CU-Boulder Develops Laser for ARPES

BOULDER, Colo., Jan. 11, 2006 -- Researchers have developed a new application for lasers in the field of condensed matter physics that they say leads the way for their use in angle-resolved photoemission, or ARPES. The team, led by University of Colorado at Boulder (CU-Boulder) physics professor Dan Dessau, has developed a system to perform ARPES using 6 eV photons from the fourth harmonic of a mode-locked Ti:sapphire laser.

Schematic of the laser ARPES system. SHG = second harmonic generation, FHG = fourth harmonic generation, and UHV = ultrahigh vacuum. (Image courtesy University of Colorado, Boulder)

ARPES is the most direct way to observe the quantum mechanical structure of electrons in solids and is one of the key tools used in the quest to understand the complex electronic interactions responsible for high-temperature superconductivity. The technique is based on Einstein’s photoelectric effect, where photons of sufficient energy eject electrons from a solid. Because the electron momentum is conserved in this process, the angular distribution of photoelectrons is representative of the initial electronic states in the solid.
Typically, ARPES experiments are performed at large multi-user synchrotron light sources costing on the order of a hundred million dollars. Instead, the CU-Boulder system uses 6 eV photons from the fourth harmonic of a Ti:Sapphire laser produced through two stages of nonlinear second harmonic generation in BBO. The resulting flux of 2 x 10¹⁴ photons/s in a bandwidth of less than 5 meV represents about two orders of magnitude of improvement over even the best synchrotron beamlines, said Jake Koralek, a graduate student working under phsycis professor Dan Dessau to develop this system. The relatively low photon energy also greatly increases the momentum resolution and decreases the background signal of ARPES relative to higher energy synchrotrons, he said.

Comparison of ARPES data taken using laser and synchrotron light sources (Image courtesy University of Colorado, Boulder)

"The pulsed nature of the Ti:sapphire laser also opens up the possibility to directly observe electron dynamics using ARPES," according to Koralek. "These advantages have enabled us to produce the clearest images yet of electrons in a high-temperature superconductor."
Koralek said the team expects the laser will be used more and more for ARPES experiments in the future, although they won't eliminate the usefulneess of synchrotrons. He said their laser sits on an 8-by-4-foot table and costs approximately $200,000; it also has the benefit of allowing researchers complete control over their experiments, in their own labs.
Their results are being published this week in Physical Review Letters and recently appeared in Science Magazine.
For more information, visit: www.colorado.edu/physics