1. Laboratory of Mechanics, Materials and Structures, Faculty of Science, Department of Physics , 107751 University of Yaounde 1 , P.O. Box 812 Yaounde , Cameroon
2. Laboratory of Condensed Matter, Electronics and Signal Processing (LAMACETS), Department of Physic, Faculty of Sciences , 107819 University of Dschang , P.O. Box 67 , Dschang , Cameroon