1. Physics and Engineering Sciences Laboratory, University Institute of the Coast, Douala P.O. Box 3001, Cameroon
2. Research Unit Condensed Matter, Electronics and Signal Processing, University of Dschang, Dschang P.O. Box 67, Cameroon
3. Center for Nonlinear Mechanics, COPPE—Department of Mechanical Engineering, Universidade Federal do Rio de Janeiro, Rio de Janeiro P.O. Box 68.503, Brazil