We present a detailed time-resolved differential reflectivity study of the electronic and the coherent phonon generation response of a GaAs optical microcavity after resonant picosecond laser pulse excitation. A complex behavior is observed as a function of laser–cavity-mode detuning and incident power. The observed response is explained in terms of the large dynamical variations of the optical cavity-mode frequency induced by the ultrafast laser excitation, related to the optical modulation of the GaAs-spacer index of refraction due to photoexcited carriers. It is demonstrated that this effect leads to a strong optical dynamical tuning of the coherent phonon detection sensitivity of the device.