Dietrich von der Linde

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.

Femtosecond melting and ablation of semiconductors studied with time of flight mass spectroscopy

Using time-of-flight mass spectroscopy, we have investigated melting and ablation of gallium arsenide and silicon irradiated by femtosecond pulses. Below the ablation threshold the maximum surface temperature is obtained from the collisionless time-of-flight distributions of evaporated or sublimated particles. At the melting threshold, we estimate a temperature for the silicon surface which is approximately 500 K higher than the equilibrium melting temperature. In the fluence regime where melting is known to be a nonthermal process, we measure maximum surface temperatures in excess of 2500 K for both silicon and gallium arsenide, indicating rapid thermalization after nonthermal melting. At the ablation threshold, we estimated for both materials surface temperatures between 3000 and 4000 K. We observed a clear threshold-like effect in the number of detected particles, indicating the occurrence of a bulk effect. The flow parameters above the ablation threshold are discussed and compared to the different mod...

Ultrafast imaging interferometry at femtosecond- laser-excited surfaces

�2 rad and amplitude changes 1% with micrometer spatial resolution 1 m. Interferograms are processed using a 2D-Fourier transform algorithm. We discuss the image formation and the physical interpretation of the measured interferograms. The technique is applied to measure transient changes of a GaAs surface irradiated with intense femtosecond laser pulses with fluences near the ablation threshold.

Picosecond acoustic response of a laser-heated gold-film studied with time-resolved x-ray diffraction

We apply time-resolved x-ray diffraction using ultrashort x-ray pulses from a laser-produced plasma to probe the picosecond acoustic response of a thin laser-heated gold film. Measurements of the temporal changes in the angular distribution of diffracted x-rays provide direct quantitative information on the transient evolution of lattice strain. This allows to disentangle electronic and thermal pressure contributions driving lattice expansion after impulsive laser excitation. The electron-lattice energy equilibration time τE=(5±0.3) ps as well as the electronic Gruneisen parameter γe=(1.48±0.3) have been determined.

Ultrafast phase transitions and lattice dynamics probed using laser-produced x-ray pulses

When intense femtosecond laser pulses are focused on solid targets short-lived microplasmas are formed which emit bursts of x-rays with kilovolt photon energies. Under the proper conditions x-ray pulses as short as a few hundred femtoseconds can be produced. These x-ray pulses enable ultrafast x-ray spectroscopy using pump–probe schemes where the x-ray pulses serve as probe pulses. This article describes time-resolved x-ray diffraction experiments which reveal changes in the atomic structure with a time resolution of a few hundred femtoseconds. In particular, we have studied solid-to-liquid phase transitions in semiconductors induced by femtosecond photoexcitation and the accompanying thermoacoustic phenomena. We were able to monitor the changes in the atomic position underlying a coherent optical phonon mode. These and a number of other lattice dynamics experiments discussed here demonstrate the feasibility and usefulness of ultrafast time-resolved x-ray diffraction. Future applications in many other fields of science can be foreseen.

Dynamics of femtosecond-laser-induced ablation from solid surfaces

Femtosecond laser induced ablation from solid surfaces has been investigated by means of time resolved microscopy. On transparent materials ablation is initiated by dielectric breakdown and formation of a dense and hot surface plasma. Measurements of the plasma threshold yield values of a few times 1013 W/cm2 with little variation among different materials. This indicates that microscopic surface properties are responsible for surface breakdown. On absorbing semiconductors and metals near-threshold ablation is brought about by hydrodynamic expansion of the laser generated hot and pressurized matter. Upon expansion into vacuum initially metallic materials transform into a transparent state with a high refractive index. The observed behavior is related to general properties of matter in the liquid-gas coexistence regime.

Dynamics of ultrashort pulse-laser ablation: equation-of-state considerations

Ultrafast time resolved microscopy of femtosecond laser irradiated surfaces reveals a universal feature of the ablating surface on nanosecond time scale. All investigated materials show rings in the ablation zone, which were identified as an interference pattern (Newton fringes). Optically sharp surfaces occur during expansion of the heated material as a result of anomalous hydrodynamic expansion effects. Experimentally, the rings are observed within a certain fluence range which strongly depends on material parameters. The lower limit of this fluence range is the ablation threshold. We predict a fluence ratio between the upper and the lower fluence limit approximately equal to the ratio of critical temperature to boiling temperature at normal pressure. This estimate is experimentally confirmed on different materials (Si, graphite, Au, Al).

Short-pulse-laser-induced optical damage and fracto-emission of amorphous, diamond-like carbon films

Short-pulse-laser-induced damage and ablation of thin films of amorphous, diamond-like carbon have been investigated. Material removal and damage are caused by fracture of the film and ejection of large fragments. The fragments exhibit a delayed, intense and broadband emission of microsecond duration. Both fracture and emission are attributed to the laser-initiated relaxation of the high internal stresses of the pulse laser deposition-grown films.

Transient (000)-order attenuation effects in ultrafast transmission electron diffraction

We discuss the observation of a transient (000)-order attenuation in time-resolved transmission electron diffraction experiments. It is shown that this effect causes a decrease of the diffraction intensity of all higher diffraction orders. This effect is not unique to specific materials as it was observed in thin Au, Ag and Cu films.

Ionization mechanisms in dielectrics irradiated by femtosecond laser pulses (Poster Award Paper)

In order to investigate the ultrafast dynamics of free carriers generated in bulk dielectrics by intense femtosecond laser pulses we have designed a setup for ultrafast time-resolved imaging Mach-Zehnder interferometry. The application of the 2D-Fourier-transform technique allows us to accurately reconstruct the actual laser-induced phase shifts and transmission changes for the probe pulses, which provide the properties of free carriers. Interferometric measurements in high-purity fused silica clearly demonstrate that the dominant ionization mechanism for intensities below 10 TW/cm2 is multiphoton ionization.