahertz remote spectroscopic sensing system succeeded in realtime remotemonitoring of hazardous gases generated by a fire.
The main purpose of the experiments reported here
was to verify the basic operation of active remote
spectroscopic sensing in the terahertz band. We therefore used an artificial metal corner reflector to reflect
the terahertz electromagnetic waves effectively.
However, this does not mean that such a reflector isalways required in actual situations. For example, theintersections of three orthogonal planes at room corners or at a protruding wall also form corner-reflector
structures. Such non-metallic corner reflector structures made by the wall’s materials certainly exist on
the interior and exterior walls of structures. We
believe that terahertz electromagnetic waves of sufficient intensity can be returned when we irradiatesuch structures with terahertz electromagnetic waves.
We have already confirmed that a corner reflector
made of concrete panels effectively reflects terahertz
waves.
5. Concluding remarks
We have reported on remote detection of hydrogencyanide in smoke by using terahertz waves. We willcontinue our investigation and clarify the sensing
system’s potential, such as the detectable gas species,
detectable minimum gas concentrations, and maximum distance to the target. We will also clarify theneeds of emergency responders and develop a morepractical remote gas detection system.
References
[1]

[2]

[3]

N. Shimizu, H.–J. Song, Y. Kado, T. Furuta, A. Wakatsuki, and Y.
Muramoto, “Gas Detection Using Terahertz Waves,” NTT Technical
Review, Vol. 7, No. 3, 2009.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2009
03sf7.html
N. Shimizu, T. Ikari, K. Kikuchi, K. Matsuyama, A. Wakatsuki, S.
Kohjiro, and R. Fukasawa, “Remote Gas Sensing in Full-scale Firewith Sub-terahertz Waves,” 2011 IEEE MTT-S International Microwave Symposium Digest, Baltimore, MD, USA, 2011.
A. Wakatsuki, Y. Muramoto, and T. Ishibashi, “Development of Terahertz-wave Photomixer Module Using a Uni-traveling-carrier Photodiode,” NTT Technical Review, Vol. 10, No. 2, 2012.

NTT Technical Review

Feature Articles

[4]

[5]

https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2012
02fa5.html
S. Kohjiro, K. Kikuchi, M. Maezawa, T. Furuta, A. Wakatsuki, H. Ito,
N. Shimizu, T. Nagatsuma, and Y. Kado, “0.2–0.5 THz Single-band
Heterodyne Receiver Based on a Photonic Local Oscillator and aSuperconductor-insulator-superconductor Mixer,” Appl. Phys. Lett.,Vol. 93, No. 9, p. 093508, 2008.
K. Aihara and T. Kukutsu, “NTT R&D of Systems and Devices for

Naofumi Shimizu

Senior Research Engineer, Microwave andPhotonics System Research Group, Smart
Devices Laboratory, NTT Microsystem Integration Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in engineering physics from Kyoto University in
1986, 1988, and 2004, respectively. He joined
NTT LSI Laboratories in 1988 and engaged in
R&D of III-V high-speed devices. From 1998, he
was with NTT Lightwave Communications
Laboratory, where he was engaged in research on
high-speed lightwave transport systems. In April
2004, he moved to NTT Microsystem Integration
Laboratories and is currently engaged in R&D ofterahertz spectroscopy systems. He is a memberof IEEE, the Institute of Electronics, Information
and Communication Engineers (IEICE), and theJapan Society of Applied Physics (JSAP). He
received the 2002 IEICE Electronics Society
Prize and the Best Paper Award of 2006 from theAsia-Pacific Microwave Photonics Conference.

Naoya Kukutsu

Senior Research Engineer, Supervisor, Group
Leader, Smart Devices Laboratory, NTT Microsystem Integration Laboratories.
He received the B.E., M.E., and D.E. degrees inelectrical engineering from Hokkaido University
in 1986, 1988, and 1991, respectively. His D.E.
dissertation described research for a time-domain
electromagnetic wave numerical analysis method. In 1991, he joined NTT Applied Electronics
Laboratories in Musashino. His current research
involves millimeter-wave and terahertz-wave
transmission, as well as imaging systems. He is amember of the IEEE MTT and COM Societies
and IEICE.

Vol. 10 No. 2 Feb. 2012

[6]

Safety,” NTT Technical Review, Vol. 10, No. 2, 2012.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2012
02fa1.html
N. Shimizu, K. Kikuchi, T. Ikari, K. Matsuyama, A. Wakatsuki, S.
Kohjiro, and R. Fukasawa, “Absorption Spectra of Smoke Emitted
from Heated Nylon Fabric Measured with a Continuous-Wave SubTerahertz Spectrometer," Appl. Phys. Exp., Vol. 4, No. 3, pp. 032401–
032401-3, 2011.

Atsushi Wakatsuki

Senior Research Engineer, Terabit Device
Laboratory, NTT Photonics Laboratories.
He received the B.E. and M.E. degrees in electronics from Tohoku University, Miyagi, in 1983
and 1985, respectively. He joined NTT in 1985
and developed telemetry and command equipment for communication systems on satellites.
During 1989–1997, he moved to NTT OptoElectronics Laboratories and ATR (Advanced
Telecommunications Research Institute International) and studied semiconductor optical waveguide devices, MQW lasers, ultrashort-pulse
generation, and two-light-mixing in nonlinear
crystals. Since 2007, he has been with NTT Photonics Laboratories, where he has been active inresearch on ultrahigh-speed photodiodes andTHz-wave photomixer modules. He is a memberof the Japan Society of Applied Physics (JSAP).

Yoshifumi Muramoto

Senior Research Engineer, Terabit Device
Laboratory, NTT Photonics Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from Osaka Prefecture University in 1990 and 1992, respectively. He joined
NTT Opto-Electronics Laboratories in 1992 andhas been engaged in research on monolithicallyintegrated photoreceivers and high-speed photodetectors. He is a member of IEICE and JSAP.

Feature Articles: Imaging and Sensing Technologies for Safety and Security

Development of Terahertz-wave
Photomixer Module Using a
Uni-traveling-carrier PhotodiodeAtsushi Wakatsuki†, Yoshifumi Muramoto,and Tadao Ishibashi
Abstract

As a promising power source for various terahertz-wave (THz-wave) applications, we have developed
a compact, lightweight, high-power, and ultrawideband THz-wave photomixer module using a uni-traveling-carrier photodiode. The module operates at room temperature and exhibits output power of over
0.5 mW at 0.35 THz and a 10-dB-down bandwidth of 260 GHz. Its output frequency can be swept continuously over an extremely wide frequency range in the terahertz band. In this article, we report on thephotomixing technique using an ultrahigh-speed, high-power photodiode to generate THz-waves and thedevelopment of two versions of the photomixer module.

1. Introduction
In both communications and non-communications
fields, there have been many trials for using an unexploited frequency range, the terahertz range, for highfrequency applications. Terahertz-waves (THzwaves) are electromagnetic waves in the frequency
range from 0.1 THz to 10 THz (Fig. 1). This frequency range is positioned between radio waves andlight waves, so the characteristics of THz-waves areeither intermediate between them or a combination of
their advantages. The main features of interest arealso described in Fig. 1.
Recently, various high-frequency applications utilizing these superior features of THz-waves havebeen actively proposed and investigated. Possible
applications include an ultrahigh-capacity wirelesscommunication system, non-destructive inspection/
imaging system, spectroscopic remote sensing/analysis system, and astronomical radio telescope system,
as shown in Fig. 2. THz-waves have been enthusiastically investigated especially for systems concerning† NTT Photonics Laboratories
Atsugi-shi, 243-0198 Japanhuman safety and rescue in light of the frequent disasters that have occurred in recent years.
A compact, lightweight, and easy-to-handle THzwave generator is a key device for such high-frequency systems. The development of a generator thatsimultaneously provides high power, a wide bandwidth, and frequency-sweeping capability has been
one of the most important targets in this field.
2. THz-wave generation using a photodiodeNTT Photonics Laboratories invented the uni-traveling-carrier photodiode (UTC-PD) [1] as a key
device for high-speed optical communication systems and has continued to investigate it to make
improvements and develop applications. The UTCPD simultaneously achieves both ultrahigh speed andhigh power compared with the conventional pin-PD.
This excellent feature is extremely suitable not only
for a high-speed optical communication receiver but
also for an electromagnetic-wave generator. The
development of the THz-wave photomixer module
was based on the same technology as used in the
UTC-PD-based millimeter-wave photomixer modulethat we developed earlier [2]. The structures of both
NTT Technical Review

Feature Articles

Frequency range of terahertz waves
Microwaves

Millimeterwaves

THz-waves(0.1−10 THz)

Infrared

VisibleBoundary

Radio waves

Ultraviolet

X-raysLight waves

Features similar to radio waves, light waves, and radiation- Transparent for certain kinds of materials- Absorption spectra (fingerprints) of many materials- High information capacity compared with microwaves and millimeter waves- High-spatial resolution compared with microwaves and millimeter waves- Small beam scattering compared with infrared and visible light- Safer for the human body than x-rays and gamma rays

Fig. 1. Frequency range and interesting features of THz-waves.

- High-capacity wirelessinformation delivery- Local signal supply for radio telescope- Non-destructive inspection- Remote imaging- Spectroscopic analysis- Remote sensing

Non-compressedhigh-definition TV

LAN

LAN

Video
network

Short-range high-capacity
wireless communication

Pricelessartwork

Dangerous gases
Frequency

Video
network

10-Gbit/s
Ethernetf3

THz-wave
illumination

f2
f1

Absorption
intensity
Absorption
spectrumTHz-waverefrection

Disaster spot

Spectroscopic
remote gas sensing

Discoveryof historical drafts

Artist’s draft
hidden behind
painting
Nondestructive
inspection & imaging

LAN: local area network

Fig. 2. Possible applications utilizing the features of THz-waves.

the UTC-PD and the module were improved and optimized for use in a frequency range of 0.2 THz to 0.65
THz. At present, our module has a center frequencyof 0.35 THz for application as the power source of aspectroscopic remote sensing system [3].
The PD-based photomixing technique is explained
Vol. 10 No. 2 Feb. 2012

in Fig. 3. THz-waves are generated by ultrahighspeed optical-to-electrical (O/E) conversion in a
UTC-PD. An optical beat-signal with a beating frequency of fb (= f1 - f2) is obtained by combining twolights of different frequencies (f1, f2; f1>f2) in an optical coupler. For example, by using lights with

Feature Articles

Waveguide orantennaTHz-waves
Modulatedcurrent

Light 1 (f1)

Opticalbeat-signal

Coupler
fb = f1-f2(terahertz frequency)

Ultrahigh-speed
OE-conversion
by UTC-PD

Light 2 (f2)Fig. 3. PD-based photomixing technique.

p-dopedabsorption layer Electron
p-contactplayerElectronp-contact
p
layer

i
Hole
Undopedabsorption layer

Velocity
overshoot

p

Light

Light
Hole
n

n-contactlayer(a) Band structure of pin-PD

Undoped carriercollection layerinn-contactlayer(b) Band structure of UTC-PD

Fig. 4. Differences between band structures of conventional pin-PD and UTC-PD.

