Jean-Claude Kieffer

Evidence for a structurally-driven insulator-to-metal transition in VO2 : a view from the ultrafast timescale

We apply ultrafast spectroscopy to establish a time-domain hierarchy between structural and electronic effects in a strongly correlated electron system. We discuss the case of the model system

Laser-Induced Electron Tunneling and Diffraction

{\mathrm{VO}}_{2}

ULTRAFAST X-RAY ABSORPTION PROBING OF A CHEMICAL REACTION

, a prototypical nonmagnetic compound that exhibits cell doubling, charge localization, and a metal-insulator transition below 340 K. We initiate the formation of the metallic phase by prompt hole photo-doping into the valence band of the low-

Microwave guiding in air by a cylindrical filament array waveguide

T

Compression of 1.8 μm laser pulses to sub two optical cycles with bulk material

insulator. The insulator-to-metal transition is, however, delayed with respect to hole injection, exhibiting a bottleneck time scale, associated with the phonon connecting the two crystallographic phases. This structural bottleneck is observed despite faster depletion of the

Optical switching in VO2 films by below-gap excitation

d

Laser-based microfocused x-ray source for mammography: feasibility study.

bands and is indicative of important bandlike character for this controversial insulator.

Guiding large-scale spark discharges with ultrashort pulse laser filaments

Molecular structure is usually determined by measuring the diffraction pattern the molecule impresses on x-rays or electrons. We used a laser field to extract electrons from the molecule itself, accelerate them, and in some cases force them to recollide with and diffract from the parent ion, all within a fraction of a laser period. Here, we show that the momentum distribution of the extracted electron carries the fingerprint of the highest occupied molecular orbital, whereas the elastically scattered electrons reveal the position of the nuclear components of the molecule. Thus, in one comprehensive technology, the photoelectrons give detailed information about the electronic orbital and the position of the nuclei.

Generation of Intense Terahertz Radiation via Optical Methods

Ultrafast x‐ray techniques can, in principle, allow us to more directly watch the time evolution of matter, with atomic spatial resolution and with time resolution on the scale of atomic motions such as the making and breaking of chemical bonds, in order to more directly observe the fundamental molecular dynamics underlying the concept of ‘‘mechanism’’ in inorganic, organic, and biochemical reactions. As a step toward this goal, we have observed a chemical reaction process, photoinduced dissociation of gas phase SF6 molecules, detected by ultrafast near‐edge x‐ray absorption spectroscopy with time resolutions of 1.5–3 ps, near the sulfur K edge at a photon energy of 2.48 keV (4.98 A).

CEP stable 1.6 cycle laser pulses at 1.8 μm

Microwave guiding was demonstrated over 16cm in air using a large diameter hollow plasma waveguide. The waveguide was generated with the 100TW femtosecond laser system at the Advanced Laser Light Source facility. A deformable mirror was used to spatially shape the intense laser pulses in order to generate hundreds of filaments judiciously distributed in a cylindrical shape, creating a cylindrical plasma wall that acts as a microwave waveguide. The microwaves were confined for about 10ns, which corresponds to the free electron plasma wall recombination time. The characteristics of the plasma waveguide and the results of microwave guiding are presented.