Faculty Profiles - OZAKI Nobuhiko

Published Papers - OZAKI Nobuhiko

Division display >> ／ All the affair displays1 - 104 of about 104

Numerical validation of low-coherence interferometers using low-group-velocity and low-dispersion photonic crystal waveguides for ultra-compact OCT applications

Nanaho Oda, Hisaya Oda, Naoki Ikeda, and Nobuhiko Ozaki （Part： Last author,　Corresponding author )

Japanese Journal of Applied Physics ( IOP publishing ) 64 ( 4 ) 04SP58 1-6 2025.04 [Refereed]

In this study, an ultra-compact optical coherence tomography (OCT) system based on integrated photonic crystal waveguides (PhC-WGs) was proposed and validated numerically. A low-velocity and low-dispersion (LVLD) PhC-WG is employed in the reference arm, enabling significant reduction in its length because of the high group index achieved with a broadband light source. Optical delays in the reference arm were found to increase with the designed group index of the LVLD-PhC-WG, confirming the low-coherence interference. Additionally, similar findings were observed in a PhC-WG-based OCT (PhC-OCT) model integrating a directional coupler and the LVLD-PhC-WG. The PhC-OCT with a considerably small footprint, e.g. 20 × 100 μm2, for the interferometer including the reference arm indicated interferometric signals with an axial resolution of approximately 40 μm. These results demonstrate the feasibility of an ultra-compact OCT system using PhC-WGs.

Monolithic vertical cavities via selective-area growth using a rotational metal mask and InAs quantum dots for near-infrared multiple-wavelength surface-emitting light source applications

Yuuki C. Hodson, Tatsuki Yokota, Eiichiro Watanabe, Naoki Ikeda, and Nobuhiko Ozaki （Part： Last author,　Corresponding author )

Applied Physics Express ( IOP Publishing ) 18 ( 3 ) 035501 1-5 2025.03 [Refereed]

We herein report a method for fabricating near-infrared multiple-wavelength surface-emitting light sources. After growing a GaAs layer containing stacked InAs quantum dots (QDs) on a GaAs/AlAs distributed Bragg reflector by molecular beam epitaxy, additional GaAs layers were grown via selective-area growth using a rotational metal-mask. The GaAs thickness variation generated different vertical cavity (VC) modes that resonated with different emission wavelengths within the broadband emission from the InAs QDs. Photoluminescence measurements confirmed monolithic VCs formation in the selected areas. This method offers a simple fabrication process and extends the emission wavelength range of near-infrared multiple-wavelength surface-emitting light sources.

In segregation influence on properties of InAs quantum dots in dots-in-a-well

Koki Okuno, Naoki Okada, Kosuke Iwaide, and Nobuhiko Ozaki （Part： Last author,　Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 63 ( 5 ) 055507 1-5 2024.05 [Refereed]

We investigated the growth of InAs quantum dots (QDs) in a dots-in-a-well (DWELL) from the perspective of the influence of In segregation from the InGaAs layers in the DWELL. Reflection high-energy electron diffraction (RHEED) measurements during the growth of the lower InGaAs layer indicated that In segregation increased with the In composition of the InGaAs layer. The estimated In segregation values were consistent with the decreases in the critical thickness for the QDs growth and the total volume variations of the grown QDs. These results illustrate that the segregated In from the lower InGaAs layer contributes to the QD growth in the DWELL, and their density increases. Furthermore, RHEED measurements during the growth of the upper InGaAs layer indicated the suppression of the deformation of embedded QDs , which could partially contribute to the longer emission wavelength of the QDs in the DWELL.

Proposal and numerical verification of an ultrasmall terahertz source using integrated photonic crystal waveguides for highly efficient differential frequency generation

Yota Koyama, Hisaya Oda, Naoki Ikeda, Yoshimasa Sugimoto, Nobuhiko Ozaki （Part： Last author,　Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 62 ( SG ) SG1033 1-5 2023.03 [Refereed]

We propose and numerically investigate integrated photonic crystal waveguides (PhC-WGs) formed in a semiconductor slab to realize an ultrasmall and highly efficient terahertz (THz) wave source. The structure consists of a straight PhC-WG with low-group-velocity and low-dispersion (LVLD) for efficient difference frequency generation (DFG) connected to two PhC-WGs to introduce two fundamental lights into the LVLD PhC-WG. The fundamental light propagating through each PhC-WG designed to enhance their electric fields by the slow-light effect is efficiently coupled to the LVLD PhC-WG owing to the reduced refractive index differences at the boundaries of the heterostructures. The DFG from the two fundamental lights was numerically simulated, and a temporal intensity oscillation corresponding to the difference in frequency was clearly observed. By comparing the DFG intensities of the integrated structures with an LVLD PhC-WG and a strip WG, the estimated DFG intensity from the LVLD PhC-WG was more than 100 times higher than that from the strip WG. These results indicate the effectiveness of the proposed heterostructure in the application of a highly efficient THz source with an ultrasmall footprint compared with conventional materials.

Broadband optical wavelength conversion through four-wave mixing in W3-type AlGaAs photonic crystal waveguides

Hisaya Oda, Youhei Hosokawa, Kazuki Hayashi, Nobuhiko Ozaki, Naoki Ikeda, Yoshimasa Sugimoto

Applied Physics Express ( IOP Publishing ) 16 ( 2 ) 022004 1-4 2023.02 [Refereed]

We realized a wide-band wavelength conversion method through four-wave mixing in W3-type AlGaAs photonic crystal waveguides. AlGaAs exhibits a large third-order nonlinearity. Furthermore, because of its large bandgap, two-photon absorption can be avoided in the 1550-nm range. A four-wave mixing efficiency of −9.03 dB was obtained for a pump peak power of 7 W. Furthermore, by utilizing the two even guided bands of the W3-type photonic crystal waveguide, a conversion bandwidth greater than 38 nm was achieved with a conversion efficiency of −22 dB.

In-situ estimation of emission wavelength of embedded InAs QDs using RHEED intensity measurements

Nobuhiko Ozaki, Daigo Ikuno （Part： Lead author,　Corresponding author )

Journal of Crystal Growth ( Elsevier BV ) 588 126657 1-4 2022.06 [Refereed]

We propose a method for estimating the emission wavelength of self-assembled InAs quantum dots (QDs) embedded in a capping layer using reflection high-energy electron diffraction (RHEED) intensity measurements. As QDs are capped with a Ga(In)As layer, the RHEED intensity of QDs decays, and the decay degree depends on the growth conditions of capping. The RHEED intensity decay was analyzed, and a relationship between the decay degree and photoluminescence peak energy of embedded QDs was determined. This relationship can be used to predict the emission wavelength of embedded QDs during capping, which is particularly useful for QD-based optical device fabrication.

Near-infrared dual-wavelength surface-emitting light source using InAs quantum dots resonant with vertical cavity modes

Jin Oshima, Nobuhiko Ozaki, Hisaya Oda, Eiichiro Watanabe, Hirotaka Ohsato, Naoki Ikeda, Yoshimasa Sugimoto, Richard A. Hogg （Part： Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 61 ( SD ) SD1003 1-5 2022.03 [Refereed]

We developed a compact dual-wavelength surface-emitting light source using InAs quantum dots (QDs) embedded in a vertical cavity (VC). The VC was designed to possess two optical cavity modes that resonate with the discrete emission lines of the QDs. The fabricated light source exhibited significant enhancements in the vertical light emission corresponding to the VC modes. In addition, the light source demonstrated selectivity to the enhanced emission wavelengths with changes in temperature. Compared to conventional dual-wavelength vertical external cavity surface-emitting lasers, these QD-based dual-wavelength emission devices allow for the realization of simple structures because the InAs QDs act as dual-light-emitting materials. These results can be applied to simple dual-wavelength surface-emitting light sources.

Emission wavelength control of InAs/GaAs quantum dots using an As2 source for near-infrared broadband light source applications

Nobuhiko Ozaki, Yuma Hayashi, Shunsuke Ohkouchi, Hirotaka Ohsato, Eiichiro Watanabe, Naoki Ikeda, Yoshimasa Sugimoto, Richard A. Hogg （Part： Lead author,　Corresponding author )

Applied Physics Express ( IOP Publishing ) 14 ( 5 ) 055501 1-6 2021.04 [Refereed]

Herein, we report an emission wavelength control technique for self-assembled InAs quantum dots (QDs) grown via molecular beam epitaxy using an As2 source (As2-QDs). The As2-QDs exhibited photoluminescence with a shorter center wavelength and larger bandwidth than those of the QDs grown using an As4 source. In addition, the emission center wavelength could be controlled by adjusting the time between the growth and capping of the As2-QDs. We utilized the multilayer stack of emission-wavelength-controlled As2-QDs to fabricate an electrically-driven light source and demonstrated its broadband (approximately 130 nm) emission in the 1–1.3 μm wavelength range.

1.1 μm waveband tunable laser using emission-wavelength-controlled InAs quantum dots for swept-source optical coherence tomography applications

Toshiya Tsuji, Nobuhiko Ozaki, Sho Yamauchi, Katsuya Onoue, Eiichiro Watanabe, Hirotaka Ohsato, Naoki Ikeda, Yoshimasa Sugimoto, David T. D. Childs, and Richard A. Hogg （Part： Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 60 ( SB ) SBBE02 1-6 2021.02 [Refereed]

In this study, an optical gain chip using emission-wavelength-controlled self-assembled InAs quantum dots (QDs) was developed for swept-source optical coherence tomography (SS-OCT) applications. The optical characterizations indicated that the QDs emission wavelength and optical gain spectra were controlled in the 1.1μm waveband by optimizing the QDs growth conditions. This waveband is useful for obtaining a large imaging depth of OCT because of an optimal balance between absorption and scattering in biological samples. In addition, continuous tunable lasing in the waveband was achieved by introducing the QD-based gain chip into a grating-coupled external cavity. This tunable laser was introduced into an SS-OCT setup, and the point spread function (PSF) was evaluated. The PSF position was observed to vary according to the optical path length differences. These results demonstrate the feasibility of the application of emission-wavelength-controlled QDs for SS-OCT.

Bandwidth enhancement in an InGaN/GaN three-section superluminescent diode for optical coherence tomography

Graham R. Goldberg, Dae-Hyun Kim, Richard J. E. Taylor, David T. D. Childs, Pavlo Ivanov, Nobuhiko Ozaki, Kenneth L. Kennedy, Kristian M. Groom, Yukihiro Harada, and Richard A. Hogg

Applied Physics Letters ( AIP Publishing ) 117 ( 6 ) 061106 1-5 2020.08 [Refereed]

In this paper, the optoelectronic properties of InGaN-based blue (430 nm) superluminescent light-emitting diodes with a multi-section, three contact design are reported. The bias conditions of the rear absorber section and gain sections are explored in terms of enhancing and maximizing spectral bandwidth. We demonstrate that broader emission can be obtained with a short circuit, rather than an open circuit absorber section, and with two active regions at different current densities as opposed to a single active contact. Under optimal drive conditions, a −3 dB linewidth of 20 nm is obtained at 430 nm. Analysis of the device emission spectrum indicates that an axial resolution of ∼3.4 μm may be obtained in an optical coherence tomography system.

