1. Department of Electrical and Computer Engineering, University of California San Diego, La Jolla, CA 92093
2. Department of Physics, Center for Advanced Nanoscience, University of California San Diego, La Jolla, CA 92093
3. Department of Physics and Astronomy, University of Denver, Denver, CO 80208
4. Departamento de Física, Facultad de Ciencias, Universidad de Chile, Santiago 7800024, Chile