Optical limiting capability of thick nonlinear absorbers

A theoretical model describing the fluence-dependent absorption of Gaussian shaped laser beams by optically thick nonlinear media is applied to the interpretation of Z-scan measurements with thick samples of chloro-aluminium-phthalocyanine. The results obtained from the measurements and the modeling have produced an enhanced understanding of the nonlinear processes operating within this type of material, as well as the role of the relaxation mechanisms and saturation effects in shaping the nonlinear absorption, and the optical limiting capability of this type of absorber. An exact first-order analysis of a five-level molecular model has been developed and explored for the case where the incident beam possesses a Bat temporal profile. The predictions of the model are in excellent agreement with experimental observations and with results produced from a numerical modeling technique, previously developed, where a simple "relaxation-free" and "population-saturation-free" three-level model is used to predict the transmission properties of a thick five-level system.