Author: Karkare, S.S.
Paper Title Page
WEPAB099 Near-Threshold Nonlinear Photoemission From Cu(100) 2822
 
  • C.J. Knill, S.S. Karkare
    Arizona State University, Tempe, USA
  • H.A. Padmore
    LBNL, Berkeley, California, USA
 
  Funding: National Science Foundation Grant No. PHY-1549132
Photocathodes that have a low mean transverse energy (MTE) are crucial to the development of compact X-ray Free Electron Lasers (XFEL) and ultrafast electron diffraction (UED) experiments. For FELs, low MTE cathodes result in a lower requirement for electron energy when lasing at a defined energy, and for a defined electron energy result in lasing at higher energy. For UED experiments, low MTE cathodes give a longer coherence length, allowing measurements on larger unit cell materials. A record low MTE of 5 meV has been recently demonstrated from a Cu (100) surface when measured near the photoemission threshold and cooled down to 30 K with liquid Helium [*]. For UED and XFEL applications that require a high charge density, the low quantum efficiency of Cu (100) near threshold necessitates the use of a high laser fluence to achieve the desired charge density [**]. At high laser fluences the MTE is limited by nonlinear effects, and therefore it is necessary to investigate near photoemission threshold at these high laser fluences. In this paper we report on nonlinear, near-threshold photoemission from a Cu (100) cathode, and its effect on the MTE.
* S. Karkare et al, Phys. Rev. Lett. 125, 054801 (2020)
** J. Bae et al, J. Appl. Phys., 124, 244903 (2018)
 
poster icon Poster WEPAB099 [0.829 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB099  
About • paper received ※ 19 May 2021       paper accepted ※ 21 July 2021       issue date ※ 29 August 2021  
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WEPAB169 Towards Ultra-Smooth Alkali Antimonide Photocathode Epitaxy 3001
 
  • E.J. Montgomery
    Private Address, Bolingbrook, USA
  • O. Chubenko, G.S. Gevorkyan, S.S. Karkare, P. Saha
    Arizona State University, Tempe, USA
  • R.G. Hennig, J.T. Paul
    University of Florida, Gainesville, Florida, USA
  • C. Jing, S. Poddar
    Euclid Beamlabs, Bolingbrook, USA
  • H.A. Padmore
    LBNL, Berkeley, California, USA
 
  Funding: Work supported by Department of Energy, Office of Science, Office of Basic Energy Sciences, under grant number DE-SC0020575.
Photocathodes lead in brightness among electron emitters, but transverse momenta are unavoidably nonzero. Ultra-low transverse emittance would enable brighter, higher energy x-ray free-electron lasers (FEL), improved colliders, and more coherent, detailed ultrafast electron diffraction/microscopy (UED/UEM). Although high quantum efficiency (QE) is desired to avoid laser-induced nonlinearities, the state-of-the-art is 100 pC bunches from copper, 0.4 mm-mrad emittance. Advances towards 0.1 mm-mrad require ultra-low emittance, high QE, cryo-compatible materials. We report efforts towards epitaxial growth of cesium antimonide on lattice matched substrates. DFT calculations were performed to downselect from a list of candidate lattice matches. Co-evaporations achieving >3% QE at 532 nm followed by atomic force and Kelvin probe microscopy (AFM and KPFM) show ultra-low 313 pm rms (root mean square) physical and 2.65 mV rms chemical roughness. We simulate roughness-induced mean transverse energy (MTE) to predict <1 meV from roughness effects at 10 MV/m in as-grown optically thick cathodes, promising low emittance via epitaxial growth.
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB169  
About • paper received ※ 19 May 2021       paper accepted ※ 02 June 2021       issue date ※ 11 August 2021  
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THPAB142 Optical and Surface Characterization of Alkali-Antimonide Photocathodes 4037
 
  • P. Saha, O. Chubenko, G.S. Gevorkyan, A.H. Kachwala, S.S. Karkare, C.J. Knill
    Arizona State University, Tempe, USA
  • E.J. Montgomery, S. Poddar
    Euclid Beamlabs, Bolingbrook, USA
  • H.A. Padmore
    LBNL, Berkeley, California, USA
 
  Alkali-antimonides, characterized by high quantum efficiency and low mean transverse energy in visible light, are excellent electron sources to drive x-ray free electron lasers, electron cooling and ultrafast electron diffraction applications etc. Existing studies of alkali-antimonides have focused on quantum efficiency and emittance, but information is lacking on the fundamental aspects of the electronic structure, such as the energy gap of the semiconductor and the density of defects as well as the overall nano-structure of the materials. We are, therefore, conducting photoconductivity measurements to measure fundamental semiconductor properties as well as using atomic force microscope (AFM) and kelvin probe force microscope (KPFM) to measure the nanostructure variations in structure and surface potential.  
poster icon Poster THPAB142 [1.211 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-THPAB142  
About • paper received ※ 16 May 2021       paper accepted ※ 14 July 2021       issue date ※ 13 August 2021  
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