Optical limiting and nonlinear absorption of excited states in metalloporphyrin-doped sol gels

Optical limiting and upconverted luminescence have been studied in different chromophore-doped aluminosilicate gel materials and the effects of the dynamic processes from different central ions are discussed. Chromophores are metalloporphyrins of tetra-4-sulfonatophenylporphyrinato-copper(II) and tetra-4-sulfonatophenylporphyrinato-zinc(II) (CuTPPS and ZnTPPS, respectively) which are embedded in aluminosilicate gel hosts. Photo-upconverted luminescence was observed and believed to result from the radiative recombination of the second excited singlet state. Saturated behavior of upconverted emission was found as a function of incident excitation intensity. An optical limiting phenomenon associated with reverse saturated absorption was studied. These effects are attributed to higher excited state absorption. The observation of upconverted luminescence indicates that the upper excited state absorption occurs while nonlinear absorption is being observed. Dynamic analysis of excited state processes indicates that intersystem crossing and triplet-triplet (quartet-quartet) absorption dominates nonlinear processes and suggests that saturation absorption or reverse saturation absorption is dependent on the ratio of the effective absorption cross section for the excited states to that for the ground state. Absorption cross sections for the excited states are estimated by the data fitting to upconverted emission intensity and nonlinear absorption.