Experimental observation of electron density bifurcation in plasma-metamaterial composites in microwave range

Metamaterials, which are composed of designed microstructures and show extraordinary electromagnetic responses, match plasmas so well, and high-power microwaves induce bifurcation phenomena in this plasma-metamaterial composite. Since dielectric constant or permittivity of plasmas varies from positive to negative values at microwave frequencies, the composite with negative permeability becomes a reconfigurable negative refractive index material [1]. Furthermore, as indicated by our recent report [2,3], this composite shows strong nonlinear properties. Bifurcation of permittivity (or electron density) was predicted by a theory [2], and we have verified it in our recent experiments; using double split ring resonators whose array showed negative permeability at 2.45 GHz, clear bifurcation with hysteresis was observed in electron density evolutions with input power \textless 300 W. This result implies that this composite is a nonlinear microwave metamaterial.\\[4pt] [1] O. Sakai et al., Plasma Sources Sci. Technol. 13, 013001 (2013).\\[0pt] [2] O. Sakai, Journal of Applied Physics 109, 084914 (2011).\\[0pt] [3] Y. Nakamura and O. Sakai, Jpn. J. Appl. Phys. 53, 03DB04 (2014).