Development of photoinjectors is a critical research area for modern accelerators. Photoinjectors produce the high brightness electron beams that have enabled the realization among others of Ultrafast Electron Diffraction and Microscopy setups, X-ray Free Electron Lasers, Energy Recovery Linacs, high average current sources for bunched electron beam cooling and spin polarized electron beams used in colliders. All these accelerator machines rely on photocathodes to produce electron beams with precisely controlled temporal and spatial shapes, often with stringent requirements on emittance, temporal response, and spin polarization. This 3-day workshop hosted by Brookhaven National Laboratory (October 3–5, 2023) will explore the current state of the art in accelerator photocathodes, from operational, theoretical and a materials science perspective, aiming also at identifying directions for future research and new opportunities for collaboration.
Topics Include:
The organizers envision a workshop format that will allow plenty of time for discussion.
Event ID: 44380
Highly efficient spin-polarized electron sources play a pivotal
role in advancing particle physics experiments, spin-polarized electron microscopy, and materials sciences. Developing such sources requires in-depth modeling of the mechanisms influencing their performance. The Monte Carlo approach is one of the very few tools capable of addressing the subtleties of material band structure and enabling simultaneous study of essential yet often competing photocathode characteristics, including quantum efficiency (QE), electron spin polarization (ESP), mean transverse energy (MTE), and response time.
In this presentation, we delve into the Monte Carlo approach to modeling spin-polarized photoemission from p-doped bulk GaAs activated to negative electron affinity (NEA). We also consider possibilities of implementing both low temperature and lattice strained adjustments to the Monte Carlo approach using Density Functional Theory (DFT) calculations. This work greatly facilitates our ability to improve the efficiency of photocathodes by shedding light on the fundamental mechanisms that constrain the performance of GaAs spin-polarized electron sources, thus establishing a model for future investigations of photoemission characteristics of other materials and more intricate heterostructures, with the potential to surpass GaAs in performance.
The development of high brightness photocathodes is of significant importance in advancing the field of accelerator applications1, 2. One of the most prominent materials for such photocathodes is Cesiated-GaAs, owing to its high quantum efficiency and negative electron affinity3. To increase the efficiency of light coupling into the photoemissive surface, a waveguide in combination with a grating coupler has been introduced to facilitate the integration of the photoemission process. Two different systems were studied; i) the first structure was achieved by transferring a 40 nm thick layer of p-GaAs onto a nanofabricated Si3N4 waveguide through the epitaxial transfer method. ii) we directly grow Cs3Sb thin film on top of waveguide for the second structure. The grating coupler was designed and positioned in front of the waveguide to optimize the transmission efficiency and effectively couple light into the waveguide. Furthermore, wavelength-dependent grating designs allow for the coupling of various wavelengths of light into the photoemissive surface. Absorption of photons by the absorber films through evanescent coupling can initiate new photoemission mechanisms, leading to brighter electron beams and enabling unprecedented shaping of emitted electrons. This research lays the groundwork for integrating photonics and nanofabrication advancements with photocathodes to create high brightness electron sources for accelerator applications.
(1) Karkare, S.; Dimitrov, D.; Schaff, W.; Cultrera, L.; Bartnik, A.; Liu, X.; Sawyer, E.; Esposito, T.; Bazarov, I. Monte Carlo charge transport and photoemission from negative electron affinity GaAs photocathodes. Journal of Applied Physics 2013, 113 (10), 104904.
(2) Maruyama, T.; Brachmann, A.; Clendenin, J.; Desikan, T.; Garwin, E. L.; Kirby, R.; Luh, D.-A.; Turner, J.; Prepost, R. A very high charge, high polarization gradient-doped strained GaAs photocathode. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 2002, 492 (1-2), 199-211.
(3) Blankemeier, L.; Rezaeifar, F.; Garg, A.; Kapadia, R. Integrated photonics for low transverse emittance, ultrafast negative electron affinity GaAs photoemitters. Journal of Applied Physics 2019, 126 (3), 033102.
