[1] Queens Univ Belfast, Dept Phys & Astron, Belfast BT7 1NN, Antrim, North Ireland
[2] Helmholtz Inst Jena, Frobelstieg 3, D-07743 Jena, Germany
[3] Friedrich Schiller Univ Jena, Inst Opt & Quantenelekt, Max Wien Pl 1, D-07743 Jena, Germany
[4] SLAC Natl Accelerator Lab, 2575 Sand Hill Rd, Menlo Pk, CA 94025 USA