Recent Projects

Matter in Intense X-ray Fields

We are currently constructing an ultrafast, high intensity, soft x-ray source based on high harmonics of a TW laser system. With this source, we can begin to study light matter interaction in the unexplored regime of high field (E > 1V/Angstrom) and high frequency (ω > 1 a.u.). We are particularly interested in discovering nonlinear ways to manipulate x-ray scattering and absorption from atoms and large macromolecules.

Femtosecond X-ray Beamlines 6.0.1 and 6.0.2

The femtosecond soft x-ray beamline 6.0.1 has just come online with its hard x-ray counterpart scheduled to come online this winter. The beamlines use laser slicing to deliver 100 fs pulses of x-ray light at 20 kHz repitition rate, or a usable fs x-ray flux of 3E6 photons/s/0.1% bandwidth.

Laser Induced Melting

Our group recently participated in a collaboration at the Sub Picosecond Pulsed Source (SPPS) to look at laser induced, non-thermal melting in InSb. The sample is pumped by a femtosecond laser pulse and probed simultaneously by the roughly ~80 fs x-ray pulse of SPPS at grazing incidence.

The two beams cross at an angle, so that some parts of the sample are hit with the laser first and other parts with the x-rays first. Time between pump and probe is projected along the long axis of the x-ray spot. Diffraction occurs if the sample is still ordered but disappears as the sample melts and long range order is lost.

Laser excitation excites more than 10 percent of the valence electrons to the conduction band of the semiconductor, creating a dense, high temperature, electron hole plasma. However, as electron-phonon coupling times are on the order of one picosecond, the lattice remains cool for the first few hundred femtoseconds. Still, we observed the (220) Bragg diffraction signal to fall by 80 percent in 280 fs, indicating that the lattice has disordered. By comparing data from different Bragg reflections, we determined that for the first few hundred femtoseconds, ion core motion is dominated by inertia, and the atoms drift towards disorder at their original room temperature velocities.