Research Act iv it ies in N TT B asi c R esear ch Labo ra t o rie s Vo l u m e 25

Research Activities in
NTT Basic ResearchLaboratories

J u l y 201 5
N T T B a s i c R es ea rc h L a b o ra t o ri es ,N i p p o n T el eg ra p h a nd T el ep ho ne C o rp o ra tion
http://www.brl.ntt.co.jp/

Fiscal 2014

Vol.

25

Cover Photograph: Over-100-bit Photonic Crystal Optical-RAM Chip
We are working on large-scale integration of optical-memory (o-RAM) by photonic crystal
technology which can break through the limit of size and power consumption in conventional
o-RAMs. Here we fabricated a 1.1-mm silicon photonic crystal chip in which over 100
nanocavities were integrated and realized successfully over-100-bit o-RAM operation with low
power for the first time (See page 45).

Microscope image of 128 nanocavities integrated in a silicon chip. Lower: Schematic of multi-bit o-RAM
action with wavelength division multiplexing.

Research Activities in NTT BRL
Editorial Committee
Principal of o-RAM operation. A bistable memory that holds/switches “0” and “1” states is realized by
fast carrier-plasma effect greatly enhanced in a high quality factor nanocavity.

NTT Basic Research Laboratories,
NTT Corporation  
3-1 Morinosato Wakamiya, Atsugi,Write and read-out operations of 4-bit random pulses in two different sets of sequential 4 o-RAMs(demonstrated in a similar integrated o-RAM chip in an InP-based photonic crystal).

  Kanagawa, 243-0198 Japan  

URL: http://www.brl.ntt.co.jp/

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, andmedicine, conduct basic research in the fields ofmaterials science, physical science and opticalscience.
Since our management principle is based on an"open door" policy, we are collaborating withmany universities and research institutes in Japan,
the US, Europe, and Asia as well as other NTT
laboratories. NTT-BRL regularly organizes
international conferences related to quantum
physics and nanotechnology at NTT Atsugi R&D
Center and also holds a "Science Plaza" to enhance public understanding of our activities and toencourage a frank exchange of opinions. Moreover, one of our missions is the education ofyoung researchers and we sponsor the biennial "BRL School", which boasts distinguishedresearchers as lecturers.
These activities enable us to realize our missions with respect to the promotion of advances inscience and the development of groundbreaking technology for NTT's business. Your continued
support will be greatly appreciated.

July, 2015

Tetsuomi Sogawa
Director
NTT Basic Research Laboratories

Contents
page◆ Cover♦ Over-100-bit Photonic Crystal Optical-RAM Chip
◆ Message from the Director◆ NTT Basic Research Laboratories Organogram ......................................................................... 1◆ Member List ....................................................................................................................................... 2
I. Research Topics◇ Overview of Research in Laboratories ................................................................................................. 17◇ Materials Science Laboratory .............................................................................................................. 19♦ Suppression of Self-Heating Effect in AlGaN/GaN HEMTs by Mechanical Transfer Using an
h-BN Release Layer♦ MBE Growth of Transition Metal Dichalcogenide Thin Films♦ Superconducting and Insulating Ground States in La2CuO4 Structural Isomers♦ Simultaneous Light Emissions from Erbium-Thulium Silicates on Silicon in the Second and Third
Telecommunications Bands♦ Raman Spectroscopic Visualization of Topological Defects in Chemical Vapor Deposition-Grown
Graphene by Isotope Labeling♦ On-Chip FRET Graphene Aptasensor for Multiple Protein Detection♦ Formation of a Suspended Lipid Membrane on a Microcavity Covered by a Thin SiO2 Layerwith a Nanohole Array♦ Cross-Sectional SEM Observation of the Interface between a Neuron and a Conductive Material♦ Platform for Low Noise Biosensors Using Microwells Sealed with Lipid Bilayers◇ Physical Science Laboratory ................................................................................................................ 28♦ Deviation from the Law of Energy Equipartition in a Small Dynamic-Random-Access Memory:
Analysis about Thermal Noise with Single-Electron Resolution♦ Electric Tuning of Direct/Indirect Optical Transitions in Silicon♦ High-Speed Single-Electron Transfer via a Single Trap Level in Si♦ Dynamic Control of Phonon Vibrations in a One-Dimensional Phononic Crystal♦ Two-Mode Squeezing in an Electromechanical Resonator♦ New Approach for a Long-Lived Quantum Memory♦ Resonant Edge Magnetoplasmons and Their Decay in Graphene♦ Josephson Coupling in Semiconductor/Superconductor Hybrid Quantum Point Contacts♦ NMR Probing of Wigner Solids in High Magnetic Fields◇ Optical Science Laboratory ................................................................................................................. 37♦ Flat-Band Ferromagnetism in the Multilayer Lieb Optical Lattice♦ Quantum Key Distribution over a 72 dB Channel Loss Using Ultralow Dark Count Superconducting
Detectors♦ On-Chip Generation and Demultiplexing of Quantum Correlated Photons Using Si-Silica
Monolithic Waveguide Platform♦ Loss-Tolerant Quantum Cryptography with Imperfect Sources♦ The Design of a Scalable and Distributed Quantum Information Processor in Diamond♦ Characterizing Inner-Shell Electron Motion with Isolated Attosecond Pulse♦ Multi-Stacked InAs/InP Heterostructure Nanowires Grown by Self-Assisted Vapor-Liquid-Solid Mode
♦ Energy Transfer in Er2O3 Epitaxial Thin Layers♦ Over-100-bit Integrated Optical RAM Chip
♦ Nanocavity-Enhanced Raman Scattering of Single-Walled Carbon Nanotubes♦ Emission Enhancement in a Nanowire Coupled to a Nanoantenna◇ Nanophotonics Center ......................................................................................................................... 48♦ Continuous-Wave Operation of Lambda-Scale Embedded Active-Region Photonic-Crystal
Lasers on Silicon♦ Mode MUX/DEMUX on Three-Dimensional SiOx-Waveguide Platform
II. Data◇ The 8th Advisory Board ....................................................................................................................... 51◇ Science Plaza 2014 .............................................................................................................................. 52◇ List of Visitors’ Talks ........................................................................................................................... 53◇ List of Award Winners ........................................................................................................................... 56◇ List of In-House Award Winners ........................................................................................................... 57◇ Numbers of Papers, Presentations and Patents (Fiscal 2013) .............................................................. 58◇ Numbers of Papers, Presentations and Patents (Fiscal 2014) .............................................................. 59◇ Publication List..................................................................................................................................... 61◇ List of Invited Talks at International Conferences (Jan. 2014 - Mar. 2015) ........................................ 68

NTT Basic Research Laboratories Organogram
As of March 31, 2015

Director
Dr. Tetsuomi Sogawa
Research Planning Section

Quantum and Nano Device Research

Executive Manager
Dr. Hideki Yamamoto

Senior Distinguished Researcher
Dr. Hiroshi Yamaguchi
Materials Science Laboratory
Executive Manager
Dr. Hiroki Hibino

Physical Science Laboratory
Executive Manager
Dr. Akira Fujiwara

Optical Science Laboratory
Executive Manager
Dr. Hideki Gotoh

Nanophotonics Center
Project Manager
Dr. Masaya Notomi

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)1

Member List
As of March 31, 2015(*/ left NTT BRL in the middle of the year)

NTT Basic Research Laboratories
Director

Dr. Tetsuomi Sogawa

Quantum and Nano Device Research

Senior Distinguished Researcher

Dr. Hiroshi Yamaguchi

Research Planning Section

2

Senior Research Scientist, Supervisor

Dr. Hideki Yamamoto

Senior Research Scientist, Supervisor

Dr. Hiroshi Nakashima

Senior Research Scientist, Supervisor

Dr. Shiro Saito

NTT Research Professor

Prof. Yasuhiro Tokura (University of Tsukuba)

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Materials Science Laboratory

Executive Manager

Dr. Hiroki Hibino
Dr. Fumihiko Maeda

Thin-Film Materials Research Group:
Dr. Hideki Yamamoto (Group Leader)
Dr. Kazuhide Kumakura
Dr. Hisashi Sato
Dr. Yoshitaka Taniyasu
Dr. Koji Onomitsu
Dr. Masanobu Hiroki
Dr. Shin-ichi Karimoto
Dr. Junichi Nishinaka
Dr. Ryan G. Banal*

Dr. Tetsuya Akasaka
Dr. Yoshiharu Krockenberger
Dr. Kazuyuki Hirama
Dr. Hironori Okumura

Low-Dimensional Nanomaterials Research Group:
Dr. Hiroki Hibino (Group Leader)
Dr. Fumihiko Maeda
Dr. Yuko Ueno
Dr. Satoru Suzuki
Dr. Hiroo Omi
Dr. Yoshiaki Sekine
Dr. Makoto Takamura
Dr. Carlo M. Orofeo*
Dr. Adel Najar

Dr. Kazuaki Furukawa
Dr. Ken-ichi Sasaki
Dr. Shengnan Wang
Dr. Manabu Ohtomo

Molecular and Bio Science Research Group:
Dr. Koji Sumitomo (Group Leader)
Dr. Shingo Tsukada
Dr. Nahoko Kasai
Dr. Toichiro Goto
Dr. Aya Tanaka
Dr. Tetsuhiko Teshima

Dr. Yoshiaki Kashimura
Dr. Azusa Oshima

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)3

Physical Science Laboratory

Executive Manager

Dr. Akira Fujiwara
Dr. Toshiaki Hayashi
Takeshi Karasawa

Nanodevices Research Group:
Dr. Akira Fujiwara (Group Leader)
Toru Yamaguchi
Dr.Toshiaki Hayashi
Hirotaka Tanaka
Dr. Jinichiro Noborisaka
Dr. Kensaku Chida
Hybrid Nanostructure Physics Research Group:
Dr. Hiroshi Yamaguchi (Group Leader)
Dr. Yuichi Harada
Dr. Kenji Yamazaki
Dr. Hajime Okamoto
Dr. Kosuke Kakuyanagi
Daiki Hatanaka
Dr. Hiraku Toida
Quantum Solid State Physics Research Group:
Dr. Koji Muraki (Group Leader)
Dr. Kiyoshi Kanisawa
Dr. Satoshi Sasaki
Dr. Kyoichi Suzuki
Dr. Takeshi Ota
Dr. Keiko Takase
Dr. Hiroshi Irie
Dr. Francois Couedo

4

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Dr. Katsuhiko Nishiguchi
Dr. Gento Yamahata

Dr. Imran Mahboob
Dr. Yuichiro Matsuzaki
Dr. Ryuichi Ota

Dr. Hiroyuki Tamura
Dr. Norio Kumada
Dr. Trevor David Rhone

Optical Science Laboratory

後藤部長

Executive Manager

Dr. Hideki Gotoh
Dr. Tetsuya Mukai

Quantum Optical State Control Research Group:
Dr. Kaoru Shimizu (Group Leader)Dr. Hiroki Takesue
Kazuhiro Igeta
Dr. Makoto Yamashita
Dr. Tetsuya Mukai
Dr. Nobuyuki Matsuda
Daisuke Hashimoto
Dr. Takahiro Inagaki
Dr. Kazuto Noda
Theoretical Quantum Physics Research Group:
Dr. William John Munro (Group Leader)
Dr. Kiyoshi Tamaki
Dr. Fumiaki Morikoshi
Dr. George Knee
Dr. Fabian Furrer
Quantum Optical Physics Research Group:
Dr. Hideki Gotoh (Group Leader)
Dr. Kouta Tateno
Dr. Takehiko Tawara
Dr. Atsushi Ishizawa
Dr. Guoqiang Zhang
Dr. Keiko Kato
Dr. Hiroki Mashiko
Dr. Yoji Kunihashi
Photonic Nano-Structure Research Group:
Dr. Masaya Notomi (Group Leader)
Dr. Akihiko Shinya
Dr. Atsushi Yokoo
Dr. Hideaki Taniyama
Dr. Hisashi Sumikura
Dr. Masato Takiguchi
Dr. Masaaki Ono
Dr. Devin Smith
Dr. Feng Tian

Dr. Hiroyuki Shibata
Dr. Kensuke Inaba
Dr. Hiromitsu Imai

Dr. Koji Azuma

Dr. Katsuya Oguri
Dr. Haruki Sanada
Dr. Kenichi Hitachi

Dr. Eiichi Kuramochi
Dr. Kengo Nozaki
Dr. Abdul Shakoor*

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

5

Nanophotonics Center

Project Manager

Dr. Masaya Notomi

Photonic Nano-Structure Research Team:
Dr. Akihiko Shinya
Dr. Atsushi Yokoo
Dr. Hideaki Taniyama
Dr. Hisashi Sumikura
Dr. Masato Takiguchi
Dr. Masaaki Ono
Dr. Takehiko Tawara
Dr. Hiroyuki Shibata
Dr. Kouta Tateno
Dr. Guoqiang Zhang
Nanostructured Device Research Team:Dr. Shinji Matsuo
Dr. Koji Yamada
Dr. Takaaki Kakitsuka
Dr. Masakazu Arai
Dr. Koichi Hasebe
Dr. Koji Takeda
Rai Kou (Rai Takahashi)
Dr. Kotaro Takeda
Tatsurou Hiraki
Ryo Nakao

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Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Dr. Eiichi Kuramochi
Dr. Kengo Nozaki
Dr. Hiroo Omi
Dr. Nobuyuki Matsuda

Dr. Tai Tsuchizawa
Dr. Tomonari Sato*
Hidetaka Nishi
Dr. Kota Okazaki
Takuro Fujii

Senior Distinguished Researchers
Masaya NOTOMI received his B.E., M.E. and Ph.D. degrees in applied
physics from The University of Tokyo, Japan in 1986, 1988, and 1997,
respectively. He joined NTT Optoelectronics Laboratories, Nippon
Telegraph and Telephone Corporation in 1988 and moved to NTT Basic
Research Laboratories in 1999. 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. In 1996-1997, he was a visiting researcher
of Linköping University, Sweden. He was a guest associate professor ofApplied Electronics in 2003-2009 and is currently a guest professor of
Physics in Tokyo Institute of Technology. He was appointed as SeniorDistinguished Researcher of NTT since 2010. He is currently a director of
NTT Nanophotonics Center and a group leader of Photonic Nanostructure
Research Group. He received IEEE/LEOS Distinguished Lecturer Awardin 2006, Japan Society for the Promotion of Science (JSPS) prize in 2009,
Japan Academy Medal in 2009, The Commendation for Science andTechnology by the Minister of Education, Culture, Sports, Science andTechnology (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 received his B.E., M.S. in physics and Ph.D.
degrees in engineering from Osaka University in 1984, 1986, and 1993,
respectively. He joined NTT Basic Research Laboratories, Nippon
Telegraph and Telephone Corporation in 1986 and has been engaged in
the study of compound semiconductor surfaces using electron diffraction
and scanning tunneling microscopy. His current interests aremicro/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(JSAP) in 2008 and 2009. He served as more than 40 committee membersof academic societies and international conferences. He was appointed as
Senior Distinguished Researcher of NTT since 2011. He is currently an
executive manager of Quantum and Nano Device Research and a groupleader of Hybrid Nano-Structure Physics Research Group. He received
the Paper Awards of Japan Society of Applied Physics in 1989, 2004, and2010, MNC2008 Outstanding Paper Award in 2009, SSDM2009 Paper
Award in 2010, Inoue Prize for Science in 2012, and Commendation for
Science and Technology by MEXT in 2013. He was made a Fellowship of
Institute of Physics (IOP) in 2011 and JSAP in 2013. He is a member of
JSAP, the Physical Society of Japan, Institute of Physics (IOP), American
Physical Society (APS), and IEEE.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)7

Koji MURAKI 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. He joined NTT Basic Research Laboratories, Nippon
Telegraph and Telephone Corporation in 1994. Since then, he has been
engaged in the growth of high-mobility heterostructures and the study ofhighly correlated electronic states realized in such structures. He was aguest researcher at Max-Planck Institute, Stuttgart, Germany during
2001-2002. He served as a program committee/chair of international
conferences on High Magnetic Fields in Semiconductor Physics (HMF)
and Electronic Properties of Two-Dimensional Systems (EP2DS). He wasa leader of physics research and epitaxy group of ERATO Nuclear Spin
Electronics Project, Japan Science and Technology, during 2008-2015. He
was appointed as Distinguished Researcher of NTT in 2009 and SeniorDistinguished Researcher of NTT in 2013. He is currently a group leaderof Quantum Solid State Physics Research Group. He is a member of the
Physical Society of Japan and Japan Society of Applied Physics.
Shingo TSUKADA received his M.D. degrees from Toyama Medical
and Pharmaceutical University, Japan and his medical license in 1990. He
received the Ph.D. degrees in medicine from Tsukuba University, Japan in2003 respectively. He was a visiting researcher at University of California
at San Diego, U.S.A. during 2003-2005. He joined NTT Basic ResearchLaboratories, Nippon Telegraph and Telephone Corporation in 2010 as a
Research Specialist, and from 2013 as a Senior Research Scientist. Since
then, he has been engaged in the study of mechanism and activity controlof signal transduction of brain cell. His current interests are the detectionof biomedical signals using novel wearable-type and implant-typebioelectrodes based on the composites of conductive polymers withvarious fibers and textiles. He was appointed as Senior Distinguished
Researcher of NTT in 2014. He is currently a member of Molecular and
Bio Science Research Group. He is a member of Society for Neuroscience,
The Physiological Society of Japan, The Japan Society of Applied Physics,
the Japan Neuroscience Society, the Japanese Circulation Society, and theJapanese Orthopaedic Association.

8

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Distinguished Researchers
Akira FUJIWARA received his B.S., M.S., and Ph.D. degrees inapplied physics from The University of Tokyo, Japan in 1989, 1991,
and 1994, respectively. He joined NTT LSI Laboratories, Nippon
Telegraph and Telephone Corporation in 1994 and moved to NTT
Basic Research Laboratories in 1996. Since 1994, he has been engaged
in research on silicon nanostructures and their application to
single-electron devices. He was a guest researcher at the National
Institute of Standards and Technology (NIST), Gaithersburg, MD, USA
during 2003-2004. He was a director of the Japanese Society ofApplied Physics in 2010 and 2011 and a visiting professor of Hokkaido
University in 2013. He was appointed as Distinguished Researcher of
NTT in 2007. He is currently a senior manager of Physical Science
Laboratory and a group leader of Nanodevices Research Group. He
received the SSDM Young Researcher Award in 1998, SSDM Paper
Award in 1999, and Japanese Journal of Applied Physics (JJAP) Paper
Awards in 2003 , 2006, and 2013. He was awarded the Young Scientist
Award from the Minister of MEXT (Ministry of Education, Culture,
Sports, Science, and Technology) in 2006. He is a member of the Japan
Society of Applied Physics and the IEEE.
Yoshitaka TANIYASU received 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 ResearchLaboratories, Nippon Telegraph and Telephone Corporation in 2001.
Since then, he has been engaged in research on wide-bandgap nitride
semiconductors. He was a visiting researcher at Ecole polytechnique
fédérale de Lausanne (EPFL) during 2011-2012. He was appointed asDistinguished Researcher of NTT in 2010. He is currently a member ofThin-Film materials Research Group. He received the Young Scientist
Presentation Award at the Japan Society of Applied Physics (JSAP) in
2001, the Young Scientist Award at the 14th Semiconducting andInsulating Materials Conference in 2007, the Young Scientists’ Prize
from the Minister of Education, Culture, Sports, Science andTechnology, the Young Scientist Award at the 38th International
Symposium on Compound Semiconductors in 2011, and the Best Paper
Award at the International Workshop on Nitride Semiconductors in
2012. He is a member of the Japan Society of Applied Physics.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

9

Norio KUMADA received his B.S., M.S., and Ph.D. degrees inphysics from Tohoku University, Japan, in 1998, 2000, and 2003,
respectively. He joined NTT Basic Research Laboratories, Nippon
Telegraph and Telephone Corporation in 2003. Since then, he has been
engaged in the study of highly correlated electronic states insemiconductor heterostructures. He was a visiting researcher at CEA
Saclay during 2013-2014. He was appointed as Distinguished
Researcher of NTT in 2010. He is currently a member of Quantum Solid
State Physics Research Group. He received the Young Scientist Award of
the Physical Society of Japan in 2008, and the Young Scientists’ Prize
from the Minister of Education, Culture, Sports, Science and Technology
in 2012. He is a member of the Physical Society of Japan.
Katsuhiko NISHIGUCHI received his B.E., M.E., and Ph.D. inelectrical engineering from Tokyo Institute of Technology, Japan in
1998, 2000, and 2002, respectively. He joined NTT Basic ResearchLaboratories, Nippon Telegraph and Telephone Corporation in 2002.
Since then, he has been engaged in the research on physics andtechnology of Si nanometer-scale devices for LSI applications withlow power consumption and new functions. He was an invitedresearcher at the National Center for Scientific Research (CNRS),France during September 2008 and also a guest researcher at Delft
University of Technology, Delft, the Netherlands during 2012-2013. He
was appointed as Distinguished Researcher of NTT in 2011. He iscurrently a member of Nanodevices Research Group. He received
IUPAP Young Author Best Paper Award at the International
Conference on Physics of Semiconductors 2000, Graduate Student
Award Silver at the Materials Research Society 2000 Fall Meeting,Young Scientist Award at the Japan Society of Applied Physics Spring
Meeting in 2000, JSAP Outstanding Paper Award 2013, and The
Commendation for Science and Technology by the Minister of
Education, Culture, Sports, Science and Technology of Japan (theYoung Scientists' Prize) in 2013. He is a member of the Japan Societyof Applied Physics.
Shiro SAITO received his B.S., M.S., and Dr. Eng. degrees inapplied physics from The University of Tokyo, Japan, in 1995, 1997,
and 2000, respectively. He joined NTT Basic Research Laboratories,
Nippon Telegraph and Telephone Corporation in 2000. Since then, he
has been engaged in quantum information processing using
superconducting circuits. He was a guest researcher at Delft Universityof Technology, Delft, the Netherlands during 2005-2006. He is a guest
associate professor in Tokyo University of Science from 2012. He wasappointed as Distinguished Researcher of NTT in 2012. He is currently
a member of Hybrid Nano-Structure Physics Research Group. He
received the Young Scientist Presentation Award at the Japan Societyof Applied Physics (JSAP) Spring Meeting in 2004. He is a member of
the Physical Society of Japan and the Japan Society of Applied
Physics.

