  At carrier densities above the Mott density Coulomb screening destroys the
exciton resonance. This, together with band-gap renormalization and band
filling, severely affects the optical spectra. We have experimentally studied
these effects by ultrafast pump-probe reflectivity measurements on a ZnO single
crystal at various wavelengths around the exciton resonance and in a broad
carrier-density range. Theoretically we determined the Mott density in ZnO to
be $1.5\times10^{24}$ m$^{-3}$ at 300 K. Taking a field-theoretical approach,
we derived and solved the Bethe-Salpeter ladder equation and we computed the
density-dependent reflectivity and absorption spectra. A carrier dynamics model
has been developed, containing three-photon absorption, carrier cooling, and
carrier trapping near the surface. The agreement between the theoretical
reflectivity based on our model and the experimental data is excellent.