Numerical investigation of highly efficient and tunable terahertz-wave generation using a low-group-velocity and low-dispersion two-dimensional GaAs photonic crystal waveguide

Teruyuki Nakahama, Nobuhiko Ozaki, Hisaya Oda, Naoki Ikeda, and Yoshimasa Sugimoto （Part： Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 59 ( 9 ) 090903 1-7 2020.08 [Refereed]

Noninvasive High-axial-resolution Profile Imaging Using Near Infrared Broadband Light Source Based on Self-assembled Quantum Dots

Nobuhiko Ozaki （Part： Lead author,　Corresponding author )

Journal of the Imaging Society of Japan 58 ( 6 ) 617 - 625 2019.12 [Refereed] [Invited]

Development of a broadband superluminescent diode based on self-assembled InAs quantum dots and demonstration of high-axial-resolution optical coherence tomography imaging

Nobuhiko Ozaki, Sho Yamauchi, Yuma Hayashi, Eiichiro Watanabe, Hirotaka Ohsato, Naoki Ikeda, Yoshimasa Sugimoto, Kenji Furuki, Yoichi Oikawa, Kunio Miyaji, David T D Childs, and Richard A Hogg （Part： Lead author,　Corresponding author )

Journal of Physics D: Applied Physics ( IOP Publishing ) 52 ( 22 ) 225105 1-9 2019.05 [Refereed]

Tunable external cavity laser diode based on wavelength controlled self-assembled InAs quantum dots for swept-source optical coherence tomography applications at 1100 nm wavelength band

Nobuhiko Ozaki, David T. D. Childs, Aleksandr Boldin, Daigo Ikuno, Katsuya Onoue, Hirotaka Ohsato, Eiichiro Watanabe, Naoki Ikeda, Yoshimasa Sugimoto, Richard A. Hogg （Part： Lead author,　Corresponding author )

SPIE Proceedings ( SPIE ) 10939 1093911 1-6 2019.03 [Refereed]

Preparation of Ag2Se QDs with excellent aqueous dispersion stability by organic coating with aqueous ATRP

Yoshio Nakahara, Yuki Kunitsu, Nobuhiko Ozaki, Mutsuo Tanaka, Setsuko Yajima

Polymer Bulletin ( Springer Science and Business Media LLC ) 76 ( 9 ) 4753 - 4768 2018.12 [Refereed]

Non-destructive and non-contact measurement of semiconductor optical waveguide using optical coherence tomography with a visible broadband light source

Kazumasa Ishida, Nobuhiko Ozaki, Hirotaka Ohsato, Eiichiro Watanabe, Naoki Ikeda, Yoshimasa Sugimoto （Part： Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 57 ( 8S2 ) 08PE03 1-5 2018.08 [Refereed]

Reflection Spectra from SiC Substrate with Circular-Slot Antennas and Possible Interaction between Surface-Plasmon Polaritons and Surface-Phonon Polaritons

N. Umemori, K. Kasahara, T. Yaji, N. Ozaki, N. Ikeda, Y. Sugimoto

2018 12th International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials) ( IEEE ) 204 - 206 2018.08

Non-destructive inspection of semiconductor optical waveguide using optical coherence tomography with visible broadband light source

Kazumasa Ishida, Nobuhiko Ozaki, Naoki Ikeda, Yoshimasa Sugimoto

22nd Microoptics Conference, MOC 2017 ( Institute of Electrical and Electronics Engineers Inc. ) 2017- 242 - 243 2018.01 [Refereed]

Growth of quantum three-dimensional structure of InGaAs emitting at ~1 µm applicable for a broadband near-infrared light source

Nobuhiko Ozaki, Shingo Kanehira, Yuma Hayashi, Shunsuke Ohkouchi, Naoki Ikeda, Yoshimasa Sugimoto, Richard A. Hogg （Part： Lead author,　Corresponding author )

Journal of Crystal Growth ( Elsevier BV ) 477 230 - 234 2017.11 [Refereed]

Gallium Nitride Superluminescent Light Emitting Diodes for Optical Coherence Tomography Applications

Graham R. Goldberg, Aleksandr Boldin, Sophia M. L. Andersson, Pavlo Ivanov, Nobuhiko Ozaki, Richard J. E. Taylor, David T. D. Childs, Kristian M. Groom, Kenneth L. Kennedy, Richard A. Hogg

IEEE Journal of Selected Topics in Quantum Electronics ( Institute of Electrical and Electronics Engineers (IEEE) ) 23 ( 6 ) 2000511 1-11 2017.07 [Refereed]

Strain Balancing of Metal-Organic Vapour Phase Epitaxy InAs/GaAs Quantum Dot Lasers

Timothy S. Roberts, Benjamin J. Stevens, Edmund Clarke, Ian Tooley, Jonathan Orchard, Ian Farrer, David T. D. Childs, Nasser Babazadeh, Nobuhiko Ozaki, David Mowbray, Richard A. Hogg

IEEE Journal of Selected Topics in Quantum Electronics ( Institute of Electrical and Electronics Engineers (IEEE) ) 23 ( 6 ) 1901208 1-8 2017.05 [Refereed]

Ultra-small near-infrared multi-wavelength light source using a heterojunction photonic crystal waveguide and self-assembled InAs quantum dots

Sho Uchida, Nobuhiko Ozaki, Teruyuki Nakahama, Hisaya Oda, Naoki Ikeda, Yoshimasa Sugimoto （Part： Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 56 ( 5 ) 050303 1-4 2017.05 [Refereed]

Gallium nitride light sources for optical coherence tomography

Graham R. Goldberg, Pavlo Ivanov, Nobuhiko Ozaki, David T. D. Childs, Kristian M. Groom, Kenneth L. Kennedy, Richard A. Hogg

SPIE Proceedings ( SPIE ) 10104 101041X 1-7 2017.02 [Refereed]

High-resolution and nondestructive profile measurement by spectral-domain optical coherence tomography with a visible broadband light source for optical-device fabrication

Tsuyoshi Nishi, Nobuhiko Ozaki, Yoichi Oikawa, Kunio Miyaji, Hirotaka Ohsato, Eiichiro Watanabe, Naoki Ikeda, Yoshimasa Sugimoto （Part： Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 55 ( 8S3 ) 08RE05 1-5 2016.08 [Refereed]

Operation of an InAs quantum-dot embedded GaAs photonic crystal slab waveguide laser by using two-photon pumping for photonics integrated circuits

H. Oda, A. Yamanaka, N. Ozaki, N. Ikeda, Y. Sugimoto

AIP Advances ( AIP Publishing ) 6 ( 6 ) 065215 1-6 2016.06 [Refereed]

Emission wavelength variation of InAs quantum dots grown on GaAs using As2 molecules in molecular beam epitaxy

Yuma Hayashi, Nobuhiko Ozaki, Shunsuke Ohkouchi, Hirotaka Ohsato, Eiichiro Watanabe, Naoki Ikeda, Yoshimasa Sugimoto （Part： Corresponding author )

2016 Compound Semiconductor Week (CSW) [Includes 28th International Conference on Indium Phosphide & Related Materials (IPRM) & 43rd International Symposium on Compound Semiconductors (ISCS)] ( IEEE ) 1 - 2 2016.06 [Refereed]

Electrically Driven Near-Infrared Broadband Light Source with Gaussian-Like Spectral Shape Based on Multiple InAs Quantum Dots

Takuma YASUDA, Nobuhiko OZAKI, Hiroshi SHIBATA, Shunsuke OHKOUCHI, Naoki IKEDA, Hirotaka OHSATO, Eiichiro WATANABE, Yoshimasa SUGIMOTO, Richard A. HOGG （Part： Corresponding author )

IEICE Transactions on Electronics ( Institute of Electronics, Information and Communications Engineers (IEICE) ) E99.C ( 3 ) 381 - 384 2016.03 [Refereed]

Superluminescent diode with a broadband gain based on self-assembled InAs quantum dots and segmented contacts for an optical coherence tomography light source

Nobuhiko Ozaki, David T. D. Childs, Jayanta Sarma, Timothy S. Roberts, Takuma Yasuda, Hiroshi Shibata, Hirotaka Ohsato, Eiichiro Watanabe, Naoki Ikeda, Yoshimasa Sugimoto, Richard A. Hogg （Part： Lead author,　Corresponding author )

Journal of Applied Physics ( AIP Publishing ) 119 ( 8 ) 083107 1-7 2016.02 [Refereed]

Gallium Nitride Super-Luminescent Light Emitting Diodes for Optical Coherence Tomography Applications

Graham R. Goldberg, Pavlo Ivanov, Nobuhiko Ozaki, David T. D. Childs, Kristian M. Groom, Kenneth L. Kennedy, Richard A. Hogg

2016 INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE (ISLC) ( IEEE ) 2016 [Refereed]

Optical characterization of In-flushed InAs/GaAs quantum dots emitting a broadband spectrum with multiple peaks at ~1 μm

Shigehiro Kitamura, Masaya Senshu, Toshio Katsuyama, Yuji Hino, Nobuhiko Ozaki, Shunsuke Ohkouchi, Yoshimasa Sugimoto, Richard A Hogg （Part： Corresponding author )

Nanoscale Research Letters ( Springer Science and Business Media LLC ) 10 231 1-6 2015.05 [Refereed]

Abstract

We investigated optical properties of In-flushed InAs quantum dots (QDs) grown on a GaAs substrate by molecular beam epitaxy. By using the In-flush technique for setting the height of self-assembled InAs QDs, we have tuned the emission wavelength of InAs QDs to the ~1 μm regime, which can be utilized as a non-invasive and deeply penetrative probe for biological and medical imaging systems. The controlled emission exhibited a broadband spectrum comprising multiple peaks with an interval of approximately 30 meV. We examined the origin of the multiple peaks using spectral and time-resolved photoluminescence, and concluded that it is attributed to monolayer step fluctuations in the height of the In-flushed QDs. This feature can be advantageous for realizing a broadband light source centered at the ~1 μm regime, which is especially suitable for the non-invasive cross-sectional biological and medical imaging system known as optical coherence tomography.

Imaging of spectral-domain optical coherence tomography using a superluminescent diode based on InAs quantum dots emitting broadband spectrum with Gaussian-like shape

Hiroshi Shibata, Nobuhiko Ozaki, Takuma Yasuda, Shunsuke Ohkouchi, Naoki Ikeda, Hirotaka Ohsato, Eiichiro Watanabe, Yoshimasa Sugimoto, Kenji Furuki, Kunio Miyaji, Richard A. Hogg （Part： Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 54 ( 4S ) 04DG07 1-5 2015.04 [Refereed]

Spectral and temporal photoluminescence behavior of colloidal PbS quantum dots

S. Kitamura, M. Senshu, H. Tokushige, T. Katsuyama, N. Ozaki, I. Tanaka, Y. Sugimoto

Superlattices and Microstructures ( Elsevier BV ) 79 123 - 134 2015.03 [Refereed]

SPECTRAL-DOMAIN OPTICAL COHERENCE TOMOGRAPHY WITH A WHITE LIGHT DEVELOPED FOR OPTICAL DEVICE FABRICATION

T. Nishi, N. Ozaki, H. Ohsato, E. Watanabe, N. Ikeda, Y. Sugimoto （Part： Corresponding author )

2015 20TH MICROOPTICS CONFERENCE (MOC) ( IEEE ) 180 - 181 2015 [Refereed]

Integration of Emission-Wavelength-Controlled InAs Quantum Dots for Ultra-Broadband Near-Infrared Light Source

Nobuhiko Ozaki, Koichi Takeuchi, Yuji Hino, Yohei Nakatani, Takuma Yasuda, Shunsuke Ohkouchi, Eiichiro Watanabe, Hirotaka Ohsato, Naoki Ikeda, Yoshimasa Sugimoto, Edmund Clarke, Richard A. Hogg （Part： Lead author,　Corresponding author )

Nanomaterials and Nanotechnology ( SAGE Publications ) 4 26 1-17 2014.09 [Refereed] [Invited]

Near-infrared (NIR) light sources are widely utilized in biological and medical imaging systems owing to their long penetration depth in living tissues. In a recently developed biomedical non-invasive cross-sectional imaging system, called optical coherence tomography (OCT), a broadband spectrum is also required, because OCT is based on low coherence interferometry. To meet these operational requirements, we have developed a NIR broadband light source by integrating self-assembled InAs quantum dots (QDs) grown on a GaAs substrate (InAs/GaAs QDs) with different emission wavelengths. In this review, we introduce the developed light sources and QD growth techniques that are used to control the emission wavelength for broadband emission spectra with center wavelengths of 1.05 and 1.3 μm. Although the strain-induced Stranski-Krastanov (S-K) mode-grown InAs/GaAs QDs normally emit light at a wavelength of around 1.2 μm, the central emission wavelength can be controlled to be between 0.9–1.4 μm by the use of an In-flush technique, the insertion of a strain-reducing layer (SRL) and bi-layer QD growth techniques. These techniques are useful for applying InAs/GaAs QDs as NIR broadband light sources and are especially suitable for our proposed spectral-shape-controllable broadband NIR light source. The potential of this light source for improving the performance of OCT systems is discussed.

