Lattice image and scattering intensityof ultrathin step-free InN SQW.

Micro-PL spectrumof ultrathin step-free InN SQW.

Extremely Narrow Violet Photoluminescence Line
from Ultrathin InN Single Quantum Well on Step-free GaN Surface
We have successfully fabricated an ultrathin step-free InN single quantum well (SQW) that emits
highly monochromatic ultraviolet light. The step-free InN SQW has one-monolayer (1ML)-thick InN
sandwiched by GaN barrier layers and has abrupt hetero-interfaces without any monolayer steps. The
abrupt hetero-interfaces enable us to control precisely the emission wavelength and line width byutilizing the quantum size effect. 2ML- and 3ML-thick step-free InN SQWs will emit sharp green andred lights as predicted by theoretical calculations. (Page 18)(a) AFM phase image of graphene grown on SiC. (b) Time evolution of the plasmon signal.
(c) Plasmon velocity as a function of the magnetic field.

Plasmon Transport in Graphene Investigated by Time-Resolved Electrical
Measurements
We showed that the velocity of plasmons in graphene can be controlled over two orders of magnitude
by applying the magnetic field, screening the plasmon electric field with a gate metal, and changing
the carrier density. The wide tunability of the plasmon velocity encourages designing graphenenanostructures for plasmonic circuits. (Page 33)－Ⅰ－(a) Schematic diagram of atoms trapped on a persistent-supercurrent atom chip.
(b) TOF images after evaporative cooling. vf represents the final radio frequency of the evaporative cooling.
(c) Cross-sections of the optical density along the dotted line in Fig. (b).

Bose-Einstein Condensate on a Persistent-Supercurrent Atom Chip
We achieved 87Rb Bose-Einstein condensate (BEC) on a persistent supercurrent atom chip by a radio
frequency induced evaporative cooling. The condensation was verified by the time-of-flight (TOF) imaging of the bimodal momentum distribution obtained at vf = 1.750 MHz. (Page 39)

Scanning electron microscope image

Light-current characteristic
and spectrum

Lambda-Scale Embedded Active-Region Photonic-Crystal (LEAP) Lasers
We have developed a photonic-crystal (PhC) nanocavity laser in which a small active region is embedded within an InP-based line defect waveguide. We call this a Lambda-scale Embedded Active-region PhC (LEAP) laser. By using InAlAs for the sacrificial layer in order to reduce the leakage current,
we realized a LEAP laser with the lowest threshold current and energy consumption of any semiconductor laser operating at above room temperature. (Page 45)－Ⅱ－Message from the Director
We at NTT Basic Research Laboratories (BRL)
are extremely grateful for your interest and support
with respect to our research activities. BRL's
missions are to promote progress in science andinnovations in leading-edge technology to advance
NTT's business. To achieve these missions,researchers in fields including physics, chemistry,
biology, mathematics, electronics, informatics,
and medicine, conduct basic research on materials
science, physical science and optical science.
Since our management principle is based on an"open door" policy, we are collaborating with many
universities and research institutes in Japan, US,
Europe, and Asia as well as other NTT laboratories.
NTT-BRL organizes international conferences
related to quantum physics and nanotechnology
and also holds a "Science Plaza" to enhance public
understanding of our activities and to ensure a frank exchange of opinions. These activities
enable us to realize our missions with respect to the promotion of advances in science and thedevelopment of groundbreaking technology for NTT's business. Your continued support will begreatly appreciated.

July, 2013

Tetsuomi Sogawa
Director
NTT Basic Research Laboratories－Ⅲ－

Contents
page◆ Cover♦ Coherent Manipulation of Moving Electron Spins Using Magnetic-Field-Free Electron Spin

Resonance◆ Color Frontispiece ............................................................................................................Ⅰ♦ Extremely Narrow Violet Photoluminescence Line from Ultrathin InN Single Quantum Wellon Step-free GaN Surface♦ Plasmon Transport in Graphene Investigated by Time-Resolved Electrical Measurements♦ Bose-Einstein Condensate on a Persistent-Supercurrent Atom Chip
♦ Lambda-Scale Embedded Active-Region Photonic-Crystal (LEAP) Lasers◆ Message from the Director ................................................................................................Ⅲ◆ NTT Basic Research Laboratories Organogram ................................................................ 1◆ Member List ....................................................................................................................... 2
I. Research Topics◇ Overview of Research in Laboratories ............................................................................................... 15◇ Materials Science Laboratory ............................................................................................................ 17♦ Nitride-Based Semiconductor Light-Emitting Transistors♦ Extremely Narrow Violet Photoluminescence Line from Ultrathin InN Single Quantum Wellon Step-Free GaN Surface♦ Doping Control of AlInN/GaN Lattice-Matched Heterostructure♦ Universal Superconducting Ground State in Nd2CuO4 and Nd1.85Ce0.15CuO4♦ Label-Free Protein Detection on Graphene Oxide Surface♦ Epitaxial Trilayer Graphene Mechanical Resonators Obtained by Electrochemical Etching

Combined with Hydrogen Intercalation♦ Self-Organization of Quasi-Free-Standing Monolayer Graphene Nanoribbon Networks♦ Reconstitution of Homomeric GluA2 Receptors in Supported Lipid Membranes: Functionaland Structural Properties♦ Electrostatic Control of Artificial Cell Membrane Spreading by Tuning the Thickness of an

Electric Double Layer in a Nanogap♦ Conductive Polymer Combined Wearable Electrodes for Electrocardiography Recordings◇ Physical Science Laboratory ............................................................................................................... 27♦ Donor-Based Single Electron Pump
♦ Signal Detection Via a Transistor with Steep Current Characteristics

♦ Cavity-Electromechanics with Phonons♦ Ferromagnetic-Induced Component in Piezoresistance of GaMnAs♦ An Electromechanical Membrane Resonator♦ Shot Noise Spectroscopy on a Semiconductor Quantum Dot in the Elastic and Inelastic

Cotunneling Regimes♦ Plasmon Transport in Graphene Investigated by Time-Resolved Electrical Measurements♦ Impact of Graphene Quantum Capacitance on Transport Spectroscopy♦ Energy Relaxation Process of a Quantum Memory Coupled with a Superconducting Qubit♦ Evidence of a Crossover from Trions to an Electron-Hole Gas◇ Optical Science Laboratory ................................................................................................................ 37♦ Fundamental Limit to Qubit Control with Coherent Field♦ Monolithic Source of Polarization Entanglement Using Silicon Photonics♦ Bose-Einstein Condensate on a Persistent-Supercurrent Atom Chip
♦ Ultrafast Surface Electron Dynamics on GaAs(001) Using Time-Resolved Photoelectron

Spectroscopy Based on High-Order Harmonic Source♦ Coherent Manipulation of Moving Electron Spins Using Magnetic-Field-Free Electron Spin

Resonance♦ High-Efficiency Detection of Carrier-Envelope Offset Frequency with a Dual-Pitch PPLN

Ridge Waveguide◇ Nanophotonics Center ........................................................................................................................ 43♦ Ultrafast Spontaneous Emission from Copper-Doped Silicon Nanocavity♦ Purcell-Enhanced Single Photon Emission at Telecom Wavelengths from Quantum-Dot

Nanocavities♦ Lambda-Scale Embedded Active-Region Photonic-Crystal (LEAP) Lasers♦ 22-Gbit/s × 16-ch WDM Receiver Based on a Si-Ge-silica Monolithic Photonic Platform

II. Data◇ Science Plaza 2012.......................................................................................................................... 47◇ The 7th Advisory Board (Fiscal 2012)◇ Award Winners’ List (Fiscal 2012)............................................................................................ 48................................................................................................. 49◇ In-house Award Winners’ List (Fiscal 2012).................................................................................. 51◇ Numbers of Papers, Presentations and Patents (2012)................................................................... 53◇ List of Invited Talks at International Conferences (2012).............................................................. 55

NTT Basic Research Laboratories Organogram
As of March 31, 2013

Director

Toshiki Makimoto, Dr.

Planning Section
Nano Bio Research

Executive Manager

Tetsuomi Sogawa, Dr.

Executive Manager

Toshiki Makimoto, Dr.
Nano-photonic Center
Quantum and Nano Device Research
Senior Distinguished Researcher

Executive Manager

Masaya Notomi, Dr.

Hiroshi Yamaguchi, Dr.
Materials Science Laboratory
Executive Manager

Hiroki Hibino, Dr.

Physical Science Laboratory
Executive Manager

Akira Fujiwara, Dr.

Optical Science Laboratory
Executive Manager

Tetsuomi Sogawa, Dr.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)1

Member List
As of March 31, 2013(*/ left NTT BRL during the year)

NTT Basic Research Laboratories
Director, Dr. Toshiki Makimoto

Nano Bio Research

Executive Manager,

Dr. Toshiki Makimoto
Dr. Keiichi Torimitsu*

Quantum and Nano Device Research

Senior Distinguished Researcher,2

Dr. Hiroshi Yamaguchi

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)Research Planning Section

Executive Research Scientist,

Dr. Tetsuomi Sogawa

Senior Research Scientist,

Dr. Hiroshi Nakashima
Dr. Hiroki Takesue*

Senior Research Scientist,

Dr. Katsuya Oguri
Dr. Akihiko Shinya*

Nano-Photonic Center

Executive Manager,
NTT Research Professor,

Dr. Masaya Notomi
Prof. Yasuhiro Tokura (University of Tsukuba)

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)3

Materials Science Laboratory

Executive Manager,

Dr. Hiroki Hibino
Dr. Tetsuya Akasaka
Dr. Nahoko Kasai*

Thin-Film Materials Research Group:
Dr. Hideki Yamamoto (Group Leader)
Dr. Yasuyuki Kobayashi
Dr. Kazuhide Kumakura
Dr. Tetsuya Akasaka
Dr. Yoshitaka Taniyasu
Dr. Yoshiharu Krockenberger Dr. Masanobu Hiroki
Dr. Kazuyuki Hirama
Dr. Chia-Hung Lin
Low-Dimensional Nanomaterials Research Group:
Dr. Hiroki Hibino (Group Leader)
Dr. Fumihiko Maeda
Dr. Kazuaki Furukawa
Dr. Hiroo Omi
Dr. Ken-ichi Kanzaki*
Dr. Makoto Takamura
Dr. Shengnan Wang
Dr. Carlo M. Orofeo
Molecular and Bio Science Research Group:
Dr. Koji Sumitomo (Group Leader)
Dr. Akiyoshi Shimada*Dr. Hiroshi Nakashima*
Dr. Yoshiaki Kashimura
Dr. Touichiro Goto
Dr. Shingo Tsukada

4

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Dr. Hisashi Sato
Dr. Koji Onomitsu*
Dr. Ryan Banal

Dr. Satoru Suzuki
Dr. Shin-ichi Tanabe
Dr. Yuya Murata

Dr. Nahoko Kasai
Dr. Aya Tanaka

Physical Science Laboratory

Executive Manager,

Dr. Akira Fujiwara
Dr. Toshiaki Hayashi
Toru Yamaguchi*
Takeshi Karasawa

Nanodevices Research Group:
Dr. Akira Fujiwara (Group Leader)Dr. Hiroyuki Kageshima
Dr. Katsuhiko Nishiguchi
Dr. Gento Yamahata
Dr. Gabriel Lansbergen*
Nanostructure Technology Research Group:
Dr. Hiroshi Yamaguchi (Group Leader)
Dr. Kenji Yamazaki
Toru Yamaguchi
Dr. Koji Onomitsu
Dr. Hajime Okamoto
Dr. Yuma Okazaki
Junzo Hayashi
Quantum Solid State Physics Research Group:
Dr. Koji Muraki (Group Leader)
Dr. Satoshi Sasaki
Dr. Kiyoshi Kanisawa
Dr. Toshiaki Hayashi
Dr. Takeshi Ohta
Dr. Ken-ichi Hitachi*
Dr. Keiko Takase
Dr. Trevor David Rhone
Superconducting Quantum Physics Research Group:
Dr. Hiroshi Yamaguchi (Group Leader)
Dr. Kouichi Semba*
Dr. Hayato Nakano
Dr. Shin-ichi Karimoto
Hirotaka Tanaka
Dr. Yuichiro Matsuzaki
Dr. Xiaobo Zhu*
Spintronics Research Group:
Dr. Akira Fujiwara (Group Leader)
Dr. Tatsushi Akazaki*
Dr. Hiroyuki Tamura
Dr. Yuichi Harada
Dr. Yoshiaki Sekine
Dr. Hiroshi Irie

Dr. Jin-ichiro Noborisaka

Dr. Imran Mahboob
Daiki Hatanaka

Dr. Kyoichi Suzuki
Dr. Norio Kumada
Dr. Takashi Kobayashi

Dr. Shiro Saito
Dr. Kousuke Kakuyanagi

Dr. Masumi Yamaguchi

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)5

Optical Science Laboratory

Executive Manager,

Dr. Tetsuomi Sogawa
Dr. Tetsuya Mukai
Dr. Takehiko Tawara*

Quantum Optical State Control Research Group:
Dr. Kaoru Shimizu (Group Leader)Dr. Hiroki Takesue
Dr. Masami Kumagai
Dr. Hiroyuki Shibata
Dr. Makoto Yamashita
Dr. Fumiaki Morikoshi
Dr. Kiyoshi Tamaki
Daisuke Hashimoto
Dr. Nobuyuki Matsuda
Dr. Hiromitsu Imai
Dr. Takahiro Inagaki
Dr. Toshihiro Kubo*
Quantum Optical Physics Research Group:
Dr. Tetsuomi Sogawa (Group Leader)Dr. Hideki Gotoh
Dr. Kouta Tateno
Dr. Katsuya Oguri*
Dr. Atsushi Ishizawa
Dr. Haruki Sanada
Dr. Keiko Kato
Dr. Yoji Kunihashi
Dr. Ken-ichi Sasaki
Photonic Nano-Structure Research Group:
Dr. Masaya Notomi (Group Leader)
Dr. Atsushi Yokoo
Dr. Eiichi Kuramochi
Dr. Hideaki Taniyama
Dr. Hisashi Sumikura
Dr. Masato Takiguchi
Dr. Masaaki Ono
Dr. Danang Birowosuto
Dr. Xu, Hao

6

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Kazuhiro Igeta
Dr. Tetsuya Mukai
Dr. Kensuke Inaba
Dr. Koji Azuma
Dr. William John Munro

Dr. Takehiko Tawara
Dr. Guoquiang Zhang
Dr. Ken-ichi Hitachi
Dr. Hiroki Mashiko

Dr. Akihiko Shinya
Dr. Kengo Nozaki
Dr. Jimyung Kim*

Nano-Photonic Center

Executive Manager,

Dr. Masaya Notomi

Photonic Nano-Structure Research Team:
Dr. Masaya Notomi
Dr. Akihiko Shinya
Dr. Eiichi Kuramochi
Dr. Hideaki Taniyama
Dr. Kengo Nozaki
Dr. Masato Takiguchi
Dr. Hiroo Omi
Dr. Takehiko Tawara
Dr. Nobuyuki Matsuda

Dr. Atsushi Yokoo
Dr. Hisashi Sumikura
Dr. Masaaki Ono
Dr. Hiroyuki Shibata

InP Compound Device Research Team:Dr. Shinji Matsuo
Dr. Takaaki Kakitsuka
Dr. Koji Takeda
Dr. Koichi Hasebe

Dr. Tomonari Sato

Silicon Photonics Research Team:
Dr. Koji Yamada
Dr. Housei Akazawa
Hiroshi Fukuda
Rai Kou (Rai Takahashi)
Tatsurou Hiraki

Dr. Tai Tsuchizawa
Hidetaka Nishi

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

7

Senior Distinguished Researcher
Masaya Notomi was born in Kumamoto, Japan, on 16 February 1964. Hereceived his B.E., M.E. and Ph.D. degrees in applied physics from Universityof Tokyo, Japan in 1986, 1988, and 1997, respectively. In 1988, he joined NTT
Optoelectronics Laboratories. Since then, his research interest has been to control
the optical properties of materials and devices by using artificial nanostructures,and engaged in research on quantum wires/dots and photonic crystal structures.
He has been in NTT Basic Research Laboratories since 1999, and currently agroup leader of Photonic Nanostructure Research Group and a director of NTT
Nanophotonics Center. He is also entitled as Senior Distinguished Scientistof NTT since 2010. In 1996-1997, he was a visiting researcher of Linköping
University (Sweden). He was a guest associate professor of Applied Electronics
in 2003-2009 and is currently a guest professor of Physics in Tokyo Instituteof Technology. He received IEEE/LEOS Distinguished Lecturer Award in2006, JSPS (Japan Society for the Promotion of Science) prize in 2009, Japan
Academy Medal in 2009, The Commendation for Science and Technology by
the Minister of Education, Culture, Sports, Science and Technology (Prize for
Science and Technology, Research Category) in 2010, and IEEE Fellow grade in
2013. He is serving as a member of National University Corporation Evaluation
Committee in the Japanese government. He is also a member of the Japan
Society of Applied Physics, APS, IEEE, and OSA.

