1. School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, Singapore 639798; OPTIMUS, Centre for OptoElectronics and Biophotonics, Nanyang Technological University, 50 Nanyang Avenue, Singapore, Singapore 639798; and LUMINOUS! Centre of Excellence for Semiconductor Lighting and Displays, Nanyang Technological University, 50 Nanyang Avenue, Singapore, Singapore 639798