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Chinese Physics B

Generation of multiple electron beams by cold atom electron source

H.B. Wang, Haoyu Huang, Jianing Sun, Chunkai Xu, Enliang Wang, Xu Shan, Xiangjun Chen

Published Apr 27, 2026• 0 citations
Publisher

Abstract

Abstract We report the generation of multiple electron beams using a cold atom electron source (CAES) combined with a spatial light modulator (SLM). In the experiment, neutral ${ }^{87} \mathrm{Rb}$ atoms are confined in a magneto-optical trap and cooled to approximately $700 \mu \mathrm{~K}$. By shaping the intensity profile of the excitation laser via the SLM, the spatial distribution of the excited atoms can be precisely controlled. The excited atoms are then ionized by a dye laser, and the ionized electrons are extracted and focused by an electric field to form multiple electron beams. As a demonstration, we generate regular arrays of $3 \times 3,4 \times 4,5 \times 5,6 \times 6$ beams, each with a spot size of about $100 \mu \mathrm{~m}$ and a normalized transverse emittance of $\sim 0.4 \times 10^{-9} \mathrm{~m} \cdot \mathrm{rad} \cdot \sqrt{\mathrm{eV}}$. This programmable electron beam technique holds potential for applications in electron-beam lithography and wafer inspection.

DOI
10.1088/1674-1056/ae64d2

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Abstract We report the generation of multiple electron beams using a cold atom electron source (CAES) combined with a spatial light modulator (SLM). In the experiment, neutral 87Rb{ }^{87} \mathrm{Rb}87Rb atoms are confined in a magneto-optical trap and cooled to approximately 700μ K700 \mu \mathrm{~K}700μ K. By shaping the intensity profile of the excitation laser via the SLM, the spatial distribution of the excited atoms can be precisely controlled. The excited atoms are then ionized by a dye laser, and the ionized electrons are extracted and focused by an electric field to form multiple electron beams. As a demonstration, we generate regular arrays of 3×3,4×4,5×5,6×63 \times 3,4 \times 4,5 \times 5,6 \times 63×3,4×4,5×5,6×6 beams, each with a spot size of about 100μ m100 \mu \mathrm{~m}100μ m and a normalized transverse emittance of ∼0.4×10−9 m⋅rad⋅eV\sim 0.4 \times 10^{-9} \mathrm{~m} \cdot \mathrm{rad} \cdot \sqrt{\mathrm{eV}}∼0.4×10−9 m⋅rad⋅eV​. This programmable electron beam technique holds potential for applications in electron-beam lithography and wafer inspection.
We report a programmable multiple ion beam system developed by combing a magneto-optical trap ion source with a spatial light modulator (SLM). Neutral rubidium atoms are trapped into a magneto-optical trap and cooled to 700 μK. An excitation laser beam, the intensity distribution of which is shaped into a square array by the SLM, selectively excites the cold atoms, which are subsequently ionized by an ionization laser beam. The ions are then extracted by an electric field to produce multiple ion beams. A 3×3 ion beam array has been generated. The size of each beams is 180 μm, and the transversal reduced emittance of multiple ion beams at the source is evaluated to be 1.65 × 10−9 m radeV. We also demonstrate the generation of ion bunches with a user-defined spatial pattern. In the future, by integrating this programmable multiple ion beam system with high reduction ion optics, it is possible to generate nanoscale multiple focused ion beams for the potential applications in maskless patterning and direct writing lithography.
The authors describe a new method for creating a high quality focused ion beam using laser-cooled neutral atoms in a magneto-optical trap as an ion source. They show that this new technique can provide spot resolutions and brightness values that are better than the state of the art in focused ion beams. The source can be used with a range of different ionic species and can be combined with laser cooling techniques to exert unprecedented control over the ion emission, for example, producing single ions “on demand.” The beam quality is a result of a high brightness and a narrow energy distribution, both of which stem from the cold temperature (≈100μK) of the atoms. The ions are produced by subjecting the cold neutral atoms to a photoionization laser, after which they become a compact source of nearly monoenergetic ions. With the application of a potential gradient, the ions form a beam that can be focused via standard ion optical techniques. They discuss estimations based on the initial size of the ion cloud and the energy distribution and show that the resulting beam has a low emittance.
