Beam propagation in reverse saturable absorbing dyes

In a reverse saturable absorbing dye the absorption coefficient gets greater as the dye absorbs more light. In some of these dyes, where triplet state absorption dominates, the absorption coefficient can change by more than a factor of 10 in response to the laser fluence, F(J/cm²). Organic macrocyclic dyes such as porphyrins, phthalocyanines and naphthalocyanines have this property and have been studied extensively over the last decade. In order to understand the effects of the various contributing photo-physical processes, detailed beam propagation models have been developed. These have usually concentrated on modelling the interaction of a weakly focused laser beam with a thin sample but more recently interest in the low threshold tight focusing geometry has increased. The interaction of a tightly focused beam with a non-linear material exhibits some unusual effects. This paper describes how these effects may be modelled using ray optic Monte-Carlo methods and more accurate beam propagation techniques. The evolution of the nonlinear absorbing region within the sample is modelled using both methods. Both approaches reveal the growth of two distinct regions of high absorption within the laser beam and the nonlinear transmission curves obtained in the two cases agree well