1. Department of Physics; National University of Singapore, 2 Science Drive 3, Singapore, 117542, Singapore; Department of Chemistry, National University of Singapore; 3 Science Drive 3 Singapore 117543 Singapore
2. Department of Physics; National University of Singapore, 2 Science Drive 3, Singapore, 117542, Singapore;; Shanghai Institute of Applied Physics, Chinese Academy of Sciences; P. O. Box 800-204 Shanghai 201800 P. R. China
3. Department of Physics; National University of Singapore; 2 Science Drive 3 Singapore 117542 Singapore
4. Department of Physics; La Trobe University; Victoria Australia 3086