Hiroshi Yamaguchi was born in Osaka on October 30, 1961. He received
the B.E., M.S. in physics and Ph.D. degrees in engineering from the Osaka
University in 1984, 1986 and 1993, respectively. He joined NTT Basic ResearchLaboratories in 1986 and has engaged in the study of compound semiconductor
surfaces using electron diffraction and scanning tunneling microscopy. His
current interests are micro/nanomechanical devices using semiconductor
heterostructures. He was a visiting research fellow in Imperial College,
University of London, U.K. during 1995-1996 and a visiting research staff
in Paul Drude Institute, Germany in 2003. He is a guest professor in Tohoku
University from 2006 and a director of the Japanese Society of Applied Physics
in 2008 and 2009. He served as more than 40 committee members of academic
societies and international conferences. He received Inoue Prize for Science(2012), Institute of Physics Fellowship (2011), SSDM2009 Paper Award (2010),
MNC2008 Outstanding Paper Award (2009), and the Paper Awards of Japan
Society of Applied Physics (1989, 2004, 2010). He is currently a group leaderof Nanostructure Technology Research Group and Superconducting Quantum
Physics Research Group. He is a member of the Japan Society of Applied
Physics, the Physical Society of Japan, Institute of Physics (UK), and IEEE.

8

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Distinguished Researchers
Akira Fujiwara was born in Tokyo, Japan on March 9, 1967. He received
his B.S., M.S., and Ph.D. degrees in applied physics from The University ofTokyo, Japan in 1989, 1991, and 1994, respectively. In 1994, he joined NTT
LSI Laboratories and moved to NTT Basic Research Laboratories in 1996.
Since 1994, he has been engaged in research on silicon nanostructures and theirapplication to single-electron devices. He is an executive manager of PhysicalScience Laboratory and a group leader of Nanodevices Research Group sinceApril 2012. He was a guest researcher at the National Institute of Standards andTechnology (NIST), Gaithersburg, MD, USA during 2003-2004. He received
the SSDM Young Researcher Award in 1998, SSDM Paper Award in 1999, andJapanese Journal of Applied Physics (JJAP) Paper Awards in 2003 and 2006. He
was awarded the Young Scientist Award from the Minister of MEXT (Ministry of
Education, Culture, Sports, Science, and Technology) in 2006. He was a directorof the Japanese Society of Applied Physics in 2010 and 2011. He is a member of
the Japan Society of Applied Physics and the IEEE.

Koji Muraki was born in Tokyo, Japan in 1965. He received his B.S., M.S.,
and Ph.D. degrees in applied physics from The University of Tokyo, Japan, in
1989, 1991, and 1994, respectively. In 1994, he joined NTT Basic ResearchLaboratories, Kanagawa, Japan. Since then, he has been engaged in the growthof high-mobility heterostructures and the study of highly correlated electronic
states realized in such structures. He was a guest researcher at Max-Planck
Institute, Stuttgart, Germany during 2001-2002. He is a member of the PhysicalSociety of Japan and Japan Society of Applied Physics.

Yoshitaka Taniyasu was born in Toyama, Japan on June 10, 1973. Hereceived his B.E., M.E., and Dr. Eng. degrees in electrical engineering from
Chiba University, Chiba, Japan in 1996, 1998, and 2001, respectively. He
joined NTT Basic Research Laboratories in 2001. He has been engaged inresearch on epitaxial growth and device application of wide bandgap nitride
semiconductors. He was a visiting researcher at Ecole polytechnique fédérale deLausanne (EPFL) during 2011-2012. He received the Young Scientist Award at
the 14th Semiconducting and Insulating Materials Conference (SIMC-XIV) in
2007, and the Young Scientists’ Prize from the Minister of Education, Culture,
Sports, Science and Technology in 2011, the Young Scientist Award at the 38th
International Symposium on Compound Semiconductors (ISCS2011) in 2011,
the Best paper Award at the International Workshop on Nitride Semiconductors(IWN2012) in 2012. He is a member of the Japan Society of Applied Physics(JSAP).

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)9

Norio Kumada was born in Gifu, Japan in 1975. He received his B.S., M.S.,
and Ph.D. degrees in physics from Tohoku University, Japan, in 1998, 2000,
and 2003, respectively. In 2003, he joined NTT Basic Research Laboratories,
Kanagawa, Japan. Since then, he has been engaged in the study of highly
correlated electronic states realized in semiconductor heterostructures or ingraphene. He received Young Scientist Award of the Physical Society of Japan
in 2008 and Young Scientists’ Prize from the Minister of Education, Culture,
Sports, Science and Technology in 2012. He is a member of the Physical Societyof Japan.

Katsuhiko Nishiguchi was born in Hiroshima, Japan in 1975. He received
the B.E., M.E., and Ph.D. in electrical engineering in 1998, 2000, and 2002,
respectively, from Tokyo Institute of Technology, Tokyo, Japan. Since joining
NTT Basic Research Laboratories in 2002, he has been engaged in the researchon physics and technology of Si nanometer-scale devices for LSI applications
with low power consumption and new functions. He was an invited researcher at
the National Center for Scientific Research (CNRS), France during September
2008 and a visiting scientist at Delft University of Technology, the Netherlands
during 2012-2013. He received IUPAP Young Author Best Paper Award at theInternational Conference on Physics of Semiconductors 2000, Graduate Student
Award Silver at the Materials Research Society 2000 Fall Meeting, and Young
Scientist Award at the Japan Society of Applied Physics Spring Meeting in 2000.
He is a member of the Japan Society of Applied Physics.

Shiro Saito was born in Tokyo, Japan in 1972. He received his B.S., M.S.,
and Dr. Eng. degrees in applied physics from the University of Tokyo, Japan,
in 1995, 1997, and 2000, respectively. In 2000, he joined NTT Basic ResearchLaboratories. Since then, he has been engaged in quantum information
processing using superconducting circuits. He was a guest researcher at Delft
University of Technology, Delft, the Netherlands during 2005-2006. He has
been a guest associate professor in Tokyo University of Science since 2012. Hereceived Young Scientist Award at the Japan Society of Applied Physics Spring
Meeting in 2004. He is a member of the Physical Society of Japan and the Japan
Society of Applied Physics.

10

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Advisory Board (2012 Fiscal Year)
Name

Affiliation

Prof. Gerhard Abstreiter

Walter Schottky Institute and Physik Department
Technishe Universität München, Germany

Prof. John Clarke

Physics Department, University of California, Berkeley,
U.S.A.

Prof. Evelyn Hu

School of Engineering and Applied Sciences,
Harvard University, U.S.A.

Prof. Mats Jonson

Department of Physics, Göteborg University andHeriot-Watt University, Sweden

Prof. Sir Peter KnightPhysics Department, Imperial College/The Kavli Royal
Society International Centre Chicheley Hall, U.K.

Prof. Anthony J. Leggett

Department of Physics, University of Illinois, U.S.A.

Prof. Allan H. MacDonald

Department of Physics, The University of Texas, Austin,
U.S.A.

Prof. Andreas Offenhäusser

Institute of Complex Systems, Forschungszentrum Jülich,
Germany

Prof. Halina Rubinsztein-Dunlop

School of Physical Sciences, University of Queensland,
Australia

Prof. Klaus von Klitzing

Max-Planck-Institut für Festkörperforschung, Germany

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)11

Invited / Guest Scientists (2012 Fiscal Year)

12

Name

Affiliation

PeriodDr. Trevor David Rhone

Japan Science and Technology
Agency (JST), Japan

Feb. 2012 – Feb. 2013

Prof. Jeremy L O'Brien

University of Bristol, U.K.

Mar. 2012 – May 2012

Prof. Toshihiro Kubo

University of Tsukuba, Japan

Apr. 2012 – May 2012

Prof. Koichi Semba

National Institute of Informatics(NII), Japan

Apr. 2012 – Mar. 2013

Prof. Jorg P. Kotthaus

Ludwig Maximilian University of
Munich, Germany

May 2012 – May 2012

Dr. Kenichiro Kusudo

National Institute of Informatics(NII), JapanMay 2012 – Mar. 2013

Dr. Rais Shaikhaidarov

Royal Holloway University of
London, U.K.

Oct. 2012 – Nov. 2012

Prof. David Cox

University of Surrey, U.K.

Oct. 2012 – Nov. 2012

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Overseas Trainees (2012 Fiscal Year)
Name

Affiliation

PeriodHenri Juhani Suominen

The University of Edinburgh, U.K.

Aug. 2011– Jul. 2012

Andy Berry

University of Victoria, Canada

Sep. 2011 – Apr. 2012

Michael Firka

University of Victoria, Canada

Sep. 2011 – Aug. 2012

Elan Michael Grossman

Georgia Institute of Technology, U.S.A.

Jan. 2012 – Aug. 2012

Bennett Eleazer

Georgia Institute of Technology, U.S.A.

Jan. 2012 – Aug. 2012

Yong Fan Jiang

The University of British Columbia, Canada

Jan. 2012 – Aug. 2012

Gediminas Dauderis

Vilnius University, Lithuania

Jan. 2012 – Aug. 2012

Maria Anagosti

University of Gent, Belgium

Jan. 2012 – Aug. 2012

Roger Molto Pallares

Chemical Institute of Sarria (IQS), Spain

Jan. 2012 – Aug. 2012

Sanna Maria Rauhamaki University of Jyväskylä, FinlandJan. 2012 – Aug. 2012

Alex Yang

The University of British Columbia, Canada

Jan. 2012 – Aug. 2012

Shibin Thomas

Mahatma Gandhi University, India

Mar. 2012 – Mar. 2013

Ahmet Taspinar

Delft University of Technology, Netherlands

Apr. 2012 – Sep. 2012

Robert Amsuss

Vienna University of Technology, Austria

Apr. 2012 – May 2012

David Tregurtha

University of Bath, U.K.

Jun. 2012 – Sep. 2012

Mickael Mounaix

ESPCI ParisTech (École supérieure de physique et Jul. 2012 – Dec. 2012de chimie industrielles de la ville de Paris), France

Romain Dubourget

ESPCI ParisTech (École supérieure de physique et Jul. 2012 – Dec. 2012de chimie industrielles de la ville de Paris), France

Ruaridh Forbes

The University of Edinburgh, U.K.

Jul. 2012 – Apr. 2013

Paul Koecher

Oxford University, U.K.

Oct. 2012 – Dec. 2012

Justin Yan

The University of British Columbia, Canada

Jan. 2013 –Joey Chau

University of Toronto, Canada

Jan. 2013 –Thomas Ziebarth

University of Victoria, Canada

Jan. 2013 –Punn Augsornworawat

McGill University, Canada

Jan. 2013 –Pawel Pactwa

AGH University of Science and Technology,
PolandJan. 2013 –Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)13

Domestic Trainees (2012 Fiscal Year)

14

Name

Affiliation

PeriodTomohiro Sakai

Keio University

Apr. 2012 – Mar. 2013

Takayuki Watanabe

Tohoku University

Apr. 2012 – Mar. 2013

Shun Dai

University of Tokyo

Apr. 2012 – Mar. 2013

Tatsuya Baba

Tokyo University of Science

Apr. 2012 – Mar. 2013

Rento Osugi

Tohoku University

Apr. 2012 – Mar. 2013

Shun-ichi Matsumoto

Tokyo University of Science

Apr. 2012 – Mar. 2013

Hajime Suzuki

Tokyo University of Science

Apr. 2012 – Mar. 2013

Takanobu Tsunoi

Yokohama National University

Apr. 2012 – Mar. 2013

Takahiko Sato

University of Tokyo

May 2012 – Mar. 2013

Ryonsoku O

The University of TokushimaJun. 2012 – Dec. 2012

Keigo Furuta

University of Toyama

Jul. 2012 – Aug. 2012

Wataru Nakayama

Keio University

Jul. 2012 – Aug. 2012

Hiromitsu Chatani

The University of Tokushima

Aug. 2012– Sep. 2012

Takahito Hodumi

Hokkaido University

Aug. 2012– Sep. 2012

Yuji Saito

Tokyo Denki University

Aug. 2012– Mar. 2013

Nuremy Binti Che Ani

Tokyo Denki University

Aug. 2012– Mar. 2013

Miho Yamazaki

Tokyo Denki University

Aug. 2012– Mar. 2013

Huynh Dief Phuoc

Nagaoka University of Technology

Oct. 2012 – Feb. 2013

Shingo Isobe

Nagaoka University of Technology

Oct. 2012 – Feb. 2013

Nobutoshi Yamaguchi

Nagaoka University of Technology

Oct. 2012 – Feb. 2013

Masashi Tachi

Tokyo University of Science

Oct. 2012 – Mar. 2013

Tomoyuki Tahara

Toyohashi University of Technology

Jan. 2013 – Feb. 2013

Shuhei Nakagawa

Toyohashi University of Technology

Jan. 2013 – Feb. 2013

Takahiro Gotoh

Tokyo Denki University

Feb. 2013 – Mar. 2013

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Ⅰ．Research Topics

Overview of Research in Laboratories
Materials Science Laboratory

Hiroki Hibino

This laboratory aims at contributing to progress in materials science and revolutionizing
information communication technology by creating new materials and functions throughmaterials design at the atomic and molecular levels.
This laboratory consists of three research groups investigating a wide range of materials
such as nitride semiconductors, graphene, superconductors, and biological molecules. We areconducting innovative materials research based on the technologies of growing high-quality
thin films and precisely measuring the structure and physical properties of materials.
This year, we succeeded in measuring extremely narrow violet photoluminescence line fromultrathin InN single quantum well on step-free GaN surface, demonstrating label-free protein
detection on graphene oxide surface, and achieving electrocardiography simply by wearing apiece of clothing that has textile electrodes combined with conductive polymer.

Physical Science Laboratory

Akira Fujiwara

The aim of this laboratory is to develop semiconductor- and superconductor-based solid-state
devices, which will have a revolutionary impact on future communication and information
technologies. Utilizing high-quality crystal growth techniques and nanolithography techniques
we have developed, five research groups are working on nanodevices, quantum information
processing devices, and high-sensitivity sensors based on new degrees of freedom such as
single electrons, mechanical oscillations, quantum coherent states, and spins.
This year we succeeded in measuring and controlling the velocity of plasmons in graphene
and realizing "cavity-electromechanics" with a nanomechanical resonator, which is a classicalanalogy of cavity quantum electrodynamics. We also demonstrated the operations of novel
devices such as a logic device with an electromechanical membrane resonator and a sensor
based on stochastic resonance with silicon nanotransistors.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)15

Optical Science Laboratory

Tetsuomi Sogawa

This laboratory aims for the development of core-technologies that will innovate on opticalcommunications and optical signal processing, and seeks fundamental scientific progresses.
The groups in our laboratory are working for the quantum state control by very weak light,
the search for intriguing phenomena using very intensive and short pulse light, and control
of optical properties by using photonic crystals and ultrasonic techniques, based on uniqueproperties of semiconductor nanostructures such as quantum dots and nanowires.
In this year, we demonstrated a Bose-Einstein condensate of cold atoms on the
superconducting atom chip. We succeeded in generating a polarization entangled photon pairwith a high quality by use of an integrated silicon photonic circuit. We also demonstrated thatelectron spins in semiconductors can be transported and manipulated by using surface acoustic
waves and that electron spin resonance can be achieved in the absence of real magnetic fields.

Nanophotonics Center

Masaya Notomi

Nanophotonics Center (NPC) was just established in April 2012 by several researchgroups involved with nanophotonics in NTT Basic Research Laboratories, NTT Photonics
Laboratories, and NTT Microsystem Integration Laboratories. We are aiming for developing
a full-fledged large-scale photonic integration technology by which we will be able to densely
integrate a large number of nano-scale photonic devices with various functions in a single
chip. Furthermore, we are targeting extreme reduction of the consumption energy for photonic
information processing by taking advantage of the nanophotonics technology.
Currently, we are conducting studies of photonic crystals to reduce the footprint andconsumption energy of various photonic devices, such as all-optical switches, all-opticalmemories, and lasers. We are also studying various photonic nanostructures to ultimately
enhance light-matter interactions, and exploiting photonic integrated circuits and devices
based on the silicon photonic platform. Furthermore, we are developing ultra-fine nanofabrication technologies based on electron beam lithography and sophisticated integration
technologies of active semiconductors for photonics.

