POLAR OPTICAL MODES IN GAAS-ALAS SUPERLATTICES

The normal modes of GaAs-AlAs superlattices are studied analytically b y assuming that each constituent slab is perfectly rigid for the mecha nical vibrations with the normal frequencies of the other one. This, w hich is a very good approximation for GaAs-AlAs, effects a considerabl e simplification and allows for an analytical study in terms of explic it basis functions which incorporate the symmetries of different modes . An application is made to a 12-12 superlattice. The results display the type of anisotropy of the modes near the zone centre found in micr o-Raman experiments.