1. Department
of Physics and I3N, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
2. Optical
Materials Engineering Laboratory, ETH Zurich, 8092 Zurich, Switzerland
3. Walter
Schottky Institut and Physik-Department, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany