1. Department of Physics and Research Center OPTIMAS , Technische Universität Kaiserslautern , Erwin-Schrödinger-Strasse 46, 67663 Kaiserslautern , Germany
2. Institut für Nanophotonik Göttingen e.V. , Hans-Adolf-Krebs-Weg 1, 37077 Göttingen , Germany
3. Quantum Electronics Division , Lebedev Physical Institute , 119991 Moscow , Russia
4. Theoretical Physics Department , University of Kassel , 34132 Kassel , Germany