Laser ARPES
Laser-based ARPES represents the next step in the evolution of this venerable experimental technique. Traditionally, UV-ARPES has been performed with light from either a synchrotron source or from a helium lamp. The application of laser as a light source for ARPES experiments has been limited due to the lack of a suitable nonlinear crystals which can produce UV photons with high enough energy for ARPES measurements. It was not until recently that KBBF, a nonlinear crystal capable of generating UV photons with energy higher than 7eV, has become available. Fig. 1(a) shows a prism-KBBF-prism device which was made in our lab, and is used in a second harmonic generation geometry for 7 eV photon.
By pushing to lower photon energies we are able to make many experimental gains over higher-energy sources. One distinct advantage is an inherently higher momentum resolution, as the detector-angle-to-momentum conversion scales directly with photo-electron kinetic energy. In Fig. 1 (b), we plot the corresponding 1 deg. x 1 deg. detector region at different excitation photon energies in unit of Å-1. As a reference, the first quadrant of the Brillouin zone and the Fermi surface of optimally doped Bi-2212, as measured by laser ARPES, are also plotted. It is clear that at lower photon energy, one can sample the momentum space in a much finer grid, given an identical angular range of the detector.
Second, these laser sources are at least an order of magnitude brighter than a similarly monochromatized line from a synchrotron source or He lamp, which in turn allows us to work at finer energy resolutions and collect higher quality data in a shorter amount of time. Fig. 2 shows the nodal spectra of an optimally-doped high-Tc cuprate Bi-2212, which were taken by our Laser ARPES system. In this measurement, the energy resolution and momentum resolution used are 3 meV and better than 0.003 Å-1, respectively. Because of the high resolution of the experiment, sharp quasi-particle peaks in both momentum space (MDC) and energy space (EDC) can be obtained. These high resolution spectra will be crucial for quantitative analyses of cuprate physics.
Finally, it has been shown that electron mean free paths inside a solid increase dramatically with decreasing kinetic energy, andthis mean free path determines the depth which ARPES is able to probe in the sample being measured. Therefore, the lower photoenergy provided by laser ARPES allows us to make this traditionally surface-sensitive measurement more bulk-sensitive.
