Apparatus and method for preparing a vortex electron beam based on a highly coherent super cold electron beam

Abstract

The application relates to the technical field of vortex electron beam preparation, and particularly discloses a device and a method for preparing a vortex electron beam based on a high-coherence super-cold electron beam, which comprises the following steps: capturing a cold atom group by using an atomic capturing mechanism to obtain a captured cold atom group; irradiating the captured cold atom group by using an ionizing laser to generate electrons; starting an acceleration electric field to guide out the electrons by using the acceleration electric field to obtain an electron beam; transporting the electron beam to a coherence increasing mechanism to obtain an electron beam with increased coherence; and transmitting the electron beam with increased coherence to a vortex generating mechanism to diffract, so that the vortex electron beam is generated by means of orbital angular momentum after the diffraction. The application provides a preparation method for generating a vortex electron beam based on a magnetic optical trap cold atom, the controllability is high, the device is relatively simple, a new approach is provided for generating the vortex electron beam, and the application is favorable for promoting the research and application of the vortex electron beam.

Description

Technical Field

[0001] This invention relates to the field of vortex electron beam preparation technology, and in particular to an apparatus and method for preparing vortex electron beams based on highly coherent ultracold electron beams. Background Technology

[0002] Vortices are ubiquitous phenomena in nature, such as ocean circulation, typhoon vortices, and even liquid helium superfluid quantum vortices and superconducting quantum magnetic vortices. Among them, vortex light with orbital angular momentum (OAM) plays an important role in many fields such as quantum entanglement, space optical communication, optical microscopy, and particle manipulation. It can be used to interact with matter to provide more information about the sample, opening up new avenues for describing the crystal structure, magnetic and chiral properties of matter, as well as the internal dynamics of atomic systems. It can also provide new degrees of freedom for exploring the structure of microscopic matter.

[0003] Currently, the process of generating vortex electron beams almost entirely occurs in electron microscopes. Electron microscopes possess field emission electron sources with good coherence and excellent beam optics and electronics systems. However, electron microscopes are expensive, and generating vortex electron beams is costly, complex, and inflexible. Therefore, more and more people are researching other methods and equipment that can achieve electron beam preparation. However, the electron beams produced by methods and equipment other than electron microscopes have low coherence and are difficult to meet the needs of users.

[0004] Therefore, there is an urgent need to develop a new device and method to prepare highly coherent vortex electron beams. Summary of the Invention

[0005] The purpose of this invention is to provide an apparatus and method for preparing vortex electron beams based on highly coherent ultracold electron beams, so as to solve the problem of low coherence in the preparation of existing vortex electron beams.

[0006] To address the aforementioned technical problems, this invention provides two main solutions. Firstly, this invention provides a method for preparing vortex electron beams based on highly coherent ultracold electron beams, comprising the following steps:

[0007] S1, using an atomic trapping mechanism to trap cold atomic clusters, and obtaining the trapped cold atomic clusters;

[0008] S2, using an ionizing laser to irradiate the captured cold atom cluster to generate electrons;

[0009] S3, start the accelerating electric field, and use the accelerating electric field to extract the electrons to obtain the extracted electron beam;

[0010] S4, the electron beam is delivered to the coherence enhancement mechanism to obtain an electron beam with enhanced coherence;

[0011] S5, the electron beam with increased coherence is transmitted to the vortex generation mechanism for diffraction, and the vortex electron beam is collected and obtained.

[0012] In some embodiments of the first aspect, between step S3 and step S4, the following step is further provided: constraining and aligning the extracted electron beam using a collimation constraint mechanism to obtain a constrained electron beam, and then performing step S4 on the constrained electron beam.

[0013] In some embodiments of the first aspect, in step S1, the temperature of the captured cold atom cluster is 100–999 μK.

[0014] In some embodiments of the first aspect, in step S3, the accelerating electric field is a DC electric field.

[0015] In some embodiments of the first aspect, in step S5, the grating pattern preparation process of the vortex generation mechanism includes the following steps: interfering the vortex beam and the plane wave to obtain an interference pattern of a forked dislocation; and converting the amplitude of the interference pattern of the forked dislocation into a phase holographic grating pattern according to the phase difference between the vortex wave and the plane wave.

