Study of the luminescence power of excitons and impurity–defect centers excited via two-photon absorption

The effect of the average power of femtosecond laser radiation on the average luminescence power of excitons and impurity–defect centers on the two-photon excitation of the electron system of a crystal is studied experimentally and theoretically using ZnSe : Fe$^{2+}$ single crystals as an example. It is experimentally shown that, in the region of excitation powers under consideration, the average luminescence power of excitons in the crystal is proportional to the 4th power of the average excitation-radiation power. The average luminescence power of impurity–defect centers is of quadratic character. A theory is developed to interpret the experimentally observed dependences. It is noted that the dependence of the luminescence power of a crystal on the pump power upon two-photon excitation can be used to estimate the degree of contamination of the crystal with impurity–defect centers.