1. T. J. Watson Laboratory of Applied Physics and Kavli Nanoscience Institute, California Institute of Technology, 1200 E. California Boulevard, Pasadena, California 91125, United States
2. Department of Electrical Engineering, California Institute of Technology, 1200 E. California Boulevard, Pasadena, California 91125, United States
3. Department of Electrical and Computer Engineering, University of Massachusetts Amherst, 151 Holdsworth Way, Amherst, Massachusetts 01003, United States