1. Key Laboratory of Transparent Opto-functional Inorganic Materials; Shanghai Institute of Ceramics; Chinese Academy of Sciences; Shanghai 200050 China
2. Department of Physics; The University of Texas at Arlington; Arlington Texas 76019
3. Department of Life and Physical Sciences; Fisk University; Nashville Tennessee 37208-3051
4. Department of Electrical Engineering; The University of Texas at Arlington; Arlington Texas 76019