Ultrafast nonlinear optical response of quantum wells with one and two conduction subbands

Abstract

The response of a quantum well to an ultrafast optical pulse is examined theoretically using the Liouville equation in the electric dipole approximation. In particular a comparison is made between a system with a valence subband and a single conduction subband, and one with an additional conduction subband positioned to provide extra contributions to the resonant nonlinear polarization. The various contributions to the nonlinear polarization are identified and a simple physical picture is presented. It is shown that interference between the individual contributions is important in determining the overall nonlinear response, and that there are situations for which the addition of the second conduction subband is not beneficial.