wavelengths of 1.5000 μm (201 THz) and 1.5075 μm(200 THz), we can get a beat signal with a frequencyof exactly 1 THz. The optical beat-signal that is input
into the UTC-PD is converted into a modulated electric current with the same frequency as fb. THz-waves
can be obtained by radiating the modulated current
into space from a waveguide or antenna.
Details of the UTC-PD used in the photomixing
technique are given in the next section.
3. UTC-PD
3.1 Features
Although the most widely used PD is the conventional pin-PD, there are two limitations regarding its
performance (pin: positive, intrinsic, negative layers).
One concerns the maximum operating speed. Whenlights are input into the pin-PD’s absorption layer,
electron-hole pairs, which are the electric current carriers, are generated (Fig. 4(a)). In a pin-PD, bothelectrons and holes contribute to its O/E-conversion
mechanism. Because a hole’s effective mass is largerthan an electron’s and its velocity is much lower, themaximum operating speed of the pin-PD is limited by
the hole velocity. The other limitation concerns the
maximum output current. When the optical input
power becomes higher, the band in the undoped
absorption layer is bent by the space charge due tocarrier accumulation. This band bending causes a
drastic reduction in carrier velocity, which results insaturation of the output current. To overcome theselimitations of the pin-PD, we studied the UTC-PD.
In the UTC-PD, a p-doped absorption layer is separated from a wide-gap undoped carrier collection
layer (Fig. 4(b)), which is in clear contrast to the pinPD. This structure means that only electrons are theactive carriers running through the depleted undoped
carrier collection layer. Most of the photogenerated
holes relax extremely quickly into the absorption
layer where holes are the major carrier. Consequently,
the maximum operating speed of the UTC-PD isdetermined only by the electron velocity, which ismuch higher than the hole velocity. The space charge
effect is also much smaller than in the pin-PD because
NTT Technical Review

Feature Articles

Stub-line

UTC-PDCPW

GND padSignalpad

300 µmBias pad

GND pad450 µm
CPW: coplanar waveguide
GND: groundFig. 5. Photograph of the fabricated UTC-PD chip.

carrier accumulation is extremely small in principle
compared with the pin-PD. For these reasons, theUTC-PD can exhibit high performance, that is, ultrahigh-speed and high saturation output current simultaneously.
We have achieved a 3-dB-down bandwidth of 310
GHz and a 10-dB-down bandwidth of 750 GHz as theoperational frequency bandwidth [4]. We have also
obtained a maximum output current of 30 mA forhigh optical input power.

power of the THz-waves is designed to be as high as
possible at around the designed center frequency. In
this development, the center frequency was set to0.35 THz. The output current is expected to increaseat around the center frequency through use of theresonant effect. We measured the pulse photoresponse to evaluate the chip’s resonant design. Fromthe measured pulse waveform oscillating at about
0.35 THz, we ascertained that the stub-line acted
properly as a resonant circuit.

3.2 Optimization for THz-wave photomixer module
It is essential to optimize the UTC-PD’s band structure in order to obtain an output current with a terahertz frequency. We designed a thinner absorption
layer than the one in the UTC-PD for the previous
millimeter-wave photomixer; this reduced the UTCPD’s carrier transit time when it operates at higher
frequencies. The thicknesses and materials of theother layers were also optimized for compatibility
with the thin absorption layer.
Besides the band structure, we also improved thephysical structure of the UTC-PC chip. The fabricated UTC-PD chip designed to operate in the terahertz
frequency range is shown in Fig. 5. The chip is 300 ×
450 μm2.
The frequency response of the PD output current
shows a generally broadband-shape caused by theresistor-capacitor time constant. The response
decreases with increasing operating frequency. For
the THz-wave photomixer, a stub-line was integrated
on the chip to increase the output current. It acts as aresonant circuit for the UTC-PD to compensate forthe roll-off of the frequency response. The output

4. Waveguide-output-type photomixer module

Vol. 10 No. 2 Feb. 2012

4.1 Optimization
For practical use, in addition to being high-performance, the photomixer module is required to be compact, lightweight, and easy to handle. Moreover, since
the module will be applied to various high-frequency
systems, it should be easy to assemble on a board orin a package. As a result of our investigation, we
determined that a butterfly-package would be an
excellent form for the module. It is a generic packagewith a good track record for many optical devices, so
it is cheap and readily available.
For the module structure investigation, there are
two main issues: they concern optical implementation and electrical implementation. The former covers how efficiently light is input to the UTC-PD, andthe latter covers how efficiently ultrahigh-frequency
current from the UTC-PD is converted into THzwaves.
Today’s high-frequency coaxial connectors havepronounced propagation loss for current with a frequency range above 0.1 THz, so it is essential tochose either the waveguide technique or directFeature Articles

Quartz coupler

UTC-PD chip
Opticalbeat-signal

Lenses

Optical fiber

Waveguide

Metal mountTHz-wavesFig. 6. Schematic cross-section of the module’s internal structure.

radiation technique (quasi-optical technique) to output the current from the module. We developed awaveguide-based module structure during our development of the previous millimeter-wave photomixer
module and confirmed its excellent characteristics.
The THz-wave photomixer module is based on an
extension of that technique.
The cross-section of the internal structure is schematically shown in Fig. 6. The optical part consists ofan optical fiber and collimation lenses. The collimation lens system has good versatility for an opticaldesign, and it also has excellent optical alignment
stability among the optical fiber, lenses, and UTCPD. The UTC-PD chip is mounted on a metal blockwith a rectangular waveguide whose aperture size is0.864 mm × 0.432 mm. The UTC-PD chip and thewaveguide are connected by a quartz coupler consisting of an impedance converter, a micro-strip line, anda radiator. The module is made by assembling thesecomponents in the butterfly-package with the same
waveguide.
The design of the quartz coupler is extremely
important for high emission efficiency of the ultrahigh-frequency current as THz-waves into the waveguide. The design policies were as follows:(1) Reflection power as low as possible(2) Transmission power as high as possible(3)	Propagation of higher-order modes in the
waveguide suppressed as much as possible fromthe UTC-PD’s current output point to the waveguide’s THz-wave output point.
The quartz coupler was designed using a threedimensional electromagnetic wave simulator to
obtain flattened characteristics from 0.2 THz to 0.65
THz. As a result of simulation for various designs, theoptimized one achieved reflection power of less than
-30 dB and transmission power loss of less than 3
dB.
4.2 Performance
We fabricated a THz-wave photomixer module(Fig. 7) with the optimized design described in theprevious section. The module is 3 × 1.65 × 1 cm3.
The output power dependence on frequency when
the optical beat signal’s frequency was swept from
0.2 THz to 0.65 THz is shown in Fig. 8. The output
power was clearly enhanced at around 0.35 THz(designed center frequency of the stub-line) with the
maximum value being over 0.5 mW [5]. This is thehighest value ever reported for a photomixer with asingle PD operating in this frequency range. The 3and 10-dB-down bandwidths were as wide as 120 and
260 GHz, respectively, showing extremely wideband
performance.
The main feature of this module is that it simultaneously achieves both high-output power and extremely
wide bandwidth. In addition to this high performance,
it is very easy to use because it operates in continuous-wave operation at room temperature without any
cooling.
5. Quasi-optical photomixer module
For diversification of the THz-wave output method,
we have been also investigating a quasi-optical photomixer module (Fig. 9). The THz-wave generation
principle is the same as for the waveguide-outputtype module. The quasi-optical module consists of a
NTT Technical Review

Feature Articles

Optical beat-signal
Optical fiber
Outer waveguideTHz-waves

DC cable
ModuleFig. 7. Photograph of the fabricated THz-wave photomixer module.

0–2.7 dBm

Output power (dBm)–5

Photocurrent=20 mA
Bias voltage=-2.5 V

Optical fiber

120 GHz1 cm–10260 GHz–15

THz-waves–20

Si lens–25–30

DC biasFig. 9. Photograph of the quasi-optical photomixer module.
0.2

0.3

0.4
0.5
Frequency (THz)

0.6

Fig. 8. Fabricated module’s output power dependence on
frequency.

UTC-PD chip integrated with an antenna and a metal
package with a silicon lens for collimating the radiated THz-waves.
In the waveguide-output-type module, as the operation frequency becomes higher, the waveguide’s
aperture must become smaller. Therefore, in the terahertz range, it becomes difficult to fabricate a precise
rectangular waveguide with a small aperture by ametal-cutting process. The quasi-optical module issuitable for increasing the operating frequency
because it does not require high-precision metal-cutting. There are also some other good features:Vol. 10 No. 2 Feb. 2012(1)	It is easy to make either a broadband- or resonant-type module by choosing the type of integrated antenna.
(2)	It is easy to fabricate an airtight module, which
is beneficial for module reliability because themodule does not have an aperture.
(3)	The electrical implementation is easy becausethat there is no quartz coupler requiring precise
positional alignment.
Although the maximum output power is lower thanthat of the waveguide-output-type module at present,
we confirmed THz-wave generation up to a frequencyof 1.5 THz [6].
5. Conclusion
The features of the UTC-PD were described,
including the operating principles and characteristics.
Feature Articles

Compared with the conventional pin-PD, the UTCPD has the advantages of extremely wide bandwidthand high power owing to its unique carrier transportmechanism. One application for the UTC-PD is THzwave generation. Our developed THz-wave photomixer module using a UTC-PD, fabricated as a waveguide-output-type module, exhibits excellent performance. It has a maximum output power of over 0.5
mW at 0.35 THz and a 10-dB-down bandwidth of 260
GHz. This maximum output power is the highest
value ever reported for direct generation from a photomixer operating in a frequency range of 0.2 THz to
0.65 THz. Our UTC-PD-based quasi-optical module
generates THz-waves up to a maximum frequency of
1.5 THz.

References
[1]
[2]

[3]

[4]
[5]

Acknowledgment
Part of this work was supported by the National
Institute of Information and Communications Technology, Japan.

Atsushi Wakatsuki

Senior Research Engineer, Terabit Device
Laboratory, NTT Photonics Laboratories.
He received the B.E. and M.E. degrees in electronics from Tohoku University, Miyagi, in 1983
and 1985, respectively. He joined NTT in 1985
and developed telemetry and command equipment for communication systems on satellites.
During 1989–1997, he moved to NTT OptoElectronics Laboratories and ATR (Advanced
Telecommunications Research Institute International) and studied semiconductor optical waveguide devices, MQW lasers, ultrashort-pulse
generation, and two-light-mixing in nonlinear
crystals. Since 2007, he has been with NTT Photonics Laboratories, where he has been active inresearch on ultrahigh-speed photodiodes andTHz-wave photomixer modules. He is a memberof the Japan Society of Applied Physics (JSAP).

[6]

T. Ishibashi and H. Ito, “Uni-traveling-carrier Photodiodes,” Tech.
Dig. Ultrafast Electronics and Optoelectronics, Lake Tahoe, CA,
USA, pp. 83–87, 1997.
H. Ito, T. Nagatsuma, A. Hirata, A. Sasaki, Y. Hirota, and T. Ishibashi,“High-power Photonic Millimeter-wave Generation at 100 GHz
Using Matching-circuit-integrated Uni-travelling-carrier Photodiodes,” Proc. of Inst. Elect. Eng. Optoelectron., Vol. 150, pp. 138–142,
2003.
N. Shimizu, N. Kukutsu, A. Wakatsuki, and Y. Muramoto, “Remote
Detection of Hazardous Gases in Full-scale Simulated Fire by Using
Terahertz Electromagnetic Waves,” NTT Technical Review, Vol. 10,
No. 2, 2012.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2012
02fa4.html
H. Ito, T. Furuta, S. Kodama, and T. Ishibashi, “InP/InGaAs Uni-traveling-carrier Photodiode with 310 GHz Bandwidth,” Electron. Lett.,Vol. 36, No. 21, pp. 1809–1810, 2000.
A. Wakatsuki, T. Furuta, Y. Muramoto, T. Yoshimatsu, and H. Ito,
“High-power and Broadband Sub-terahertz Wave Generation Using a
J-band Photomixer Module with Rectangular-waveguide Output
Port,” Proc. of the 33rd IRMMW-THz2008, Pasadena, CA, USA,
M4K2.1199, 2008.
H. Ito, T. Furuta, F. Nakajima, K. Yoshino, and T. Ishibashi, “Photonic Generation of Continuous THz Wave Using Uni-traveling-carrier Photodiode,” J. Lightwave Technol., Vol. 23, No. 12, pp. 4016–
4021, 2005.

