1. Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, Massachusetts 02139, USA
2. Photon Science Institute and School of Electrical and Electronic Engineering, University of Manchester, Manchester M13 9PL, United Kingdom
3. Ion Beam Centre, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom
4. Dipartimento di Fisica e Astronomia, Università di Padova and CNR-IMM, Via, Marzolo 8, Padova I-35131, Italy
5. Harvard John A. Paulson School of Engineering and Applied Sciences, Cambridge, Massachusetts 02138, USA
6. Microphotonics Center, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, Massachusetts 02139, USA