[0016] In some embodiments of the first aspect, the expression for the vortex beam is: a(r,z)exp(ilφ); the expression for the plane wave is: b(r,z)exp(ikx sinθ+ikz cosθ); where i represents a complex number, θ is the angle between the plane wave and the Z-axis along the propagation direction, a(r,z) is the amplitude of the vortex beam, b(r,z) is the amplitude of the plane wave, l is the topological charge, (r,φ,z) are cylindrical coordinates, and k is the wave number.

[0017] In some embodiments of the first aspect, the size of the grid pattern is 1-100 μm.

[0018] Secondly, the present invention provides an apparatus for preparing vortex electron beams based on highly coherent ultracold electron beams, which applies a method for preparing vortex electron beams based on highly coherent ultracold electron beams according to the first aspect, comprising an atom trapping mechanism, an ionization laser mechanism, an electrode mechanism, a coherence enhancement mechanism, and a vortex generation mechanism; the atom trapping mechanism includes multiple laser elements and multiple magnetic field elements, the emitted beams of the multiple laser elements are perpendicular to each other and converge at the convergence point, the multiple magnetic field elements are arranged outside the convergence point to form a trapping magnetic field, and the convergence point is located within the trapping magnetic field; the emitted beam of the ionization laser mechanism is aligned with the convergence point; the electrode mechanism includes multiple electrode plates, the multiple electrode plates are arranged outside the convergence point, the multiple electrode plates are arranged opposite each other to form an accelerating electric field, and the convergence point is located within the accelerating electric field; the coherence enhancement mechanism and the vortex generation mechanism are arranged sequentially along the electron transport path.

[0019] In some embodiments of the second aspect, a collimation mechanism is also included; the collimation mechanism includes a beam tube and a magnetic lens; along the electron transmission path, the magnetic lens and the coherence enhancement mechanism are sequentially disposed within the beam tube.

[0020] In some embodiments of the second aspect, a detection mechanism is also included; the detection mechanism includes a microchannel plate detector, a fluorescent screen, and a CCD camera (charge coupled device); along the electron transport path, the microchannel plate detector, the fluorescent screen, and the CCD camera are sequentially disposed on the side of the vortex generation mechanism away from the coherence enhancement mechanism.

[0021] The most prominent beneficial effects of this invention are as follows:

[0022] Firstly, since the electrons in this scheme are generated by ionization of cold atomic groups by laser irradiation, the generated electrons have a low temperature, which makes the transverse coherence length of the electron beam larger. The transverse coherence length can reach the order of 10 nm, which has very good strong coherence.

[0023] Second, in the first aspect, since the electrons in this scheme are drawn out by the accelerating electric field, the electrons form an electron beam, and the electron beam is transmitted to the coherence enhancement mechanism. In this process, a highly coherent ultracold electron beam with the required energy is formed.

[0024] Thirdly, in the first aspect, since the electron beam in this scheme is delivered to the coherence enhancement mechanism, which adopts a slit skimmer, the transverse coherence length of the electron beam formed by cold atoms is increased from the nm level to the μm level, which greatly improves the coherence of the electron beam.

[0025] Fourthly, in the second aspect, since this scheme uses magnetic fields and laser fields to capture cold electrons and irradiates cold atoms through an ionization laser mechanism to form an electron beam, the electron beam is transmitted to a coherence enhancement mechanism and a vortex generation mechanism to form a vortex electron beam with good coherence. This realizes the preparation of vortex electron beams using a lower-cost and more flexible device, breaking away from the previous limitation that vortex electrons could only be generated in an electron microscope. It has important research significance and broad application prospects, and can be directly applied to linear electron accelerators to obtain higher-energy vortex electron beams. Attached Figure Description

[0026] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of the overall method flow provided by a preferred embodiment of the present invention;

[0028] Figure 2 This is a schematic diagram of the overall device structure layout provided by a preferred embodiment of the present invention;

[0029] Figure 3 This is a schematic diagram of a fork-shaped diffraction holographic grating for generating a vortex electron beam according to a preferred embodiment of the present invention.

