C. Dietrich

Femtosecond X-ray measurement of coherent lattice vibrations near the Lindemann stability limit

The study of phase-transition dynamics in solids beyond a time-averaged kinetic description requires direct measurement of the changes in the atomic configuration along the physical pathways leading to the new phase. The timescale of interest is in the range 10-14 to 10-12 s. Until recently, only optical techniques were capable of providing adequate time resolution, albeit with indirect sensitivity to structural arrangement. Ultrafast laser-induced changes of long-range order have recently been directly established for some materials using time-resolved X-ray diffraction. However, the measurement of the atomic displacements within the unit cell, as well as their relationship with the stability limit of a structural phase, has to date remained obscure. Here we report time-resolved X-ray diffraction measurements of the coherent atomic displacement of the lattice atoms in photoexcited bismuth close to a phase transition. Excitation of large-amplitude coherent optical phonons gives rise to a periodic modulation of the X-ray diffraction efficiency. Stronger excitation corresponding to atomic displacements exceeding 10 per cent of the nearest-neighbour distance—near the Lindemann limit—leads to a subsequent loss of long-range order, which is most probably due to melting of the material.

Generation and application of ultrashort X-ray pulses

Relatively small-scale laser-driven sources of short wavelength radiation covering a range from the extreme ultraviolet to the hard X-ray regime are now available. Because the duration of the X-ray pulses is comparable to, or shorter than the laser pulse width, it is possible to carry out X-ray measurements with picosecond or femtosecond time resolution.

'Ultrafast' Extended to X-Rays: Femtosecond Time-Resolved X-Ray Diffraction

For a long time the capability to perform measurements with femtosecond time-resolution belonged exclusively to the domain of optics. However, in the last few years laser-driven X-ray sources have been developed which enable femtosecond time-resolution to be extended to the X-ray regime.

Plasma mirror distortions and parametric instabilities induced by high intensity femtosecond pulses on solid targets

The interaction of femtosecond laser pulses with a plasma “mirror” in a weak relativistic regime with a controlled plasma gradient is investigated. The characteristic scale lengths for the decay of high order harmonic generation, increase of the divergence of the reflected radiation, and development of plasma instabilities are determined.

Femtosecond time resolution in X-ray spectroscopy

For a long time the capability to perform measurements with femtosecond time-resolution belonged exclusively to the domain of optics. However, in the last few years laser-driven X-ray sources have been developed which enable femtosecond time-resolution to be extended to the X-ray regime.

Ultrafast time‐resolved X‐ray diffraction

Femtosecond laser‐generated plasmas emit ultrashort X‐ray pulses in the multi‐keV range, which allow the extension of X‐ray spectroscopy into the ultrafast time‐domain. We report here on the generation of such short X‐ray pulses and their application for time‐resolved diffraction as a means to directly study ultrafast structural dynamics in laser‐excited solids.

Relativistic surface harmonics

Experiments and computer simulations of high harmonic generation from surface plasmas using femtosecond laser pulses are presented. By increasing the pump intensity above 1019 W/cm2 and varying the plasma scale length the mechanism of harmonic generation are changed.

Plasma mirror distortions and instabilities induced by high intensity femtosecond pulses

Summary from only given. We report on the experiments on laser-solid interaction in a weakly relativistic regime (a/sub o/ = 0.6) using high-contrast-ratio femtosecond pulses with the intensity of about 10/sup 18/ W/cm/sup 2/. A titanium sapphire laser producing 200 mJ pulses of 120 fs duration at a wavelength 800 nm has been used.

Structural dynamics in laser-excited solids investigated with femtosecond x-ray diffraction

Summary form only given. Ultrashort x-ray pulses offer a unique combination of atomic-scale spatial and temporal resolution, which permits direct measurements of structural transients on an ultrafast time scale. Using time-resolved X-ray diffraction with femtosecond, multi-keV X-ray pulses we have studied transient lattice dynamics in optically excited semiconductors and metals. In an optical pump/X-ray probe configuration transient changes in X-ray diffraction from [111]-oriented, single-crystalline thin films of Ge and Bi have been measured.

Observation of ultrafast structural transitions in solids using femtosecond x-ray pulses

Summary form only given. Time-resolved x-ray diffraction using multi-keV x-ray pulses reveals nonthermal changes of the lattice structure in less than 300 fs in a semiconductor and, for the first time, in a metal.