Faculty Profiles - OZAKI Nobuhiko

Published Papers - OZAKI Nobuhiko

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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.

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