Graduating School 【 display / non-display

  • Kyoto University  Faculty of Engineering  1990.03

Graduate School 【 display / non-display

  • Kyoto University  Graduate School, Division of Engineering  Doctor's Course  1995.03

Campus Career 【 display / non-display

  • University of Shiga Prefecture  School of Engineering  Department of Electronic Systems Engineering  Professor   2014.08 - Now


Papers 【 display / non-display

  • Independent control method for plasmonic skin depth based on transformation from spoof surface plasmon polaritons to bound states in the continuum

    Itami G., Sakai O.

    Physical Review B  Physical Review B  106 (24)    2022.12

    10.1103/PhysRevB.106.245406  Joint Work  


    A metal plate array (MPA) is the complementary structure of a metal hole array and forms spoof surface plasmon polaritons (SSPPs) supported by parallel plate modes in a unit cell. A ring dipole array, which is a periodic structure designed by providing an aperture in the conductive part of the unit cell of an MPA, has the potential to produce bound states in the continuum (BICs). In this paper, the transformation from SSPPs to BICs is confirmed using these two structures by broadening the aperture in the unit cell. Making use of this phenomenon, control of the skin depth of surface modes is numerically demonstrated while retaining the resonant frequency, and the fitting values of the plasmonic skin depth are in good agreement with theoretical values.

  • Foundations of plasma photonics: lamps, lasers, and electromagnetic devices

    Nijdam S., Desai K.V., Park S.J., Sun P.P., Sakai O., Lister G., Eden J.G.

    Plasma Sources Science and Technology  Plasma Sources Science and Technology  31 (12)    2022.12

    10.1088/1361-6595/ac8448  Joint Work  


    The enduring contributions of low temperature plasmas to both technology and science are largely a result of the atomic, molecular, and electromagnetic (EM) products they generate efficiently such as electrons, ions, excited species, and photons. Among these, the production of light has arguably had the greatest commercial impact for more than a century, and plasma sources emitting photons over the portion of the EM spectrum extending from the microwave to soft x-ray regions are currently the workhorses of general lighting (outdoor and indoor), photolithography for micro- and nano-fabrication of electronic devices, disinfection, frequency standards (atomic clocks), lasers, and a host of other photonic applications. In several regions of the EM spectrum, plasma sources have no peer, and this article is devoted to an overview of the physics of several selected plasma light sources, with emphasis on thermal arc and fluorescent lamps and the more recently-developed microcavity plasma lamps in the visible and ultraviolet/vacuum ultraviolet regions. We also briefly review the physics of plasma-based metamaterials and plasma photonic crystals in which low temperature plasma tunes the EM properties of filters, resonators, mirrors, and other components in the microwave, mm, and sub-mm wavelength regions.

  • Complexity visualization, dataset acquisition, and machine-learning perspectives for low-temperature plasma: A review

    Sakai O., Kawaguchi S., Murakami T.

    Japanese Journal of Applied Physics  Japanese Journal of Applied Physics  61 (7)    2022.07

    10.35848/1347-4065/ac76fa  Joint Work  


    Low-temperature plasma plays various roles in industrial material processing as well as provides a number of scientific targets, both from theoretical and experimental points of view. Such rich features in variety are based on its complexities, arising from diverse parameters in constituent gas-phase species, working gas pressure, input energy density, and spatial boundaries. When we consider causalities in these complexities, direct application of machine-learning methods is not always possible since levels of complexities are so high in comparison with other scientific research targets. To overcome this difficulty, progresses in plasma diagnostics and data acquisition systems are inevitable, and the handling of a large number of data elements is one of the key issues for this purpose. In this topical review, we summarize previous and current achievements of visualization, acquisition, and analysis methods for complex plasma datasets which may open a scientific and technological category mixed with rapid machine-learning advancements and their relevant outcomes. Although these research trends are ongoing, many reports published so far have already convinced us of various expanding aspects of low-temperature plasma leading to the potential for scientific progress as well as developments of intellectual design in industrial plasma processes.

  • Synthesis of Electromagnetic Wave-Absorbing Co-Ni Alloys and Co-Ni Core-Shell Structured Nanoparticles

    Ishijima M., Takada T., Cuya Huaman J.L., Mizutomi T., Sakai O., Shinoda K., Uchikoshi M., Mamiya H., Suzuki K., Miyamura H., Balachandran J.

