[1] Eötvös University, Dept. of Solid State Physics, Múzeum krt. 6-8, H-1088 Budapest, Hungary
[2] Res. Inst. Solid State Phys. Optics, Hungarian Academy of Sciences, P.O.B. 49, H-1525 Budapest, Hungary
[3] Dept. of Mech. Engineering Tchnology, Technical University of Budapest, Bertalan L. utca 2, H-1111 Budapest, Hungary