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Laser ARPES

FIG. 1  (a) An in-house prism-KBBF-prism device. (b)Schematics showing the momentum space corresponding to a 1° x 1° detector region reached by various photon energies.

Laser-based ARPES represents the next step in the evolution of this venerable experimental technique. Traditionally, ultraviolet-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 crystal that can produce photons in the ultraviolet (UV) range with sufficiently high flux for ARPES measurements. It was not until recently that KBBF (potassium beryllium fluoroborate), a nonlinear crystal capable of generating UV photons with energy higher than 7 eV, 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 the 7 eV photon.

Momentum Resolution

One distinct advantage of using photons with lower energy from a laser source as compared to using higher energy sources is an inherently higher momentum resolution, as the detector-angle-to-momentum conversion scales directly with photoelectron kinetic energy. In Fig. 1(b), we plot the corresponding 1° x 1° detector region at different excitation photon energies in unit of Å-1. For reference, the first quadrant of the Brillouin zone and the Fermi surface of optimally doped Bi2212, 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.

A natural compromise of using low-energy photons is the shrunken momentum space that can be probed by ARPES. For example, the 7 eV laser source is unable to reach the antinodal region of Bi2212, where interesting physics such as pseudogap is suggested to manifest in ARPES spectra. Hence, in addition to a 7 eV laser source, we have recently commissioned an 11 eV laser source that largely preserves the superior momentum resolution and in the meantime, it has been demonstrated to cover a much larger momentum space, including the entire first Brillouin zone of Bi2212. These two laser sources hold the promising possibilities for investigating finer details that are previously missed due to low resolution.

FIG. 2  Band dispersion, MDC, and EDC for an optimally-doped Bi2212.

Energy Resolution

The laser sources we are using are at least an order of magnitude brighter than a similarly monochromatized line from a synchrotron source or Helium 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 Bi2212, which were taken by our 7 eV 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.

Bulk Sensitivity

It has been shown that electron mean free paths inside a solid increase dramatically with decreasing kinetic energy [1]. This means that the mean free path determines the depth at which ARPES can probe into the sample. Therefore, the lower photon energy provided by laser ARPES allows us to make this traditionally surface-sensitive measurement more bulk-sensitive.

References

[1] M. P. Seah and W. A. Dench. Quantitative electron spectroscopy of surfaces: A standard data base for electron inelastic mean free paths in solids. Surface and Interface Analysis 1, 2 (1979)