Tadao Ishibashi

Research Professor at NTT Photonics Laboratories and Engineering Manager at NTT Electronics Corporation.
He received the B.E., M.E., and Ph.D. degrees
in applied physics from Hokkaido University in
1971, 1973, and 1986, respectively. Since joining
Nippon Telegraph and Telephone Public Corporation (now NTT) in 1973, he has been involved
in research on high-speed and optoelectronic
semiconductor devices, including IMPATT
diodes, field-effect transistors, heterostructure
bipolar transistors, high-speed photodiodes, avalanche photodiodes, THz photomixers, andMach-Zehnder modulators. He received the
Ichimura Award in 1992 and IEICE Electronics
Society Award in 2001. He is a fellow of IEICE,
JSAP, and IEEE.

Yoshifumi Muramoto

Senior Research Engineer, Terabit Device
Laboratory, NTT Photonics Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from Osaka Prefecture University in 1990 and 1992, respectively. He joined
NTT Opto-Electronics Laboratories in 1992 andhas been engaged in research on monolithicallyintegrated photoreceivers and high-speed photodetectors. He is a member of the Institute ofElectronics, Information and Communication
Engineers (IEICE) and JSAP.

NTT Technical Review

Feature Articles: Imaging and Sensing Technologies for Safety and Security

Visualization of Pharmaceutical
Drug Molecules by
Terahertz Chemical Imaging
Katsuhiro Ajito†, Yuko Ueno, and Ho-Jin Song
Abstract

Terahertz spectroscopy allows the detection and identification of various molecular networks ofchemical bond interactions, including those of hydrogen bonds. In this article, we explain terahertz timedomain spectroscopy, the principles of terahertz chemical imaging, and a multicomponent quantitative
analysis method for amino acids that can visualize pharmaceutical crystalline polymorphs as an exampleof this technology.

1. Introduction
Terahertz waves (THz-waves), which possess anenergy level between that of radio waves and lightwaves, can provide various types of information,
including crystal phonon modes, low-wavenumber
frequencies, and gas molecule rotation modes. The
specific absorption frequencies of a chemical substance are shown in Fig. 1 [1]. Mid-wavelength infrared absorption spectroscopy deals with the high-frequency vibration mode, so it can identify the functional groups of molecules. By contrast, terahertz
spectroscopy can determine low-frequency vibration
modes such as those existing in van der Waals interactions and hydrogen bonding. Terahertz peaks
appear owing to resonance in the hydrogen bonds inmost amino acid crystals. Terahertz spectroscopy canidentify various types of information from molecular
networks, including biomolecules dissolved in aqueous solution, organic molecular crystals, higher-orderprotein structures, and the DNA (deoxyribonucleicacid) double helix. The hydrogen bonding of biomolecule clusters and molecular networks in water isshown in Fig. 2. From the terahertz spectrum, we canobtain the continuous frequency corresponding toeach individual peak; that is, we can obtain the chem† NTT Microsystem Integration Laboratories
Atsugi, 243-0198 Japan

ical image. Molecular clusters may contain pharmaceutical drug molecules and hydrogen molecules,
amongst others. These molecular networks areimportant in the biomedical field because they can
influence the effectiveness of medicines. However,the details of molecular networks are difficult toobtain because the cluster size is on the nanometerscale and there are few methods for measuring such
sizes accurately, particularly when the cluster
includes water as a component.
The resonance frequency of molecular networks isin the terahertz band, which makes such networks
amenable to terahertz spectroscopy and imaging.
Terahertz spectroscopy is a useful tool that can identify hydrogen bonds present in molecules that comprise various bio-specimens, including organic acids,
amino acids, sugars, drugs, polypeptides, DNA, andproteins as well as macrostructures such as cancer
cells. Since most amino acids and pharmaceutical
drug molecules contain hydrogen bonds, quantitative
analysis from terahertz spectrum peaks is possible.
The applications of terahertz spectroscopy are wide
ranging and many practical uses are expected. In
addition to fundamental research applications such as
pathology testing or the identification of different
types of explosives, other applications of the technology have been receiving attention. In contrast to Xray imaging, terahertz chemical imaging (TCI) can
identify molecules by their molecular network
NTT Technical Review

Feature Articles

30 cm

3 cm

3 mm 300 µm 30 µm

3 µm

300 nm30 nm

Near-infrared light
Radio waves Microwaves Terahertz waves Mid-infrared light Ultraviolet light
X-raysVisible light
109

1010

1011

1012

1013

1014

1015

1016 Hz

Hydrogen bond vibration mode
Distortion mode
Low-frequency molecular vibration mode
Crystalline phonon vibrations
Molecular rotations

Fig. 1. Specific chemical absorption frequencies.

Terahertz waves

Terahertz chemical imaging

Frequency

Degree ofabsorption

Molecule clusters

Biomolecules

Hydrogen bonding

Frequency
Terahertz spectrum

Fig. 2. Terahertz spectrum of molecular network.

spectrum. Because the technique can reveal themolecular network rather than only the molecular
distribution, we believe that TCI can provide a new
method for both drug identification and pathologicalexamination. Terahertz waves penetrate many typesof materials and can be used to confirm molecular
uniformity as well as to identify crystal polymorphs.
Crystal polymorphism arises from different types ofhydrogen bonding occurring within crystals and canVol. 10 No. 2 Feb. 2012lead to differences in physical and chemical properties, including differences in solubility, melting point,
and bioavailability. Bioavailability, rather thandescribing physiochemical properties of pharmaceutical drug molecules, describes the way in which
chemicals are absorbed in the human body and isimportant in determining the effective dosages ofparticular medications.

Feature Articles

2. Principle of terahertz time-domain
spectroscopy (THz-TDS) and THz-TDS system

and recorded and then the Fourier transform is used
to change each frequency. Samples such as medicinal
tablets are fixed in a vacuum chamber, which can
attenuate the absorption of atmospheric water.

Terahertz time-domain spectroscopy (THz-TDS) is
the most popular type of terahertz spectroscopy.
Advances in terahertz technology have allowed thecommercialization of THz-TDS, which is also beginning to be recognized as a new analytical chemistry
method. A THz-TDS system involves coherent signal
generation and detection. The terahertz pulse repetition frequency is around 80 MHz. A typical THzTDS system configuration is shown in Fig. 3 [2]. An
ultrashort (10–100 fs) laser pulse is split so that boththe terahertz generator and receiver are irradiated.
The most common terahertz generator and receiver isa photoconducting antenna. A sub-picosecond terahertz pulse wave irradiates the sample and when it
propagates, the electric field strength changes. Rather
than measuring the light intensity in the frequency
domain, we use the time domain to measure the electric field strength. The measurement of terahertz
waves in the frequency domain usually uses a delayline. Repetitive terahertz pulse waves are measured
and converted into intensity and phase by Fouriertransformation. THz-TDS can be performed at room
temperature and it has better sensitivity than farinfrared spectroscopy for frequency domain measurement. An example of a TCI system using THz-TDS
and a stage that can be moved in three dimensions isshown in Fig. 4 [3]. While the stage is scanned, thewaveform of the terahertz pulse wave is measured

3. Quantitative chemical analysis by THz-TDS
Until now, there has been no discussion of the quantitative relationship between the degree of absorption(absorbance) gained from THz-TDS and the sample
concentration or sample optical path length measured
from actual spectra. To illustrate the usefulness of
THz-TDS as a general analytical sensing method, it isMirrors
Samples (tablets)THz-wave pulses

THz-wavegenerator

Detector
Delay line

Femtosecond laser
Controller(personal computer)Fig. 3. THz-TDS system configuration.

yCoodinates(Xn, Yn)x
THz-wave generator

y

Delay
line

z

Samples (tablets)x

Vacuum chamberTHz-wave generator

Femtosecondlaser
3D movable stage

Fig. 4. TCI system using a three-dimensionally movable stage.



NTT Technical Review

Feature Articles

Supplement tablets (no packaging)Experimental value
Calculated value
4=2.03 THz

4

L-glutamic acid2

Degree of absorption

2
Degree of absorption

Degree of absorption

0.314 mol/L
0.585 mol/L
0.837 mol/L

04=1.22 THz

L-arginine

L-isoleucine

L-leucine

Maltitol
Citric acidL-valine

Supplement tablets (paper packaging)2

Maltitol

0

0
1

2
3
Frequency (THz)4(a) THz absorption spectra of L-glutamic
acid tablets.

0.0

0.5

L-glutamic acid L-isoleucine
L-arginine
L-valine

1.0

Concentration (mol/L)(b) Calibration curve for the two spectral
peaks.

L-leucine
Citric acid
(THz)

0.5

1.0

1.5
2.0
Frequency

2.5

3.0

(c) Spectra of supplement tablets and results ofquantification analysis based on standard spectra.

Fig. 5. THz quantification analysis results for supplement tablets containing various types of amino acids.

necessary to conduct a quantitative discussion of therelationship between absorbance and sample concentration or sample optical path length and the relationship between noise level and detection limit, inparallel with qualitative discussion of the terahertz
absorption peaks. The absorption spectra of three different concentrations of medicinal tablets that include
crystals of the essential amino acid L-glutamic acidare shown in Fig. 5(a) [4]. Amino acid crystals forma stable crystal structure via hydrogen bonding
among amino acid molecules or bonding between
amino acid molecules and hydrated water molecules.
It is believed that multiple instances of hydrogenbonding exist between molecules, which can bereflected in a single spectrum. The calibration curves
for the two main peaks are shown in Fig. 5(b). The
peak intensity (including background absorption) is
proportional to the concentration, which shows thatquantitative analysis is possible. To determine thetypes of different amino acids present, we calculated
the molar absorption coefficients in the region from
0.5–3.0 THz. These coefficients were then used as thestandard spectra. We then carried out quantitative
chemical analysis of sample tablets containing a mixture of several amino acids. The upper part of Fig. 5(c)shows the spectrum of commercially available supplements containing various types of amino acids [5].
Vol. 10 No. 2 Feb. 2012

Five types of amino acid were detected, including Lglutamic acid, and L-leucine, and the concentrationof each amino acid could be quantified by automatic
analysis using a standard spectrum database. The
lower part of Fig. 5(c) shows the spectrum and analysis result for pills wrapped in medical paper madefrom starch. Since THz-waves have high permeability
for such substances even when they are wrapped in
medical paper, there was almost no resulting effect onpeak position and strength, and quantitative analysis
was possible with standard spectra. We expect thequantitative accuracy to improve in the future as the
database quality and the fitting techniques are
improved.
4. Visualization of pharmaceutical
drug molecules by TCI
TCI technology can identify the two- or threedimensional distribution of each type of molecule on
the basis of the molecular network as well as molecular concentrations from the terahertz absorption spectrum. An example of an application to pharmaceutical
drug molecules—identification of two polymorphs offamotidine, which is a component of stomach medicine and a histamine receptor antagonist (H2 blocker)—is shown in Fig. 6.
Feature Articles

Polyethylene

A-typecrystalsB-typecrystals

0.9 THz
1.2 THz
Test tablets
1.4 THz

Degree of absorption

1.6 THz
2.0

B-typecrystals(1.2 THz)

Commerciallyavailable drugs
B-typecrystals

1.5

A-typecrystals(1.6 THz)

A-typecrystals
Degree of absorption
Low
High

1.0
0.5
0.0
0.5

1.0
1.5
Frequency

2.0 (THz)(a) Terahertz absorption spectra and transmittanceimages of a tablet of crystalline polymorphic
famotidine.

(b) Terahertz absorbance images of test and commercial
tablets.

Fig. 6. TCI mapping of test and commercial drug tablets.

