[1] Tech Univ Kaiserslautern, Fachbereich Phys, D-67663 Kaiserslautern, Germany
[2] Tech Univ Kaiserslautern, Forschungszentrum OPTIMAS, D-67663 Kaiserslautern, Germany
[3] Taras Shevchenko Natl Univ Kyiv, Fac Radiophys, Kiev, Ukraine
[4] Oakland Univ, Dept Phys, Rochester, MI 48309 USA