Faculty Profiles - SAKAI Osamu

Papers - SAKAI Osamu

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A tunable double negative device consisting of a plasma array and a negativepermeability metamaterial Reviewed International coauthorship

Iwai A., Righetti F., Wang B., Sakai O., Mark A. Cappelli, M. A.

Physics of Plasmas 27 2020.2

Mobilities in network topology and simulation reproducibility of sightseeing vehicle detected by low-power wide-area positioning system Reviewed

Yamamoto K., Yoshida J., Miyagi S., Minami S., Minami D., Sakai O.

Electronics (Switzerland) 9 ( 1 ) 2020.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Electronics (Switzerland)

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. Vehicle mobilities for passengers in a city’s downtown area or in the countryside are significant points to characterize their functions and outputs. We focus on commercial sightseeing vehicles in a Japanese city where many tourists enjoy sightseeing. Such mobilities and their visualizations make tourist activities smoother and richer. We design and install a low-power, wide-area positioning system on a rickshaw, which is a human-pulled, two-or three-wheeled cart, and monitor its mobility in Hikone City. All the spatial locations, which are recorded in a time sequence on a cloud server, are currently available as open data on the internet. We analyze such sequential data using graph topology, which reflects the information of corresponding geographical maps, and reproduce it in cyberspace using an agent-based model with similar probabilities to the accumulated rickshaw records from one spatial node to another. Although the numerical results of the agent traced in a simulated city are partially consistent with the rickshaw’s record, we identify some significant differences. We conclude that the rickshaw’s mobility observed at the actual sightseeing sites is partially in the random motion; some cases are strongly biased by memory routes. Such non-randomness in the rickshaw’s mobility indicates the existence of specific features in tourism sources that are identified for each sightseeing activity and affected by local sightseeing resources.

DOI： 10.3390/electronics9010116

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Localized breaking of parameter uniformity by macroscopically-negative-permeability metamaterial in low-pressure microwave plasma

Akinori Iwai, Yoshihiro Nakamura, Osamu Sakai, Yoshiharu Omura

Plasma Sources Science and Technology 29 ( 3 ) 2020

Publishing type：Research paper (scientific journal)

Measurements of plasma parameters and transmitted microwaves are performed in a composite of double-split-ring resonators (DSRRs) making magnetic permeability negative and plasmas excited by a 2.45 GHz microwave. We launch the microwave at 100-200 W, modulated by a pulse wave at a low frequency and a low duty ratio to detect time evolutions of plasma parameters. We perform the measurements at different positions with varying distance from the supporting plate of the DSRRs, and find enhanced wave transmission in the composite with non-uniform profiles of electron temperature. To reinforce the measurements of electron temperature, we evaluate the electron energy distribution function (EEDF) by deriving the second deviation of electron current, upon which data smoothing methods are applied, and the estimated EEDF becomes spatially non-uniform with a similar profile to electron temperature. In terms of the validity of results on the enhanced wave transmission, one-dimensional electromagnetic particle simulations reveal sneaking microwaves inside the composite. In comparison with cases of the negative-permeability and the passive (positive-permeability) DSRRs, we confirm that the non-uniformity is attributed to the magnetic resonance of the DSRRs, which is a microscopic effect of the DSRRs to plasma generation, whereas their macroscopic effect makes permeability negative and enhances wave energy transfer inside the composite.

DOI： 10.1088/1361-6595/ab2d74

Detection of bending motion at waist of kitchen workers by using a single commercial accelerometer

Ryosuke Tsuji, Katsumi Furuta, Osamu Sakai, Shigeyuki Miyagi

2019 IEEE 8th Global Conference on Consumer Electronics, GCCE 2019 1172 - 1173 2019.10

Publishing type：Research paper (international conference proceedings)

Investigating the motion of kitchen workers is one of the important elements for reducing the time for serving food to customers in fast food restaurants. We show that a single 3-axis accelerometer attached to kitchen workers is useful for detecting their back bending motion at the waist and estimating time or time period by using only a simple technique. The introduced technique was applied to the practical data acquired from a real situation in a fast food restaurant. The precision of the detected back bending motion reached approximately 70%.

