Ultrafast broadband reverse saturable absorption based on two-photon induced singlet state

The broadband reverse absorption in a multi-branched conjugated compound TPPh is investigated. Transient absorptive spectra of TPPh solution is recorded and a broadband excited-state absorption (475~780 nm) is discovered. The lifetime of this broadband excited-state absorption was measured to be about 20 ns. Transient fluorescence experiment was conducted to confirm that the long-lived broadband excited-state absorption is established on the first singlet state. Optical limiting with extremely high linear transmittance is achieved under the excitation of 532 nm, 21 ps pulses. Theoretically analysis showed that both two-photon absorption and excited-state absorption played a part in it, optical limiting capability is thus enhanced via cooperating effects of two-photon absorption and excited-state absorption. All of the findings suggested that TPPh is a good broadband nonlinear absorptive material and could be further optimized for optical limiting applications.