Ultrafast optical responses in one-dimensional Mott insulators of halogen-bridged Ni compounds

Halogen-bridged nickel compounds have attracted much attention because they show large optical nonlinearity and photoinduced insulator-metal transition. We report a new fabrication method for thin film samples of the Ni compounds. Using these thin film samples, we have performed femtosecond pump-probe spectroscopy. Photomodulated absorption induced by resonant and nonresonant photoexcitation on charge-transfer band have revealed that excitons decay very fast with the time constant τ less than 2 picoseconds and charge carriers decay relatively slow with τ > 20 ps. The slower decay of the charge carriers is attributable to the formation of polarons and the resultant decrease of their mobility.