Two-photon absorption spectra of GaAs with2ℏω1near the direct band gap

The two-photon absorption spectrum of GaAs was measured at 300 and 4 °K by monitoring the resulting band gap luminescence. For $2\hslash {\omega }_1$ more than 0.01 eV above the direct band gap $E_g$ the luminescence has an approximately ${(2\mathrm{}\hslash {\omega }_1-E_g)}^{\frac{3}{2}}$ dependence characteristic of the allowed-forbidden mechanism corresponding to the two-band model. A weaker spectral dependence is observed when $2\hslash \omega -E_g<0.01\mathrm{}$ eV. This is tentatively identified with a contribution from the allowed-allowed mechanism of the three-band model which has an $\sim {(2\mathrm{}\hslash \omega -E_g)}^{\frac{1}{2}}$ dependence, and exciton contributions in the continuum. The transition to the $n=1\mathrm{}$ exciton is weak, and is attributed to the allowed-allowed transition of the three-band model.