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Two-dimensional arrays of microplasmas form “plasma photonic crystals”, which have new functions that have not been present in bulk plasma. Two features of abnormal electromagnetic wave propagation on the first photonic band above the electron plasma frequency are demonstrated. Near the electron plasma frequency, unidirectional propagation takes place, and divergent propagation occurs near the band gap in the Γ-X direction. These phenomena are verified in experiments, and the details are confirmed by equi-frequency contours on a two-dimensional wavenumber plane. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Functional arrays of microplasmas were designed for the purpose of controlling electromagnetic waves in the range 10–100 GHz. A two-dimensional square-lattice array of microplasma columns was proved to have photonic-crystal-like properties showing the presence of photonic band gaps and flat bands. A one-dimensional linear array of cold cathode fluorescent lamps (CCFLs) embedded in between the truncated strip lines showed the resonant transmission characteristics due to the surface-wave modes corresponding to the flat-band frequencies. Another one-dimensional configuration of CCFLs vertically aligned along both sides of a coplanar strip line showed peculiar behavior at the lower and higher edges of the band gap. These unique wave-propagation properties are attributed to the periodic structures of which pitches are comparable to or smaller than the wavelengths of electromagnetic waves. Therefore, these artificial arrays are potential metamaterials, which can be used for plasma devices controlling electromagnetic waves.

Propagation of electromagnetic waves in several types of microplasmas has been examined experimentally in a frequency range 10?75?GHz. Firstly, the fundamental characteristics of the propagation were investigated using a planar geometry of microplasma assembly, and the electron density was derived by a comparison of the transmittance with the theoretical analyses using a Drude type model with collisional effects. Secondly, an extraordinary propagation phenomenon such as the focusing effect was observed in a two-dimensional periodical microplasma array. This kind of anomalous refraction cannot be interpreted only by predictions based on the dielectric property of bulk plasma, and it is suggested that a photonic-crystal-like periodical dielectric structure may play a significant role. Thirdly, it was demonstrated that the T-junction formed by a microplasma connected to a microstrip line can control the transmission of microwaves. An attenuation (or modulation) depth of about 35% was obtained with a series of two T-junctions connected to the strip line at the right-angled corners. All the above features come from (a) the relatively high electron density of the microplasmas near 1013?cm?3, (b) the complex dispersion relation with collisional effects and (c) the spatial arrangement with a characteristic scale of the same order of the wavelength of microwaves.

Advances in high-efficiency, large-area, surface-emitting photonic-crystal lasers have been developed to achieve peak emission powers of 35W with a slope efficiency of 0.7W/A for a total device area of ∼0.4mm2.

We experimentally verified that a microplasma assembly can create a functional dielectric layer for the propagation of electromagnetic waves as a “plasma photonic crystal.” A two-dimensional array in a square lattice was composed of columnar plasmas of about 2mm in diameter, and the transmitted microwaves at 70–75GHz showed a change of energy flow direction. This result is attributed to the fact that periodical structure is composed of individual plasma columns with a different dispersion than the ambient part and the experimental frequency range lies in the vicinity of the lowest band gap of the photonic crystal calculated theoretically.

Plasma photonic band gaps have been observed in a two-dimensional microplasma array, and we have characterized their properties by both experimental and theoretical results. Microplasma columns ignited in helium near atmospheric pressure formed crystal-like structures in a square lattice with a lattice constant from 1.5to2.5mm. Microwaves in the millimeter range transmitting through the array region attenuated at frequencies of photonic band gap in the Γ‐X direction, as predicted by the modified plane-wave expansion method. Frequency dependence around the band gap was clarified in the numerical analysis of electromagnetic wave propagation and agreed with experimental results. Electron density in microplasmas was estimated to be 1×1013cm−3 from the attenuation rate at the band gap in the Γ‐X direction. Variation of the lattice constant induced frequency shift of the band gap in the millimeter and subterahertz regions, and so plasma photonic crystal can perform as a dynamically controllable band-stop filter.