1. Institute of Fundamental and Frontier Sciences; University of Electronic Science and Technology of China; Chengdu 610054 P. R. China
2. Centre Énergie Matériaux et Télécommunications; Institut National de la Recherche Scientifique; 1650 Boul. Lionel Boulet Varennes QC J3X 1S2 Canada
3. Department of Physics and Astronomy; Ohio University; Athens OH 45701 USA