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Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Hiroki TAKESUE received his B.E., M.E., and Ph.D. degrees inengineering science from Osaka University, Japan, in 1994, 1996, and2002, respectively. He joined NTT Access Network Systems
Laboratories, Nippon Telegraph and Telephone Corporation in 1996
and moved to NTT Basic Research Laboratories in 2003. Since then,he has been engaged in research on optical access networks using
wavelength division multiplexing and experimental quantum optics
and quantum communications. He was appointed as Distinguished
Researcher of NTT in 2013. He is currently a member of Quantum
Optical State Control Research Group. He received several awards
including the ITU-T Kaleidoscope Conference 2nd Best Paper Awardin 2008 and The Commendation for Science and Technology by the
Minister of Education, Culture, Sports, Science and Technology ofJapan (the Young Scientists' Prize) in 2010. He was a Visiting Scholar
at Stanford University, Stanford, CA from 2003 to 2004, and a guest
researcher at the National Institute of Standards and Technology(NIST), Boulder, CO in 2014. He is a member of IEEE and the Japan
Society of Applied Physics.
Imran MAHBOOB received a combined B.Sc. and M.Sc. degree in
Theoretical Physics from The University of Sheffield, U.K., in 2001
and Ph.D. degree in Physics studying the electronic properties ofnitride semiconductors from The University of Warwick, U.K., in2004, respectively. He joined NTT Basic Research Laboratories,
Nippon Telegraph and Telephone Corporation in 2005 as a Research
Associate, from 2008 as a Research Specialist, and from 2012 as a
Senior Research Scientist. His current interests are developing
electromechanical resonators for digital signal processing applications
and to study their non-linear dynamics. He was appointed asDistinguished Researcher of NTT in 2013. He is currently a member ofHybrid Nano-Structure Physics Research Group. He received the
Clarke Prize in Physics from the University of Sheffield in 2001 andthe Young Scientist Award at the 2003 Physics of Semiconductors andInterfaces conference. He is a member of the American PhysicalSociety.
Haruki SANADA received his B.E., M.E., and Ph.D. degrees inelectrical engineering from Tohoku University, Japan, in 2001, 2002,
and 2005 respectively. He joined NTT Basic Research Laboratories,
Nippon Telegraph and Telephone Corporation in 2005. His researchinterests are optical and spin properties of low-dimensional
semiconductor nanostructures, and their application to solid-state
quantum information processing. He is a visiting researcher at
Chalmers University of Technology, Sweden in 2015. He wasappointed as Distinguished Researcher of NTT in 2014. He is currently
a member of Quantum Optical Physics Research Group. He received
the Young Scientist Presentation Award at the Japan Society of Applied
Physics (JSAP) Autumn Meeting in 2004, the SSDM Paper Award in
2010, and the RIEC Award from Tohoku University in 2014. He is amember of the Japan Society of Applied Physics.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)11

Yoshiharu KROCKENBERGER received his diploma in physics
from The University of Technology of Munich, Germany, studying
tunneling spectroscopy on superconductors. Working at the Max
Planck Institute for Solid State Research in Stuttgart, Germany, ontransition metal oxides with strong electron correlations he received his
PhD degree from The University of Technology of Darmstadt,
Germany in 2006. At the end of 2006 he joined the Correlated Electron
Research Center at The Institute for Advanced Industrial Science andTechnology in Tsukuba, Japan as a research scientist. In 2008, hemoved to RIKEN in Wako, Japan where he was engaged as a researchscientist at the Cross-Correlated Materials Research group. He joined
NTT Basic Research Laboratories, Nippon Telegraph and Telephone
Corporation in 2010. His interests are development of novel
superconducting materials and competing order parameters instrongly-correlated electronic systems. He was appointed asDistinguished Researcher of NTT in 2013. He is currently a member of
Low-Dimensional Nanomaterials Research Group. He received theYoung Scientist Award for an Excellent Article from Superconductivity
Division of Japan Society of Applied Physics in 2012. He is a memberof American Physical Society, Materials Research Society, and The
Japanese Society of Applied Physics.

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Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Advisory Board
Name

Affiliation

Prof. Gerhard Abstreiter

Walter Schottky Institute, Germany

Prof. John Clarke

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

Prof. Evelyn Hu

Harvard University, U.S.A.

Prof. Mats Jonson

University of Gothenburg, Sweden

Prof. Sir Peter Knight

Imperial College London, U.K.

Prof. Anthony J. Leggett

University of Illinois at Urbana-Champaign, U.S.A.

Prof. Allan H. MacDonald

The University of Texas at Austin, U.S.A.

Prof. Andreas Offenhäusser

Forschungszentrum Jülich, Germany

Prof. Halina Rubinsztein-Dunlop

The University of Queensland, Australia

Prof. Klaus von Klitzing

Max Planck Institute for Solid State Research, Germany

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)13

Invited / Guest Scientists
Name

Affiliation

PeriodÉcole Normale Supérieure / The Centre
Prof. Christos Flytzanis

National de la Recherche ScientifiqueJune 2014 - July 2014

(ENS/CNRS), France
Dr. Paulo Santos
Dr. Jason Brown

Paul Drude Institute for Solid StateElectronics, Germany
University of Oxford, U.K.

Oct. 2014 - Dec. 2014
June 2014 - June 2014

Overseas Trainees
Name

Affiliation

PeriodLouise Waterston

University of Edinburgh, U.K.

July 2013 - July 2014

Rick Lu

University of Waterloo, Canada

Sep. 2013 - Apr. 2014

Anna Fomitcheva Khartchenko

University of Barcelona, Spain

Sep. 2013 - Aug. 2014

Jose Alberto Rodriguez Santamaria

University of Burgos, Germany

Sep. 2013 - Aug. 2014

Krzysztof Jan Gibasiewicz

Warsaw University of Technology, Poland

Sep. 2013 - Aug. 2014

Gianfranco D'Ambrosio

Politecnico di Milano, Italy

Sep. 2013 - Aug. 2014

Logan G. Blackstad

Geogia Institute of Technology, U.S.A.

Sep. 2013 - Aug. 2014

Peter Karkus

14

Budapest University of Technology andEconomics, Hungary

Sep. 2013 - Aug. 2014

Adrian Salmon

Geogia Institute of Technology, U.S.A.

Jan. 2014 - July 2014

Henry Pigot

The University of British Columbia, Canada

Jan. 2014 - Aug. 2014

Ryan Neufeld

University of Waterloo, Canada

Jan. 2014 - Aug. 2014

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Name

Affiliation

PeriodAndrew Tin

McGill University, Canada

Feb. 2014 - Dec. 2014

Roxana Filip

University of Ottawa, Canada

May 2014 - Dec. 2014

Josh Kuo

The University of British Columbia, Canada

May 2014 - Dec. 2014

Christophe Rainville

McGill University, Canada

May 2014 - Dec. 2014

Shane Dooley

University of Leeds, U.K.

June 2014 - Sep. 2014

Matt Edmonds

University of Bath, U.K.

June 2014 - Dec. 2014

Thomas McManus

University of Bath, U.K.

July 2014 - Dec. 2014

Flore Castellan

ESPCI ParisTech, France

July 2014 - Dec. 2014

Pierre-Alix Carles

ESPCI ParisTech, France

July 2014 - Dec. 2014

Amaury DodelESPCI ParisTech, France

July 2014 - Dec. 2014

Remi Dupuy

ESPCI ParisTech, France

July 2014 - Dec. 2014

Tony JinESPCI ParisTech, France

July 2014 - Dec. 2014

Hadrien Duprez

École Polytechnique de Montreal, Canada

May 2014 -Sophia Chan

University of Edinburgh, U.K.

June 2014 -Aleksandra Krajewska

University of Edinburgh, U.K.

July 2014 -Clemens Todt

Technical University Dresden, Germany

Sep. 2014 -Aleix Llenas

Polytechnic University of Catalonia Barcelona
Tech, Spain

Sep. 2014 -Dorota Kowalczyk

Gdansk University of Technology, Poland

Sep. 2014 -Silviu Dinulescu

University Politehnica of Bucharest, Romania

Sep. 2014 -Akie Watanabe

The University of British Columbia, Canada

Jan. 2015 -Mats Powlowski

Corcodia University, Canada

Jan. 2015 -Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)15

Domestic Trainees (Fiscal 2014)

16

Name

Affiliation

PeriodHan CaiTohoku University

Apr. 2014 - Mar. 2015

Ahmad YoshinariTokyo University of Science

Apr. 2014 - Mar. 2015

Kazuki Moriya

The University of Tokyo

Apr. 2014 - Mar. 2015

Kosuke Sato

Kyoto University

Apr. 2014 - Mar. 2015

Toru Tanaka

Waseda University

Apr. 2014 - Mar. 2015

Takahiko SatoThe University of Tokyo

Apr. 2014 - Mar. 2015

Saki Tanaka

Keio University

Apr. 2014 - Mar. 2015

Rento Osugi

Tohoku University

Apr. 2014 - Mar. 2015

Tomohiro Tamaki

Toyo University

Apr. 2014 - Mar. 2015

Takahiro Goto

Tokyo Denki University

Apr. 2014 - Mar. 2015

Ken-ichi Suzuki

Tokyo Denki University

Apr. 2014 - Mar. 2015

Tomohiko Yamaguchi

Tokyo University of Science

Apr. 2014 - Mar. 2015

Masato Tsunekawa

Tokyo Institute of Technology

Apr. 2014 - Mar. 2015

Keisuke Noguchi

Tokyo Institute of Technology

Apr. 2014 - Mar. 2015

Akihiro Fushimi

Keio University

Apr. 2014 - Mar. 2015

Takuya Ohrai

Tokyo University of Science

May 2014 - Mar. 2015

Yamato Ashikawa

Tohoku University

Aug. 2014 - Mar. 2015

Masafumi Horio

The University of Tokyo

Sep. 2014 - Nov. 2014

Ayumi Ishijima

Kyushu University

Aug. 2014 - Sep. 2014

Kohei Nakanishi

Mie University

Aug. 2014 - Sep. 2014

Ryousuke Tanaka

Hokkaido University

Aug. 2014 - Sep. 2014

Daichi Morita

University of Tsukuba

Aug. 2014 - Sep. 2014

Takuya Aritsuki

Tokushima University

Aug. 2014 - Sep. 2014

Asahiko Sakai

Nagaoka University of Technology

Oct. 2014 - Feb. 2015

Tran Minh Tien

Nagaoka University of Technology

Oct. 2014 - Feb. 2015

Masafumi Horio

The University of Tokyo

Dec. 2014 - Mar. 2015

Ryousaku Sakamoto

Toyohashi University of Technology

Jan. 2015 - Feb. 2015

Takaya Nakane

Toyohashi University of Technology

Jan. 2015 - Feb. 2015

Motoki Itoh

Toyohashi University of Technology

Jan. 2015 - Feb. 2015

Takeshi Kamiya

Osaka University

Jan. 2015 - Mar. 2015

Suguru Endo

Keio University

Jan. 2015 - Mar. 2015

Yuki Nagakubo

The University of Tokyo

Feb. 2015 - Feb. 2015

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Ⅰ．Research Topics

Overview of Research in Laboratories

Materials Science Laboratory

Hiroki Hibino

The aim of this laboratory is to contribute to progress in materials science and to
revolutionize information communication technology by creating novel materials andfunctions through material design at the atomic and molecular levels.
This laboratory consists of three research groups investigating a wide range ofmaterials: e.g., typical compound semiconductors including GaAs and GaN,
two-dimensional materials such as graphene, high-Tc oxide superconductors, andbiomolecules. We are conducting innovative materials research based on advanced
thin-film growth technologies along with high-precision and high-resolution
measurements of structures and properties.
This year, we succeeded in the CVD growth of single-layer graphene composed of
mm-sized single-crystalline domains. The self-heating effect has been suppressed in
GaN-based electronic devices by transferring them to thermally conductive materials
using our original epitaxial lift-off technique based on the cleavable nature of h-BN. In
addition, the sensing fabric “hitoe”, which was developed last year in collaboration with
Toray Industries, Inc., has been marketed by GOLDWIN Co. Ltd.

Physical Science Laboratory

Akira Fujiwara

The aim of the Physical Science Laboratory is to develop semiconductor- andsuperconductor-based devices and/or hybrid-type devices, which will have arevolutionary impact on the future ICT society. Research groups in our laboratory areusing both high-quality crystal growth and nanolithography techniques that we havedeveloped to explore novel properties that can lead to nanodevices for ultimate
electronics and novel information processing devices based on new degrees of freedom
such as single electrons, mechanical oscillations, quantum coherent states, electron
correlation, and spins.
This year we realized the dynamic control of one-dimensional phononic crystals,
observed Wigner solids in high magnetic fields, achieved high-speed single-electrontransfer via a single-trap level in silicon, and observed dark states in a superconductor
diamond quantum hybrid system. Progress was also made in the research on two-mode
squeezing in an electromechanical resonator, the electric tuning of direct/indirect opticaltransitions in silicon, and resonant edge magnetoplasmons in graphene.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)17

Optical Science Laboratory

Hideki Gotoh

The aims of this laboratory are to develop innovative core technologies for opticalcommunications and optical signal processing, and to make fundamental scientificprogress.
The groups in our laboratory are working to achieve quantum state control andquantum information processing by using very weak light, to discover intriguing
phenomena by using very intense short pulse light, to control optical properties by using
photonic crystals and ultrasonic techniques, and to characterize the unique properties ofsemiconductor nanostructures such as quantum dots and nanowires.
This year, we have succeeded in monitoring inner-shell electron motion with anultrafast strobe light source using attosecond optical pulse creation techniques. We havealso made progress on quantum key distribution employing superconducting detectors
with an ultra-low-dark count rate and undertaken theoretical studies on quantum
cryptography assuming realistic environments. In addition, we have clarified thequantum optical properties of silicon integrated optical circuits, revealed energy transfer
processes in Er oxides, and fabricated a new type of semiconductor hetero-nanowire
exploiting original self-catalyst growth techniques.

Nanophotonics Center

Masaya Notomi

The Nanophotonics Center was established in April 2012, and is now composed ofseveral nanophotonics-related research groups from NTT Basic Research Laboratories
and NTT Device Technology Laboratories. Our aim is to develop a full-fledgedlarge-scale photonic integration technology that will allow us to densely integrate alarge number of nano-scale photonic devices with various functions in a single chip.
Furthermore, we are targeting an extreme reduction in energy consumption for photonic
information processing by taking advantage of nanophotonics technology.
This year, we demonstrated an optical random access memory of over 100 bits basedon ultrasmall integrated photonic crystal nanocavities. As a novel step, we realized InP
nanowire and plasmonic nanoantenna coupled systems that showed substantial lightemission enhancement. We also achieved large Raman enhancement in Si photonic
crystal nanocavities with carbon nanotubes. As regards device applications, we achieved
the room-temperature operation of on-Si photonic crystal nanolasers by employing awafer-bonding technique, and demonstrated a spatial mode multiplexer/demultiplexer
based on 3D multi-layer waveguides.

18

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Suppression of Self-Heating Effect in AlGaN/GaN HEMTs
by Mechanical Transfer Using an h-BN Release Layer
Masanobu Hiroki, Kazuhide Kumakura, Yasuyuki Kobayashi*, Tetsuya Akasaka,
Toshiki Makimoto**, and Hideki Yamamoto
Materials Science Laboratory
III-nitride semiconductor materials have potential for high-power device applications by
exploiting their high critical electric field. Under high-current operation, however, power
performance is limited because of the generation of Joule heat (the so-called self-heating
effect). Therefore, thermal management in the GaN-based devices is necessary to further
improve high-power performance. One approach to overcome this problem is to grow device
structures on highly thermal conductive substrate. However, the lattice mismatch or thedifference in thermal expansion coefficients between substrates and GaN often limit the
crystal quality and thickness of the layers. We propose a different approach, where thedevices are transferred to foreign materials with high thermal conductivity. Recently, we havedeveloped a mechanical transfer technique using release layer (MeTRe mothod), in which
h-BN is inserted between a GaN layer and sapphire substrate as a release layer [1]. Using thismethod, we have demonstrated strong emission from transferred InGaN/GaN multiple
quantum well light emitting diodes (LEDs) [2]. In this study, we applied the same approachto electronic devices [3]. We fabricated AlGaN/GaN high-electron-mobility transistors(HEMTs) on h-BN/sapphire substrates and transferred them to copper plates, which led to an
improvement of device performance due to enhanced heat dissipation.
Figure 1 shows typical I-V characteristics of an AlGaN/GaN HEMT before release fromthe substrate and after transfer to the copper plate. We obtained good pinch-off and saturation
characteristics before release and after transfer. Before release, a large reduction in draincurrent Id with increasing Vds, i.e., negative differential resistance, is observed in thesaturation region. In contrast, the negative resistance is reduced for the transferred HEMT.
The negative resistance is commonly attributed to the self-heating effect. Transfer fromsapphire (thermal conductivity κ = 40 W/m K) to the copper plate (κ = 390 W/m K) improves
the heat dissipation efficiency. We took temperature images of devices during operation with
1-W power dissipation (Fig. 2). Before release, a hot spot with a temperature of about 50°C isobserved. In contrast, the temperature is as low as 30°C for the transferred HEMT. Our
results indicate that the transfer technique using the h-BN release layer can enhance the heatdissipation and significantly improve the power performance of GaN-based devises.
Present address:* Hirosaki University, ** Waseda University[1] Y. Kobayashi et al., Nature 483, 223 (2012).
[2] T. Makimoto et al., Appl. Phys. Express 5, 07102 (2012).
[3] M. Hiroki et al., Appl. Phys. Lett. 105, 193509 (2014).
Probe

Probe

Probe

Probe

60
50
40

Sapphire sub.
Before release

2 mm

2 mm
After transfer

30
20°C

Fig. 2. Temperature map of devices before release and after transfer.
Fig. 1. I-V characteristics of HEMT
before release and after transfer.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)19

MBE Growth of Transition Metal Dichalcogenide Thin Films
Koji Onomitsu, Aleksandra Krajewska, Ryan Neufeld, Fumihiko Maeda,
Kazuhide Kumakura, and Hideki Yamamoto
Materials Science Laboratory
Transition metal dichalcogenides (TX2) belong to a family of two-dimensional layeredmaterials, where adjacent layers are weakly bound by van der Waals forces, like in graphite.
These materials offer a large variety of properties, from insulating/semiconducting to metallic
ones, depending on the combination of T and X. Among them, we have been focusing on
semiconducting MoSe2.
We grew wafer-scale MoSe2 thin films on Se terminated GaAs(111)B substrates using themolecular beam epitaxy (MBE) method. The Scanning transmission electron microscope(STEM) image in Fig. 1 shows that two-monolayer (2L)-thick MoSe2 is grown in a layer-bylayer manner. It also indicates that the MoSe2 layers can climb over the monolayer step of
GaAs. Though Fig. 1 shows an STEM image for a double-layer MoSe2, it should be notedthat the layer number can be varied with growth time. We carried out Raman spectroscopy
measurements and investigated layer number dependence. Figure 2 shows Raman spectra
around the A1g peak for single-layer (1L), double-layer (2L), and more-than-four-layer (> 4L)
MoSe2 thin films. The spectrum for Se-terminated GaAs is also shown as a reference.
According to a previous report on MoSe2 mechanically cleaved from bulk crystal [1], the A1g
peak shifts to a higher wavenumber with an increasing number of layers because of multiple
interlayer interactions. The A1g peaks in our MoSe2 thin films also show the blue shift (Fig. 2)
when the layer number increases. The layer number of our films estimated from the positionsof A1g peaks, using reported values for cleaved samples as references [1], well coincides withthat directly determined by STEM observation. On the other hand, splitting of the A1g peak is
not clearly observed in our film samples, which is in contrast to the cleaved ones withthicknesses of > 3L [1]. The peak splitting originates from the minute difference in vibration
frequencies between the in-phase and out-of-phase vibrations and is very sensitive to thelayer number [1]. Therefore, the absence of the peak splitting in our films likely stems from afluctuation of the layer number (±1 layer) due to partial formation of growth nuclei on
atomically flat surfaces.
Our results indicate that wafer-scale MoSe2 thin films with a controlled layer number canbe prepared by MBE, which is widely used for growth of high-quality thin films of
conventional semiconductors. Application of this technique to other TX2 materials may allow
fabrication of heterostructures consisting of various TX2 materials.

2 nm

�𝟏𝟏]
× [𝟎𝟎𝟏𝟏

Fig. 1. Cross-sectional STEM image of MoSe2
around a GaAs step. Incident electron beam isparallel to [01� 1] direction. MoSe2 is epitaxiallygrown on the GaAs(111)B surface.
20

Intensity (arb. units)[1] P. Tonndorf, et al., Opt. Express 21, 4908 (2013).
Se-terminated GaAs
~1L
~2L
>4L
230

235

240

245
-1

250

Wavenumber (cm )Fig. 2. Layer number dependence of Raman
spectra around A1g peak.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Superconducting and Insulating Ground States
in La2CuO4 Structural Isomers
Yoshiharu Krockenberger and Hideki Yamamoto
Materials Science Laboratory
Superconductivity in cuprate superconductors is associated to a doped Mott insulator
scenario and this antiferromagnetic insulating state is considered to be universal. The phase
diagram of cuprate superconductors aims at the fundamental comparison of hole- and electron
doping into CuO2 planes. While these CuO2 planes are, indeed, common among all cuprate
superconductors, variations of the coordination of the Cu2+ ions are entirely ignored. It is wellestablished that superconductivity is induced by hole doping to the CuO2 planes with 5- or 6fold coordinated copper. The induction of superconductivity in cuprates with 4-foldcoordinated copper is – against common understanding – not associated to doping but toelimination of defects. Cuprates with 4-fold coordinated copper are metals per se and this fact
has not been recognized [1]. It would therefore be revealing if one could alter the copper
coordination by keeping chemical elements as well as their stoichiometry constant whilesolely varying the coordination of copper. That is what we did using La2CuO4. Molecularbeam epitaxy allows to explore thermodynamic synthesis conditions entirely inaccessible byother synthesis methods. In particular, low synthesis temperatures are key. We synthesized
La2CuO4 with three different copper coordination: 6-fold coordination (T-phase), 5-foldcoordination (T*-phase), and 4-fold coordination (T’-phase). All three phases were growncoherently onto SrLaAlO4, DyScO3, and PrScO3 substrates, respectively. Among the threeisomers of La2CuO4 the T-phase (6-fold coordinated copper) is the thermodynamically stable
polymorph and its electronic response is well known. So, it is not surprising that T-La2CuO4
is insulating [Fig. 1(a)]. The T*-phase of La2CuO4 is, however, a new material as it is not athermodynamic stable polymorph. So far we shed some light on its electronic response (it isinsulating) and magnetic ordering phenomena are currently investigated by low-energy muon
spin resonance spectroscopy. The T’-phase of La2CuO4 is also only accessible by lowtemperature synthesis methods and its electronic response strongly deviates from its isomers.
T’-La2CuO4 is a superconductor [2, 3].
[1] Y. Krockenberger et al., Sci. Rep. 3, 2235 (2013).
[2] Y. Krockenberger et al., J. Phys. Soc. Jpn. 83, 114602 (2014).
[3] Y. Krockenberger et al., Appl. Phys. Express 7, 063101 (2014).

