1. Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125, USA
2. Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, California 91125, USA
3. Department of Physics, University of Crete, GR-70 013 Heraklion, Greece
4. NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA