1. Integrated Optoelectronics and Microoptics Research Group, Physics Department, Technische Universität Kaiserslautern (TUK), PO Box 3049, D-67653 Kaiserslautern, Germany
2. Nanostructuring Center (NSC), Physics Department, Technische Universität Kaiserslautern (TUK), PO Box 3049, D-67653 Kaiserslautern, Germany