Alcohol additive effect in hydrogen generation from water with carbon by photochemical reaction

Kosuke Maeda, Nobuhiko Ozaki, Ikuko Akimoto

Japanese Journal of Applied Physics ( IOP Publishing ) 53 ( 5S1 ) 05FZ03 - 05FZ03 2014.03 [Refereed]

Near-infrared superluminescent diode using stacked self-assembled InAs quantum dots with controlled emission wavelengths

Nobuhiko Ozaki, Takuma Yasuda, Shunsuke Ohkouchi, Eiichiro Watanabe, Naoki Ikeda, Yoshimasa Sugimoto, Richard A. Hogg （Part： Lead author,　Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 53 ( 4S ) 04EG10 - 04EG10 2014.02 [Refereed]

Growth and Optical Characterizations of InAs-QDs Emitting at 1μm with a Broadband Spectrum for a Light Source for Biomedical Optical Coherence Tomography

Nobuhiko OZAKI, Yuji HINO, Shunsuke OHKOUCHI, Naoki IKEDA, Yoshimasa SUGIMOTO （Part： Lead author,　Corresponding author )

Journal of the Society of Materials Science, Japan ( Society of Materials Science, Japan ) 62 ( 11 ) 679 - 682 2013.11 [Refereed]

Broadband emission centered at ~1 µm with a Gaussian-like spectrum by stacking In-flushed QD layers

Nobuhiko Ozaki, Yuji Hino, Shunsuke Ohkouchi, Naoki Ikeda, Yoshimasa Sugimoto （Part： Lead author,　Corresponding author )

Physica Status Solidi C: Current Topics in Solid State Physics ( WILEY-V C H VERLAG GMBH ) 10 ( 11 ) 1361 - 1364 2013.10 [Refereed]

We have grown a stack of In-flushed InAs quantum dot (QD) layers to realize a broadband emission centered at similar to 1 mu m with a Gaussian-like spectrum as a light source for optical coherence tomography (OCT). The In-flush process, which enables control of the height of the QD, was optimized to control the emission wavelength and intensity of self-assembled InAs/GaAs QDs. A partial capping layer of GaAs was deposited on as-grown InAs/GaAs QDs with various thicknesses (d(cap)), and a rapid annealing of the sample was executed with the proper annealing temperature. By optimizing d(cap) and the annealing temperature for each QD layer, a broadband Gaussian-like spectrum with a 102-nm-bandwidth was obtained. The coherence function of the emission spectrum exhibits a 4.5-mu m axial resolution for OCT images without distinct side lobes. These results demonstrate the effectiveness of the In-flushed QDs as a light source to obtain high-quality OCT images.

Hydrogen generation by laser irradiation of carbon powder in water

Ikuko Akimoto, Kousuke Maeda, Nobuhiko Ozaki

Journal of Physical Chemistry C ( American Chemical Society ({ACS}) ) 117 ( 36 ) 18281 - 18285 2013.09 [Refereed]

We report the photochemical activity of carbon powder in the generation of hydrogen from water by laser irradiation, without any electrodes and photocatalysts. The gas was obtained by laser irradiation in the VIS-NIR range and was dependent nonlinearly on the laser power density. From a gas component analysis and a repeated irradiation experiment, it was found that the carbon powder was oxidized and acted as a sacrificial reagent in the photochemical hydrogen generation. In addition, a highly carbonized charcoal, known as Bincho-tan, was found to effectively work for the hydrogen generation. © 2013 American Chemical Society.

Extending emission wavelength of InAs/GaAs quantum dots beyond 1.3 mu m by using quantum dot bi-layer for broadband light source

N. Ozaki, Y. Nakatani, S. Ohkouchi, N. Ikeda, Y. Sugimoto, K. Asakawa, E. Clarke, R. A. Hogg （Part： Lead author,　Corresponding author )

Journal of Crystal Growth ( ELSEVIER SCIENCE BV ) 378 553 - 557 2013.09 [Refereed]

We have grown bi-layer InAs quantum dots (QDs) on GaAs substrates for extending the emission wavelength of InAs/GaAs QDs beyond 1.3 mu m. The QD bi-layer, which comprises two QDs layers (seed- and active-QDs) separated with a GaAs spacer layer of 10 nm in thickness, exhibits light emission from active-QDs with longer wavelength. An enlargement of the active-QDs occurred by optimizing several growth parameters: growth temperature of seed-QDs, amount of InAs supplied for seed- and active-QDs layers. These optimized parameters lowered the density of the seed-QDs strain spreading upward, which resulted in an enlargement of the active-QDs. We achieved a control of the extension of emission wavelength up to approximately 1.4 mu m from 1.2 mu m. These results indicate that the QD bi-layer can be applied to a broadband light source exhibiting an emission spectrum centered at 1.3 mu m with a bandwidth of 200 nm. (c) 2013 Elsevier B.V. All rights reserved.

Growth of InAs/GaAs quantum dots with central emission wavelength of 1.05 μm using In-flush technique for broadband near-infrared light source

Yuji Hino, Nobuhiko Ozaki, Shunsuke Ohkouchi, Naoki Ikeda, Yoshimasa Sugimoto （Part： Corresponding author )

Journal of Crystal Growth ( Elsevier B.V. ) 378 501 - 505 2013.09 [Refereed]

Self-assembled InAs quantum dots (QDs) emitting at around 1.05 μm in wavelength were grown on a GaAs substrate by using the In-flush technique. A rapid annealing (In-flush) process after the growth of the GaAs capping layer, which partially covers InAs-QDs, reduces the height of the InAs-QDs to the thickness of the capping layer. Using this approach, the emission wavelengths of the QDs were precisely controlled by varying the thickness of the capping GaAs layer. The central emission wavelength was suitably controlled in the range of approximately 1.22-0.95 μm. This method enables the realization of a broadband 1.05 μm light source with a bandwidth of beyond 200 nm via a combination of In-flushed QDs. Such a broadband light source of 1.05 μm in wavelength is applicable to optical coherence tomography (OCT), thereby enabling high-resolution and large penetration depth in the OCT images. In addition, we found that a higher emission intensity of the In-flushed QDs can be obtained by reducing the annealing temperature of the In-flush process. © 2013 Elsevier B.V. All rights reserved.

Monolithically grown multi-color InAs quantum dots as a spectral-shape-controllable near-infrared broadband light source

Nobuhiko Ozaki, Koichi Takeuchi, Shunsuke Ohkouchi, Naoki Ikeda, Yoshimasa Sugimoto, Hisaya Oda, Kiyoshi Asakawa, Richard A. Hogg （Part： Lead author,　Corresponding author )

Applied Physics Letters ( AIP Publishing ) 103 ( 5 ) 051121 1-4 2013.07 [Refereed]

Enhancement upconversion luminescence in InAs-quantum dots embedded GaAs photonic-crystal slab line-defect waveguide

H. Oda, A. Yamanaka, N. Ozaki, N. Ikeda, Y. Sugimoto

2013 Conference on Lasers and Electro-Optics Europe and International Quantum Electronics Conference, CLEO/Europe-IQEC 2013 ( IEEE Computer Society ) 2013 [Refereed]

Photonic crystals are very suitable for controlling radiation field and propagation characterization of light. As an important application, it is ultra compact and ultrafast optical integrated circuits (OIC) based on photonic crystals slab waveguide (PhC-WG) composing of the line-defects [1, 2]. The PhC-WG is also attractive for laser lasing, because very small group velocity of near the Brillouin zone (BZ) edge should enhance interactions between the radiation field and matter. Indeed, the lasing spectrum has been observed from optically pumped InAs-quantum dots (InAs-QDs) embedded PhC-WG [3]. And a small group velocity (Vg) in the PhC-WG gives rise to enhancement intensity and, therefore, is advantageous to nonlinear optical effect. In this work, we present the 1.55 μm to 1.3 μm upconversion luminescence (UL) based on two-photon absorption in InAs-QDs GaAs PhC-WG. © 2013 IEEE.

Size dependent optical properties of quinacridonequinone nanoparticles prepared by liquid laser ablation in water

Ikuko Akimoto, Masahiro Ohata, Nobuhiko Ozaki, Ping Gu

Chemical Physics Letters ( Elsevier BV ) 552 102 - 107 2012.11 [Refereed]

Nanophotonics Based on Semiconductor-Photonic Crystal/Quantum Dot and Metal-/Semiconductor-Plasmonics

Kiyoshi ASAKAWA, Yoshimasa SUGIMOTO, Naoki IKEDA, Daiju TSUYA, Yasuo KOIDE, Yoshinori WATANABE, Nobuhiko OZAKI, Shunsuke OHKOUCHI, Tsuyoshi NOMURA, Daisuke INOUE, Takayuki MATSUI, Atsushi MIURA, Hisayoshi FUJIKAWA, Kazuo SATO

IEICE Transactions on Electronics ( Institute of Electronics, Information and Communications Engineers (IEICE) ) E95-C ( 2 ) 178 - 187 2012.02 [Refereed] [Invited]

Broadband Light Source Based on Four-Color Self-Assembled InAs Quantum Dot Ensembles Monolithically Grown in Selective Areas

Nobuhiko OZAKI, Koichi TAKEUCHI, Shunsuke OHKOUCHI, Naoki IKEDA, Yoshimasa SUGIMOTO, Kiyoshi ASAKAWA, Richard A. HOGG （Part： Lead author,　Corresponding author )

IEICE Transactions on Electronics ( Institute of Electronics, Information and Communications Engineers (IEICE) ) E95-C ( 2 ) 247 - 250 2012.02 [Refereed]

Formation of nanoparticles of organic molecules by liquid laser ablation

Ikuko Akimoto, Masahiro Ohata, Nobuhiko Ozaki, Gu Ping

Materials Research Society Symposium Proceedings 1455 55 - 60 2012 [Refereed]

We carried out laser ablation of three organic molecules, rubrene (Rb), Oralith Brilliant Pink R (BP) and quinacridonequinone (QQ) in a poor solvent, water. As a result, nanoparticles of BP and QQ were formed, but those of Rb were not formed because of photodissociation. For a rigid molecule, QQ, optical properties of colloidal solutions were investigated in relation to the size of the included nanoparticles. A linear correlation between the blue shift of the absorption peak energy and the decrease in the diameter of the nanoparticles was found, indicating that the nanoparticle diameter can be easily estimated from the absorption spectrum of a colloidal solution. From the solution, a nanoparticle film was fabricated on an electrode by the electrophoretic deposition method. © 2012 Materials Research Society.

