Excitonic optical nonlinearity of quantum-well structures in a static electric field

The excitonic nonlinear optical susceptibility ${\chi }^{\mathrm{}\left(n\right)\mathrm{}}$, of multiple-quantum-well structures subject to a static electric field normal to the layers is theoretically investigated when the incident photon energy is lower than, but comparable to, the absorption edge. We have taken into account both the effect of giant dipole moment (GDM) and that of virtual charge (VC) of excitons. For $n=2\mathrm{}$, the VC effect is absent, and ${\chi }^{\mathrm{}\left(2\right)\mathrm{}}$ is enhanced solely by the GDM effect, resulting in a value about 10 times larger than that of the bulk. For $n=3\mathrm{}$, on the other hand, both the GDM and the VC effects contribute to large ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$. The former is dominant near the two-photon resonance, while the latter is dominant near the one-photon resonance. Resulting enhanced values of ${\chi }^{\mathrm{}\left(2\right)\mathrm{}}$ are 10-${10}^3$ times larger than that of the bulk.