102

101
0

100 200 300 400
T (K)

106
104
102
100
0

20 T = 300 K-20-1

ρ (mΩcm)(b)ρ (Ωcm)ρ (Ωcm)

103

1.2

108

(a)

I (pA)

104

V (V) 1

100 200 300 400
T (K)

(c)

∝ T2

0.6

0.0

0

∝T
100 200 300
T (K)

400

Fig. 1. Temperature dependent resistivity. (a) T-phase La2CuO4. (b) T*-phase La2CuO4. (c) T’-phase.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)21

Simultaneous Light Emissions from Erbium-Thulium Silicates on Siliconin the Second and Third Telecommunications Bands
Hiroo Omi1,3, Maria Anagnosti1, and Takehiko Tawara2,3
1
Materials Science Laboratory, 2Optical Science Laboratory, 3NTT Nanophotonics Center
The increasing demand for small-footprint and cost-effective devices in fiber opticalcommunications links has prompted significant research activity into developing efficientsilicon-based lasers. The operating wavelengths of interest are in and neighboring the C-bandlines (1530-1565 nm). Some effort has been focused on combining the function of two lasers
into one by achieving dual-wavelength emission from separate dopant ions. Dual-band lasers
using the emissions of erbium (Er) and other neighboring ions such as Tm, Nd, Yb and Ho in
glasses have been reported. However, there are few reports on the co-doping of Er and Tm
into crystalline hosts for realization of broadband optical waveguide amplifiers and dualwavelength light emitters on silicon in the telecommunications bands.
We succeeded in observing sharp light emissions from Tm3+ ions in the wavelength regionsof 1300 and 1470 nm and from Er3+ ions at 1530 nm from a mixture of polycrystalline
Er2xTm2-2xSiO5 and Er2xTm2-2xSi2O7 formed on Si [1]. We demonstrated that Er3+ ions in themixed compounds act as not only 1530 nm wavelength light emitters but also as effective
sensitizers to the 1300 nm wavelength light emissions from Tm3+ ions (Fig.1). The resonant
energy transfers between higher energy levels of Er3+ and Tm3+ ions play an important role inthe simultaneous sharp light emissions in O and S+C bands when excited with λexc = 532 nm(Fig. 2). Additionally, we demonstrated the dual-wavelength sharp light emissions in the S
and C bands at λexc = 785 nm.
A future objective of this work will be the deeper understanding and optimization of the
mixture formation on silicon and the lifetime measurements of the light emissions, which isimportant for obtaining high-quantum-efficiency dual-wavelength light emitters in thetelecommunications bands.
[1] M. Anagnosti, H. Omi, and T. Tawara, Opt. Mat. Express 4, 1747 (2014).

22

Fig. 1. Wavelength excitation dependence

Fig. 2. Energy diagrams and possible transitionsof PL spectra from Er-Tm silicates.

between Er3+ and Tm3+ ions in Er-Tm silicates.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Raman Spectroscopic Visualization of Topological Defects in Chemical
Vapor Deposition-Grown Graphene by Isotope Labeling
Shengnan Wang, Satoru Suzuki, and Hiroki Hibino
Materials Science Laboratory
In the past decade, chemical vapor deposition (CVD) growth of graphene has been
emerged as a promising route to scale up the fabrication of graphene-based applications [1].
However, the CVD graphene samples are always polycrystalline, composed of singlecrystalline domains having various in-plane rotations and related topological defects. These
topological defects, especially grain boundaries, dominate the electrical and mechanicalperformance of large-scale CVD-grown graphene. Thus, an efficient method for elucidating
the topological structure in CVD graphene is important for its fundamental research andpotential technological applications in nanoelectronics and related fields.
Here, for the first time we demonstrate an isotope labeling method to identify the
topological defects in CVD-grown graphene by micro-Raman spectroscopy [2]. In a stepwise
CVD process, the hydrogen and isotopic carbon sources are sequentially introduced after the
standard growth of full-coverage monolayer graphene. Owing to the dual role of hydrogen in
the catalytic reaction of graphene growth, the surface exchange of 12C-13C atoms occurs on aheated copper substrate. Taking the advantages of the distinct phonon energy between 12C
and 13C lattice, we observed the coexistence of 12C- and 13C-graphene in an isotope-labeledgraphene sample by Raman spectroscopy [Fig. 1(a)]. Moreover, as Fig. 1(b) shows, the 13Crich regions form a pronounced network-like structure, isolating the 12C-graphene regions
into micrometer-sized islands. The Raman characteristics of isotope-labeled graphene
indicate that the carbon atom substitution is energetically favored along the grain boundaries
of polycrystalline CVD graphene, which is further evidenced by low-energy electron
microscopy (LEEM) characterization [Fig. 1(c)]. The carbon isotope labeling method
provides a facile and effective way to characterize the topological defects in CVD-growngraphene, and also gives new insight for understanding the growth mechanism of graphene on
copper catalyst.
[1] S. Wang et al., Appl. Phys. Lett. 103, 253116 (2013).
[2] S. Wang et al., Nanoscale 6, 13838 (2014).

Fig. 1. (a) Raman spectra from coexistence area of 12C- and 13C- graphene and area of pure 12C-graphenein an isotope-labeled graphene sample. (b) Raman 13C-2D intensity mapping of an isotope-labeledgraphene sample. (c) Color-coded dark-field LEEM image of the sample in (b). The different color andnumber shows the various lattice orientations of grains with different shapes.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)23

On-Chip FRET Graphene Aptasensor for Multiple Protein Detection
Yuko Ueno and Kazuaki Furukawa
Materials Science Laboratory
Graphene and its chemical derivatives behave as efficient acceptors for fluorescenceresonance energy transfer (FRET) between a graphene surface and dye molecules locatedclose to it. This makes graphene a promising material for realizing a unique type of
biomolecular interface for visualizing an invisible biological response to a measurable
physical quantity such as fluorescence. We have successfully demonstrated a graphene oxide(GO) aptasensor for selective and highly sensitive protein detection. We realized this sensor
by modifying the GO surface with a pyrene-aptamer-dye probe that we developed. The
segments work as a linker to the GO surface, a protein recognition part, and a fluorescence
detection tag, respectively [Fig. 1 (left)] [1]. The system allows us to perform molecular
detection on a solid surface, which is a powerful tool for realizing an on-chip sensor, andespecially for forming a multichannel configuration and for micropatterning probes [2, 3].
The on-chip sensor allows us to evaluate the sensor response quantitatively by using one of
the channels/patterns as an internal standard.
Aptamers offer many advantages as molecular recognition probes because they arechemically stable and have a wide variety of targets. We confirmed the versatility of the GO
aptasensor through the detection of three different proteins, namely, thrombin, prostate
specific antigen (PSA), and hemagglutinin, simply by changing the aptamers but with thesensor composition remaining the same. We also demonstrated the simultaneous detection ofmultiple proteins, thrombin and PSA, on a single chip by using a multichannel linear-array
GO aptasensor [Fig. 1 (right)] [3]. Another attractive feature of aptamers is that they can be
flexibly designed without loss of activity. We designed a biomolecular probe for highly
sensitive protein detection by modifying an aptamer with a DNA spacer. The spacer controls
the distance between a fluorescence dye and a quencher, which is crucial for FRET-based
sensors. We successfully demonstrated an improvement in the sensitivity of an on-chip GO
aptasensor [2]. Our findings will advance the field of on-chip microsensors.
[1] K. Furukawa et al., J. Mater. Chem. B 1, 1119 (2013).
[2] Y. Ueno et al., Chem. Commun. 49, 10346 (2013): Featured on cover.
[3] Y. Ueno et al., Anal. Chim. Acta 866, 1 (2015): Featured on cover.

Fig. 1. (Left) Design of a protein recognition system of GO FRET aptasensor.
(Right) Demonstration of multiple protein detection on a single chip.

24

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Formation of a Suspended Lipid Membrane on a Microcavity
Covered by a Thin SiO2 Layer with a Nanohole Array
Aya Tanaka1, Yoshiaki Kashimura1, Eiichi Kuramochi2, and Koji Sumitomo1
1
Materials Science Laboratory, 2Optical Science Laboratory
By fusing nanotechnology and biotechnology, we aim to fabricate a nanobiodevice thatworks with membrane proteins on a Si substrate. We succeeded in fabricating a microcavity
array sealed with a lipid bilayer on a Si substrate for analyzing ion channel activity [1].
However, it is essential that we improve the lifetime of the nanobiodevice. In this study, wefabricated microcavities on a Si/SiO2 substrate covered by a thin SiO2 layer with nanohole
arrays that we call a “pepper shaker substrate” [2].
The nanohole arrays were fabricated in a thin SiO2 layer on a Si substrate using an electron
beam lithography and dry etching technique. The Si substrate beneath the SiO2 overlayer was
selectively etched through the nanohole arrays with a KOH aqueous solution, and thisresulted in the formation of a microcavity covered by a thin SiO2 layer with a nanohole arrayas shown in Fig. 1. The microcavity volume was made comparable to that of a living neuronor synapse by choosing suitable etching conditions. The nanohole diameter (100 nm) was
similar to the domain size of a cell membrane supported by cytoskeletons. The total area ofthe lipid membrane suspended over a nanohole array was kept large enough for the insertionof membrane proteins, although the area of a single nanohole was small. Figure 2 shows
fluorescence images of a lipid membrane patch on a pepper shaker substrate. Calcein
fluorescence [Fig. 2(b)] was observed at the microcavity where fluorescence from the lipid
patch [Fig. 2(a)] was observed (for example, white arrow A). There were more than 100
nanoholes in each microcavity. If even one of the lipid membranes sealing the nanoholes
were broken, fluorescent probes would flow out (white arrow B). Nevertheless, the small
diameter of the nanoholes makes the suspended lipid membrane more stable, and almost all
the microcavities were successfully sealed. This stabilization also increases the lifetime of the
nanobiodevice. We confirmed that the green fluorescence was still confined in the
microcavities after 10 days.
The stable sealing of the microcavities allows us to reconstitute and analyze membraneproteins embedded in suspended lipid membranes using fluorescence microscopy, AFM, andelectrophysiology. We believe that this system could be employed for mimicking andstudying the behavior of biological systems.
[1] K. Sumitomo et al., Biosens. Bioelectron. 31, 445 (2012).
[2] A. Tanaka et al., Appl. Phys. Express 7, 017001 (2014).

(a)

(b)

Fig. 1. Schematic illustration of a pepper shaker

Fig. 2. Fluorescence images of pepper shaker

substrate. (a) Top view and (b) cross-sectional

substrate after rupture of GUVs. (a) Lipid

view. (c) SEM image of microcavity cross-section.

membrane. (b) Calcein confined in microwells.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)25

Cross-Sectional SEM Observation of the Interface between a Neuron
and a Conductive Material
Toichiro Goto, Nahoko Kasai, and Koji Sumitomo
Materials Science Laboratory
Recently, neuron-nanostructure integrated devices have been attracting a lot of interest in
relation to understanding signal transfer mechanisms. In such devices, signals from single
neurons, e.g., neuron action potentials, are transmitted extracellularly via conductive
materials, and hence the interface between single neurons and conductive materials plays animportant role. However, there have been few investigations of this interface because of the
difficulty involved in direct and high-resolution observation with conventional optical
microscopy. We employed a focused ion beam (FIB)/SEM combined system, which enabled
the cross-sectional observation of neurons cultivated on the surfaces of conductive materials.
We also examined the relationship between the interface structure and neuron-substrateaffinity by using fluorescence microscopy.
Neurons were prepared from rat cortex and cultivated on two types of conductive thinfilms: indium tin oxide (ITO) and Ti. The cultivated neurons were immunostained andobserved with a fluorescent microscope. For the FIB/SEM experiments, the cultivatedneurons were dehydrated, fixed and finally freeze-dried. Figures 1 (a) and (b) arefluorescence images of the neurons on ITO and Ti, respectively. On the ITO surface, somas
tend to aggregate and dendrites grow linearly, indicating the low affinity between neurons
and ITO. In contrast, on a Ti surface, very little soma aggregation or straight dendrite growthis observed, implying a high affinity between neurons and Ti. Figures 1 (c) and (d) are crosssectional SEM images of single neurons on ITO and Ti surfaces, respectively. Although theneurons hardly make contact with the ITO surface as indicated by the wide interspace
between the neurons and the ITO, they come into close contact with the Ti surface. These
results obtained in fluorescence and FIB/SEM experiments indicate that Ti has the higher
biocompatibility.
Our approach allowed high-resolution observation of the interface between single neurons
and substrates, which will provide important implications for the control of neuron growth,leading to a quantitative analysis of the neuronal signals from neuron-nanostructure
integrated devices.
[1] T. Goto et al., J. Nanosci. Nanotechnol. (accepted).

ITO

50µm

Ti

Soma

(a)

50µm

Soma

(b)

ITO

Cross section

Ti

2µm

Substrate surface

2µm

(c)

Cross section

Substrate surface(d)Fig. 1. Fluorescence images of neurons on (a) ITO and (b) Ti thin films. Cross-sectional SEM images of single
neurons on (c) ITO and (d) Ti thin films.

26

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Platform for Low Noise Biosensors Using Microwells Sealed with Lipid
Bilayers
Yoshiaki Kashimura and Koji Sumitomo
Materials Science Laboratory
A sealed microwell with a lipid bilayer is a promising candidate for constructing artificial
cells. If channel proteins can be incorporated into the membrane, their functional properties
can be detected using electrophysiological or microscopic techniques. We have fabricatedmicrowell structures sealed with lipid bilayers on a Si substrate, and succeeded in observing
Ca2+ ion transport through α-hemolysin channels with fluorescent microscopy [1]. However,
ion leakage through the water layer between the lipid bilayer and the substrate poses a
problem when we try to achieve a low-noise biodevice. Here we present a newly designed
microwell structure that uses a self-assembled monolayer (SAM) on a Au surface to prevent
ion leakage from/into the microwells [2].
Figure 1 shows the device structure used in this study. We fabricated microwells with aslightly offset Au ring [Fig. 1(a)]. An octadecanethiol SAM was formed on the Au surface.
The microwells were filled with calcein or Ca2+ indicators (fluo-4) and then sealed with lipid
bilayers by rupturing giant unilamellar vesicles. Figure 1(b) shows a fluorescence image of asealed microwell, in which calcein is confined. There is no fluorescence on the Au ring due tothe energy transfer between the dyes and gold. Fluorescence recovery after photobleachingobservation revealed that lateral fluidity is maintained across the Au ring. This result supports
the structural geometry predicted in Fig. 1(c). Figure 2 shows the time course of thefluorescence intensity of fluo-4 confined in the microwells when CaCl2 solution was added tothe outer solution. Unmodified substrates (without a Au ring) caused a significant increase in
the fluorescence intensity within 20 min. However, with a modified substrate (with a Au ring),
there was no increase in fluorescence intensity during the observation time. By separating the
microwells and the outer regions, we can effectively reduce ion diffusion through the waterlayer. We estimated the membrane resistance with a view to realizing the electrophysiologicaldetection of channel protein activity. The membrane resistances for the modified microwells
were one order higher than those of the unmodified microwells, leading to a reduction in the
background noise. The improved membrane resistance and background noise for the modified
microwells resulted from the reduction in ion leakage. The device structure used in this study
has the potential to provide a platform for measuring the very low signal-to-noise ratios ofchannel proteins.
[1] K. Sumitomo, Y. Kashimura et al., Biosens. Bioelec. 31, 445 (2012).
[2] Y. Kashimura, K. Sumitomo et al., ISSS-7, 3PN-27 (2014).

Fig. 1. (a) SEM image of the Au ring substrate. (b) Fluorescence imageof a sealed microwell. (c) Schematic illustration of the modified
microwell sealed with a lipid bilayer.

Fig. 2. Change in fluorescenceintensity from fluo-4 confined within
microwells as a function of time.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)27

Deviation from the Law of Energy Equipartition in a Small DynamicRandom-Access Memory:
Analysis about Thermal Noise with Single-Electron Resolution
Katsuhiko Nishiguchi, Pierre-Alix Carles, and Akira Fujiwara
Physical Science Laboratory
The performance of integrated circuits used in a range of electronic devices has been
improved by downsizing transistors in the circuits. Such downsizing, however, increases thenoise-related degradation of circuit performance, and the importance of an analysis of noisewith single-electron resolution has therefore been growing. Indeed, random telegraph noise
originating from single-electron trapping in a transistor gives rise to errors in memory circuits
composed of small transistors. In this work, using a dynamic random access memory(DRAM), we demonstrate an analysis of thermal noise, one of the most fundamental noises inelectronic devices, with a single-electron resolution.
For the analysis, the DRAM is combined with a charge sensor based on a transistor [Figs.
1(a) and (b)]. Since this sensor has charge sensitivity of single-electron resolution, thenumber of electrons in the capacitor of the DRAM can be counted. The output signal from thesensor [Fig. 1(c)] indicates that single electrons enter and leave the capacitor randomly andthus the number of electrons in the capacitor fluctuates, which is exactly single-electron
thermal noise. Using the characteristics of the sensor, we can convert the fluctuation of thenumber of electrons into voltage fluctuation. It is well known that according to the law ofenergy equipartition in which each electron has thermal energy kBT/2 on average, a varianceVvar2 of voltage fluctuation can be given by kBT/C, where kB is the Boltzmann constant, Ttemperature, C capacitance. Figure 2 shows the change in Vvar2 as a function of C. While Vvar2
is close to kBT/C at C > 10 aF, C smaller than 10 aF causes a deviation between Vvar2 andkBT/C. When charging energy EC (= e2/2C) for injecting one electron into the capacitor issmaller than thermal energy kBT/2, the thermal energy helps electrons in enter and leave thecapacitor, which means that kBT/C or the law of energy equipartition is satisfied. On the other
hand, when EC > kBT/2 at smaller C, the thermal energy is not large enough to help electrons
in enter and leave the capacitor, which leads to smaller Vvar2. Additionally, voltage V applied
to the DRAM can adjust the energy for injecting and rejecting one electron to be zero. This
allows even small thermal energy to help one electron to enter and leave the capacitor, whichmakes Vvar2 larger than kBT/C. These features at EC > kBT/2 represent a deviation from the lawof energy equipartition. Since our demonstration is carried with DRAM of very small size,
our results are very important for all electronic devices with small dimensions.
[1] K. Nishiguchi et al., Nanotechnology 25, 275201 (2014).

(a) DRAM

SensorFluctuation of numberof electrons in the capacitor
26

C

(b)

Outputsensor

DRAM

W
100 L
nm

Output (nA)

V

(c)

-3
-2
-1
0
1
2
3
4

25
24
0

5
Time (s)10Fig. 1. (a) Equivalent circuit and (b) scanning electron microscope
image of the device. (c) Single-electron detection.

28

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Thermal noise Vvar2 (10-4 V2)[2] P.-A. Carles et al., Jpn. J Appl. Phys. 54, 06FG03 (2015).
10
60 K
Experimental
values
1

0.1
1

kBT/C

Expected values
10
Capacitance C (aF)

Fig. 2. Change in thermal noise as afunction of capacitance C.

Electric Tuning of Direct/Indirect Optical Transitions in Silicon
Jinichiro Noborisaka, Katsuhiko Nishiguchi, and Akira Fujiwara
Physical Science Laboratory
Bulk silicon has an indirect band gap and a multiple degenerate valley structure in the
conduction band. Since photons do not carry significant momentum and a dipole transition
requires momentum conservation, a direct optical transition is not allowed in bulk silicon.
Here we demonstrate electric tuning of direct/indirect optical transitions with speciallyprepared Si/SiO2 interfaces [1], where the valley splitting is tuned by a gate electric field andits value is at least one order of the magnitude larger than that at an ordinary metal oxide
semiconductor (MOS) interface [2]. For conventional Si MOS interfaces, the valley splitting
is well explained by the extended zone effective mass theory [3], where real space
confinement causes valley interaction. By extending this theory, it is expected that the valley
splitting will lead to phononless direct optical transitions in Si. We therefore anticipate thatthe direct optical transition is significant and it can be controlled by the gate electric field in
the interface with the large valley splitting.
Devices are Si MOS field effect transistors with 4.3- or 6-nm-thick Si quantum well (QW)
fabricated on SIMOX (separation by implantation of oxygen) (001) wafer annealed at1350 °C for 40 hours. It is known that large valley splitting appears when electrons are
squeezed to the buried oxide interface of SIMOX. Heavily doped p- and n-type contacts areformed in order to inject holes and electrons into a channel (Fig. 1). The front (FG) and backgate (BG) control the distribution of both carriers in the Si QW. Figure 2 shows typicalelectroluminescence (EL) spectra at various back-gate voltages (VBG). For VBG < 0, an
indirect TO phonon mediated optical transition dominates the EL spectra, where the valley
splitting (2∆) is at most several mili-electron volts. In contrast, for VBG > 0, the EL intensitiesof phononless direct optical transition (NP) become dominant with increasing VBG. The
intensity ratio of NP to bulk NP (the NP intensity at VBG = 0 V for the device with thicker
QW) reached 800 at VBG = 80 V, where the valley splitting is estimated to be 30 meV. Thus
we demonstrate electric tuning of direct/indirect optical transitions in Si just by changing gate
voltage.
This work was partly supported by the Funding Program for Next Generation WorldLeading Researchers of JSPS.
[1] J. Noborisaka et al., Sci. Rep. 4, 6950 (2014).
[2] K. Takashina et al., Phys. Rev. Lett. 96, 236801 (2006).
[3] F. J. Ohkawa and Y. Uemura, J. Phys. Soc. Jpn. 43, 907 (1977).

Fig. 1. Device structure.

