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Ultrafast all-optical switch is proposed and demonstrated using picoseconds spin-polarization relaxation in a multiple-quantum-well (MQW) etalon structure. The decay time of the conventional all-optical switching in MQW etalon is restricted by the carrier lifetime, typically in nanoseconds. Using carrier spin relaxation, the polarization change of the probe beam has been demonstrated to be switched with a pulse width of 4 ps and a contrast of 4:1 at a pump pulse energy of 50 fJ//spl mu//sup 2/. In the present device, the contrast is determined by the polarization rotation angle of the probe beam, and the polarization-rotation angle has been shown to be proportional to the total well thickness. It is predicted that a contrast can be improved over 13 dB by optimizing the MQW etalon structure, indicating potential applicability to ultrafast optical communication systems. The optimization would improve the transmission from a …