Modification of optical response in quantum dots embedded in a photonic crystal waveguide via photonic band engineering

Nobuhiko Ozaki, Hayato Yoneda, Koichi Takeuchi, Hisaya Oda, Naoki Ikeda, Yoshimasa Sugimoto, Yoshinori Watanabe, Kiyoshi Asakawa （Part： Lead author,　Corresponding author )

Materials Research Society Symposium Proceedings 1438 13 - 18 2012 [Refereed]

We propose to use Purcell effect emerged at slow light regions in photonic crystal waveguide (PC-WG) modes for controlling the relaxation time of excited carriers in QDs. Straight GaAs PC-WGs including InAs-QDs with various lattice constants of PC were prepared in order to control the wavelength of the slow light in the PC-WG modes. PL measurements of the PC-WGs indicated enhancements of emission from QDs at the localized wavelength of slow light regions due to the Purcell effect. The enhanced emission peak wavelength was continuously shifted with the PC lattice constant. These results suggest that the PC-WG can be utilized to modify the spontaneous emission rate and carrier relaxation time of the embedded QD. This modification can be applied and useful for various QD-based optical devices as well as our proposed all-optical switching device based on PC-WG/QD. © 2012 Materials Research Society.

Multi-color quantum dot ensembles grown in selective-areas for shape-controlled broadband light source

N. Ozaki, K. Takeuchi, S. Ohkouchi, N. Ikeda, Y. Sugimoto, K. Asakawa, R. A. Hogg （Part： Lead author,　Corresponding author )

Journal of Crystal Growth ( ELSEVIER SCIENCE BV ) 323 ( 1 ) 191 - 193 2011.05 [Refereed]

Multi-color quantum dot (QD) ensembles were grown by selective-area growth method to realize a shape-controlled broadband light source. By using a metal-mask, QD ensembles and strain reducing layer (SRL) were formed in selective areas on a wafer. The SRL thickness was varied to achieve appropriate shifts in the peak wavelength of the QD emission spectrum up to 90 nm. A summation of PL spectra obtained from the multi-color QD ensembles shows a broadband emission spectrum with a width of approximately 120 nm, even though this spectrum is attributed to the ground state emissions of these QD ensembles. A current-induced broadband light source such as a superluminescent diode (SLD) based on the multi-color QD ensembles is expected to have an emission spectrum with a width of more than 120 nm owing to the combination of excited state emissions. Furthermore, a desired shape of the SLD spectrum can be obtained by controlling the injection current applied to each QD ensemble. This approach is promising for a shape-controlled broadband SLD, and it is particularly applicable to optical coherence tomography (OCT). (C) 2010 Elsevier B.V. All rights reserved.

InAs quantum-dots laser utilizing GaAs W1 type photonic-crystal slab line-defect waveguide

H. Oda, N. Hamada, A. Yamanaka, N. Ozaki, N. Ikeda, Y. Sugimoto

2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011 CB. P.4 THU 2011

Photonic crystals are very suitable for controlling radiation field and propagation characterization of light. As an important application, it is ultra compact and ultrafast optical integrated circuits (OIC) based on photonic crystals slab waveguides (PC-WGs) composing of the line-defects [1, 2]. The PC-WG is also attractive for laser lasing, because very small group velocity of near the Brillouin zone (BZ) edge should enhance interactions between the radiation field and matter. Indeed, the lasing spectrum has been observed from optically pumped InAs-quantum dots (InAs-QDs) embedded in PC-WG of the multimode W3 type (three rows missing line-defect) [3]. In the present study, we observe laser action in InAs-QDs PC-WG of the single-mode W1 type (single row missing line-defect). © 2011 IEEE.

WIDEBAND OPERATION OF 2D PHOTONIC CRYSTAL DIRECTIONAL COUPLER WITH TOPOLOGY OPTIMIZED WAVEGUIDE BENDS

YOSHINORI WATANABE, NAOKI IKEDA, YOSHIAKI TAKATA, YOSHINORI KITAGAWA, NOBUHIKO OZAKI, YOSHIMASA SUGIMOTO, KIYOSHI ASAKAWA

Journal of Nonlinear Optical Physics & Materials ( World Scientific Pub Co Pte Lt ) 19 ( 04 ) 543 - 550 2010.12 [Refereed]

Nanophotonics technology for advanced quantum dot / photonic crystal device and metal / semiconductor plasmonic device

Y. Sugimoto, N. Ikeda, D. Tsuya, N. Ozaki, H. Oda, A. Yamanaka, A. Miura, T. Nomura, D. Inoue, H. Fujikawa, S. Ohkouchi, Y. Watanabe, Y. Koide, K. Satoh, K. Asakawa

2010 Photonics Global Conference, PGC 2010 1月5日 2010

Nano-photonic structures/materials such as surface plasmon and negative-index material as well as quantum dots (QD) and photonic crystals (PCs) are discussed from potentials for new photonic devices and technologies. PC slab waveguides and QDs were developed for key photonic devices such as ultra-small and ultrafast symmetrical Mach-Zehnder (SMZ)-type all-optical switch (PC-SMZ) and an optical flip-flop device (PC-FF). We have studied energy-saving light/matter interaction in a PC waveguide with emphasis on virtual-excitation-based nonlinear effects such as optical Kerr effect. We have also demonstrated a new RGB color filter on Al film with sub-wavelength holes in a large area with a high precision and optical beam-steering device with a wide steering angle.

Evolution of nanophotonics from semiconductor photonic crystal device to metal/semiconductor plasmonic device

K. Asakawa, Y. Sugimoto, N. Ikeda, D. Tsuya, Y. Koide, Y. Watanabe, N. Ozaki, D. V. Kumar, T. Nomura, D. Inoue, A. Miura, H. Fujikawa, K. Sato

Conference Proceedings - 5th International Conference on Advanced Optoelectronics and Lasers, CAOL' 2010 18 - 24 2010 [Refereed]

This paper reviews our recent activities on nanophotonics based on a photonic crystal (PC)/quantum dot (QD)-combined structure for an all-optical device and a metal/semiconductor composite structure using surface plasmon (SP) and negative refractive index material (NIM). The former structure contributes to an ultrafast signal processing component by virtue of new PC design and QD selective-area-growth technologies, while the latter provides a new RGB color filter with a high precision and optical beam-steering device with a wide steering angle. © 2010 IEEE.

Monolithic Fabrication of Two-Color InAs Quantum Dots for Integrated Optical Devices by Using a Rotational Metal Mask

Nobuhiko Ozaki, Shunsuke Ohkouchi, Yoshiaki Takata, Naoki Ikeda, Yoshinori Watanabe, Yoshimasa Sugimoto, Kiyoshi Asakawa （Part： Lead author,　Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 48 ( 6 ) 065502 1-4 2009.06 [Refereed]

Sequential Operations of Quantum Dot/Photonic Crystal All-Optical Switch With High Repetitive Frequency Pumping

Y. Kitagawa, N. Ozaki, Y. Takata, N. Ikeda, Y. Watanabe, Y. Sugimoto, K. Asakawa

Journal of Lightwave Technology ( Institute of Electrical and Electronics Engineers (IEEE) ) 27 ( 10 ) 1241 - 1247 2009.05 [Refereed]

Advanced quantum dot and photonic crystal technologies for integrated nanophotonic circuits

Y. Sugimoto, N. Ikeda, N. Ozaki, Y. Watanabe, S. Ohkouchi, T. Kuroda, T. Mano, T. Ochiai, K. Kuroda, N. Koguchi, K. Sakoda, K. Asakawa

MICROELECTRONICS JOURNAL ( ELSEVIER SCI LTD ) 40 ( 4-5 ) 736 - 740 2009.04 [Refereed]

Two types of nanophotonic technologies-two-dimensional photonic crystal (2D PC) slab waveguides (WGs) and quantum dots (QDs)-were developed for key photonic device Structures in the future. For an ultrafast digital photonic network, an ultrasmall and ultrafast symmetrical Mach-Zehnder (SMZ)type all-optical switch (PC-SMZ) and an optical flip-flop device (PC-FF) have been developed. To realize these devices, one method is to develop a selective-area molecular beam epitaxial growth QD technique by employing a metal mask method. Another method is to establish a new design method, i.e., topology optimization of the 2DPC WG with a wide and flat bandwidth, high transmittance, and low reflectivity. We also fabricated an optical microcavity in a photonic crystal slab embedded with GaAs QDs by droplet epitaxy. The Purcell effect on the exciton emission of GaAs QDs was confirmed by microphotoluminescence and lifetime measurements. (C) 2008 Elsevier Ltd. All rights reserved.

Site-controlled InAs quantum dot formation grown on the templates fabricated by the Nano-Jet Probe method

S. Ohkouchi, N. Ozaki, Y. Sugimoto, H. Ishikawa, K. Asakawa

JOURNAL OF CRYSTAL GROWTH ( ELSEVIER SCIENCE BV ) 311 ( 7 ) 1819 - 1821 2009.03 [Refereed]

We have developed a nanoprobe-assisted bottom-up technique that allows to fabricate site-controlled quantum dots (SC-QDs), using a specially designed atomic force microscope probe, referred to as the Nano-jet Probe (NJP). Beginning with the fabrication of regular QD arrays by using the NJP, we vertically aligned self-assembled InAs QDs using the strain-induced stacking method and produced assembled QD stacks. In order to improve the optical property of the SC-QDs, an AlGaAs barrier layer was introduced in the spacer layer of the stacked structures. The photoluminescence measurements of the fabricated structures revealed that they had good crystallographic qualities. (C) 2008 Elsevier B.V. All rights reserved.

Nanophotonic technologies for innovative all-optical signal processor using photonic crystals and quantum dots

Y. Sugimoto, N. Ikeda, N. Ozaki, Y. Watanabe, S. Ohkouchi, S. Nakamura, K. Asakawa

TRANSPORT AND OPTICAL PROPERTIES OF NANOMATERIALS ( AMER INST PHYSICS ) 1147 179 - + 2009 [Refereed]

GaAs-based two-dimensional photonic crystal (2DPC) slab waveguides (WGs) and InAs quantum dots (QDs) were developed for key photonic device structures in the future. An ultrasmall and ultrafast symmetrical Mach-Zehnder (SMZ)-type all-optical switch (PC-SMZ) and an optical flip-flop device (PC-FF) have been developed based on these nanophotonic structures for an ultrafast digital photonic network. To realize these devices, two important techniques were developed. One is a new simulation method, i.e., topology optimization method of 2DPC WGs with wide/flat bandwidth, high transmittance and low reflectivity. Another is a new selective-area-growth method, i.e., metal-mask molecular beam epitaxy method of InAs QDs. This technique contributes to achieving high-density and highly uniform InAs QDs in a desired area such as an optical nonlinearity-induced phase shift arm in the PC-FF. Furthermore, as a unique site-controlled QD technique, a nano-jet probe method is also developed for positioning QDs at the centre of the optical nonlinearity-induced phase shift arm.

Photonic crystal and quantum dot technologies for ultra-small and ultra-fast all-optical flip-flop

N. Ozaki, N. Ikeda, Y. Watanabe, Y. Takata, Y. Kitagawa, S. Ohkouchi, S. Nakamura, A. Watanabe, A. Mizutani, Y. Sugimoto, K. Asakawa （Part： Lead author )

ECS Transactions 16 ( 41 ) 31 - 37 2009 [Refereed]

Nano-photonic technologies of GaAs-based two-dimensional photonic crystal (2DPC) slab waveguides and InAs quantum dots (QDs) are reviewed for symmetric Mach-Zehnder type, ultra-small and ultra-fast all-optical switch (PC-SMZ) and optical flip-flop device (PC-FF). In the 1st phase of this work, ultra-fast (∼ps) and ultra-low energy (∼100 fJ) switching has been demonstrated using the PC-SMZ chip with 600 μm x 300 μ in size. In the 2nd phase, the concept of the PC-FF based on the dual PC-SMZs for providing a latch function has been proposed for a future ultra-fast optical digital processor. One of the priority subjects is to establish a new design method, i.e., topology optimization method of the 2DPC waveguide with wide/flat bandwidth, high transmittance and low reflectivity. Another one is to develop a selective-area-MBE growth technique with a metal-mask method for high-density and highly uniform InAs QDs with different absorption wavelengths in different areas. ©The Electrochemical Society.

