1. Institute of Experimental Physics I, Justus-Liebig University, Heinrich-Buff-Ring 16, 35392 Giessen, Germany
2. Institute of Physical Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, 35392 Giessen, Germany
3. Research Center for Integrated Quantum Electronics, Hokkaido University, North 13, West 8, Sapporo 060-8628, Japan