Fig. 2. Electroluminescence spectra.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)29

High-Speed Single-Electron Transfer via a Single Trap Level in Si
Gento Yamahata, Katsuhiko Nishiguchi, and Akira Fujiwara
Physical Science Laboratory
Single-electron (SE) transfer is a technique for accurately conveying an SE insynchronization with a clock signal, which is expected to be applied to a current standard andto low-power-consumption information processing. For the current standard, high-speed
operation (more than gigahertz) with an error rate below 10-8 is necessary. While SE transfer
using an electrically formed quantum dot has been widely studied, SE transfer using a traplevel with a large activation energy is expected to have higher accuracy. However, thepossibility of high-speed trap-mediated SE transfer is not obvious. Here, we report theachievement of high-speed SE transfer at 3.5 GHz using a trap level in Si [1].
Figure 1(a) shows a schematic of the device. Double-layer polycrystalline-Si gateelectrodes on a Si wire with a width of a few ten nanometers were formed using electron
beam lithography. Each lower gate (G1, G2) is used to form a potential barrier in the Si wire.
The upper gate is used to modulate the potential between G1 and G2. We selected a device
that has a single trap level (most likely an interface trap) under the right edge of G1 andmeasured SE transfer current via the trap level at 17 K. To perform the SE transfer, we
modulate a potential barrier by applying a high-frequency signal (frequency f) to G1, with afixed negative voltage applied to G2 to create another potential barrier [Fig. 1(b)]. An SE is
captured by the trap level from the source when the barrier under G1 is low. After that, when
the barrier under G1 is high, the captured SE is emitted to the drain. This results in a transfer
current level of ef (e is the elementary charge). Detailed measurements reveal that the captureand emission can become fast by lowering the barrier height under G1 during the capture
phase and by applying a strong electric field at the trap level during the emission phase,respectively. This can be achieved when the high-frequency signal has a large amplitude.
Under this condition, we achieved high-speed operation at 3.5 GHz [Fig. 1(c)], in which thetransfer error rate is below the level that can be measured using a commercial current meter(~ 10-3). Theoretically, we found that 1-GHz operation with an error rate below 10-8 may bepossible. In the future, we will evaluate the absolute accuracy [2] of the trap-mediated SE
transfer to realize high-accuracy and high-speed SE transfer toward application to the current
standard.
This work was partly supported by the Funding Program for Next Generation WorldLeading Researchers of JSPS (GR 103).
[1] G. Yamahata, K. Nishiguchi, and A. Fujiwara, Nature Commun. 5, 5038 (2014).
[2] G. Yamahata, K. Nishiguchi, and A. Fujiwara, Phys. Rev. B 89, 165302 (2014).

(c)

G1

SE emission

1.0

Current (nA)

SiO2

(b)

SE

SE capture

G2

SourceSi wireDrain

Trap level

0.8
0.6

1

0.4

f = 3.5 GHz

0.2
0

-0.45

-0.25
VUG (V)

Current (ef )(a)0Fig. 1. (a) Schematic of the device. (b) Electron potential diagram during the trapmediated transfer. (c) High-speed trap-mediated SE transfer (T = 17 K).

30

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Dynamic Control of Phonon Vibrations
in a One-Dimensional Phononic Crystal
Daiki Hatanaka1, Imran Mahboob1, Koji Onomitsu2, and Hiroshi Yamaguchi1
1
Physical Science Laboratory, 2Materials Science Laboratory
Phononic crystals (PnCs) are one of the most promising candidates for controlling phonon
propagation [1]. The periodically modulated elastic structure of the PnC can sustain a phonon
bandgap that enables the transmission of phonons such as sound, mechanical vibration, andeven heat to be spatially controlled. This ability to control phonons has led to the concept of anovel phononic system in which information signals encoded in phonons can be processed.
The key challenge to realizing this tantalizing prospect is the dynamic control of the phonons
in the PnC, whereas to date, almost all PnC devices reported have been passive structures. To
overcome this limitation, we have developed a unique class of PnCs via electromechanicalresonators and have demonstrated dynamically controlled phonon vibrations [2].
The PnC waveguide (WG) consists of a one-dimensional array of GaAs/AlGaAs-based
membrane resonators as shown in Fig. 1(a) which are suspended via the periodically arranged
air-holes. At the center of the WG, a control mechanical resonator is created by increasing the
air-hole separation to the adjacent membranes. Applying an alternating voltage with 5.74
MHz to the electrode on the right edge membrane can piezoelectrically excite phonon waves
which travel down to the WG through the control mechanical resonator and are opticallymeasured at the left edge membrane [dashed line in Fig. 1(b)]. Simultaneously, the control
mechanical resonator is excited at 1.86 MHz which generates a localized phonon vibration.
The strongly confined phonons in the control mechanical resonator induce a nonlinear elastic
effect that causes the WG’s transmission spectrum to shift to higher frequencies, resulting inthe suppression of the phonons at 5.74 MHz [solid line in Fig. 1(b)].
The electromechanical resonator based PnC WG permits the phonon transmission to be
dynamically controlled which opens up the possibility of developing highly functional signal
processing systems utilizing mobile phonons.
This work was supported by KAKENHI.
[1] M. Maldovan, Nature 503, 209 (2013).
[2] D. Hatanaka et al., Nature Nanotech. 9, 520 (2014).

(b)

10 µm 25 µm

30 µm

Displacement (nm)

(a)

0.4

0.3

0.2

0.1

0.0
5.72

WaveguideControl
Waveguidemechanical resonator

5.74

5.76

Frequency (MHz)Fig. 1. (a) The control mechanical resonator-embedded one-dimensional PnC measured at room temperatureand in a high vacuum. (b) WG’s transmission switching via control mechanical resonator.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)31

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Two-Mode Squeezing in an Electromechanical Resonator
Imran Mahboob, Hajime Okamoto, and Hiroshi Yamaguchi
Physical Science Laboratory
The widespread availability of quantum entanglement with photons, in the guise of twomode squeezed states, can be attributed to the phenomenon of parametric down-conversion. A
reinterpretation of this effect with macroscopic mechanical objects can offer a route towards apurely mechanical entanglement and the unique possibility of probing the quantum
mechanical nature of our everyday classical world. In spite of this prospect, mechanical twomode squeezed states have remained elusive due to the inability to recreate the nonlinear
interaction at the heart of this phenomenon in the mechanical domain.
To address this we have developed a non-degenerate parametric down-converter, in amechanical resonator integrated with electrical functionality, which enables mechanicalnonlinearities to be dynamically engineered to emulate this interaction [1]. In thisconfiguration, phonons are simultaneously generated in pairs in two macroscopic vibration
modes which results in the amplification of their motion by more than 20 dB. In parallel,mechanical two-mode squeezed states are also created which exhibit fluctuations 5 dB below
the thermal level of their constituent modes as well as harbouring correlations between themodes that become almost perfect as their amplification is increased. This remarkable
observation of correlations between two massive phonon ensembles paves the way towards anentangled macroscopic mechanical system at the single phonon level.
This work was supported by KAKENHI[1] I. Mahboob et al., Phys. Rev. Lett. 112 167203 (2014).

Fig. 1. (a) An electron micrograph of the coupled GaAs based electromechanical resonator, sustaining asymmetric (S) and an asymmetric mode (A), whose profiles extracted from a finite element analysis are also
depicted. The mechanical elements are integrated with piezoelectric transducers that can induce tension andparametrically modulate the spring constant. (b) The thermal Langevin force driven motion of the symmetric
XS:YS (red points) and asymmetric XA:YA (not shown) modes yield a circular distribution in phase spaceindicating their uncorrelated random nature. Piezoelectrically activating the non-degenerate parametric downconversion, at the sum frequency of both modes (ωS+ωA), results in two-mode squeezing which can be observedin the cross-quadratures namely XA:YS (black points) and XS:YA (not shown). (c) The two-mode squeezed statecorresponds to the fluctuations in both modes becoming correlated which can be readily observed via the nonzero off-diagonal elements in the corresponding Pearson correlation coefficient matrix.

32-

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

New Approach for a Long-Lived Quantum Memory
Xiaobo Zhu1, Yuichiro Matsuzaki1, Robert Amsüss1,3,
Kosuke Kakuyanagi1, Takaaki Shimo-Oka4, Norikazu Mizuochi4, Kae Nemoto5,
Kouichi Semba1,5, William J. Munro2, and Shiro Saito1
1
Physical Science Laboratory, 2Optical Science Laboratory,
3
TU Wien, 4Osaka University, 5NII
A hybrid approach to couple different quantum systems is considered as a promising way
to realize quantum computation, because such an approach has a potential advantage tocombine the best properties of each system. One of the attractive hybrid approaches is to use
a superconducting flux qubit and a spin ensemble in diamond [1-3]. The former system plays
a role of a processor while the latter system is useful for a memory. In this hybrid system, an
unknown long-lived state was observed [1, 2]. Although this long-lived state has a potential
application for a memory, there was no theoretical explanation why the long-lived system
was observed, which made it difficult to use this state for quantum computation.
We have found the mechanism how the state was so-long-lived, and have shown that this isan evidence of a dark state in the diamond [4]. A dark state has a destructive interference sothat the signal from this state should be significantly suppressed. However, we theoreticallyshow that, in this hybrid system, magnetic fields and strain variations decrease the effect ofthe destructive interference, which leads to a detectable signal from the dark state. We
experimentally show that the lifetime of the dark state was around 150 ns [4], which is much
longer than the previously observed lifetime of 20 ns in the diamond [3].
Since we understand the origin of this long-lived state, our results open a new way torealize a quantum memory for the storage of information by using this dark state. If such aquantum memory is realized, we can reduce the resource and cost for quantum computation
to provide us with ultra-fast calculation. This work was supported by FIRST and NICT.
[1] Y. Kubo et al., Phys. Phys. Lett. 105, 140502 (2010).
[2] X. Zhu et al., Nature 478, 221 (2011).
[3] S. Saito et al., Phys. Rev. Lett. 111, 107008 (2013).
[4] X. Zhu et al., Nature Commun. 5, 3424 (2014).

Fig. 1. Energy diagram and spectroscopy of the superconductor diamond system. A sharp peak is observed
between the broader peaks. The former one denotes a long-lived dark state while the latter one corresponds toa short-lived state that was observed before.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)33

Resonant Edge Magnetoplasmons and Their Decay in Graphene
Norio Kumada1,3, Preden Roulleau3, Benoit Roche3, Masayuki Hashisaka4, Hiroki Hibino2,
Ivana Petković3, and D. C. Glattli3
1
Physical Science Laboratory, 2 Materials Science Laboratory, 3CEA Saclay,
4
Tokyo Institute of Technology
In quantum Hall states, which appear in two-dimensional systems under perpendicularmagnetic field, current flows along sample edge. The edge channels provide unique onedimensional systems to perform a variety of quantum transport experiments. Edgemagnetoplasmons (EMPs) are collective charge excitations in the edge channels.
Investigation of EMP properties, in particular, the decay mechanism is essential to obtainrobust quantum effects in such systems. In this work, we show the dispersion relation anddissipation mechanisms of EMPs in graphene [1].
We prepared graphene by thermal decomposition of SiC. The samples used have disk
shaped graphene. High-frequency transmission between two electrodes, which couple tographene only capacitively, was investigated in frequency and time domains [Fig. 1(a)]. The
dispersion relation of EMPs obtained by the frequency-domain measurement is nonlinear due
to carrier interactions [Fig. 1(b)]. The velocity, which corresponds to the slope of thedispersion relation, is 1.7 × 106 m/s for smaller wave number, consistent with that obtainedby time-of-flight measurement [2]. This value is larger than the Fermi velocity in graphene(106 m/s), demonstrating that charges propagate in collective modes, that is, EMPs. The
EMP decay time is directly obtained by the time-domain measurement [Fig. 1(c)]. From its
frequency and temperature dependence, EMP dissipations are found to be caused bycapacitive and resistive couplings to localized states in the bulk graphene. We suggest that,
owing to the linear band structure and the strong edge confinement, EMP dissipation ingraphene can be lower than that in GaAs systems. Our results encourage using graphene forquantum transport experiments and plasmonic applications.
[1] N. Kumada et al., Phys. Rev. Lett. 113, 266601 (2014).
[2] N. Kumada et al., Nature Commun. 4, 1363 (2013).

Fig.1. (a) Device structure. Graphene is circular shaped with the perimeter of 200 or 1000 µm. Two highfrequency lines to inject and detect EMPs are capacitively coupled to graphene. (b) Dispersion relation of
EMPs determined by frequency domain measurement. The temperature is 4 K and the magnetic field is 10 T.
(c) Example of time domain measurement. Motion along the circular path with decay of EMPs is recorded asa function of time.

34

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Josephson Coupling in Semiconductor/Superconductor
Hybrid Quantum Point Contacts
Hiroshi Irie1, Yuichi Harada1, Hiroki Sugiyama2, and Tatsushi Akazaki1
1
Physical Science Laboratory, 2NTT Device Technology Laboratories
The conductance of a ballistic point contact linking two reservoirs at thermal equilibrium isquantized in multiples of the conductance quantum 2e2/h. The origin of this phenomenon is
the quantization of the transverse momentum in the narrow constriction. Intriguingly, theconductance of the contact remains finite even though no scattering is assumed at theconstriction. This raises the question as to what happens if we replace the reservoirs withsuperconductors. This question was answered by Beenakker and van Houten a few years after
the discovery of the conductance quantization [1]. They theoretically analyzed asuperconducting quantum point contact (SQPC) made of a smooth and impurity-freesuperconducting constriction, and showed that superconducting Josephson current flows up toe∆/ћ (∆: superconducting gap) per quantized mode. Even more than 20 years after thetheoretical prediction, there have been only a limited number of reports on its experimental
verification [2]. In this work, we study a semiconductor/superconductor hybrid QPC thatexhibits staircase variation of the Josephson critical current Ic with respect to the number ofquantized mode n, which provides a compelling evidence of the quantization of Ic [3].
The hybrid QPC consists of a QPC formed in a high-In-content In0.75Ga0.25As twodimensional electron gas (2DEG) and two Nb electrodes in the vicinity of the QPC [Fig. 1].
Josephson coupling is attained via Andreev reflection of Bogoliubov quasiparticles confinedin the 2DEG. The transport properties of the quasiparticles, and hence the Josephson junction
characteristics, can be controlled by means of the external electric field from a gate electrode.
Figure 2 shows the gate voltage Vg dependence of Ic measured at 20 mK. Ic shows a clear
stepwise variation as n changes with Vg, demonstrating a Josephson coupling through thequantized mode. The experimental value of Ic per mode is 10.3 nA, which is one order ofmagnitude smaller than the theoretical value of 125 nA. This discrepancy is accounted for by
the presence of a proximity layer having smaller ∆ at the InGaAs/Nb interface. The
suppressed stepheights for n = 1 and 2 are caused by the finite temperature effect. From thoseobservations, we experimentally prove the quantization of Josephson current and demonstrate
a single-channel SQPC for the first time.
This work was supported by KAKENHI.
[1] C. W. J. Beenakker and H. van Houten, Phys. Rev. Lett. 66, 3056 (1991).
[2] H. Takayanagi et al., Phys. Rev. Lett. 75, 3533 (1995).
[3] H. Irie et al., Phys. Rev. B 89, 165415 (2014).

80

Ic (nA)

60

Experimental
Calculated

4020
Temp: 20 mK
0-2.5

Fig. 1. Hybrid quantum point contact.

-2.0

Vg (V)-1.5

Fig. 2. Quantization of the Josephson critical current.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)35

NMR Probing of Wigner Solids in High Magnetic Fields
1

Trevor David Rhone1,2, Lars Tiemann1,2, Naokazu Shibata3, and Koji Muraki1,2
Physical Science Laboratory, 2Japan Science and Technology Agency, 3Tohoku University

In two-dimensional electron systems subject to strong magnetic fields, various quantum
phases emerge due to electron interaction. While it is believed that electrons form Wigner
solids in the low filling factor regime, where the sequence of fractional quantum Hall statesterminates, experimental evidence for Wigner solids has been limited to microwave
absorption measurements which show the presence of pinning-mode resonances characteristicof a solid. Here we focus on another defining feature of a solid - translational symmetry
breaking. We use NMR to probe on a nanometer scale the spatial variation of probability
density resulting from electron solidification, and to obtain microscopic information
including the spatial extent of lattice electrons [1].
Figure 1 shows the filling factor dependence of longitudinal resistance and resistively
detected NMR spectra of 75As taken at various filling factors at 6.4 T. At  = 1/3, where theelectron system is in a fractional quantum Hall liquid phase, the measured NMR spectrum
matches a simulation assuming a uniform electron system. In contrast, at  < 1/3 and in thevicinity of  = 2, the measured spectra cannot be explained by the uniform model. Instead, amodel assuming the formation of Wigner crystal domains of electrons or electrons/holes
added to  = 2 reproduces the measured spectra exceedingly well. The difference in the
spectral lineshapes for  = 1.9 and 2.1, which share the same effective filling of 0.1, is amanifestation of the distinct single-particle wavefunctions for the respective Landau levels.
This highlights NMR’s ability to resolve probability density variation on a nanometer scale.
[1] L. Tiemann*, T. D. Rhone*, N. Shibata, and K. Muraki, Nature Phys. 10, 648 (2014). (*: These authors
contributed equally to this work.)

Fig. 1. (a) Filling factor dependence of longitudinal resistance. (b)-(d) Resistively detected NMR spectraof 75As taken at various filling factors. The horizontal axis is the frequency shift with respect to theresonance frequency at  = 2. Dashed lines represent simulations assuming uniform electron systems.
Solid lines are numerical fits using a model incorporating the presence of Wigner crystal domains. Insets:
illustrations of occupied Landau levels for (b)  = 0.2, (c) 1.9, and (d) 2.1.

36

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Flat-Band Ferromagnetism in the Multilayer Lieb Optical Lattice
Kazuto Noda1,2, Kensuke Inaba1,2, and Makoto Yamashita1,2
1
Optical Science Laboratory, 2Japan Science and Technology Agency
Ultracold atom gases can be trapped in an optical lattice using lasers. The highcontrollability of atoms provides us with an ideal tool for investigating quantum many-bodysystems, such as mimicking electrons in bulk materials. Thus, the system can be regarded as aquantum simulator, which stimulates theoretical and experimental efforts to observe quantum
magnetism using fermionic atoms. The Kyoto university group, led by Prof. Yoshiro
Takahashi, has successfully created the Lieb optical lattice [Fig. 1(a)], which attracts muchattention as mentioned below. The band structure of this two-dimensional Lieb lattice showsa dispersionless band (flat band), which may induce the exotic magnetic ordering state (flatband ferromagnetism). The emergence of this state is exactly shown (Lieb theorem [1]),which has attracted theoretical attention. The realization of the Lieb optical lattice provides anideal opportunity for the first observation of the flat-band ferromagnetism.
In the experiment, a multilayer Lieb optical lattice [Fig. 1(b)] (𝐿𝐿 represents the number oflayers) is inevitably realized. To observe the flat-band ferromagnetism, we theoreticallyinvestigate how this multilayer structure affects magnetic ordering [2]. We reveal that theproperty of magnetic ordering is quite different between odd layers (𝐿𝐿 = 2𝑙𝑙 − 1, where 𝑙𝑙 is apositive integer) and even layers (𝐿𝐿 = 2𝑙𝑙). Figure 2 shows average magnetization, which is anorder parameter of the magnetic ordering, as a function of atom-atom interaction strength. For
odd layers (𝐿𝐿 = 2𝑙𝑙 − 1), Fig. 2 (a) shows that finite magnetizations appear with quite weakinteraction strength. This indicates the emergence of the flat-band ferromagnetism, which canbe regarded as a generalized consequence of that on a single layer (𝐿𝐿 = 1, the twodimensional Lieb lattice mentioned above). Further, we also find the gradual change of themagnetic ordering states from the two dimensions (𝐿𝐿 = 1) to the three dimensions (𝐿𝐿 → ∞).
This result exhibits that the flat-band magnetism, well-known for the two dimensions, appears
even in the three dimensions. On the other hand, for even layers (𝐿𝐿 = 2𝑙𝑙) [Fig. 2 (b)], we
reveal that magnetization develops at finite interaction strength. As 𝐿𝐿 increases, this criticalinteraction strength decreases. Thus, in the limit of 𝐿𝐿 → ∞ (three dimensions), the difference
between odd and even layers disappears and the flat-band magnetism appears. In experiment,a multilayer Lieb optical lattice that consists of several tens of layers will show the threedimensional flat-band magnetism, which is revealed in our study.
This work was supported by JST-CREST and KAKENHI.
[1] E. H. Lieb, Phys. Rev. Lett. 62, 1201 (1989).
[2] K. Noda, K. Inaba, and M. Yamashita, Phys. Rev. A. 90, 043624 (2014).

Fig. 2. Average magnetization (magnetic order parameter) vs.
Fig. 1. (a) Lieb lattice and (b)multilayer Lieb lattice.

interaction of the multilayer Lieb lattice with 𝐿𝐿 [(a) 𝐿𝐿 = 2𝑙𝑙 − 1and (b) 𝐿𝐿 = 2𝑙𝑙]. Results at zero temperature and half filling [2].

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)37

Quantum Key Distribution over a 72 dB Channel Loss Using Ultralow
Dark Count Superconducting Detectors
1

Hiroyuki Shibata1, Toshimori Honjo2, and Kaoru Shimizu1
Optical Science Laboratory, 2NTT Secure Platform Laboratories

Quantum key distribution (QKD) offers ultimately secure communication based on thelaws of quantum mechanics. For the realization of QKD network, it is necessary to increase
the maximum distance of QKD. Previously, we have demonstrated the QKD over 200 km(42.1 dB loss) in fiber using superconducting nanowire single photon detectors (SSPDs) [1].
Here, we have successfully improved the S/N ratio of SSPDs by 30 dB [2], and demonstrated
the QKD over 336 km (72 dB loss) in fiber [3], which is the today’s world record of QKD
distance.
In QKD, faint single photons are usually used as a carrier of information. When the fiber
length increases, the loss of the fiber increase and the signals of single photons are masked by
the noise. It is impossible to distribute the secure key when the quantum bit error rate
(QBER) exceeds a certain level. Therefore, we need a single-photon detector with higher S/Nratio for the long distance QKD. We find that the origin of the dark count rate (DCR) of
SSPD is the blackbody radiation at room temperature which propagate through an opticalfiber, and introduce the cold optical bandpass filter to block the radiation [2]. It is possible toimprove the S/N ratio of SSPDs more than 30 dB using the cold optical filter. Figure 1 shows
the system detection efficiency () and DCR of the SSPD. At a bias current of 21.5 A,  =4.4% and DCR < 0.01 cps is achieved.
Using the SSPDs with ultralow DCR, we perform the QKD experiment under thedifferential phase shift QKD scheme using a weak coherent light. Figure 2 summarizes theresults of our experiment. At the loss of 72 dB (336 km fiber), we can generate the securekeys from the sifted keys since the QBER is still below 4.1% [3].
[1] H. Takesue et al., Nature Photon. 1, 343 (2007).
[2] H. Shibata, K. Shimizu, H. Takesue, and Y. Tokura, Appl. Phys. Express 6, 023101 (2013).

