Effect of continuum states on two-photon absorption in quantum wells

The contribution, due to continuum states, to the two-photon subband-subband transition rate in semiconductor quantum wells is calculated for transition energies below the band gap of the barrier material. Unlike the case of one-photon absorption, we have found that for two-photon absorption the continuum contribution cannot always be neglected. In the case of the polarization of the radiation beam along the growth axis, we have found this contribution to be comparable to the contribution due to transitions between discrete states. On the contrary, for polarizations in the layer planes the effect of the continuum is negligible. A new mathematical formulation, based on the formalism of the distribution theory, has been developed in order to correctly take the continuum states into account.