A high-brightness source of femtosecond polarized electrons for electron microscopy
Overview
Magnetic materials underpin a large and growing range of modern technologies, from data storage to emerging spintronic and quantum devices. The ability to probe magnetic surface phenomena on ultrafast timescales, however, remains an important experimental challenge. While scanning electron microscopy (SEM) can be used to probe the electrostatic potential of a sample, techniques such as spin-polarized low energy electron microscopy (SPLEEM) are needed to examine the spin or magnetic texture.
Scientists at Xelera Research LLC, in collaboration with Cornell University, are working to develop a compact, high-performance, ultrafast polarized electron gun that will be suitable for table-top SEM and SPLEEM installations. We aim to provide a complete electron source system (including cathode activation, drive laser delivery, beam generation, acceleration, steering, and one focusing stage) for the generation of femtosecond polarized beams with electron-microscope-scale beam quality.
Our goal is to bring accelerator-grade polarized source reliability into a microscope-compatible package.
Schematic of polarized source system
Nano-emitter Arrays
Beam quality in electron microscopy is strongly limited by source size, which sets the initial transverse emittance. Together with the charge per pulse, this determines the brightness and hence the spatial resolution and/or acquisition time for a given microscope. Traditional photoemission sources are constrained by optical diffraction to sizes above about 1 micron.
A key advancement in our design is the use of lithographically patterned GaAs nano-emitter photocathode arrays with emitters of varying sizes, down to about 200 nm. Such an array will enable users to select the emitter that provides the best balance of beam current, quality, time resolution, and energy spread for their application.
The photocathode concept
The cathode fabrication procedure begins by applying a thin layer of platinum on top of a GaAs wafer. The Pt layer is then lithographically patterned and etched to create micro- and nano-scale holes, exposing the GaAs underneath.
Sources developed in this way show polarizations up to 43% as measured by Mott polarimetry and quantum efficiencies approaching 1%. To shorten the ultimate pulse duration and achieve the target response time of 100 fs, the emitters must additionally be thinned to < 100 nm.
The photocathodes will be illuminated from the back of the emitter (in “transmission mode”) so that we may utilize a strong focusing lens to collect light onto the active area. Drive laser power requirements will thereby be reduced to enable ~100 MHz repetition rate operation, which is compatible with commercially available Ti:Sapphire laser oscillators.
Example tiled GaAs nano-well array, individual close-up, and image of single 500 nm emitter:
All of our GaAs cathode fabrication is performed at the Cornell Nanoscale Facility (CNF) at Cornell University. CNF is a world-leading nanofabrication facility featuring a complete suite of lithography and thin-film deposition (sputter and evaporation) tools, including an array of sample characterization tools such as atomic force microscopy, TEM, and SEM.
Compact DC Electron Gun Design
We are in the process of building a prototype based on our full optical, vacuum, and mechanical design of a compact 15–20 kV dc gun appropriate for SEM and SPLEEM deployment. The complete system will include:
Compact form factor suitable for tabletop installation
In-situ cathode annealing and activation
Removable photocathode wafer and optional load lock
Drive laser delivery with transmission-mode illumination
Beam steering and one focusing stage
The GaAs photocathodes in our design require ultra-high vacuum to ensure long cathode lifetimes. Our prototype includes standard UHV design with all-metal ConFlat sealing flanges and all-metal angle and gate valves, which will enable operation at ≤ 10⁻¹¹ Torr (after cleaning and baking). An integrated heater will be used for cathode processing.
Section view of initial prototype
In the section view shown, the laser enters the gun optical system from the top right. In this configuration, the laser first passes through a polarized beam splitter. The resulting forward-transmitted beam passes through a Neutral Density (ND) filter and is captured by a CCD camera located at a virtual cathode location. The downward-transmitted beam passes through a quarter-wave plate, creating circularly polarized light, and then through a vacuum window into the dc gun.
Activation of the cathode is performed with the cathode in place with the cesium source downstream of the anode. In case frequent cathode swaps are desired, we have also designed an optional load-lock system which avoids venting the gun and can be used to store several cathodes at a time.
A prototype design with a load-lock system, shown below, also includes additional ports for optional front-side illumination:
Electrostatic Design
Our design work includes complete electrostatic modeling of the gun geometry to ensure that unwanted field emission is avoided and triple-point fields are within reasonable limits. Here is an example electrostatic solution, performed using the LANL Poisson Superfish code, for an early prototype model of the gun and a cathode potential of −15 kV:
Beam Dynamics Design and Optimization
We perform comprehensive beam dynamics simulations, along with multi-objective genetic algorithm (MOGA) optimization, to map the tradeoffs between different source characteristics. With fields generated from the gun model and with a single emittance-compensation solenoid included, our simulations show that higher brightness solutions couple the energy spread, emittance, and pulse length. Preliminary results indicate that for the right combination of charge and pulse length, beams are achievable that have transverse normalized emittances as low 20–200 pm, bunch lengths on the order of 100 fs, and energy spreads of a few hundred meV. The cathode nano-emitter array approach will allow the user to select various working points along these trade-off curves.
Examples of trade-off curves between bunch length and brightness, energy spread, and emittance for several different bunch charges, with a repetition rate of 80 MHz:
Diagnostic Beamline
A short beamline will be constructed at Cornell University to verify the performance goals of the electron gun. The beamline footprint is small enough that it can be directly mounted on the same optical table as the drive laser. In addition to the electron gun, there will be a solenoid focusing magnet, two pairs of dipole coils for steering, a pinhole aperture, and finally an MCP-based imaging detector. The full 4D transverse phase space of the beam will be measured using an aperture scan method, and the bunch length will be measured using a ponderomotive scattering technique.
Proposed test beamline with emittance and bunch-length diagnostics
Parameter Table
The table below summarizes the main design targets for our polarized source:
| Vacuum level in gun | 10−12–10−11 Torr |
| Gun voltage | 15–30 kV |
| Average beam current | ~1 nA |
| Bunch repetition rate | ~100 MHz |
| Bunch charge | ≤ 100 e− |
| GaAs emitter diameter | as small as 200 nm |
| Drive laser wavelength | 780 nm |
| Spin polarization | up to 40% |
| Transverse norm. emittance | 10–100 pm |
| RMS bunch length | as low as 100 fs |
| RMS energy spread | ~200 meV |
3D Render of prototype tabletop polarized source system
This work is supported by DOE STTR grant number DE-SC0024845.