1. Materials Science and Engineering, University of California, 9500 Gilman Drive, La Jolla, San Diego, California 92093, USA
2. Department of Electrical and Computer Engineering, University of California, 9500 Gilman Drive, La Jolla, San Diego, California 92093, USA
3. College of Optical Sciences, The University of Arizona, 1630 E University Blvd., Tucson, Arizona 85719, USA