NONLINEAR RESPONSE OF QUANTUM-CONFINED ELECTRONS IN NONPARABOLIC SUBBANDS

We show that quantum confinement can dramatically alter the density-de pendence of the third-order susceptibility, chi(NP)((3)) that arises f rom band nonparabolicity. Our results predict an oscillatory dependenc e of the efficiencies for third-harmonic generation and four-wave mixi ng on the subband occupation of quantum wells, and for narrow wells wi th high charge densities predict an enhancement over the bulk suscepti bility. We also make a simple estimate of the fields required to satur ate this nonparabolicity contribution to chi((3)). We discuss these re sults in light of recent experiments on third-harmonic generation from narrow-gap quantum wells at frequencies of similar to 1 THz, and show that nonparabolicity may not be the only nonlinearity contributing to the large chi((3)) observed. (C) 1996 American Institute of Physics.