Topology optimization of a wavelength-selective Y-junction for 2D photonic crystal waveguides

Yoshinori Watanabe, Naoki Ikeda, Yoshiaki Takata, Yoshinori Kitagawa, Nobuhiko Ozaki, Yoshimasa Sugimoto, Kiyoshi Asakawa

JOURNAL OF PHYSICS D-APPLIED PHYSICS ( IOP PUBLISHING LTD ) 41 ( 17 ) 2008.09 [Refereed]

The topology optimization ( TO) technique has been employed to design a wavelength-selective Y-junction for two-dimensional photonic crystal slab waveguides. The TO design exhibits high-pass and low-pass filtering spectra for two different paths in the Y-junction. Large cross-talk suppression of -20 dB at wavelength separation of 30 nm has also been obtained. These calculated characteristics have been experimentally verified by using a GaAs-based air-bridge-type photonic crystal waveguide. The TO-designed Y-junction contributes to an essential component for a new photonic crystal-based all-optical flip-flop, PC-FF.

Selective-area-growth of InAs-QDs with different absorption wavelengths via developed metal-mask/MBE method for integrated optical devices

N. Ozaki, Y. Takata, S. Ohkouchi, Y. Sugimoto, N. Ikeda, K. Asakawa （Part： Lead author )

APPLIED SURFACE SCIENCE ( ELSEVIER SCIENCE BV ) 254 ( 23 ) 7968 - 7971 2008.09 [Refereed]

The selective-area-growth (SAG) of InAs quantum dots (QDs) with different emission wavelengths on different areas was carried out using the metal-mask (MM) method during conventional molecular beam epitaxy (MBE) growth. Two SAG areas (SAG-1 and SAG-2) of the QDs were obtained using the rotational MM having square windows; first, the MM was mounted on a substrate for SAG-1, and then the same MM was used for SAG-2 after being rotated it by 180 degrees. The absorption wavelengths of the selectively grown QDs were controlled by inserting strain-reducing layers of a different thickness on each grown QD layer. Photoluminescence measurements revealed the successful SAG of QDs in the neighboring a few hundred-micron-square areas with different emission wavelengths, e.g., 1250 and 1300 nm. These techniques are promising for applications of QD-based optical and electronic devices, including our proposed photonic crystal (PC) and QD-based all-optical digital flip-flop (FF) device, i.e., PC-FF.

Selective growth of stacked InAs quantum dots by using the templates formed by the Nano-Jet Probe

S. Ohkouchi, Y. Sugimoto, N. Ozaki, H. Ishikawa, K. Asakawa

APPLIED SURFACE SCIENCE ( ELSEVIER SCIENCE BV ) 254 ( 23 ) 7821 - 7823 2008.09 [Refereed]

We have demonstrated the selective area growth of stacked self-assembled InAs quantum dot (QD) arrays in the desired regions on a substrate and confirmed the photoluminescence (PL) emission exhibited by them at room temperature. These InAs QDs are fabricated by the use of a specially designed atomic force microscope cantilever referred to as the Nano-Jet Probe (NJP). By using the NJP, two-dimensional arrays with ordered In nano-dots are fabricated in the desired square regions on a GaAs substrate and directly converted into InAs QD arrays through the subsequent annealing by the irradiation of As flux. By using the converted QD arrays as strain templates, self-organized InAs QDs are stacked. These stacked QDs exhibit the PL emission peak at a wavelength of 1.02 mu m. (C) 2008 Elsevier B.V. All rights reserved.

Optical-nonlinearity-induced phase shift via selective-area grown InAs quantum dots in a photonic crystal waveguide

Yoshinori Kitagawa, Nobuhiko Ozaki, Yoshiaki Takata, Naoki Ikeda, Shunsuke Ohkouchi, Yoshinori Watanabe, Yoshimasa Sugimoto, Myoshi Asakawa

JAPANESE JOURNAL OF APPLIED PHYSICS ( JAPAN SOCIETY APPLIED PHYSICS ) 47 ( 4 ) 2893 - 2896 2008.04 [Refereed]

The selective-area growth (SAG) of InAs quantum dots (QDs) in a GaAs two-dimensional photonic crystal waveguide (2DPC-WG) slab was performed by a metal-mask molecular beam epitaxy (MBE) method, and phase shifts induced by an optical nonlinearity (ONL) of the QD were characterized by two-color pump and probe measurements. The phase shift is caused by the refractive index change (Delta n) in the PC-WG, while the Delta n is induced by the absorption saturation as a third-order ONL effect of the QD. For the samples with different SAG-QD lengths, it was confirmed that the phase shift is proportional to the length of the SAG area and to the pump energy. For application to the PC/QD-based symmetrical Mach-Zehnder (SMZ)-type all-optical switch, that is, PC-SMZ, a bright prospect for the design criterion was obtained such that the 180 degrees phase shift, necessary for the PC-SMZ, can be achieved using the four-stacked-layer-QD WG with a 250 mu m length at a pump energy of 1000 f(J) a slow light with a group velocity of 0.05c (c is the light velocity in vacuum) is used.

In situ metal mask for selective area growth of thin epitaxial layers

Shunsuke Ohkouchi, Nobuhiko Ozaki, Yoshiaki Takata, Yoshinori Kitagawa, Yusui Nakamura, Naoki Ikeda, Yoshimasa Sugimoto, Kiyoshi Asakawa

JAPANESE JOURNAL OF APPLIED PHYSICS ( JAPAN SOCIETY APPLIED PHYSICS ) 47 ( 4 ) 2987 - 2990 2008.04 [Refereed]

We have developed an in situ metal mask that enables the selective formation of molecular beam epitaxially grown layers in narrow regions and the subsequent formation of marker layers. It can be fitted to a sample holder and removed in an ultrahigh-vacuum environment; therefore, device structures can be fabricated without exposing the sample surfaces to air. To explore the effectiveness of the mask, we used it to grow quantum dot (QD) layers in narrow regions and verified the perfect selectivity of the QD growth. The grown QDs exhibited high optical quality with a photoluminescence peak at approximately 1.29 mu m and a linewidth of 30 meV at room temperature. Furthermore, on the selectively grown layers, we have fabricated waveguide structures that showed a high transmittance near 1.3 mu m wavelength with a dip due to the absorption by the embedded QDs. The developed mask can be used for the integration of microstructures into optoelectronic functional devices.

Molecular beam epitaxial growth of site-controlled InAs quantum dot arrays using templates fabricated by the Nano-Jet Probe method

S. Ohkouchi, Y. Sugimoto, N. Ozaki, H. Ishikawa, K. Asakawa

PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES ( ELSEVIER SCIENCE BV ) 40 ( 6 ) 1794 - 1796 2008.04 [Refereed]

We have demonstrated the selective area growth of self-assembled InAs quantum dots (QDs) in the desired regions by using a template composed of InAs QD arrays. These InAs QDs were fabricated by the use of a specially designed atomic force microscope cantilever, referred to as the Nano-Jet Probe (NJP). Using the NJP, two-dimensional arrays of ordered In nano-dots were fabricated in the selected square regions on a GaAs substrate and directly converted to InAs QD arrays by subsequent annealing by the irradiation of As flux. By using the converted QD arrays as strain templates, self-organized InAs QDs were formed in the selected square regions. These InAs QD arrays were used for the further stacking of QDs, and photoluminescence emission was confirmed from the fabricated stacked QD structure. (c) 2007 Elsevier B.V. All rights reserved.

Precise control of dry etching for nanometer scale air-hole arrays in two-dimensional GaAs/AlGaAs photonic crystal slabs

Naoki Ikeda, Yoshimasa Sugimoto, Yoshinori Watanabe, Nobuhiko Ozaki, Yoshiaki Takata, Yu Tanaka, Kuon Inoue, Kiyoshi Asakawa

OPTICS COMMUNICATIONS ( ELSEVIER SCIENCE BV ) 275 ( 1 ) 257 - 267 2007.07 [Refereed]

Precise control of highly anisotropic reactive-ion-beam-etching (RIBE) for GaAs/AlGaAs-based two-dimensional photonic crystals (2DPCs) is investigated in terms of the substrate temperature, T-s, ion accelerating voltage, V-i, and Cl-2 gas pressure, p. T-s is shown to influence the shape of the sidewall, while the balance of the physical etching dominated by the value of V and the chemical etching dominated by the value of p is essential for keeping smooth and vertical sidewalls of 100-nm-scale air-holes. 2DPC air-hole patterns are defined by an electron beam (EB) lithography machine and air-holes are dry-etched with the EB resist as an etching mask. The optimized balance between the V-i and p for 0.5-1.0-mu m-deep air-holes results in the high-rate-etching regime given at V-i = 500 V and p = 8 x 10(-4) Torr using a 650-nm-thick resist mask, while the optimized balance for 50-nm-scale fine air-holes results in the low-rate-etching regime given at V-i = 330 V and p = 5 x 10(-4) Torr using a 350-nm-thick resist mask. In particular, the latter condition is essential for fabricating topology-optimized 2DPC air-hole arrays with the minimum air-hole-size of 50 nm or less. These process conditions definitely contribute to excellent measured transmission spectra in good agreement with the calculated one in a near-infrared range. (c) 2007 Elsevier B.V. All rights reserved.

Topology optimization of waveguide bends with wide, flat bandwidth in air-bridge-type photonic crystal slabs

Yoshinori Watanabe, Naoki Ikeda, Yoshimasa Sugimoto, Yoshiaki Takata, Yoshinori Kitagawa, Akio Mizutani, Nobuhiko Ozaki, Kiyoshi Asakawa

JOURNAL OF APPLIED PHYSICS ( AMER INST PHYSICS ) 101 ( 11 ) 2007.06 [Refereed]

High transmission recovery of slow light in a photonic crystal waveguide using a hetero group velocity waveguide

Nobuhiko Ozaki, Yoshinori Kitagawa, Yoshiaki Takata, Naoki Ikeda, Yoshinori Watanabe, Akio Mizutani, Yoshimasa Sugimoto, Kiyoshi Asakawa （Part： Lead author )

OPTICS EXPRESS ( OPTICAL SOC AMER ) 15 ( 13 ) 7974 - 7983 2007.06 [Refereed]

High transmission of slow-light in a photonic crystal (PC) waveguide (WG) using a hetero group-velocity (Ht-V-g) PC-WG was proposed and experimentally investigated. The Ht-V-g WG, which comprises a low-group- velocity (L-V-g) PC-WG section between two identical high-group-velocity (H-V-g) PC-WGs, is designed to decrease the impedance mismatch of the L-V-g PC-WG. The increase in transmittance of a propagating pulse was confirmed in the Ht-V-g PC-WG even in the vicinity of the band-gap, whereas the homogeneous PC-WG showed a gradual decrease in transmittance with the pulse wavelength approaching the bandgap. The group index (ng) of the L-V-g region in the Ht-V-g PC-WG was measured by the cross-correlation method and attained a value above 20. On the other hand, the transmittance of the Ht-V-g structure recovered approximately 16dB compared to the homogeneous L-V-g WG having same ng, 17. This recovery is mainly dominated by the coupling improvement due to the Ht-V-g structure, around 12dB. These results indicate the effectiveness of the Ht-V-g structure to use slow light in a PC- WG, which leads to various applications in PC-based optical devices. (C) 2007 Optical Society of America

