1. Department of Physics and Optical Science, and Center for Optoelectronics and Optical Communications, The University of North Carolina at Charlotte, 9201 University City Boulevard, Charlotte, North Carolina 28223, USA
2. Department of Electrical and Computer Engineering, and Center for Optoelectronics and Optical Communications, The University of North Carolina at Charlotte, 9201 University City Boulevard, Charlotte, North Carolina 28223, USA