1. Department of Physics, Santa Clara University, 500 El Camino Real, Santa Clara, California 95053-0315, USA
2. Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Onna-son, Kunigami, Okinawa 904-495, Japan
3. Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, China
4. Department of Physics and Engineering Physics, Tulane University, New Orleans, Louisiana 70118, USA
5. Max Planck Institute for Solid State Research, Heisenbergstasse 1, 70569 Stuttgart, Germany
6. Department of Chemistry, Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13, 81377 München, Germany
7. Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany