1. Department of Physics , Faculty of Science, University of Kordofan , El-Obeid 51111 , Sudan
2. Beijing Key Lab of Nanophotonics and Ultrafine Optoelectronic Systems and School of Physics, Beijing Institute of Technology , Beijing 100081 , China
3. Department of Physics , College of Science, Sudan University of Science and Technology , Khartoum 11113 , Sudan
4. Department of Physics , College of Science, Imam Mohammad Ibn Saud Islamic University (IMSIU) , Riyadh 11623 , Saudi Arabia