Polaritons are the mixed modes that result from the interaction between material excitations (phonons, excitons, $\ldots$) and the retarded electromagnetic field. The semiclassical theory of polaritons was developed in the fifties by Huang [1] and Born and Huang [2] in the context of long-wavelength lattice vibrations in ionic crystals. Exciton-polaritons and their quantum-mechanical theory were introduced by Fano [3], Hopfield [4] and Agranovich [5]. The concept of polaritons has proven to be a very fruitful one and has been subject to numerous experimental and theoretical investigations for more than 40 years. The developments of nanostructure physics led to several new phenomena and concepts related to radiation-matter interaction and stimulated novel studies of excitons and polaritons in confined electron and photon systems. In particular, the demonstration of the strong-coupling regime between excitons and photons in planar microcavities and the recent discovery of stimulated scattering and amplification of cavity polaritons have opened exciting and very active areas of research.

In the present lectures I shall review a few main concepts related to excitons and polaritons in bulk and quantum-confined semiconductor systems. I shall concentrate on the following topics: polaritons in bulk semiconductors, excitons and polaritons in thin films and quantum wells, and finally radiation-matter interaction in microcavities, especially with three-dimensional photonic confinement. A unifying theme in these lectures may be the evolution of basic quantities like the exciton oscillator strength or the polariton coupling in going from the bulk to different kinds of confined systems.

1. Polaritons in bulk semiconductors

2. Excitons in thin films and quantum wells

3. Polaritons in thin films and quantum wells

4. Radiation-matter interaction in microcavities

5. Conclusions