Monte Carlo simulations are a powerful tool for modeling photoemission from photocathodes, enabling the prediction of key parameters such as quantum efficiency, mean transverse energy, electron spin polarization, and photocathode response time. However, these simulations require material band structure parameters, which are not always available from experiments. This work aims to establish a reliable framework for calculating electronic band structure parameters using Density Functional Theory (DFT). Specifically, we apply this framework to investigate the effects of lattice strain and temperature on the electronic band structure and electron transport in GaAs. This approach will be further extended to explore band structure modifications in heavily p-doped semiconductors and to calculate electronic band structures of novel spin-polarized photocathode materials.

We report on the quantum efficiency (QE) and mean transverse energy (MTE) of photoemitted electrons from cadmium arsenide(Cd₃As₂), a three-dimensional Dirac semimetal (3D DSM) of interest for photocathode applications due to its unique electronic band structure, characterized by a 3D linear dispersion relation at the Fermi energy. Samples were synthesized at the National Renewable Energy Laboratory (NREL) and transferred under ultra-high vacuum to Arizona State University (ASU) for measurement using a photoemission electron microscope (PEEM). The maximum QE was measured to be 3.37 × 10⁻⁴ at 230 nm, and the minimum MTE was 55.8 meV at 250 nm. These findings represent the first reported QE and MTE measurements of Cd₃As₂ and are an important step in evaluating the viability of 3D DSMs as low-MTE photocathodes. Such photocathodes, constrained to lower MTEs by the electronic band structure, may prove effective in advancing beam brightness in next-generation instruments and techniques.

High quantum efficiency (QE) semiconductor photocathodes are essential for generating high average beam current and brightness. One class of semiconductor photocathodes considered for use in photoinjectors for unpolarized and polarized electron beams are III-nitride heterostructures. These materials can exhibit negative electron affinity at the surface, utilizing intrinsic polarization fields to engineer the band structure without the need for additional surface treatments. In this study, we investigate the effects of light exposure on the surface of III-nitride photocathodes and the resulting changes in QE and photoemission, using photoemission electron microscopy (PEEM) for characterization. We demonstrate that exposing a GaN photocathode to a 240 nm wavelength laser at 870 µW for 15 minutes increases the QE by two orders of magnitude, with a maximum QE of 2.34 × 10⁻⁴ observed. Although III-nitride photocathodes are known for their robustness, our findings indicate that laser exposure can significantly alter their QE. Our observations reveal the need for a detailed investigation of photo-induced effects on QE in III-Nitride photocathodes."

Generation of ultralow-emittance electron beams with high brightness is critical for several applications such as ultrafast electron diffraction, microscopy, and advanced accelerator techniques. By leveraging the differences in work function and electronic structure between different materials, we enabled spatially localized photoemission, resulting in picometer-scale emittance from a flat photocathode. We also investigated space charge effects by measuring how the emission spot size, as measured in a photoemission electron microscope, changes with the number of electrons emitted per laser pulse. When more than one electron is emitted simultaneously, Coulomb repulsion causes a substantial broadening of the observed source size, enabling us to investigate the limitations imposed by vacuum space charge forces during pulsed photoemission. Our results highlight the potential of nanoscale photoemitters as high-brightness electron sources and offer new insights into electron correlations that emerge after ultrafast photoemission.

Accurate measurement of electron beam emittance is essential for optimizing high-brightness electron sources. The Pinhole Scan Technique measures the 4D phase space and hence the emittance by measuring the beam profile after clipping the beam using a pinhole followed by a drift section and then scanning the beam over the pinhole. This technique has been implemented in low (< 200 keV) beamlines at both Cornell university and Arizona State University. However, the technique poses several practical challenges. In this work, we analyze and address key issues affecting the 4D phase space and emittance measurements using this technique. We identify and investigate sources of inaccuracies like the pinhole aspect ratio, beam divergence, position-momentum correlations in the phase space, and the point-spread-function of the detector and suggest techniques to minimize them. Our findings offer a pathway to more accurate 4D phase space characterization in advanced electron beam systems.

Generation of ultralow-emittance electron beams with high brightness is critical for several applications such as ultrafast electron diffraction, microscopy, and advanced accelerator techniques. By leveraging the differences in work function and electronic structure between different materials, we enabled spatially localized photoemission, resulting in picometer-scale emittance from a flat photocathode. We also investigated space charge effects by measuring how the emission spot size, as measured in a photoemission electron microscope, changes with the number of electrons emitted per laser pulse. When more than one electron is emitted simultaneously, Coulomb repulsion causes a substantial broadening of the observed source size, enabling us to investigate the limitations imposed by vacuum space charge forces during pulsed photoemission. Our results highlight the potential of nanoscale photoemitters as high-brightness electron sources and offer new insights into electron correlations that emerge after ultrafast photoemission.

