Ultrafast optical nonlinearity in the quasi-one-dimensional Mott insulatorSr2CuO3

We report strong instantaneous photoinduced absorption in the quasi-one-dim ensional Mott insulator Sr2CuO3 in the IR spectral region. The observed pho toinduced absorption is to an even-parity two-photon state that occurs imme diately above the absorption edge. Theoretical calculation based on a two-b and extended Hubbard model explains the experimental Features and indicates that the strong two-sphoton absorption is due to a very large dipole coupl ing between nearly degenerate one- and two-photon stares. Room temperature picosecond recovery of the optical transparency suggests the strong potenti al of Sr2CuO3 for all-optical switching.