[0030] The attached figures are labeled as follows:

[0031] 1. Atom trapping mechanism; 10. Laser component; 11. Magnetic field component; 12. Mirror; 2. Ionizing laser mechanism; 3. Electrode mechanism; 30. Electrode plate; 4. Coherence enhancement mechanism; 5. Vortex generation mechanism; 6. Collimation mechanism; 60. Magnetic lens; 7. Detection mechanism; 70. Microchannel plate detector; 71. CCD camera; 72. Fluorescent screen. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0033] Vortexes are ubiquitous phenomena in nature, such as ocean circulation, typhoon vortices, and even superfluid quantum vortices in liquid helium and superconducting quantum magnetic vortices. Vortex phenomena can also manifest in sound waves, electromagnetic waves, and single-particle matter waves. In recent years, optical vortices and related scientific research have attracted widespread attention. Vortex light with orbital angular momentum (OAM) has played a crucial role in numerous fields, including quantum entanglement, space optical communication, optical microscopy, and particle manipulation. Furthermore, the universality of the wave equation suggests that vortex waves with angular momentum should also apply to matter waves, such as electrons. Vortex electron beams possess a unique phase structure and quantized orbital angular momentum, making them significantly different from plane-wave electron beams. In electron microscopy, due to the larger magnetic moment of vortex electron beams, their interaction with matter can provide more information about the sample, opening new avenues for depicting the crystal structure, magnetic and chiral properties of matter, and the internal dynamics of atomic systems. In addition, vortex electron beams also have important applications in nuclear physics and particle physics, with their intrinsic orbital angular momentum providing new degrees of freedom for exploring the structure of microscopic matter. Currently, the main methods for generating vortex electron beams include holographic gratings, spiral phase plates, and magnetic monopoles. When a highly coherent electron beam passes through these devices, the phase of the electron beam wave function changes, thereby generating a vortex.

[0034] Currently, the process of generating vortex electron beams almost exclusively occurs in electron microscopes. Electron microscopes possess field emission electron sources with good coherence and excellent beam optics and electronics systems. However, electron microscopes are expensive, and generating vortex electron beams is costly, complex, and inflexible. Therefore, there is an urgent need to develop a new device and technology capable of generating vortex electron beams.

[0035] To address the aforementioned problems, this solution provides a novel method and apparatus for preparing vortex electron beams. Specific embodiments are described below; please refer to them. Figures 1 to 3 Specifically, it includes the following steps:

[0036] S1, cold atom clusters are captured using the atom trapping mechanism 1 to obtain the captured cold atom clusters. With this setup, the characteristics of cold atoms, such as low temperature, slow speed, long de Broglie wavelength and very good coherence, can be used to prepare highly coherent vortex electron beams.

[0037] In this embodiment, a magneto-optical trap is formed within the atom trapping mechanism 1. The magneto-optical trap is generated by three mutually orthogonal cooled laser beams and a gradient magnetic field generated by a pair of anti-Helmholtz coils. The atomic clusters are trapped within the magneto-optical trap. In this embodiment, rubidium atoms are preferably used as atomic clusters. Other cold atoms that can be used to prepare vortex electron beams or to prepare vortex electron beams using similar devices are all within the protection scope of this invention. Those skilled in the art can choose according to their actual needs.

[0038] In some embodiments of this application, the temperature of the captured cold atom clusters is 100–999 μK, which is achieved using Doppler cooling to reduce the temperature of the captured atom clusters to several hundred μK, with a cluster density of approximately 10. 9 cm -3 With this setup, the temperature of the electrons generated by the ionization of cold atomic clusters in subsequent electron generation steps will also be controlled in the order of several K, which helps in the preparation of highly coherent electron beams.

[0039] S2 utilizes ionizing laser irradiation to capture cold atom clusters, generating electrons. With this setup, when the laser acts on the cold atoms, the atoms are ionized to produce electrons. During the photoionization process, the cold atoms follow the conservation of momentum and energy. When cold atom clusters at temperatures of several hundred μK undergo near-threshold ionization, the electron temperature can be controlled within the order of several K, as shown by the following formula: in m is Planck's constant. e Let T be the electron mass and T be the electron source temperature. The transverse coherence length of the electron is directly related to the electron temperature. The calculated transverse coherence length is on the order of tens of nm, which has high coherence. Therefore, highly coherent electron pulses can be generated based on cold atoms.

[0040] It should be understood that the transverse coherence length of the electron beam Therefore, the electron beam temperature is a key factor determining the coherence of the electron source; the lower the temperature, the greater the transverse coherence length of the beam.

[0041] The ionizing laser used is a continuously tunable ionizing laser, with the wavelength selected near the ionization threshold. In this scheme, a pulsed laser with a wavelength range of 460–490 nm can be output, with a pulse width of 5 ns and a linewidth of 0.02 cm. -1 The advantage of continuously tunable wavelength of ionizing lasers is that, because the photoionization process satisfies the conservation of momentum and energy, after photoionization occurs, electrons and ions distribute energy exceeding the ionization threshold. Since electrons are lightweight, they obtain most of the energy. Choosing near-threshold ionization allows for easy control of the initial temperature of electrons, laying the foundation for ensuring subsequent coherence.

