  Multidimensional coherent spectroscopy (MDCS) is used to separate coherent
and incoherent many-body contributions to the population-time dynamics in a
GaAs-based semiconductor microcavity encapsulating a single InGaAs quantum
well. In a three-pulse four-wave-mixing scheme, the second delay time is the
population time that in MDCS probes excited-state coherences and population
dynamics. Nonlinear optical interactions can mix these contributions, which are
isolated here for the lower- and upper-exciton-polariton through the self- and
mutual-interaction features. Results show fast decays and oscillations arising
from the coherent response, including a broad stripe along the absorption
energy axis, and longer time mutual-interaction features that do not obey a
simple population decay model. These results are qualitatively replicated by
Bloch equation simulations for the 1s exciton strongly coupled to the
intracavity field. The simulations allow for separation of coherent and
incoherent Pauli-blocking and Coulomb interaction terms within the
chi^(3)-limit, and direct comparison of each feature in one-quantum rephasing
and zero-quantum spectra.
