Contribution of two-photon and excited state absorption in 'axial-bonding' type hybrid porphyrin arrays under resonant electronic excitation

Two-photon absorption (2PA) and excited state absorption (ESA) properties of 'axial-bonding' type hybrid porphyrin arrays are investigated in the picosecond regime under single-photon resonant excitation condition. A crossover from reverse saturable absorption (RSA) to saturable absorption (SA) and back to RSA is observed with the increase of excitation intensity. In the corresponding intensity ranges, third- and fifth-order phase conjugate signals from degenerate four wave mixing are observed. These observations are explained using rate equations for population densities in a three-level energy scheme. At higher intensities, 2PA is found to be dominant contributor to nonlinear absorption compared to ESA.