[0042] It should be noted that the ionizing laser is generated by the ionizing laser mechanism 2. Preferably, the ionizing laser mechanism 2 is a pulsed dye laser. Of course, other lasers that can generate ionizing lasers can also be selected by those skilled in the art according to their actual needs.

[0043] S3, start the accelerating electric field, use the accelerating electric field to extract electrons, the accelerating electric field obtains the extracted electron beam, the electric field voltage is 100 to 1000 volts, after adopting this setting method, electrons with coherence on the order of nm are extracted by the accelerating electric field and form an electron beam with a certain speed and energy. Under the acceleration of the electric field, the electron beam can be transmitted to the next mechanism for processing.

[0044] It should be noted that the accelerating electric field is only activated when electrons need to be accelerated; otherwise, it is in the off state.

[0045] Specifically, a high-voltage power supply is connected to the electrode plates 30 at both ends of the action area to obtain an accelerating electric field. Electrons are drawn out through the potential difference of the accelerating electric field, so that the electron beam has the required energy.

[0046] In some embodiments of this application, the accelerating electric field is a DC electric field.

[0047] In some embodiments of this application, between step S3 and step S4, the following step is provided: the extracted electron beam is constrained and collimated using a collimation constraint mechanism to obtain a constrained electron beam, and then step S4 is performed on the constrained electron beam. With this setting, the collimation constraint mechanism can limit the divergence of the coherent electron beam, especially the lateral diffusion of the electron beam, thereby maintaining the lateral coherence length.

[0048] In some embodiments of this application, the collimation constraint mechanism preferably adopts a magnetic lens 60. Other devices that can achieve collimation constraint are also within the protection scope of this application, and those skilled in the art can choose according to their actual needs.

[0049] S4, the electron beam is delivered to the coherence enhancement mechanism 4 to obtain an electron beam with increased coherence. After adopting this setting method, the coherence of the electron beam is increased after the collimated electron beam is passed into the coherence enhancement mechanism 4, thereby enhancing the coherence of the electron beam a second time.

[0050] Among them, the coherence enhancement mechanism 4 is about 20 cm away from the accelerating electric field.

[0051] Preferably, the coherence enhancement mechanism 4 adopts a slit skimmer, which is a horn-shaped conical structure with an aperture of 100 micrometers. According to diffraction theory, the coherence length is inversely proportional to the aperture of the slit skimmer. The smaller the aperture, the longer the coherence length of the beam. After the collimated electron beam is passed into the slit skimmer, the coherence of the electron beam is increased from tens of nanometers to micrometers, which greatly increases the coherence of the electron beam.

[0052] It should be understood that the pore size of the slit skimmer includes, but is not limited to, 100 micrometers, and those skilled in the art can choose a suitable pore size according to their own experimental needs.

[0053] S5, the electron beam with increased coherence is transmitted to the vortex generation mechanism 5 for diffraction, and a vortex electron beam is collected and obtained. With this setting, the electron beam with increased coherence is diffracted by the vortex generation mechanism 5, and a highly coherent, low emittance vortex electron beam with orbital angular momentum is generated after diffraction.

[0054] In step S5, the pattern preparation of the vortex mechanism is carried out as follows:

[0055] The interference of the vortex beam and the plane wave is used to obtain the interference pattern of the forked dislocation. Based on the phase difference between the vortex wave and the plane wave, the amplitude of the interference pattern of the forked dislocation is converted into a phase holographic grating pattern. Finally, the designed diffraction hologram is obtained by etching a silicon nitride thin film with a focused electron beam. With this setup, the pattern of the vortex mechanism can be prepared.

[0056] The expression for the vortex beam is: a(r,z)exp(ilφ);

[0057] The expression for a plane wave is: b(r,z)exp(ikx sinθ+ikz cosθ);

[0058] In the formula, i represents a complex number, θ is the angle between the plane wave and the Z-axis along the propagation direction, a(r,z) is the amplitude of the vortex beam, b(r,z) is the amplitude of the plane wave, l is the topological charge, (r,φ,z) are cylindrical coordinates, and k is the wave number.

