Faculty Profiles - SAKAI Osamu

Papers - SAKAI Osamu

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Fractal-like ray trajectories traced in three-dimensional array of photoelectrochemical cells

Osamu Sakai, Yoshihiko Ueda, Tomohide Akiyama, Josuke Nakata

Journal of Physics D: Applied Physics 46 ( 18 ) 2013.5

Publishing type：Research paper (scientific journal)

Underwater spherical photoelectrochemical cells aligned in three-dimensional electrolyte space convert injected sunlight into driving force for water splitting, which is analysed by a model calculation and verified in experiments. Sunlight is well absorbed via a number of bounces on Si, and trajectories of its rays are in a fractal-like scale-free system. Similar systems are applicable to general photoelectrochemical cells. © 2013 IOP Publishing Ltd.

DOI： 10.1088/0022-3727/46/18/182001

Scopus

Hydrazine generation for the reduction process using small-scale plasmas in an argon/ammonia mixed gas flow Reviewed

Keiichiro Urabe, Hiraoka Yu, Sakai Osamu

PLASMA SOURCES SCIENCE & TECHNOLOGY 22 ( 3 ) 2013.5

Diagnostic of electrophoretic particles by dielectric dispersion spectra

Ayumi Ando, Ryuichi Kiyoe, Yuusuke Nishio, Hiroshi Inoue, Osamu Sakai

Proceedings of the International Display Workshops 2 1318 - 1321 2013

Two-dimensional metamaterial for surface wave propagation with scanning metallic defect for permittivity imaging

Osamu Sakai, Go Itami, Tomohide Akiyama, Yoshinori Harada

2013 International Symposium on Electromagnetic Theory, EMTS 2013 - Proceedings 719 - 721 2013

Publishing type：Research paper (international conference proceedings)

A perforated metal plate, working as a two-dimensional metamaterial to support surface wave propagation, plays a role of an imaging head of permittivity mapping at millimeter-wave ranges. One noninvasive metallic probe is swept two-dimensionally along the metamaterial surface to induce near field to detect the corresponding local permittivity. This configuration is applicable to detection of disease tissues for medical uses. © 2013 IEICE.

Scopus

Absorption spectroscopy using interference between optical frequency comb and single-wavelength laser Reviewed

Keiichiro Urabe, Osamu Sakai

APPLIED PHYSICS LETTERS 101 ( 5 ) 2012.7

Microcavity array plasma system for remote chemical processing at atmospheric pressure

Dae Sung Lee, Satoshi Hamaguchi, Osamu Sakai, Sung Jin Park, J. Gary Eden

Journal of Physics D: Applied Physics 45 ( 22 ) 2012.6

Publishing type：Research paper (scientific journal)

A microplasma system designed for chemical processing at atmospheric pressure is fabricated and characterized with flowing He/O 2 gas mixtures. At the heart of this microcavity dielectric barrier discharge (MDBD) system are two arrays of half-ellipsoidal microcavities engraved by micropowder blasting into dielectric surfaces facing a flowing, low-temperature plasma. Experiments demonstrate that the ignition voltage is reduced, and the spatially averaged optical emission is doubled, for an MDBD flowing plasma array relative to an equivalent system having no microcavities. As an example of the potential of flowing atmospheric microplasma systems for chemical processing, the decomposition of methylene blue (as evidenced by decoloration at 650.2nm) is shown to proceed at a rate as much as a factor of two greater than that for a non-microcavity equivalent. © 2012 IOP Publishing Ltd.

DOI： 10.1088/0022-3727/45/22/222001

Scopus

Temporally and spectrally resolved observation of a crossed-flow DBD plasma jet using pure helium and argon/acetone mixed gases Reviewed

Keiichiro Urabe, Brian L. Sands, Biswa N. Ganguly, Osamu Sakai

PLASMA SOURCES SCIENCE & TECHNOLOGY 21 ( 3 ) 2012.6

Plasma microchannel and jet enhanced by an array of ellipsoidal microcavities

D. S. Lee, O. Sakai, S. J. Park, J. G. Eden, S. Hamaguchi

IEEE Transactions on Plasma Science 39 ( 11 PART 1 ) 2690 - 2691 2011.11

Publishing type：Research paper (scientific journal)

Plasma jet discharges are shown to be enhanced significantly, with respect to emission intensity, in the presence of ellipsoidal microcavities dispersed at uniform intervals along the gas flow channel. In these experiments, 25 microcavities were fabricated in the top and bottom faces of a gas flow channel having a rectangular cross section. Conducting film electrodes exterior to the glass structure, powered by a 20-kHz sinusoidal voltage, produced plasma along the entire channel as well as within the microcavities. Stronger optical emission and longer plasma jets were observed in this system than in an equivalent plasma jet system without microcavities. © 2006 IEEE.

