1. Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, Michigan 48109, USA
2. Naval Research Laboratory, Plasma Physics Division, 4555 Overlook Ave. SW, Washington, DC 20375, USA
3. Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA
4. Physics Department, Lancaster University, Bailrigg, Lancaster LA1 4YW, United Kingdom