1. Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Trieste 34149, Italy
2. Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, Sofia 1784, Bulgaria
3. Optics Lab, The Abdus Salam International Centre for Theoretical Physics, Trieste 34151, Italy
4. Laboratoire de Physique des Composants à Semi-conducteurs (LPCS), Université de Lomé, 01 BP 1515 Lomé, Togo
5. “LOTIS TII” Ltd., 17, Oktyabrskaya Str., Minsk 220030, Belarus
6. Elettra-Sincrotrone, S.C.P.A, Trieste 34149, Italy
7. Mathematics and Informatics Department, Udine University, Udine 33100, Italy