We present a theory of ultrafast time-resolved pump and probe spectroscopy
in semiconductor microcavities. The numerical solutions are in excellent ag
reement with experimental data. An analytical study based on the Weisskopf-
Wigner approximation gives physical insight into the transient optical nonl
inearities of semiconductor microcavites. It is able to explain the physica
l origin of the unusual transient shifts and asymmetric-excitation-induced
dephasing observed experimentally. This theory demonstrates that the nonper
turbative regime provides a unique tool for measuring the spectrum of four-
particle correlations.