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Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Nitride-Based Semiconductor Light-Emitting Transistors
Kazuhide Kumakura, Hideki Yamamoto, and Toshiki Makimoto
Materials Science Laboratory
Light-emitting transistors (LETs), which are based on high-speed heterojunction bipolartransistors (HBTs) with a quantum well (QW) in the base layer, are electrical devices as well
as optical ones [1]. According to the principles of their operation, LETs can run faster thanconventional light-emitting diodes (LEDs). Since nitride-based semiconductor LEDs arecommercially available for lighting, it is expected that the nitride-based semiconductor LETs
will work as light sources for visible light communication [2]. In this work, we fabricatednitride-based semiconductor LETs and investigated their optical output characteristics.
We fabricated pnp AlGaN/InGaN/GaN LETs with a 3-nm-thick QW in the base layer.
The In mole fraction of the InGaN QW was higher than that of the InGaN base layer. When
we apply forward bias at the emitter-base junction of the LETs, holes are injected from theemitter to the base layer. The injected holes diffuse toward the collector and are captured at
the QW. Some of the uncaptured holes reach the collector. Figure 1 shows a photograph of theemission from the LETs under forward bias conditions. We can observe the strong purplish
emission with the peak wavelength of 410 nm. Next, we fabricated three types of LETs with
the QW position varied in the base layer: near the emitter, at the center of the base, and near
the collector. Figure 2 shows the optical output power as a function of the base current for
the three LETs. We also show the output power for a QW-free LET (standard HBT) as areference. The output power with the QW near the collector is weaker than with it near theemitter. This indicates that the probability of the hole capture depends on the position of the
QW. The velocity of the holes diffusing in the base becomes larger as the holes approach thecollector, which results in a lower hole capture probability near the collector. We achieved 0.3mW optical output power from the LET with the QW near the emitter at the base current of
0.3 mA, corresponding to the external quantum efficiency of 3.3 %. We will investigate theelectrical and optical modulation properties of the LETs for high-speed operation, which may
open an avenue to their application as high-speed visible light sources.
[1] M. Feng et al., Appl. Phys. Lett. 84 (2004) 1952.
[2] Y. Tanaka et al., IEICE Trans. Commun. E86-B (2003) 2440.
[3] K. Kumakura et al., Int. Conf. Solid State Devices and Materials, 2011 Nagoya, A-4-2.

Fig. 1. Photograph of the emission from the LET underforward bias conditions.

Fig. 2. Optical output power as a function of thebase current for the three LETs and a HBT.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)17

Extremely Narrow Violet Photoluminescence Line
from Ultrathin InN Single Quantum Well on Step-Free GaN Surface
Tetsuya Akasaka, Hideki Gotoh*, Yasuyuki Kobayashi, and Hideki Yamamoto
Materials Science Laboratory, *Optical Science Laboratory
Blue and green light emitting diodes (LEDs) have been achieved with InGaN quantum
well (QW) active layers.For a longer wavelength emission, e.g., red emission, InGaN QWs
havingmuch higher In composition arerequired, butthe higher In concentrationcausessevere
deterioration of crystalline quality. High-efficiency emission in near-ultraviolet, visible, andnear-infrared regions, in principle,can also be achieved by using InN QWs, if the thicknessof the ultrathin InN QWs [typical ly several monolayers (MLs)] can be controlled with anaccuracy of a single ML. However, abrupt hetero-interfaces without a single-ML step have notbeen obtained so far. Recently, we have succeeded in fabricating step-free GaN surfaces 50µm in diameter without any ML steps by selective-area metalorganic vapor phase epitaxy [1].
In the present study, we investigated the fabrication of step-free InN QWs using step-free GaN
surfaces as a template.
Partially coalesced two-dimensional nuclei of InN were formed on a step-free GaN surface
by a 10-s InN growth. The coalescence was completed after 30 s and 1-ML-thick step-free
InN was eventually formed. Subsequently, a step-free InN single quantum well (SQW)
hetero-structure was fabricated by depositing a GaN cap layer onto the step-free InN surface.
Figure 1 shows a cross-sectional lattice image of the step-free InN SQW taken by the highangle annular dark field (HAADF) method in scanning transmission electron microscopy(STEM). In atoms are brighter than Ga atoms, since heavier atoms have stronger scattering
intensity. The horizontally integrated scattering intensity of a single line at the center ishigher than the others, indicating theformation of 1-ML-thick step-free InN SQW. A microphotoluminescence spectrum was measured at 4 K for the 1-ML-thick step-free InN SQW(Fig. 2). A sharp single peak is observed with a full width at half maximum of 9 meV at 3.03
eV (violet color) [2]. The peak width is much narrower than that (~ 60 meV) of conventional
InGaN QWs emitting a violet light, which is also a hallmark of the abrupt hetero-interfaces
and homogenous composition in the step-free InN SQW. 2ML- and 3ML-thick step-free InN
SQWs will emit sharp green and red light as predicted by theoretical calculations.
This work was supported by KAKENHI.
[1] T. Akasaka, Y. Kobayashi, and M. Kasu, Appl. Phys. Express 3 (2010) 075602.
[2] T. Akasaka, H. Gotoh, Y. Kobayashi, and H. Yamamoto, Adv. Mater. 24 (2012) 4296.

Fig. 1. Lattice image and horizontally integrated scattering
intensity of 1-ML-thick step-free InN .SQW.
18

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Fig. 2. Micro-photoluminescence spectrum of
1-ML-thick InN SQW measured at 4 K.

Doping Control of AlInN/GaN Lattice-Matched Heterostructure
Yoshitaka Taniyasu, Jean-François Carlin*, Antonino Castiglia*,
Raphaël Butté*, and Nicolas Grandjean*Materials Science Laboratory,*Ecole Polytechnique Fédérale de LausanneAmong III-nitride semiconductors, AlInN is the only ternary alloy that can be grownlattice-matched to GaN at an In composition of 17 % [1]. Lattice-matched AlInN/GaN
heterostructures should be free from cracks, strain-driven composition inhomogeneities, andstrain-related defects, which are limiting the performance of ultraviolet/visible light-emitting
diodes (LEDs) and laser diodes (LDs) when using conventional lattice-mismatched AlGaN/
GaN or InGaN/GaN heterostructures. In addition, an AlInN/GaN structure has a large bandgap
discontinuity and a large refractive index contrast even at the lattice-matched condition [1],which leads to strong carrier and optical confinements. Thus, AlInN/GaN structures are
expected not only to improve the device properties but also to increase the design flexibility
for novel III-nitride devices.
Doping control of AlInN lattice-matched to GaN is crucial for device applications. However,
non-intentionally doped AlInN has so far shown n-type conduction with high residual donor
concentration, which is a significant obstacle to p-type doping as well as to intentional n-type
doping. To reduce the residual donor concentration, we grew AlInN under an In-rich condition
to benefit from the In surfactant effect. Then, n-type conduction of AlInN was intentionallycontrolled by Si doping. On the other hand, we found that one of the compensating defects for
p-type doping is related to the presence of surface pits. By decreasing the pit density, p-type
AlInN was successfully obtained by Mg doping (Fig. 1) [2].
Using p-type AlInN, we fabricated InGaN/GaN multiple quantum wells (MQWs) LEDs.
The LEDs show light emission at a wavelength of 445 nm, which corresponds to electron-hole
recombinations occurring in the InGaN/GaN MQWs (Fig. 2). This confirms the efficient hole
injection from the p-type AlInN layer to the MQWs. The achievement of p-type as well as
intentional n-type AlInN will open further new possibilities for III-nitride devices.
[1] J.-F. Carlin and M. Ilegems, Appl. Phys. Lett. 83 (2003) 668.
[2] Y. Taniyasu, J.-F. Carlin, A. Castiglia, R. Butté, and N. Grandjean, Appl. Phys. Lett. 101 (2012) 082113.

Fig. 1. Net acceptor concentration NA – ND of Mg-doped
AlInN with high and low pit density as a functionof Mg flow rate. Inset: AFM image of AlInN
surface.

Fig. 2. Electroluminescence of InGaN/GaN MQW
LEDs using p-type Mg-doped AlInN withhigh and low pit density.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)19

Universal Superconducting Ground State in Nd2CuO4 andNd1.85Ce0.15CuO4Yoshiharu Krockenberger, Hideki Yamamoto, and Michio Naito*
Materials Science Laboratory, *Tokyo Univ. of Agricul. & Technol.
The induction of superconductivity in electron-doped cuprates is associated to an annealing
process, quite in contrast to hole doped cuprates. Hence, while the copper coordination
number of hole doped cuprates is five or six, it is four for electron doped cuprates. Thesefour-fold coordinated copper ions form two-dimensional, square-planar CuO 2 plaquettes. So
far it has been believed that the superconducting phase diagram of electron and hole doped
cuprates is nearly symmetric with respect to doping in antiferromagnetic-insulating andsuperconducting orders. For the understanding of high temperature superconductivity it wastherefore assumed that the ubiquitous insulating state is prerequisite in the absence of doping.
Here we show that the existence of an antiferromagnetic- and insulating order for electron
doped cuprates is not mandatory but a consequence of the annealing treatment. We applied
the so-called "two-step annealing" process to Nd2CuO4 for the induction of superconductivity.
The plain fact that superconductivity can be induced into insulting Nd2CuO4 is currently being
scrutinized by several methods. Here, we only focus on the superconducting state itself.
Nd2CuO4 (NCO) and Nd1.85Ce0.15CuO4 (NCCO) thin films have been grown by molecularbeam epitaxy on (001)SrTiO3 (STO) substrates. NCCO films were annealed in situ whereas
NCO films have been annealed ex situ by two-step annealing procedure [1-4]. Thin films wereanalyzed by high-resolution reciprocal space mapping at each annealing step. A monotonous
decrease in c-axis lengths together with a constant in-plane lattice length are strong indicatorsfor effectively evacuating apical oxygen sites of as-grown NCO and NCCO. The electronic
properties of NCO and NCCO are nearly identical, i.e., both are metals and the temperaturedependence of the resistivity appears to be Fermi
liquid like. The superconducting response withrespect to external magnetic field is shown in Fig. 1
for NCO and NCCO. The superconducting transition
temperature in the absence of magnetic field aswell as the magnetic field dependence shows that inboth cases, NCO and NCCO, the superconducting
response is quantitatively and qualitatively the same.
Such a response can be interpreted as follows: The
square-planar coordinated CuO2 plane is a metal andthe additional doping only modulates quantities ofband filling rather than a destruction of a long-range
antiferromagnetic order.
[1] Y. Krockenberger et al., Phys. Rev. B85 (2012) 184502.
[2] Y. Krockenberger et al., Jpn. J. Appl. Phys. 51 (2012)
010106.
[3] H.Yamamoto et al., Physica C470 (2010) 1025.
[4] H.Yamamoto et al., Solid State Commun. 151 (2011) 771.

20

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Fig. 1. Resistivity vs temperature of NCCO
(top) and NCO (bottom) thin films
grown on (001) STO substrates after
theannealing process for magnetic fields
B = 0, 1, 2, 4, 6, 8, 10, 12, and 14 T.

Label-Free Protein Detection on Graphene Oxide Surface
Kazuaki Furukawa and Yuko Ueno*
Materials Science Laboratory, *NTT Microsystem Integration Laboratories
Graphene oxide (GO) is an analogue of an oxidized form of graphene, which contains
a number of C-O bonds generated by the oxidization of C=C double bonds. GO holds anatomically thin sheet-like structure like graphene, but at the same time it shows many different
features from graphene. For instance GO is an insulator, water-dispersive material and efficient
fluorescence quencher. We designed and built the label-free protein recognition system on the
GO surface using these unique properties [1].
We used GO fixed on a solid surface. The GO surface was modified using pyrene linker tosp2 domains left in GO and the thrombin aptamer with a probe dye (FAM) bonded at the otherterminus (Fig. 1, top). Here the aptamer is a single strand DNA that forms a complex with a
specific target molecule. In this study, we used the aptamer for thrombin, an important protein
for blood clotting, to demonstrate the idea.
The recognition mechanism is schematically shown in Fig. 1. Fluorescence from the dye
is quenched by the GO at the initial stage when the modified molecules adsorbed on the GO
surface with the strong interaction between single strand DNA and GO. The fluorescencerecovers when the aptamer recognizes and forms a complex with thrombin (Fig. 1, bottom).
This is because the dye is separated from the GO surface by the recognition. The validity ofthe system was confirmed using a single piece of GO. Fluorescence microscope observations
showed that the fluorescence recovers only on the GO surface after the thrombin addition(Fig. 2, top). The thickness of GO was increased about 2.9 nm upon the thrombin recognition,
which was traced by atomic force microscope observations for the same GO piece (Fig. 2,
bottom). The experiments demonstrated the successful label-free detection of thrombin. Our
system is advantageous for the operation in microfluidics because the GO piece is firmly fixed
on the solid surface. With an increase variety of detectable proteins by choosing different
aptamers, it provides a common platform of GO aptasensors.
[1] K. Furukawa et al., J. Mater. Chem. B 1 (2013) 1119.

Fig. 1. Schematic drawing of aptamer-modified
graphene oxide surface and its molecular
detection mechanism.

Fig. 2. Fluorescence microscope (top) and atomic
force microscope (bottom) images of a GO
piece before and after thrombin addition.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)21

Epitaxial Trilayer Graphene Mechanical Resonators Obtained byElectrochemical Etching Combined with Hydrogen Intercalation
Makoto Takamura, Kazuaki Furukawa, Hajime Okamoto*,
Shinich Tanabe, Hiroshi Yamaguchi*, and Hiroki Hibino
Materials Science Laboratory, *Physical Science Laboratory
Graphene is an ideal material for use in nano-electromechanical systems due to its low
mass density and exceptional mechanical properties, such as its high Young’s modulus.
The fundamental mechanical resonance properties of graphene resonators are now being
investigated using systems fabricated from monolayer exfoliated graphene flakes. Exfoliation
is a simple way of producing high-quality graphene flakes, but it does not provide good
control of the shape and thickness of the graphene. Consequently, there have been few
studies on bilayer and trilayer graphene resonators. In this article, we report the creation oftrilayer epitaxial graphene resonators through strict control of the number of layers and theirmechanical resonance properties.
In this study, we created trilayer epitaxial graphene resonators by using bilayer graphene
epitaxially grown on SiC(0001) substrate as a starting material [1]. The epitaxial growthmethod creates a buffer layer, where carbon atoms are partially bound to silicon atoms of thesubstrate. To make the buffer layer into a graphene layer, we decoupled the Si-C bonds byhydrogen intercalation and performed electrochemical wet etching of the SiC substrate. These
procedures allow us to control the number of layers precisely. A scanning electron microscopy(SEM) image [Fig. 1(a)] verifies that the double-clamp graphene beam is suspended between
the Au/Cr pads. A cross-sectional transmission electron microscope (TEM) image [Fig. 1(b)]shows that the suspended graphene is trilayer. We can thus conclude that the buffer layer andthe bilayer graphene turned into trilayer graphene. We next investigated mechanical resonanceproperties of the trilayer graphene. The amplitude-versus-frequency curve for the trilayer
graphene resonator (Fig. 2) shows the resonant frequency ( f0) to be 7.52 MHz and the quality
factor to be ~600 at room temperature in a vacuum of 10–5 Pa. However, we calculated f0 to be
0.4 MHz on the basis of standard beam theory. Another SEM image showed that the resonatorhas a buckled-shape deformation. This enhances the stiffness of the resonator, and as a result,
f0 was higher than expected. The inverse of the quality factor, Q–1, of the trilayer graphene
showed the typical temperature dependence of monolayer graphene with a doubly clamped
structure. This implies that doubly clamped graphene resonators have a common mechanismof energy loss irrespective of the number of layers.
[1] M. Takamura et al., Jpn. J. Appl. Phys. 52 (2013) 04CH01.

Fig. 1. (a) An SEM image and (b) a cross-sectional
TEM image of suspended trilayer graphene.
22

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Fig. 2. The amplitude-versus-frequency curvefor the trilayer graphene resonator.

