Optical mode mixing in an isotropic elastic layer

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Abstract
Elastic boundary conditions are shown to emerge naturally from a simple unifying theory describing long-wavelength excitations in bulk semiconductors. Their application to the cases of a freely vibrating layer and to a layer enclosed by an infinitely rigid medium (roughly approximated by a GaAs layer in vacuum and by a GaAs/AlAs quantum well respectively) shows that the disappearance of relevant dilational and shear stresses in the first case and the inhibition of displacement in the second 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, as distinct from hydrodynamic, boundary conditions brings the continuum model much closer to the predictions of microscopic theory.