    Inorganic Chemistry  Inorganic Chemistry    2022.01

    10.1021/acs.inorgchem.2c02633  Joint Work  


    Co-Ni alloy nanoparticles, a potential candidate for microwave absorption material, were successfully synthesized by tuning the reduction timing of Co and Ni ions by introducing oleylamine as a complexing agent and 1-heptanol as a reducing solvent. The formation mechanism elucidated using time-resolved sampling and in situ X-ray absorption spectroscopy (XAS) and ultraviolet-visible (UV-vis) spectrophotometry measurements suggested that the delay in the reduction of Co ions via complexation with oleylamine facilitated the co-reduction of Co with Ni ions and led to the formation of Co-Ni alloys. The successful synthesis of Co-Ni alloys experimentally confirmed the differences in magnetic properties between alloy and core-shell structured Co50Ni50 particles. Further, the syntheses of Co-Ni alloys with different compositions were also possible using the above technique. In addition, the microwave absorption properties were measured using the free-space method utilizing a vector network analyzer of Co50Ni50-polyethylene composite with different sheet thicknesses. A reflection loss (RL) value of -25.7 dB at 13.6 GHz for the alloy structure was more significant than the core-shell counterpart. The above values are high compared to results reported in the past. The validity of the measurements was confirmed by utilizing the parameter retrieval method to extract permittivity and permeability from the scattering parameter (S) and recalculation of the RL as a function of frequency.

  • Logistics Route Planning in Agent-Based Simulation and Its Optimization Represented in Higher-Order Markov-Chain Networks

    Ikai R., Miyagi S., Sakai O.

    Springer Proceedings in Complexity  Springer Proceedings in Complexity   38 - 50  2021.07

    10.1007/978-3-030-81854-8_4  Joint Work  


    Route planning in logistics, in which multiple pickup and delivery positions exist in a road network, is a complicated task with many choices in a path selection and their influences on the following procedures. Solving this task by multi-agent simulations, we examine the route optimization process by monitoring motions in networks based on simple or higher-order Markov chains (MCs). Agent footprints in the networks, which spread over the entire network at the initial phase, converge on small number of edges as the transportation path gets shortened. When we increase the order of MCs in agent mobilities, the MC networks are enlarged and possess a large number of nodes and edges with structural regularity so that one node contains partial trace history, while the optimized route that frequently overlaps edge groups with high transition probabilities is equivalent to a smaller and more noticeable subgraph around a local optimal solution. In other words, this localization of the traces indicates a convergence level in optimization, which can be a measure for route planning in logistics.

  • In-vacuum active colour sensor and wireless communication across a vacuum-air interface

    Sakai O., Kitagawa T., Sakurai K., Itami G., Miyagi S., Noborio K., Taguchi K.

    Scientific Reports  Scientific Reports  11 (1)    2021.01

    10.1038/s41598-020-80501-z  Joint Work  


    © 2021, The Author(s). In situ sensing with wireless digital-data transfer is a potential processing scheme that works very closely to the location of an event monitored by a sensor and converts the sensor’s raw output into digitized and informative small-volume bits, as suggested by recent proposals for edge computing and the Internet of Things (IoT). Colour perception may be a target of in situ sensor data acquisition; however, in contrast to from other sensing devices, colour sensors that detect visible light signals are usually located away from light-emitting sources, collecting light transmitting through the space and attenuating it in some manner. For example, in a vacuum chamber whose gas pressure is much less than the ambient atmosphere in which the sensors usually work, there are many veiled light sources, such as discharge plasma, for various industrial purposes including nanoscale manufacturing. In this study, we designed an in-vacuum colour sensor that can work with analogue-to-digital conversion and transfer data by wireless communication; this sensor is active in a low-pressure plasma chamber, detecting light signals and transferring them to a personal computer located outside the vacuum chamber. In addition to detecting lights with controlled spectra from outside successfully, we achieved complete operation of our in-vacuum active sensor for plasma emissions generated at 100 Pa. Comparing the signals with data from simultaneous monitoring by a monochromator, we established that the recorded signals arose from the plasma, confirming successful direct detection of low-pressure plasma emissions without any filtering effects between the sensor and the target object.

  • Statistics of Growing Chemical Network Originating from One Molecule Species and Activated by Low-Temperature Plasma

    Mizui Y., Miyagi S., Sakai O.