Development of Air Stable Silicon Photoemitter with Electronically Tunable Negative Electron Affinity
Abstract:
Semiconductor photocathodes with negative electron affinity (NEA) surfaces can generate high quantum efficiency electron beams while illuminated by visible wavelength photons. Traditionally, NEA surfaces are created by coating the semiconductor with an extremely polar material such as cesium oxide. This chemically modified NEA surface require ultrahigh vacuum for stable operation. Here, we have bypassed these stringent requirements by using a silicon/oxide/graphene heterostructure where a bias voltage between graphene and silicon can electrostatically generate a tunable NEA surface with an emission current density of ~3.4 A/m2 at an external quantum efficiency of ~0.1%. This hot electron laser assisted cathode (HELAC) device can operate at relaxed vacuum conditions with a pressure as high as 1 mtorr and can be stored in atmospheric conditions. We have developed a theoretical model for the emission characteristics of the device that has good agreement with the experimental results. The model allows us to predict the performance limits of HELAC and also propose more optimal designs for the device structure.
The interplay between a photocathode electron source, the laser driving the photoemission and the electric field into which those electrons emerge define the electron source emittance. The need to characterise photocathode performance and specifically to measure the transverse energy spread of photoemitted electrons is therefore crucial to minimise emittance and maximise beam brightness.
The TESS concept has been developed over the last decade, and has proved to be a simple and powerful method to characterise photoemissive performance. TESS offers insights into the performance of these materials at any illumination wavelength, at room temperature or under cryogenic conditions. TESS also allows photocathode materials to be studied while subjected to progressive degradation, allowing us to assess their robustness. Furthermore, the high gain available with the TESS detector facilitates photocathode studies at the emission threshold where the energy spread approaches the thermal limit and the quantum efficiency is vanishingly small.
I will present an overview of the TESS system, highlighting the substantial developments which have been made in recent years, and will present data on some of our more recent work using the TESS in our development of a CsTe deposition capability at Daresbury, and in the use of ultra-thin metal oxide films to enhance photoemission.
One of the simplest models of tunneling makes use of a δ-function barrier V (x) = ¯h2γδ(x)/2m which depends on a single parameter γ, making it a useful tool to examine a variety of technologically important applications (i) theoretical predictions of photoemission from materials such as cesiated GaAs surfaces often include a high and thin barrier at the surface attributed to a submonolayer coating of an alkali metal such as cesium to bring theoretically predicted yields in line with photoemission experiments; (ii) time evolution simulations of quantum mechanical (QM) tunneling effects using the Wigner distribution function (WDF) and Schrodinger equation (SE) use the δ-function barrier differently to gauge how abruptness and symmetry affect numerical time evolution and because the δ-function barrier demonstrates that the passage of a wave packet incident through it is delayed, even though the barrier is of zero thickness; (iii) the δ-function describes IV characteristics of normal-superconducting (NS) point contacts where the transition from the metallic to the superconducting limit is governed by a strength γ of the barrier. Left and right states are coupled, affect the crossover from metallic to small-area tunnel-junction behavior and impact current calculations and charge imbalance processes at the normal-superconducting (NS) interface; and (iv) the interest in conduction, transport, and/or emission when either the barriers or wells have a time dependent behavior, for which using steady state emission equations may be insufficient demand the creation of new exact methods that we consider. In such cases, an examination of the physics of tractable models gives a means to examine methods useful for more complex configurations.
