1. Department of Physics, University of Maryland, College Park, Maryland 20742, USA
2. Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742, USA
3. Department of Electrical Engineering, University of Maryland, College Park, Maryland 20742, USA
4. Plasma Physics Division, Naval Research Laboratory, 4555 Overlook Ave., SW, Washington, District of Columbia 20375, USA