1. NEST, Scuola Normale Superiore and Istituto Nanoscienze—CNR, Piazza
S. Silvestro 12, I-56127 Pisa, Italy
2. Istituto Officina dei Materiali—CNR, Basovizza S.S. 14 km 163.5,
I-34149 Trieste, Italy
3. Optoelectronics
Research Centre, Tampere University of Technology, P.O. Box 692, FIN-33101
Tampere, Finland