1. Physics Department and Center for Optoelectronics and Photonics Paderborn (CeOPP), Paderborn University, Warburger Straße 100, 33098 Paderborn, Germany
2. 3. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany
3. Electrical Engineering Department, Paderborn University, Warburger Straße 100, 33098 Paderborn, Germany