1. Department of Physics and Material Sciences Center, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, Germany
2. NAsPIII/V GmbH, Am Knechtacker 19, 35041 Marburg, Germany
3. Nonlinear Control Strategies Inc, 3542 N. Geronimo Ave., Tucson, AZ 85705, USA