All‐optical picosecond switching of a quantum well etalon using spin‐polarization relaxation

All‐optical picosecond gate operation is demonstrated using the spin relaxation in a quantum well etalon. Electron spin‐polarization photoexcited by circularly polarized light in GaAs quantum wells decays in picoseconds. With the use of the spin relaxation in a quantum well etalon, all‐optical full switching is achieved with a decay time of as fast as 7 ps, by adopting a simple optical differential method, which uses only a quarter‐wave plate and a polarizer in addition to the conventional all‐optical switching setup.