Selective area growth of InAs quantum dots with a metal mask towards optical integrated circuit devices

N. Ozaki, Y. Takata, S. Ohkouchi, Y. Sugimoto, Y. Nakamura, N. Ikeda, K. Asakawa

JOURNAL OF CRYSTAL GROWTH ( ELSEVIER SCIENCE BV ) 301 771 - 775 2007.04 [Refereed]

A metal-mask (MM) molecular beam epitaxy technique for selective-area-growth (SAG) of self-assembled InAs-quantum dots (QDs) on a GaAs substrate was developed for realizing ultra-small and ultra-fast all-optical switches (PC-SMZ). Growth conditions of the QD were optimized by virtue of the MM with a large window enabling real-time observations of reflection-high-energy-electron-diffraction patterns and growth temperatures. Successful SAG was confirmed by atomic force microscope observations of the high-density (4 x 10(10) cm(-2)) QD grown only on the unmasked area, while high uniformity of the QD was examined by room-temperature photoluminescence (PL) measurement. It resulted in a 1250 nm peak in the center wavelength, 38 meV in FWHM, comparable to that of the QD grown without the NM. In addition, insertion of a strain-reducing layer of GaInAs on the QD reported previously was confirmed to be effective for controlling the PL peak wavelength of the QD between 1250 and 1320 nm without degrading the optical quality of QDs. The MM method shown here provides a possibility of an advanced PC-SMZ, i.e., an ultra-fast all-optical flip-flop (PC-FF), which requires a QD ensemble with a different absorption-peak wavelength in a different area. (c) 2006 Elsevier B.V. All rights reserved.

Selective growth of InAs quantum dots using In nano-dot arrays formed by nano-jet probe method

S. Ohkouchi, Y. Sugimoto, N. Ozaki, H. Ishikawa, K. Asakawa

JOURNAL OF CRYSTAL GROWTH ( ELSEVIER SCIENCE BV ) 301 726 - 730 2007.04 [Refereed]

We have demonstrated the selective area growth of high density InAs quantum dots (QDs) by using site-controlled InAs dots that were formed in the desired regions as templates. The fabrication of the InAs dots for the templates was enabled by the use of a specially designed atomic-force-microscope (AFM) cantilever, referred to as the nano-jet probe (NJP). Using the NJP, two-dimensional (213) arrays of ordered In nano-dots were reproducibly fabricated in the desired regions on a GaAs substrate. These In nano-dots were directly converted to InAs QD arrays by subsequent annealing with irradiation of arsenic flux. By using the converted QD arrays as strain templates, self-organized InAs QDs were formed in the selected regions. (c) 2006 Elsevier B.V. All rights reserved.

InAs quantum dots grown on selective areas with a metal mask for photonic-crystal-based ultra-small and ultra-fast all optical devices

N. Ozaki, Y. Takata, S. Ohkouchi, Y. Sugimoto, N. Ikeda, Y. Watanabe, Y. Kitagawa, A. Mizutani, K. Asakawa

2007 INTERNATIONAL CONFERENCE ON INDIUM PHOSPHIDE AND RELATED MATERIALS, CONFERENCE PROCEEDINGS ( IEEE ) 493 - 496 2007

A metal-mask (MM) method for selective-area-growth (SAG) of self-assembled InAs quantum dots (QDs) on a GaAs substrate was developed for applications of ultra-small and ultra-fast all optical devices based on a combination of QD and photonic crystal waveguides (PC-WGs). Successful SAG of QDs with high density and high optical quality comparable to conventional InAs-QDs grown without the MM was confirmed by atomic force microscopy and photo luminescence (PL) measurements. The QD density was 4x10(10) cm(-2) and FWHM of the PL emission was around 30meV at room temperature. By insertion of a strain-reducing layer on the QD, the PL peak wavelength was controlled from 1240nm to 1320nm. The MM method is promising for realizing the PC-based all optical devices, which require SAG of QDs and a QD ensemble with a different absorption-peak wavelength in a different area.

Measurements of optical non-linearity induced phase-shifts of signal pulse with repetitive control pulses in photonic crystal/quantum dot waveguide

Yoshinori Kitagawa, Nobuhiko Ozaki, Yoshiaki Takata, Naoki Ikeda, Yoshinori Watanabe, Akio Mizutani, Yoshimasa Sugimoto, Kiyoshi Asakawa

2007 IEEE LEOS ANNUAL MEETING CONFERENCE PROCEEDINGS, VOLS 1 AND 2 ( IEEE ) 666 - 667 2007

Monolithic growth of InAs-QDs with different absorption wavelengths in different areas for integrated optical devices

Y. Takata, N. Ozaki, S. Ohkouchi, Y. Sugimoto, N. Ikeda, Y. Watanabe, Y. Kitagawa, A. Mizutani, K. Asakawa

2007 IEEE LEOS ANNUAL MEETING CONFERENCE PROCEEDINGS, VOLS 1 AND 2 ( IEEE ) 30 - 31 2007

We have developed a technique for selective-area-growth (SAG) of InAs quantum dots (QDs) having different absorption wavelengths, with the goal of using them in all-optical integrated devices. This technique, using the metal-mask method and the insertion of a strain-reducing-layer, provides successful SAG of QDs absorbing at different wavelengths in different areas. This SAG technique is promising for various QD applications as well as our proposed photonic-crystal-based all optical switch (PC-SMZ) and digital flip-flop device (PC-FF).

Topology Optimization for Photonic Crystal Waveguide Bends with Wide and Flat Bandwidths in Air-Bride type Photonic Crystal Slabs

Y. Watanabe, N. Ikeda, Y. Sugimoto, Y. Takata, Y. Kitagawa, A. Mizutani, N. Ozaki, K. Asakawa

2007 CONFERENCE ON LASERS & ELECTRO-OPTICS/QUANTUM ELECTRONICS AND LASER SCIENCE CONFERENCE (CLEO/QELS 2007), VOLS 1-5 ( IEEE ) 1027 - 1028 2007

Selective-area growth of self-assembled InAs-QDs by metal mask method for optical integrated circuit applications

Ozaki N, Takata Y, Ohkouchi S, Sugimoto Y, Ikeda N, Watanabe Y, Kitagawa Y, Mizutani A, Asakawa K

Materials Research Society Symposium Proceedings 959 65 - 70 2007 [Refereed]

Fluorescence XAFS study on local structure around Cr atoms doped in ZnTe

Hironori Ofuchi, Nobuhiko Ozaki, Nozomi Nishizawa, Hidekazu Kinjyo, Shinji Kuroda, Koki Takita

X-RAY ABSORPTION FINE STRUCTURE-XAFS13 ( AMER INST PHYSICS ) 882 517 - + 2007 [Refereed]

The geometric structures for ferromagnetic Zn1-xCrxTe films grown by molecular beam epitaxy were investigated by fluorescence XAFS measurements in order to elucidate the relationship between the geometric structure and the magnetic properties. XAFS analysis suggested that the majority of Cr atoms doped in CrTe substituted the Zn-site in the ZnTe lattice below the Cr content x=0.048, and formed Cr-Te compounds such as Cr2Te3 and CrTe above x=0.090. It is suggested that ferromagnetism of the Zn1-xCrxTe films above x=0.090 is due to the formation of Cr-Te compounds.

Selective formation of high density InAs quantum dot arrays using templates fabricated by the nano-jet probe

S. Ohkouchi, Y. Sugimoto, N. Ozaki, H. Ishikawa, K. Asakawa

2007 INTERNATIONAL CONFERENCE ON INDIUM PHOSPHIDE AND RELATED MATERIALS, CONFERENCE PROCEEDINGS ( IEEE ) 466 - 469 2007

We have demonstrated the selective area growth of high density InAs quantum dots (QDs) in the square regions by using site-controlled InAs dots that were formed in the desired regions as templates. These fabricated InAs dots for the templates were enabled by the use of a specially designed atomic-force-microscope (AFM) cantilever, referred to as the Nano-Jet Probe (NJP). Using the NJP, two-dimensional (2D) arrays of ordered In nano-dots were fabricated in the desired square regions on a GaAs substrate. These In nano-dots were directly converted to InAs QD arrays by subsequent annealing with irradiation of arsenic flux. By using the converted QD arrays as strain templates, self-organized InAs QDs with high density were formed in the selected square regions.

Topology Optimization for Photonic Crystal Waveguide with Wide and Flat Bandwidths in Ultra-Fast All-Optical Switch (PC-SMZ)

Yoshinori Watanabe, Yoshimasa Sugimoto, Naoki Ikeda, Nobuhiko Ozaki, Akio Mizutani, Yoshiaki Takata, Yoshinori Kitagawa, Jakob Jensen, Ole Sigmund, Peter Borel, Martin Kristensen, Kiyoshi Asakawa

2006 IEEE LEOS Annual Meeting Conference Proceedings ( IEEE ) 2006.10

Broadband waveguide intersection with lowcrosstalk in two-dimensional photonic crystal circuits by using topology optimization

Yoshinori Watanabe, Yoshimasa Sugimoto, Naoki Ikeda, Nobuhiko Ozaki, Akio Mizutani, Yoshiaki Takata, Yoshinori Kitagawa, Kiyoshi Asakawa

OPTICS EXPRESS ( OPTICAL SOC AMER ) 14 ( 20 ) 9502 - 9507 2006.10 [Refereed]

Photonic crystal and quantum dot technologies for all-optical switch and logic device

Kiyoshi Asakawa, Yoshimasa Sugimoto, Yoshinori Watanabe, Nobuhiko Ozaki, Akio Mizutani, Yoshiaki Takata, Yoshinori Kitagawa, Hiroshi Ishikawa, Naoki Ikeda, Koichi Awazu, Xiaomin Wang, Akira Watanabe, Shigeru Nakamura, Shunsuke Ohkouchi, Kuon Inoue, Martin Kristensen, Ole Sigmund, Peter Ingo Borel, Roel Baets

NEW JOURNAL OF PHYSICS ( IOP PUBLISHING LTD ) 8 2006.09 [Refereed]

Nano-photonic technologies of GaAs-based two-dimensional photonic crystal (2DPC) slab waveguides (WGs) and InAs-based quantum dots (QDs) are reviewed for a symmetrical Mach - Zehnder (SMZ) type, ultra-small and ultra-fast all-optical switch (PC-SMZ) and logic device. As the first phase, ultra-fast (similar to ps) and ultra-low energy (similar to 100 fJ) switching has been demonstrated using a chip 600 mu m x 300 mu m in size. The second phase is to create a PC-SMZ-based ultra-fast photonic logic switch with a latch function for a future ultra-fast photonic digital processor. One of the priority subjects is to establish a new design method, i.e., topology optimization (TO) method of 2DPC-WGs with wide/flat bandwidth, high transmittance and low reflectivity. Another one is to develop selective-area-grown, high-density and highly uniform InAs QDs with large optical nonlinearity (ONL) by using a metal-mask (MM) molecular beam epitaxy (MBE) growth method. Recent results regarding these two subjects encourage us to reach the final goal.