This thesis presents details of a new type of cold atom electron source (CAES) that uses an alternating current magneto-optical trap (AC-MOT). Cold atom electron sources produce electron bunches by threshold photoionisation from a population of laser-cooled atoms. The electrons extracted from the atoms are made into a beam which has an ultra-low transverse velocity distribution. The low velocity distribution increases the brightness and reduces the energy spread of the beam, making CAESs potential sources for future electron diffraction and electron spectroscopy studies. The newly commissioned source detailed here produces focused electron bunches with characteristic temperatures as low as 12.6 ± 0.8 K, at a rate of up to 10 Hz, and with a bunch charge of 2.2±0.2fC, comparable to other state-of-the-art cold atom electron sources. The AC-MOT CAES can also produce cold bunches with energies <200 eV, which is lower than has previously been reported for this type of source. The electrons originate from a population of laser-cooled 85Rb atoms held in an AC- MOT. The AC-MOT CAES design allows the magnetic trapping fields to be switched to zero in 32 μs, more than �ˆ�175 times faster than is possible using a comparable DC-MOT source. This means that electron bunches can be created and extracted in a magnetic field-free environment without any reduction in trap density and hence bunch charge, as occurs in conventional DC-MOT sources. The design, development, characterisation, and operation of the systems needed for the new AC-MOT CAES are detailed in this thesis. This includes details of the atom trapping system, the photoionisation system, the electron extraction system and the various timing systems for creation and characterisation of the resultant cold electron bunches. Finally, the characterisation of the AC-MOT CAES itself is presented, and the measured beam metrics are used to compare the new AC-MOT CAES with other CAES designs which are under development.
The ultrafast and ultracold electron source, based on near-threshold photoionization of a laser-cooled and trapped atomic gas, offers a unique combination of low transverse beam emittance and high bunch charge.Its use is however still limited because of the required cold-atom laser-cooling techniques.Here we present a compact ultracold electron source based on a grating magneto-optical trap (GMOT), which only requires one trapping laser beam that passes through a transparent accelerator module.This makes the technique more widely accessible and increases its applicability.We show the GMOT can be operated with a hole in the center of the grating and with large electric fields applied across the trapping region, which is required for extracting electron bunches.The calculated values of the applied electric field were found to agree well with measured Stark shifts of the laser cooling transition.The electron beams extracted from the GMOT have been characterized.Beam energies up to 10 keV were measured using a time-of-flight method.The normalized root-mean-squared transverse beam emittance was determined using a waist scan method, resulting in ϵ ¼ 1.9 nm rad.The root-mean-squared transverse size of the ionization volume is 30 μm or larger, implying an electron source temperature in the few-10 K range, 2-3 orders of magnitude lower than conventional electron sources, based on photoemission or thermionic emission from solid state surfaces.
We analyze a method for serial writing of arbitrary two-dimensional patterns using optical focusing of a collimated atomic beam. A spatial light modulator is used in a side illumination geometry to create a localized optical spot with secondary maxima that are well separated from the central peak. Numerical simulation of a lithography experiment using a magneto-optical trap as a source of cold Cs atoms, collimation and cooling in a magnetic guide, and optical focusing predicts FWHM pixel sizes of 110×110 nm and writing times of ∼20 ms/pixel.
The evolving field of nanofabrication demands that more precise fabrication and evaluation tools be developed. We describe a method for creating a high quality focused ion beam with enhanced capabilities using an ion source based on laser-cooled neutral atoms in a magneto-optical trap. This technique will improve resolution and brightness beyond the current state of the art, can be used with the full range of atomic species that can be laser cooled and trapped, and will allow unprecedented control over the ion emission, allowing, for example, the production of single ions ``on demand.'' We give estimates for the emittance and present a realistic ray tracing analysis of a basic focusing system demonstrating the feasibility of focusing the beam to a spot size of less than 10nm10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}10nm.