10

2

10

6

10

4

10
10

2

-2
-3

21

22

23

10
24

Key Rate (bit/s)

0
-1

20

4

10

1

DCR = 0.01 cps

25

3

10

2

10

Sifted20

Secure

15

1

10

0

10

306 km fiber-1

336 km fiber

10
10

10
5

-2

20

30

30

40

50

QBER (%)

10

10

38

5

3

14
12
10
8

System DCR (cps)

System  (%)[3] H. Shibata, T. Honjo, and K. Shimizu, Opt. Lett. 39, 5078 (2014) : Selected for Spotlight on Optics.

60

70

80

0

Bias (Channel Loss (dB)

Fig. 1. Bias current dependences of detection
efficiency and dark count rate of our SSPD.

Fig. 2. Channel loss dependence of sifted key rate,secure key rate, and QBER of the experiment.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

On-Chip Generation and Demultiplexing of Quantum Correlated Photons
Using Si-Silica Monolithic Waveguide Platform
Nobuyuki Matsuda1,2, Hidetaka Nishi2,3, Tai Tsuchizawa2,3, William J. Munro1,
Hiroki Takesue1, and Koji Yamada2,3
1
Optical Science Laboratory, 2NTT Nanophotonics Center,3
NTT Device Technology Laboratories
Integrated photonics technology has proven useful for the large-scale integration ofquantum information systems on photonic chips thanks to its compactness and interferometricstability. Intense study is under way on the development of on-chip quantum components,
such as quantum light sources [1], quantum circuits [2], and single photon detectors [3]. The
next step is the integration and interconnection of these components on a single substrate.
This time we have realized the monolithic integration of a Si waveguide photon pair source
and a photon-pair demultiplexer employing a silica-based arrayed waveguide grating (AWG)[4], which can facilitate interconnecting the quantum light source and the circuit.
The circuit is schematically depicted in Fig. 1(a). In the silicon waveguide part, acorrelated pair of signal and idler photons is created via spontaneous four-wave mixing
process following the annihilation of two photons inside the pump pulse. Thanks to the highmaterial nonlinearity of Si as well as the strong light confinement of the waveguide, nondegenerate photon pairs are efficiently created in the telecommunication wavelength. The
photon pairs are subsequently spectrally separated [Fig. 1(b)] by AWG into different output
channels. We employed silica-based material (SiOx), which is a low nonlinear material, as thecore of the AWG in order to mitigate noise photon generation outside of the photon pairsource. The integration of the two quantum photonic components has been realized with themonolithic photonic integration technology [5]. Using the device, we have successfully
demonstrated the on-chip generation and demultiplexing of quantum correlated photons on achip [Fig. 1(c)]. Furthermore, we have confirmed that the silica part of the device does notcontribute to noise photon generation.
The device can be used as a compact correlated photon pair source, and will be useful formany quantum information applications including wavelength-division multiplexing quantum
communication technologies and heralded single photon sources. Moreover, the silica-based
AWG can provide an interface between a Si-based photon pair source and silica-based
lightwave circuits, which are useful as linear-optics-based quantum circuits.
[1] N. Matsuda et al., Sci. Rep. 2, 817 (2012).
[2] A. Politi et al., Science 320, 646 (2008).
[3] W. H. Pernice et al., Nature Commun. 3, 1325 (2012).
[4] N. Matsuda et al., Opt. Express 22, 22831 (2014).
[5] H. Nishi et al., Appl. Phys. Express 3, 102203 (2010).
Signal channel

PumppulsesSignalphotons

Si waveguide

Silica-based AWG

Idler
photons

Transmission intensity(arb. units in dB)(b)

Idler channel

(c)

0

-1

10

-10
-20
-30
-40

1555

1560
1565
Wavelength (nm)Net pair generation
rate (/pulse)

(a)

-2

10

-3

10

-4

10

-5

10

1

10

100

Pump peak power (mW)

Fig. 1. (a) Monolithic chip housing a Si wire photon pair source and a silica-based arrayed waveguidegrating. (b) Transmission spectra of AWG output ports. (c) Photon pair generation rate.
Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)39

Loss-Tolerant Quantum Cryptography with Imperfect Sources
1

K. Tamaki1 M. Curty3 G. Kato2 Hoi-Kwong Lo4, and K. Azuma1
Optical Science Laboratory, 2NTT Communication Science Laboratories,
3
University of Vigo, 4University of Toronto

Quantum key distribution (QKD) is a way to distribute a random bit sequence, which iscalled a key, between a sender (Alice) and a receiver (Bob). Its striking advantage is that theleakage of the information on the key to an eavesdropper (Eve) can be arbitrary set according
to Alice and Bob’s demand. In order to achieve such high security, the QKD devices must
operate as the security theory requires. For instance, in the case of the most well-known
protocol, BB84 [1], the phase modulator is required to apply an exact phase modulation,
which is randomly chosen from 0, π/2, π, 3π/2, for each pulse. However, such exact values
can never be achieved due to the imperfections in the modulator.
This kind of modulation error would not seem to be so problematic to affect the security,
unfortunately however, this is not the case. In a typical QKD communication, due to theinevitable photon losses in the quantum channel, a single photon is detected only once out of1000 emission pulse. Eve may exploit these losses in such a way that only when she succeeds
in exploiting the modulation error, she sends out a pulse to Bob to cause the detection event.
In other words, she may enlarge the effective modulation errors. This is exactly what
happened in the existing theory [2]. Figure 1(a) is a plot of the key generation rate against the
distance between Alice and Bob, and the solid, dashed and dotted lines respectively represent
the case with perfect modulation, the modulation error case with about 3.6 degree, and themodulation error case with about 7.2 degree. One can see in the figure that the small errors
have a severe impact on the achievable communication distance.
In our work [3], we have proposed a technique to drastically decrease the effect that theimperfect state preparation has on the communication distances. This is made possible byutilizing the data, which was used to be discarded in the BB84 protocol. Figure 1 (b) is theplot of the key rate against the distance based on the same experimental data as used in Fig.
1(a). The three lines are almost superposed, implying that the state preparation flaw is notproblematic. Moreover, it can be concluded from our observation that sending three states isenough in the BB84 where sending four states was considered to be needed.
Our research is in part supported by FIRST and NICT.
[1] C. H. Bennett and G. Brassard, Proceedings of IEEE International Conference on Computers, Systems, andSignal Processing, IEEE Press (New York), 1984, pp. 175-179.
[2] D. Gottesman et. al., Quant. Inf. Comput. 5, 325-360 (2004).
[3] K. Tamaki et.al., Phys. Rev. A 90, 052314 (2014).
Three-state protocol with decoy states

log10R

log10R

Decoy-state BB84 protocol based on GLLP(a)Distance (km)(b)

Distance (km)Fig. 1. The key rate vs. the communication distance. (a) based on [1]. (b) based on our technique [2].
40Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

The Design of a Scalable and Distributed Quantum Information
Processor in DiamondWilliam J. Munro1, Michael Trupke2, Simon J. Devitt3, Ashley M. Stephens3,
Burkhard Scharfenberger3, Kathrin Buczak2, Tobias Nöbauer2, Mark S. Everitt3,
Jörg Schmiedmayer2, and Kae Nemoto3
1
Optical Science Laboratory, 2TU Wien, 3NII
Physics and information are intimately connected together. It is now widely
accepted that the ultimate “information” processing devices will be those that harness
and utilize the principles of quantum mechanics. There have been many physicalsystems identified that can be used for such processing [1]. Many proof of principle
experiments have been performed, but none of these are immune from errors. One
needs to find a path from today’s experiments to tomorrow’s scalable quantum
computers and communication networks. Here, we develop an architecture based on asimple quantum module comprising an optical cavity containing a single negatively
charged nitrogen vacancy center (NV-) in diamond. The modules can be connected
together by photons propagating in an optical fiber network and thus collectively used
to generate a topological cluster state, a robust substrate for quantum information
processing. We show that this architecture enables us to start with a few modules andscale up to large capacity quantum information processing with existing technology [2].
The module at the center of this approach is depicted in Fig. 1(a) where the cavities
mediate interactions between the photons and the electron spins. The electron spins arecoupled to nuclear spins: the later constitute long-lived quantum memories where
quantum information is stored and processed. The electron spin is acting as aninterface between the nuclear spin and optical photons. Entanglement can be generated
between two modules using a single photon propagating in the fiber-optical network[Fig. 1(a)]. The scheme works by dividing a photon on a 50/50 beam splitter andsending each half to a module where it interacts (gaining a phase shift if the electron
spin is in its |1> state). The reflected photon parts from each module are thenrecombined on the beam splitter and the dark port measured. If the detector clicks,
entanglement has been generated between the electron spins in each module [2]. Thisentanglement is then transferred to the nuclear spins creating a long-lived link between
the modules. Once this has been done entanglement generation can be attempted withfurther modules allowing a large-scale cluster state to be generated [Fig. 1(b)]. Such a
resource is useful for universal quantum computation and long-range quantum
communication.
This work was supported in part by NICT.
[1] T. D. Ladd et al., Nature 464, 45 (2010).
[2] K. Nemoto et al., Phys. Rev. X 4, 031022 (2014).

Fig. 1. NV center based quantum architecture. (a) Entanglement of two quantum modules using a single
photon interferometer (inset shows the energy level structure of the NV center). (b) Entanglement
between many modules creating a 3D lattice for general quantum computation and communication.
Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)41

Characterizing Inner-Shell Electron Motion with Isolated Attosecond
Pulse
Hiroki Mashiko, Tomohiko Yamaguchi, Katsuya Oguri, and Hideki Gotoh
Optical Science Laboratory
We aim to observe and manipulate ultrafast electron motion using attosecond (10-18 s) laser
pulses, which is shortest optical pulse in the world. In an atom, electrons are classified into
outer shell (valance-shell) and inner shell (core-shell) types. In common devices, the outershell with a low energy band gap (a few electronvolts) is used. However, since the inner shell
has a larger band gap (a few orders magnitude higher than the outer shell), the electron
motion is much faster. For instance, the decay time of an excited inner-shell electron is on ascale from a few attoseconds to several hundred femtoseconds (that of an outer-shell electron
is on the scale of nanosecond).
We successfully observed electron motion with the dipole response in the inner shell using
the combination of an IAP (isolated attosecond pulse) and SPIDER (spectral phaseinterferometry for direct electric field reconstruction) method [1]. We generated the IAP with
192-as duration –one of the shortest laser pulses in the world− and this IAP can approach thetime scale of the electron motion. Figure 1 shows the dipole response in neon atom (2s-3p
states) induced by the IAP. The dipole response produces the coherent photoemission, whichis interfered with the IAP and detected by a spectrometer. The SPIDER method can fully
characterize the dipole response (the decay time, dipole phase, and periodicity of dipole
oscillation). The decay time indicates 35 fs (10-15 s), which is million times shorter than
general outer-shell decay time. In addition, the periodicity of dipole oscillation corresponds to
90 as. Principally, the periodicity produces 10 PHz (1015 Hz) responses for the optical device.
The achievement for inner shell with ultrafast motion may pave the way for the developmentof new types of ultrafast devices.
[1] H. Mashiko, T. Yamaguchi, K. Oguri, A. Suda, and H. Gotoh, Nature Commun. 5, 5599 (2014).

Fig. 1. Characterizing dipole response with inner shell in neon atom using the IAP.
42

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Multi-Stacked InAs/InP Heterostructure Nanowires Grown
by Self-Assisted Vapor-Liquid-Solid Mode
Guoqiang Zhang, Kouta Tateno, and Hideki Gotoh
Optical Science Laboratory
Semiconductor nanowires (NWs) have provided a new platform for fundamental physics
research and have become next-generation building blocks in photonics, electronics, andenergy conversion. One crucial challenge for NWs has been the development of a CMOScompatible synthesis approach which produces semiconductor heterostructure NWs withexcellent optical properties. This remains challenging because gold (Au), which is widelyused as a catalyst particle when NWs are synthesized with the bottom-up vapor-liquid-solid(VLS) approach, is not permitted in the mainstream CMOS process.
Here, we report gold-free multi-stacked InAs/InP heterostructure NWs with controlled
luminescence wavelength in 1.1-1.6 µm. We realized the NW by using an indium-particleassisted VLS synthesis approach [1-3]. The growth temperature as low as 320°C enables in
the formation of an atomically abrupt InP/InAs interface by supressing the diffusion andweakening the reservoir effect in the indium droplet (Fig. 1). The low growth temperature
also enables us to grown multi-stacked InAs/InP NWs in the axial direction without any
growth on the NW side face in the vapor phase epitaxy mode.
The high controllability of the growth technology ensures that the luminescence can be
tailored by the thickness of InAs segment and cover the 1.3-1.5 μm telecommunication
window range. By using the nanoscale-spatial-resolution technology combing CL with SEM,
we directly correlated the site of different-thickness InAs segments with its luminescence
property in a single NW and demonstrate the InAs-thickness controlled energy of opticalemission in 1.1-1.6 μm (Fig. 2).
This work was supported by KAKENHI.
[1] G. Zhang et al., Appl. Phys. Express 5, 055201 (2012).
[2] G. Zhang et al., AIP Advances 3, 052107 (2013).
[3] G. Zhang et al., Nanotechnology 26, 115704 (2015).

Fig. 1. Multi-stacked InAs/InP hetero-NW. (a)Fig. 2. SEM-CL analysis. (a) CL spectra of a single

and (b) SEM images (tilt: 38º) of the InP/InAs

InP/InAs NW. (b) SEM image of an InP/InAs NW

NW grown on an InP (111)B substrate. (c)and the schematic diagram of the NW structure. (c)-HAADF-STEM

InP/InAs(f) Monochromatic mapping images of the four peak-heterointerface. The dashed arrow indicates theenergy values shown in (a). The arrows indicate theatomically-abrupt interface.

positions of the InAs layers with varied thickness.

image

of

the

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)43

Energy Transfer in Er2O3 Epitaxial Thin Layers
1Takehiko Tawara1,3, Hiroo Omi2,3, and Hideki Gotoh1
Optical Science Laboratory, 2Materials Science Laboratory, 3NTT Nanophotonics Center

The intra-4f orbitals in rare-earth ions are attractive as a robust material platform for the
manipulation of quantum information in a solid because they form deterministic and discrete
quantum levels that do not depend on either host material or temperature. We concentrate on
erbium oxide (Er2O3) epitaxial thin films, which can match the lattice constant of Si(111) andinteract with telecom-band photons. In this study, we investigate the dynamics of excited
populations in the intra-4f orbital of Er ions that are indispensable if we are to realize
quantum information manipulation.
Figure 1 shows a cross-sectional TEM image of a grown Er2O3 sample and a schematic ofa unit cell. The TEM image proves that the Er2O3 is epitaxially grown on the Si(111) surface
without crystalline defects [1]. A unit cell of Er2O3 contains 32 erbium ions, 24 at sites withC2 (non-inversion) symmetry and 8 at sites with C3i (inversion) symmetry. Since these energy
levels differ at different sites and exhibit narrow inhomogeneous broadening, we can selectthe excitation site and the level precisely by tuning the laser frequency. Figure 2 showsenergy diagrams of the ground [Z(’)1] and 1st excited [Y(’)1-5] states and a color plot ofphotoluminescence excitation (PLE) spectra. When the energy level of the C2 site is excitedresonantly (e.g. Y3 level), the PL is not from the same C2 site, and it is observed only fromthe spatially separate C3i site (e.g. Y’1-Z’1 transition). This means that energy migrates
between distant C2 and C3i sites as a result of a resonant energy transfer arising from a dipoledipole interaction or wave function overlap. We determined that the time constant of theenergy transfer from C2 to C3i is 8 µs by performing a simple rate-equation analysis. This ismuch shorter than the emission decay from the Y1 to Z1 state (100 µs), but is sufficiently longcompared with the time required to complete quantum information manipulation in Er2O3
crystals [2]. These results show that the Er2O3 epitaxial thin layers constitute a promisingmaterial platform for the manipulation of quantum information in a solid.
This work was undertaken in collaboration with Hokkaido University and was supported
by KAKENHI.
[1] H. Omi et al., Jpn. J. Appl. Phys. 51, 02BG07 (2012).
[2] T. Tawara et al., Appl. Phys. Lett. 102, 241918 (2013).

Fig. 1. Cross-sectional TEM image
and schematic of unit cell.

44

Fig. 2. Energy diagrams and PLE color plot.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Over-100-bit Integrated Optical RAM Chip
Eiichi Kuramochi1,2, Kengo Nozaki1,2, Akihiko Shinya1,2, Koji Takeda2,3, Tomonari Sato2,3,
Shinji Matsuo2,3, Hideaki Taniyama1,2, Hisashi Sumikura1,2, and Masaya Notomi1,2
1
Optical Science Laboratory, 2NTT Nanophotonics Center,3
NTT Device Technology Laboratories
Optical memory is a key device for all-optical routing/signal-processing. The former isapplicable for both broadband networks in telecommunication and data centers and on/offchip optical interconnects for large-scale CMOS electronic chips. The transportation ofphotonics over electronics is considered to be a way of dealing with the huge data-transfer
cost (power, heat, space, overhead), which becomes serious as the integration density
increases. We have previously demonstrated an ultra-small optical random access memory (oRAM) with an ultra-small operating power (30 nW) by using a high-Q photonic crystal
nanocavity [1]. However, the highly dense integration of many o-RAMs is difficult to achievewith the previous nanocavity design.
In this study, we employed a novel three-missing-hole nanocavity (L3) with the systematictuning of multiple holes [Fig. 1(a)], which enhanced the Q factor 10-fold compared with theconventional L3 [2]. Thanks to the narrow cavity mode linewidth, enhanced carrier-plasma
bistability, and wide free spectrum range, we could employ wavelength-division-multiplexing
technology to realize write/read-out access to every o-RAM via one common side-coupling
waveguide, and eventually 128-bit o-RAMs were integrated as a simple tandem array of 128
L3s [Fig. 1(b)] [3]. The lattice constant a at every L3 was changed from 408 to 424 nm in
0.125 nm steps to set a wavelength spacing of ~30 GHz and the cavity spacing was 20a. The
total length of the 128-bit integrated o-RAM was only 1.1 mm. We fabricated this precise 128
L3 array in a Si photonic crystal using electron beam lithography. In the experiment, werealized o-RAM operation by injecting bias light at the wavelength of the target o-RAM, andwe switched the “1” state and “0” state with set/reset pulses. As shown in Fig. 1(c), we
successfully demonstrated the o-RAM operation of 105 L3s. In addition to thegroundbreaking jump in bit number from the several bits of conventional o-RAMs, the total
operating power of 30 mW was surprisingly low. We applied the same technology to a moresophisticated InP-based nanocavity with an embedded buried heterostructure and realized
multi-bit o-RAM operation on lower power consumption and a second time scale [3].
The success of state-of-the-art nanophotonics has opened the way to the monolithic
integration of 1000 nanophotonics devices (or 1000 bits) and even one million devices on achip by employing parallel integration.
[1] K. Nozaki et al., Nature Photon. 6, 248 (2012).
[2] E. Kuramochi et al., Opt. Lett. 39, 5780 (2014).
[3] E. Kuramochi et al., Nature Photon. 8, 474 (2014).

(a)

(b)
(c)
Fig. 1. Over-100-bit integrated o-RAM chip. (a) L3 with novel hole-tuning. (b) Microscope images.
(c) Temporal output of 105 bits.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)45

Nanocavity-Enhanced Raman Scattering ofSingle-Walled Carbon Nanotubes
Hisashi Sumikura1,2, Eiichi Kuramochi1,2, Hideaki Taniyama1,2, and Masaya Notomi1,2
1
Optical Science Laboratory, 2NTT Nanophotonics Center
Carbon nanotubes emit intense spontaneous Raman scattering thanks to a strong electronphonon interaction under low-dimensional quantum confinement, and they are expected to
exhibit a large Raman gain. To observe the stimulated Raman scattering and Raman laser
action of carbon nanotubes, we study the nanocavity-enhanced Raman scattering ofsemiconducting single-walled carbon nanotubes (SWCNTs) placed in high-Q silicon
photonic crystal (Si PhC) nanocavities. Figure 1(a) shows a fabricated point-defect Si PhC
nanocavity covered with randomly oriented SWCNTs. The experimental cavity Q value was
1100. Figure 1(b) shows the Raman spectrum of a PhC sample when it was excited with a
940-nm-wavelength laser. An intense Stokes Raman peak is seen at around 1600 cm-1, whichis assigned to the G + mode of the SWCNTs. Figures 1(c) and (d) show the Raman spectra andpeak intensities of the G + mode, respectively, as the wavelength of the excitation laserchanges. Figure 1(c) also shows the PhC cavity mode at 1117 nm. When the Raman
scattering wavelength of the G + mode is adjusted to the cavity resonance, the peak Raman
intensity is resonantly increased. This result shows that the resonant PhC nanocavity
enhances the spontaneous Raman scattering of SWCNTs [1]. When we took the excitationand emission extraction efficiencies of our samples into consideration, we found that the
Raman intensity was two orders of magnitude larger than the intensity of SWCNTs on a flat
Si film. This enhanced Raman intensity results from a 20-fold increase in the emission
extraction efficiency and a 5-fold increase in the density of the optical state, both of which areprovided by the resonant PhC nanocavity. Further enhancement of the Raman scattering
intensity will need novel PhC nanocavities with an air slot holding many SWCNTs.
The nanocavity-enhanced spontaneous Raman scattering demonstrated here is the first step
towards realizing low-threshold and nanometer-scale Raman lasers based on a high-Q Si PhC
cavity and carbon nanotubes.

