Laser light vs. synchrotron light in ARPES measurements

  1. In ARPES measurements, people say that the laser light cannot cover a wide area of momentum space whereas the synchrotron light can (M. Hashimoto et al., Nat. Phys. 10 (2014) 483). Why is that?

    (Figure 3a in the paper is a combination of laser and synchrotron data to plot spectra along the Fermi surface from the nodal region to the antinodal region)
  2. jcsd
  3. UltrafastPED

    UltrafastPED 1,919
    Science Advisor
    Gold Member

  4. At a glance it looks like they have a "psuedo" band gap .

    Since the laser imparts negligible linear momentum, the laser excited electron from below to above the band gap must have the same linear momentum. This is because the transition matrix element (going back to Fermi's rule here)


    is always zero except when k_initial = k_final. This restricts the allowed transitions, and is especially noticeable for single crystals.

    The core states excited by x-rays in ARPES experiments are a linear combo of momentum eigen states. That is the transition matrix element


    will always be none zero because psi(k_core,E_core) has a component along almost every possible final state.
  5. The maximum momentum you can reach is k=2 pi/lambda, where lambda is the wave length of the light.

    Laser light is typically visible with wave lengths of 300 nm and above, synchrotron "light" is typically soft or hard x-rays, i.e. wave lengths of 20 Angstroms or less.

    Brightness has nothing to do with this. In fact, lasers have much higher brightness than synchrotrons. There is a lot of work going on these days on developing x-ray free electron lasers to get the best of both worlds.
  6. Thank you
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