[1] Oklahoma State Univ, Sch Elect & Comp Engn, Stillwater, OK 74078 USA
[2] Chinese Acad Sci, Inst Phys, Beijing Natl Lab Condensed Matter Phys, Beijing 100080, Peoples R China
[3] Univ Jena, Inst Condensed Matter Theory & Solid State Opt, Abbe Ctr Photon, D-07743 Jena, Germany