DOI： 10.1109/GCCE46687.2019.9015606

Detection of swallowing times using a commercial RGB-D camera

Jun Yoshida, Keiko Kozawa, Sueyoshi Moritani, Shin Ichi Sakamoto, Osamu Sakai, Shigeyuki Miyagi

2019 IEEE 8th Global Conference on Consumer Electronics, GCCE 2019 1154 - 1155 2019.10

Publishing type：Research paper (international conference proceedings)

For assisting early detection and treatment for dysphagic patients, an assessment system for swallowing disorders is necessary. Detecting the swallowing time is one of the key components for developing such an assessment system. Hence, we propose a swallowing detection algorithm using a commercial depth camera. The proposed method considers movements in the front of the neck, before and during swallowing, to determine the swallowing time. We then experimentally demonstrate the performance of the algorithm. The results show that the proposed algorithm achieves a recall and precision of 0.958 and 0.813, respectively. Thus, this algorithm can assist in the early detection of dysphagia.

DOI： 10.1109/GCCE46687.2019.9015325

Low-cost color-sensitive optical sensor and its calibration by neural network

Shinya Ueno, Osamu Sakai

2019 IEEE 8th Global Conference on Consumer Electronics, GCCE 2019 1156 - 1157 2019.10

Publishing type：Research paper (international conference proceedings)

Although sensor systems play various roles in consumer electronics, high-precision sensors with high-quality calibration are not suitable for all cases in industrial processes and in our daily life uses because of their price and size. Otherwise, one relies on subjective and personal evaluations when judgments are required. In particular, color data is one of the essential information in many industrial and daily cases, and, for example, color sensitive optical sensors are useful for estimations of fruits and vegetables in agriculture and in purchases of such products. In this research, in order to lower the price of the sensors with sufficient precision, which is one of the obstacles to extend the application fields, we propose a method to make software calibration for color spectra detected by color sensors using a neural network. When the low-cost sensors with adequate calibration are available, they work well for not only engineers in industrial sites and in agriculture fields but also general consumer customers.

DOI： 10.1109/GCCE46687.2019.9015596

Analytical formulation for radiation characteristics of a surface wave sustained plasma antenna

T. Naito, O. Sakai

Physics of Plasmas 26 ( 7 ) 2019.7

Publishing type：Research paper (scientific journal)

This paper reports the analytical formulation of a surface wave sustained plasma antenna based on the antenna theory and the theory for a surface wave sustained plasma. The antenna (tube) structure is a basic quarter-wavelength monopole antenna, comparable to a wavelength of a surface wave. The theory for a surface wave sustained plasma in a semi-infinitely long tube is extended to a finite length plasma. The electron density distribution is calculated based on the extended theory, and antenna characteristics are obtained from the electron density distribution. The analytical results obtained from the formulation show good agreement with the numerical results. The electron density in the finite length tube depends on the existence of the interference between forward- and backward-traveling waves and the boundary condition at the tube end. The electron density distribution is determined based on the balance between the forward- and backward-traveling waves. In the case that the plasma does not reach the tube end, which is equivalent to the case of a semi-infinitely long tube, the antenna gain is constant at a low value due to high plasma resistance and low radiation resistance, without the propagation of the backward-traveling wave. Once the plasma reaches the tube end, due to the coexistence of the forward- and backward-traveling waves with their interference, the electron density is elevated fairly beyond the critical density for sustaining surface wave propagation. Consequently, the antenna gain increases with the synergistic effect of the decrease in plasma resistance and the increase in radiation resistance, and the surface wave sustained plasma works as an antenna.

DOI： 10.1063/1.5088855

Symmetrical estimation method for skin depth control of spoof surface plasmon polaritons using dispersed waves from a metallic hole array

Go Itami, Osamu Sakai

Journal of Applied Physics 125 ( 21 ) 2019.6

Publishing type：Research paper (scientific journal)

A metallic hole array (MHA) supports the propagation of spoof surface plasmon polariton (SSPP) modes on its surface. We observe enhancement of dispersed waves at a specific angle when SSPP modes propagate. We propose a symmetrical estimation method for SSPP skin depth using waves dispersed from an MHA. The proposed method provides an experimental demonstration of the skin depth of the electric fields of SSPP modes. Using the dispersed waves emitted from the plane of the MHA, we are able to detect confined electric fields with decay lengths that resemble those theoretically predicted around a specific frequency of SSPP mode excitation.