The absorption spectra of tablets containing the Atype and B-type crystalline polymorphs of famotidinewith polyethylene binder as well as transmittance
TCI images are shown in Fig. 6(a) [6]. Type B is adigestive-tract medicine that works as a histamine
H2-receptor antagonist. Types A and B are easily
identified in the terahertz spectrum because there aredifferences in their absorption peaks. Absorption
images of the test tablets, which exhibited the characteristic A-type and B-type peaks, and of commercial
drug tablets are shown in Fig. 6(b) [7]. The A-type
and B-type tablets were 10 mm and 7.6 mm, respectively, in diameter. Types A and B showed peaks close
to 1.6 THz and 1.2 THz, respectively. In the test tablets, regions of A type on the left and regions of B
type on the right could be clearly identified. By contrast, in the commercial drug tablet images, there isalmost no A type, whereas B type is widely distributed as the active ingredient. This is the first time thatpharmaceutical drug crystal polymorphs have beenvisualized inside tablets. There are several tablet
molding methods, so the method used for molding
the test tablets investigated here may not be used for
all tablets in practice; nevertheless, it is very important to pioneer new testing methods while achieving
the desired goal of pharmaceutical product safety.


5. Conclusion
We introduced molecular pharmaceutical drugs andsupplements as examples of substances suitable forquantitative analysis by terahertz spectroscopy andchemical imaging. Even though the number of
researchers involved in terahertz wave technology
has increased markedly in recent years, potential
applications of terahertz wave chemical analysis are
only just being identified. The reasons for thisinclude, but are not limited to, the difficulty inmeasuring the 1–3-THz range, which is effective fordetecting hydrogen bonding and molecular networks,
the fact that a theory for terahertz spectral analysis
methods has not yet been established, the complexityof THz-TDS spectral analysis, and the cost of theequipment used. Nevertheless, the ability of terahertz
waves to visualize new structures and interactions
that were previously unobservable is a powerful toolin the fields of biology, medicine, and life sciences. In
future, we would like to continue exploring anddeveloping this revolutionary technology.

NTT Technical Review

Feature Articles

References
[1]
[2]

[3]

Y. Ueno and K. Ajito, “Analytical Terahertz Spectroscopy,” Analytical
Sciences, Vol. 24, No. 2 pp. 185–192, 2008.
K. Ajito, Y. Ueno, T. Haga, and N. Kukutsu, “Terahertz Spectroscopy
Technology for Molecular Networks,” NTT Technical Review, Vol. 7,
No. 3, 2009.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2009
03sf6.html
Y. Ueno, R. Rungsawang, I. Tomita, and K. Ajito, “Quantitative Measurements of Amino Acids by Terahertz Time-domain Transmission
Spectroscopy,” Analytical Chemistry, Vol. 78, No. 15, pp. 5424–5428,
2006.

Katsuhiro Ajito

Senior Research Scientist, Smart Devices
Laboratory, NTT Microsystem Integration Laboratories, and Distinguished Technical Member,
NTT Science and Core Technology Laboratory
Group.
He received the Ph.D. degree in applied chemistry from the University of Tokyo in 1995. He
joined NTT Basic Research Laboratories in
1995. From 1995 to 2007, he studied glutamate
in single synapses using Raman spectroscopy
and laser tweezers. His current research interests
include nano- and biochemistry studied by terahertz spectroscopy and imaging techniques. He
received the Young Scientist Award for the Presentation of an Excellent Paper from the Japan
Society of Applied Physics (JSAP) in 1999. He
has been the Executive Director and the Chair of
the Terahertz Spectroscopy Division of the Spectroscopical Society of Japan (SSJ) since 2005,
the Secretary of the Terahertz Interest Group inIEEE 802.15 Working Group for wireless personal area networks since 2011, and the Chair ofthe Micro/Nano Electromechanical Systems andBio/Medical Analyses Area of the International
Conference on Solid State Devices and Materials
in JSAP since 2012. He is a member of IEEE,
SSJ, JSAP, the Chemical Society of Japan, and
the American Chemical Society.

[4]

[5]
[6]
[7]

Y. Ueno, K. Ajito, N. Kukutsu, and E. Tamechika, “Quantitative
Analysis of Amino Acids in Dietary Supplements Using Terahertz
Time-domain Spectroscopy,” Analytical Sciences, Vol. 27, No. 4, pp.
351–356, 2011.
K. Ajito, Y. Ueno, H.-J. Song, E. Tamechika, and N. Kukutsu, “Terahertz Chemical Imaging of Molecular Networks for Pharmaceutical
Applications,” ECS Trans., Vol. 35, No. 7, pp. 157–165, 2011.
K. Ajito and Y. Ueno, “THz Chemical Imaging for Biological Applications,” IEEE Trans. on Terahertz Science and Technology, Vol. 1,
No. 1, pp. 293–300, 2011.
K. Ajito, Y. Ueno, H.-J. Song, E. Tamechika, and N. Kukutsu, “Terahertz Spectroscopic Imaging of Polymorphic Forms in Pharmaceutical Crystals,” Molecular Crystals and Liquid Crystals, Vol. 538, No.
1, pp. 33–38, 2011.

Ho-Jin SongResearch Engineer, NTT Microsystem Integration Laboratories.
He received the B.S. degree in electronics
engineering from Kyungpook National University, Korea, in 1999 and the M.S. and Ph.D.
degrees in information and communications
engineering from Gwangju Institute of Scienceand Technology (GIST), Korea, in 2001 and2005, respectively. From 2005 to 2006, he was at
the Center for Hybrid Optical Access Networks(CHOAN) in GIST, Korea, as a research professor and engaged in research on millimeter-wave
communications systems utilizing radio-overfiber technologies. Since joining NTT Microsystem Integration Laboratories in 2006, he has beenworking on the development of millimeter-wave
and sub-terahertz wave systems for communications, sensing, imaging, and measurement applications using photonic technologies and highspeed electronics. He is a member of IEEE and
IEICE.

Yuko Ueno

Senior Research Scientist, Network Hardware
Integration Laboratory, NTT Microsystem Integration Laboratories.
She received the B.S., M.S., and Ph.D. degrees
in chemistry from the University of Tokyo in
1995, 1997, and 2002, respectively. She joined
NTT Integrated Information & Energy Systems
Laboratories in 1997. She was a post-doctoral
fellow at the University of California at Berkeley
and at Lawrence Berkeley National Laboratory,
Berkeley, CA, USA, from 2004 to 2005. She
received the Encouragement Award (award for ayoung scientist) from the Japan Society for Analytical Chemistry.

Vol. 10 No. 2 Feb. 2012Feature Articles: Imaging and Sensing Technologies for Safety and Security

Portable Sensor for Determining
Benzene Concentration from Airborne/
liquid Samples with High Accuracy
Serge Camou†, Emi Tamechika, and Tsutomu Horiuchi
Abstract

Our previously developed portable monitoring system for detecting airborne benzene down to the levelof one part per billion (1 ppb) has been extended to a portable sensor for liquid samples through thedevelopment of modules for extraction from the liquid phase to the vapor phase based on two protocols:
bubbling and pervaporation. This portable sensor has demonstrated a high selectivity to benzene or a
detection limit below the one-part-per-billion-by-volume (1-ppbV) level depending on the extraction
protocol.

1. Introduction
Benzene, toluene, and xylenes, sometimes referred
to as BTX, have similar chemical compositions but
exhibit specific properties that have led to their extensive usage in various industries (chemical industries
and petroleum refineries) and in commonly usedproducts (e.g., gasoline). However, all three compounds, but especially benzene, represent threats tohuman health even at trace-level concentrations. On
the basis of scientific studies, developed countries
have set regulatory levels that should not be exceededin gas and liquid samples in order to prevent any
impact on the human population or environmental
ecosystems. Regulatory levels in Japan, the USA, andEurope are given in Table 1. Because of its large carcinogenic effect, benzene has the lowest regulatorylevels, in the low-parts-per-billion (ppb, 10-9) range.
Standard protocols for detecting BTX compounds
have been developed, mostly on the basis of the gas
chromatography and mass spectrometry (GC/MS)
technique. This setup provides high reliability andsensitivity; however, it cannot be operated in the field,
requires a highly skilled operator, and is costly andbulky.
† NTT Microsystem Integration Laboratories
Atsugi-shi, 243-0198 Japan

Between the two main potential exposure media,
liquid and gas, airborne BTX is considered to be themajor threat because car exhaust has been identified
as the main source of emission. This is particularly
relevant in densely populated areas with huge numbers of cars on the roads.
For simultaneous detection of BTX compounds onsite with a portable sensor, absorption spectroscopy
in the ultraviolet (UV) optical wavelength range represents a promising alternative. In the UV wavelength
range, almost all of the molecules absorb light atvarious wavelengths depending on the structure of
their outer electron clouds. For simultaneous detection of multiple compounds, the wavelength range
can be tuned to where compounds of interest exhibit
strong and characteristic absorption peaks. For example, benzene, toluene, and o-xylene exhibit very distinct and well-resolved absorption signatures in the
230–290-nm range (Fig. 1) despite their similar
chemical compositions. When operating in this wavelength range, this technique therefore enables simultaneous detection of the three compounds. However,the sensitivity is usually limited by the length of thelight path through the sample, with the typical detection limit being in the parts-per-million (ppm, 10-6)
range. To reach sensitivity in the parts-per-trillion(ppt, 10-12) range and satisfy the requirements, one
usually uses a pre-concentration cell before the
NTT Technical Review

Feature Articles

Table 1. D atasheet for benzene, toluene, and xylene (o-, p-, and m-xylene all together) for Japan,
the USA, and Europe.
Air (ppb)

Waste water (ppbV)

Drinking water (ppbV)

Boiling point(°C)

Solubility inwater (g/L)

Japan

USA* Europe Japan

USA

Benzene

80.5

1.8

1

103

1.7

110

500

−

11

5

1

Toluene

110.8

0.47

70

2x105

−

−

−

−

−

1000

−

Xylene138–144

ca. 0

200

105

−

−

−

−

−

10,000−Europe Japan

USA

Europe* In the workplace (8-hour workday or 40-hour workweek exposure)

0.02

BenzeneTolueneo-Xylene

Absorption (a.u.)