The analytic model of a δ-function barrier inside a confining well is extended to exactly evaluate the eigenstates and show their dependencies. The time evolution of a superposition of the lowest eigenstates is considered for barriers having comparable Gamow tunneling factors so as to quantify the impact of barrier height and shape on time evolution in a simple and exact system, and thereby serve as a proxy for tunneling time. Lastly, density profiles and associated quantum potentials are examined for coupled wells to show changes induced by weaker and wider barriers. The single parameter δ-function model is useful for the rapid evaluation of current density and is demonstrated. The simplicity of the barrier is further useful in drawing a connection between quantum well bound state problems to tunneling and transport problems, but it is a blunt tool that forces the eigen-energies in a closed system to take on simple limiting values. In the present work, we examine the eigenstates associated with a δ-function barrier in a confining well with infinite walls to identify how the δ-function barrier relates to the more complicated barrier representations. The methods are to be generalized to examine barriers of other shapes that differ from the rectangular barrier by a shape factor to simulate how barrier parameters affect either the dynamic behavior associated with density migration from one side of the well to the other through the barrier or the parameterization of the NS interface.
References
[1] K. L. Jensen et al., “A delta barrier in a well and the exact time evolution of its eigenstates,”
J. Appl. Phys., vol. 133, no. 17, pp. 174402, 2023
The adjoint method promises to greatly improve the calculation of the effect of fabrication tolerances and may also lead to highly efficient optimization algorithms for general charged-particle beamline design. The adjoint method applied to an electron beam source design is discussed. We note that such methods vastly improve computational efficiency by requiring only two or three runs of the simulation code to calculate the sensitivities (derivatives) of a figure of merit to variations of a number of physical parameters, as compared with (N+1) runs required to compute sensitivities of a figure of merit to N physical parameters. The computational savings can be enormous for large values of N. Additionally, in codes that employ computational grids matched to their geometries or volumetric features of interest, remeshing is not required for the subsequent runs, often-times allowing the second run to be restarted from the first, reducing overall run time. The other benefit is a higher-accuracy sensitivity reductions by using the same mesh.
Advanced semiconductor manufacturing requires high-end metrology to have successful semiconductor process control.
Currently, high-end metrology requires lab tools to assist in semiconductor metrology. However, this is slow, expensive, and, often destructive. Fabs need inline metrology systems.
New memory and logic semiconductor designs and architectures are transitioning to 3-D structures. In the near future, as device structures become more complicated and require more process steps, process control becomes more critical. Conventional metrology is challenged to characterize the tiny features, dimensions, and material composition with the required sensitivity and require depth. Electron beam inspection and metrology is expected and can become an important part of the metrology process control. Photoelectron beam metrology is one of the most attractive technologies for semiconductor process control.
This talk will give an overview of the metrology challenges in the semiconductor process. It will also review some top-level requirements for Photoelectron beam metrology requirements.
The excitation laser polarization and coupling geometry metastructures define the spin-orbit interaction of surface plasmon polaritons at metal-vacuum interfaces. We image the topological plasmonic fields by imaging nonlinear electron emission by photoemission electron microscopy. We record with nanometer spatial and femtosecond temporal resolution the generated space-time spin textures of plasmonic vortex fields and how they can dress the space-time invariance of matter. The angular momentum of light is transferred to photoemitted electrons in a coherent two-photon emission. We conclude that the angular momentum must be carried away by the generated electron beam. The plasmonic vortex cores act as sources of magnetoelectric interaction on 10 nm scale.
Abstract:
Photocathodes based on GaAs and other III–V semiconductors are capable of producing highly spin-polarized electron beams. GaAs/GaAsP superlattice photocathodes exhibit high spin polarization; however, the quantum efficiency (QE) is limited to less than 1%. To increase the QE, GaAs/GaAsP superlattice photocathodes with a Distributed Bragg Reflector (DBR) are usually used underneath. This configuration creates a Fabry–Pérot cavity between the DBR and the GaAs surface, enhancing the absorption of incident light and, consequently, the QE. However, the peak QE often deviates from the design requirements of around 780 nm. To investigate this, we have used transmission electron microscopy and improved heat cleaning techniques to mitigate temperature-induced diffusion of atoms from one layer to another. Secondary Ion Mass Spectroscopy is also performed to evaluate the effect of heat treatment on these photocathodes.