Simplified modeling of optical absorption and nonlinear index change in semiconductors

A simplified model of the two-level system is used to show the nonlinear optical behaviors of absorption and change in refractive index. The optical absorption spectrum and the dispersion of index change are calculated by means of the density matrix theory. The dependence of the absorption coefficient and the refractive index change on the optical intensity is shown. The mechanism of nonlinear optical effects is illuminated. Theoretical analysis shows that the nonlinear absorption and index change result from the change of photogenerated carrier population. It has been generalized into the study of nonlinear optical properties in semiconductors