1. Fachrichtung Physik, Universität des Saarlandes, Campus E2.6, 66123 Saarbücken, Germany
2. Technische Physik, Physikalisches Institut and Wilhelm-Conrad-Röntgen-Research Center for Complex Material Systems, Am Hubland, D-97074 Würzburg, Germany
3. State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, Sun Yat-Sen University, Guangzhou 510275, China
4. Institut für Halbleiteroptik und Funktionelle Grenzflächen (IHFG), Center for Integrated Quantum Science and Technology (IQST) and SCoPE, University of Stuttgart, 70569 Stuttgart, Germany
5. Kirchhoff-Institut for Physics, University of Heidelberg, 69120 Heidelberg, Germany
6. Institute of Physics, University of Münster, 48149 Münster, Germany
7. CNR-INO and European Laboratory for Non-Linear Spectroscopy (LENS), Via Nello Carrara 1, Sesto Fiorentino 50019, Italy