1. Laboratory for Nanophotonics, Department of Physics and Astronomy, M.S. 61, Rice University, Houston, Texas 77005-1892, Experimental Solid State Group, Physics Department, Imperial College, London SW7 2AZ, United Kingdom, IMEC, Kapeldreef 75, 3001 Leuven, Belgium, Department of Electrical and Computer Engineering, M.S. 366, Rice University, Houston, Texas 77005-1892, Department of Chemistry, M.S. 60, Rice University, Houston, Texas 77005-1892