Methodology and Implementation of a Tunable Deep-Ultraviolet Laser Source for Photoemission Electron Microscopy.

Photoemission electron microscopy (PEEM) is a unique and powerful tool for studying the electronic properties of materials and surfaces. However, it requires intense and well-controlled light sources with photon energies ranging from the UV to soft X-rays for achieving high spatial resolution and image contrast. Traditionally, many PEEMs were installed at synchrotron light sources to access intense and tunable soft X-rays. More recently, the maturation of solid-state lasers has opened a new avenue for laboratory-based PEEMs using laser-based UV light at lower photon energies. Here, we report on the characteristics of a laser-based UV light source that was recently integrated with a PEEM instrument. The system consists of a high repetition rate, tunable wavelength laser coupled to a harmonics generation module, which generates deep-UV radiation from 192 nm to 210 nm. We comment on the spectral characteristics and overall laser system stability, as well as on the effects of space charge within the PEEM microscope at high UV laser fluxes. Further, we show an example of imaging on gallium nitride, where the higher UV photon energy and flux of the laser provides considerably improved image quality, compared to a conventional light source. These results demonstrate the capabilities of laser-based UV light sources for advancing laboratory-based PEEMs.