[1] Univ Basel, Dept Phys, CH-4056 Basel, Switzerland
[2] Univ Tubingen, Inst Phys, D-72076 Tubingen, Germany
[3] Univ Tubingen, Ctr Collect Quantum Phenomena LISA, D-72076 Tubingen, Germany
[4] Ecole Polytech Fed Lausanne, Lab Mat Semicond, Inst Mat, CH-1015 Lausanne, Switzerland
[5] Phys Tech Bundesanstalt, Fachbereich Quantenelekt 2 4, D-38116 Braunschweig, Germany
[6] Tech Univ Munich, Lehrstuhl Phys Funktionaler Schichtsyst, Phys Dept E10, D-85747 Garching, Germany
[7] Ecole Polytech Fed Lausanne, Inst Mat, Fac Sci & Tech Ingenieur, CH-1015 Lausanne, Switzerland