1. Department of Physics and Astronomy, University of Rochester, Rochester, New York 14620, USA
2. Center for Coherence and Quantum Optics, University of Rochester, Rochester, New York 14620, USA
3. Institute of Optics, University of Rochester, Rochester, New York 14620, USA
4. Materials Science, University of Rochester, Rochester, New York 14620, USA
5. Solid State Physics Laboratory, ETH Zurich, 8093 Zurich, Switzerland