1. Centre for Biomedical Materials Applications and Technology (BIOMAT), Department of Mechanical Engineering, National University of Singapore, 10 Kent Ridge Crescent, 119260 Singapore, Optical Materials and Systems Division, A*STAR Data Storage Institute (DSI), 5 Engineering Drive 1, 117608 Singapore, and Department of Electrical and Computer Engineering, National University of Singapore, 10 Kent Ridge Crescent, 119260 Singapore