  We study the time-resolved reflectivity spectrum of a switched planar
GaAs-AlAs microcavity. Between 5 and 40 ps after the switching (pump) pulse we
observe a strong excess probe reflectivity and a change of the frequency of
light trapped in the cavity up to 5 linewidths away from the cavity resonance.
This frequency change does not adiabatically follow the fast-changing cavity
resonance. The frequency change is attributed to an accumulated phase change
due to the time-dependent refractive index. An analytical model predicts
dynamics in qualitative agreement with the experiments, and points to crucial
parameters that control future applications.
