29aA02
プラズマ・メタマテリアルによる機能性電磁波媒質の実現
Plasma Metamaterials: Functional Media for Electromagnetic Waves
酒井 道
Osamu Sakai
京都大学大学院 工学研究科 電子工学専攻
Department of Electronic Science and Engineering, Kyoto University
Metamaterials, whose concept was proposed
about 10 years ago, exhibit extraordinary properties
by their spatially periodic micro structure; theproperties such as perfect lens and cloaking [1, 2]
cannot be achieved using conventional bulk
materials with good microscopic homogeneity.
When we make composites of plasmas and such
metamaterials that are usually made of metallic
and/or dielectric materials, we can expect furtherfunctions which are beyond conventional materials,plasmas themselves, and the ordinary metamaterials,
and we call them “plasma metamaterials” hereafter[3], as shown in Fig. 1 as a conceptual view.
In the plasma metamaterials, embedded plasmas
play several important roles. First, tunability of
on/off states is significant in comparison with othersolid-state materials, and a pattern inside an array of
microplasmas is variable. This indicates that we canobtain materials with variable lattice constant. Inaddition to this simple feature, we can mention
further advantages of plasma metamaterials asdescribed below.
For instance, dielectric constant is quite uniqueincomparison
withothermaterials.
Non-magnetizedplasmashavecomplexpermittivity  whose real part is both positive
and negative and whose imaginary part is adjustable.
The electron plasma frequency depends on electron
density that varies with input electric power, andthe resultant change in Re   leads to phase shiftof propagating waves and to achievement of anegative permittivity state, which is favorable inmetamaterials with negative refractive index. Theelectron elastic collision frequency is functions ofgas species and pressure, and the resultant changein Im  affects attenuation of the waves.
Another highly potential feature of the embedded
plasmas is nonlinearity. As mentioned earlier, theelectron plasma frequency is a key factor for  anddepends on electron density. Electron density is as afunction of electric fields for plasma generation;
ionization coefficient has a nonlinear function on
reduced electric fields. That is, plasmas are fairly
nonlinear, and many kinds of such researches as
nonlinear wave propagation have been studied so far.
Nowadays nonlinear metamaterials have attracted
significant attention, and plasmas are very suitable forthis role.
Briefly, we summarized our recent relevant

Figure 1. Conceptual view of plasma metamaterials.

studies in the following. We verified several
schemes of plasma metamaterials in experiments,including
dynamic
negative-refractive-index
materials [4, 5], tunable supporters for spoof
surface plasmon polaritons [6], and chain-likestructures of localized-surface-plasmon-like modes[7, 8] as well as simple plasma photonic crystals [7,9, 10]. Recently, we predicted saddle-nodebifurcation of nonlinear plasma metamaterials withnegative permeability  [11]. Our recent studies
also include identification of a “defect” effect onmetamaterial functions and its application to
imaging of dielectric constant for detection ofirregular tissues for medical application [12].

References[1]J. B. Pendry, “Negative refraction makes aperfect lens,” Physical Review Letters, vol. 85,
p.3966, 2000.
[2]
J. B. Pendry, D. Schurig, and D. R. Smith,“Controlling electromagnetic fields,” Science, Vol.
312, p.1780, 2006.
[3]
O. Sakai and K. Tachibana, “Plasmas asmetamaterials: a review,” Plasma Sources Scienceand Technology, vol. 21, p.013001, 2012.
[4]
O. Sakai, T. Shimomura and K. Tachibana,“Negative refractive index designed in a periodic
compositeoflossy
microplasmas
andmicroresonators,” Physics of Plasmas, vol. 17,p.123504, 2010.
[5]
O. Sakai, T. Naito, T. Shimomura and K.
Tachibana, “Microplasma array with metamaterialeffects,” Thin Solid Films, vol. 518, p.3444, 2010.
[6]D.-S. Lee, O. Sakai and K. Tachibana,“Microplasma-induceddeformationofananomalous response spectrum of electromagnetic
waves propagating along periodically perforated
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[7]
O. Sakai, T. Sakaguchi, T. Naito, D.-S.
Lee and K. Tachibana, “Characteristics ofmetamaterials composed of microplasma arrays,”
Plasma Physics and Controlled Fusion, vol. 49,
p.B453, 2007.
[8]
O. Sakai, T. Naito and K. Tachibana,“Microplasma array serving as photonic crystals
and plasmon chains,” Plasma and Fusion Research,
vol. 4, p.052, 2009.
[9]
O. Sakai, T. Sakaguchi and K. Tachibana,“Photonic bands in two-dimensional microplasma
arrays. I. Theoretical derivation of band structuresof electromagnetic waves,” Journal of Applied
Physics, vol. 101, p.073304, 2007.
[10]
T. Sakaguchi, O. Sakai and K. Tachibana,“Photonic bands in two-dimensional microplasma
arrays. II. Band gaps observed in millimeter andsub-terahertz ranges,” Journal of Applied Physics,
vol. 101, p.073305, 2007.
[11]
O. Sakai, “Transition between positive
and negative permittivity in field-dependent
metamaterial,” Journal of Applied Physics, vol. 109,
p.084914, 2011.
[12] O. Sakai and Y. Harada, “Dielectric constantimaging by millimeter-wave metamaterial withscanning noninvasive probe,” 14th International
Congress of Histochemistry and Cytochemistry (Kyoto,Japan, August 26-29, 2012) (Abstracts, p. 127).