0.015

0.01

0.005

0

–0.005
230

240

250

260

270

280

290

Wavelength (nm)Fig. 1. R eference absorption spectra of benzene, toluene, and o-xylene
in the 230−290-nm wavelength range.

detection cell. The sample gas is enriched by accumulating molecules from the carrier gas over a period oftime (concentration time) and releasing them instantaneously. The release of a large number of molecules
within a small volume of gas usually leads to a concentration increase by a factor of 1000 to 10,000,
which opens the way to gas detection at the ppt-levelat the cost of a longer response time. A few years ago,
we developed a portable airborne-BTX sensor [1] bycombining UV spectroscopy measurements and apre-concentration stage; it has sensitivity in the lowppb range that satisfies the requirements [2].
However, BTX compounds diluted in liquid samples also represent a threat and there are numerous
potential sources, such as industrial spillage contaminating river basins and leaking underground storageVol. 10 No. 2 Feb. 2012

tanks at aging gasoline stations contaminating theunderground drinking water network. Despite the low
solubility of BTX compounds in water, which limits
their concentrations to low levels, the values given in
Table 1 clearly show that such contamination remains
a serious issue. Standard protocols for monitoringwater quality have been developed using the same
aforementioned techniques, so they exhibit the same
limitations. To further extend the use of our portableairborne BTX sensor to liquid samples, we havedeveloped modules that can be attached to the airborne system to provide an adequate interface withliquid samples. From theoretical considerations, UV
spectroscopy can characterize liquid samples. However, because the molecules of interest interact withthe solvent matrix, the absorption peaks are broader
Feature Articles

In

Bubbling system

Microfluidic
tube

Concentration
cell
PassivedryingOut

Pump

Detection cell

Extraction system

Opticalfiber

Liquid sample module
Pervaporation system

SpectrometerUV light source
Personal computer
Airborne detection system

Fig. 2. Schematic view of the portable airborne BTX sensor combined with the liquid sample module, where thered arrows show the extraction path of BTX molecules.

and smaller, which makes sensitive and selective
measurements much more challenging. This protocol
thus operates much more efficiently with vapor-phasesamples than liquid ones. To extend the use of ourportable, high-sensitivity BTX sensor, we have developed two different extraction systems with specificproperties (there is a tradeoff between sensitivity andselectivity) and a passive drying system that allows
efficient use of the same pre-concentration stage as in
the airborne system (Fig. 2).
2. Bubbling-based extraction module
As mentioned in the previous section, benzene,
toluene, and o-xylene exhibit low affinity with water,
so they are almost certain to evaporate as soon as theirmolecules are in close proximity to any air-liquidinterface. The bubbling extraction technique takes
advantage of this characteristic by increasing the
air-liquid surface area to further enhance the evaporation of BTX molecules.
The concept has been extensively described elsewhere [3], but can be summarized as follows. Air as acarrier gas is pushed deep inside the sample liquidcontained in a closed container. Air bubbles are thencreated deep inside the solution. As the bubbles riseto the surface, benzene molecules evaporate at the air
bubble surface and accumulate within the bubble gas
volume. When the bubbles reach the surface, they
release their contents into the air in the overheadspace, which is gradually transferred to the detection
cell. With a large number of small bubbles, we candrastically increase the air-liquid surface ratio andextract the benzene from the entire volume of thesample liquid without any limitation from diffusion
to the top surface. Experiments were performed withsample solutions prepared at known concentrations
by diluting an adequate amount of pure solutions.
Sample solutions were prepared in deionized water atconcentration levels of 0.45, 3, and 3 ppmV (ppm byvolume) for benzene, toluene, and o-xylene, respectively, where the benzene-to-toluene concentration
ratio was fixed to match the levels expected fromgasoline spillage. Experimental results obtained withthe bubbling extraction module combined with thedetection cell are shown in Fig. 3. The data show
absorption peak heights at 253, 267, and 272 nm versus time for benzene, toluene, and o-xylene, respectively, with the time origin of 0 corresponding towhen we started the carrier gas flow. The overall
shapes of the curves in Fig. 3 are similar for the three
BTX compounds, with maximum response reachedafter only a few minutes. This is consistent with analmost instantaneous extraction of BTX molecules,
where the slow decrease in the response with elapsed
time comes from the delay effect of the overheadvolume. The extraction process is easy and fast, andits efficiency provides highly sensitive measurementof aqueous BTX.
However, this method has one issue concerning the
NTT Technical Review

Feature Articles

7

BenzeneTolueneo-Xylene

Absorption peak height (a.u.)

6
5
4
3
2
1
0

0

10

20

30

40

50

60

Time (min.)Fig. 3. R aw results for BTX sample solution with bubbling extraction
and continuous monitoring by UV-spectroscopic measurements(no concentration stage).

selectivity to benzene. Many compounds exhibit low
solubility in water and can thus be extracted efficiently
to the gas phase by the bubbling protocol. Toluene is
an ingredient of ordinary gasoline at a concentrationlevel several times higher than that of benzene. Since
toluene and o-xylene have absorption spectra thatoverlap that of benzene in the 230–290-nm range,
their presence, as well as the presence of the potentially many other confounding molecules in real
samples, may strongly interfere with benzene measurement, which requires better accuracy.
3. Pervaporation-based extraction module
An alternative extraction protocol based on a pervaporation technique [4] that provides better selectivity to benzene has been developed. Pervaporation
occurs when a membrane separates the liquid phasefeed and the vapor gas permeate and acts as a selective
barrier. It combines two different basics processes:(1) from the liquid interface, permeation of the compound of interest through the membrane and (2) its
evaporation into the vapor phase at the gas interface.
Components are then separated on the basis of thedifference in their transport rates through the membrane. Stronger affinity between the component andthe membrane material yields faster extraction. As aresult, hydrophobic membranes are commonly used
to remove traces of organic compounds from aqueous
solutions.
Vol. 10 No. 2 Feb. 2012

A typical pervaporation setup is shown in the left
bottom of Fig. 2. A tube connects the gas inlet to theoutlet and acts as a selective barrier between the liquid and gas phases. While flowing inside the tube, thecarrier gas gathers the benzene molecules that havemigrated through the tube wall and transports them
towards the detection stage. Here, silicone rubber
tubes, available on the market in various sizes at low
cost, were used as pervaporation membranes. Experiments with calibrated BTX solutions were performed
and typical results for BTX concentrations of 0.45
ppmV of benzene, 3 ppmV of toluene, and 3 ppmV ofo-xylene are shown in Fig. 4. This time the extraction
profiles strongly differed depending on the compound: benzene was extracted at a fast pace with aplateau reached after 30 min, toluene was gradually
released from the silicone rubber at a slow and constant pace, and o-xylene was completely blocked by
the membrane. As a consequence, within the first 15
min, the responses to benzene and toluene were about
the same order of magnitude despite the huge disparities in their initial concentration levels. However,the plateau reached by the sensor response to benzene
occurred at a level about three times lower than the
maximum reported in Fig. 3, indicating a consequentloss of sensitivity.

Feature Articles

Absorption peak height (a.u.)

2.5

BenzeneTolueneo-Xylene

2
1.5
1
0.5
0
–0.5

0

10

20

30
Time (min.)

40

50

60

Fig. 4. R aw results for BTX sample solution with tube extraction andcontinuous monitoring by UV-spectroscopic measurements(no concentration stage).

4. Passive drying system forpre-concentration module
As a standard protocol in the gas-trace detection
field, the pre-concentration technique has previously
been implemented in our airborne-BTX portable sensor. However, after extraction from the liquid phase,the carrier gas coming out of the extraction module
exhibited a relative humidity (RH) close to the saturation level, which strongly disturbed the efficiency ofthe pre-concentration cell through capillary condensation. To solve this problem, we added a passive
drying system before the pre-concentration cell toselectively extract water molecules from the carriergas. The passive drying system consists of a tube
made of Nafion material [5]. When the carrier gas
flows through this tube, the BTX molecules are
retained inside the tube, while the water molecules aswell as alcohols and a few other compounds can easily migrate through the Nafion membrane. This phenomenon, which is also called permeation, is equivalent to the pervaporation described earlier, but without the phase transition. As mentioned earlier, theextraction provides a carrier gas (air) with artificiallyhigh RH levels close to 100%, while the outer surfaceof the Nafion tube is exposed to ambient air at a RH
of about 40–50%. To counterbalance the RH difference at the two sides of the Nafion membrane, thewater molecules migrate through the membrane,from the inner space to the outer space. Because this

phenomenon is fast and a small volume of carrier gas
cannot change the RH value of the atmospheric gas
surrounding the Nafion tube, this method enables one
to get rid of excess water molecules, bringing the RH
back to the atmospheric level, with a tube that is only
a few centimeters long.
The typical gain in sensitivity when the pre-concentration stage is used in combination with bubbling
extraction is shown in Fig. 5. Despite a short concentration time of 5 min, the overall response is shifted
towards the low concentration levels by more than
two orders of magnitude, leading to a detection limit
in the pptV range, which far exceeds the requirements. The same effect has been reported with the
tube-extraction system, but improvements are limited
to low-ppbV levels. However, a better understandingof how the pre-concentration works would make possible further improvements in sensitivity. As mentioned above, the pre-concentration cell specificallyaccumulates the molecules of interest from the carriergas over the so-called concentration time and theninstantaneously releases them in a small volume ofcarrier gas for detection. The process can therefore beviewed as integration over the first few minutes of thechromatographic extraction response, as shown in
Figs. 3 and 4 for bubbling and pervaporation extractions, respectively. In both cases, increasing the concentration time will improve the sensitivity at the costof sensor response time but also impact the response
selectivity to benzene. Therefore, from Figs. 3 and 4,
NTT Technical Review

Absorption peak height @ 253 nm (a.u.)

Feature Articles

With pre-concentration
moduleWithout
pre-concentration
module

S/N=3
0.1

1

10

100

1000

Benzene concentration (ppbV)
S/N: signal-to-noise ratio
ppbV: parts per billion by volume

Fig. 5. E ffect of the pre-concentration stage on sensor response withbenzene aqueous solution and bubbling extraction(concentration time: 5 min).

we can expect two different behaviors: (1) with bubbling extraction, increasing the concentration time
will slightly improve the sensitivity, and the selectivity to benzene will remain unchanged and (2) with
pervaporation extraction, increasing the concentration
time will drastically improve the sensitivity, but the
selectivity to benzene will slightly decrease over the
first 20 min and then decrease more rapidly for longer
concentration times. Therefore, concentration timeand choice of the extraction module are two factorsthat can be selected according to the application
requirements in terms of sensor response, sensitivity,and selectivity to benzene.

time. However, when selectivity to benzene is the key
feature, and other compounds such as toluene and oxylene are also present at several times higher concentration levels, then the tube extraction systemshould be preferred. In future, field tests should be
conducted to further validate the approach and demonstrate the ability of our sensor to perform realtime,
on-site measurement at levels satisfying national
regulations.
References[1]

5. Concluding remarks
We have developed a portable, high-sensitivity
aqueous BTX sensor by adding two modules to theairborne-detection system: a system for extraction
from the liquid phase to the vapor phase, followed bya passive drying system. Two extraction systems fordifferent applications have been implemented andsuccessfully tested. When high sensitivity is a priority, bubbling extraction should be chosen: the sensor
response is relatively fast, and sensitivity in the pptV
range can be tuned by means of the concentration

Vol. 10 No. 2 Feb. 2012

[2]
[3]

[4]
[5]

T. Horiuchi, Y. Ueno, S. Camou, T. Haga, and A. Tate, "Portable Aromatic VOC Gas sensor for Onsite Continuous Air Monitoring with
10-ppb Benzene Detection Capability," NTT Technical Review, Vol.
4, No. 1, pp. 30-37, 2006.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2006
01030.pdf
S. Camou, T. Horiuchi, and T. Haga, “Ppb Level Benzene Gas Detection by Portable BTX Sensor Based on Integrated Hollow Fiber
Detection Cell,” Proc. of IEEE Sensors 2006, Daegu, Korea, 2006.
S. Camou, A. Shimizu, T. Horiuchi, and T. Haga, “Ppb-Level Detection of Benzene Diluted in Water with Portable Device Based on
Bubbling Extraction and UV Spectroscopy,” Sens. and Act. B, Vol. 32,
No. 2, pp. 601–607, 2008.
J. Amador-Hernandez and M. D. Luque de Castro, “Pervaporation: aUseful Tool in Food Analysis,” Food Chemistry, Vol. 68, No. 4, pp.
387–394, 2000.
P. W. Majsztrik, M. B. Satterfield, A. B. Bocarsly, and J. B. Benziger,“Water Sorption, Desorption and Transport in Nafion Membranes,” J.
of Membrane Science, Vol. 301, No. 1-2, pp. 93–106, 2007.

Feature Articles

Serge Camou

Research Scientist, Network Hardware Integration Laboratory, NTT Microsystem Integration Laboratories.
He received the B.S., M.S., and Ph.D. degrees
in physics from Cergy-Pontoise University, Paris
7 University, and Franche-Comte University,
France, in 1994, 1996, and 2000, respectively.
After two years as a JSPS (Japan Society for the
Promotion of Science) fellow at the University ofTokyo doing research on DNA lab-on-a-chip
devices, he moved to NTT Microsystem Integration Laboratories (Biosensing Group) in 2003 asa postdoctoral researcher and was involved in the
airborne-BTX portable sensor development. In
2005, he was hired as a permanent researcher andis currently involved in the development of anoninvasive and continuous blood glucose system based on continuous-wave photoacoustic
technology. He is a member of the IEEE Society.