DOI： 10.1063/1.5088116

Direction-of-arrival estimation from scattering patterns in a subwavelength periodic structure of a conductive scatterer

G. Itami, T. Iizuka, Y. Toriumi, J. Kato, O. Sakai

2019 URSI International Symposium on Electromagnetic Theory, EMTS 2019 2019.5

Publishing type：Research paper (international conference proceedings)

A novel direction-of-arrival (DOA) estimation method that uses subwavelength periodic structures of conductive scatterers (SPCS) is proposed. There is a possibility that scattering patterns with frequency and angular responses can be generated by using subwavelength structures on the basis of metamaterials. Therefore, the frequency and angular dependencies of scattering patterns generated by SPCS are investigated analytically and experimentally. In the analytical studies, the 2D spatial distributions of electric field intensities in SPCS are analyzed. And it is confirmed that the patterns have frequency and angular dependencies. In the experimental studies, the 1D spatial distributions of the scattering patterns in SPCS were measured, confirming the frequency and angular dependence of their spatial distribution. Thus, the proposed method has the potential for application in an advanced DOA estimation device.

DOI： 10.23919/URSI-EMTS.2019.8931530

Two-dimensional imaging of permittivity distribution by an activated meta-structure with a functional scanning defect Reviewed

Itami G., Sakai O., Harada Y.

Electronics (Switzerland) 8 ( 2 ) 2019.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Electronics (Switzerland)

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. A novel 2D imaging method for permittivity imaging using a meta-structure with a functional scanning defect is proposed, working in the millimeter wave-range. The meta-structure we used here is composed of a perforated metal plate with subwavelength-holes and a needle-like conductor that can scan two-dimensionally just beneath the plate. The metal plate, which is referred to as a metal hole array (MHA) in this study, is known as a structure supporting propagation of spoof surface plasmon polaritons (SSPPs). High-frequency waves with frequencies higher than microwaves, including SSPPs, have the potential to detect signals from inner parts embedded beneath solid surfaces such as living cells or organs under the skin, without physical invasion, because of the larger skin depth penetration of millimeter wave-bands than optical wave-bands. Focused on activated SSPPs, the localized distortion of SSPP modes on an MHA is used in the proposed method to scan the electromagnetic properties of the MHA with a needle-like conductor (conductive probe), which is a kind of active defect-initiator. To show the validity of the proposed method, electromagnetic analyses of the localized distortions of wave fields were performed, and one-and two-dimensional imaging experiments were conducted with the aim of detecting both conductive and dielectric samples. The analytical results confirmed the localized distortion of the electric field distribution of SSPP modes and also indicated that the proposed method has scanning ability. In experimental studies, the detection of conductive and dielectric samples was successful, where the detected dielectrics contained pseudo-biological materials, with an accuracy on the order of millimeters. Finally, a biomedical diagnosis in the case of a rat lung is demonstrated by using the experimental system. These results indicate that the proposed method may be usable for non-invasive and low-risk biomedical diagnosis.

DOI： 10.3390/electronics8020239

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Functional Composites of Discharge Plasmas and Solid Metamaterials Reviewed

Sakai O., Iwai A.

Springer Series in Materials Science 287 151 - 167 2019.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Springer Series in Materials Science

© Springer Nature Singapore Pte Ltd 2019. Discharge plasmas are composed of electrons and ions, and their permittivity is dynamic and tunable. Conventional metamaterials are composed of designed functional microstructures of solid materials, and become extraordinary wave media such as negative-permeability materials. The composites of the plasmas and the metamaterials are well mixed to show dynamic properties coming from plasmas and extraordinary outputs based on metamaterials. Here, we describe their theoretical basis and topical features observed in microwave experiments. Beyond properties of tunability, such composite “plasma metamaterials” work well as nonlinear and high-energy-carrier metamaterials, unlike conventional solid-state metamaterials.

DOI： 10.1007/978-981-13-8649-7_10

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Topological network properties of fractal-like metallic nanoparticle patterns and their effects on optical resonances Reviewed

Kihara N., Sakai O.

