1. Center for Bio/Molecular Science and Engineering Code 6900, U.S. Naval Research Laboratory, Washington, D.C., 20375, USA
2. Optical Sciences Division Code 5600, U.S. Naval Research Laboratory, Washington, D.C., 20375, USA
3. National Research Council, Washington, D.C., 20001, USA
4. College of Science, George Mason University, Fairfax, Virginia, 22030, USA