Tsutomu Horiuchi

Senior Research Scientist, Network Hardware
Integration Laboratory, NTT Microsystem Integration Laboratories.
He received the B.S. and M.S. degrees in physics from Waseda University, Tokyo, in 1983 and1985, respectively. He received the Ph.D. degree
in chemistry from Keio University, Kanagawa, in
1994. He joined NTT Ibaraki Electrical Communication Laboratories in 1985. He has been
engaged in research on electrochemistry, biosensors, and chemical sensors. He was a visiting
scientist at New Mexico State University, Las
Cruces, NM, USA, in 1997. He is a member of
JSAP.

Emi Tamechika

Senior Research Engineer, Supervisor, Leaderof Microsensor Research Group, NTT Microsystem Integration Laboratories.
She received the B.S. and M.S. degrees inphysics from Tokyo University of Science in1983 and 1985, respectively, and the Ph.D.
degree from Tokyo Metropolitan University in
1989. She joined NTT LSI Laboratories, Kanagawa, in 1989. She has been engaged in R&D of
super-high-resolution improvement of opticallithography for LSI fabrication. She has also
engaged in studies on silicon photonics technology. She received the Award of the Japanese
Agency of Science and Technology for prominent patents in 1995. She is a member of theJapan Society of Applied Physics (JSAP), thePhysical Society of Japan, the Electrochemical
Society of Japan, and the Science Council ofJapan.



NTT Technical Review

Regular Articles

Acoustically Controlled Spin Precession
Revealed by Two-dimensional Imaging
of Spin Transport in GaAs
Haruki Sanada†, Hideki Gotoh, Koji Onomitsu, andTetsuomi Sogawa
Abstract

Magneto-optic Kerr microscopy was used to investigate the spin precession of electrons traveling insemiconductor quantum wells by using surface acoustic waves (SAWs). Two-dimensional images of the
SAW-induced spin flow reveal anisotropic spin precession behavior caused by the coexistence ofdifferent types of spin-orbit interactions. The dependence of spin-orbit effective magnetic fields on SAW
intensity indicates the existence of acoustically controllable spin-orbit interactions resulting from SAWinduced strain and SAW-induced Rashba contributions. These phenomena will provide the versatility
needed for spin manipulation in future spintronic applications such as spin transistors and quantum
computers.

1. Introduction
In conventional semiconductor electronics, electron spins have been ignored because they are randomly oriented and cancel out completely. The
research field called semiconductor spintronics aims
to extract spin properties in order to use them for bothclassical and quantum information technologies,
which will achieve higher processing speeds, lower
electrical power consumption, and higher integration
densities than conventional charge-based devices [1].
Understanding the spin dynamics of electrons moving in semiconductors is one of the keys to developing spintronic technologies because it will enable
electrical spin manipulation without external magnetic fields in ways that have been proposed for spin
field-effect transistors [2], spin filters [3], and quantum computers [4]. In a system lacking inversion
symmetry, moving electrons feel effective magnetic
fields even in the absence of real magnetic fields. Thisis caused by spin-orbit interactions (SOIs) arising† NTT Basic Research Laboratories
Atsugi-shi, 243-0198 Japan

from relativistic effects. In general, magnetic fields
induce spin precession, which is the rotation of thespin angular momentum around the magnetic field.
Because of this, moving spins precess with a frequency proportional to the strength of the SOIs. So
far, spin transport experiments using a DC-electricfield [5] have succeeded in observing the spin precessions induced by SOIs in the absence of a real magnetic field [6]. Surface acoustic waves (SAWs)
applied to semiconductor quantum wells (QWs),
which confine electrons in thin layers sandwiched
between barrier layers made of other materials, provide another way to transport electron spins [7]–[9].
This method has the advantage of effectively suppressing the dominant spin relaxation process causedby the electron-hole exchange interaction in undoped
semiconductors [10] owing to the spatial separationof electrons and holes in the type-II-like lateral potential modulation created by the SAWs [7]. As a result,it has become possible to transport electrons by using
SAWs over distances of nearly 100 µm while maintaining their spin coherence during precession aroundthe effective magnetic fields caused by SOIs [8].
Here, we used magneto-optic Kerr rotation (KR)NTT Technical Review

Regular Articles

(a)

(b)

[110]

SAW

[010]
[110]

AlGaAsGaAs
SAW

AlGaAsRF
GaAs QW

IDTElectron energy

(c)

- --

-- -

-- -

Fig. 1. (a) Schematic view of the sample. (b) SAW-induced displacement of atoms in the sample’s cross section.
Electrons are confined in a GaAs/AlGaAs QW. (c) Conduction band energy for the QW shown in (b).
Electrons are trapped in the valleys of the modulated potential and transported by the SAWs.

microscopy to explore the two-dimensional (2D)
dynamics of traveling spins under SAWs over a wide
range of acoustic amplitudes. The present experiment
revealed the existence of spatially anisotropic SOIs as
well as their dependence on SAW strength. A theoretical analysis of the experimental results elucidated
the mechanisms of the acoustically controllable spin
dynamics. The phenomena will provide a new
approach for the spin-manipulation as well as spintransport without the application of any magnetic
fields or DC electric fields.

Pump

SAW

Probe
θK

n

Sca

[110]
[110]

SAW

2. ExperimentFig. 2. S etup for spatially resolved KR measurements.

A schematic view of the sample is shown in
Fig. 1(a). It was a 20-nm-thick undoped GaAs single
QW with short-period GaAs/AlAs (average Al content: 30%) barriers grown by molecular-beam epitaxy
on a (001) semi-insulating GaAs substrate. The QW
was located 485 nm below the surface. A 50-nm-thickAl film deposited on top of the sample was processedby electron-beam lithography into interdigital transducers (IDTs), which were designed for operation at
a SAW wavelength of 2.55 µm and frequency of
1.154 GHz. Rayleigh SAWs propagate along either[110] or [110], depending on the direction of theapplied radio frequency (RF) signal, with a SAW
velocity |vSAW| = 2.9 km/s. The single SAW beams
produce moving wires, which are formed by the oneVol. 10 No. 2 Feb. 2012

dimensional (1D) lateral confinement of the SAWinduced piezoelectric potential (Figs. 1(b) and (c)).
Interference between two orthogonal SAW beams
forms moving dots traveling along [010] with a velocity of 
√2 |vSAW| [8], as shown in Fig. 1(a).
The spin dynamics during transport was measured
by temporally and spatially resolved KR microscopyusing a mode-locked Ti:sapphire laser (1.5 ps, 82
MHz, 1.527 eV), as illustrated in Fig. 2. Circularly
polarized pump pulses (average power: 1.1 µW) werefocused at a fixed position on the sample. Linearlypolarized probe pulses (0.9 µW) with a time delayRegular Articles

3. Results and discussion
The spatiotemporal evolution of photoinjected
spins trapped in moving dots traveling in the [010]
direction is shown in Fig. 3. The slope of the KR signal indicates that the spin-polarized electrons moved
with the expected velocity of 
√2 |vSAW| = 4.14 km/s.
The oscillations with a period of about 4.5 ns are
attributed to spin precession around the spin-orbit
effective magnetic field, as shown in the inset. The
data thus clearly demonstrate that our method successfully extracted information about the spin
dynamics, including spin transport and SOI-induced
spin precession.
Spatial maps of steady-state spin densities for moving wires are shown in Figs. 4(a) and (c) and that formoving dots is shown in Fig. 4(b). In the two-color
measurement, the pump energy was fixed at 1.527 eV,
whereas the probe energy was tuned to 1.525, 1.526,
and 1.528 eV for [110], [010], and [110], respectively,
because the bandgap energies at the electron-trapping
positions were modulated by SAW fields [11]. The
pump (probe) power was 20 (0.9) µW. In contrast to
the well-confined carrier transport achieved by moving dots (Fig. 4(b)), carriers in moving wires diffused
rapidly along the wire axis (Figs. 4(a) and (c)).
Although the KR signal for the wires was reduced by
the carrier diffusion, we could access the momentum
direction dependences of spin precessions (Fig. 4(d)),
50
Pump-probe distance (µm)

relative to the pump pulses were scanned over thesurface and the KR angle q K of their reflected light
was measured by a balanced detection technique. The
pump light’s polarization was modulated between
left- and right-circular polarizations at 50.1 kHz, andthe probe light was chopped with an acousto-optic
modulator at 52.0 kHz. The difference frequency (1.9 kHz) was used as a reference for lock-in
detection. The full width at half maximum (FWHM)
spot size of the normally incident probe beam wasapproximately 3 µm, whereas the waist size of theobliquely incident pump beam was 6 µm and its spot
on the sample was slightly elongated in the [100]direction. The position of the probe light spot wasscanned in the QW plane for spatially resolved KR
measurements. For the 2D mapping of steady-statespin distributions, we chose to use a two-color pumpprobe method using a pair of continuous-wave Ti:
sapphire lasers. All the measurements were carriedout at a low temperature (30 K), where long-distancespin transport is expected [7], [8] in the absence of
applied magnetic fields.

40

θK (arb. units)
-1.0

0.0

Moving direction

1.0

30
20
10
0

Effective magnetic field
0

5
10
Delay time (ns)15

Fig. 3. S patiotemporal evolution of the KR signal formoving dots traveling along [010]. The dashed line
shows the slope determined from the estimatedvelocity of the moving dots. The inset shows the
motion of the spin precessing around the effective
magnetic field during transport.

where the sum of the mapping data (Figs. 4(a)–(c))
shows that the isophase lines have clear ellipticalshapes. In Fig. 4(e), the KR angles q K (opencircles)along particular axes ([110], [010], and [110]) are fitted with a function q K(d)=q 0 cos(2pkd) exp(–d/Ls),
where d is the pump-probe distance and Ls and k are
fitting parameters representing the spin decay lengthand spatial precession frequency, respectively. We
obtained k[110] = 0.0589 ± 0.0008, k[010] = 0.05369 ±
0.00009, and k[110]= 0.0460 ± 0.0006 µm–1 for thetraveling directions [110], [010], and [110], respectively. Since k is proportional to the spin-orbit effective magnetic field, these results demonstrate that the
SOIs are spatially anisotropic in the present system.
The SOI dependence on the traveling direction iscaused by the coexistence of different types of SOIs
[6], [12]–[14]. For electrons confined in (001) QWs,
the momentum-dependent effective magnetic field
Wso(k)is primarily determined by k-linear terms,
which are classified into two types [15]:Wa(k) =

2a  kY  β
2β  –kX  –kX , W (k) =  kY ,h 0 
h 0 

(1)

where we used a coordinate system with base vectors
X̂ // [100], Ŷ // [010], and Ẑ // [001]. The orientation
dependences of Wa(k) and Wβ(k) in k space areshown in Fig. 5. Since these vectors have different
dependences on the direction of k, their coexistence
NTT Technical Review

Regular Articles

[010]
[110]

[110]

θK (arb. units)
(a)

(b)

40

40
30

20

Y (µm)

[110]

10

10
0

-10

-20

-10

0
10
X (µm)

20

-10

0.0

1.0

30

[010]

20

0

-1.0

40

Y (µm)30
Y (µm)

(c)

20

[110]

10
0

-20

-10

0
10
X (µm)

-10

20

-20

-10

0
10
X (µm)

20

(e)

(d)

40
0
[110]

θK (arb. units)

Y (µm)

30

20

10

0

0
[010]

0
[110]

-10

-20

-10

0
X (µm)

10

20

0

20

40
60
80
Pump-probe distance (µm)

100Fig. 4. (a)–(d) 2D images of spin densities for moving wires traveling along [110] (a) and [110] (c) and for moving
dots traveling along [010] (b). The sum of the data (a)–(c) is plotted in (d), where the dashed ellipses are
guides for the eye. (e) Kerr rotation signals along the three directions in the data (a)–(c) are plotted (black
open circles) and the red lines are fitting curves. The dashed lines represent precession phases of p/2, (3/2)p,(5/2)p, ....

leads to twofold symmetry of |Wtotal(k)|=|Wa(k)+Wβ(k)|, as illustrated in Fig. 5. In general, Wa(k) andWβ(k) are mainly induced by the structural inversion
Vol. 10 No. 2 Feb. 2012

asymmetry caused by a static electric field (Rashba
SOI [16]) and the intrinsic bulk inversion asymmetry(Dresselhaus SOI [17]), respectively. In addition toRegular ArticlesΩβ (k)Ωα (k) + Ωβ (k)kY // [010]Ωα (k)

kX // [100]Fig. 5. E ffective magnetic fields Wa, Wb, and Wa + Wb plotted as vectors
in k space. a < 0, b > 0, and b > |a| are assumed in this figure.

function of the SAW intensity for moving dots. We
estimated the SAW intensities from the bandgap shiftobserved in photoluminescence spectra [21]. As the
SAW intensity increased, the precession frequency
increased monotonically, suggesting that the SOI is
acoustically controllable. The experimental data were
well reproduced by simulating the precession frequency (solid lines in Fig. 6) [22], where each SOI
contribution to the spin precessions was estimatedfrom the calculation of the Rayleigh SAW fields
including the piezoelectric coupling [23]. This analysis revealed the contributions of SAW-dependent
Rashba and strain SOIs to the total SOI, indicatingthat the spin rotation angle can be tuned by adjusting
the SAW intensity.

