Radiative and non-radiative polariton structure of superlattices

Abstract

Optical properties of multilayered materials, including synthesized superlattices received special attention in the last few years. To some extent, reflectance and absorption of these materials are adjustable: the polariton structure depends sensitively on the layer geometry.Though the approach considered here is applicable to any stratified structure, we emphasize on polaritons of semi-infinite superlattices, that is superlattices which exhibit a free surface. In these thin-film structures, the accumulation of interfaces gives rise to peculiar electromagnetic eigenmodes, distributed as continuous frequency bands. The termination of the superlattice at the surface modifies the density of these modes, as compared with the mode density of an infinite, truly periodic superlattice. In particular, one notices the appearance of isolated branches analogous to surface polariton modes of semi-infinite homogeneous materials.We approach the description of radiative and non-radiative modes, including finite life-time effects, from the point of view of scattering theory, using a Green's function technique. The local density of states of the polariton modes is calculated and provides a complete information on allowed electromagnetic excitations, as a function of wavelength, at any depth in the superlattice. These results will be used to discuss reflectance and attenuated total reflection experiments on superlattices.