Temporal response of nonlinear transmission in optically thick saturable absorbers and determination of saturable absorption parameters

The temporal response of nonlinear transmission in saturable absorbers is theoretically investigated on the basis of a four-energy-level model including an excited-state absorption, and is experimentally employed for determining saturable absorption parameters in saturable-dye-doped films. The approximation of optically thin absorbers is shown to be inadequate to this end. In theoretical calculations, optically thick saturable absorbers are treated by a set of simultaneous partial differential equations: a rate equation governing the dynamics of population densities of energy levels and a propagation equation governing the propagation of optical fields in the optically thick absorbers. By comparing experimental results with numerical ones, the saturable absorption parameters can be determined on the basis of the intensity dependence of response time. In numerical calculations, the effect of random orientation is also considered for optically anisotropic molecules rigidly fixed in saturable absorbing media. The characterization of the saturable absorption parameters is conducted by using uranine-doped poly(vinyl alcohol) films.