We report the first observation of optical nonlinearities due to subband structures in the growth direction of ann‐type superlattice by four‐wave mixing in an end‐firing configuration. We observe a nearly eightfold increase in the third‐order optical susceptibility when the electric fields of the CO2laser are changed from a parallel to a perpendicular orientation to the plane of an Al0.08In0.92Sb/InSb superlattice. No change is observed in a control InSb epilayer under similar conditions.