[1] Rice Univ, Dept Phys & Astron, Lab Nanophoton, Houston, TX 77005 USA
[2] Univ London Imperial Coll Sci Technol & Med, Dept Phys, Expt Solid State Grp, London SW7 2AZ, England
[3] IMEC, B-3001 Leuven, Belgium
[4] Rice Univ, Dept Elect & Comp Engn, Houston, TX 77005 USA
[5] Rice Univ, Dept Chem, Houston, TX 77005 USA