Virtual photoconductivity due to intense optical radiation transmitted through a semiconductor

Virtual photoconductivity is the change in static dielectric susceptibility due to the virtual excitation of electrons and holes by an optical beam in the transparent region of a semiconductor, near the band edge. It is the inverse of the Franz-Keldysh effect, which is the change in optical constants due to a strong static field. Previously, only the low-optical-intensity limit of virtual photoconductivity had been theoretically analyzed. We provide in this paper a nonlinear theory of virtual photoconductivity valid for strong optical fields. The main approximation is that the optical matrix element be strong enough to overwhelm the Coulomb interaction between electron and hole, but weak enough to avoid two-photon absorption. At high optical intensities, we find that the change of static dielectric constant saturates at \ensuremath{\approxeq}0.5 units. The saturated regime of virtual photoconductivity, while difficult to attain experimentally, is best explored on the red side of the exciton absorption tail in high-quality GaAs crystals at very low temperatures.