1. Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA
2. Department of Physics, Stanford University, Stanford, California 94305, USA
3. Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
4. SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
5. Department of Applied Physics, Stanford University, Stanford, California 94305, USA
6. Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794, USA