One-photon Keldysh absorption in direct-gap semiconductors

We have adapted the Keldysh multiphoton absorption theory to one-photon transitions in semiconductors. We find that the Keldysh theory is in good agreement with both the absolute values and with the frequency dependence of the absorption coefficients of InSb and GaAs. We have also reexamined the often used first-order perturbation approach and derived an expression for the absorption coefficient using the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{p}}$ method. A comparison between theory and experimental data has been carried out without adjustable parameters.