Nonlinear absorption and refraction process of fluorene-based molecules via picosecond and femtosecond measurements

Abstract

The nonlinear optical performance of several Fluorene-based molecules was studied using different measurement methods and pulse durations. We used picosecond pulses at 532 nm and, femtosecond pulses tunable from 532-810 nm for performing open and closed aperture Z-scans, and we used femtosecond 570-930 nm pulses for two-photon induced fluorescence (2PF) spectroscopy. The observed nonlinear losses were compared using the three methods. The results exhibit much stronger nonlinear absorption with picosecond pulses due to the additional excited-state absorption processes involved. Also the nonlinear refractive index was found to be higher for the picosecond measurements. In addition using a femtosecond white-light continuum (WLC) pump-probe method we showed that by proper tuning of pump and probe wavelengths an intermediate state resonance enhancement (ISRE) of the 2PA can be observed yielding the same overall nonlinear absorption observed using picosecond pulses.