Applied Sciences (Switzerland) 8 ( 8 ) 2018.8

Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Sciences (Switzerland)

© 2018 by the authors. Fractal-like nanoparticle two-dimensional patterns forming in diffusion-limited aggregation show variant spatial patterns. However, they have invariant statistical properties in their network topologies, even though their formation is completely in self-assembled processes. One of the outputs from these topological properties is optical resonances at invariant frequencies, which is a required feature of a metamaterial alternative. Fractal-like metallic patterns studied here in both experiments and theoretical models exhibit similar resonance frequencies in the infrared-ray range, and they depend on the unit length of nanoparticles composing arbitrary fractal-like structures. The scheme of analysis applied here using complex network theory does not only reveal the topological properties of the nanoparticle network, but points out their optical and possibly other physical potentials arising from their geometrical properties.

DOI： 10.3390/app8081310

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Invariance of parameter identification in multiscales of meta-atoms in metamaterials Reviewed

Sakai O., Iwai A., Omura Y.

Advances in Physics: X 3 ( 1 ) 339 - 356 2018.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Advances in Physics: X

© 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. The concept of metamaterials has led to extraordinary schemes of wave propagation, which has been verified using various meta-atoms, constituent units of metamaterials, as well as applied to a number of categories in physics. Although its definition clarifies the maximum size of meta-atoms as a fraction of one wavelength, the size may vary by several orders, like from millimeters (‘macroscopic level’) to atomic scales (‘microscopic level’) for microwaves. This review surveys several patterns of parameter combinations, like permittivity and permeability in electromagnetic metamaterials, which have been achieved at either macroscopic or microscopic levels, with the similar analogy under the concept of metamaterials. Various experimental and theoretical efforts reported so far and shown here verify that the parameter identification of these values (permittivity, permeability, and refractive index) is independent of meta-atom sizes, with importance of spatial integration procedure on the order of a wavelength.

DOI： 10.1080/23746149.2018.1433551

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Spectroscopic ellipsometry analysis of perovskite manganite films for resistance switching devices

Masaki Yamada, Osamu Sakai, Toshihiro Nakamura

Thin Solid Films 571 ( P3 ) 597 - 600 2014.11

Publishing type：Research paper (international conference proceedings)

Pr0.5Ca0.5MnO3 (PCMO) films were deposited on LaAlO3 (100) substrates changing process pressure from 1.33 to 5.33 Pa by RF magnetron sputtering. Current-voltage characteristic measurements and spectroscopic ellipsometry measurements were carried out to investigate the mechanism of resistance switching in PCMO films. Resistance switching was observed in the devices composed of the PCMO films deposited at low pressures of 1.33 and 2.67 Pa. The deposition pressure dependence of the electronic structure of PCMO films was detected as a difference in dielectric functions by spectroscopic ellipsometry. Spectroscopic ellipsometry data indicated that the PCMO films exhibiting resistance switching had large oscillator strength of the electric dipole charge transition in (MnO6)9 - and (MnO6)8 - octahedral complexes, small oscillator strength of d-d transitions in Mn3 + and Mn4 + ions, and large high frequency dielectric constant. The formation of (MnO6)9 - and (MnO6)8 - octahedral complexes and oxygen vacancies might be required for obtaining large resistance switching.

DOI： 10.1016/j.tsf.2013.11.145

Localized electromagnetic distortion in 2D metal hole array and its application to imaging of permittivity distribution

G. Itami, T. Akiyama, O. Sakai, Y. Harada

2014 Asia-Pacific Microwave Conference Proceedings, APMC 2014 1253 - 1255 2014.3

Publishing type：Research paper (international conference proceedings)

We performed two-dimensional imaging of conductive and dielectric samples above a metal plate with a two-dimensional array of subwavelength holes (metal hole array) with electromagnetic distortion formed by inserting a needle-like conductive probe into holes in it. A metal hole array has a special electromagnetic response such as localization of electromagnetic waves at a specific frequency near it. Generally, the phenomenon described above is called 'spoof surface plasmon resonance.' And such special media can be regarded as 'metamaterial' from a point of view that it realizes a macroscopically uniform medium by the subwavelength-size holes in the metal plate. We conducted numerical analyses of electric field profiles in order to clarify how electric field distributions near the metamaterial change by the electromagnetic distortion, and confirmed the phenomenon that these uniform electric fields around the 'metamaterial' are distorted locally by using the needle-like conductive probe. Electromagnetic waves response to the permittivity at the space, so that it is considered that the change of the permittivity at the space is reflected as that of the electromagnetic distribution at the space. Based on the above, the formation of the electromagnetic distortion can be a detecting method of samples which have dielectric responses. Therefore, we consider that this two-dimensional imaging system is applicable to detecting something which has abnormal permittivity inside bulk material, such as a cancer in biomaterial tissues.

