1. Research Center for Development of Far-Infrared
Region, University of Fukui, Fukui 910-8507,
Japan
2. Institute of Applied Physics of the Russian
Academy of Sciences, Nizhny Novgorod 603950,
Russia
3. O. Ya. Usikov Institute of Radiophysics and
Electronics, National Academy of Sciences of Ukraine, Kharkov 61000,
Ukraine
4. Institute of Electronics of the Bulgarian Academy
of Sciences, Sofia 1784, Bulgaria