Hisato Yamaguchi

Reducing the mean transverse energy (MTE) of electrons emitted from photocathodes will improve the performance of linear accelerator applications like x-ray free electrons lasers (XFELs) and ultrafast electron diffraction (UED) and microscopy (UEM) experiments. Metallic photocathodes are popular for these applications due to their convenience of use. However, they typically have a very low quantum efficiency and require a large laser fluence in order to extract the desired charge density for applications like XFELs and single-shot UED/UEM experiments. Recent theoretical investigations have shown that nonlinear photoemission effects of multiphoton emission and electron heating increase the MTE dramatically at these large laser fluences. In this paper, we report on measurements of nonlinear near-threshold photoemission from a graphene-coated Cu(110) photocathode at laser pulse lengths of 130 fs, 1 ps, and 10 ps. We extrapolate our measured data to find the ideal irradiating photon energy that results in a minimum MTE from such metallic photocathodes for charge densities relevant to photoinjectors and specify quantum efficiency requirements to obtain the thermally limited MTE at these charge densities.

We report a lowering of work function for lanthanum hexaboride (LaB6) by monolayer hexagonal boron nitride (hBN) coating. Photoemission electron microcopy (PEEM) and thermionic emission electron microscopy (TEEM) both revealed that the hBN coated region of a LaB6 (100) single crystal has a lower work function compared to the bare (i.e., non-coated) and graphene coated regions. A broad and uniform brighter image of the hBN coated region in PEEM was quantitatively supported by a 0.4 eV decrease in the work function in photoelectron spectra compared to the bare region. TEEM results were consistent in that the hBN coated region exhibited thermionic emission at 905 °C, whereas the bare and graphene coated regions did not. A larger decrease in the work function for hBN coated LaB6 (100) compared to graphene coated LaB6 (100) was qualitatively supported by our density functional theory calculations. Adding an oxide layer in the calculations improved consistency between the calculation and experimental results. We followed up our calculations with synchrotron-radiation x-ray photoelectron spectroscopy and confirmed the presence of an oxide layer on our LaB6.

Protection of free-electron sources has been technically challenging due to lack of materials that transmit electrons while preventing corrosive gas molecules. Two-dimensional materials uniquely possess both of required properties. Here, we report three orders of magnitude increase in active pressure and factor of two enhancement in the lifetime of high quantum efficiency (QE) bialkali photocathodes (cesium potassium antimonide (CsK2Sb)) by encapsulating them in graphene and thin nickel (Ni) film. The photoelectrons were extracted through the graphene protection layer in a reflection mode, and we achieved QE of similar to 0.17% at similar to 3.4 eV, 1/e lifetime of 188 h with average current of 8.6 nA under continuous illumination, and no decrease of QE at the pressure of as high as similar to 1 x 10(-3) Pa. In comparison, the QE decreased drastically at 10(-6) Pa for bare, non-protected CsK2Sb photocathodes and their 1/e lifetime under continuous illumination was similar to 48 h. We attributed the improvements to the gas impermeability and photoelectron transparency of graphene.