Optical switching in a thermally expanding Si e´talon has been demonstrated. In this experiment a pulsed CO2laser beam is used to heat a Si e´talon and shift the interference fringe of a 1.5 &mgr;m probe beam. It is shown that the switching time can be greatly reduced from ms to &mgr;s by choosing a probe beam of shorter wavelength in an external switching configuration. By aligning one e´talon at switch‐on and another at switch‐off, we have demonstrated a write and erase logic system.