Collective oscillations driven by correlation in the nonlinear optical regime

We present an analytical and numerical study of the coherent exciton polari zation including exciton-exciton correlation. The time evolution after exci tation 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 pr operties of the retarded two-exciton correlation function, which includes C oulomb effects beyond the mean-held approximation. The overall time evoluti on depends on the low-frequency spectral behavior. The collective mode, wel l separated from the slower coherent density evolution, manifests itself in the coherent emission of a resonantly excited excitonic system, as demonst rated numerically.