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We present an analytical and numerical study of the coherent excitonpolarization including exciton-exciton correlation. The time evolution afterexcitation with ultrashort optical pulses can be divided into a slowly varyingpolarization component and novel ultrafast collective modes. The frequency anddamping of the collective modes are determined by the high-frequency propertiesof the retarded two-exciton correlation function, which includes Coulombeffects beyond the mean-field approximation. The overall time evolution dependson the low-frequency spectral behavior. The collective mode, well separatedfrom the slower coherent density evolution, manifests itself in the coherentemission of a resonantly excited excitonic system, as demonstrated numerically.Comment: 4 pages, 4 figures, accepted for publication in Physical Review  Letter

Collective Oscillations Driven by Correlation in the Nonlinear Optical Regime