[0059] Preferably, the fork-shaped diffraction holographic grating that generates the vortex electron beam is, for example... Figure 3 As shown, the size of the grating pattern is 1-100μm. After adopting this setting method, the size of the grating and the lateral coherence length of the electron beam are both on the order of μm, i.e., 1-100μm.

[0060] S6, the obtained vortex electron beam is transmitted to the detection mechanism 7 for collection and recording. With this setting, the generated vortex electron beam can be detected.

[0061] The basic method and principle of this scheme can be understood from the above. The following text will give the apparatus for applying the above method to realize the preparation of vortex electron beam.

[0062] Please refer to Figure 2It includes an atom trapping mechanism 1, an ionization laser mechanism 2, an electrode mechanism 3, a coherence enhancement mechanism 4, and a vortex generation mechanism 5. The atom trapping mechanism 1 is used to form a magneto-optical trap to trap cold atoms; the ionization laser mechanism 2 is used to emit an ionization laser, which is aligned with the magneto-optical trap; the electrode mechanism 3 is arranged outside the atom trapping mechanism 1 and is used to form an accelerating electric field outside the magneto-optical trap; the coherence enhancement mechanism 4 and the vortex generation mechanism 5 are arranged sequentially on the electron transport path. With this arrangement, the magneto-optical trap can trap cold atom clusters. After the ionization laser irradiates the cold atom clusters, it can form electrons with a transverse coherence length of nanometers. After the electrons are extracted by the accelerating electric field, they form an electron beam. After the electron beam passes through the coherence enhancement mechanism 4, the transverse coherence length of the electrons will increase from nanometers to micrometers. The electron beam passes into the vortex generation mechanism 5 to generate a vortex electron beam, thereby obtaining a higher-energy vortex electron beam.

[0063] For atomic trapping mechanism 1, such as Figure 2 As shown, the atom trapping mechanism 1 includes three laser elements 10 and two magnetic field elements 11. The emitted light from the three laser elements 10 is perpendicular to each other and converges at the intersection point. The two magnetic field elements 11 are arranged outside the intersection point to form a trapping magnetic field. The intersection point is located within the trapping magnetic field. The optical intersection point and the trapping magnetic field together form a magneto-optical trap. The magneto-optical trap can trap cold atom clusters, and through Doppler cooling, the temperature of the atom clusters can be cooled to several hundred μK, with an atom cluster density of approximately 10⁻⁶. 9 cm -3 After adopting this setup, the light pressure of three mutually orthogonal circularly polarized cooling laser beams and the gradient magnetic field of the anti-Helmholtz coil are used to capture atoms. Under the interaction of light pressure and magnetic field, cold atoms are tightly bound in a very small spatial region, and the cold atom clusters can reach hundreds of μK, so that the transverse coherence length of electrons reaches the nanometer level.

[0064] Among them, the three laser elements 10 emit three continuous laser beams (corresponding to the three dimensions in the rectangular coordinate system respectively) to form a laser field. The reflector 12 is arranged relative to the three laser elements 10. The three continuous laser beams emitted by the three laser elements 10 are reflected by the reflector 12 to form three pairs of mutually orthogonal light paths that propagate in opposite directions. The magnetic field element 11 is preferably generated by an anti-Helmholtz coil. The anti-Helmholtz coils are placed symmetrically, and the input currents are equal in magnitude but opposite in direction.

[0065] For ionizing laser mechanism 2, such as Figure 2 As shown, the emitted light of the ionizing laser mechanism 2 is aligned with the intersection point, and the emitted laser is an ionizing laser with continuously adjustable wavelength. The wavelength of the ionizing laser is selected near the ionization threshold. Preferably, the ionizing laser mechanism 2 adopts a pulsed dye laser.

[0066] For electrode mechanism 3, such as Figure 2 As shown, the electrode mechanism 3 includes a pair of electrode plates 30, which are arranged outside the intersection point to form an accelerating electric field. The accelerating electric field will be turned on when electrons need to be extracted. With this setting, after the ionizing laser completes electron ionization, the accelerating electric field is activated, and the ionization products (electrons) are accelerated and extracted under the action of the extraction electric field to form the required highly coherent ultracold electron beam.

