We show that quantum confinement can dramatically alter the density‐dependence of the third‐order susceptibility, χ(3)NP, that arises from band nonparabolicity. Our results predict an oscillatory dependence of the efficiencies for third‐harmonic generation and four‐wave mixing on the subband occupation of quantum wells, and for narrow wells with high charge densities predict an enhancement over the bulk susceptibility. We also make a simple estimate of the fields required to saturate this nonparabolicity contribution to χ(3). We discuss these results in light of recent experiments on third‐harmonic generation from narrow‐gap quantum wells at frequencies of ∼1 THz, and show that nonparabolicity may not be the only nonlinearity contributing to the large χ(3) observed.

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