1. Department of Electrical and Computer Engineering; National University of Singapore; 4 Engineering Drive 3 Singapore 117583 Singapore
2. Institute of Materials Research and Engineering; Agency for Science Technology and Research (A*STAR); 2 Fusionopolis Way, Innovis, #08-03 Singapore 138634 Singapore
3. Department of Physics; National University of Singapore; 2 Science Drive 3 Singapore 117542 Singapore