Plasma metamaterials for novel interaction between plasmas and waves

Summary form only given. Metamaterials, which are composed of small periodic microstructures, are functional media with extraordinary properties for electromagnetic waves. When a metamaterial includes plasmas in its main part, which we refer to as a “plasma metamaterial,”¹ it can provide a novel field where plasmas and waves interact in a unique way that cannot be predicted in normal media, and plasma metamaterials can stimulates new scientific and technological interest. Furthermore, with topics on optical functional properties of plasmas, they can form a part of the new area “plasma photonics”² which widely covers increasing potential topics. One of the specific examples of plasma metamaterials is described in the following. When we install a negative-permeability material such as an array of double split ring resonators in a reactor space,¹ the microwave with low energy density cannot propagate since refractive index is imaginary. However, if the microwave energy is high enough to generate an overdense plasma with negative permittivity, the microwave can propagate due to transformation of the refractive index from imaginary to real (negative) values. Furthermore, this evolution process is quite nonlinear with two saddle-node bifurcations.^1,3 That is, if we consider them as media of electromagnetic waves, they will show strong nonlinear properties with memory effects. From the point of view of new plasma sources, they will provide overdense plasmas for high throughput processing reactors. The frequencies of plasma metamaterials are in microwaves, THz waves, may be in infrared rays, and even can be in visible lights according to the corresponding electron densities of the plasmas. A wide variety of studies related to processing plasmas, fusion plasmas and space plasmas will be activated by the concepts of plasma metamaterials.