Self-Organization of Quasi-Free-Standing Monolayer Graphene
Nanoribbon Networks
Yuya Murata, Makoto Takamura, Hiroyuki Kageshima, and Hiroki Hibino
Materials Science Laboratory
Graphene, a two-dimensional (2D) crystalline sheet of carbon, has generated considerable
attention owing to its ultrathin geometry and high carrier mobility, with potential applicationin high-performance low-power electronics. It has been shown that the band gap of graphene
can be controlled when it is tailored into a ribbon shape with a width of up to several
nanometers [1]. However, methods such as lithography do not satisfy the requirements for theideal nanoribbons in terms of production efficiency, uniformity of crystal orientation, and edge
structure. On the other hand, it has been reported that a buffer layer on a SiC(0001) surface isdecoupled from the substrate by H intercalation and it turns into graphene (quasi-free-standing
monolayer graphene, QFMLG) [2]. Although the buffer layer is a 2D sheet of carbon likegraphene, it is electrically insulating due to covalent bonds with the substrate. It is desirable todesorb H atoms partially and create insulating regions in QFMLG for fabrication of graphenenanostructures. We investigated H desorption from QFMLG and found a self-organization of
QFMLG nanoribbon networks during the H desorption process.
We performed in-situ STM observations of the H desorption process on QFMLG. Above
630°C, H-desorbed regions expand from SiC step edges (Fig. 1). Each H-desorbed region
increases in size and is split into several patches by QFMLG nanoribbons. Finally, H-desorbed
patches separated by the network of QFMLG nanoribbons cover the entire surface. The
average width and length of the QFMLG naoribbons are a few nanometers and ~10 nm,―
respectively. The nanoribbons run along the SiC[1120] directions, with uniform armchairedges. The nanoribbon shows the graphene 3 × 3 surface structure, indicating electron
scattering at the boundary between the QFMLG and H-desorbed regions (Fig. 2) [3].
[1] M. Y. Han et al., Phys. Rev. Lett. 98 (2007) 206805.
[2] C. Riedl et al., Phys. Rev. Lett. 103 (2009) 246804.
[3] K. Sakai., Phys. Rev. B 81 (2010) 235417.

Fig. 1. Series of time-lapse STM images at 590°C, withelapsed time of (a) 0, (b) 10, and (c) 64 min (182 nm
× 250 nm), and at 630°C, with (d) 0, (e) 37, and (f)69 min (273 nm × 341 nm).

Fig. 2. QFMLG nanoribbon networks (a) 200 nm
× 200 nm, (b) 30 nm × 30 nm, (c) 10 nm ×
10 nm, (d) 2.5 nm × 2.5 nm. The rhombus
shows a graphene 3 × 3 unit cell.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)23

Reconstitution of Homomeric GluA2 Receptors in Supported Lipid
Membranes: Functional and Structural Properties
Jelena Baranovic, Chandra S. Ramanujan, and Nahoko Kasai*
University of Oxford, *Materials Science Laboratory
Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) are
glutamate-gated ion channels ubiquitous in the central nervous system where they mediate
fast excitatory neurotransmission, and act as molecular determinants of memory formationand learning. Structural studies of full-length AMPARs by electron microscopy (EM) and
X-ray crystallography have provided important insights into channel assembly and function.
However, the correlation between structure and functional states of the channel remains
ambiguous, particularly since these functional states can only be assessed with the receptor
bound within an intact lipid bilayer. Our group demonstrated structural examination ofreconstituted functioning GluA3 by using atomic force microscopy (AFM) [1], however,
the reason why the observed protein height was smaller than that observed by EM was still
unknown.
In this study, to provide a basis for investigating AMPAR structure in a membrane
environment, we developed an optimized reconstitution protocol of GluR2 whose structure
has previously been characterized by EM, and examined their function and structure [2]. AFM
studies of the reconstituted samples provide high-resolution images of full-length membraneembedded AMPARs at densities comparable to those in postsynaptic membranes (Fig.1).
Single-channel recordings of reconstituted GluA2 recapitulate key electrophysiologicalparameters of the channels expressed in native cellular membranes (Fig.2). The data
demonstrate the effect of protein density on conformational flexibility and dimensions of thereceptors and provide the first structural characterization of functional, membrane-embedded
AMPARs, thus, laying the foundation for correlated structure-function analyses of the
predominant mediators of excitatory synaptic signals in the brain.
[1] N. Kasai et al., BBA General Subjects 1800 (2010) 655.
[2] J. Baranovic, C. S. Ramanujan, and N. Kasai, J. Biol. Chem. 288 (2013) 8647.

Fig. 1. AFM image of GluA2 reconstituted at highdensity (A) and zoom of a GluA2 clustercomposed of three proteins (B). Height histogramof GluA2 reconstituted at high density (C), and aheight profile of a dotted line in B (D).

24

Fig. 2. Activity of reconstituted GluA2. (A) Singlechannel current at +80 mV in 5 mM L-Glu and 250µM CTZ; the signal was absent in the presence of
NBQX (middle trace) and in the negative control(bottom trace). (B) Single-channel conductance at+80 mV with a Gaussian fit.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)Electrostatic Control of Artificial Cell Membrane Spreading by Tuning
the Thickness of an Electric Double Layer in a Nanogap
Yoshiaki Kashimura and Kazuaki Furukawa
Materials Science Laboratory
We have reported that the self-spreading of an artificial cell membrane (ACM) on a solid
support could be controlled electrostatically by modulating an electric field applied between
nanogap electrodes, where we concluded that the electric double layer played a significant
role in the trap of the ACM [1]. In this study, we describe the mechanism of the electrostatic
control of ACM self-spreading, and investigate the dependence of the ionic concentration ofan electrolyte and the nanogap spacing under finely controlled conditions [2].
A lipid mixture (molar ratio 7:3) consisting of uncharged L-α-phosphatidylcholine andnegatively charged L-α-phosphatidylglycerol containing 1 mol% Texas Red-DHPE wasprepared. A small amount of the solid was attached to the device. The self-spreading of an
ACM was initiated by immersing the device in an electrolyte solution including 1-100 mM
NaCl. A DC voltage (–50 mV) was applied between nanogap electrodes during operation.
We calculated the electric potentials in the nanogap as a function of the distance from one
side of the electrode surfaces as shown in Fig. 1. When the nanogap spacing (d) is sufficiently
larger than the thickness of the electric double layer (D), the electric field is shielded bycounterions in an electrolyte solution leading to no voltage-dependent change in self-spreading[Fig. 1(a)]. In contrast, when d ≤ 5D, the electric field can be effectively applied in the
nanogap spacing, leading to the electrostatic trapping of the ACM [Fig. 1(b, c)]. We performedexperiments to determine whether ACM self-spreading could be controlled at each ionicconcentration of NaCl (c) as a function of nanogap spacing. For NaCl solutions, the thicknessof the electric double layer is expressed as D ~ 0.304 / c . The results are summarized in Fig. 2.
The circles and triangles, respectively, show cases where self-spreading could and could not be
controlled by applying a voltage. This result is suggestive of a certain threshold for controllingthe self-spreading. Therefore, the maximum nanogap width for controlling the self-spreading
at a certain ionic concentration is given by dmax ~ α D ~ 0.304α / c , where α is the thresholdfactor. According to the above equation, dotted lines with a slope of –0.5 were drawn in Fig.
2 for each α value and the line for α = 5.5 provided the best fit with the experiments. This isin good agreement with the prediction obtained from the electric potential calculation, whered = 5D is a threshold for the control of ACM self-spreading, suggesting the validity of our
proposed mechanism.
[1] Y. Kashimura et al., J. Am. Chem. Soc. 133 (2011) 6118.
[2] Y. Kashimura et al., IEICE TRANS. ELECTRON 96-C (2013) 344.

Fig. 1. Electric potentials in the
nanogap. (a) d = 10D,(a) d = 5D, (c) d = 2D.

Fig. 2. Result showing whether ACM self-spreading
could be controlled at various ionicconcentrations. The dotted lines exhibit a slopeof –0.5 with various a values.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)25

Conductive Polymer Combined Wearable Electrodes forElectrocardiography Recordings
Shingo Tsukada, Hiroshi Nakashima, Nahoko Kasai, Keiichi Torimitsu,
and Koji Sumitomo
Materials Science Laboratory
In today’s aging society, the continuous monitoring of the heartbeat and electrocardiograms
is expected to reduce the risk of a heart attack (sudden death) through the early detectionand early treatment of any cardiac malfunction. Monitoring is also effective in preventing
the physical overload caused by improper exercise and the accumulation of mental stress.
However, conventional biomedical electrodes use an electrolyte paste or gel, which resultsin excessive wetting of the skin surface and occasionally causes skin irritation or contact
dermatitis. The conductive polymer (PEDOT-PSS) coated silk or synthetic fibers havebeen used to fabricate wearable electrodes to obtain biomedical recordings [1]. The new
bioelectrode made of conductive fiber is flexible, biocompatible, and hydrophilic andthus allows stable recording of bioelectric signals equivalent to that of a conventional
electrode,without using any electrolyte paste or gel. Experiments on human volunteers(10 able-bodied people) wearing undershirts equipped with the electrode (Fig. 1) revealed
successful long-term recording of the heartbeat and electrocardiograms (Fig. 2) [2]. Basedon this result, the heartbeat and electrocardiograms can be monitored continuously simply by
wearing the electrode shirts. We can expect the electrode to find various applications in sports,
health enhancement, and the support of medical diagnosis, as well as scientific research.
[1] S. Tsukada, H. Nakashima and K. Torimitsu, PLoS ONE 7(4) (2012) e33689.
[2] 2013 Feb 12 NTT Press release.

Fig. 1. A wearable electrode has been fabricatedby mounting a conductive textile on a shirt.

26

Fig. 2. Simply wearing the shirt enables the Holter
electrocardiograms to be monitored.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Donor-Based Single Electron Pump
Gabriel P. Lansbergen, Yukinori Ono, and Akira Fujiwara
Physical Science Laboratory
Down scaling of semiconductor devices has been providing us the opportunity to explore
a new approach to devices with a limited amount of dopants in the active device area, whosefunctionality is based on individual dopants. Recent progress in nanofabrication has led to theobservation of the charge of a single dopant in silicon nanotransistors [1] and the operation ofa single-atom electron spin qubit in silicon [2].
Recently we have investigated a novel device concept based on single dopant functionality,that is, we have demonstrated multiple-donor based single electron pumps [3]. Our devices
consist of Si nanowire MOSFETs in series fabricated on a silicon-on-insulator (SOI) wafer(see Fig. 1). A stacked gate layer structure is employed; the lower layer consists of three finegates, LG (not used), MG, and RG, defined by electron beam lithography, and the top layer
consists of a large single upper gate (UG). Donor atoms (arsenic) are locally implanted in
the nanowire channel between MG and RG using a 60 nm wide aperture in a resist mask.
For charge transfer operation (see Fig. 1 right), we apply an AC signal with a frequency f
to MG. RG is tuned such that it induces a small barrier in the Si nanowire beneath it. Whenthe barrier formed by VMG is in its low state (I), electrons enter from the source. When thebarrier is subsequently ramped up (II) to its high state (III) by V MG, electrons are captured
at donor sites, and then the gate electric field ionizes the donors and evacuates their boundelectrons to the drain region. Figure 2 shows the charge pump characteristics ISD versus VUG
at various right fine gate voltages (VRG). Current plateaus due to single-electron transfer via
multiple dopants are clearly observed. It is also shown that the number of dopants is tunable
by VRG; positively increased VRG reduces a depletion region in the island, making more donors
available for charge transfer. Temperature-dependent characterization allows us to extract theionization energy of a single arsenic donor. Dopant based SE pumps are capable of pumping
multiple electrons with the number of dopants and therefore promising for electrical standards
generating high currents.
[1] Y. Ono et al., Appl. Phys. Lett. 90 (2007) 102106.
[2] J. J. Pla et al., Nature 489 (2012) 541.
[3] G. P. Lansbergen, Y. Ono, and A. Fujiwara, Nano Lett. 12 (2012) 763.

Fig. 1. (Left) Schematic top view of Si nanowire
MOSFET (W = 80 nm, L = 100 nm). (Right)
Potential diagram of operation of charge pump.

Fig. 2. Pump current as a function of gate voltage,
showing single-electron transfer via a tunable
number of individual donors up to I = 6ef.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)27

Signal Detection Via a Transistor with Steep Current Characteristics
Katsuhiko Nishiguchi and Akira Fujiwara
Physical Science Laboratory
A field-effect transistor (FET) is a device to amplify signals applied to a gate terminal, which
can be used for a sensor with a high charge sensitivity. To maximize the sensitivity, current
characteristics as a function of gate voltage should be as steep and as non-linear as possible.
However, steepness or subthreshold swing (SS) of current characteristics is fundamentally
limited by thermal fluctuation, e.g., 60 mV/dec at room temperature. On the other hand,practical use of the sensor necessitates suppression of outside noise masking an output signal
from the FET. In this work, we demonstrate detection of a signal buried in noise with atransistor having extremely small SS [1].
The device is composed of a wire channel covered with three gates as shown in Fig. 1 [2].
An upper gate (UG) and back gate (BG) is used to induce a source and drain electrically.
Then, current is modulated by a lower gate (UG) as in a conventional LG. When drain
voltage is large enough to induce hot electrons, they generate electron-hole pairs due toimpact ionization. Generated holes flow in a body region of the channel and thus amplify
electron current, which lead to further impact ionization. This positive feedback makes current
characteristics steeper and SS reaches ~1 mV/dec regardless of thermal fluctuation (Fig. 2).
Detection of signal buried in noise is performed by using stochastic resonance (SR), which
is a unique phenomenon that takes advantage of noise to enhance the response of a system
to a weak signal [3]. Non-linear current characteristics of the FET enable SR to provide
a correlation between the noise-buried input and output current signals. It is confirmed
experimentally and theoretically that stronger non-linearity, i.e., small SS, and bistabilityof current characteristics boost an effect of SR (Fig. 3). Moreover, the dynamic bistability
enables SR effect to be enhanced by adding common noise to multiple FETs. The FET
providing such unique characteristics opens the way to use SR for practical applications.
This work was partly supported by the Funding Program for Next Generation WorldLeading Researchers of JSPS (GR103)[1] K. Nishiguchi and A. Fujiwara, Appl. Phys. Lett. 101 (2012) 193108.
[2] K. Nishiguchi and A. Fujiwara, Appl. Phys. Express 5 (2012) 085002.
[3] L. Gammaitoni et al., Rev. Mod. Phys. 70 (1998) 223.

Fig. 1. Cross-sectional views and SEM
image of the device.

28

Fig. 2. Current-voltage
characteristics.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Fig. 3. Detection of a signal
buried in noise.

Cavity-Electromechanics with Phonons
Imran Mahboob, Katsuhiko Nishiguchi, Hajime Okamoto, and Hiroshi Yamaguchi
Physical Science Laboratory
Cavity-opto/electromechanical systems exploit the parametric coupling between a photonic
cavity and a mechanical resonator [1]. The parametric coupling arises from the motion of themechanical element that can modulate the cavity’s frequency. This results in the emergence ofside-bands at the sum (blue) and difference (red) frequency of the constituent resonators. By
optically pumping on the side-bands, the coupling between the two systems can be enhanced.
This in turn can induce an EIT-like transparency in the cavity where energy is transferredto the mechanical resonator. Alternatively, the cavity can also manipulate the oscillation
dynamics of the mechanical resonator and even cool it into its quantum ground state i.e.
energy is transferred to the cavity from the mechanical resonator.
In this work, we demonstrate an all-phonon analogue of a photonic cavity-opto/
electromechanical system where the mechanical oscillation of interest is composed by the
1st oscillation mode and role of the phonon cavity is played by the 2nd oscillation mode.
The parametric coupling between the modes is created by modulating the tension in theelectromechanical resonator at the sideband frequencies. This in turn modifies the oscillation
dynamics of the constituent modes that can drive the parametric coupling between them.
The experiments were performed in a piezoelectrically active GaAs/AlGaAs
electromechanical resonator which sustained the 1st and 2nd modes at 171.3 kHz and470.9 kHz respectively (Fig. 1). The piezoelectric transducer enabled the tension in themechanical resonator to be modulated at the sidebands. The creation of parametric coupling isdemonstrated by probing the 2nd mode whilst pumping on the red-side band (see Fig. 2). This
results in energy from the 2nd mode being transferred to the 1st mode. As the pump amplitude
is increased, the coupling between modes is made so strong that it results in parametric normal
mode splitting where the modes are no longer distinct but rather a hybrid. We exploit the
dynamic nature of this parametric coupling to create a transparency in the phonon cavity andto cool the 1st mode [2].
[1] T. Kippenberg and K. Vahala, Science 321 (2008) 1172.
[2] I. Mahboob et al., Nature Physics 8 (2012) 387.

Fig. 1. An SEM image of the electromechanicalresonator and simple circuit schematic.