DOI： 10.1109/TPS.2011.2165561

Scopus

Temporally Resolved Imaging of Jet-Type Dielectric Barrier Discharge Using He and Ar/Acetone Crossed Gas Flows Reviewed

Keiichiro Urabe, Brian L. Sands, Osamu Sakai, Biswa N. Ganguly

IEEE TRANSACTIONS ON PLASMA SCIENCE 39 ( 11 ) 2011.11

Discharge-Mode Transition in Jet-Type Dielectric Barrier Discharge Using Argon/Acetone Gas Flow Ignited by Small Helium Plasma Jet Reviewed

Keiichiro Urabe, Keitaro Yamada, Osamu Sakai

JAPANESE JOURNAL OF APPLIED PHYSICS 50 ( 11R ) 2011.10

Chemical filters by non-thermal atmospheric pressure plasmas for reactive fields Reviewed

Osamu Sakai, Tadasuke Morita, Yoshihiko Ueda, Noriaki Sano, Kunihide Tachibana

THIN SOLID FILMS 519 ( 20 ) 6999 - 7004 2011.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

Non-thermal plasmas in electrode configurations are designed in this study and investigated at atmospheric pressure for generation of reactive fields. A combination of insulated wire electrodes and bare metal wire electrodes makes it possible to obtain a filter-like assembly of microplasmas, even in ambient air or in aqueous solutions. Oxidation and reduction fields are obtained by controlling the gas supply of hydrogen, oxygen, and water vapor, and these fields benefit various functional processes such as recycling CO(2), decomposing organic molecules, and increasing the work function on metal surfaces. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tsf.2011.01.225

Web of Science

Transition between positive and negative permittivity in field-dependent metamaterial

Osamu Sakai

Journal of Applied Physics 109 ( 8 ) 2011.4

Publishing type：Research paper (scientific journal)

Permittivity in a metamaterial, which depends on the electric field of a propagating electromagnetic wave, induces anomalous responses against electromagnetic waves. When we consider discharge plasma an example of a metamaterial with field-dependent permittivity and negative permeability, a transition takes place between positive and negative permittivity, leading to reflection of waves in a low-field branch and transmission by the creation of a negative-refractive-index medium in a high-field branch. Nonlinearity in the phase space of the permittivity and the electric field in this system trigger this dynamic transition, which is suggested in the numerical results and clarified in an analytical model. © 2011 American Institute of Physics.

DOI： 10.1063/1.3574921

Scopus

Combined spectroscopic methods for electron-density diagnostics inside atmospheric-pressure glow discharge using He/N-2 gas mixture Reviewed

Keiichiro Urabe, Osamu Sakai, Kunihide Tachibana

JOURNAL OF PHYSICS D-APPLIED PHYSICS 44 ( 11 ) 2011.3

Enhancement of visible He<inf>2</inf> emission by infrared laser excitation of He metastable atoms in atmospheric-pressure plasma jet

Keiichiro Urabe, Hideki Motomura, Osamu Sakai, Kunihide Tachibana

Journal of Physics D: Applied Physics 44 ( 4 ) 2011.2

Publishing type：Research paper (scientific journal)

We observed bright red emission at an intersection of an infrared (IR) laser beam and an atmospheric-pressure plasma jet (APPJ). The APPJ had a coaxial dielectric barrier discharge configuration and generated a plasma plume in a He gas flow ejected into air from a glass capillary. Although the laser-beam wavelength was 1083 nm corresponding to He I (23P-23S) atomic transition, the enhanced red emission peaked in the visible region at 640 nm and had broad bandwidth corresponding to He2 (d 3Σ+u -b 3IIg) molecular transition. This result indicates that the IR laser excitation of He metastable (23S) atoms leads to selective formation of excited He2 (d 3Σ+u ) molecules from excited He (23P) atoms. The 640 nm emission intensity decreased as the laser beam moved downstream in the plasma plume, because of the quenching effects on the excited He atoms and the excited He2 molecules by air entrainment. We believe that the He2 (d 3Σ+u ) molecular formation follows two potential curves of He 2 (f 3Σu or f 3IIu) and (d 3Σ+u ) states, overcoming a potential hump by three-body collision processes. © 2011 IOP Publishing Ltd.

DOI： 10.1088/0022-3727/44/4/042001

Scopus

Emerging aspects in a plasma-metamaterial composite

Osamu Sakai

2011 30th URSI General Assembly and Scientific Symposium, URSIGASS 2011 2011

Publishing type：Research paper (international conference proceedings)

When plasmas are immersed in a metamaterial structure, such a composite shows various properties which are not expected in an ordinary metamaterial. Plasmas are either generated by external powers or present in nature as a space plasma, and composites of metamaterials and such plasmas serve as elements of electromagnetic wave controllers as well as negative-refractive-index materials. This report describes the fundamental properties of the composites with specific examples, and some fields of potential applications for human space activities are surveyed. © 2011 IEEE.

