About

Siddharth Karkare is an associate professor in the Department of Physics at Arizona State University. He earned his Ph.D. in physics from Cornell University in 2015 and subsequently worked as a post-doctoral researcher at the Lawrence Berkeley National Laboratory before joining ASU in 2018. Prof. Karkare's research lies at the intersection of accelerator physics and nanoscience, focusing on the generation and manipulation of bright electron beams. His work spans applications from meter-scale electron microscopes to large kilometer-scale particle colliders and free-electron lasers. At ASU, he established the Photoemission and Bright Beams lab, which investigates the fundamental physics of photoemission and aims to enhance the brightness of electron beams. Prof. Karkare was honored with the Department of Energy Early Career Award in 2020 and was a Moore Inventor Fellow Finalist in 2021.

Research topics

• Atomic physics
• Physics
• Nuclear physics
• Materials science
• Optics
• Quantum mechanics
• Condensed matter physics

Selected publications

• Picometer-scale emittance and space charge effects in nanostructured photocathodes

  Open MIND · 2026-01-29

  articleSenior author

  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.

  DOI

• Investigating Dirac semimetal cadmium arsenide as a potential low-MTE photocathode

  Open MIND · 2026-01-29

  articleSenior author

  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.

  DOI

• Enhanced quantum efficiency from optical interference in alkali antimonide photocathodes: Modeling and experimental results

  Journal of Applied Physics · 2026-02-12

  articleOpen access

  We present measurements of enhanced quantum efficiency (QE) in thin film alkali antimonide photocathodes from optical interference in the cathode-substrate multilayer. Modulations in the spectral response are observed over a range of visible wavelengths and are shown to increase the QE by more than a factor of two at specific wavelengths. We present a model describing the QE modulations based on the three step photoemission process incorporating cases of both constant density of states and density functional theory-derived density of states and show that the calculated results are in good agreement with the measurements. Model predictions demonstrate that QE can be enhanced by more than a factor of 5 by optimization of cathode and substrate layer thicknesses. Additionally, these calculations reveal that optical interference can yield higher quantum efficiencies in thin films compared to thick, optically dense films. We model the QE vs excitation wavelength of multiple alkali antimonide compounds at different thicknesses. We then discuss the advantages of this interference effect for electron accelerators.

  PublisherDOI

• Supplementary Material

  AIP Publishing · 2026-01-01

  datasetOpen access

  Supplemental data set including density of states calculation results for the studied materials.

  PublisherDOI

• Optimizing 4D emittance measurements using the pinhole scan technique

  JACOW · 2026-01-29

  articleOpen accessSenior author

  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.

  PublisherDOI

• Density functional theory approach for calculating electronic band structure parameters for Monte Carlo simulations of photoemission

  Open MIND · 2026-01-29

  articleSenior author

  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.

  DOI

• Supplementary Material

  Open MIND · 2026-01-01

  dataset

  Supplemental data set including density of states calculation results for the studied materials.

  DOI

• Enhanced quantum efficiency from optical interference in alkaliantimonide photocathodes: modeling and experimental results

  Open MIND · 2026-01-01

  other

  We present measurements of enhanced quantum efficiency (QE) in thin film alkali antimonide photocathodes fromoptical interference in the cathode-substrate multilayer. Modulations in the spectral response are observed over a rangeof visible wavelengths, and are shown to increase the QE by more than a factor of two at specific wavelengths. Wepresent a model describing the QE modulations based on the three step photoemission process incorporating cases ofboth constant density of states and density functional theory (DFT)-derived density of states and show that the calculatedresults are in good agreement with the measurements. Model predictions demonstrate that QE can be enhanced by morethan a factor of 5 by optimization of cathode and substrate layer thicknesses. Additionally, these calculations revealthat optical interference can yield higher quantum efficiencies in thin films compared to thick, optically dense films.We model the QE versus excitation wavelength of multiple alkali antimonide compounds at different thicknesses. Wethen discuss the advantages of this interference effect for electron accelerators.

  DOI

• Enhanced quantum efficiency from optical interference in alkaliantimonide photocathodes: modeling and experimental results

  AIP Publishing · 2026-01-01

  otherOpen access

  We present measurements of enhanced quantum efficiency (QE) in thin film alkali antimonide photocathodes fromoptical interference in the cathode-substrate multilayer. Modulations in the spectral response are observed over a rangeof visible wavelengths, and are shown to increase the QE by more than a factor of two at specific wavelengths. Wepresent a model describing the QE modulations based on the three step photoemission process incorporating cases ofboth constant density of states and density functional theory (DFT)-derived density of states and show that the calculatedresults are in good agreement with the measurements. Model predictions demonstrate that QE can be enhanced by morethan a factor of 5 by optimization of cathode and substrate layer thicknesses. Additionally, these calculations revealthat optical interference can yield higher quantum efficiencies in thin films compared to thick, optically dense films.We model the QE versus excitation wavelength of multiple alkali antimonide compounds at different thicknesses. Wethen discuss the advantages of this interference effect for electron accelerators.

  PublisherDOI

• Light-induced enhancement of quantum efficiency in III-nitride photocathodes

  Open MIND · 2026-01-29

  article

  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.

  DOI

Frequent coauthors

• Ivan Bazarov

  Cornell University

  61 shared

• H. A. Padmore

  48 shared

• Jared Maxson

  Cornell University

  33 shared

• Jai Kwan Bae

  Cornell University

  30 shared

• Dimitre Dimitrov

  27 shared

• L. Cultrera

  Brookhaven National Laboratory

  26 shared

• Oksana Chubenko

  Northern Illinois University

  26 shared

• J. Smedley

  University of Rochester

  20 shared

Education

• Ph.D.

  Cornell University

  2015

Awards & honors

• Department of Energy Early Career Award (2020)
• Moore Inventor Fellow Finalist (2021)