Kali Prasanna Mondal

The performance of electron accelerators and photon detectors heavily depends on the efficiency and stability of the photocathode. The emitted beam current and brightness can be limited by surface and bulk disorder in polycrystalline photocathode materials. Epitaxial growth offers a promising route to mitigate these limitations and enable high-current, high-brightness electron beams. In this work, we present the first demonstration of the epitaxial growth of a bi-alkali antimonide photocathode, K2CsSb, utilizing both pulsed laser deposition-assisted and thermal evaporation techniques. In our study, the growth of the epitaxial layer with a flat surface was confirmed by reflection high-energy electron diffraction for the K2CsSb thin film. In situ and real-time x-ray characterization were utilized to confirm the film stoichiometry, surface roughness, and crystallinity. Quantum efficiency (QE) of 3% at 530 nm and 9% at 450 nm was obtained from an epitaxial K2CsSb thin film with a film thickness as thin as 6 nm with an ultrasmooth surface of 0.4 nm roughness on a Gr/4H–SiC substrate. Increasing the thickness to 20 nm enhanced the QE to 4.5% and 14.5% at 530 and 450 nm, respectively, with the surface roughness remaining under 1 nm. In a follow-up experiment, QE of over 9% (530 nm) and 21% (450 nm) were achieved from an epitaxial K2CsSb thin film grown on a 4H–SiC single-crystal substrate.

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.

GaAs-based photocathodes are the primary choice for polarized electron sources, commonly used in polarized electron microscopes and polarized positron sources. GaAs photocathodes are typically activated with cesium and oxygen, which are highly reactive and require an ultra-high vacuum (∼10−11 Torr or lower) to operate reliably, resulting in substantial operational difficulties. A short exposure to a mediocre vacuum results in an instantaneous loss of cathode quantum efficiency (QE) due to the chemical reaction of the active layer with residual gas molecules or back-bombardment ions during operation. Covering the GaAs cathode with a 2D material, such as monolayer graphene, could provide protection against such damage due to the inhibition of chemical reactions with residual gas molecules. In this paper, we have incorporated a method known as intercalation to pass the active material underneath the graphene and activate the superlattice GaAs/GaAsP (SL-GaAs) photocathode. X-ray photoelectron spectroscopy, low-energy electron microscopy, and Mott scattering measurements were performed to evaluate the formation of the photocathode under graphene, as well as its spectral response and electron spin polarization. Our results demonstrate that the successful activation of the SL-GaAs photocathode with a graphene protection layer is achieved with a moderate QE. Furthermore, we found that the electron spin polarization of the cathode with a surface protection layer is higher than the conventional cathode without a protection layer.

Polarized electron beams play critical role in fundamental physics research by providing additional observables and opening new channels of discoveries. This discovery potential is well-known and is successfully used in high-energy and nuclear physics research. Recently, more conventional branches of science, such as ultra-fast electron microcopy, started exploitation unique features of polarized electrons. Currently GaAs crystals illuminated by circular polarized IR lasers remain the best choice for generating polarized electrons. All existing polarized sources are an electrostatic electron guns providing extremely high vacuum (XHV) conditions for survival of GaAs photo-emissivity. But limits in accelerating voltage and its gradient limits both the quality and quantity of available beams from these guns. These are the reasons why accelerator community was and is attempting to extend this technology to the radio-frequency electron guns, which are capable of accelerating beams with significantly higher accelerating gradients and total accelerating voltage. Unfortunately, all previous attempts of operating GaAs photocathodes in RF guns were unsuccessful. In this paper, we report on successful operation of GaAs photocathode in superconducting RF gun, describe in detail the accelerator system, used techniques, evolution of the GaAs quantum efficiency, and parameters of the generated electron beam.

The quality of organic semiconductor tris-(8-hydroxyquinoline)aluminum (Alq ₃ ) thin films, deposited at the rate of 5.0 Å s ⁻¹ , 2.5 Å s ⁻¹ , and 1.0 Å s ⁻¹ respectively, have been investigated using x-ray reflectivity (XRR) and grazing incidence small angle x-ray scattering (GISAXS) studies. While XRR provides information about structural details, GISAXS provides information about the morphology of the films. The film quality is very good with a surface roughness of only 8 Å. The electron density of the film increases from 0.39 e Å ⁻³ to 0.43 e Å ⁻³ due to reduction of growth rate from 5 Å s ⁻¹ to 1 Å s ⁻¹ . The higher electron density (0.43 e Å ⁻³ ) of the film deposited at the lower deposition rate (1 Å s ⁻¹ ) indicates the presence of less defects and/or porosity in the film. Average separation between pores increases from 200 Å to 500 Å and simultaneously pore depth decreases from 300 Å to 120 Å due to reduction of growth rate from 5 Å s ⁻¹ to 1 Å s ⁻¹ . The mixing between Alq ₃ and Co at the Alq ₃ /Co interface and diffusion of Co into Alq ₃ layer through Co/Alq ₃ interface have been studied by combined XRR and grazing incidence x-ray standing wave (GIXSW) measurements for the three Alq ₃ /Co/Alq ₃ /W/Si multilayers in which Alq ₃ films were deposited at the above mentioned three different rates. It is found that there is about 30 Å thick (∼ two monolayer of Alq ₃ ) mixing region at the Alq ₃ /Co interface in three multilayer samples, which suggests that mixing at this interface is independent of the growth rate of Alq ₃ films. The depth of diffusion of Co into Alq ₃ through Co/Alq ₃ interface reduces from 300 Å to 160 Å due to the reduction of deposition rate from 5 Å s ⁻¹ to 1 Å s ⁻¹ . This study suggests that by controlling the growth rate of Alq ₃ , porosity as well as the diffusion of ferromagnetic Co in an organic semiconductor Alq ₃ can be reduced, which plays an important role in the performance of organic spin valve devices.