Resonant light scattering mediated by excitonic polaritons in semiconductors

The field of polariton-mediated light scattering is obviously a very rich one. It first gives a direct visualisation of those fundamental hcoupled-mode excitations, the excitonic polaritons. It helps to obtain evidence for the predictions made by Hopfield and others of some extraordinary properties of these objects. Let us mention, for instance, the dispersive, multiple Brillouin peaks, the AS/S ratio in RBS larger than unity, the disappearence of sharp RRS peaks into luminescence bands at resonance, the huge cross sections of RIRS and its doubleresonance behavior on the n = 2 exciton-polariton peak. In this latter case, we encounter once more the situation where a third-order process with two strong interactions overcomes a two-step process involving weak interactions. But polariton-mediated light scattering is much more than just another theoretical description of the light-matter interaction. It provides a huge amount of information on exciton parameters [masses, L-T splittings (i.e., polarizability), fine structure parameters (exchange energy)], and semiconductor parameters (refractive indices, sound velocities, phonon energies, K-linear energy terms, impurity energy levels and concentration). Exciton-phonon (acoustic as well as optical phonons) and exciton-impurity interactions can be studied in great detail with this type of resonance scattering. Due to this wealth, and also due to the relative ease with which such experiments can be performed, polaritonmediated light scattering still has a bright future. There is a number of unresolved and challenging problems. In the first place, of course, the ABC problem due to its generality and fundamental character. The extremely complicated, somewhat uncontrolled, experimental situation at the crystal interface has been discussed in various portions of the text. We want to stress again that only complete sets of measurements, i.e., transmission, reflectivity, RBS (efficiency, linewidth, hlineshapes, high-resolution kinematics) can provide useful information to shed some light onto this delicate problem. Other remaining tasks concern realistic calculations of efficiencies and multiple scattering properties (in particular, thermalization of RRS events). Interesting experiments would be the observation of RBS for surface exciton polaritons, which would, as in the case of bulk polaritons, bring welcome information on these otherwise rather indirectly evidenced excitations.