0.056

Precession frequency (µm-1)

Dots moving along [010]

0.054

0.052

4. Conclusion
1020
SAW intensity (W/m)

30

Fig. 6. SAW intensity dependence of precession frequency
for moving dots. The solid line was obtained by
calculation. The error bars for the data represent
two standard errors of the parameters obtained
from a least-squares fitting.

these static contributions, the present system shouldhave a dynamic Rashba SOI induced by the verticalcomponent of the SAW piezoelectric field as well as
strain-induced SOIs [18]–[20].
As expected, the precession frequency varied with
the SAW intensity, which is defined as the acoustic
power flux per unit length along a cross-section of the
SAW beam. The symbols in Fig. 6 represent theexperimentally obtained precession frequency as a

We studied the SOIs of electrons traveling in semiconductor QWs by using SAW fields. The temporally
and/or spatially resolved KR technique enabled 2D
imaging of the traveling spins, which revealed anisotropic spin precession behavior resulting from thecoexistence of different types of SOIs. The dependence of the precession frequencies on SAW intensity, which was analyzed on the basis of a theoreticalmodel, indicates that the strengths of the SOIs due tostrain and Rashba contributions can be tuned byadjusting the SAW intensity. Our experimental results
will be beneficial for further investigation of acoustically induced SOIs in semiconductors. They will also
provide the versatility needed for spin manipulation
via dynamically controlled SOIs in future spintronic
applications such as spin transistors and quantum
computers.

NTT Technical Review

Regular Articles

Acknowledgments
We thank P. V. Santos of Paul-Drude-Institut für
Festkörperelektronik, Berlin, Germany, and J. Nitta
and M. Kohda of Department of Materials Science,
Tohoku University, Sendai, Japan, for useful discussions. This work was partly supported by the Japan
Society for the Promotion of Science (JSPS).
References
[1]

[2]
[3]

[4]
[5]

[6]
[7]

[8]
[9]

S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. vonMolnár, M. L. Roukes, A. Y. Chtchelkanova, and D. M. Treger, “Spintronics: A Spin-based Electronics Vision for the Future,” Science, Vol.
294, pp. 1488–1495, 2001.
S. Datta and B. Das, “Electronic Analog of the Electro-optic Modulator,” Appl. Phys. Lett., Vol. 56, No. 7, p. 665, 1990.
T. Koga and S. Datta, “Spin-filter Device Based on the Rashba EffectUsing a Nonmagnetic Resonant Tunneling Diode,” Phys. Rev. Lett.,Vol. 88, No. 12, p. 126601, 2002; J. Ohe, M. Yamamoto, T. Ohtsuki,
and J. Nitta, “Mesoscopic Stern-Gerlach Spin Filter by Nonuniform
Spin-orbit Interaction,” Phys. Rev. B, Vol. 72, No. 12, p. 041308,
2005.
D. Stepanenko and N. E. Bonesteel, “Universal Quantum Computation through Control of Spin-orbit Coupling,” Phys. Rev. Lett., Vol.
93, No. 14, p. 140501, 2004.
J. M. Kikkawa and D. D. Awschalom, “Lateral Drag of Spin Coherence in Gallium Arsenide,” Nature (London) Vol. 397, No. 6715, p.
139, 1999; D. Hägele, M. Oestreich, W. W. Rühle, N. Nestle, and K.
Eberl, “Spin Transport in GaAs,” Appl. Phys. Lett., Vol. 73, No. 11, p.
1580, 1998; H. Sanada, I. Arata, Y. Ohno, Z. Chen, K. Kayanuma, Y.
Oka, F. Matsukura, and H. Ohno, “Relaxation of Photoinjected Spinsduring Drift Transport in GaAs,” Appl. Phys. Lett., Vol. 81, No. 15, p.
2788, 2002.
Y. Kato, R. C. Myers, A. C. Gossard, and D. D. Awschalom, “Coherent Spin Manipulation without Magnetic Fields in Strained Semiconductors,” Nature (London), Vol. 427, No. 6969, p. 50, 2004.
T. Sogawa, P. V. Santos, S. K. Zhang, S. Eshlaghi, A. D. Wieck, andK. H. Ploog, “Transport and Lifetime Enhancement of Photoexcited
Spins in GaAs by Surface Acoustic Waves,” Phys. Rev. Lett., Vol. 87,
No. 27, p. 276601, 2001.
J. A. H. Stotz, R. Hey, P. V. Santos, and K. H. Ploog, “Coherent Spin
Transport through Dynamic Quantum Dots,” Nature Mater., Vol. 4,
No. 8, p. 585, 2005.
O. D. D. Couto, Jr., F. Iikawa, J. Rudolph, R. Hey, and P. V. Santos,“Anisotropic Spin Transport in (110) GaAs Quantum Wells,” Phys.
Rev. Lett., Vol. 98, No. 3, p. 036603, 2007.

Vol. 10 No. 2 Feb. 2012[10] G. L. Bir, A. G. Aronov, and G. E. Pikus, “Spin Relaxation of Electrons due to Scattering by Holes,” Zh. Eksp. Teor. Fiz. Vol. 69, No. 4,
p. 1382, 1975 [Sov. Phys. JETP, Vol. 42, p. 705, 1975].
[11] F. Alsina, P.V. Santos, and R. Hey, “Spatial-dispersion-induced
Acoustic Anisotropy in Semiconductor Structures,” Phys. Rev. B, Vol.
65, No. 19, p. 193301, 2002.
[12] L. Meier, G. Salis, I. Shorubalko, E. Gini, S. Schön, and K. Ensslin,“Measurement of Rashba and Dresselhaus Spin-orbit Magnetic
Fields,” Nature Phys., Vol. 3, No. 9, p. 650, 2007.
[13] B. M. Norman, C. J. Trowbridge, J. Stephens, A. C. Gossard, D. D.
Awschalom, and V. Sih, “Mapping Spin-orbit Splitting in Strained(In,Ga)As Epilayers,” Phys. Rev. B, Vol. 82, No. 8, p. 081304(R),2010.
[14] S. D. Ganichev, V. V. Bel’kov, L. E. Golub, E. L. Ivchenko, P. Schneider, S. Giglberger, J. Eroms, J. De Boeck, G. Borghs, W. Wegscheider, D. Weiss, and W. Prettl, “Experimental Separation of Rashba
and Dresselhaus Spin Splittings in Semiconductor Quantum Wells,”
Phys. Rev. Lett., Vol. 92, No. 25, p. 256601, 2004; M. Scheid, M.
Kohda, Y. Kunihashi, K. Richter, and J. Nitta, “All-electrical Detection of the Relative Strength of Rashba and Dresselhaus Spin-orbit
Interaction in Quantum Wires,” Phys. Rev. Lett., Vol. 101, No. 26, p.
266401, 2008; M. Studer, G. Salis, K. Ensslin, D. C. Driscoll, and A.
C. Gossard, “Gate-controlled Spin-orbit Interaction in a Parabolic
GaAs/AlGaAs Quantum Well,” Phys. Rev. Lett., Vol. 103, No. 2, p.
027201, 2009.
[15] R. Winkler, “Spin-orbit Coupling Effects in Two-dimensional Electron and Hole Systems,” Springer, Berlin, 2003.
[16] Y. A. Bychkov and E. I. Rashba, “Oscillatory Effects and the Magnetic Susceptibility of Carriers in Inversion Layers,” J. of Physics C:
Solid State Physics, Vol. 17, No. 33, p. 6039, 1984.
[17] G. Dresselhaus, “Spin-orbit Coupling Effects in Zinc Blende Structures,” Phys. Rev. Vol. 100, No. 2, pp. 580–586, 1955.
[18] G. E. Pikus and A. N. Titkov, in “Optical Orientation,” edited by F.
Meier and B. P. Zakharchenya, Elsevier, Amsterdam, 1984.
[19] G. C. La Rocca, N. Kim, and S. Rodriguez, “Effect of Uniaxial Stresson the Electron Spin Resonance in Zinc-blende Semiconductors,”
Phys. Rev. B, Vol. 38, No. 11, pp. 7595–7601, 1988.
[20] B. A. Bernevig and S.-C. Zhang, “Spin Splitting and Spin Current in
Strained Bulk Semiconductors,” Phys. Rev. B, Vol. 72, No. 11. p.
115204, 2005.
[21] T. Sogawa, P. V. Santos, S. K. Zhang, S. Eshlaghi, A. D. Wieck, andK. H. Ploog, “Dynamic Band-structure Modulation of Quantum Wells
by Surface Acoustic Waves,” Phys. Rev. B, Vol. 63, No. 12, p.
121307(R), 2001.
[22] H. Sanada, T. Sogawa, H. Gotoh, K. Onomitsu, M. Kohda, J. Nitta,
and P. V. Santos, “Acoustically Induced Spin-orbit Interactions
Revealed by Two-dimensional Imaging of Spin Transport in GaAs,”
Phys. Rev. Lett., Vol. 106, No. 21, p. 216602, 2011.
[23] S. H. Simon, “Coupling of Surface Acoustic Waves to a Two-dimensional Electron Gas,” Phys. Rev. B, Vol. 54, No. 19, p. 13878, 1996.



Regular Articles

Haruki Sanada

Research Scientist, Quantum Optical Physics
Research Group, Optical Science Laboratory,
NTT Basic Research Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in electrical engineering from Tohoku University,
Miyagi, in 2001, 2002, and 2005, respectively.
He joined NTT Basic Research Laboratories in2005. His research interests include the opticalproperties of low-dimensional semiconductor
nanostructures and carrier spin dynamics modulated by surface acoustic waves and their application to solid-state quantum information processing. He is a member of the Japan Society ofApplied Physics (JSAP).

Hideki Gotoh

Senior Research Scientist, NTT Basic ResearchLaboratories.
He received the B.E., M.E., and Ph.D. degrees
in engineering from Hiroshima University in
1991, 1993, and 2000, respectively. Since joining
NTT Basic Research Laboratories in 1993, hehas been working on optical physics and device
applications of semiconductor nanostructures.
He is a member of JSAP and the Optical Societyof America.