Fig. 2. The 2nd mode being probed at large
red-pump amplitudes which undergoes
parametric normal mode splitting indicatingthat it is strongly coupled to the 1st mode.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)29

Ferromagnetic-Induced Component in Piezoresistance of GaMnAs
Koji Onomitsu1, 2, Imran Mahboob1, Hajime Okamoto1, Yoshiharu Krockenberger2,
and Hiroshi Yamaguchi1
1
Physical Science Laboratory, 2Materials Science Laboratory
The piezoresistive effect (PR) is defined as the change in resistance of a material due to an
applied stress and is a fundamental property of semiconductors and metals. In conventional
semiconductors, PR originates from a change in either carrier density or mobility. PR canalso be observed in metals, including ferromagnetic materials, due to geometrical effects. PR
provides a powerful tool for the investigation of electronic transport properties and can be used
for sensing applications. In this study, we report a new mechanism of PR in GaMnAs where
the ferromagnetic ordering plays an essential role. We characterized PR by incorporating a
GaMnAs piezoresistor into a micromechanical cantilever and investigated its temperatureand magnetic field dependence at the ferromagnetic transition. A clear ferromagnetic-induced
piezoresistance component (FMPR) was found below the Curie temperature (Tc) of our
GaMnAs (Tc ~ 48 K). Figure 1 shows the schematic illustration of the cantilever together
with electrode on PZT and measurements setup. The frequency response of PR is detected
via down-mixing technique. The reference frequency can be tuned by adjusting bias-current
frequency ( f1) and actuation frequency ( f2). Figure 2 shows the temperature dependence of PR
around Tc without external magnetic field: (a) real part and (b) imaginary part. Data (dots)show the measured frequency response, and the straight lines were obtained by calculations.
Data and calculated lines have been offset by ∆ = 0.04 Ω for clarity.
These results indicate that the FMPR of GaMnAs arises from the perturbation of
spontaneous spin ordering by the applied strain. Moreover, the change in resistance is delayedwith respect to mechanical strain. We deduced a delay time of 230 ± 35 ns. The experimental
results presented indicate that the micromechanical method of characterizing spin dynamics iscomplementary to conventional electrical and optical methods [1].
[1] K. Onomitsu et al., Phys. Rev. B 87 (2013) 060410 (R).

Fig. 1. Schematic illustration of the cantilever
on PZT and measurements setup.

30

Fig. 2. Temperature dependence of PR around Tc.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

An Electromechanical Membrane Resonator
Daiki Hatanaka, Imran Mahboob, Hajime Okamoto, Koji Onomitsu, and Hiroshi Yamaguchi
Physical Science Laboratory
Membrane-based mechanical resonators (MMR) have received much interest because they
have high quality-factors [1]. In addition, they also have a large surface area that not only
makes them easier to use in optical architectures, but also it makes them more responsive
as detectors. However, to date most MMRs have been passive structures with only opticalcontrol. The lack of electrical control prevents the development of the MMR for morepractical applications. Here, we report a GaAs/AlGaAs-based piezoelectric MMR that enables
all electrical transduction of the mechanical motion [2].
The MMR consists of GaAs (5 nm) / Al0.27Ga0.73As (95 nm) / n-type GaAs (100 nm)
heterostructure on an Al0.65Ga0.35As sacrifice layer (3.0 µm). The circular membrane with adiameter of 30 µm was suspended by removing the sacrifice layer through the center hole withhydrofluoric acid as shown in Fig. 1. All measurements in this report were performed in a highvacuum and at room temperature.
Mechanical oscillations were induced by applying an oscillating voltage to electrode A or
B due to the piezoelectric effect and the resulting oscillations were detected by measuring
the piezovoltage generated from electrode C. In order to demonstrate its electromechanicalfunctionality, a mechanical logic gate is built with the fundamental (0, 1) mode in the MMR.
In the experiment, electrodes A and B are used for binary inputs and electrode C is used for――a binary output where electrodes A and C are along the [110] orientation and electrode B―
is along the [110] orientation as shown in Fig. 1. An excitation of the (0, 1) mode via input
A or B results in membrane oscillations with opposite phase due to the opposite sign of the―――piezoelectric constants in the [110] and [110] orientations in AlGaAs. Therefore, when an
actuation of the same magnitude is simultaneously applied to both inputs A and B, mechanicaloscillations can be cancelled out, resulting in no piezovoltage at output C. This response fromthe (0, 1) mode enables the MMR to implement an XOR (A⊕B) logic gate as shown in Fig. 2.
Furthermore, we can also demonstrate an OR (A B) logic gate in the first (1, 1) mode using
a similar method. Thus, the electrically-active MMR promises functions and performance
beyond conventional passive MMRs which could be used to develop a broad range ofapplications for mechanical resonators.
∩[1] J. D. Thompson et al., Nature 452 (2008) 72.
[2] D. Hatanaka et al., Appl. Phys. Lett. 101 (2012) 063102.

Fig. 1. The piezoelectric MMR and its
measurement set-up.

Fig. 2. An XOR logic gate (A⊕B) in the (0, 1) mode.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)31

Shot Noise Spectroscopy on a Semiconductor Quantum Dot
in the Elastic and Inelastic Cotunneling Regimes
Yuma Okazaki1, 2, Satoshi Sasaki1, 2, and Koji Muraki1
1
Physical Science Laboratory, 2Tohoku University
Shot noise is a time-dependent current fluctuation reflecting the discreteness of charge
carriers. Measurements of shot noise in coherent conductors such as a quantum dot (QD) are
expected to reveal dynamical mechanisms and underlying correlation in electron transport.
Electron cotunneling is a higher-order tunneling process observed in the Coulomb blockade
regime. Cotunneling is classified into elastic and inelastic processes; the former involves only
the QD ground state, while the latter accompanies dynamical charge fluctuation between the
ground and excited states. Shot noise measurements have been reported only for the inelastic
cotunneling regime in a carbon nanotube QD [1]. For semiconductor QDs, on the other hand,shot noise measurements in the cotunneling regime are made more challenging by the evenlower cotunneling current inherent to semiconductor QDs [2].
Here we study the shot noise in a semiconductor QD in the cotunneling regime. We fabricate
a small QD [Fig. 1(a)], in which the level spacing ∆E can be increased up to 1 meV. Owing tothis large ∆E, we can obtain cotunneling current high enough as compared to the resolutionof our noise measurement setup. Figure 1(b) shows the conductance G as a function of source
drain bias Vsd. For a small bias range (|Vsd| < 1 mV), we find small but finite conductance due
to elastic cotunneling. For a large bias range (|Vsd| > 1 mV), on the other hand, G is strongly
enhanced due to the strong inelastic cotunneling. Figure 1(c) shows the Fano factor F, which
is determined from the measured current noise Sout and current I [F = Sout / 2eI with e being
the electron charge]. We observe the Poissonian Fano factor F ~ 1 in the elastic cotunnelingregime. On the other hand, we observe an enhancement of the Fano factor up to F ~ 2.5(super-Poissonian Fano factor) in the inelastic cotunneling regime. This large Fano factor
results from the fact that the inelastic cotunneling is accompanied by an emission of excess
electrons, which leads to electron bunching and enhances the shot noise [3]. The observed
difference in the value of the Fano factor can be utilized as a new spectroscopic measurement,by which we can quantitatively distinguish the microscopic origins of charge transport.
[1] E. Onac et al., Phys. Rev. Lett. 96 (2006) 026803.
[2] S. Gustavsson et al., Phys. Rev. B 78 (2008) 155309.
[3] Y. Okazaki, S. Sasaki, and K. Muraki, Phys. Rev. B 87 (2013) 041302(R).

Fig. 1. (a) Device and the setup. The current noise is fed to the LC resonator, and is measured
using cold-amplifier. (b) Conductance G and (c) Fano factor F as a function of Vsd.

32

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Plasmon Transport in Graphene Investigated by Time-Resolved Electrical
Measurements
Norio Kumada, Shinichi Tanabe, Hiroki Hibino, Hiroshi Kamata*, Masayuki Hashisaka*,
Koji Muraki, and Toshimasa Fujisawa*
Physical Science Laboratory, *Tokyo Institute of Technology
Plasmons, which are collective charge oscillations, could provide a means of confining
electromagnetic field to nanoscale regions. Recently, plasmonics in graphene have attracted
interest, particularly because of the tunable plasmon dispersion. In this work, we carried out
time-resolved electrical measurements of edge magnetoplasmons (EMPs), which are plasmons
localized at graphene edge in a high magnetic field, and demonstrate that the velocity of EMPs
can be controlled over two orders of magnitude [1].
Graphene used was grown on SiC substrate. We used two samples, one with and the otherwithout a large top gate. All measurements were carried out at 1.5 K. Plasmons are injected
into graphene by applying a voltage step to the injection gate and detected through the detector
Ohmic contact fabricated 1.1 mm away from the injection gate [Fig. 1(a)]. From the time offlight between the injector and the detector, the plasmon velocity is determined. In the ungatedsample, as the magnetic field is increased, the velocity decreases from 6000 km/s to 2000 km/
s [Fig. 1(b)]. In the gated sample, on the other hand, the velocity is about 100 km/s, which
is one order of magnitude smaller than that in the ungated sample [Fig. 1(c)]. The smaller
velocity is due to the gate screening effect of the electric field in plasmons. At a fixed magnetic
field (12 T), as the gate bias and thus the carrier density are decreased, the velocity decreases
to 10 km/s with oscillations. These results indicate that the velocity of plasmons in graphene
can be controlled over two orders of magnitude by applying the magnetic field, screening theplasmon electric field with a gate metal, and changing the carrier density. The wide tunabilityof the plasmon velocity encourages designing graphene nanostructures for plasmonic circuits.
This work was supported by KAKENHI.
[1] N. Kumada et al., Nature Commun. 4 (2013) 1363.

Fig. 1. (a) Sample structure and the experimental setup. (b) Plasmon velocity in the ungated sample as a functionof the magnetic field. (c) Plasmon velocity in the gated sample as a function of the gate bias at a magnetic
field of 12 T.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)33

Impact of Graphene Quantum Capacitance on Transport Spectroscopy
Keiko Takase, Shinichi Tanabe*, Satoshi Sasaki, Hiroki Hibino*, and Koji Muraki
Physical Science Laboratory, *Materials Science Laboratory
Graphene, a two-dimensional honeycomb lattice of carbon atoms, is known to have therelativistic energy band called Dirac cone. Accordingly, the density of states in graphenedepends on the Fermi energy, whereas those in the conventional two-dimensional systems(2DESs) do not rely on the Fermi level. This then indicates that the quantum capacitance,
which is defined to be proportional to the density of states, is quite different for graphene andconventional 2DESs. In this study, using a top-gated device fabricated from epitaxial grapheneon SiC, we demonstrate that the interplay between quantum capacitances of graphene andinterface states in the device changes the appearance of the fan diagram in epitaxial graphene,
which consequently represents the relativistic graphene Landau level. This indicates that ourtransport measurements serve as a kind of energy spectroscopy [1], and thus enables us todeduce the energy broadening of the Landau levels in graphene [1].
The sample is a top-gated Hall-bar device, fabricated from graphene on SiC. At low
temperature and low fields, we observe the quantum Hall (QH) state, where the Hall resistancebecomes quantized at h/{(4N+2)e2} and longitudinal resistance R xx becomes zero at theLandau level filling factor ν = 4N + 2 (N: integer). The observed QH states are illustrated ina fan diagram [Fig. 1(a)], where Rxx is mapped as a function of gate voltage Vg and magnetic
field B. At ν = 2, a wide region with zero Rxx, indicating the ν = 2 QH state, appears and clear
Rxx peaks are observed at ν = 0, 4, and 8. In contrast to the usual case, in which Rxx peaks
appear as an equidistant linear fan diagram in the Vg – B plane, in our epitaxial graphenedevice, the trajectories of the Rxx peaks are curved and unequally spaced. This can be
explained by our model [Fig. 1(c)], in which not only graphene but also the interface states
nearby graphene, such as dangling-bond states at SiC or those in the gate insulator, serveas charge reservoirs when Vg is swept. As a result, the quantum capacitance of grapheneand that of the interface states associated with the density of states play a significant role inthe Vg dependence of the graphene carrier density. Analyzing the Vg dependence with thismodel allows us to deduce the interface state densities. Furthermore, when the interface state
density is much larger than the graphene carrier density, the Fermi level in graphene becomes
proportional to Vg. Consequently, Rxx peaks in the Vg – B plane represents the graphene
Landau-level structure. Filling factor calculated vs. Vg and B in Fig. 1(b) nicely reproduces theexperimentally observed peak positions of Rxx in Fig. 1(a).
[1] K. Takase et al., Phys. Rev B 86 (2012) 165435.

Fig. 1. (a) Rxx mapped vs. Vg and B. (b) Filling factors calculated vs. Vg and B. (c) Schematic illustration of thegraphene device with nearby interface states.

34

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)Energy Relaxation Process of a Quantum Memory
Coupled with a Superconducting Qubit
Yuichiro Matsuzaki and Hayato Nakano
Physical Science Laboratory
For quantum information processing, each physical system has a different advantage asregards implementation and so hybrid systems that benefit from the advantage of several
systems would provide a promising approach. One common hybrid approach involves
combining a superconducting qubit as a controllable qubit and another quantum system witha long coherence time as a memory qubit [1]. The use of a superconducting qubit gives us
an excellent controllability of the quantum states and the memory qubit is capable of storing
information for a long time. It has been believed that selective coupling can be realized
between a superconducting qubit and a memory qubit by tuning the energy splitting between
them.
However, we have shown that this detuning approach has a fundamental drawback as regards
energy leakage from the memory qubit [2]. Even if the superconducting qubit is effectively
separated by reasonable detuning, energy relaxation time in the memory qubit decreases via
residual weak coupling when the superconducting qubit is affected by severe dephasing (Fig.
1). This energy transport from the memory qubit to the control qubit can be interpreted as theappearance of the anti Zeno effect induced by the fluctuation in the superconducting qubit. We
have suggested possible ways to avoid this energy relaxation process by using decoherence
free subspaces, which is feasible with existing technology [2].
This work was supported by KAKENHI.
[1] X. Zhu et al., Nature 478 (2011) 221.
[2] Y. Matsuzaki et al., Phys. Rev. B 86 (2012) 184501.

Fig. 1. Relationship between relaxation time of the memory qubit and the dephasing time ofthe superconducting qubit. We set the coupling strength as 25 MHz and set the detuning
energy as 1 GHz.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)35

Evidence of a Crossover from Trions to an Electron-Hole Gas
Masumi Yamaguchi, Shintaro Nomura*, Hiroyuki Tamura, and Tatsushi Akazaki
Physical Science Laboratory, *University of Tsukuba
A trion, the bound state of two electrons and a hole, cross over to the unbound state of thetwo-dimensional electron gas plus a hole state (2DEG-h) as the excess electron density is
increased (see Fig. 1). This is because the Coulomb attraction between the electron and holeis screened by the degenerate 2D electron gas. In studying this crossover, a difficulty lies in
the unavoidable spatial disorder of the electrostatic potential existing in samples. In the lowelectron-density regime, the electrons are localized at the potential valleys. The electrondensity regime where the crossover occurs often overlaps the electron-density regime where
the electron localized-delocalized transition occurs.
In this work, we studied the crossover by analyzing the evolution of the PL spectrum using
an undoped GaAs QW [1]. Unlike in modulation-doped QWs, which are commonly used forstudying 2D electron system, the electron localization occurs at much lower density than the
crossover regime since the typical length scale of the disorder potential for our sample is long.
Therefore, the electron localization and the crossover are clearly separated in our sample.
By measuring the photoluminescence linewidth under perpendicular electric fields, we havetraced the variation of the effective radius a* of the trion as a function of electron density(Fig. 2). The a* increases sharply above ns = 2 × 1014 m–2, at which the screening length drops.
Here, the screening length is predicted by nonlinear-screening theory. This is a clear evidenceof the trion-2DEG-hole crossover related to the dissociation of the trion state screened by the2D electron gas.
This work was supported by KAKENHI.
[1] M. Yamaguchi et al., Phys, Rev. B 87 (2013) 081310(R).

Fig. 1. Tron state and two-dimensional electron
gas plus hole state.

36

Fig. 2. Electron density dependence of the trion
radius a*. Inset: Screening length.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Fundamental Limit to Qubit Control with Coherent Field
Kazuhiro Igeta
Optical Science Laboratory
Controlling matter qubits with coherent electromagnetic field is regarded as the mostpromising building block of quantum information processing systems. In practice, we canuse the classical field theory which enables us to control a qubit perfectly as described by the
Bloch rotation according to the pulse area of the field. However, classical fields exist only
as an infinitely strong intensity limit. In reality, strong but finite strength coherent fields areavailable that will cause control errors because of their lack of infiniteness.
In this work, we formulate the full quantum mechanical interaction between a pure coherent
field and a qubit in the general initial state including mixed states. While the fidelity error
accompanied by π/2 pulse control is shown to be inversely proportional to the average photon
number in a way similar to that revealed by the former results [1, 2] our results show that theerror depends strongly on the initial state of the qubit [3]. Our result (Fig. 1) reveals that thethe entanglement built up between the field and the qubit is the origin of the error.
This work was partly supported by CREST of the Japan Science Technology Agency.
[1] J.Gea-Banacloche, Phys. Rev. A 65 (2002) 022308.
[2] M.Ozawa, Phys. Rev. Lett. 89 (2002) 057902.
[3] K. Igeta, N. Imoto, and M. Koashi, Phys. Rev. A 87 (2013) 022321.