Intensity (arb. unit)

(b)
4000

G+

3000
2000

Excitation:
940.0 nm

-

G

1000
0

1400 1500 1600 1700 1800
Raman shift (cm-1)

G+ peak intensity (arb. unit)

954
952
950
948
946
944
942
940

(d)
8000
6000

(c)

3

x10
10
8
6
4
2

4000
3000
2000
1000
0

PL intensity (arb. unit)

Pump wavelength (nm)(a)

Raman intensity (a. u.)[1] H. Sumikura et al., Appl. Phys. Lett. 102, 231110 (2013).

1120
1140
1100
Emission wavelength (nm)PhC cavity
Flat Si
Fitted curve
Baseline

4000
2000
0
1105 1110 1115 1120 1125
Emission wavelength (nm)Fig. 1. (a) Scanning electron microscope image of a silicon PhC nanocavity covered with SWCNTs. (b)Stokes Raman spectrum of a fabricated PhC sample. (c) Raman spectra of a PhC sample as a function of
the excitation laser wavelength. The photoluminescence of the PhC nanocavity is also shown. (d) G +Raman peak intensity as a function of the Raman scattering wavelength.

46

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Emission Enhancement in a Nanowire Coupled to a Nanoantenna
Masaaki Ono1,3, Eiichi Kuramochi1,3, Guoqiang Zhang1, Hisashi Sumikura1,3, Yuichi Harada2,
David Cox4, and Masaya Notomi1,3
1
Optical Science Laboratory, 2Physical Science Laboratory, 3NTT Nanophotonics Center,4
National Physical Laboratory, U.K.
Nanoemitters such as quantum dots and nanowires have small active regions, and are goodcandidates for realizing optical devices with low power consumption, a small footprint, andhigh-speed responsivity. On the other hand, it is difficult to couple them efficiently to lightbecause they are much smaller than the wavelength of light. Therefore, we have focused on
plasmonics, which overcomes the diffraction limit of light, and studied a bowtie nanoantenna
to enhance the light-matter interaction. The modes of bowtie nanoantennas are localized in aregion of the order of 10 nm in size, which is close to the size of the nanoemitters. However,
it is very difficult to place nanoemitters in the subwavelength region precisely, and adispersion technique has been mainly used for nanoemitter placement [1]. In this study, wefabricated a nanowire-nanoantenna coupled system, where we placed a single InP nanowire
within the gap of a gold bowtie nanoantenna by using a nanomanipulator installed in a
focused ion beam system [Fig. 1(a)]. Moreover, we observed significant emission
enhancement in the fabricated coupled system [2].
We positioned the InP nanowire in the gap during in-situ scanning electron microscopeobservation [Fig. 1(b)]. We investigated the emission characteristics at a temperature of 80 K
by photoluminescence (PL). The excitation and emission wavelengths were 636 and 875 nm,
respectively. Figure 2 shows a map of the PL intensity. Here, E⊥ (E//) is the light polarized
perpendicular (parallel) to the nanoantenna. The black dashed line (red solid line) was
obtained under E⊥ (E//) excitation and E⊥ (E//) detection. The PL intensity (I) was normalized
by the intensity at a point R (IR), which is far from the nanoantenna. Although I/IR was close
to unity at the antenna position for E⊥ polarization, that for E// polarization was 6.1, whichindicates that the emission was strongly enhanced by the nanoantenna. When we consider thatthe diameter of excitation laser was much larger than the antenna mode, the intrinsic
enhancement factor was as large as 110. The obtained enhancement is due to the double
enhancement of the excitation and emission, and our numerical analysis agrees very well withthe experimental results. Moreover, a numerical analysis indicated that nanowirenanoantenna interplay leads to additional enhancement in our coupled system. Here, weestablished a nanomanipulation technique for subwavelength nanowires, and achieved
emission enhancement by efficiently utilizing the plasmonic structure.
[1] A. Kinkhabwala et al., Nature Photon. 3, 654 (2009).
[2] M. Ono et al., 2014 IEEE Photonics Conference, WH2.1 (2014).

100 nm
Au
InP

R

8

(b)

SiO2
50 nm

80 nm

100 nmFig. 1. (a) Schematic structure of the coupled system. (b)SEM image of the fabricated sample. We inserted ananowire with a diameter of 60 nm and a length of 7 µm.

Normalized intensity(a)Antenna position

6

E//

4

E⊥

2
0

-2

0
2
4
6
x (µm)
Fig. 2. Normalized PL intensity map on the
nanowire. Inset: SEM image that shows theantenna and reference positions.

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)47

Continuous-Wave Operation of Lambda-Scale Embedded Active-Region
Photonic-Crystal Lasers on Silicon
Koji Takeda2,3, Tomonari Sato2, Takuro Fujii2,3, Eiichi Kuramochi1,2,
Masaya Notomi1,2, Koichi Hasebe2,3, Takaaki Kakitsuka2,3, and Shinji Matsuo2,3
1
Optical Science Laboratory, 2NTT Nanophotonics Center,3
NTT Device Technology Laboratories
Recent CMOS processors employ multi-core architectures, and the network ofinterconnections between cores is very important. However, there is a bandwidth bottleneckin current electrical interconnects. To overcome it, many researchers are attempting to form
optical interconnects on CMOS chips because they provide a larger bandwidth with lower
energy consumption. We have studied lambda-scale embedded active-region photonic-crystal(LEAP) lasers on InP substrates as light sources for on-CMOS-chip optical interconnects [2].
Here we report the continuous-wave (CW) operation of LEAP lasers on Si substrates as thefirst step to realizing on-CMOS-chip light sources.
Device fabrication procedures were similar to those we have reported elsewhere [3]. We
carried out crystal (re)growths and lateral p-i-n junction formation on InP substrates to form
an ultrasmall buried heterostructure (BH). After SiO2 had been deposited on the InP
substrates, the surfaces were planarized by chemical mechanical polishing (CMP) and the 2inch InP substrates were bonded to 2-inch Si substrates. After the bonding, the host InP
substrates were removed by mechanical polishing and wet etching so that a 250-nm-thick IIIV membrane was transferred onto the Si. Two-dimensional photonic-crystal (PhC) holes werethen formed by electron-beam lithography and dry etching, followed by metallization
processes. Figure 1 shows a schematic of the fabricated device.
We injected current and measured light output from the direction normal to the wafer atroom temperature. The measured output power and applied voltage versus injected current(L-I-V characteristics) are shown in Fig. 2. We achieved a CW operation of the LEAP lasers
on Si substrates with a threshold current of 31 µA, which is the record low value among
semiconductor lasers on Si. The device showed single-mode operation with a lasing
wavelength of 1501 nm. This achievement reveals that the LEAP lasers can operate not only
on InP but also on Si substrates, and we believe this is an important step to realizing LEAP
lasers as on-CMOS-chip light sources.
[1] D. A. B. Miller, Proc. IEEE 97, 1166 (2009).
[2] K. Takeda et al., Nature Photon. 7, 569 (2013).
[3] K. Takeda et al., Opt. Express, 22, 702 (2014).
22

0.3
0.3
p-InP

BuriedInGaAsP 6-QW

2.87 x 0.3 x 0.15 μm3

1.5
1.5
0.2
0.2
0.15
0.15

0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0

0.1
0.1
0.05
0.05

SiO2
bonding/sacrificial layer
Si substrate

Bonding interface

00
Lattice constant
410 nmFig. 1. Schematic of the LEAP lasers on Si.

48

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

00

50
50

11

0.5
0.5

0 10 20 30 40 50 60

100
150
100
150
Current (µA)(A)
Current

00
200
200

Fig. 2. L-I-V characteristics.

Voltage
Voltage (V)(V)

Current-blocking
trenches

0.25
0.25
OutputOutput power
power (µW)
(W)

n-InP

Mode MUX/DEMUX on Three-Dimensional SiOx-Waveguide Platform
Tatsurou Hiraki1, 2, Tai Tsuchizawa1, 2, Hidetaka Nishi1, 2, Tsuyoshi Yamamoto2, andKoji Yamada1, 2
1
NTT Nanophotonics Center, 2NTT Device Technology Laboratories
Space division multiplexing (SDM) can increase the spectral efficiency per fiber bymultiplexing the signals in orthogonal LP modes in a few-mode fiber. In SDM systems, mode
multiplexer/demultiplexer (MUX/DEMUX) selectively launches each LP mode. The ModeMUX/DEMUX should be integrated on a cost-effective platform, because the spatial
parallelism lets network cost grow linearly with the number of multiplicities. However, it isdifficult to build an integrated and low-cost system with reported fiber- and free-space optics
devices. In this work, using a three-dimensional SiOx waveguide platform, we demonstrated
an integrated mode MUX/DEMUX on a Si chip.
Fig. 1(a) shows a schematic of an integrated mode MUX/DEMUX for three modes. We
used two layers of SiOx film with different refractive indices: 1.5 for the lower layer and 1.49
for the upper one. The interlayer clad thickness is 2.0 µm. The calculated electrical field
patterns are shown in Fig. 1(a). At the fiber-chip interface (0 mm), the mode field patterns of
the waveguides match the fiber-mode patterns with low losses. On the other hand, withchanging the core width, the power of each mode converges in a different core (3 mm). The
cores are separated (3.2 mm), and cores 3 and 4 are coupled into one single-mode port with amulti-mode interference coupler. Consequently, LP01, LP11a, and LP11b travel throughsingle-mode ports 1, 3, and 2, respectively. The device has an area of 0.6 mm2, and thecalculated loss and intermodal crosstalk are less than 2.3 dB and -11 dB, respectively. The
fabrication process features the low-temperature formation of controlled-refractive-index
SiOx films, which prevents thermal damage in the vertical stacking process, and iscompatible with the silicon photonics platform [1, 2].
Figure 1(b) shows the measured near-field patterns (NFPs) of a fabricated device withdifferent fiber-mode inputs. As designed, the LP01, LP11a, and LP11b modes were mainly
demultiplexed to output ports 1, 3, and 2, respectively. The estimated intermodal crosstalk isless than -6.4 dB. The results show the feasibility of the integrated mode MUX/DEMUX. In
future work, we plan to monolithically integrate it with Si/Ge active devices.
[1] T. Hiraki et al., Elec. Lett. 51, 74 (2014).
[2] T. Hiraki et al., Proc. in OFC2015 W1A.2 (2015).

(a)

(b)

Fig. 1. (a) Schematic of the three-mode MUX/DEMUX and calculated electrical field patterns. (b)Measured NFPs of outputs of fabricated device with different fiber-mode inputs.
Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)49

Ⅱ．Data
Fiscal 2014: April 1, 2014 ～ March 31, 2015

The 8th Advisory Board
The NTT BRL Advisory Board, first convened in 2001, held its 8th meeting during January 26 - 28
(2015). The focus of this Advisory Board was to provide an objective evaluation of our research
activities, plans, and directions, so as to enable us to utilize strategic management to systematicallycoordinate and align our resources and actions in a timely manner. On this occasion, the board wascomposed of nine distinguished scientists.
Over the course of the three days, the board evaluated our work and made valuable suggestions andcomments in relation to our research and its management. They indicated that our research level was
globally leading, and but suggested that it was important for us to disseminate and publicize ourresearch achievements more. They also raised several issues related to human resources, researchbudgets, and collaborations (both internal and external). We are actively addressing all these issues
currently.
At this meeting, our BRL researchers had lunch with the board members and also met them during a
lively poster session, where they had chances to present their research. The poster session also
included a dinner party, so the board members and the BRL researchers were able to interact in acasual atmosphere. For the BRL and NTT executives, a Japanese style dinner was arranged. This
provided an excellent good opportunity to discuss the future management strategy of NTT BRL withan international perspective in mind.
The next board meeting will be held in two years.

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

Affiliation
Walter Shottky Inst.
Univ. California, Berkeley
Harvard Univ.
Univ. Gothenburg
Imperial College London
Univ. Illinois at Urbana-Champaign
The Univ. Texas at Austin
Forschungszentrum JülichThe Univ. Queensland
Max Planck Inst.

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

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

51

Science Plaza 2014
“Science Plaza 2014”, an open-house event of NTT Basic Research Laboratories (BRL), was held at
NTT Atsugi R&D Center on Friday, November 21st, 2014. Under the banner “Frontier Science: Open
Door to the Future”, Science Plaza aimed to disseminate our latest research accomplishments throughvarious sections of people inside and outside of NTT and to gather diverse opinions.
Following an opening address from Dr. Tetsuomi Sogawa, the director of BRL, the project manager
of NTT Nanophotonics Center, Dr. Masaya Notomi gave a lecture on “Extreme investigation of opticaldevices using photonic crystals”. In the afternoon session, Professor / Chief Scientist Hidetoshi Katoriof The University of Tokyo and Quantum Metrology Laboratory in RIKEN, gave a special lecture
entitled “The relativistic space-time embossed by optical lattice clocks”. Each lecture waswell-attended and followed by heated question and answer sessions.
As regards the poster exhibits, 38 posters, including 16 from Nanophotonics Center, Device
Innovation Center, Device Technology Laboratories and Communication Science Laboratories
presented our latest research accomplishments. Universities and the research institutes also presented
29 posters of their joint research with NTT in the poster exhibits. While explaining the originality andimpact—as well as the future prospects—, 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 that has 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. After all lectures, presentations, andexhibitions, a banquet was held in Center’s dining room, where lively conversation among participants
deepened their amity.
About 220 people from research institutes, universities, and general industries, as well as from NTT
Group, attended Science Plaza 2014. Thanks to the efforts of all participants, the conference ended ona high note. We would thus like sincerely to express our gratitude to all of the participants.

52

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

List of Visitors’ Talks
Date

Apr. 10

Speakers

Affiliation
Title

Prof. Katsushi Fujii

The University of Tokyo / RIKEN, Japan

Dr. Shinichiro Nakamura

Artificial photosynthesis for hydrocarbon generation system
International Center for Materials Nanoarchitectonics,

Apr. 11

Dr. Yutaka Wakayama

National Institute for Materials Science, Japan
Control of tunneling current by molecules
iTHES in RIKEN, Japan

Apr. 17

Dr. Robert Johansson

Entangled-state generation in nanomechanical resonators andoptomechanical-like coupling in microwave circuits
CEA Saclay, France

Apr. 22

Dr. Yuimaru Kubo

Hybrid Quantum Circuit with a Superconducting Qubit andan Electron Spin Ensemble
École Polytechnique Fédérale de Lausanne (EPFL),
Switzerland

May 8

Mr. Naotomo Takemura

Photon correlation measurement of VCSEL, and nonlinear
optical spectroscopy of semiconductor minute resonatorpolariton

May 14

Prof. Alberto Morpurgo

University of Geneva, Switzerland
Transport through suspended graphene
École Normale Supérieure, France

June 27

Prof. Christos Flytzanis

Ultrafast high THz-field photo-driven carrier dynamics andtransport

July 2

Prof. Christos Flytzanis

École Normale Supérieure, France
Nanophononics: a nonlinear battleground par excellence
Keio University, Japan

July 16

Prof. Junko Ishi-Hayase

Quantum interfaces and sensors using two-level systemensembles in semiconductors
University of Colorado at Boulder, U.S.A.

July 17

Dr. Justin Waugh

Spin-Orbital Texture of Topological Insulators Bi2X3 (X =Se,Te)
Shizuoka University, Japan

July 28

Prof. Jiro Temmyo

Compound ZnO Growth with Oxygen Radical Assisted
MOCVD and Optical Device (LED/PD/PV) Application
Tokyo University of Science, Japan

Aug. 4

Prof. Akira Suda

High-order harmonic generation by non-collinear wave
mixingResearch Activities in NTT Basic Research Laboratories Vol. 25 (2014)

53

Date

Speakers

Affiliation
Title
Mie University, Japan

Sep. 1

Dr. Kanta Tsumoto

Preparation of giant recombinant proteoliposomes bybaculovirus membrane fusion

Sep. 4

Prof. Takahiro Sagawa

The University of Tokyo, JapanMr. Sosuke Ito

Information and thermodynamics
Keio University, Japan

Sep. 5

Ms. Kana Iwakuni

Dual-comb spectroscopy with narrow linewidth opticalfrequency combs
Weizmann Institute of Science, Israel

Oct. 8

Dr. Hiroyuki Inoue

Observation of neutral modes in fractional quantum Hall
regimeOct. 23

Dr. Paulo V. Santos

Oct. 31

Dr. Takeshi FukuharaPaul Drude Institute for Solid State Electronics, Germany
Acoustic control and manipulation of quantum wells excitons
RIKEN, Japan
Quantum dynamics of magnons in optical lattices
The National Enterprise for nanoScience andOct. 31

Dr. Stefan HeunnanoTechnology, Italy
Exploring the physics of one-dimensional systems byscanning gate microscopy
The National Enterprise for nanoScience andOct. 31

Dr. Yuya Murata

nanoTechnology, Italy
Correlation between morphology and transport properties ofquasi-free-standing monolayer graphene
Paul Drude Institute for Solid State Electronics, Germany

Nov. 10

Dr. Stefan Fölsch

Generating and probing semiconductor quantum structureswith single-atom precisionNov. 19

Nov. 26

54

Prof. Klaus Ensslin
Dr. Koji Usami
Dr. Rekishu Yamazaki

Nov. 27

Prof. Barbaros Özyilmaz

Jan. 8

Dr. Peter S. Turner

Eidgenössische Technische Hochschule Zürich, Switzerland
Non-local transport in InAs-GaSb quantum wells
RCAST, Research Center for Advanced Science andTechnology at the University of Tokyo, Japan
Quantum transducers with collective excitations in solid
National University of Singapore, Singapore
From Graphene to Phosphorene
University of Bristol, U.K.
Experimental Demonstration of Quantum Data Compression

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Date

Jan. 9Jan. 14

Speakers

Prof. Yuji Mochizuki
Prof. Kaori Fukuzawa
Dr. Akihisa Goban

Affiliation
Title
Rikkyo University / Nihon University, Japan
Fragment molecular orbital method and its application
examplesCalifornia Institute of Technology, U.S.A.
Strong atom-light interactions in 1D photonic crystals
Chiba University, Japan

Jan. 14

Prof. Ken Morita

Terahertz pulse generation for electron spin control insemiconductors

Jan. 22

Prof. Silvija GradečakJan. 22

Dr. Rao Tatavarti

Massachusetts Institute of Technology, U.S.A.
Semiconductor nanowires for energy applications
MicroLink Devices, U.S.A.
Large Area Inverted Metamorphic (IMM) ELO Solar Cells
Imperial College London, U.K.

Mar. 2Mr. Samuele Grandi

Waveguide coupling of single photons from a solid state
emitter: from room temperature to cryogenic condition

Mar. 10

Prof. Qiang Zhang

University of Science and Technology of China, China
Quantum Key Distribution with Realistic Devices
Japan Atomic Energy Agency
Researches on ultrafast dynamics in VUV region:

Mar. 11

Dr. Ryuji Itakura

Time-resolved reflection spectroscopy of highly excited solid
surface and coherence of atomic ions created in ultrafast
ionization
University of Leeds, U.K.

Mar. 17

Dr. Nicolò Lo Piparo

Long-distance quantum key distribution with imperfectdevices

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)55

List of Award Winners
AwardAward Winners Title

Date

The Commendation for
Science and Technology by
the Minister of Education,
Culture, Sports, Scienceand Technology, The Young
Scientists’ Prize

H. Okamoto

A study of the quantum
nanomechanical sensing technology

Apr. 15, 2014

Conference on LED and
Its Industrial Application(LEDIA '14)
Young Researcher's
Paper Award

R. G. Banal

Nonpolar M-Plane AlGaN
Deep-UV LEDs

Apr. 24, 2014

Multidisciplinary Digital
Publishing InstituteEntropy Best Paper Award 2014

K. L. Brown
W. J. Munro
V. M. Kendon

Using Quantum Computers for
Quantum Simulation

Apr. 30, 2014

Spectroscopy & Innovation
Award

Y. UenoDemonstration of industrial
applicability of spectroscopy
through R&D at NTT Laboratories

May 29, 2014

Fellowship of the OpticalSociety of America (OSA)W. J. Munro

Research in Quantum Optics andQuantum Information Processing.

Oct. 27, 2014

Award for Encouragement ofResearch in International Unionof Materials Research Societies,
International Conference in Asia
(IUMRS-ICA)

K. Hirama

Nitride/diamond heterostructure
systems - from growth to devices -Oct. 31, 2014

Research Institute of ElectricalCommunication
RIEC Award

H. Sanada

Electron spin resonance using
spin-orbit interaction

Nov. 28, 2014

IEICE Electronics Society LQE
Young Researchers Award

K. Nozaki

InGaAs photodetectors based on
photonic crystal waveguide including
ultrasmall buried heterostructure

Dec. 19, 2014

JSAP Young Scientist
Oral Presentation Award

T. Inagaki

Low-voltage optical phase modulation
by electric-field-induced phasetransition of KTN crystal

Mar. 11, 2015

56

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

List of In-House Award Winners
AwardAward Winners

Title

Date

NTT Science andCore Technology
Laboratory Group
Director Award

G. Yamahata
K. Nishiguchi
A. Fujiwara

Research on high-accuracy andhigh-sensitivity electronics using
single-electron manipulation anddetection

Dec. 5, 2014

NTT Science andCore Technology
Laboratory Group
Director Award

K. Fujii
K. OnoT. Ogasawara
M. Seyama
H. Koizumi
S. Tsukada
H. Nakashima
N. Kasai
K. Sumitomo
S. KunoDevelopment andcommercialization of functional
materials, "hitoe", that enables
continuous measurement of
biological information just by
wearing

Dec. 5, 2014

BRL Director Award
Award for Achievements

D. Hatanaka
I. Mahboob
K. Onomitsu
H. Yamaguchi

Proposal for dynamic phononic
crystal using compound
semiconductors and its
demonstration

Mar. 27, 2015

BRL Director Award
Award for Achievements

T. D. Rhone
L. Tiemann
G. Gamez
K. Muraki

For the realization ofhigh-mobility two-dimensional
electron systems exhibiting
highly correlated electronic
phases and for the elucidation of
their properties

Mar. 27, 2015

BRL Director Award
Award for
Excellent Papers

J. Noborisaka
K. Nishiguchi
A. Fujiwara“Electric tuning of direct-indirect Mar. 27, 2015
optical transitions in silicon”
Scientific Reports 4, 6950 (2014)

BRL Director Award
Award for Best Paper
for Environmental
Contribution

Y. Taniyasu“An aluminium nitride
light-emitting diode with awavelength of 210 nanometres”
Nature 441, 325 (2006)

Mar. 27, 2015

BRL Director Award
Award for Performance

T. OtaM. Yamaguchi
T. Hayashi
S. Sasaki

Contribution for renewal ofhelium gas purification systemand continuous supply of liquid
heliumMar. 27, 2015

BRL Director Award
Award for Encouragement

H. Sumikura

Large cavity QED effect on copper
impurities in siliconMar. 27, 2015

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)57

Numbers of Papers, Presentations and Patents (Fiscal 2013*)
The numbers of papers published in international journals, presentations at international conferences
and applied patents in fiscal 2013 are 118, 283, and 61, respectively. The numbers for each researcharea are as follows;*April 1, 2013-March 31, 201458

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Numbers of Papers, Presentations and Patents (Fiscal 2014)
The numbers of papers published in international journals, presentations at international conferences
and applied patents in fiscal 2014 are 103, 171, and 61, respectively. The numbers for each researcharea are as follows;

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

59

The numbers of research papers published in the major journals are shown below.
Journals

IF2013*

Numbers

Physical Review B

3.664

10

Japanese Journal of Applied Physics

1.057

9

Optics Express

3.525

7

Applied Physics Letters

3.515

7

Physical Review A

2.991

7

Applied Physics Express

2.567

6

Nature Communications

10.742

4

Journal of Crystal Growth

1.693

4

Physical Review Letters

7.728

3

Scientific Reports

5.078

3

Optics Letters

3.179

3

Nature Nanotechnology

33.265

2

Nature Photonics

29.9582

Nanotechnology

3.672

2

New Journal of Physics

3.671

2

Journal of Lightwave Technology

2.862

2

Journal of Applied Physics

2.185

2

Journal of the Physical Society of Japan

1.475

2

Nature Physics

20.603

1

Nano Letters12.9401

ACS Nano

12.033

1

Chemistry of Materials

8.535

1

Physical Review X

8.463

1

Nanoscale

6.739

1

Analytica Chimica Acta
4.517
*IF2013: Impact Factor 2013
The average IF2013 for all research papers from NTT Basic Research Laboratories is 4.927.

