1. Mechanical
Engineering Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
2. IBM T.J. Watson
Research Center, 1101 Kitchawan Road, Yorktown Heights, New York 10598, United States
3. Department of Electrical & Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States
4. Department
of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China