Ultrafast structural changes measured by time-resolved X-ray diffraction

High-intensity X-ray pulses from third-generation synchrotron sources make it possible to study the temporal dynamics of rapidly evolving materials. We report a study of rapid and reversible disordering of the structure of an InSb crystal induced by an ultrashort laser pulse. A novel crosscorrelation detection technique is described, which allowed us to observe rapid changes in X-ray diffraction that occur on a time-scale of less than 2 ps. Time-resolved X-ray diffraction experiments were undertaken at the Advanced Light Source synchrotron. We irradiated an InSb crystal with a 100 fs laser pulse, and probed the irradiated region with a 70 ps X-ray pulse. A rapid reduction in the X-ray diffraction signal was observed, followed by recovery on a timescale of about 100 ns. In order to distinguish between various mechanisms that could cause the reduction in X-ray signal, ultrafast time resolution is required. We therefore developed a cross-correlation technique using two laser-irradiated crystals. This technique will allow studies with a temporal resolution limited only by the laser pulse duration. In our current work, we were limited by a low signal-to-noise ratio, which results in a temporal resolution of about 2 ps. Additionally, and from measurements of the angular dependence of the diffraction efficiency (rocking curve), we conclude that the ultrashort laser pulse induced reversible disorder, or some other phase transition, in the bulk InSb crystal. ∗ Corresponding author. 1 Previous studies of crystalline structure dynamics The structural properties of materials following irradiation with ultrashort light pulses has been studied for two decades [1–11]. One motivation for this work has been the study of ultrafast disordering (e.g., melting) and other potential light-induced phase transitions (e.g., to novel ordered structures). Pump-probe techniques have generally been employed. On ultrafast timescales (< 1 ps), studies have involved time-resolved reflectivity and second-harmonic generation, both using optical laser pulses. Subpicosecond timescale is fundamental, and consistent with atomic motion on the scale of a typical molecular bond length. On longer timescales, electron [12, 13] and X-ray [14–20] diffraction have provided insight into phenomena including thermallyinduced melting, shock propagation, strain, heat diffusion, crystal regrowth, and annealing. As an example of ultrafast structural changes in materials, theoretical studies suggest that optical excitation of about 10% of the valence electrons in crystals with a diamond or zincblende structure should result in a structural instability which gives rise to ultrafast disordering on a time-scale shorter than the electron-phonon relaxation time [21–24]. It has also been speculated that such structural instabilities could result in a transient, ordered phase [24].