Fig. 1. Solid lines represent exact fidelity error rates for the initially up and initially down
qubit states, respectively, plotted to the average photon number (N) of the field on
a log10 scale, along with the asymptotic curves for large N (~1/N) and results of theformer studies[1,2] as shown by dotted lines.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)37

Monolithic Source of Polarization Entanglement Using Silicon Photonics
Nobuyuki Matsuda1, 3, Hanna Le Jeannic1, Hiroshi Fukuda2, 3, Tai Tsuchizawa2, 3,
Koji Yamada2, 3, and Hiroki Takesue1
1
Optical Science Laboratory, 2NTT Microsystem Integration Laboratories,
3
Nanophotonics Center
Integrated photonic waveguides, which provide a highly-stable and ultra-compact opticalcircuitry, is a promising candidate for a platform of scalable photonic quantum information
processing (QIP) system [1]. Many QIP protocols are based on photonic quantum states
encoded in polarization degree of freedom. Therefore, it is essential to implement integrated
subsystems to generate, manipulate and measure polarization-encoded quantum states of light.
This time we realized the first monolithic source of polarization entangled photon pairs by
employing the silicon photonics technology [2].
The device consists of two silicon-wire waveguides (SWWs) connected by a siliconpolarization rotator (SPR) [Fig. 1(a)]. In a SWW, a correlated pair of signal and idler photons
is created following the annihilation of two pump photons via spontaneous four-wave mixing
process. Here the pump, signal and idler photons are horizontally (H) polarized. The SPR
has an off-axis double core structure of a Si inner core and a SiOxNy outer core. The SPR
exhibits two orthogonal eigenmodes, which have different effective refractive indices andeigen-axes tilted at 45° with respect to the normal to the substrate [3]. The birefringence in the
eigenmodes causes the polarization plane to rotate by an amount that depends on the length of
the SPR. We obtain polarization-entangled photon pairs from the device by using optical pumppulses with + 45° linear polarization as shown in Fig. 1. The polarization-mode dispersion andthe polarization-dependent loss in a SWW, which potentially degrade the purity of polarization
entanglement, can be automatically cancelled out in our device. We successfully obtained amaximally entangled state from the chip with a state fidelity as high as 94 %, which is well
above the classical limit.
This work was supported by KAKENHI.
[1] A. Peruzzo et al., Science 329 (2010) 1500.
[2] N. Matsuda et al., Sci. Rep. 2 (2012) 817.
[3] H. Fukuda et al., Opt. Express 16 (2008) 2628.

Fig. 1. (a) The monolithic polarization entanglement source and (b) its working principle. V represents
the vertical polarization.
38

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)Bose-Einstein Condensate on a Persistent-Supercurrent Atom Chip
Hiromitsu Imai, Kensuke Inaba, Makoto Yamashita, and Tetsuya Mukai
Optical Science Laboratory
Ultra-cold atoms manipulated by electromagnetic field have been used widely for theapplications of quantum information processing and quantum metrology. As a promisingquantum device for full quantum control of an atomic state, a persistent-supercurrent atom
chip allows us to trap atoms in extremely stable and tight potential. So far we have trapped
atoms in a magnetic field of the persistent-supercurrent atom chip [1, 2]. In this year, weachieved 87Rb Bose-Einstein condensate (BEC) by radio frequency induced evaporative
cooling. This result paves the way for the development of quantum devices such as quantum
memories and atom interferometers.
As shown in Fig. 1, Rb atoms were trapped in a potential generated by the magnetic fieldof a persistent supercurrent in a closed loop circuit and a bias magnetic field. To verify thecondensation, we employed the time-of flight (TOF) imaging technique, and measured themomentum distribution of atoms released from the trapping potential after the evaporative
cooling. Figure 2 (a) and (b) show the TOF image after 15 ms and the cross-section along
the dotted line. These images were taken when a final radio frequency of the evaporative
cooling vf was 1.770 MHz. The broad distribution indicates that the atomic gas remained in
the thermal state. In Fig. 2 (c) and (d), a bimodal momentum distribution was observed as theonset of the BEC transition at vf = 1.750 MHz. When vf was further decreased to 1.735 MHz[Figure 2 (e) and (f)], the thermal distribution was not observed and an almost pure condensate
with a narrow distribution was obtained.
This work was supported by FIRST program, KAKENHI, and JST-CREST.
[1] T. Mukai et al., Phys. Rev. Lett. 98 (2007) 260407.
[2] C. Hufnagel et al., Phys. Rev. A 79 (2009) 053641.

Fig. 1. Schematic diagram of atoms trapped ona persistent-supercurrent atom chip.

Fig. 2. TOF images after 15 ms (upper panels) andcross-sections of the optical density along thedotted lines (lower panels).

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)39

Ultrafast Surface Electron Dynamics on GaAs(001) Using Time-Resolved
Photoelectron Spectroscopy Based on High-Order Harmonic Source
Katsuya Oguri, Keiko Kato, and Hiroki Mashiko
Optical Science Laboratory
High-harmonic generation (HHG) provides an attractive ultrashort light source of coherent
radiation in the extreme-ultraviolet (EUV) and soft-x-ray regions of the spectrum, which isbased on a highly nonlinear interaction between gaseous atoms and high peak power laser
pulse with intensity of 1013 – 1015 W/cm2 [1]. For the last two decades from the discovery of
HHG, the HHG technology has made rapid progress resulting in a unique light source thatpossesses different characteristics from synchrotron radiation source and x-ray free electron
laser. This opens the door to new widely applications of HHG source [2]. We have developed
a technique of ultrafast surface photoelectron spectroscopy utilizing three characteristics of
HHG including sharp spectral shape, ultrashort pulse duration, and wavelength of surface
sensitivity. With this technique, we successfully measured a time-evolution of surface
photovoltage effect on photo-excited GaAs(001) surface.
The photoelectron spectroscopy system uses a 100-fs titanium:sapphire laser system witha central wavelength of 790 nm operating at a 10 Hz repetition rate (Fig. 1). We selected the59th harmonic pulse as the EUV probe by a pair of Mo/Si multilayer mirrors, and measured aphotoelectron spectrum of GaAs(001) sample that was chemically etched. When the sample
is excited with the laser pulse, we observed an energy shift of Ga 3d-core level photoelectron
peak with about 200 meV towards higher binding energy, and its relaxation (Figs. 2(a) and (b)).
This indicates that the surface potential changed due to the spatial separation of the electronhole pair that was generated by the laser excitation, which is called as surface photovoltage(SPV) effect. Improving the temporal resolution ofthis scheme will enable us to investigate dynamics of
SPV effect on semiconductor surface in femtosecond
region [3].
[1] C. Winterfeldt et al., Rev. Mod. Phys. 80 (2008) 117.
[2] C. La-O-Vorakiat et al., Phys. Rev. Lett. 103 (2009)
257402.
[3] K. Oguri et al., Jpn. J. Appl. Phys. 51 (2012) 072401.

Fig. 1. Schematic illustration of the time-resolved
surface photoelectron spectroscopy system.

40

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Fig. 2. Photo-induced transient change in the
Ga-3d peak at various time delays (a), andits temporal evolution (b), respectively.

Coherent Manipulation of Moving Electron Spins Using
Magnetic-Field-Free Electron Spin Resonance
Haruki Sanada, Yoji Kunihashi, and Koji Onomitsu*
Optical Science Laboratory, *Physical Science Laboratory
The most promising method of quantum information processing with electron spins isbased on electron spin resonance (ESR), a technique commonly used both in research andfor practical applications. However, the previously proposed spin manipulation methods
require external magnetic fields, which are generated in spaces that greatly exceed the sizeof individual electrons, thus making these complex and inefficient techniques unsuitable fordevice applications. Here we demonstrate magnetic-field-free ESR achieved by using spinorbit effective magnetic fields (BSO) [1].
The sample was an undoped 20-nm-thick GaAs/AlGaAs (001) quantum well (Fig. 1). A
surface acoustic wave (SAW) beam generated by an interdigital transducer (IDT) produces
an array of potential wires that move along y (|| [-110]) with a velocity of 2.97 km/s. A Ti
film with 3-µm-wide slits deposited on the wafer partially screens the piezoelectric field andproduces moving dots that travel along the channels formed beneath the slits. We performed
Kerr microscopy to investigate spin transport along a straight and winding channels, the latterof which was designed to be close to the resonance condition. A circularly polarized pumplight generates spin polarized electrons at certain position (y = 0) on the channels and linearlypolarized light probes the Kerr rotation, which is proportional to the spin density at the probe
position [2]. The sample was placed in an electro-magnet that generated a magnetic field (Bext)
along [110].
Figure 2 shows Kerr signals as a function of y and Bext for the two channels. The oscillations
observed for the straight channel are attributed to the spin precession induced by static
magnetic fields. The effects induced by an oscillating spin-orbit field appear in the data for thewinding channel; the precession phases show peculiar features at around Bext = 0 and 46 mT.
The Bloch simulations under BSO well reproduce the experimental results, and this proves thefeasibility of the magnetic-field-free ESR. The technique will provide an efficient and flexible
approach for the coherent control of flying spin information in solid-state devices.
This work was supported by KAKENHI.
[1] H. Sanada et al., Nature Physics 9 (2013) 280.
[2] H. Sanada et al., Phys. Rev. Lett. 106 (2011) 216602.

Fig. 1. Schematic view of the sample (left) and opticalreflectivity images of the strait and winding
channels (right).

Fig. 2. Kerr rotation signals for the straight(upper) and winding (lower) channels
plotted as a function of Bext and y.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)41

High-Efficiency Detection of Carrier-Envelope Offset Frequency with a
Dual-Pitch PPLN Ridge Waveguide
Kenichi Hitachi, Atsushi Ishizawa, and Masaki Asobe
Optical Science Laboratory
A carrier-envelope offset (CEO) stabilized optical frequency comb provides dramatic
progress in the field of precision spectroscopy and femtosecond pulse shaping. We havestudied a 25-GHz-mode-spacing optical frequency comb provided by an externally phasemodulated laser diode, in which each frequency mode can be separated. This is an advantage
compared to a common mode-locked laser whose repetition rate is smaller than a few hundred
megahertz. For stabilizing a frequency comb, it is common to utilize a f-to-2f self-referencing
interferometer (SRI), in which one-octave bandwidth of the supercontinuum (SC) spectrum
is required [1]. However, it is difficult to use this method for a phase-modulated frequency
comb, because phase noise in the SC spectrum increases as the frequency departs from theoriginal frequency. For this reason, we focus on a 2f-to-3f SRI, in which 2/3-octave bandwidthof the SC spectrum is used. By utilizing a fiber laser for generating a SC spectrum and a dualpitch (DP) periodically poled lithium niobate (PPLN) ridge waveguide for third-harmonic (TH)generation, we succeeded in detecting the CEO frequency with a high signal-to-noise ratio(SNR) (> 30 dB).
A DP-PPLN ridge waveguide device consists of two monolithically integrated segmentswith different quasi-phase matching (QPM) pitch sizes (Λ1, Λ2) [Fig. 1(a)]. First, the secondharmonic (SH) light of the SC component at the frequency of f1 is generated in the first
segment with the pitch size of Λ1. Then, the sum frequency of the SH light (2 f1) and SC
component of f1’ is generated in the second segment with Λ2 [2]. Confinement of the SC
light with a ridge waveguide structure and monolithically integrated design free from optical
coupling loss enables TH generation with high efficiency.
In the experiment, a SC light (~300 mW) was generated by injecting the output from thefiber laser into a high nonlinear fiber, and spectrally separated with a dichroic mirror at the
wavelength of 1500 nm. The SH (TH) light is generated from the short- (long-) wavelengthcomponent [~1200 (1800) nm] of the SC light. By detecting the heterodyne beat of the two
outputs with a photodetector, we detected CEO signals with a SNR > 30 dB, which is highenough for stabilizing the CEO frequency [Fig. 1(b)]. In the future, we will detect and stabilize
the CEO frequency of our developed externally phase-modulated laser with this technique.
[1] A. Ishizawa et al., Opt. Express 16 (2008) 4706.
[2] K. Hitachi et al., Electron. Lett. 49 (2013) 145.

Fig. 1. (a) The schematic of a DP-PPLN ridge waveguide and (b) the detection of CEO signal.
42

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Ultrafast Spontaneous Emission from Copper-Doped Silicon Nanocavity
Hisashi Sumikura1,2, Eiichi Kuramochi1,2, Hideaki Taniyama1,2, and Masaya Notomi1,2
1
Nanophotonics Center, 2Optical Science Laboratory
Impurities binding an exciton in Si show intense emission without phonon emission andattract a lot of attention as quantum bits (qubits) manipulated by light because of theirlong-lived spins. However, the light-impurity interaction is inefficient due to nonradiative
processes including the Auger process. In this study, we focused on the combination of copper
isoelectronic centers (Cu-IECs) and photonic crystal (PhC) nanocavities. The IEC originatesfrom a bound exciton of deep impurities. Since the IEC is free from the Auger process, itsemission quantum efficiency (QE) is high [1]. In addition, the high-Q nanocavity accelerates
spontaneous emission from the Cu-IECs, which is known as the Purcell effect, and enhances
the light-impurity interaction.
We have developed a novel method for doping the Cu-IECs to a SOI wafer. The Si PhC
nanocavities were fabricated on the Cu-doped SOI wafer. Figure 1 shows photoluminescence(PL) spectra for the cavity with Q = 7200 and PL decays of the Cu-IECs. At 1227.5 nm,
the emission peak of the Cu-IECs is found. The blue-side peak originates from the cavity
resonance. When the cavity resonance approaches to the Cu-IEC line, the PL intensity andthe decay rate of the Cu-IECs increase and take maximum values at zero detuning. Figure 2
shows PL decays and decay rates of the Cu-IECs in resonant cavities as a function of the Q/
V value of the cavity. It is found that the decay rate is almost proportional to Q/V. This result
proves the Purcell effect on the Cu-IECs because the theoretical estimation agrees well to theexperimental data. In the highest-Q cavity (Q = 16000), the PL lifetime of the Cu-IECs is 1.1
ns, which is 30 times shorter than that in an unpatterned SOI sample. Since this PL lifetime issufficiently smaller than the nonradiative lifetime around 40 ns, the QE may close to unity.
These results enable to develop novel fast and energy-efficient Si light emitting devices andquantum optical devices based on impurity qubits in Si.
[1] S. P. Watkins et al., Solid State Commun. 43 (1982) 687.

Fig. 1. (a) PL spectra for Cu-doped nanocavity. Inset
shows the intensity of the Cu-IEC line.
(b) PL decays and decay rates of the Cu-IECs.

Fig. 2. (a) PL decays of the Cu-IECs in resonantcavities. (b) PL decay rates as a function of
the Q/V values of the cavities. V is a mode
volume.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)43

Purcell-Enhanced Single Photon Emission at Telecom Wavelengths from
Quantum-Dot Nanocavities
Muhammad Danang Birowosuto1, Hisashi Sumikura1,2, Shinji Matsuo2,3, Hideaki Taniyama1,2,
and Masaya Notomi1,2
1
2
Optical Science Laboratory, Nanophotonics Center, 3NTT Photonics Laboratories
A telecom-band (1.55 µm) single photon source is an important feature for the quantum
communication networks, quantum repeaters, and all optical quantum information processing
via optical fibers as this wavelength provides the lowest loss for long-distance transmission.
Single InAs/InP quantum dot (QD) is an attractive emitter while a coupling of this single QD
with a cavity yields a faster and brighter single photon source [1]. Single photon emission at
1.55 µm with shortest radiative recombination time of 1.12 ns from a single InAs QD in InP
optical horn was previously reported [2]. However, there is no report for the single photon at
1.55 µm from the coupling between a single QD and a resonant cavity.
We demonstrate a fast single photon source at 1.55-µm telecom band from the couplingof the single QD with the cavity of the photonic crystals [3]. We combine the coupling of
the biexciton (XX) of the single QD and the cavity, which may yield a faster single photon
source than that of the exciton (X). Fig. 1 exhibits temperature-dependent photoluminescence
spectra. The cavity is a line defect with hole-distance modulations. From the fit of the cavity
peak, we obtain an experimental Q of 2,000. We observe that the QD biexciton and the cavity
are in resonance at 22 K. At this temperature, we measure a very fast emission of 0.2 ns dueto the Purcell effect with the five-fold emission rate enhancement, see Fig. 2. Photon intensity(2)
correlation measurement shows an antibunching with g (0) of 0.1. Such fast single photon
source at telecom band has a potential to improve the modulation rate of the long-distancequantum networks.
[1] A. J. Shields, Nature Photon. 1 (2007) 215.
[2] K. Takemoto et al., Appl. Phys. Express 3 (2010) 092802.
[3] M. D. Birowosuto et al., Sci. Rep. 2 (2012) 312.

