Femtosecond response from two copolymers with intense two-photon absorption

Under excitation of femtosecond laser pulses at 800 nm, intense two-photon absorption induced fluorescence was observed from two copolymers, linear structure copolymer M2 and tri-branched copolymer M3. In a one-color pump–probe experiment at 800 nm, an ultrafast transient absorption was observed, which was mainly from the simultaneous absorption of the one photon from the pump beam and another photon from the probe beam. This analysis was further confirmed by a two-color pump–probe measurement with a pump at 800 nm and a probe at 556 nm, respectively. The other two decaying processes in transient absorption have a lifetime of about 14 and 126 ps, which reflects the intraband relaxation and the decay of the excited state via intersystem crossing or the solvation effect, respectively.