1. Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany
2. Columbia Astrophysics Laboratory, Columbia University, New York, New York 10027, USA
3. Dilas Diode Laser, Inc., Tucson, Arizona 85715, USA
4. Institute of Condensed Matter and Nanosciences, Université Catholique de Louvain, Louvain-la-Neuve B-1348, Belgium