Thibault Dartigalongue

The Formation of Warm Dense Matter : Experimental Evidence for Electronic Bond Hardening in Gold

Under strong optical excitation conditions, it is possible to create highly nonequilibrium states of matter. The nuclear response is determined by the rate of energy transfer from the excited electrons to the nuclei and the instantaneous effect of change in electron distribution on the interatomic potential energy landscape. We used femtosecond electron diffraction to follow the structural evolution of strongly excited gold under these transient electronic conditions. Generally, materials become softer with excitation. In contrast, the rate of disordering of the gold lattice is found to be retarded at excitation levels up to 2.85 megajoules per kilogram with respect to the degree of lattice heating, which is indicative of increased lattice stability at high effective electronic temperatures, a predicted effect that illustrates the strong correlation between electronic structure and lattice bonding.

Grating enhanced ponderomotive scattering for visualization and full characterization of femtosecond electron pulses.

Real time views of atomic motion can be achieved using electron pulses as structural probes. The requisite time resolution requires knowledge of both the electron pulse duration and the exact timing of the excitation pulse and the electron probe to within 10 - 100 fs accuracy. By using an intensity grating to enhance the pondermotive force, we are now able to fully characterize electron pulses and to confirm many body simulations with laser pulse energies on the microjoule level. This development solves one of the last barriers to the highest possible time resolution for electron probes.

Femtosecond electron pulse characterization using laser ponderomotive scattering

We demonstrate a method for the measurement of the instantaneous duration of femtosecond electron pulses using the ponderomotive force of an intense ultrashort laser pulse. An analysis procedure for the extraction of the electron pulse duration from the transient change of the transverse electron beam profile is proposed. The durations of the electron pulses generated in our setup were determined to be 410+/-30 fs.

Femtosecond electron diffraction: an atomic perspective of condensed phase dynamics

Femtosecond electron diffraction (FED) is a new technique within the still-developing field of ultrafast diffraction. This paper presents an outline of the basic features of FED, including a brief history of its development in terms of the technical challenges of working with femtosecond electron pulses and the ultrathin samples required. Application of FED to melting in aluminium and gold excited by intense femtosecond laser pulses will be discussed. The interplay of experiment and theory will be explored, particularly with respect to molecular dynamics simulations of the same processes we experimentally observe. Homogeneous nucleation emerges as an important melting mechanism under the strongly-driving conditions that we employ. Future applications of FED will be discussed in terms of progress to date.

Experimental basics for femtosecond electron diffraction studies

This paper provides a practical introduction to the current practice of femtosecond electron diffraction. We emphasize a general implementation suitable for a wide class of photoactive samples, including nonreversible samples. High density femtosecond electron pulses are required and the consequences for diffractometer design and sample preparation and handling are discussed. Finally, the real space structural analysis of strongly-driven melting in metals possible with our implementation is outlined.

Atomic View of the Photoinduced Collapse of Gold and Bismuth

Two different mechanisms of photoinduced melting were studied by femtosecond electron diffraction. The structural response of gold indicates an electronically-induced increase of the melting temperature. Bismuth was found to disorder within one vibrational period.

Femtosecond Electron Diffraction Study on the Melting Dynamics of Gold

The melting process in gold was resolved using femtosecond electron diffraction. The results support a thermally-driven melting mechanism with homogeneous nucleation, which is in qualitative agreement with previous work on aluminum.

Grating Enhanced Ponderomotive Scattering for Characterization of Femtosecond Electron Pulses

We demonstrate a method for measuring the duration of femtosecond electron pulses capable of 10 fs accuracy, using the ponderomotive force of the intensity grating produced by counterpropagating laser pulses in the microjoule energy range.

Electronically Driven Structural Dynamics of Si Resolved by Femtosecond Electron Diffraction

Femtosecond electron diffraction studies on (001)-oriented single crystalline Si found that at low excitation, longitudinal and transverse [001] acoustic phonon modes were generated. At ~11% valence excitation, the lattice collapsed non-thermally in <500 fs.

Femtosecond Electron Diffraction Study on the Heating and Melting Dynamics of Gold

The heating and melting dynamics of a 20 nm thick gold film were studied using femtosecond electron diffraction. After heating to 30% above the melting point the gold film underwent a thermally-driven heterogeneous phase transition by melting front propagation.