1. The Blackett Laboratory, Department of Physics, Imperial College London, London SW72AZ, UK.
2. Photonic Centre of Excellence (OPTIMUS), School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue 639798, Singapore.
3. National Science Foundation Nanoscale Science and Engineering Center, 3112 Etcheverry Hall, University of California, Berkeley, CA 94720, USA.