1. Department of Physics, Faculty of Science, University of Douala, Douala, Cameroon
2. Faculty of Science, University of Maroua, Maroua, Cameroon
3. Department of Mathematics, Scottish Church College, Kolkata, India
4. Department of Physics and Astronomy, University College London, UK
5. Laboratoire Ondes et Milieux Complexes, UMR 6294 CNRS and Université du Havre, Le Havre, France