1. Laboratoire de Physique Appliquée, Faculté des Sciences, Université de Sfax, B.P. 1171, 3000 Sfax, Tunisie
2. University of Coimbra, CFisUC, Physics Department, Rua Larga, P-3004-516 Coimbra, Portugal
3. I3N and Physics Department, University of Aveiro, 3810-193, Aveiro, Portugal