[0067] For coherence enhancement mechanism 4, such as Figure 2 As shown, the coherence enhancement mechanism 4 is disposed on the electron transport path. The coherence enhancement mechanism is a cone with both ends open. Preferably, the coherence enhancement mechanism 4 adopts a slit skimmer, that is, the slit skimmer is a horn-shaped cone structure with an aperture of 100 micrometers. According to diffraction theory, the coherence length is inversely proportional to the aperture of the slit skimmer. The smaller the aperture, the longer the coherence length of the beam. After the collimated electron beam is passed into the slit skimmer, the coherence of the electron beam is increased from the tens of nanometers level to the micrometer level.

[0068] It should be understood that the pore size of the slit skimmer includes, but is not limited to, 100 micrometers, and those skilled in the art can choose a suitable pore size according to their own experimental needs.

[0069] For the vortex generating mechanism 5, such as Figure 2 As shown, the vortex generation mechanism 5 is located on the side of the coherence enhancement mechanism 4 away from the atom trapping mechanism 1, and the vortex generation mechanism 5 is also located on the electron transport path. With this arrangement, after the electrons reach the vortex generation mechanism 5, they are diffracted to generate a highly coherent, low-emittance vortex electron beam with orbital angular momentum.

[0070] In some embodiments of this application, the vortex generation mechanism 5 is preferably a holographic diffraction grating. The pattern of the holographic diffraction grating is obtained through special design. Specifically, a vortex beam a(r,z)exp(ilφ) propagating along the Z-axis and a plane wave are set, wherein the plane wave makes an angle θ with the Z-axis along the propagation direction, and the wave function is b(r,z)exp(ikx sinθ+ikz cosθ), where a(r,z) and b(r,z) are amplitudes, l is the topological charge, (r,φ,z) are cylindrical coordinates, and k is the wave number. After the vortex wave and the plane wave interfere, an interference pattern of fork dislocations is obtained. Then, according to the phase difference between the two, the amplitude of the fork interference pattern in the fork interference distribution is converted into a phase hologram. The designed diffraction hologram is obtained by etching a silicon nitride thin film with a focused electron beam, thereby obtaining a holographic diffraction grating.

[0071] In some possible embodiments of this application, the method of using a vortex generating mechanism 5 to replace a holographic diffraction grating with a spiral phase plate or a magnetic needle to obtain a vortex electron beam is also within the scope of protection of this invention.

[0072] In some embodiments of this application, the device of the present invention further includes a collimation mechanism 6, which includes a beam tube and a magnetic lens 60. Along the electron transmission path, the magnetic lens 60 and the coherence enhancement mechanism 4 are sequentially disposed in the beam tube. With this arrangement, a set of magnetic lenses 60 and slit skimmers are installed on the electron beam transmission path. The magnetic lens 60 can limit the divergence of the coherent electron beam, and the slit skimmer can increase the lateral coherence length to the order of several μm.

[0073] In some embodiments of this application, the holographic diffraction grating is fixed on the target holder and placed in a vacuum chamber.

[0074] In some embodiments of this application, the device of the present invention further includes a detection mechanism 7, which includes a microchannel plate detector 70, a fluorescent screen 72, and a CCD camera 71. Along the electron transport path, the microchannel plate detector 70, the fluorescent screen 72, and the CCD camera 71 are sequentially arranged on the side of the vortex generation mechanism 5 away from the coherence enhancement mechanism 4. With this arrangement, the spatial distribution of the vortex electron beam is recorded by the imaging detector 71. The forked diffraction grating that generates the vortex electron beam is as follows: Figure 3 As shown, it can generate orbital angular momentum of The vortex electron beam has a grating size on the order of micrometers.

[0075] It should be noted that the direction along the electron transport path mentioned throughout the text is from... Figure 2 The middle refers to the direction from left to right.

[0076] In summary, the ultracold electron beam generated by cold atom photoionization in this scheme has a lateral coherence length on the order of tens of nm. When the electron beam passes through a slit skimmer set behind the focusing lens, its coherence length increases to several μm, greatly enhancing the coherence and energy of the generated vortex electron beam. Compared with existing technologies, the method and apparatus provided by this invention have lower costs and greater flexibility, breaking through the previous limitation of only being able to generate vortex electrons in electron microscopes. It has significant research value and broad application prospects, and can be directly applied to linear electron accelerators to obtain higher-energy vortex electron beams.

