Third‐harmonic generation in an AlGaAs/AlAs superlattice is observed for the first time at a pump‐light wavelength of around 2 mgr;m. The value of third‐order optical susceptibility khgr;(3), for the polarization parallel to the superlattice layer in a nonresonant region, is close to that reported for bulk GaAs. Enhancement of khgr;(3), giving more than three times the nonresonant value, is observed near a three‐photon resonance region. A characteristic third‐harmonic signal is observed when the pump‐light polarization has a component perpendicular to the superlattice layer. This is considered to be another khgr;(3)enhancement which is caused by nonparabolicity in the superlattice.
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