  A laser pulse, several meV red-detuned from the excitonic line of a quantum
well, has been shown to induce an almost instantaneous and rigid shift of the
lower and upper polariton branches. Here we demonstrate that through this
shift, ultra-fast all-optical control of the polariton population in a
semiconductor microcavity should be achievable. In the proposed setup a Stark
field is used to bring the lower polariton branch in or out of resonance with a
quasi-resonant continuous-wave laser, thereby favoring or inhibiting the
injection of polaritons into the cavity. Moreover we show that this technique
allows for the implementation of optical switches with extremely high
repetition rates.
