1. Quantitative Light Imaging Laboratory; Department of Electrical and Computer Engineering; Beckman Institute of Advanced Science and Technology; University of Illinois at Urbana-Champaign; Urbana IL 61801 USA
2. Photonic Systems Laboratory, Department of Electrical and Computer Engineering, Micro and Nanotechnology Laboratory; University of Illinois at Urbana-Champaign; Urbana IL 61801 USA
3. Laser Biomedical Research Center; G. R. Harrison Spectroscopy Laboratory; Massachusetts Institute of Technology; Cambridge Massachusetts 02139 USA