Koji Onomitsu

Research Scientist, Nanostructure Technology
Research Group, Physical Science Laboratory,
NTT Basic Research Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from Aoyama Gakuin University, Tokyo, in 1998 and 2000, respectively, and
the Ph.D. degree in electrical engineering fromWaseda University, Tokyo, in 2004. He joined
NTT Basic Research Laboratories in 2006. His
current interests are crystal growth of modulation
doped structures for high-mobility two-dimensional electron systems using MBE, mechanicaldevices incorporating GaMnAs, and on-chip
amplifiers of mechanical vibration signals using
HEMTs. He is a member of JSAP.

Tetsuomi Sogawa

Executive Manager of Research Planning Section, and Group Leader of Quantum Optical
Physics Research Group, NTT Basic ResearchLaboratories.
He received the B.E., M.E., and Ph.D. degrees
in electrical engineering from the University ofTokyo in 1986, 1988, and 1991, respectively. He
joined NTT Basic Research Laboratories in1991. From 2004 to 2006, he worked for theCouncil for Science and Technology Policy,
Cabinet Office, Japan, as a deputy director forpolicy planning. His current research interests
include fabrication technology for low-dimensional nanostructures, optical properties of quantum dots/wires and photonic crystals, spin-related phenomena, and SAW application to nanostructures. He is a member of JSAP.

NTT Technical Review

Practical Field Information about
Telecommunication Technologies

Damage to Plastic Components
Caused by Solvent Cracking
Abstract

This article introduces two case studies of damage to plastic components caused by solvent cracking
and the countermeasures developed by NTT EAST. It is the ninth in a bimonthly series on the theme ofpractical field information about telecommunication technologies. This month’s contribution is from theMaterials Engineering Group, Technical Assistance and Support Center, Maintenance and Service
Operations Department, Network Business Headquarters.

1. Introduction

2. Damage to latching tab of modular plug

Telecommunication facilities use various materials
such as metal, concrete, and plastic. Among these,
plastic is widely used in equipment that comes into
direct contact with customers. This article introduces
two case studies of how plastic components can bedamaged and the countermeasures that were taken toprevent reoccurrence.

2.1 Problem
At a certain metalworking plant in Japan, the modular plug of the helical cord of a particular telephonereceiver had to be replaced repeatedly because theplug’s latching tab broke away from its base threetimes over the course of a five-year period (Fig. 1).
The telephone set sat on a desk on the work floor andtraces of cutting oil used for cutting metal plates on alathe could be seen splattered about the desk. Grease
from this cutting oil was also found on the telephone’s
helical cord (Fig. 2). The damaged modular plug was

Location of break

Fig. 1. Modular plug with damaged latching tab.

Broken modular plug

Fig. 2. Helical cord stained with grease from cutting oil.
† NTT EAST
Ota-ku, 144-0053 JapanNTT Technical Review

Practical Field Information about Telecommunication Technologies

made of acrylic resin.
2.2 Cause of damage
Some plastic materials are known to suffer fromcracking when solvents or oils adhere to locations
subjected to stress. This solvent cracking is a type ofenvironmental-stress cracking that affects plastic
molded products. If a large amount of internal stress(strain) exists in a molded product that comes into
contact with solvents or oils, those substances may
penetrate into the product and cause intermolecularslippage and crack generation. This is accompanied
by molecular exfoliation that gives rise to a highly
glossy, mirror-like fractured surface.
In this case study, the plug’s acrylic resin had low
resistance to solvent cracking and cutting oil was
found on the plug, so two of the key conditions forcrack generation were present.
The fractured surface of the damaged plug wasobserved with a digital microscope and found to behighly glossy and mirror-like without any unevenness(Fig. 3), which is a characteristic feature of solvent
cracking.
To continue our investigation, we conducted anexperiment to reproduce such solvent cracking using
the same type of cutting oil as used at the metalworking plant. When the receiver of a telephone set islifted off its hook, the cord is stretched, which applies
stress to the latching tab, which pushes it away fromits base. To simulate this process, we coated the latching tab of a new modular plug with cutting oil, appliedstress to the tab’s base, and heated the tab at 80°C
using a Geer oven to accelerate the reaction. After
two weeks, cracks occurred at the base. Digital
microscope observation of the fractured surface
revealed a smooth, mirror-like surface similar to thefractured surface of the actual damaged tab at themetalworking plant (Fig. 4).
2.3 Measures to prevent reoccurrence
Connecting a modular plug to the modular socket
on a receiver that has been splashed with cutting oil
may cause the plug’s tab to eventually break away
owing to ongoing damage caused by remnant oil.
Therefore, the socket should be wiped clean of remnant grease with a cotton swab or other implement
before the plug is inserted. We also recommend using
a gel to fill the gap in the plug’s aperture where thespiral cord is inserted to prevent cutting oil fromentering the socket.

Vol. 10 No. 2 Feb. 2012

Mirror-like fractured surface

Fig. 3. F ractured surface of modular plug damaged atmetalworking plant.

Mirror-like fractured surface

Fig. 4. Experimentally reproduced fractured surface.

3. Damage to connector of
AC adaptor for ISDN modem
3.1 Problem
In response to a fault report stating that power could
not be supplied to an ISDN (integrated services digital network) modem, an inspection found that theconnector of the modem’s AC adaptor was damaged(Fig. 5). Further inspection also found the remains ofants in the socket-pin area and on the nearby circuitboard, as shown in Fig. 6. There had been no reportsPractical Field Information about Telecommunication Technologies

SocketISDN modem

Damagedconnector

AC adaptor

Fax

Fig. 5. Location of damaged connector.

Fig. 8. Sores on conductor sheaths inside connector.

Socket-pin area

Circuit board (near socket)

Fig. 6. Ant that penetrated ISDN modem.

Conductor sheath

Table 1. R esults of ion chromatography analysis of sub­stance in damaged connector.
Unit: ppm
–

CI

NO

–
2

F

HCOO

NO3

1st run

4.96

2nd run

5.15

28.60

20.61

3.43

30.45

21.55

3.80

Average

5.15

29.53

21.55

3.80

–

–

3–
4

PO

SO42–

2.72

5.40

7.32

3.21

5.21

7.75

3.21

5.30

7.75

–

was found inside the damaged connector and thecharred remains of an ant were found on the circuitboard near the socket.
Damaged pieces of connector

Full view of damaged connector

Fig. 7. Damaged AC-adaptor connector.

of lightning strikes or power outages since the installation of this equipment.
3.2 Inspection results
The connector (made of polycarbonate material)
was found to have fallen to pieces, exposing thesheathing of its conducting wires (Fig. 7). Moreover,the sheathing (made of flexible polyvinyl chloride)
was also found to have surface sores (Fig. 8). A
brownish adherent thought to be the remains of ants

3.3 Cause of damage
We analyzed the brownish adherent in the damaged
connector by ion chromatography and found the presence of formic acid (HCOO–), as indicated in Table 1.
While connector damage due to formic acid wastherefore suspect, microscopic observation of theconnector at the breaks revealed smooth, mirror-likefractured surfaces. We therefore investigated the possibility that solvent cracking was the cause of thisdamage. The abovementioned charred ant remains
found on the circuit board near the socket led us tobelieve that an ant had entered through gaps in themodem’s enclosure and made contact with metal pins
causing an electrical short and consequently generating high temperatures. Furthermore, taking into
NTT Technical Review

Practical Field Information about Telecommunication Technologies

account the presence of sores on conductor sheaths
near that circuit board, we considered that thosesheaths were also heated when the electrical circuit
shorted. We therefore surmised that the heating ofconductor sheaths in this way could cause plasticizing
agent to separate from the sheath and come into contact with the polycarbonate connector, thereby generating solvent cracking. Infrared reflection spectroscopic measurements of these conductor sheaths
confirmed the presence of a phthalate ester plasticizer. The penetration of phthalate ester plasticizer into
polycarbonate resin is known to facilitate solvent
cracking. We therefore concluded that solvent cracking was probably the reason for the damage to theVol. 10 No. 2 Feb. 2012

AC-adaptor connector in this case study.
4. Conclusion
This article introduced case studies of damage toplastic components caused by solvent cracking.
When materials that contain solvents (such as adhesives, paints, and oils) are used near equipment containing plastic components, special care must be
taken to ensure that such materials do not adhere to
the equipment. We hope that this article has been useful in helping the reader obtain a better understandingof material deterioration.

Information

NTT Receives Award fromthe Thomson Reuters 2011 Top 100
Global Innovators Program
Nippon Telegraph and Telephone Corporation(NTT, Tokyo) has received an award from theThomson Reuters 2011 Top 100 Global Innovators program [1], [2]. This program launched byThomson Reuters (New York) is an initiative thatanalyzes patent data and related metrics using aproprietary methodology to identify the companies and institutions that lead the world in innovation. The methodology is based on four principal
criteria: patent approval success rate*1, global
reach of patent portfolio*2, patent influence inliterature citations*3, and overall patent volume.

Thomson Reuters state that “being recognized asa Top 100 Global Innovator … confirms an organization’s commitment to progressing innovation
globally, to the protection of ideas and to thecommercialization of inventions.”
NTT understands that this award certifies thatits R&D activities are pioneering and its inventions are valuable in the global market. It will
make every effort to promote patent acquisition
based on the outputs of R&D activities and makethe most use of owning intellectual properties.

Haruhiko Kojima (left), Managing Director, NTT Intellectual Property
Center, receiving a trophy from Masaki Nagao (right), Managing Director,
Thomson Reuters Professional KK, a Thomson Reuters Company.

References[1]

NTT Technical Review.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=nt
r201201ext.html*1 Patent approval success rate: the ratio of published applications to granted patents over the most recent three years.
*2 Global reach of patent portfolio: the metric calculated byusing the number of innovative patents that have quadrilateral
patents (China, Europe, Japan, and the USA). An innovative[2]

Thomson Reuters Names World’s 100 Most Innovative Companies
http://thomsonreuters.com/content/press_room/legal/515197

patent is defined as the first publication in a patent documentof a new technology etc.
*3 Patent influence in literature citations: the metric determinedby counting citations to each organization’s patents over themost recent five years, excluding self-citations.

NTT Technical Review

External Awards
SAINT2011 Best Paper Award
Winners: Mitsuaki Akiyama, Takeshi Yagi, and Mitsutaka Ito, NTT
Information Sharing Platform Laboratories
Date: July 19, 2011
Organization: IEEE/IPSJ
For “Searching Structural Neighborhood of Malicious URLs to
Improve Blacklisting”.
Filtering based on blacklists is a major countermeasure against
malicious websites. However, blacklists must be updated because
malicious URLs (uniform resource locators) tend to be short-lived
and their sub strings may be partially mutated to avoid blacklisting.
Due to these characteristics, it can be assumed that unknown malicious URLs exist in the neighborhood of known malicious URLs,
created by the same adversary. We propose an effective blacklist URL
generation method. We try to discover URLs in the neighborhood ofa malicious URL by using a search engine. Those suspicious neighborhoods around malicious URLs require further investigation todetermine their blacklisting candidacy. We experimentally evaluated

this generation method by using real blacklisted URLs for both driveby-download and click-download infection. The results showed that
our method can effectively improve identification of malicious URLs
and maintenance of the coverage of blacklists.
Published as: M. Akiyama, T. Yagi, and M. Itoh, “Searching Structural Neighborhood of Malicious URLs to Improve Blacklisting,”
Proc. of the 2011 IEEE/IPSJ 11th International Symposium on Applications and the Internet (SAINT), pp. 1–10, Munich, Bavaria.
IEICE Distinguished Service Award
Winner: Takashi Ikebe, Service Control Platform Development
Project, Third Promotion Project, NTT Network Service Systems
Laboratories
Date: Sep. 14, 2011
Organization: IEICE Communications Society
For contributions to conducting conferences for the technical committee on network systems as an assistant to the Secretary General.

NTT Technical Review