Topology optimised photonic crystal waveguide intersections with high-transmittance and low crosstalk

N. Ikeda, Y. Sugimoto, Y. Watanabe, N. Ozaki, A. Mizutani, Y. Takata, J. S. Jensen, O. Sigmund, P. I. Borel, M. Kristensen, K. Asakawa

ELECTRONICS LETTERS ( INSTITUTION ENGINEERING TECHNOLOGY-IET ) 42 ( 18 ) 1031 - 1033 2006.08 [Refereed]

Significant enhancement of ferromagnetism in Zn1-xCrxTe doped with iodine as an n-type dopant

Nobuhiko Ozaki, Nozomi Nishizawa, Stephane Marcet, Shinji Kuroda, Osamu Eryu, Koki Takita （Part： Lead author )

PHYSICAL REVIEW LETTERS ( AMERICAN PHYSICAL SOC ) 97 ( 3 ) 2006.07 [Refereed]

The effect of additional doping of charge impurities was investigated in a ferromagnetic semiconductor Zn1-xCrxTe. It was found that the doping of iodine, which is expected to act as an n-type dopant in ZnTe, brought about a drastic enhancement of the ferromagnetism in Zn1-xCrxTe, while the grown films remained electrically insulating. In particular, at a fixed Cr composition of x=0.05, the ferromagnetic transition temperature T-C increased up to 300 K at maximum due to the iodine doping from T-C=30 K of the undoped counterpart, while the ferromagnetism disappeared due to the doping of nitrogen as a p-type dopant. The observed systematic correlation of ferromagnetism with the doping of charge impurities of both the p and n type, suggesting a key role of the position of Fermi level within the impurity d state, is discussed on the basis of the double-exchange interaction as a mechanism of ferromagnetism in this material.

New design for wide/flat bandwidth in photonic crystal-based SMZ all-optical device (PC-SMZ)

K. Asakawa, Y. Sugimoto, Y. Watanabe, N. Ozaki, A. Mizutani, N. Ikeda, O. Sigmund, P. I. Borel, M. Kristensen

ICTON 2006: 8TH INTERNATIONAL CONFERENCE ON TRANSPARENT OPTICAL NETWORKS, VOL 2, PROCEEDINGS ( IEEE ) 146 - 149 2006

A two-dimensional photonic crystal (2DPC) slab waveguide-based symmetric Mach-Zehnder type all-optical switch (PC-SMZ) is promising for a future ultra-fast photonic network system. An ultra-fast (similar to ps) and ultra-low energy (similar to 100 fJ) switching has already been demonstrated as the 1(st) phase. The 2(nd) phase is to create a PC-SMZbased ultra-fast photonic logical switch with a latch function for an ultra-fast photonic digital processor. One of the steps towards the goal is to develop a new design method of waveguides with wide/flat bandwidth, high transmittance and low reflectivity for improving switching performances. Another one is to develop a selective area growth of quantum dots. This paper dominantly reviews these two recent topics as well as briefing the PC-SMZ performances.

Topology optimization for photonic crystal waveguide intersection with wide and flat bandwidths in ultra-fast all-optical switch (pc-smz)

Y. Sugimoto, Y. Watanabe, N. Ikeda, N. Ozaki, A. Mizutani, Y. Takata, J. S. Jensen, O. Sigmund, P. I. Borel, M. Kristensen, H. Ishikawa, K. Asakawa

2006 European Conference on Optical Communications Proceedings, ECOC 2006 1 - 2 2006

Planar focusing lens grating for vertical coupling on 2D photonic crystal slato waveguide

Akio Mizutani, Naoki Ikeda, Yoshinori Watanabe, Nobuhiko Ozaki, Yoshiaki Takata, Yoshinori Kitagawa, Frederik Laere, Roel Baets, Yoshimasa Sugimoto, and Kiyoshi Asakawa

2006 IEEE LEOS ANNUAL MEETING CONFERENCE PROCEEDINGS, VOLS 1 AND 2 ( IEEE ) 843 - 844 2006

Photonic crystals and quantum dots: Towards integrated optics for advanced ultra-fast all-optical signal processing

K. Asakawa, Y. Sugimoto, Y. Watanabe, N. Ozaki, A. Mizutani, Y. Takata, Y. Kitagawa, N. Ikeda, S. Ohkouchi, S. Nakamura, A. Watanabe, X. Wang, M. Kristensen, O. Sigmund, P. I. Borel, R. Baets

2006 European Conference on Optical Communications Proceedings, ECOC 2006 4801132 1-4 2006

Magneto-optical study of ferromagnetic semiconductor (Zn,Cr)Te

N. Nishizawa, S. Marcet, N. Ozaki, S. Kuroda, K. Takita

PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 3, NO 12 ( WILEY-V C H VERLAG GMBH ) 3 ( 12 ) 4102 - 4105 2006 [Refereed]

Magneto-optical properties of a ferromagnetic semiconductor (Zn,Cr)Te were investigated through magnetic circular dichroism (MCD) measurements in the visible spectral range. (Zn,Cr)Te films without and with an additional nitrogen (N) doping as an acceptor exhibited different MCD spectra around the F point; a broad MCD band extending over a few hundred meV was observed in the undoped (Zn,Cr)Te while a relatively sharp peak appeared in the heavily N-doped (Zn,Cr)Te. The different MCD spectra are considered to be correlated with the change in the Cr valence state due the acceptor doping.

Suppression of ferromagnetism due to hole doping in Zn1-xCrxTe grown by molecular-beam epitaxy

N Ozaki, Okabayashi, I, T Kumekawa, N Nishizawa, S Marcet, S Kuroda, K Takita （Part： Lead author )

APPLIED PHYSICS LETTERS ( AMER INST PHYSICS ) 87 ( 19 ) 2005.11 [Refereed]

Electric and magnetic properties were investigated on p-type Zn1-xCrxTe doped with nitrogen (N) as an acceptor. Thin films of p-Zn1-xCrxTe (x <= 0.09) were grown by molecular-beam epitaxy with the supply of N-2 gas excited by rf plasma. With the increase of Cr composition x at an almost fixed N concentration of the order of 10(20) cm(-3), the temperature dependence of resistivity changed from metallic behavior to an insulating one, accompanied with a significant decrease of the hole concentration. The magnetization measurements revealed that ferromagnetic behaviors observed in undoped Zn1-xCrxTe were suppressed due to the nitrogen doping; with N concentrations of the order of 10(20) cm(-3), hysteresis loops in the magnetization curve disappeared, the magnitude of magnetization decreased, and the ferromagnetic transition were not observed down to 2 K according to the Arrott plot analysis. These experimental findings are discussed on the basis of the ferromagnetic double exchange interaction which is considered to work on the Cr 3d impurity level formed in the band gap of ZnTe. (C) 2005 American Institute of Physics.

Growth and magnetic properties of novel ferromagnetic semiconductor (Zn, Cr)Te

S Kuroda, N Ozaki, N Nishizawa, T Kumekawa, S Marcet, K Takita

SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS ( NATL INST MATERIALS SCIENCE ) 6 ( 6 ) 558 - 564 2005.09 [Refereed]

Magnetic properties of a novel ferromagnetic semiconductor (Zn, Cr)Te were investigated. Zn1-xCrxTe thin films, both without and with the additional hole doping by nitrogen, were grown by molecular beam epitaxy. In the magnetization measurement on Zn1-xCrxTe without carrier doping, the ferromagnetic behaviors such as a hysteresis loop in the magnetization vs. magnetic field curve were observed. Similar hysteretic behaviors in the field dependence were reproduced in the magnetic circular dichroism measurement. The ferromagnetic transition temperature TC deduced from Arrott plot increased almost linearly with Cr composition with the maximum T-C = 275 K at a Cr composition of x = 0. 17. The ferromagnetic behaviors observed in the undoped samples were found to be suppressed upon the p-type doping with nitrogen. These experimental findings are discussed based on the double exchange mechanism and the suppression of ferromagnetism by the hole doping is interpreted as due to the shift of the Fermi level in the Cr 3d level with the acceptor doping. (c) 2005 Elsevier Ltd. All fights reserved.

Growth of silicon nanowires on H-terminated Si {111} surface templates studied by transmission electron microscopy

N Ozaki, Y Ohno, J Kikkawa, S Takeda （Part： Lead author )

JOURNAL OF ELECTRON MICROSCOPY ( OXFORD UNIV PRESS ) 54 I25 - I29 2005.09 [Refereed]

We have studied the growth of silicon nanowires (SiNWs) by means of transmission electron microscopy. SiNWs are grown from nanocatalysts via the Vapor-Liquid-Solid (VLS) mechanism using silane (SiH4) gas as a source gas. The nanocatalysts are prepared on a hydrogen (H)-terminated Si surface. We have examined the formation mechanism of nanocatalysts on H-terminated surface and have observed several structural variants of SiNWs. According to the study we have suggested that many structural variations of SiNWs are possible, which modify the structural properties of SiNWs to great extents.

Magnetic behaviors of ferromagnetic semiconductor Zn1-xCrxTe grown by MBE

N Ozaki, N Nishizawa, S Marcet, S Kuroda, K Takita （Part： Lead author )

JOURNAL OF SUPERCONDUCTIVITY ( SPRINGER/PLENUM PUBLISHERS ) 18 ( 1 ) 29 - 32 2005.02 [Refereed]

The magnetic and structural properties of NIBE-grown films of Zn1-xCrxTe were investigated. The magnetization versus magnetic field (M-H) measurement of Zn1-xCrxTe (x = 0.01-0.17) showed clear hysteresis loop at low temperatures. The ferromagnetic transition temperature (T-C) estimated from the Arrott-plot analysis increased almost linearly with the Cr composition (x) up to 275 K at x = 0.17. However, in the magnetization versus temperature (M-T) measurement, the irreversibility between the zero-field-cooled (ZFC) and field-cooled (FC) processes was observed. This is typically observed in the magnetic random system such as spin-glass or superparamagnetic phase. In the high resolution transmission microscopy (HRTEM) observations, structural defects such as stacking faults and polycrystalline-like structure were observed at high Cr compositions, whereas any apparent precipitates of different phases were not seen in all the range of Cr compositions examined. The correlation of the observed magnetic randomness with the local structural defects was discussed.

Magnetic properties of undoped and N-doped Zn1-xCrxTe grown by MBE

N Ozaki, N Nishizawa, S Kuroda, K Takita （Part： Lead author )

Physics of Semiconductors, Pts A and B ( AMER INST PHYSICS ) 772 345 - 346 2005 [Refereed]

Nitrogen-doped Zn1-xCrxTe (x approximate to 0.02) thin films were gown by molecular beam epitaxy (MBE) with rf-plasma excited nitrogen source. p-type conductivity was confirmed through the Hall measurements with the highest hole concentration up to 1.8x10(18) cm(-3). The magnetization measurements of the hole-doped samples revealed the suppression of the ferromagnetism compared to the undoped one. The correlation of the magnetic properties with the position of Fermi level is discussed.

Nucleation and growth processes of silicon nanowires

S Takeda, N Ozaki, K Ueda, H Kohno, J Kikkawa, Y Ohno

Group-IV Semiconductor Nanostructures ( MATERIALS RESEARCH SOCIETY ) 832 257 - 267 2005 [Refereed]

We have studied the nucleation and growth processes of silicon nanowires (SiNWs) by means of transmission electron microscopy and scanning tunneling microscopy. SiNWs are grown on hydrogen-terminated Si surface via the VLS (Vapor-Liquid-Solid) mechanism using silane (SiH4) as source gas. We have classified the growth process of SiNWs into three stages: the formation of nanocatalysts on a substrate, the nucleation of SiNWs in nanocatalysts, followed by the growth of SiNWs. We have shown that the structures of SiNWs are varied in several ways in each stage, and accordingly the structural properties of grown SiNWs can be modified to great extents. At the present moment, the phenomena at the each stage are not fully controlled, and this prevents us utilizing silicon nanowires more effectively.