    Studies in Computational Intelligence  Studies in Computational Intelligence  944   398 - 409  2021.01

    10.1007/978-3-030-65351-4_32  Joint Work  


    Chemistry in plasma is complicated because it has so many reactions in parallel and in series. A complex network is suitable for the visualization and the analysis of its complexity. A numerical calculation based on hundreds of rate equations is a typical tool for plasma chemistry, but such a computational process does not clarify the undergoing physical and chemical properties that stabilize many industrial plasma processes for a number of applications. In this study, we focus on low-temperature plasma in which high-energy electrons are activators for chemical reactions, and investigate the origin of the stability by examining the statistical properties of networks for silane (SiH ) plasma. There is only one seed species in the initial space, SiH , which is surrounded by high-energy electrons. SiH is decomposed into several fragments composed of Si and/or H atoms with possible charges, and such radical and ion species are decomposed or synthesized into other species, leading to the formation of temporal reaction networks in chemistry. With the effects of rate constants that determine chemical reaction rates, we create temporal networks and observe preferential attachments that induce a new reaction in a transient state. The centrality indices for participant species and degree distributions reveal what is occurring in this complex system, and during the sequential process we observe an exponential-tail degree distribution, which is a significant source of reaction stability. 4 4 4

  • Order Estimation of Markov-Chain Processes in Complex Mobility Network Embedded in Vehicle Traces

    Yamamoto K., Miyagi S., Sakai O.

    Studies in Computational Intelligence  Studies in Computational Intelligence  944   231 - 242  2021.01

    10.1007/978-3-030-65351-4_19  Joint Work  


    Vehicle mobility in urban traffic systems is complex, partly because it reflects mobility of a human who drives a vehicle, and partly because it depends on many roles which the vehicle plays. Previous studies on human mobility revealed that it includes Lévy-flights-like motions and memoryless deterministic walks as well as random walks, but the mobility of vehicles may be more biased due to their functions. Focusing our research target on a sightseeing vehicle with sufficiently limited functions, we show a method to measure regularity of visitation patterns, quantified by order(s) of Markov chains in their mobility. Graphs of higher-order Markov chains, which are representatives of mobility in a network style, possess statistical properties; in our observation dataset, they include degree distributions similar to scale-free networks. The detection of mobility in real social experiments, which is also assumed on these graphs, yields the order of Markov chains inside it with its comparison with the results of agent-based simulations. Centrality indices of the mobility networks well coincide with prediction of these analytical and numerical results.

  • Analysis and observation of the breakdown of Babinet’s principle in complementary spoof surface plasmon polariton structures

    Itami G., Sakai O.

    Scientific Reports  Scientific Reports  10 (1)    2020.12

    10.1038/s41598-020-67923-5  Joint Work  


    © 2020, The Author(s). A metal plate array (MPA) which is a structure complimentary to a metal hole array (MHA), supports spoof surface plasmon polaritons (SSPP) as well as an MHA does. Babinet’s principle attributes the phenomenon of duality to transmission characteristics of the complimentary impedance surfaces because of the symmetry of the behaviors of electric and magnetic fields. However, it is also a fact that the complimentary structures do not follow this principle if they have wavelength-size thickness, because electromagnetic waves do not treat such thick structures as a boundary surface but as propagation spaces with the specific boundaries such as a waveguide which shows SSPP modes. If the thickness is so small that it is negligible, Babinet’s principle is still valid, while it has been uncertain how the layer thickness works to break the principle as it is increased. The unconfirmed transformation is revealed analytically and experimentally with the use of MPAs and MHAs of varying thicknesses.

  • Rescaling the complex network of low-temperature plasma chemistry through graph-theoretical analysis

    Murakami T., Sakai O.

    Plasma Sources Science and Technology  Plasma Sources Science and Technology  29 (11)    2020.11

    10.1088/1361-6595/abbdca  Joint Work  


    © 2020 IOP Publishing Ltd. We propose graph-theoretical analysis for extracting inherent information from complex plasma chemistry and devise a systematic way to rescale the network under the following key criteria: (1) maintain the scale-freeness and self-similarity in the network topology and (2) select the primary species considering its topological centrality. Network analysis of reaction sets clarifies that the scale-freeness emerging from a weak preferential mechanism reflects the uniqueness of plasma-induced chemistry. The effect of chemistry rescaling on the dynamics and chemistry of the He + O2 plasma is quantified through numerical simulations. The present chemical compression dramatically reduces the computational load, whereas the concentration profiles of reactive oxygen species (ROS) remain largely unchanged across a broad range of time, space and oxygen admixture fraction. The proposed analytical approach enables us to exploit the full potential of expansive chemical reaction data and would serve as a guideline for creating chemical reaction models.

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