OPTICAL PROPERTIES OF SEMICONDUCTORS: EXCITON-POLARITONS.

摘要

Exciton-polaritons are composite elementary excitations consisting of excitons and photons. Unique physical situations arise as a consequence of the shape of the exciton-polariton dispersion. For example, the dispersion does not have a well defined minimum and it is not obvious whether and how thermal equilibrium can be established. In addition, two modes can propagate in the same direction with the same energy and polarization, but with different wavelengths.;This thesis presents a series of experimental and theoretical studies of the optical properties of semiconductors in the exciton-polariton region. These optical properties include reflectivity, luminescence lineshape and time-resolved emission. A model is presented for the calculation of these optical properties. This model includes the two polariton branches as well as the additional boundary conditions necessary to describe the electrodynamics of spatially dispersive media. The role of the additional boundary conditions in the reflectivity and luminescence spectra is discussed, and the continuity of energy flow across the sample surface is examined. Quantitative comparison is made between the experimental and theoretical luminescence spectra. Two additional boundary conditions were considered and one is found to result in good agreement between theory and experiment.;Picosecond laser spectroscopy was employed as the experimental technique for studying the time-resolved emission of exciton-polaritons. The theoretical time-resolved spectra for different temperatures and sample purities were also calculated with the same model. The model reveals the way in which thermal equilibrium is established among exciton-polaritons. Quantitative comparison between the experimental and theoretical results for time-resolved emission is presented. The signature of the bottleneck can be seen in the observed lifetimes. In short, the model provides, for the first time, a basis for the interpretation of the experimental time-resolved emission spectra.;An extention of the model is considered which can account for other optical properties of exciton-polaritons such as absorption lineshape, linewidths of the peaks in resonant Brillouin scattering, and the structures in anomalous wave interference. It is concluded that the model is useful in interpreting experimental data whenever the polariton effects are important.