  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.
