1. Center for Nanotechnology and Molecular Materials, Department of Physics, Wake Forest University, Winston-Salem, North Carolina 27109, Instituto de Investigación en Comunicación Optica, Universidad Autonoma de San Luis Potosí, Alvaro Obregon 64, San Luis Potosí, Mexico, IPCYT Advanced Materials Department, Department of Physics, San Luis Potosí, Mexico, and New Mexico State University, Las Cruces, New Mexico