OPTICAL-MODE MIXING IN AN ISOTROPIC ELASTIC LAYER

Elastic boundary conditions are shown to emerge naturally from a simpl e unifying theory describing long-wavelength excitations in bulk semic onductors. Their application to the cases of a freely vibrating layer and to a layer enclosed by an infinitely rigid medium (roughly approxi mated by a GaAs layer in vacuum and by a GaAs/AlAs quantum well respec tively) shows that the disappearance of relevant dilational and shear stresses in the first case and the inhibition of displacement in the s econd case can only be obtained by s-polarized TO modes without mixing -LO and p-polarized TO modes are forced to mix coherently. It is also necessary for Fuchs-Kliewer interface polaritons to mix with LO modes in order to satisfy elastic boundary conditions. The use of elastic, a s distinct from hydrodynamic, boundary conditions brings the continuum model much closer to the predictions of microscopic theory.