Department of Physics, University of California, Berkeley
Materials Sciences Division, Lawrence Berkeley National Lab
Our group is on the frontiers of time-resolved and angle-resolved photoemission spectroscopy (trARPES). Our home-built trARPES system is modified from a state of the art laser-ARPES system. A titanium sapphire laser emits near-infrared pulses with energy 1.48 eV, which is enough to perturb a sample, but not enough to photoemit electrons. Using two Beta-barium borate crystals, we quadruple the frequency of the laser pulses to ultraviolet (5.93 eV), which allows the laser to eject electrons for us to measure.
The infrared pulse is called the pump, and the ultraviolet pulse is called the probe. The time interval between the pump and probe can be controlled by mirrors on a moving stage. By taking data at different time intervals, we can assemble a "film" of a sample's electronic structure as it responds to the pump (hover mouse over image below to see animation). This allows us to observe the dynamics of the system when it is out of equilibrium.
These experiments may help answer some key questions in condensed matter physics. For example, we have done experiments on high-temperature cuprate superconductors measuring the relaxation rate of electron quasiparticles. The experiment shows that the quasiparticles relax faster in the presence of greater quasiparticle density, indicating that the quasiparticles are recombining into superconducting pairs. We are also interested in doing experiments on other materials such as iron pnictides, topological insulators, and normal metals.
Our group has both a time-resolved ARPES system and a spin-ARPES system. But so far these two experiments have not yet been combined. We are now working on adding time-resolution to our spin-ARPES system, which will allow us to observe directly electron spin dynamics of materials.
An ultrafast angle-resolved photoemission apparatus for measuring complex materials
Christopher L. Smallwood, Christopher Jozwiak, Wentao Zhang, and Alessandra Lanzara
Rev. Sci. Instrum. 83, 123904 (2012)
Tracking Cooper Pairs in a Cuprate Superconductor by Ultrafast Angle-Resolved Photoemission
Christopher L. Smallwood, James P. Hinton, Christopher Jozwiak, Wentao Zhang, Jake D. Koralek, Hiroshi Eisaki, Dung-Hai Lee, Joseph Orenstein, and Alessandra Lanzara
Science 336, 1137 (2012)
Nodal quasiparticle meltdown in ultrahigh-resolution pump-probe angle-resolved photoemission
J. Graf, C. Jozwiak, C. L. Smallwood, H. Eisaki, R. A. Kaindl, D-H. Lee, and A. Lanzara
Nature Physics 7, 805-809 (2011)