Fig. 1. Photoluminescence spectra at various
temperatures for single QD in the cavity of
PhC. X and XX denote the emission fromexciton and biexciton,respectively.

44

Fig. 2. Time resolved luminescence for single QD in
the reference (squares) and the cavity (circles)of the PhC. The white lines are the exponential
fits. The inset shows an antibunching at 22 K.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Lambda-Scale Embedded Active-Region Photonic-Crystal (LEAP) Lasers
Tomonari Sato1,2, Koji Takeda1,2, Akihiko Shinya1,3, Kengo Nozaki1,3, Hideaki Taniyama1,3,
Koichi Hasebe1,2, Takaaki Kakitsuka1,2, Masaya Notomi1,3, and Shinji Matsuo1,2
1
Nanophotonics Center, 2NTT Photonics Laboratories, 3Optical Science Laboratory
Photonic networks on CMOS chips have attracted a lot of attention as regards increasingtransmission capacity, which is limited by the power consumption of electrical interconnects.
The optical devices used for a photonic network on a CMOS chip are required to operate atan energy consumption of less than 10 fJ/bit. In this context, we have developed a photoniccrystal (PhC) nanocavity laser in which a small active region is embedded within an InP-basedline defect waveguide. We call this a Lambda-scale Embedded Active-region PhC (LEAP)
laser [1]. In this report, we report a LEAP laser with the lowest threshold current and energy
consumption of any semiconductor laser operating at above room temperature, which werealized by reducing the leakage current.
Figure 1 shows a scanning electron microscope (SEM) image of a LEAP laser. An ultrasmall active region, which consists of an InGaAlAs quantum-well structure, is embedded
in a line defect in a 250 nm-thick InP PhC slab. The volume is 0.12 µm3. To reduce theleakage current through the sacrificial layer and the substrate, we change the sacrificial layerfrom InGaAs to InAlAs, because the bandgap of an InAlAs layer is larger than those of InP
and InGaAs layers. We fabricate a lateral p-i-n junction by using Zn diffusion and Si ion
implantation to inject carriers into the active region.
Figure 2 shows the light versus current characteristic and the spectrum under continuouswave (CW) operation at 25°C. The threshold current and the maximum output power of theoutput waveguide were 7.8 µA and 9 µW, respectively. The lasing wavelength was 1567.8
nm at an injection current of 20 µA. We significantly reduced the threshold current by using
InAlAs for the sacrificial layer because the threshold current of a LEAP laser with an InGaAs
sacrificial layer was 390 µA. In addition, 12.5-Gbit/s direct modulation was achieved at an
injection power of 174 µW and then the energy consumption was estimated to be 14 fJ/bit.
Furthermore, we achieved high temperature operation up to 95°C with a LEAP laser [2].
This work was supported by NEDO.
[1] S. Matsuo et al., Optics Express 20 (2012) 3773.
[2] T. Sato et al., IEEE Photonics Conference (2012) WF-2.

Fig. 1. SEM image of LEAP laser.

Fig. 2. Light-current characteristic of LEAP laser.
Inset shows a spectrum at a current of 20 µA.
Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)45

22-Gbit/s × 16-ch WDM Receiver
Based on a Si-Ge-silica Monolithic Photonic Platform
Hidetaka Nishi1,2, Rai Kou1,2, Tatsuro Hiraki1,2, Tai Tsuchizawa1,2, Hiroshi Fukuda1,2,
Kotaro Takeda1,2, Yasuhiko Ishikawa3, Kazumi Wada3, and Koji Yamada1,2
1
Nanophotonics Center, 2NTT Microsystem Integration Laboratories, 3University of Tokyo
Towards the next generation of broad-band optical networks, systems based on wavelengthdivision multiplexing (WDM) are being discussed to offer flexible bandwidth allocation,
which contributes to energy-efficient network operation. Silicon (Si) photonics WDM
receivers are expected to provide small, cost-effective photonic-electronic integrated circuitswith multiple channels and functionalities. Recently, we have developed an integrated WDM
receiver on our unique Si-germanium (Ge)-silica monolithic platform [1], and demonstrated
long-distance signal transmission with the fabricated device.
Figure 1 is a top-view of a fabricated WDM receiver. The arrayed-waveguide grating (AWG)
wavelength filter comprises silicon-oxide (SiOx) waveguides with a reflective index contrastof ~3 %. The waveguides are suitable for telecommunications applications because of their
low loss and small temperature and polarization dependence. The AWG was connected to Si
waveguides with the Ge photodetectors (PDs) using spot-size converters [2]. The Ge layer
was grown by the UHV-CVD method [3]. SiOx and SiO2 layers for the AWG were depositedby the ECR plasma-enhanced CVD method, in which the process temperature lower than
200 degrees Celsius ensures that the AWG was constructed over the Ge layer without thermal
damage. The footprint is about 1 cm2. The insertion loss of the AWG is about 5.1 dB. The
performance of Ge PD is as follows: dark current of 245 nA at -2 V, responsivity of about
1.2 A/W, and 3-dB down bandwidth of over 15 GHz, which is sufficient for over-20-Gbit/s
data transmission. Figure 2 shows measured fiber-to-PD responsivity spectra, including fibercoupling loss. The responsivity is 0.29 A/W and inter-channel crosstalk is -22 dB.
Next, we prepared 12.5-Gbit/s NRZ PRBS 231-1 signal and input it into standard singlemode fibers with dispersion of 17 ps/km/nm at distances of 20, 40, and 60 km. After that,the signal was input into the AWG-PD device by butt coupling of a high-NA fiber. Figure 3
shows measured bit-error rate (BER) curves for various distances and a received eye pattern
after 40-km transmission. We confirmed error-free signal transmission up to 40 km. The
receiver sensitivity measured at the AWG input was -6.8 dBm for the BER of 10–9 after 40-km
transmission. The experiment was performed at room temperature without thermal control. To
the best of our knowledge, this is the first demonstration of long-distance signal transmission
using Si photonic WDM receiver.
[1] H. Nishi et al., Opt. Express 20 (2012) 9312.
[2] T. Tsuchizawa et al., J. Sel. Top. Quantum Electron. 17 (2011) 516.
[3] S. Park et al., IEICE Trans. Elect. E91-C (2007) 181.

Fig. 1. Top view.
46

Fig. 2. Responsivity spectra.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Fig. 3. BER curves.

Ⅱ．DataScience Plaza 2012
"Science Plaza 2012", an open-house event of NTT Basic Research Laboratories (BRL), washeld at NTT Atsugi R&D Center on Friday, December 14th, 2012. Under the banner "Frontier
Science: Open Door to the Future", Science Plaza aimed to disseminate our latest researchaccomplishments through various sections of people inside and outside of NTT and to gather
diverse opinions.
Following an opening address from Dr. Toshiki Makimoto, the director of BRL, oneof distinguished technical members of BRL, Dr. Shiro Saito gave a lecture on "Quantum
Information Processing using Superconductivity ― Development of Quantum Memory for
Superconducting Quantum Bits". In the afternoon session, Prof. Koichi Kitazawa, Adviser to
President, Japan Science and Technology Agency, gave a special lecture entitled "Renewableenergies in the world". Each lecture was well-attended and followed by heated question-andanswer sessions.
As regards the poster exhibits, 52 posters, including 21 from NTT Microsystem Integration
Laboratories, Photonics Laboratories, Communication Science Laboratories, and Energy
and Environment Systems Laboratories presented our latest research accomplishments.
While explaining the originality and impact as well as the future prospects of our researchaccomplishments, these posters were intensively discussed, and many meaningful opinions
were heard. This year’s "Lab Tour" ― a guided tour of research facilities at NTT BRL thathas been receiving high reputation from visitors over the years ― took place at four different
labs, so that as many people as possible could join the tour. This time we also opened a boothto show the NTT R&D recruitment system for job-seeking researchers and students. After all
lectures, presentations, and exhibitions, a banquet was held in Center’s dining room, where
lively conversation among participants deepened their amity.
About 180 people from research institutes, universities, and general industries, as well asfrom NTT Group, attended Science Plaza 2012. Thanks to the efforts of all participants, theconference ended on a high note. We would thus like sincerely to express our gratitude to all of
the participants.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)47

The 7th Advisory Board (Fiscal 2012)
The Advisory Board, an external committee whose role is to evaluate the work of NTT Basic
Research Laboratories (BRL), met from January 28 - 30, 2013. This was the seventh meetingof the Advisory Board, which was first convened in 2001 to provide an objective evaluationof our research plans and activities to enable us to employ strategic management in a timely
manner. On this occasion, we were happy to welcome six new members.
Over the course of the three days the board made valuable suggestions and comments inrelation to our research and management activities. They commented that the research levelis generally high on an international scale, and that it is important for us to maintain this toplevel research and transmit information about our research achievements to the world. They
also raised several issues related to human resources, the research budget and internal andexternal collaboration. We plan to make improvements based on these valuable suggestions.
At this meeting, the BRL researchers had a lunch with the board members and a poster
session, where they had chances to present their researches to the board members. The
poster session was performed with the dinner party where the board members and the BRL
researchers were able to interact in a casual atmosphere. For the BRL and NTT executives,
we organized a Japanese style dinner, which provided a good chance to discuss the future
management strategy of NTT BRL from an international perspective. The next board meeting
will be held in two years.

Board members
Prof. Abstreiter
Prof. Clarke
Prof. Hu
Prof. Jonson
Prof. Sir Knight
Prof. Leggett
Prof. MacDonald
Prof. Offenhäusser
Prof. Rubinsztein-Dunlop
Prof. von Klitzing

48

Affiliation
Walter Schottky Inst.
Univ. California, Berkeley
Harvard Univ.
Göteborg Univ.
Imperial College
Univ. Illinois
Univ. Texas
Inst. Complex Systems
Univ. Queensland
Max-Planck-Inst.

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Research field
Low-dim. physics
Superconductor physics
Nanodevice
Condensed matter
Quantum optics
Quantum physics
Condensed matter
Nano-bio electronics
Quantum electronics
Semiconductor physics

Award Winners’ List (Fiscal 2012)
The Commendation for
Science and Technology by
the Minister of Education,
Culture, Sports, Scienceand Technology, The Young
Scientists’ Prize

JSAP Outstanding
Presentation Award ― 2012

International conference
on topological quantum
phenomena 2012
Poster award and poster
preview award

Fifth International
Conference on Optical,
Optoelectronic and Photonic
Materials and Applications
Poster Paper Award

N. Kumada

Many-body physics in twodimensional systems formedin semiconductors

Apr. 17,2012

N. Matsuda

On-chip polarizationentangled photon pair source

May 11,2012

H. Irie

Josephson characteristics ofsuperconducting quantum
point contact

May 20,2012

Fabrication of whispering
gallery mode cavities using
crystal growthJune 3,2012

H. Kudo (*)
Y. Ogawa (*)
T. Tanabe (*)
A. Yokoo(*) Keio University

JSAP Young Scientist Award

S. Tanabe

Half-Integer Quantum Hall
Effect in Gate-Controlled
Epitaxial Graphene Devices

Sep. 11,2012

Photonics in Switching 2012
Award for SC1 best paper

K. Nozaki
A. Shinya
M. Notomi
S. Matsuo
T. SatoY. Suzaki
T. SegawaR. Takahashi

First demonstration of 4-bit,
40-Gb/s optical RAM chipusing integrated photonic
crystal nanocavities

Sep. 13,
2012

H. Hibino

Characterization and growth
control of low-dimensional
materials using low-energy
electron microscopy

Sep. 27,2012

The 141st Committee on
Microbeam Analysis of Japan
Society for the Promotion of
Science Sakaki Award

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)49

International Workshop on
Nitride Semiconductors 2012
Best Paper Award

Y. Taniyasu
J.-F. Carlin*
A. Castiglia*
R. Butté*N. Grandjean*

AlInN/GaN MQW UV-LEDs

Oct. 19,2012(*) Ecole PolytechniqueFédéral de Lausanne

Fellowship of the American
Physical Society

W. J. Munro

For extensive contributions toapplied quantum information

Nov. 3,2012

26th Diamond Symposium
Oral Session Best Award

K. Hirama
H. SatoY. Harada
H. Yamamoto
M. KasuNO2-adsorbed H-terminateddiamond FETs thermallystabilized by Al2O3 passivation

Nov. 20,
2012

IEEE Fellow

M. Notomi

For leadership in thedevelopment of photonic
crystals and applications

Jan. 1,
2013

Hot Article Award, Analytical
Sciences

Y. UenoK. Furukawa
K. Hayashi
M. Takamura
H. Hibino
E. Tamechika

Graphene-modified
Interdigitated Array Electrode:
Fabrication, Characterization,
and Electrochemical
Immunoassay Application

Jan. 10,2013

33th JSAP Outstanding
Presentation Award ― 2013

H. Sumikura

Purcell effect on copper
isoelectronic centers in silicon
photonic crystal cavities

Mar. 27,
2013

Y. Krockenberger

Molecular Beam Epitaxy
and Transport Properties ofInfinite-Layer Sr0.90La0.10CuO2Thin Films

Mar. 28,
2013

Superconductivity Divisionof Japan Society of Applied
Physics
Young Scientist Award for an
Excellent Article

50

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)In-house Award Winners’ List (Fiscal 2012)
NTT Science and CoreTechnology Laboratory
Group Director Award

Y. Kobayashi
K. Kumakura
T. Akasaka
Y. Krockenberger
H. Yamamoto

Development of a process for releaseand transfer of GaN-based devices

Dec. 19,2012

NTT Science and CoreTechnology Laboratory
Group Director Award

T. SatoK. Takeda
A. Shinya
K. Nozaki
S. Matsuo

Current-injection photonic-crystal
laser for high density opticalinterconnection

Dec. 19,2012

NTT Science and CoreTechnology Laboratory
Group Director Award

S. Tsukada
H. Nakashima
A. Shimada
K. Sumitomo

Invention of the conductive polymer
combined thread for the biomedical
electrodes

Dec. 19,
2012

BRL Director Award
Award for Achievements

Y. Kobayashi
K. Kumakura
T. Akasaka

Development of a process for releaseand transfer of GaN-based devices
using layered BN

Mar. 25,
2013

BRL Director Award
Award for Achievements

S. Tsukada
H. Nakashima
A. Shimada
K. Torimitsu
K. Sumitomo

Development of the conductive
polymer combined Silk fiber for thebiomedical electrodesMar. 25,
2013

BRL Director Award
Award for Achievements

I. Mahboob
H. Okamoto
D. Hatanaka
K. Nishiguchi
K. Onomitsu
A. Fujiwara
H. Yamaguchi

Proposition and realization of novel
devices based on compound
semiconductor electromechanicalresonators

Mar. 25,
2013

BRL Director Award
Award for Encouragement

S. TanabeContribution to clarifying electronictransport properties of epitaxial
graphene on SiC

Mar. 25,
2013

BRL Director Award
Award for Excellent Papers

K. Nozaki
A. Shinya
S. Matsuo
Y. Suzaki
T. SegawaT. SatoY. Kawaguchi
R. Takahashi
M. Notomi"Ultralow-power all-optical RAM
based on nanocavities"
Nature Photonics 6, 248-252 (2012).

Mar. 25,
2013

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

51

BRL Director Award
Award for Excellent Papers

BRL Director Award
Award for Excellent Papers

52

N. Matsuda
H. Le Jeannic
H. Fukuda
T. Tsuchizawa
W. J. Munro
K. Shimizu
K. Yamada
Y. Tokura
H. Takesue"A monolithically integrated
polarization entangled photon pairsource on a silicon chip"
Scientific Reports 2, 817 (2012)

Mar. 25,
2013

K. Azuma
G. Kato"Optimal entanglement manipulation
via coherent-state transmission"
Physical Review A 85, 060303(R)(2012)

Mar. 25,
2013

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

Numbers of Papers, Presentations and Patents (2012)
The numbers of papers published in international journals, presentations at international
conferences and applied patents in year 2012 amounted to 129, 163, and 57, respectively. The
numbers for each research area are as follows;
Papers Published in International Journals(Jan. – Dec. 2012)Materials Science

42

Physical Science

41

Optical Science

31

Nanophotonics Center

15
0

10

20

30

40

50

60

Number of Papers

Presentations at International Conferences(Jan. – Dec. 2012)Materials Science44

Physical Science

67

Optical Science28

Nanophotonics Center

24
0

20

40

60

80

100

Number of Presentations

Applied Patents (Jan. – Dec. 2012)Materials Science27

Physical Science

18

Optical Science

8

Nanophotonics Center

4
0

5

10

15

20

25

30

Number of Applied Patents

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)53

The numbers of research papers published in the major journals are shown below.
(IF2011*)

Numbers

Japanese Journal of Applied Physics

1.058

15

Applied Physics Letters

3.844

11

Physical Review B

3.691

11

Applied Physics Express

3.013

10

Physical Review A

2.878

7

Journal of Applied Physics

2.168

6

Optics Express

3.587

5

Nano Letters13.1983

Nature Photonics

29.2782

Physical Review Letters

7.37

2

The Journal of Physical Chemistry Letters

6.213

2

Langmuir

4.186

2

New Journal of Physics

4.177

2

Nature36.28

1

Science31.2011Nature Nanotechnology

27.27

1

Nature Physics

18.967

1

Advanced Materials

13.877

1

ACS Nano

11.4211Nature Communications

7.396

1

Biosensors and Bioelectronics

5.602

1

Carbon5.378

1

PLoS One

4.092

1

Journals*IF2011: Impact Factor 2011 (Thomson Reuters Journal Citation Reports® 2011)

The average IF2011 for all research papers from NTT Basic Research Laboratories is 4.74.

