1. CREOL/College
of Optics, University of Central Florida, Orlando, Florida 32816, United States
2. Department
of Physics and Astronomy, San Francisco State University, San Francisco, California 94132, United States
3. Currently
with NSF Nanoscale Science and Engineering Center, University of California, Berkeley, California 94720, United States
4. TEDA
Applied Physics Institute and School of Physics, Nankai University, Tianjin 300457, China