[0077] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A method for preparing vortex electron beams based on highly coherent ultracold electron beams, characterized in that, Includes the following steps: S1, using an atomic trapping mechanism to trap cold atomic clusters, and obtaining the trapped cold atomic clusters; S2, using an ionizing laser to irradiate the captured cold atom cluster to generate electrons; S3, start the accelerating electric field, and use the accelerating electric field to extract the electrons to obtain the extracted electron beam; S4, the electron beam is delivered to the coherence enhancement mechanism to obtain an electron beam with enhanced coherence. The coherence enhancement mechanism adopts a slit interceptor, which is a trumpet-shaped conical structure. S5, the electron beam with increased coherence is transmitted to the vortex generation mechanism for diffraction to obtain a vortex electron beam; The specific process for fabricating the pattern of the vortex mechanism is as follows: The interference pattern of the forked dislocation is obtained by interfering the vortex beam and the plane wave. Based on the phase difference between the vortex wave and the plane wave, the amplitude of the interference pattern of the forked dislocation is converted into a phase holographic grating pattern. Finally, the designed diffraction hologram is obtained by etching a silicon nitride thin film with a focused electron beam. The size of the grating pattern is 1-100 μm.

2. The method for preparing a vortex electron beam based on a highly coherent ultracold electron beam according to claim 1, characterized in that, Between step S3 and step S4, the following steps are also provided: The extracted electron beam is constrained and collimated using a collimation and constraint mechanism to obtain a constrained electron beam, and then the constrained electron beam is subjected to step S4.

3. The method for preparing a vortex electron beam based on a highly coherent ultracold electron beam according to claim 1, characterized in that, In step S1, the temperature of the captured cold atom cluster is 100~999 μK.

4. The method for preparing a vortex electron beam based on a highly coherent ultracold electron beam according to claim 1, characterized in that, In step S3, the accelerating electric field is a DC electric field, and the voltage of the DC electric field is 100~1000 V.

5. The method for preparing a vortex electron beam based on a highly coherent ultracold electron beam according to claim 1, characterized in that, In step S5, the grid pattern preparation process of the vortex generation mechanism includes the following steps: By interfering the vortex beam and the plane wave, an interference pattern of fork-shaped dislocations is obtained; Based on the phase difference between the vortex wave and the plane wave, the amplitude of the interference pattern of the forked dislocation is converted into a phase holographic grating pattern.

6. The method for preparing a vortex electron beam based on a highly coherent ultracold electron beam according to claim 5, characterized in that, The expression for the vortex beam is: a(r,z)exp(ilφ); The expression for the plane wave is: b(r,z)exp(ikx sinθ + ikz cosθ); In the formula, i represents a complex number, θ is the angle between the plane wave and the Z-axis along the propagation direction, a(r,z) is the amplitude of the vortex beam, b(r,z) is the amplitude of the plane wave, l is the topological charge, φ is the azimuth angle, (r,φ,z) are cylindrical coordinates, and k is the wave number.

7. The method for preparing a vortex electron beam based on a highly coherent ultracold electron beam according to claim 5, characterized in that, The size of the grid pattern is 1-100 μm.

8. An apparatus for preparing vortex electron beams based on highly coherent ultracold electron beams, characterized in that, The method for preparing vortex electron beams based on highly coherent ultracold electron beams according to any one of claims 1 to 7, This includes an atom trapping mechanism, an ionization laser mechanism, an electrode mechanism, a coherence enhancement mechanism, and a vortex generation mechanism; The atomic trapping mechanism includes multiple laser elements and multiple magnetic field elements. The emitted light from the multiple laser elements is perpendicular to each other and converges at the convergence point. The multiple magnetic field elements are arranged outside the convergence point to form a trapping magnetic field. The convergence point is located within the trapping magnetic field. The emitted light from the ionizing laser mechanism is aligned with the intersection point; The electrode mechanism includes multiple electrode plates, which are arranged outside the junction point and opposite to each other to form an accelerating electric field. The junction point is located within the accelerating electric field. Along the electron transport path, the coherence enhancement mechanism and the vortex generation mechanism are arranged in sequence.

9. The apparatus for preparing vortex electron beams based on highly coherent ultracold electron beams according to claim 8, characterized in that, The coherence enhancement mechanism is a cone, with openings at both ends, the diameter of which is 100µm.

10. The apparatus for preparing vortex electron beams based on highly coherent ultracold electron beams according to claim 8, characterized in that, It also includes collimation mechanisms; The collimation mechanism includes a beam tube and a magnetic lens; Along the electron transport path, the magnetic lens and the coherence enhancement mechanism are sequentially disposed inside the beam tube.

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