Enhanced optical nonlinearities of superlattices within the Kronig-Penney model incorporating inherent bulk nonlinearities

Third order nonlinear optical susceptibilities χ⁽³⁾ of GaAs/Ga_1?xAlAs superlattices have been predicted which are two orders of magnitude larger than those of bulk GaAs. This enhancement is due to the band nonparabolicity arising from the additional periodicity of the superlattice. These predictions, based on a tight-binding model of the superlattice dispersion, are here extended to the more realistic Kronig-Penney (KP) model. Corrections to tight-binding are non-negligible; however, enhancements of χ⁽³⁾ are still large but reduced approximately 30%–50% over previous estimates. The KP model is also here applied to superlattices employing InSb as the quantum well material. Because of the smaller effective mass of InSb, and taking account of its bulk nonparabolicity, the minibands move to higher energy, enhancing the interwell overlap and increasing χ⁽³⁾ by about one order of magnitude over that of bulk InSb. The role of the barrier material in this case is important and is discussed. The interplay between the bulk nonparabolicity and that arising from the superlattice is also addressed.