The numbers of presentations in the major conferences are shown below.
Conferences

54

Numbers

31st International Conference on the Physics of Semiconductors

202012 International Conference on Solid State Devices and Materials

10

The 17th International Conference on Molecular Beam Epitaxy

7

International Workshop on Nitride Semiconductors 20126

11th International Conference on Quantum Communication, Measurement and Computing

42012 Materials Research Society Spring Meeting and Exhibit

4

The 20th International Conference on High Magnetic Fields in Semiconductor Physics

4

25th International Microprocesses and Nanotechnology Conference

32012 Workshop on Innovative Nanoscale Devices and Systems

3

The American Physical Society March Meeting 20123

New Diamond and Nano Carbons Coference 20123

The 7th International Conference on Physics and Applications of Spin-related Phenomena in
Semiconductors

3

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

List of Invited Talks at International Conferences (2012)
I. Materials Science Laboratory(1)

Y. Kashimura, K. Furukawa, and K. Torimitsu, "Control of molecule transport utilizing lipid bilayer selfspreading on micro/nano-patterned surface", The 6th International Symposium on Medical, Bio- and NanoElectronics, Sendai, Japan (Mar. 2012).

(2)

K. Hirama, M. Kasu, and Y. Taniyasu, "Nitride/diamond heterostructure: growth and device application",
New Diamond and Nano Carbons Conference (NDNC 2012), San Juan, Puerto Rico (May 2012).

(3)

N. Kasai, A. Shimada, S. Tsukada, A. Tanaka, Y. Kashimura, K. Sumitomo, and K. Torimitsu, "Micro/
nano-fabrication for biological interface", The 7th International Symposium on Organic MolecularElectronics (ISOME2012), Musashino, Japan (June 2012).

(4)

A. D. Malay, J. G. Heddle, S. Tomita, K. Iwasaki, N. Miyazaki, K. Sumitomo, H. Yanagi, I. Yamashita,
and Y. Uraoka, "A gold nanoparticle-catalysed artificial protein capsid", The 6th International Conference
on Gold Science Technology and its Applications (GOLD2012), Tokyo, Japan (Sep. 2012).

(5)

Y. Kobayashi, K. Kumakura, T. Akasaka, H. Yamamoto, and T. Makimoto, "Layered boron nitride asa release layer for mechanical transfer of GaN-based devices", Crystal & Graphene Science Symposium,
Boston, U.S.A. (Sep. 2012).

(6)

H. Hibino, S. Tanabe, and H. Kageshima, "Carrier transport in epitaxial and quasi-freestanding grapheneon SiC", International Union of Materials Research Societies-International Conference on Electronic
Materials (IUMRS-ICEM2012), Yokohama, Japan (Sep. 2012).

(7)

T. Akasaka, H. Goto, Y. Kobayashi, and H. Yamamoto, "Extremely narrow photoluminescence linefrom ultrathin InN single quantum well on step-free GaN surface", International Workshop on Nitride
semiconductors, Sapporo, Japan (Oct. 2012).

(8)

Y. Kobayashi, K. Kumakura, T. Akasaka, H. Yamamoto, and T. Makimoto, "Layered boron nitride
as a release layer for mechanical transfer of GaN-based devices", International Workshop on Nitride
semiconductors, Sapporo, Japan (Oct. 2012).

(9)

T. Akasaka, Y. Kobayashi, C. H. Lin, and H. Yamamoto, "Study of nucleus and spiral growth mechanisms
of GaN using selective-area MOVPE on GaN bulk substrate", Intensive Discussion on Growth of Nitride
Semiconductors, Sendai, Japan (Oct. 2012).

(10)

Y. Ueno, K. Furukawa, K. Matsuo, K. Hayashi, S. Inoue, H. Hibino, and E. Tamechika, "Aptamer
Modified Graphene Oxide Installed in a Microchannel Device: A New Lab-on-a-chip Protein Detection
System", 6th International Symposium on Nanomedicine (ISNM2012), Matsue, Japan (Nov. 2012).

(11)

K. Furukawa and H. Hibino, "AFM Study of Collision of Self-spreading Lipid Bilayers within MicroPattern", 20th International Colloquium on Scanning Probe Microscopy, Naha, Japan (Dec. 2012).

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)55

II. Physical Science Laboratory

56

(1)

S. Saito, X. Zhu, Y. Matsuzaki, W. J. Munro, M. S. Everitt, T. Shimooka, N. Mizuochi, K. Nemoto, and K.
Semba, "Coherent coupling between a superconducting flux qubit and a spin ensemble", Pathbreaking Phase
Sciences in Superconductivity 2012 (PPSS2012), Osaka, Japan (Jan. 2012).

(2)

K. Nishiguchi, "Nanoscale electronic devices with low power consumption and high functionality", The
Japan-France Frontiers of Engineering Symposium 2012 (JFFoE2012), Kyoto, Japan (Feb. 2012).

(3)

X. Zhu, S. Saito, A. Kemp, K. Kakuyanagi, S. Karimoto, H. Nakano, W. J. Munro, Y. Tokura, M. S.
Everitt, K. Nemoto, M. Kasu, N. Mizuochi, and K. Semba, "Coherent coupling of a superconducting flux
qubit to an electron spin ensemble in diamond", Quantum Information and Measurement (QIM2012), Berlin,
Germany (Mar. 2012).

(4)

L. Tiemann, G. Gamez, N. Kumada, and K. Muraki, "NMR Investigation of the v = 5/2 Fractional
Quantum Hall State", International conference on topological quantum phenomena 2012 (TQP2012),
Nagoya, Japan (May 2012).

(5)

H. Yamaguchi, I. Mahboob, and H. Okamoto, "Piezoelectric parametric resonators", NEMS-Barcelona,
Barcelona, Spain (May 2012).

(6)

A. Fujiwara, G. Yamahata, K. Nishiguchi, G. P. Lansbergen, and Y. Ono, "Silicon Single-ElectronTransfer Devices: Ultimate Control of Electric Charge", Silicon Nanoelectronics Workshop (SNW2012),
Hawaii, U.S.A. (July 2012).

(7)

L. Tiemann, G. Gamez, N. Kumada, and K. Muraki, "Nuclear magnetic resonance studies of the electron
spin polarisation in the N = 0 and N = 1 Landau levels", The 20th International Conference on High
Magnetic Fields in Semiconductor Physics (HMF20), Chamonix Mont-Blanc, France (July 2012).

(8)

H. Yamaguchi, I. Mahboob, and H. Okamoto, "Parametric mode-coupling and its application to signal
processing in GaAs/AlGaAs electromechanical resonators", 31st International Conference on the Physics of
Semiconductors (ICPS2012), Zurich, Switzerland (July 2012).

(9)

S. Saito, X. Zhu, Y. Matsuzaki, W. J. Munro, K. Kakuyanagi, T. Shimooka, N. Mizuochi, K. Nemoto,and K. Semba, "Coherent operations in a superconductor-diamond hybrid system", 19th Central European
Workshop on Quantum Optics (CEWQO2012), Sinaia, Romania (July 2012).

(10)

H. Yamaguchi, I. Mahboob, H. Okamoto, and K. Onomitsu, "Semiconductor Heterostructures for Novel
Electromechanical Devices", Materials Science Week 2012 (MSW2012), Sendai, Japan (Nov. 2012).

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

III. Optical Science Laboratory(1)

Y. Tokura, "Coherent control and detection of spin qubits in semiconductor with magnetic fieldengineering", APS March Meeting, Boston, U.S.A. (Feb. 2012).

(2)

W. J. Munro and K. Nemoto, "Inferring superposition and entanglement in evolving systems", The 2nd
Institute of Mathematical Statistics Asia Pacific Rim Meeting, Tsukuba, Japan (July 2012).

(3)

M. Yamashita and K. Inaba, "Diverse quantum phases of Bose-Fermi mixtures trapped in an opticallattice", The 21st International Laser Physics Workshop (LPHYS'12), Calgary, Canada (July 2012).

(4)

K. Oguri, H. Nakano, Y. Okano, T. Nishikawa, K. Kato, A. Ishizawa, T. Tsunoi, H. Gotoh, K. Tateno, andT. Sogawa, "Ultrafast diagnostics of photo-excited processes in solid using femtosecond laser-based soft
X-ray pulse sources", 8th International Conference on Photo-Excited Processes and Applications (ICPEPA-8),
Rochester NY, U.S.A. (Aug. 2012).

(5)

W. J. Munro and K. Nemoto, "Weak force detection using superposed coherent states", International
Workshop on Entangled Coherent State and Its Application to Quantum Information Science - Towards
Macroscopic Quantum Communication, Tamagawa University, Tokyo, Japan (Nov. 2012).

(6)

K. Azuma, "Entanglement shared via coherent-state transmission", Quantum Science Symposium 2012,
Cambridge, U.K. (Nov. 2012).

(7)

T. Mukai, "Superconducting Atom Chip in NTT", The 2nd Atom Chip Workshop in Pangkil, Indonesia(Dec. 2012).

(8)

H. Shibata, "Fabrication of Superconducting Strip Photon Detector using MgB2", International Workshop
on Superconducting Sensors and Detectors (IWSSD2012), Daejeon, Korea (Dec. 2012).

(9)

W. J. Munro, S. Saito, X. Zhu, R. Amsuss, Y. Matsuzaki, K. Kakuyanagi, T. Shimooka, N. Mizuochi, K.
Nemoto, and K. Semba, "Realizing Quantum Memories for Superconducting Qubits: Storage and Retrievalof Entangled Quantum States", EQUS Annual Workshop, Wollongong, Australia (Dec. 2012).

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)57

IV. Nanophotonics Center

58

(1)

M. Notomi, K. Nozaki, S. Matsuo, A. Shinya, T. Sato, and H. Taniyama, "fJ/bit integrated nanophotonics
towards dense photonic network on chip", SPIE Photonics West 2012, San Francisco, U.S.A. (Jan. 2012).

(2)

S. Matsuo, "High-speed directly modulated buried heterostructure photonic crystal lasers", Photonic
West 2012, San Francisco, U.S.A. (Jan. 2012).

(3)

M. Notomi, K. Nozaki, S. Matsuo, A. Shinya, T. Sato, and H. Taniyama, "fJ/bit integrated nanophotonics
for photonic network on chip", The Fourth International Conference on Metamaterials, Photonic Crystals,
and Plasmonics (META'12), Paris, France (Apr. 2012).

(4)

M. Notomi, K. Nozaki, S. Matsuo, A. Shinya, E. Kuramochi, T. Sato, H. Taniyama, and C-H. Chen,"Nonlinear Optical Functions of Photonic Crystals for Ultralow-power Photonic Processing", CLEO: 2012,
San Jose, U.S.A. (May 2012).

(5)

M. Notomi, K. Nozaki, S. Matsuo, A. Shinya, J. Kim, K. Takeda, E. Kuramochi, T. Sato, and H.
Taniyama, "Nanophotonic Devices Based on Photonic Crystal", International Conference on Photonic andElectromagnetic Structures (PECS-X), Santa Fe, U.S.A. (June 2012).

(6)

A. Yokoo, T. Tanabe, E Kuramochi, and M. Notomi, "Fabrication of photonic crystal cavity by using ananoprobe", The 7th International Symposium on Organic Molecular Electronics, Tokyo, Japan (June 2012).

(7)

K. Nozaki, A. Shinya, S. Matsuo, T. Sato, Y. Suzaki, T. Segawa, R. Takahashi, and M. Notomi,"Integrable ultralow-power nanophotonic devices on InP photonic crystals", OSA Integrated Photonics
Research, Silicon and Nano-Photonics (IPR2012), Colorado Springs, U.S.A. (June 2012).

(8)

T. Tsuchizawa, H. Nishi, R. Kou, H. Fukuda, H. Shinojima, Y. Ishikawa, K. Wada, and K Yamada,"Silicon-silica Monolithic Photonic Integration for Telecommunications Applications", Integrated Photonics
Research, Silicon and Nano Photonics (IPR2012), Colorado, U.S.A. (June 2012).

(9)

M. Notomi, K. Nozaki, S. Matsuo, A. Shinya, E. Kuramochi, T. Sato, J. Kim, K. Takeda, and H.
Taniyama, "Ultralow-power integrated nanophotonics for future chips", The 17th Opto Electronics andCommunications Conference (OECC2012), Busan, Korea (July 2012).

(10)

K. Yamada, T. Tsuchizawa, H. Nishi, R. Kou, H. Shinojima, H. Fukuda, T. Hiraki, Y. Ishikawa, andK. Wada, "Silicon Photonics for Telecommunications", The 17th Opto Electronics and Communications
Conference (OECC2012), Busan, Korea (July 2012).

(11)

M. Notomi, K. Nozaki, S. Matsuo, A. Shinya, E. Kuramochi, T. Sato, K. Takeda, J. Kim, H. Taniyama,"Ultralow-power nanophotonic devices based on buried-heterostructure photonic-crystal nanocavities",
24th International Conference on Indium Phosphide and Related Materials (IPRM2012), Santa Barbara,
U.S.A. (Aug. 2012).

(12)

M. Notomi, K. Nozaki, S. Matsuo, A. Shinya, E. Kuramochi, T. Sato, A. Yokoo, J. Kim, K. Takeda, and
H. Taniyama, "fJ/bit Integrated Nanophotonics based on Photonic Crystals", International Conference on
Electonic Materials-International Union of Material Researh Societies (IUMRS-ICEM2012), Yokohama,Japan (Sep. 2012).

(13)

K. Nozaki, A. Shinya, S. Matsuo, T. Sato, K. Takeda, C-H. Chen, Y. Suzaki, T. Segawa, R. Takahashi,
and M. Notomi, "Integrable All-Optical Random-Access Memories on InP-based Photonic Crystal
Platform", IEEE Photonics Conference (IPC2012), Burlingame, U.S.A. (Sep. 2012).

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)

(14)

K. Yamada, T. Tsuchizawa, H. Nishi, R. Kou, T. Hiraki, H. Fukuda, Y. Ishikawa, and K. Wada, "Silicon/
Ge/Silica Monolithic Photonic Integration for Telecommunications Applications", 2012 International
Conference on Solid State Devices and Materials (SSDM2012), Kyoto, Japan (Sep. 2012).

(15)

S. Matsuo, "Ultra-low operating energy lasers and switches for optical interconnection", Photonic in
Switching 2012, Ajaccio-Corsica, France (Sep. 2012).

(16)

S. Matsuo, K. Takeda, T. Sato, M. Notomi, A. Shinya, K. Nozaki, H. Taniyama, K. Hasebe, and T.
Kakitsuka, "Electrically-pumped Photonic Crystal Lasers for Optical Communications", The 38th European
Conference on Optical Communications (ECOC2012), Amsterdam, The Netherlands (Sep. 2012).

(17)

S. Matsuo, "Electrically driven photonic crystal nanocavity laser", IEEE 23rd International
Semiconductor Laser Conference (ISLC2012), San Diego, U.S.A. (Oct. 2012).

(18)

K. Yamada, T. Tsuchizawa, H. Nishi, R. Kou, H. Fukuda, T. Hiraki, Y. Ishikawa, and K. Wada,"Si/Ge/Silica Monolithic Photonic Integration Platform for Telecommunications Applications", Asia
Communications and Photonics Conference 2012 (ACP2012), Guangzhou, China (Nov. 2012).

(19)

S. Matsuo, "Electrically Driven Photonic Crystal Lasers for On-Chip Interconnect", The 2nd International
Symposium on Photonics and Electronics Convergence (ISPEC2012), Tokyo, Japan (Dec. 2012).

Research Activities in NTT Basic Research Laboratories Vol. 23 (2012)59