Magnetic and structural properties of MBE-grown Zn1-xCrxTe films

N Ozaki, N Nishizawa, S Kuroda, K Takita （Part： Lead author,　Corresponding author )

JOURNAL OF PHYSICS-CONDENSED MATTER ( IOP PUBLISHING LTD ) 16 ( 48 ) S5773 - S5776 2004.12 [Refereed]

The magnetic and structural properties of MBE-grown films of a novel diluted magnetic semiconductor (DMS), Zn1-xCrxTe, were investigated. The magnetization versus magnetic field (M-H) measurement exhibited a clear hysteresis loop at low temperatures. The ferromagnetic transition temperature (T-C) estimated from the Arrott-plot analysis increased almost linearly with the Cr concentration (x) up to 275 K at x = 0.17. In the magnetization versus temperature (M-T) measurement, irreversibility between the zero-field-cooled (ZFC) and field-cooled (FC) processes was observed. This behaviour, which is typically observed in a magnetic random system such as the spin-glass or superparamagnetic phase, is considered to be related to the local structural inhomogeneity observed by high resolution transmission electron microscopy.

Magnetotransport and magnetic properties of p-Zn1-xMnx Te : N - Carrier-induced ferromagnetism

KT Nam, S Kuroda, T Kumekawa, N Ozaki, K Takita

PHYSICA STATUS SOLIDI B-BASIC RESEARCH ( WILEY-V C H VERLAG GMBH ) 241 ( 3 ) 668 - 671 2004.03 [Refereed]

The magnetic properties of nitrogen-doped p-Zn1-chiMnchiTe grown by MBE were studied through the magnetotransport measurements. In a sample of p-Zn1-chiMnchiTe with chi = 0.034 and p = 4.7 x 10(19) cm(-3), a hysteresis loop was observed below 2.5 K in the Hall resistance and the magnetoresistivity, which indicates the ferromagnetic transition. From the plot of (R-Hall/R-sheet)(2) against B/(R-Hall/R-sheet) at several temperatures, which corresponds to the so-called Arrott plot, we can estimate the Curie temperature (T-c) of the sample as about 3K, suggesting that this sample has the higher T-c than ever reported. We also demonstrated, by means of light irradiation, that the strength of ferromagnetic interactions can be controlled by changing the hole concentration. An increase of the ferromagnetic behaviours such as a small increase of the coercive field was observed under light irradiation. This result strongly supports that the ferromagnetic transition in Zn1-chiMnchiTe is the carrier-induced one. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Magnetic properties of MBE-grown Zn1-xCrxTe

N Ozaki, N Nishizawa, KT Nam, S Kuroda, K Takita （Part： Lead author,　Corresponding author )

11TH INTERNATIONAL CONFERENCE ON II-VI COMPOUNDS (II-VI 2003), PROCEEDINGS ( WILEY-VCH, INC ) 1 ( 4 ) 957 - 960 2004 [Refereed]

We have grown Zn1-xCrxTe films by molecular-beam-epitaxy (MBE) and investigated their magnetic properties with variation of Cr concentrations (x) between 0.01-0.08. Systematic increase of the lattice constant was observed by XRD only up to around x = 0.02. Measurements of the magnetizations M-H reveal that ferromagnetic transition occurs in the films at low temperatures. The transition temperature (T-C) which was estimated from Arrott-plots increased with Cr concentration up to about 100 K for x = 0.08. However, the temperature dependence of the magnetization M-T shows different curves below T-C for field-cooled (FC) and zero-field-cooled (ZFC) processes. Moreover, their paramagnetic Curie temperatures (theta), estimated from the higher temperature M-T curves, are different from and much higher than T-C. These behaviours could be attributed to the superparamagnetism in the films. Indeed, many defects such as stacking faults are observed in the sample with x = 0.08 through HRTEM. It could give rise to the distribution of the Cr concentrations and/or the presence of precipitates including Cr. (C) 2004 WILEYNCH Verlag GinbH & Co. KGaA. Weinheim

Formation mechanism of nanocatalysts for the growth of silicon nanowires on a hydrogen-terminated Si {111} surface template

S Takeda, K Ueda, N Ozaki, Y Ohno

APPLIED PHYSICS LETTERS ( AMER INST PHYSICS ) 82 ( 6 ) 979 - 981 2003.02 [Refereed]

We have observed the formation process of nanocatalysts that act for the growth of Si nanowires by means of UHV scanning tunneling microscopy. Gold-silicon nanocatalysts that we have examined were thought to form on a hydrogen (H)-terminated [111] silicon surface and to expel Si nanowires of extremely high aspect ratio via the vapor-liquid-solid mechanism. We have observed that a nanocatalyst, that is, a droplet of melted gold-silicon alloy of about 5 nm in diameter, is actually formed in a pit on a H-terminated surface in the narrow temperature range around 500 degreesC. We have concluded that, in this specific temperature range, nanocatalysts can be melted, remain mutually isolated, absorb silicon effectively, and expel Si nanowires. Based on the result, we have proposed a method of making a thin template, which facilitates to decide the nucleation sites and the sizes of nanocatalysts, resulting in the precise control of those of Si nanowires. (C) 2003 American Institute of Physics.

Misleading fringes in TEM images and diffraction patterns of Si nanocrystallites

H Kohno, N Ozaki, H Yoshida, K Tanaka, S Takeda

CRYSTAL RESEARCH AND TECHNOLOGY ( WILEY-V C H VERLAG GMBH ) 38 ( 12 ) 1082 - 1086 2003 [Refereed]

Novel amorphization process in silicon induced by electron irradiation

J Yamasaki, Y Ohno, H Kohno, N Ozaki, S Takeda

JOURNAL OF NON-CRYSTALLINE SOLIDS ( ELSEVIER SCIENCE BV ) 299 793 - 797 2002.04 [Refereed]

We recently found that amorphization is induced in Si by MeV electron irradiation. In order to account for the steady-state diagram under electron irradiation, we propose a phenomenological theory which takes into account the two competing mechanisms, namely amorphization by higher energy recoils and recrystallization by lower energy recoils as well as thermal process. We apply the new amorphization method to fabricating artificial arrangements of the columns of a-Si in a c-Si film, and discuss its use for photonic crystal based on photonic band calculation. (C) 2002 Elsevier Science B.V. All rights reserved.

Fabrication of periodic nanohole multilayer structure on silicon surface toward photonic crystal

Y Ohno, N Ozaki, S Takeda

PHYSICA B-CONDENSED MATTER ( ELSEVIER SCIENCE BV ) 308 1222 - 1225 2001.12 [Refereed]

We have studied a porous structure on silicon surfaces that is introduced by electron irradiation. The structure consists of nanometer-sized holes arranged on silicon surfaces. Investigating the size and the distribution of surface nanoholes in a temperature range from about 4K to about 600K, we have estimated the porosity on a surface with nanoholes. We have fabricated a periodic dielectric multilayer structure on a silicon surface by introducing nanoholes periodically in one direction: alternating layers with different dielectric constants (the dielectric contrast of about 1.08), spaced by a distance of 100 nm. We can form periodic dielectric layers at arbitrary locations on surfaces by scanning an electron beam, changing the periodicity and the dielectric constant by varying irradiation condition. (C) 2001 Elsevier Science B.V. All rights reserved.

Observation of silicon surface nanoholes by scanning tunneling microscopy

N Ozaki, Y Ohno, M Tanbara, D Hamada, J Yamasaki, S Takeda （Part： Lead author )

SURFACE SCIENCE ( ELSEVIER SCIENCE BV ) 493 ( 1-3 ) 547 - 554 2001.11 [Refereed]

We have studied electron-irradiation-induced defects created on an electron exit surface of a Si thin film by means of scanning tunneling microscopy (STM). Several electron-irradiated areas with different electron doses are provided for STM observation. Transmission electron microscopy (TEM) observation reveals a number of silicon-surface-nanoholes of 2-3 nm in diameter and about 5 nm apart in an irradiated area whenever it receives the dose larger than 1.5 x 10(24) e/cm(2), while no distinctive TEM contrast of defects is observed in an area with lower dose. STM observation has shown that electron-irradiated surfaces are rougher than a nonirradiated surface. Examining the depth distribution of the areas with different doses, we have found that each irradiated surface exhibits two depth levels which are attributed to a rough surface and a bottom of surface nanoholes, respectively. Even in an area with the lowest dose (1.5 x 10(22) e/cm(2)) in this experiment we have observed distinctive STM contrasts, the arrangement and sizes of which are similar to those of the well-developed surface nanoholes observable by TEM. This STM observation shows that the arrangement of nanoholes on an electron exit surface is set up at the very early stage, followed by the excavating of nanoholes under prolonged electron irradiation. We suggest that nanoholes exist in the early stage when only a few atomic layers are removed from the initial surface. (C) 2001 Elsevier Science B.V. All rights reserved.

Optical properties of Si nanowires on a Si{111} surface

N Ozaki, Y Ohno, S Takeda （Part： Lead author )

OPTICAL MICROSTRUCTURAL CHARACTERIZATION OF SEMICONDUCTORS ( MATERIALS RESEARCH SOCIETY ) 588 99 - 103 2000 [Refereed]

We have investigated optical properties of straight silicon nanowires by means of in-situ cathodoluminescence spectroscopy in a transmission electron microscope. The nanowires, grown on a Si{111} surface via vapor-liquid-solid growth mechanism, have no structural defects such as kink, and the diameter and growth direction are controlled by varying the growth conditions. We have found that the nanowires emit intense light. These lines have not been observed in other kinds of Si nanostructures such as porous Si.

VLS growth of Si nanowhiskers on a H-terminated Si{111} surface

N Ozaki, Y Ohno, S Takeda, M Hirata （Part： Lead author )

MICROCRYSTALLINE AND NANOCRYSTALLINE SEMICONDUCTORS-1998 ( MATERIALS RESEARCH SOCIETY ) 536 305 - 310 1999 [Refereed]

We have grown Si nanowhiskers on a Si{111} surface via the vapor-liquid-solid (VLS) mechanism. The minimum diameter of the crystalline is 3nm and is close to the critical value for the effect of quantum confinement. We have found that many whiskers grow epitaxially or non-epitaxially on the substrate along the < 112 > direction as well as the < 111 > direction.
In our growth procedure, we first deposited gold on a H-terminated Si{111} surface and prepared the molten catalysts of Au and Si at 500 degrees C. Under the flow of high pressure silane gas, we have succeeded in producing the nanowhiskers without any extended defects. We present the details of the growth condition and discuss the growth mechanism of the nanowhiskers extending along the < 112 > direction.

Silicon nanowhiskers grown on a hydrogen-terminated silicon {111} surface

N Ozaki, Y Ohno, S Takeda （Part： Lead author )

APPLIED PHYSICS LETTERS ( AMER INST PHYSICS ) 73 ( 25 ) 3700 - 3702 1998.12 [Refereed]

Using a hydrogen-terminated Si{111} surface as a substrate, we have grown Si nanowhiskers along the [112] direction by the vapor-liquid-solid mechanism. The minimum silicon core diameter was 3 nm and the maximum length was about 2 mu m. The minimum silicon core diameter is close to the critical value for visible light emission due to the quantum confinement effect. In contrast to an oxidized Si surface, the hydrogen-terminated surface facilitates the formation of small molten Au-Si catalysts at a lower temperature (500 degrees C) which is slightly above the eutectic temperature. The formation of catalysts and the subsequent growth at the low temperature yield thin Si nanowhiskers on a Si substrate. (C) 1998 American Institute of Physics. [S0003-6951(98)01051-1].

　PREV-NEXT　

1