Control of work function of indium tin oxide: A surface treatment by atmospheric-pressure plasma layer on fabric-type electrodes

Yoshihiko Ueda, Junichi Abe, Hideyuki Murata, Yasuhito Gotoh, Osamu Sakai

Japanese Journal of Applied Physics 53 ( 3 SPEC. ISSUE 2 ) 2014.3

Publishing type：Research paper (scientific journal)

The surface electronic properties of metal oxides, which affect the performances of layered electronic devices, are controlled by non-thermal atmospheric-pressure plasmas generated using a fabric-type electrode and gas flow induced through this plasma layer. We specify a method to control the work function of indium tin oxide (ITO) films by this plasma, in which such reactive species as oxidant radicals are generated. These oxidants are spatially transported in the gas flow to the ITO surface and increase the ITO's work function, as verified in our experimental results. To examine the effects of the increased work function in a specific electronic device, we treat the surface of an ITO electrode in an organic lightemitting diode and observe the enhancement of the light-emitting efficiency. © 2014 The Japan Society of Applied Physics.

DOI： 10.7567/JJAP.53.03DG03

High-density microwave plasma source using negative-permeability metamaterial with tuned wave attenuation

Yoshihiro Nakamura, Osamu Sakai

Japanese Journal of Applied Physics 53 ( 3 SPEC. ISSUE 2 ) 2014.3

Publishing type：Research paper (scientific journal)

High-density plasma generated by waves with efficient wave power consumption is suitable for various dry processes. However, when we use microwaves as power sources, the electron density in a conventional microwave plasma source is limited within the range in which the permittivity of the plasma is positive. To overcome this drawback, a negative refractive index phenomenon is applicable, in which the permittivity and permeability are both negative and microwaves can propagate even in a negative-permittivity space with high electron density. In this report, we demonstrate a promising scheme for the efficient generation of high-density plasma by propagating microwaves in a negative-permeability space, and show the experimental observation of the enhancement of plasma generation using the lower imaginary part of the permeability. © 2014 The Japan Society of Applied Physics.

DOI： 10.7567/JJAP.53.03DB04

Correlation of resistance switching behaviors with dielectric functions of manganite films: A study by spectroscopic ellipsometry

Masaki Yamada, Toshihiro Nakamura, Osamu Sakai

Materials Research Society Symposium Proceedings 1633 2014

Publishing type：Research paper (international conference proceedings)

Pr0.5Ca0.5MnO3 (PCMO) films were deposited on LaAlO3 (100) substrates under pressure from 1.33 to 5.33 Pa by RF magnetron sputtering. Resistance switching and dielectric functions of PCMO films were studied by DC current-voltage characteristic measurements and spectroscopic ellipsometry (SE) measurements. Resistance switching was observed in the devices composed of PCMO films deposited under low pressures of 1.33 and 2.67 Pa. SE measurements revealed that dielectric functions also depended on deposition pressure. PCMO films deposited under lower pressure had larger high-frequency dielectric constant, larger oscillator strength of the electric dipole charge transitions in MnO6 octahedral complexes, and lower oscillator strength of d-d transitions in Mn3+ and Mn4+ ions. SE measurements suggested that oxygen vacancies and MnO6 octahedral complexes play an important role in resistance switching in PCMO films. Copyright © Materials Research Society 2014.

DOI： 10.1557/opl.2014.85

Multiheterodyne interference spectroscopy using a probing optical frequency comb and a reference single-frequency laser Reviewed

Keiichiro Urabe, Sakai Osamu

PHYSICAL REVIEW A 88 ( 2 ) 2013.8

Functional composites of plasmas and metamaterials: Flexible waveguides, and variable attenuators with controllable phase shift Reviewed

Sakai Osamu, Maeda Jun, Shimomura Takuya, Keiichiro Urabe

PHYSICS OF PLASMAS 20 ( 7 ) 2013.7

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