  We explore a planar GaAs/AlAs photonic microcavity using pump-probe
spectroscopy. Free carriers are excited in the GaAs with short pump pulses. The
time-resolved reflectivity is spectrally resolved short probe pulses. We show
experimentally that the cavity resonance and its width depend on the dynamic
refractive index of both the lambda-slab and the lambda/4 GaAs mirrors. We
clearly observe a double exponential relaxation of both the the cavity
resonance and its width, which is due to the different recombination timescales
in the lambda-slab and the mirrors. In particular, the relaxation time due to
the GaAs mirrors approaches the photon storage time of the cavity, a regime for
which nonlinear effects have been predicted. The strongly non-single
exponential behavior of the resonance and the width is in excellent agreement
to a transfer-matrix model taking into account two recombination times. The
change in width leads to a change in reflectivity modulation depth. The model
predicts an optimal cavity Q for any given induced carrier density, if the
modulation depth is to be maximized.
