Comment on "Electric field effect on the second-order nonlinear optical properties of parabolic and semiparabolic quantum wells"

Zhang and Xie [Phys. Rev. B 68, 235315 (2003)] presented their results for the second harmonic generation (SHG) susceptibility in electric-field-biased parabolic and semiparabolic quantum wells (QW's). In this Comment, we demonstrate that the results presented for the SHG coefficient in a parabolic QW with an applied electric field are not physically sound and they contradict with well-established properties of the Hermite polynomials which just happen to be the wave functions describing such a system. It is shown that in a parabolic QW with an applied electric field intersubband transitions contribute to neither second nor any higher order harmonic generation.