Monte Carlo simulations are a powerful tool for modeling photoemission from photocathodes, enabling the prediction of key parameters such as quantum efficiency, mean transverse energy, electron spin polarization, and photocathode response time. However, these simulations require material band structure parameters, which are not always available from experiments. This work aims to establish a reliable framework for calculating electronic band structure parameters using Density Functional Theory (DFT). Specifically, we apply this framework to investigate the effects of lattice strain and temperature on the electronic band structure and electron transport in GaAs. This approach will be further extended to explore band structure modifications in heavily p-doped semiconductors and to calculate electronic band structures of novel spin-polarized photocathode materials.

Accurate measurement of electron beam emittance is essential for optimizing high-brightness electron sources. The Pinhole Scan Technique measures the 4D phase space and hence the emittance by measuring the beam profile after clipping the beam using a pinhole followed by a drift section and then scanning the beam over the pinhole. This technique has been implemented in low (< 200 keV) beamlines at both Cornell university and Arizona State University. However, the technique poses several practical challenges. In this work, we analyze and address key issues affecting the 4D phase space and emittance measurements using this technique. We identify and investigate sources of inaccuracies like the pinhole aspect ratio, beam divergence, position-momentum correlations in the phase space, and the point-spread-function of the detector and suggest techniques to minimize them. Our findings offer a pathway to more accurate 4D phase space characterization in advanced electron beam systems.

We report on the quantum efficiency (QE) and mean transverse energy (MTE) of photoemitted electrons from cadmium arsenide (Cd₃As₂), a three-dimensional Dirac semimetal (3D DSM) of interest for photocathode applications due to its unique electronic band structure, characterized by a 3D linear dispersion relation at the Fermi energy. Samples were synthesized at the National Renewable Energy Laboratory (NREL) and transferred under ultra-high vacuum to Arizona State University (ASU) for measurement using a photoemission electron microscope (PEEM). The maximum QE was measured to be 3.37 × 10⁻⁴ at 230 nm, and the minimum MTE was 55.8 meV at 250 nm. These findings represent the first reported QE and MTE measurements of Cd₃As₂ and are an important step in evaluating the viability of 3D DSMs as low-MTE photocathodes. Such photocathodes, constrained to lower MTEs by the electronic band structure, may prove effective in advancing beam brightness in next-generation instruments and techniques.

High quantum efficiency (QE) semiconductor photocathodes are essential for generating high average beam current and brightness. One class of semiconductor photocathodes considered for use in photoinjectors for unpolarized and polarized electron beams are III-nitride heterostructures. These materials can exhibit negative electron affinity at the surface, utilizing intrinsic polarization fields to engineer the band structure without the need for additional surface treatments. In this study, we investigate the effects of light exposure on the surface of III-nitride photocathodes and the resulting changes in QE and photoemission, using photoemission electron microscopy (PEEM) for characterization. We demonstrate that exposing a GaN photocathode to a 240 nm wavelength laser at 870 µW for 15 minutes increases the QE by two orders of magnitude, with a maximum QE of 2.34 × 10⁻⁴ observed. Although III-nitride photocathodes are known for their robustness, our findings indicate that laser exposure can significantly alter their QE. Our observations reveal the need for a detailed investigation of photo-induced effects on QE in III-Nitride photocathodes.

Photocathodes are fundamental to the advancement of electron accelerators and photon detectors. While ultrasmooth photocathodes produced by co-deposition processes have been developed, their beam brightness remains limited by surface and bulk disorders inherent to polycrystalline structures. Epitaxial growth offers a transformative pathway to address these challenges, enabling the production of high-brightness electron beams. This work reports the pulsed laser deposition (PLD) assisted epitaxial growth of Cs2Te photocathodes on lattice-matched single-crystal substrates. Real-time growth monitoring via X-ray fluorescence (XRF) confirmed stoichiometric composition, while growth oscillations provided insights into the deposition process. The epitaxial nature of the films, characterized by a flat surface and high crystallinity, is validated through reflection high-energy electron diffraction (RHEED). Bulk crystallinity was further studied through X-ray diffraction (XRD) analysis. Spectral response of quantum efficiency (QE) with wavelength range 200 nm to 400 nm has been reported