Two-photon and excited-state absorption of pyrylium dyes studied through the Z-scan technique

Abstract

A series of pyrylium dyes is studied concerning their Non-Linear Absorption (NLA) in order to assess their applicability as optical limiters. The research aims at combining the selective material preparation through the systematic change of the molecules' substituents and the linear and nonlinear optical characterization. The open z-scan technique at various excitation intensities was used for the nonlinear absorption characterization. With this method, the third and fifth order contributions to the susceptibility can be separated and quantified. Femtosecond pulses at 760, 790 and 840 nm were used to induce the NLA. All wavelengths lay in the two-photon absorption spectral region of the dyes. In most of the cases studied an efficient Excited State Absorption (ESA), fifth order nonlinearity, following Two-Photon Absorption (TPA), third order nonlinearity, was observed above a threshold of the excitation intensity (I_thr). This was indicated through an increase of the value of the NLA coefficient above I_thr. If TPA was the only nonlinear mechanism, the NLA coefficient would be intensity independent. The third and fifth order NLA coefficients as well as the ESA cross section values were extracted. High values of the ESA cross sections of the order of 10^-15 cm² were calculated indicating that the pyrylium dyes are potential candidates in optical limiting applications.