1. Nonlinear Physics Centre; Research School of Physics and Engineering; The Australian National University; Canberra ACT 0200 Australia
2. Institute of Applied Physics; Abbe Center of Photonics; Friedrich-Schiller-Universität Jena; 07743 Jena Germany
3. Center for Integrated Nanotechnologies; Sandia National Laboratories; Albuquerque NM 87185 USA