High-frequency Franz-Keldysh effect on excitons

When the effective characteristic electro-optic energy ?|Λ_H|, which is obtained by including the electron-hole interaction in the solid, is very large compared to ?ω of the modulating electric field, the line shapes of the field-induced change of dielectric function are the same as those of Franz-Keldysh effect. However, in the low-field and high-frequency limit Δ∈₂(Ω, $\text{E}$ is nearly proportional to the first derivative of the unperturbed dielectric function. It is different from the low-field and low-frequency limit case where Δ∈₂(Ω, $\text{E}$) is proportional to the third derivative of the unperturbed dielectric function.