Near-Equilibrium Structural Dynamics on the 20ps Time Scale

We observed near-equilibrium optically-induced structural dynamics at the Stanford Synchrotron Radiation Lightsource (SSRL). Hard x-ray diffraction experiments were performed in a typically unavailable regime: low optical pump fluence (20-250 $\mu$J/cm$^{2})$ and short x ray pulses (15-60ps). We studied several different thin films (BiFeO$_{3}$ (BFO), Pb(Zr)TiO$_{3}$ (PZT), and Bi), pumping above bandgap using 343nm (BFO, PZT) or 1030nm (Bi) laser light at a 1.28MHz repetition rate in a special low alpha (short pulse) mode of the synchrotron. The corresponding small diffraction pattern changes (including fractional changes in lattice constant on order 10$^{-6}$ were made much more apparent because of the ability of the experimental apparatus to make use of the high repetition rate of the x rays; it is precisely this rapid data acquisition process that allows such small changes to be measured.