Two-photon absorption is treated theoretically using second-order perturbation theory. The exciton effect is taken into account for a crystal with degenerate valence bands. Calculations are made also including another energy band besides the conduction and valence bands. Expressions are given for the absorption of photon pairs from one radiation field and for the absorption of two photons, one from each of two different radiations. Experimental studies are made on ZnTe, GaAs, InP, and InSb. A Nd-glass laser is used for the measurements for ZnTe, GaAs, and InP; measurements on GaAs and InP are also made with the laser in combination with a Hg lamp. InSb is measured with a C${\mathrm{O}}_2$ laser. The experimental results are in good agreement with the calculations which take into account the exciton effect for degenerate valence bands, provided the two-photon energy is not too much larger than the energy gap. Calculations show that the degeneracy of valence band is important and that the importance of the exciton effect is larger the bigger the ratio of exciton binding energy to the difference between two-photon energy and the energy gap. These considerations are supported by the experimental results.

• Received 6 August 1973

DOI:https://doi.org/10.1103/PhysRevB.9.3502