1

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

60

Numbers

The Conference on Lasers and Electro-Optics (CLEO 2014)13

The 7th International Symposium on Surface Science (ISSS-7)

12

2014 International Conference on Solid State Devices and Materials (SSDM 2014)

9

APS March Meeting 2015

927th International Microprocesses and Nanotechnology Conference (MNC 2014)7

The International Symposium on Compound Semiconductors (ISCS 2014)5

32nd International Conference on the Physics of Semiconductors (ICPS)

4

International Union of Materials Research Societies (IUMRS-ICA 2014)4

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

Publication List
(1)

T. Akasaka and H. Yamamoto, "Nucleus and Spiral Growth Mechanisms of Nitride Semiconductors in
Metalorganic Vapor Phase Epitaxy", Jpn. J. Appl. Phys. 53, 100201 (2014).

(2)

H. Akazawa and Y. Ueno, "Low-Temperature Crystallization and High-Temperature Instability ofHydroxyapatite Thin Films Deposited on Ru, Ti, and Pt Metal Substrates", Surf. Coatings Technol.
266, 42-48 (2015).

(3)

M. Anagnosti, H. Omi, and T. Tawara, "Simultaneous Light Emissions from Erbium-Thulium
Silicates and Oxides on Silicon in the Second and Third Telecommunications Bands", Opt. Mater.
Express 4, 1747-1755 (2014).

(4)

R. Asaoka, H. Tsuchiura, M. Yamashita, and Y. Toga, "Density Modulations Associated with the
Dynamical Instability in the Bose-Hubbard Model", J. Phys. Soc. Jpn. 83, 124001 (2014).

(5)

R. G. Banal, Y. Taniyasu, and H. Yamamoto, "Deep-Ultraviolet Light Emission Properties ofNonpolar M-Plane AlGaN Quantum Wells", Appl. Phys. Lett. 105, 053104 (2014).

(6)

M. D. Birowosuto, G. Zhang, A. Yokoo, M. Takiguchi, and M. Notomi, "Spontaneous Emission
Inhibition of Telecom-Band Quantum Disks Inside Single Nanowire on Different Substrates", Opt.
Express 22, 11713-11726 (2014).

(7)

T. Byrnes, D. Rosseau, M. Khosla, A. Pyrkov, A. Thomasen, T. Mukai, S. Koyama, A. Abdelrahman,and E. Ilo-Okeke, "Macroscopic Quantum Information Processing Using Spin Coherent States", Opt.
Commun. 337, 102-109 (2015).

(8)

M. Curty, F. H. Xu, W. Cui, C. C. W. Lim, K. Tamaki, and H. K. Lo, "Finite-Key Analysis for
Measurement-Device-Independent Quantum Key Distribution", Nature Commun. 5, 3732 (2014).

(9)

J. Dai, R. Kometani, K. Onomitsu, Y. Krockenberger, H. Yamaguchi, S. Ishihara, and S. Warisawa,"Direct Fabrication of a W-C SNS Josephson Junction Using Focused-Ion-Beam Chemical Vapour
Deposition", J. Micromech. Microeng. 24, 055015 (2014).

(10)

M. S. Everitt, S. Devitt, W. J. Munro, and K. Nemoto, "High-Fidelity Gate Operations with the
Coupled Nuclear and Electron Spins of a Nitrogen-Vacancy Center in Diamond", Phys. Rev. A 89,
052317 (2014).

(11)

S. Fölsch, J. Martinez-Blanco, J. S. Yang, K. Kanisawa, and S. C. Erwin, "Quantum Dots with
Single-Atom Precision", Nature Nanotechnol. 9, 505-508 (2014).

(12)

A. Fushimi, H. Taniyama, E. Kuramochi, M. Notomi, and T. Tanabe, "Fast Calculation of the Quality
Factor for Two-Dimensional Photonic Crystal Slab Nanocavities", Opt. Express 22, 23349-23359(2014).

(13)

T. Goto, Y. Harada, D. Cox, and K. Sumitomo, "Fabrication of a Ring Structure at the Aperture of aHole for the Efficient Suspension of a Lipid Bilayer", Jpn. J. Appl. Phys. 53, 096503 (2014).

(14)

H. Gotoh, H. Sanada, H. Yamaguchi, and T. Sogawa, "Nonlinear Optical Spectra Having
Characteristics of Fano Interferences in Coherently Coupled Lowest Exciton Biexciton States in
Semiconductor Quantum Dots", AIP Adv. 4, 107124 (2014).
Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

61

(15)

H. A. Hafez, I. Al-Naib, K. Oguri, Y. Sekine, M. M. Dignam, A. Ibrahim, D. G. Cooke, S. Tanaka, F.
Komori, H. Hibino, and T. Ozaki, "Nonlinear Transmission of an Intense Terahertz Field ThroughMonolayer Graphene", AIP Adv. 4, 117118 (2014).

(16)

H. A. Hafez, I. Al-Naib, M. M. Dignam, Y. Sekine, K. Oguri, F. Blanchard, D. G. Cooke, S. Tanaka, F.
Komori, H. Hibino, and T. Ozaki, "Nonlinear Terahertz Field-Induced Carrier Dynamics in
Photoexcited Epitaxial Monolayer Graphene", Phys. Rev. B 91, 035422 (2015).

(17)

K. Hasebe, T. Sato, K. Takeda, T. Fujii, T. Kakitsuka, and S. Matsuo, "High-Speed Modulation of
Lateral p-i-n Diode Structure Electro-Absorption Modulator Integrated with DFB Laser", J. Lightwave
Technol. 33, 1235-1240 (2015).

(18)

M. Hashisaka, T. Ota, M. Yamagishi, T. Fujisawa, and K. Muraki, "Cross-Correlation Measurement of
Quantum Shot Noise Using Homemade Transimpedance Amplifiers", Rev. Sci. Instrum. 85, 054704(2014).

(19)

M. Hashisaka, T. Ota, K. Muraki, and T. Fujisawa, "Shot-Noise Evidence of Fractional Quasiparticle
Creation in a Local Fractional Quantum Hall State", Phys. Rev. Lett. 114, 056802 (2015).

(20)

D. Hatanaka, I. Mahboob, K. Onomitsu, and H. Yamaguchi, "Phonon Waveguides forElectromechanical Circuits", Nature Nanotechnol. 9, 520-524 (2014).

(21)

D. Hatanaka, I. Mahboob, K. Onomitsu, and H. Yamaguchi, "Mechanical Random Access Memory ina Phonon Circuit", Appl. Phys. Express 7, 125201 (2014).

(22)

T. Hiraki, T. Tsuchizawa, H. Nishi, T. Yamamoto, and K. Yamada, "Low-Loss andPolarisation-Insensitive Interlayer Coupler on Three-Dimensional SiOx-Waveguide Platform",
Electron. Lett. 51, 74-76 (2015).

(23)

T. Hiraki, H. Fukuda, K. Yamada, and T. Yamamoto, "Small Sensitivity to Temperature Variations of
Si-Photonic Mach-Zehnder Interferometer Using Si and SiN Waveguides", Front. Mater. 2, 26 (2015).

(24)

M. Hiroki, K. Kumakura, Y. Kobayashi, T. Akasaka, T. Makimoto, and H. Yamamoto, "Suppressionof Self-Heating Effect in AlGaN/GaN High Electron Mobility Transistors by Substrate-Transfer
Technology Using h-BN", Appl. Phys. Lett. 105, 193509 (2014).

(25)

T. Hoshi, N. Kashio, H. Sugiyama, H. Yokoyama, K. Kurishima, M. Ida, H. Matsuzaki, and H. Gotoh,"A Simple Method for Forming Compositionally Graded InxGa1−xAs1−ySby Base of
Double-Heterojunction Bipolar Transistors Modulating CBr4-Doping-Precursor Flow in Metalorganic
Chemical Vapor Deposition", Appl. Phys. Express 7, 114102 (2014).

(26)

T. Hoshi, N. Kashio, H. Sugiyama, H. Yokoyama, K. Kurishima, M. Ida, H. Matsuzaki, M. Kohtoku,
and H. Gotoh, "MOCVD-Grown Compressively Strained C-Doped InxGa1−xAs1−ySby with High-In/Sb
Content for Very Low Turn-on-Voltage InP-Based DHBTs", J. Cryst. Growth 404, 172-176 (2014).

(27)

H. Imai, K. Inaba, H. Tanji-Suzuki, M. Yamashita, and T. Mukai, "Bose-Einstein Condensate on a
Persistent-Supercurrent Atom Chip", Appl. Phys. B 116, 821-829 (2014).

(28)

R. Inoue, H. Takayanagi, T. Akazaki, K. Tanaka, H. Sasakura, and I. Suemune, "Carrier Flow andNonequilibrium Superconductivity in Superconductor-Based LEDs", Appl. Phys. Express 7, 073101(2014).

62

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(29)

H. Irie, Y. Harada, H. Sugiyama, and T. Akazaki, "Josephson Coupling Through One-Dimensional
Ballistic Channel in Semiconductor-Superconductor Hybrid Quantum Point Contacts", Phys. Rev. B
89, 165415 (2014).

(30)

T. Ito, H. Gotoh, M. Ichida, and H. Ando, "Evaluation of Hole-Spin Superposition in GaAs/AlGaAs
Quantum Wells Through Time-Resolved Photoluminescence Measurements", Appl. Phys. Lett. 104,
252406 (2014).

(31)

J. R. Johansson, N. Lambert, I. Mahboob, H. Yamaguchi, and F. Nori, "Entangled-State Generation
and Bell Inequality Violations in Nanomechanical Resonators", Phys. Rev. B 90, 174307 (2014).

(32)

K. Kanisawa, "Electronic Processes in Adatom Dynamics at Epitaxial Semiconductor Surfaces
Studied Using MBE-STM Combined System", J. Cryst. Growth 401, 381-387 (2014).

(33)

N. Kasai, R. Lu, T. Goto, A. Tanaka, S. Tsukada, Y. Harada, and K. Sumitomo, "Observation of
Neuronal Death in Vitro by SEM and Optical Microscopy", e-J. Surf. Sci. Nanotech. 12, 179-184(2014).

(34)

Y. Kashimura, K. Sumitomo, and K. Furukawa, "Electrostatic Control of the Dynamics of Lipid
Bilayer Self-Spreading Using a Nanogap Gate", Mater. Res. Express 1, 035404 (2014).

(35)

P. A. Knott, W. J. Munro, and J. A. Dunningham, "Attaining Subclassical Metrology in Lossy
Systems with Entangled Coherent States", Phys. Rev. A 89, 053812 (2014).

(36)

P. A. Knott, T. J. Proctor, K. Nemoto, J. A. Dunningham, and W. J. Munro, "Effect of Multimode
Entanglement on Lossy Optical Quantum Metrology", Phys. Rev. A 90, 033846 (2014).

(37)

K. M. Kojima, Y. Krockenberger, I. Yamauchi, M. Miyazaki, M. Hiraishi, A. Koda, R. Kadono, R.
Kumai, H. Yamamoto, A. Ikeda, and M. Naito, "Bulk Superconductivity in Undoped T'-La1.9Y0.1CuO4Probed by Muon Spin Rotation", Phys. Rev. B 89, 180508 (2014).

(38)

R. Kou, Y. Kobayashi, K. Warabi, H. Nishi, T. Tsuchizawa, T. Yamamoto, H. Nakajima, and K.
Yamada, "Efficient-and Broadband-Coupled Selective Spot-Size Converters with Damage-Free
Graphene Integration Process", IEEE Photon. J. 6, 6600409 (2014).

(39)

Y. Krockenberger, B. Eleazer, H. Irie, and H. Yamamoto, "Superconducting- and Insulating-Ground
States in La2CuO4 Structural Isomers", J. Phys. Soc. Jpn. 83, 114602 (2014).

(40)

Y. Krockenberger, H. Irie, J. Yan, L. Waterston, B. Eleazer, K. Sakuma, and H. Yamamoto,"Superconductivity in Cuprates with Square-Planar Coordinated Copper Driven by Hole Carriers",
Appl. Phys. Express 7, 063101 (2014).

(41)

N. Kumada, R. Dubourget, K. Sasaki, S. Tanabe, H. Hibino, H. Kamata, M. Hashisaka, K. Muraki,
and T. Fujisawa, "Plasmon Transport and Its Guiding in Graphene", New J. Phys. 16, 063055 (2014).

(42)

N. Kumada, P. Roulleau, B. Roche, M. Hashisaka, H. Hibino, I. Petkovic, and D. C. Glattli, "Resonant
Edge Magnetoplasmons and Their Decay in Graphene", Phys. Rev. Lett. 113, 266601 (2014).

(43)

E. Kuramochi, E. Grossman, K. Nozaki, K. Takeda, A. Shinya, H. Taniyama, and M. Notomi,"Systematic Hole-Shifting of L-Type Nanocavity with an Ultrahigh Q Factor", Opt. Lett. 39,
5780-5783 (2014).

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(44)

E. Kuramochi, K. Nozaki, A. Shinya, K. Takeda, T. Sato, S. Matsuo, H. Taniyama, H. Sumikura, and
M. Notomi, "Large-Scale Integration of Wavelength-Addressable All-Optical Memories on a Photonic
Crystal Chip", Nature Photon. 8, 474-481 (2014).

(45)

C. H. Lin, T. Akasaka, and H. Yamamoto, "N-Face GaN(0001) Films Grown by Group-III-Source
Flow-Rate Modulation Epitaxy", Jpn. J. Appl. Phys. 53, 11RC01 (2014).

(46)

H. K. Lo, M. Curty, and K. Tamaki, "Secure Quantum Key Distribution", Nature Photon. 8, 595-604(2014).

(47)

I. Mahboob, H. Okamoto, K. Onomitsu, and H. Yamaguchi, "Two-Mode Thermal-Noise Squeezing in
an Electromechanical Resonator", Phys. Rev. Lett. 113, 167203 (2014).

(48)

S. Mamyouda, H. Ito, Y. Shibata, S. Kashiwaya, M. Yamaguchi, T. Akazaki, H. Tamura, Y. Ootuka,
and S. Nomura, "Circularly Polarized Near-Field Optical Mapping of Spin-Resolved Quantum Hall
Chiral Edge States", Nano Lett. 15, 2417-2421 (2015).

(49)

H. Mashiko, T. Yamaguchi, K. Oguri, A. Suda, and H. Gotoh, "Characterizing Inner-Shell with
Spectral Phase Interferometry for Direct Electric-Field Reconstruction", Nature Commun. 5, 5599
(2014).

(50)

N. Matsuda, P. Karkus, H. Nishi, T. Tsuchizawa, W. J. Munro, H. Takesue, and K. Yamada, "On-Chip
Generation and Demultiplexing of Quantum Correlated Photons Using a Silicon-Silica Monolithic
Photonic Integration Platform", Opt. Express 22, 22831-22840 (2014).

(51)

N. Matsuda, E. Kuramochi, H. Takesue, and M. Notomi, "Dispersion and Light Transport
Characteristics of Large-Scale Photonic-Crystal Coupled Nanocavity Arrays", Opt. Lett. 39,
2290-2293 (2014).

(52)

S. Matsuo, T. Fujii, K. Hasebe, K. Takeda, T. Sato, and T. Kakitsuka, "Directly Modulated DFB Laser
on SiO2/Si Substrate for Datacenter Networks", J. Lightwave Technol. 33, 1217-1222 (2015).

(53)

A. Mizutani, K. Tamaki, R. Ikuta, T. Yamamoto, and N. Imoto, "Measurement-Device-Independent
Quantum Key Distribution for Scarani-Acin-Ribordy-Gisin 04 Protocol", Sci. Rep. 4, 5236 (2014).

(54)

Y. Momiuchi, K. Yamada, H. Kato, Y. Homma, H. Hibino, G. Odahara, and C. Oshima, "In Situ
Scanning Electron Microscopy of Graphene Nucleation During Segregation of Carbon on
Polycrystalline Ni Substrate", J. Phys. D 47, 455301 (2014).

(55)

Y. Murata, T. Mashoff, M. Takamura, S. Tanabe, H. Hibino, F. Beltram, and S. Heun, "Correlation
between Morphology and Transport Properties of Quasi-Free-Standing Monolayer Graphene", Appl.
Phys. Lett. 105, 221604 (2014).

(56)

M. Naito, K. Uehara, R. Takeda, Y. Taniyasu, and H. Yamamoto, "Growth of Iron Nitride Thin Films
by Molecular Beam Epitaxy", J. Cryst. Growth 415, 36-40 (2015).

(57)

A. Najar, H. Omi, and T. Tawara, "Scandium Effect on the Luminescence of Er-Sc Silicates Preparedfrom Multi-Nanolayer Films", Nanoscale Res. Lett. 9, 356 (2014).

(58)

A. Najar, H. Omi, and T. Tawara, "Effect of Structure and Composition on Optical Properties of Er-Sc
Silicates Prepared from Multi-Nanolayer Films", Opt. Express 23, 7021-7030 (2015).

64

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(59)K. Nemoto, M. Trupke, S. J. Devitt, A. M. Stephens, B. Scharfenberger, K. Buczak, T. Nobauer, M. S.
Everitt, J. Schmiedmayer, and W. J. Munro, "Photonic Architecture for Scalable Quantum Information
Processing in Diamond", Phys. Rev. X 4, 031022 (2014).

(60)

K. Nishiguchi, Y. Ono, and A. Fujiwara, "Single-Electron Thermal Noise", Nanotechnology 25,
275201 (2014).

(61)

J. Noborisaka, K. Nishiguchi, and A. Fujiwara, "Electric Tuning of Direct-Indirect Optical Transitionsin Silicon", Sci. Rep. 4, 6950 (2014).

(62)

K. Noda, K. Inaba, and M. Yamashita, "Flat-Band Ferromagnetism in the Multilayer Lieb Optical
Lattice", Phys. Rev. A 90, 043624 (2014).

(63)

K. Nozaki, E. Kuramochi, A. Shinya, and M. Notomi, "25-Channel All-Optical Gate Switches
Realized by Integrating Silicon Photonic Crystal Nanocavities", Opt. Express 22, 14263-14274 (2014).

(64)

K. Oguri, T. Tsunoi, K. Kato, H. Nakano, T. Nishikawa, K. Tateno, T. Sogawa, and H. Gotoh,"Dynamical Observation of Photo-Dember Effect on Semi-Insulating GaAs Using Femtosecond
Core-Level Photoelectron Spectroscopy", Appl. Phys. Express 8, 022401 (2015).

(65)

M. Okada, T. Sawazaki, K. Watanabe, T. Taniguchi, H. Hibino, H. Shinohara, and R. Kitaura, "Direct
Chemical Vapor Deposition Growth of WS2 Atomic Layers on Hexagonal Boron Nitride", ACS Nano
8, 8273-8277 (2014).

(66)

H. Okamoto, W. Izumida, Y. Hirayama, H. Yamaguchi, A. Riedel, and K. J. Friedland, "MechanicalResonance Characteristics of a Cylindrical Semiconductor Heterostructure Containing a
High-Mobility Two-Dimensional Electron Gas", Phys. Rev. B 89, 245304 (2014).

(67)

H. Okamoto, I. Mahboob, K. Onomitsu, and H. Yamaguchi, "Rapid Switching in High-Q MechanicalResonators", Appl. Phys. Lett. 105, 083114 (2014).

(68)

H. Okamoto, S. Suzuki, H. Narita, T. Tawara, K. Tateno, and H. Gotoh, "Deep-Level Transient
Spectroscopy Characterization of In(Ga)As Quantum Dots Fabricated Using Bi as a Surfactant", Jpn. J.
Appl. Phys. 53, 06JG11 (2014).

(69)

C. M. Orofeo, S. Suzuki, Y. Sekine, and H. Hibino, "Scalable Synthesis of Layer-Controlled WS2 andMoS2 Sheets by Sulfurization of Thin Metal Films", Appl. Phys. Lett. 105, 083112 (2014).

(70)

P. P. Rohde, K. R. Motes, P. A. Knott, J. Fitzsimons, W. J. Munro, and J. P. Dowling, "Evidence for
the Conjecture that Sampling Generalized Cat States with Linear Optics Is Hard", Phys. Rev. A 91,
012342 (2015).

(71)

O. Ryong-Sok, M. Takamura, K. Furukawa, M. Nagase, and H. Hibino, "Effects of UV Light Intensity
on Electrochemical Wet Etching of SiC for the Fabrication of Suspended Graphene", Jpn. J. Appl.
Phys. 54, 036502 (2015).

(72)

S. Sakuragi, T. Sakai, S. Urata, S. Aihara, A. Shinto, H. Kageshima, M. Sawada, H. Namatame, M.
Taniguchi, and T. Sato, "Thickness-Dependent Appearance of Ferromagnetism in Pd(100) Ultrathin
Films", Phys. Rev. B 90, 054411 (2014).

