  We have switched GaAs/AlAs and AlGaAs/AlAs planar microcavities that operate
in the "Original" (O) telecom band by exploiting the instantaneous electronic
Kerr effect. We observe that the resonance frequency reversibly shifts within
one picosecond. We investigate experimentally and theoretically the role of
several main parameters: the material backbone and its electronic bandgap, the
pump power, the quality factor, and the duration of the switch pulse. The
magnitude of the shift is reduced when the backbone of the central
$\lambda-$layer has a greater electronic bandgap; pumping with photon energies
near the bandgap resonantly enhances the switched magnitude. Our model shows
that the magnitude of the resonance frequency shift depends on the pump pulse
duration and is maximized when the duration matches the cavity storage time
that is set by the quality factor. We provide the settings for the essential
parameters so that the frequency shift of the cavity resonance can be increased
to one linewidth.
