[1] Vienna Univ Technol, Photon Inst, Gusshausstr 27-387, A-1040 Vienna, Austria
[2] Moscow MV Lomonosov State Univ, Dept Phys, Ctr Int Laser, Moscow 119992, Russia
[3] Texas A&M Univ, Dept Phys & Astron, College Stn, TX 77843 USA
[4] Heriot Watt Univ, Inst Photon & Quantum Sci, Edinburgh EH14 4AS, Midlothian, Scotland
[5] Univ Jena, Inst Opt & Quantum Elect, D-07743 Jena, Germany