1. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
2. Materials Research Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
3. Department of Mathematics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
4. Department of Mechanical Engineering, Technical University of Denmark, Nils Koppels Allé, Building 404, 2800 Kongens Lyngby, Denmark
5. NanoPhoton—Center for Nanophotonics, Technical University of Denmark, Ørsteds Plads 345A, DK-2800 Kgs. Lyngby, Denmark