1. Department of Physics, Indian Institute of Technology, Guwahati 781039, Assam, India
2. Department of Physics, University of Puerto Rico, San Juan, Puerto Rico 00936-8377, USA
3. IFW Dresden, Institute for Complex Materials, P.O. Box 270116, D-01171 Dresden, Germany
4. Department of Photonics and Display Institute, National Chiao Tung University, Hsinchu 30010, Taiwan
5. Department of Engineering Sciences, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden