In the area of ultra-fast excitation of materials, we have a long-standing collaboration with experimental groups (David Reis, Philip Bucksbaum, Roberto Merlin) at the FOCUS Center of the University of Michigan. We have calculated the coupled electronic and vibrational response of Ge thin films, in response to ultra-fast optical excitation of carriers.
This work is of interest in the development of a sub-nanosecond x-ray switch. Eamonn Murray is calculating the vibrational properties of highly photo-excited bismuth, in which up to 2% of the valence electrons in the material are briefly excited into the conduction bands. Using both frozen-phonon and density functional perturbation methods, he has calculated the softening of phonons in the photo-excited material and is currently working on the calculation of anharmonic decay. Very recent ultra-fast x-ray pump-probe experiments by David Reis's group at Stanford Linear Accelerator have provided remarkable verification of Eamonn's theoretical predictions of the photo-excited energy surface of the system.