[Investigation on the spectrum of complex self-organized plasma photonic crystal]

A complex plasma photonic crystal (PPC) was obtained by self-organization of filaments in air dielectric barrier discharge using two planar water electrodes. The PPC structure consists of many square sublattices, and each sublattice is composed of large spots, two kinds of small spots and lines, corresponding to thick plasma columns, two kinds of thin plasma columns, and plasma slices, respectively. By using the optical emission spectrum method, the electron densities and molecular vibration temperatures at different positions of the PPC were studied. The electron densities were compared by comparing the broadenings of Ar I (2P2-->1S5) spectrum line, and the molecular vibration temperatures were calculated by the spectrum line of nitrogen band of second positive system (C3Πu-->B3Πg) . It was found that the electron densities and molecular vibration temperatures at different positions are both different, showing that the plasma states at different positions are different. The descending order of the electron density is: thin plasma columns around the thick plasma columns, thick plasma columns, plasma slices, and thin plasma columns at junction of plasma slices. The descending order of the molecular vibration temperature is: thin plasma columns at junction of plasma slices, plasma slices, thick plasma columns, and thin plasma columns around the thick plasma columns, which is opposite to that of the electron density. So, the electron densities and the molecular vibration temperatures in different positions of the PPC show the opposite changing trend. As the refractive index of plasma is dependent upon the electron density, the thick plasma columns, two kinds of thin plasma columns and plasma slices in this PPC have different refractive indexes. Together with the surrounding area where no discharges occur, in which the refractive index is also different from the discharging areas, the complex PPC can be seen as a self-organized periodic structure with five different refractive indexes. The PPC has the advantages of being obtained easily, having structural diversity, and being analyzed simply, which may lead to wide applications in many scientific and technical areas.