1. Institute of Electronic Structure and Laser, Foundation for Research & Technology—Hellas, (IESL-FORTH), P.O. Box 1385, Heraklion 711 10, Greece.
2. Physics Department, University of Crete, Heraklion 714 09, Greece.
3. Lawrence Berkeley National Laboratory, Berkeley, California 94720
4. Materials Science and Technology Department, University of Crete, Heraklion 710 03, Greece