1. Department of Physics and Optical Science, Center for Optoelectronics and Optical Communications, University of North Carolina at Charlotte, Charlotte, North Carolina 28223-0001, USA
2. Air Force Research Laboratory, Sensors Directorate, Wright Patterson AFB, Ohio 45433, USA
3. Institut d'Electronique Fondamentale, UMR 8622 CNRS, Universit′e Paris-Sud XI, 91405 Orsay, France