Steven L. Johnson

A setup for ultrafast time-resolved x-ray absorption spectroscopy

We present a setup which allows the measurement of time-resolved x-ray absorption spectra with picosecond temporal resolution on liquid samples at the Advanced Light Source at Lawrence Berkeley National Laboratories. The temporal resolution is limited by the pulse width of the synchrotron source. We characterize the different sources of noise that limit the experiment and present a single-pulse detection scheme.

Coherent control of phonons probed by time-resolved x-ray diffraction

Time-resolved x-ray diffraction with picosecond temporal resolution is used to probe the product state of a coherent control experiment in which a single acoustic mode in a bulk semiconductor is driven to large amplitude or canceled out. It is demonstrated that by shaping ultrafast acoustic pulses one can coherently control the x-ray diffraction efficiency of a crystal on the time scale of a vibrational period, with application to coherent switching of x-ray beams.

Femtosecond x-ray diffraction: experiments and limits

Although the realisation of femtosecond X-ray free electron laser (FEL) X-ray pulses is still some time away, X-ray diffraction experiments within the sub-picosecond domain are already being performed using both synchrotron and laser- plasma based X-ray sources. Within this paper we summarise the current status of some of these experiments which, to date, have mainly concentrated on observing non-thermal melt and coherent phonons in laser-irradiated semiconductors. Furthermore, with the advent of FEL sources, X-ray pulse lengths may soon be sufficiently short that the finite response time of monochromators may themselves place fundamental limits on achievable temporal resolution. A brief review of time-dependent X-ray diffraction relevant to such effects is presented.

Ultrafast X-ray diffraction of laser-irradiated crystals

Abstract Coherent acoustic phonons have been observed in the X-ray diffraction of a laser-excited InSb crystal. Modeling based on time-dependent dynamical diffraction theory has allowed the extraction of fundamental constants, such as the electron-acoustic phonon coupling time. A dedicated beamline for time-resolved studies has been developed at the Advanced Light Source with special considerations toward high transmission, low scattering and a wide photon energy range. The facility combines a bend magnet beamline, time-resolved detectors and a femtosecond laser system.

Ultrafast X-ray science at the advanced light source

Our scientific understanding of the static or time-averaged structure of condensed matter on the atomic scale has been dramatically advanced by direct structural measurements using x-ray techniques and modern synchrotron sources. Of course the structure of condensed matter is not static, and to understanding the behavior of condensed matter at the most fundamental level requires structural measurements on the time scale on which atoms move. The evolution of condensed-matter structure, via the making and breaking of chemical bonds and the rearrangement of atoms, occurs on the fundamental time scale of a vibrational period, ~;100 fs. Atomic motion and structural dynamics on this time scale ultimately determine the course of phase transitions in solids, the kinetic pathways of chemical reactions, and even the efficiency and function of biological processes. The integration of x-ray measurement techniques, a high-brightness femtosecond x-ray source, femtosecond lasers, and stroboscopic pump-probe techniques will provide the unique capability to address fundamental scientific questions in solid-state physics, chemistry, AMO physics, and biology involving structural dynamics. In this paper, we review recent work in ultrafast x-ray science at the ALS including time-resolved diffraction measurements and efforts to develop dedicated beamlines for femtosecond x-ray experiments.

Atomic Motion in Laser Excited Bismuth Studied with Femtosecond X-Ray Diffraction

Asymmetric grazing incidence femtosecond x-ray diffraction is applied to investigate carrier transport, carrier relaxation and phonon coupling in laser excited bismuth crystals.

Lattice and Magnetic Dynamics of a Laser Induced Phase Transition in FeRh

We study the two coupled components of the laser induced phase transition in FeRh. We compare structural and magnetization dynamics measured with respectively time-resolved x-ray diffraction and magneto optical Kerr effect.

Laser Induced CDW Melting in TiSe 2 Optical and X-Ray Time Resolved Study

Femtosecond laser and X-ray pump/probe measurements indicate an ultrafast laser induced structural phase transition in 1T-TiSe2, mediated by an A1gamplitude mode of a CDW.

Impurity-Localized Electronic Changes in Eu:SrGa_2S_4 Studied by Ultrafast X-Ray Absorption Spectroscopy

The time-dependent electronic structure of Eu:SrGa2S4 after excitation by a femtosecond laser pulse is investigated by time-resolved x-ray absorption spectroscopy. We observe a transient shift in the Eu L3 edge consistent with Eu ionization.

Coherent phonon spectroscopy using time-resolved x-ray diffraction

Summary form only given. Time-resolved x-ray diffraction enables one to directly measure the dynamics of solids on atomic length scales in real-time. In this experiment, x-ray diffraction with 2 ps resolution is used to observe coherent acoustic vibrations at frequencies approaching 0.1 THz. The acoustic phonon dispersion relation in InSb near the Brillouin zone center is measured. Microscopic electron-phonon coupling-times are extracted. On time-scales shorter than the dephasing of the coherent oscillations, two-pulse excitation is used to coherently control the amplitude of the vibrational excitations. X-rays from the Advanced Light Source synchrotron are monochromatized to a wavelength of 5 keV and then diffracted off an InSb wafer onto a streak camera detector.