(73)

K. Sasaki, H. Gotoh, and Y. Tokura, "Valley-Antisymmetric Potential in Graphene Under Dynamical
Deformation", Phys. Rev. B 90, 205402 (2014).
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(74)

K. Sasaki and N. Kumada, "Effects of Screening on the Propagation of Graphene Surface Plasmons",
Phys. Rev. B 90, 035449 (2014).

(75)

S. Sasaki, K. Tateno, G. Zhang, H. Pigot, Y. Harada, S. Saito, A. Fujiwara, T. Sogawa, and K. Muraki,"Self-Aligned Gate-All-Around InAs/InP Core-Shell Nanowire Field-Effect Transistors", Jpn. J. Appl.
Phys. 54, 04DN04 (2015).

(76)

B. Scharfenberger, W. J. Munro, and K. Nemoto, "Coherent Control of an NV- Center with One
Adjacent 13C", New J. Phys. 16, 093043 (2014).

(77)

A. Shakoor, K. Nozaki, E. Kuramochi, K. Nishiguchi, A. Shinya, and M. Notomi, "Compact
1D-Silicon Photonic Crystal Electro-Optic Modulator Operating with Ultra-Low Switching Voltage
and Energy", Opt. Express 22, 28623-28634 (2014).

(78)

H. Shibata, "Fabrication of a MgB2 Nanowire Single-Photon Detector Using Br2–N2 Dry Etching",
Appl. Phys. Express 7, 103101 (2014).

(79)

H. Shibata, T. Honjo, and K. Shimizu, "Quantum Key Distribution over a 72 dB Channel Loss Using
Ultralow Dark Count Superconducting Single-Photon Detectors", Opt. Lett. 39, 5078-5081 (2014).

(80)

H. Sumikura, E. Kuramochi, H. Taniyama, and M. Notomi, "Ultrafast Spontaneous Emission of
Copper-Doped Silicon Enhanced by an Optical Nanocavity", Sci. Rep. 4, 5040 (2014).

(81)

M. Takamura, H. Okamoto, K. Furukawa, H. Yamaguchi, and H. Hibino, "Energy Dissipation in
Edged and Edgeless Graphene Mechanical Resonators", J. Appl. Phys. 116, 064304 (2014).

(82)

K. Takeda, T. Hiraki, T. Tsuchizawa, H. Nishi, R. Kou, H. Fukuda, T. Yamamoto, Y. Ishikawa, K.
Wada, and K. Yamada, "Contributions of Franz-Keldysh and Avalanche Effects to Responsivity of a
Germanium Waveguide Photodiode in the L-Band", IEEE J. Sel. Top. Quantum Electron. 20, 3800507(2014).

(83)

K. Takeda, T. Sato, T. Fujii, E. Kuramochi, M. Notomi, K. Hasebe, T. Kakitsuka, and S. Matsuo,"Heterogeneously Integrated Photonic-Crystal Lasers on Silicon for On/Off Chip Optical
Interconnects", Opt. Express 23, 702-708 (2015).

(84)

H. Takesue, "Entangling Time-Bin Qubits with a Switch", Phys. Rev. A 89, 062328 (2014).

(85)

K. Tamaki, M. Curty, G. Kato, H. K. Lo, and K. Azuma, "Loss-Tolerant Quantum Cryptography with
Imperfect Sources", Phys. Rev. A 90, 052314 (2014).

(86)

S. Tanabe, K. Furukawa, and H. Hibino, "Etchant-Free and Damageless Transfer of Monolayer andBilayer Graphene Grown on SiC", Jpn. J. Appl. Phys. 53, 115101 (2014).

(87)

S. Tanabe, M. Takamura, Y. Harada, H. Kageshima, and H. Hibino, "Effects of Hydrogen
Intercalation on Transport Properties of Quasi-Free-Standing Monolayer Graphene", Jpn. J. Appl.
Phys. 53, 04EN01 (2014).

(88)

K. Tateno, G. Q. Zhang, and H. Gotoh, "Etching Effect of Tertiary-Butyl Chloride During
InP-Nanowire Growth", J. Cryst. Growth 402, 299-303 (2014).

(89)

L. Tiemann, T. D. Rhone, N. Shibata, and K. Muraki, "NMR Profiling of Quantum Electron Solids inHigh Magnetic Fields", Nature Phys. 10, 648-652 (2014).

66

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(90)T. Uchida, M. Arita, A. Fujiwara, and Y. Takahashi, "Coupling Capacitance between Double
Quantum Dots Tunable by the Number of Electrons in Si Quantum Dots", J. Appl. Phys. 117, 084316(2015).

(91)

Y. Ueno, K. Furukawa, K. Matsuo, S. Inoue, K. Hayashi, and H. Hibino, "On-Chip Graphene Oxide
Aptasensor for Multiple Protein Detection", Anal. Chim. Acta 866, 1-9 (2015).

(92)

S. N. Wang, S. Suzuki, and H. Hibino, "Raman Spectroscopic Investigation of Polycrystalline
Structures of CVD-Grown Graphene by Isotope Labeling", Nanoscale 6, 13838-13844 (2014).

(93)

K. Warabi, R. Kou, S. Tanabe, T. Tsuchizawa, S. Suzuki, H. Hibino, H. Nakajima, and K. Yamada,"Optical Absorption Characteristics and Polarization Dependence of Single-Layer Graphene on
Silicon Waveguide", IEICE Trans. Electron. E97-C, 736-743 (2014).

(94)

N. Watase, M. Hashisaka, K. Muraki, and T. Fujisawa, "Single-Electron Counting Statistics with a
Finite Frequency Bandwidth", Jpn. J. Appl. Phys. 53, 04EJ01 (2014).

(95)

K. Yamada, T. Tsuchizawa, H. Nishi, R. Kou, T. Hiraki, K. Takeda, H. Fukuda, Y. Ishikawa, K. Wada,
and T. Yamamoto, "High-Performance Silicon Photonics Technology for Telecommunications
Applications", Sci. Technol. Adv. Mater. 15, 024603 (2014).

(96)

K. Yamada, T. Tsuchizawa, H. Nishi, R. Kou, T. Hiraki, K. Takeda, H. Fukuda, K. Okazaki, Y.
Ishikawa, K. Wada, and T. Yamamoto, "Silicon-Germanium-Silica Monolithic Photonic Integration
Platform for High-Performance Optical Data Communication Systems", ECS Trans. 64, 749-759(2014).

(97)

M. Yamagishi, N. Watase, M. Hashisaka, K. Muraki, and T. Fujisawa, "Spin-Dependent Tunneling
Rates for Electrostatically Defined GaAs Quantum Dots", Phys. Rev. B 90, 035306 (2014).

(98)

G. Yamahata, K. Nishiguchi, and A. Fujiwara, "Gigahertz Single-Trap Electron Pumps in Silicon",Nature Commun. 5, 5038 (2014).

(99)

G. Yamahata, K. Nishiguchi, and A. Fujiwara, "Accuracy Evaluation and Mechanism Crossover ofSingle-Electron Transfer in Si Tunable-Barrier Turnstiles", Phys. Rev. B 89, 165302 (2014).

(100)

G. Yamahata, T. Karasawa, and A. Fujiwara, "Gigahertz Single-Hole Transfer in Si Tunable-Barrier
Pumps", Appl. Phys. Lett. 106, 023112 (2015).

(101)

R. M. Yunus, M. Miyashita, M. Tsuji, H. Hibino, and H. Ago, "Formation of Oriented Graphene
Nanoribbons over Heteroepitaxial Cu Surfaces by Chemical Vapor Deposition", Chem. Mat. 26,
5215-5222 (2014).

(102)

G. Q. Zhang, K. Tateno, M. D. Birowosuto, M. Notomi, T. Sogawa, and H. Gotoh, "Controlled 1.1-1.6
μm Luminescence in Gold-Free Multi-Stacked InAs/InP Heterostructure Nanowires", Nanotechnology
26, 115704 (2015).

(103)

X. B. Zhu, Y. Matsuzaki, R. Amsüss, K. Kakuyanagi, T. Shimo-Oka, N. Mizuochi, K. Nemoto, K.
Semba, W. J. Munro, and S. Saito, "Observation of Dark States in a Superconductor Diamond
Quantum Hybrid System", Nature Commun. 5, 3524 (2014).

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List of Invited Talks at International Conferences (Jan. 2014 – Mar. 2015)
I. Materials Science Laboratory(1) H. Omi and T. Tawara, "MBE Growth and Optical Properties of Rare Earth Oxides on Si(111)",
Symposium on Surface and Nano Science 2014 (SSNS’14), Furano, Japan (Jan. 2014).
(2) Y. Kobayashi, K. Kumakura, T. Akasaka, H. Yamamoto, and T. Makimoto, "Application of BN for
GaN Devices", Photonics West 2014, San Francisco, U.S.A. (Feb. 2014).
(3) H. Yamamoto, Y. Krockenberger, and M. Naito, "Augmented Methods for Growth and Developmentof Novel Multi-Cation Oxides", Photonics West 2014, San Francisco, U.S.A. (Feb. 2014).
(4) Y. Krockenberger, H. Irie, K. Sakuma, B. Eleazer, J. Yan, L. Waterston, and H. Yamamoto, "Emerging
Superconductivity Hidden beneath Charge-Transfer Insulators", 2014 MRS Spring Meeting &Exhibit, San Francisco, U.S.A. (Apr. 2014).
(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", 2014 IEEE SiliconNanoelectronics Workshop, Honolulu, U.S.A. (June 2014).
(6) M. Hiroki, K. Kumakura, Y. Kobayashi, T. Akasaka, H. Yamamoto, and T. Makimoto, "GaN on h-BN
Technology for Release and Transfer of Nitride Devices", 2014 4th IEEE International Workshop on
Low Temperature Bonding for 3D Integration LTB-3D 2014, Tokyo, Japan (July 2014).
(7) H. Kageshima, H. Hibino, H. Yamaguchi, and M. Nagase, "Theoretical Studies of Graphene on SiC",
The 6th IEEE International Nanoelectronics Conference (IEEE INEC 2014), Sapporo, Japan (July
2014).
(8) K. Furukawa and Y. Ueno, "Graphene Oxide-Based FRET Aptasensor Fabricated and Integrated on
Solid Support", 248th ACS National Meeting & Exposition, San Francisco, U.S.A. (Aug. 2014).
(9) H. Hibino, "Low-Energy Electron Microscopy of Graphene and Hexagonal Boron Nitride",International Union of Materials Research Societies - The IUMRS International Conference in Asia
2014 (IUMRS-ICA 2014), Fukuoka, Japan (Aug. 2014).
(10) K. Hirama, Y. Taniyasu, S. Karimoto, Y. Krockenberger, M. Kasu, and H. Yamamoto,"Nitride/Diamond Heterostructure Systems - from Growth to Devices -", International Union ofMaterials Research Societies - The IUMRS International Conference in Asia 2014 (IUMRS-ICA2014), Fukuoka, Japan (Aug. 2014).
(11) H. Hibino, W. Shengnan, C. M. Orofeo, and S. Suzuki, "Growth, Characterization, andFunctionalization of Graphene and Hexagonal Boron Nitride", 2014 International Conference on
Solid State Devices and Materials (SSDM 2014), Tsukuba, Japan (Sep. 2014).
(12) K. Furukawa and Y. Ueno, "Protein Detection on Graphene Derivative Surface", KJF International
Conference on Organic Materials for Electronics and Photonics, Tsukuba, Japan (Sep. 2014).
(13) M. Hiroki, K. Kumakura, Y. Kobayashi, T. Akasaka, T. Makimoto, and H. Yamamoto, "Epitaxial Lift
Off of GaN Heterostructure by BN Insertion", 2nd Intensive Discussion on Growth of Nitride
Semiconductors (IDGN-2), Sendai, Japan (Oct. 2014).

68

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

(14) Y. Ueno and K. Furukawa, "On-Chip FRET Aptasensor Built on the Graphene Oxide Surface", The
7th International Symposium on Surface Science (ISSS-7), Matsue, Japan (Nov. 2014).
(15) K. Sumitomo, "Nanobiodevice for Mimicking Synaptic Connections", 7th International Symposium
on Advanced Plasma Science and Its Applications for Nitrides and Nanomaterials / 8th International
Conference on Plasma-Nano Technology & Science (ISPlasma2015/IC-PLANTS2015), Nagoya,Japan (Mar. 2015).

II. Physical Science Laboratory(1) K. Muraki, "Imaging Fractional Quantum Hall Liquids and Controlling Disorder Effects on the ν =5/2 State", Mauterndorf winterschool 2014, Mauterndorf, Austria (Feb. 2014).
(2) H. Yamaguchi, I. Mahboob, and H. Okamoto, "Nonlinear Dynamics and All Mechanical Phonon
Lasing in Electromechanical Resonators", 2014 IEEE International Frequency Control Symposium(IEEE IFCS), Taipei, Taiwan (May 2014).
(3) H. Yamaguchi, D. Hatanaka, H. Okamoto, and I. Mahboob, "Nonlinear Phonon Dynamics in Coupled
Electromechanical Resonators", Spin Mechanics 2, Sendai, Japan (June 2014).
(4) I. Mahboob, H. Okamoto, and H. Yamaguchi, "A Correlated Electromechanical System", NEMS 2014
- International Seminar on Nanomechanical Systems, Paris, France (June 2014).
(5) A. Fujiwara, K. Nishiguchi, and G. Yamahata, "Silicon Nanowire MOSFETs for Diverse
Applications", The 6th IEEE International Nanoelectronics Conference (IEEE INEC 2014), Sapporo,
Japan (July 2014).
(6) H. Yamaguchi, D. Hatanaka, I. Mahboob, and H. Okamoto, "Nonlinear ElectromechanicalResonators", The 6th IEEE International Nanoelectronics Conference (IEEE INEC 2014), Sapporo,
Japan (July 2014).
(7) S. Saito, "Superconducting Flux Qubit - Spin Ensemble Hybrid System", Workshop on diamondquantum science and technology, Queensland, Australia (Aug. 2014).
(8) N. Kumada, "Transport Properties of Edge Magnetoplasmons in Graphene", The International
Conference on Solid Films and Surfaces (ICSFS), Rio de Janeiro, Brasil (Sep. 2014).
(9) I. Mahboob, K. Nishiguchi, A. Fujiwara, and H. Yamaguchi, "Phonon-Lasing (and Mode Cooling) inan Electromechanical Resonator", 27th International Microprocesses and Nanotechnology
Conference (MNC 2014), Fukuoka, Japan (Nov. 2014).
(10) H. Yamaguchi, D. Hatanaka, I. Mahboob, and H. Okamoto, "Opto/Electromechanical Resonators
Based on GaAs/AlGaAs Heterostructures", 2014 International Electron Devices and Materials
Symposium (IEDMS 2014), Hualien, Taiwan (Nov. 2014).
(11) N. Kumada, "Shot Noise Generated by Graphene p-n Junctions in the Quantum Hall Effect Regime",
GDR Physique Quantique Mésoscopique, Aussois, France (Dec. 2014).
Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)69

(12) H. Yamaguchi, I. Mahboob, H. Okamoto, and D. Hatanaka, "Nonlinear Phonon Dynamics inElectromechanical Resonators", School & Discussion Meeting on Frontiers in Light-Matter
Interactions, Kolkata, India (Dec. 2014).
(13) K. Muraki, "Probing Disorder and Nnonlinear Screening in Quantum Hall Systems Using NMR",DiRT2D - 2015: Disorder and its Role in Transport in 2D systems, Okinawa, Japan (Jan. 2015).
(14) K. Muraki, "NMR Probing of Wigner Solids in High Magnetic Fields", International Symposium on
Quantum System and Nuclear Spin Related Phenomena, Miyagi, Japan (Feb. 2015).
(15) T. D. Rhone, "NMR Probing of Quantum Electron Solids in High Magnetic Fields", APS March
Meeting 2015, San Antonio, U.S.A. (Mar. 2015).

III. Optical Science Laboratory(1) W. J. Munro, A. M. Stephens, S. J. Devitt, K. A. Harrison, and K. Nemoto, "The Role of Memories in
Quantum Networks", Quantum Information and Measurement 2014, Berlin, Germany (Mar. 2014).
(2) H. Sanada, Y. Kunihashi, H. Gotoh, K. Onomitsu, M. Kohda, J. Nitta, P. V. Santos, and T. Sogawa,"Coherent Manipulation of Electron Spins in Acoustically Induced Moving Dots", International
Conference on Quantum Dots (QD 2014), Pisa, Italy (May 2014).
(3) K. Oguri, H. Mashiko, K. Kato, T. Sogawa, and H. Gotoh, "Ultrafast Spectroscopy Based on
High-Order Haromonic Sources for Sub-Cycle Dynamics in Solids", Shanghai - Tokyo Advanced
Research Symposium on Ultrafast Intense Laser Science (STAR5), Miyazaki, Japan (May 2014).
(4) K. Shimizu, "Cheat-Sensitive Commitment of a Classical Bit via Two-Way Quantum
Communication", The twenty third annual International Laser Physics Workshop (LPHYS'14), Sofia,
Bulgaria (July 2014).
(5) K. Sasaki, "Theory of Topological Raman Band in Graphene", International Union of Materials
Research Societies - The IUMRS International Conference in Asia 2014 (IUMRS-ICA 2014),
Fukuoka, Japan (Aug. 2014).
(6) H. Shibata, "Fabrication and Characterization of Superconducting Nanowire Single-Photon Detector",

27th International Symposium on Superconductivity, Tokyo, Japan (Nov. 2014).

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Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)

IV. Nanophotonics Center(1) S. Matsuo, "Directly Modulated Photonic Crystal Lasers for Computercom Applications", Photonics
West, San Francisco, U.S.A. (Feb. 2014).
(2) T. Sato and S. Matsuo, "Ultralow Power Consumption Photonic Crystal Nanocavity Lasers",
International workshop on emerging photonic technologies for access/metro networks, Paris, France(Mar. 2014).
(3) S. Matsuo, K. Takeda, and T. Sato, "Directly Modulated Photonic Crystal Wavelength-Scale Cavity
Lasers", SPIE Photonics Europe 2014, Brussels, Belgium (Apr. 2014).
(4) S. Matsuo, "Photonic Crystal Lasers for Computercom Networks", European Conference on
Integrated Optics (ECIO 17th) and the MicroOptics Conference (MOC 19th), Nice, France (June2014).
(5) K. Nozaki, E. Kuramochi, A. Shinya, S. Matsuo, T. Sato, and M. Notomi, "Ultralow-Power andIntegrated Operation of All-Optical Switches/Memories in a Photonic Crystal Chip", 16th
International Conference on Transparent Optical Networks, Graz, Austria (July 2014).
(6) A. Shinya and M. Notomi, "Integrated Nanophotonics Technology Toward fJ/bit OpticalCommunication in a Chip", 14th International Forum on Embedded MPSoC and Multicore, Margaux,France (July 2014).
(7) A. Shinya and M. Notomi, "Nanophotonics Technology Toward Optical Logic Circuits", 15th
International Forum on MPSoC for Software-Defined Hardware, Ventura, U.S.A. (July 2014).
(8) K. Nozaki, E. Kuramochi, A. Shinya, S. Matsuo, T. Sato, and M. Notomi, "Integrated All-Optical
Memories/Switches in a Photonic Crystal Chip", Advanced Photonics 2014, Barcelona, Spain (July
2014).
(9) A. Yokoo, M. D. Birowosuto, M. Takiguchi, G. Zhang, K. Tateno, E. Kuramochi, H. Taniyama, and
M. Notomi, "Semiconductor Nanowire Induced Nanocavity in Si Photonic Crystals", The 14th
International Symposium on Advanced Organic Photonics (ISAOP-14), Osaka, Japan (Nov. 2014).
(10) K. Yamada, T. Tsuchizawa, H. Nishi, R. Kou, T. Hiraki, K. Takeda, M. Usui, K. Okazaki, H. Fukuda,
Y. Ishikawa, K. Wada, and T. Yamamoto, "Si-Ge-Silica Photonic Integration Platform for
High-Performance Photonic Systems", Asia Communications and Photonics Conference, Shanghai,
China (Nov. 2014).
(11) M. Notomi, "Ultralow-Power Photonic Processing by Integrated Nanophotonics", The 5thInternational Topical Meeting on Nanophotonics and Metamaterials, Seefeld, Austria (Jan. 2015).
(12) K. Takeda, K. Honda, T. Takeya, K. Okazaki, T. Hiraki, T. Tsuchizawa, H. Nishi, R. Kou, H. Fukuda,
M. Usui, H. Nosaka, T. Yamamoto, and K. Yamada, "Comprehensive Photonics-Electronics
Convergent Simulation and Its Application to High-Speed Electronic Circuit Integration on a Si/Ge
Photonic Chip", Photonics West 2015, San Francisco, U.S.A. (Feb. 2015).

Research Activities in NTT Basic Research Laboratories Vol. 25 (2014)71

Cover Photograph: Over-100-bit Photonic Crystal Optical-RAM Chip
We are working on large-scale integration of optical-memory (o-RAM) by photonic crystal
technology which can break through the limit of size and power consumption in conventional
o-RAMs. Here we fabricated a 1.1-mm silicon photonic crystal chip in which over 100
nanocavities were integrated and realized successfully over-100-bit o-RAM operation with low
power for the first time (See page 45).

Microscope image of 128 nanocavities integrated in a silicon chip. Lower: Schematic of multi-bit o-RAM
action with wavelength division multiplexing.

Research Activities in NTT BRL
Editorial Committee
Principal of o-RAM operation. A bistable memory that holds/switches “0” and “1” states is realized by
fast carrier-plasma effect greatly enhanced in a high quality factor nanocavity.

NTT Basic Research Laboratories,
NTT Corporation  
3-1 Morinosato Wakamiya, Atsugi,Write and read-out operations of 4-bit random pulses in two different sets of sequential 4 o-RAMs(demonstrated in a similar integrated o-RAM chip in an InP-based photonic crystal).

  Kanagawa, 243-0198 Japan  

URL: http://www.brl.ntt.co.jp/

Research Act iv it ies in N TT B asi c R esear ch Labo ra t o rie s Vo l u m e 25

Research Activities in
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J u l y 201 5
N T T B a s i c R es ea rc h L a b o ra t o ri es ,N i p p o n T el eg ra p h a nd T el ep ho ne C o rp o ra tion
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