DOI： 10.1109/URSIGASS.2011.6051083

Scopus

High speed deposition of oxide materials by using plasma jet at atmospheric pressure

Yosuke Ito, Osamu Sakai, Kunihide Tachibana

AIP Conference Proceedings 1393 11 - 14 2011

Publishing type：Research paper (international conference proceedings)

We carried out plasma enhanced chemical vapor deposition using a plasma jet at atmospheric pressure for one-dimensional deposition of SiO2 films and ZnO compounds, and measured the film quality by Fourier transform infrared spectroscopy. In the case of SiO2, the deposition rate was about 400 nm/s and the breakdown voltage was 5×105 V/cm. On the other hand, in the case of ZnO, the deposition rate was about 160 nm/s. However, as-prepared films contained much carbon contamination, so it is necessary to anneal the compounds in order to obtain high-quality films. After 1-hour annealing at 500°C in air, the transparency was more than 80% in visible light range and the drop of the transparency at around 370 nm corresponding to ZnO band-edge energy. Moreover, the resistivity of the annealed ZnO compounds was estimated to be about 106 Ωcm, so we could obtain the transparent and semiconductor-like ZnO compounds successfully. © 2011 American Institute of Physics.

DOI： 10.1063/1.3653596

Scopus

Negative refractive index designed in a periodic composite of lossy microplasmas and microresonators

Osamu Sakai, Takuya Shimomura, Kunihide Tachibana

Physics of Plasmas 17 ( 12 ) 2010.12

Publishing type：Research paper (scientific journal)

A novel metamaterial with negative refractive index was designed using a spatially periodic composite of microplasmas and microresonators. Microresonators, which are double-helix metal wires in this report, work as an origin of macroscopic negative permeability material, as well as discharge electrodes. If a resonance frequency of the resonators is well below electron plasma frequency of the microplasmas where their permittivity is negative in the real part with a certain imaginary part, the macroscopic refractive index becomes negative just above the resonance frequency where the macroscopic permeability is negative, even if microplasmas are fairly lossy; due to such a loss or conductive component in permittivity, electromagnetic waves are not evanescent but propagating. This result indicates that plasmas can play important roles in parameter control of a metamaterial with a complex refractive index. © 2010 American Institute of Physics.

DOI： 10.1063/1.3524561

Scopus

FTIR study of methylene blue plasma degradation products through plasma treatment on water Reviewed

T. Shirafuji, J. Hieda, O. Takai, N. Saito, T. Morita, O. Sakai, K. Tachibana

Proc. 2010 IEEE Region 10 Conf. 2010.9

Effect of Series Capacitance and Accumulated Charge on a Substrate in a Deposition Process with an Atmospheric-Pressure Plasma Jet Reviewed

Yosuke Ito, Yutaka Fukui, Keiichiro Urabe, Osamu Sakai, Kunihide Tachibana

JAPANESE JOURNAL OF APPLIED PHYSICS 49 ( 6 ) 2010.6

Language：English Publishing type：Research paper (scientific journal) Publisher：JAPAN SOC APPLIED PHYSICS

In order to investigate the effect of accumulated charge on a substrate surface in the deposition of a SiO(2) film with an atmospheric-pressure plasma jet, we have measured the discharge current flowing into a copper substrate, which was placed 20 mm from the exit of the plasma jet and connected to a variable capacitor in series. We found that the discharge current decreased markedly when the capacitance of the substrate was below 100 pF, and the deposition rate of SiO(2) traced the variation of the capacitance. To analyze the behavior of the plasma jet, we considered an equivalent circuit of our system and verified the validity of our supposition that the accumulated charge restricts the deposition rate. Thereby, we found that the discharge current and deposition rate were determined by the capacitance of both the glass tube wall and the substrate. (C) 2010 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.49.066201

Web of Science

Experimental and numerical verification of microplasma assembly for novel electromagnetic media Reviewed

Osamu Sakai, Teruki Naito, Kunihide Tachibana

PHYSICS OF PLASMAS 17 ( 5 ) 2010.5

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

Microplasmas have a number of potential roles to control propagating electromagnetic waves. This report focuses on novel physics of periodic microplasma assembly for electromagnetic media, which is verified by experimental results and analyzed by numerical methods. Using an assembly composed of microplasmas, novel functions are expected due to its complex dielectric function arising from dielectric and lossy properties. The dielectric property creates photonic band gaps, and the lossy property drastically changes transmittance around the photonic band gaps, leading to an attenuation gap. As a result, a "complex" dispersion relation or a band diagram in the three-dimensional space of real and imaginary wavenumbers and wave frequency will open new possibilities to control electromagnetic waves by complex-variable filters composed of microplasma assemblies. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3314334]

DOI： 10.1063/1.3314334

Web of Science

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Papers - SAKAI Osamu