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Inhomogeneities

ω + χ ω
0
ω
Figure 7.11. Polariton dispersion 0
relation showing upper and lower k
branch.

polarization also due to the electronic states in the valence band and the
conduction band, the so called interband polarization. We may expect
that also this polarization couples to the light field in a similar way and
indeed the description is very much the same despite marginal differ-
ences.

7.6 Inhomogeneities

Inhomogeneities arise where material properties change abruptly. This
may be the interface region between two materials or even surfaces. We
also count lattice defects for these inhomogeneities. Those are of rather
localized character and often randomly distributed. The dimensionality
of such inhomogeneities is lower than that of the 3D bulk system. In fact
an interface is 2D and an impurity even 0D point-like. Nevertheless these
low dimensional objects indeed have influence on the bulk behavior of all
phenomena and quasi particles treated up to now. Moreover, some of
them might even be described by the quasi-particle formalism in lower
dimensions, e.g., surface phonons.

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