Collective oscillations, excitonic molecules and dephasing of excitons at low density

Th. Östreich

Summary form only given. Large-radius excitons in semiconductors suffer strong Coulomb interaction on close approach. This is a distinctive difference in comparison with two-level systems in quantum optics. A mean field theory of interacting excitons in semiconductors leads to purely repulsive interaction, and does not account for spin-dependent polarization mixing and the formation of excitonic molecules (biexcitons). We have developed a theory for the nonlinear optical response, which can account for both. The coherent polarization is driven by exciton-exciton correlation, which can be expressed in terms of spin-dependent retarded memory functions. The effective equation is a generalized Gross-Pitaevskii equation.

Hermann Haken, Haken, Hermann

Leonid V. Keldysh, Andrey Nikolaevich Kozlov

M. Dagenais

The concept of giant oscillator strength for both free and bound excitons is elucidated. It is shown that, the oscillator strength of these transitions is enhanced over the free exciton oscillator strength per molecule of the crystal by the ratio of the volume of the system divided by the volume of the crystal unit cell. An expression for the free exciton oscillator strength is obtained that can be used to describe the AC Stark shift of an excitonic resonance and also its saturation behavior.

Th. Östreich, Lu Jeu Sham

We present an analytical and numerical study of the coherent exciton polarization including exciton-exciton correlation. The time evolution after excitation with ultrashort optical pulses can be divided into a slowly varying polarization component and novel ultrafast collective modes. The frequency and damping of the collective modes are determined by the high-frequency properties of the retarded two-exciton correlation function, which includes Coulomb effects beyond the mean-field approximation. The overall time evolution depends on the low-frequency spectral behavior. The collective mode, well separated from the slower coherent density evolution, manifests itself in the coherent emission of a resonantly excited excitonic system, as demonstrated numerically.

Михаил Михайлович Глазов, Robert A. Suris