Metallic periodic gratings, electron beam pulse width characterisation apparatus, method and microscope therefor

Abstract

The application provides a metal periodic grating, an electron beam pulse width characterization device, a method and a microscope thereof. The device comprises a metal periodic grating, a light emitting electron gun, an ultrafast laser module and a camera, the ultrafast laser module comprises an ultrafast laser, a laser frequency conversion element and a delay device; the ultrafast laser module is used for converting laser generated by the ultrafast laser into pump laser and probe laser; and the light emitting electron gun is used for converting the probe laser into a pulsed electron beam. The application utilizes the metal periodic grating, and through the interaction between the pulsed electron beam and the grating near field induced by the pump laser when the pulsed electron beam passes through the grating surface, the electron beam is made to produce lateral broadening, and the pulse width of the pulsed electron beam is accurately characterized by using the broadening effect.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of ultrafast electron microscopy / electron diffraction, and particularly relates to a metal periodic grating, an electron beam pulse width characterization device, method and microscope thereof. BACKGROUND

[0002] In the field of ultrafast electron microscopy (UEM), achieving atomic-level temporal resolution is crucial for observing and understanding the dynamic structural changes of materials. To achieve this goal, accurate measurement and control of electron beam pulse width become a key technical challenge. Common measurement methods include streak camera measurement methods, but streak cameras are not accurate enough for picosecond and femtosecond ultra-short electron beam pulse width measurements. In recent years, scientists have developed various methods to characterize the duration of ultra-short electron pulses, which are of great significance for improving temporal resolution, optimizing experimental design, and deepening understanding of material dynamics. In early studies, the ponderomotive force of laser was used to characterize the duration of femtosecond electron pulses. This method analyzes the transient changes in the lateral profile of the electron beam to extract the duration of the electron pulse, and successfully determines the electron pulse duration of 410 ± 30 femtoseconds. Subsequently, by using the standing wave generated by counter-propagating lasers to enhance the ponderomotive force effect, comprehensive characterization of electron pulses, including pulse duration and accurate timing with the excitation pulse, was achieved. Although the above methods have made significant progress in electron pulse characterization, there are still some challenges in practical applications. For example, the existing technology requires a high energy density of femtosecond laser pulses. This limits its application in ultrafast electron microscopy experiments. In addition, the electron beam will widen in pulse width due to space charge effects when propagating to the sample, and the specific value depends sensitively on the number of electrons, so an in situ measurement method is needed to characterize the properties of the electron pulse. The use of photon-induced near-field electron microscopy (PINEM) effect can quickly characterize the electron beam pulse width, but this requires a complex and expensive electron energy filtering system. Recently, some existing technologies use a method to accurately determine the time zero and electron beam pulse width in ultrafast transmission electron microscopy without an electron energy filtering system. When the electron and laser pulses overlap in time, the profile of the nanoparticle becomes blurred, and this phenomenon can also be used to measure the electron beam pulse width. Studies have shown that the root cause of this blurring phenomenon is also the photon-induced near-field electron microscopy effect, which causes the electrons interacting with the near-field of the nanoparticle to have a large energy spread, thus blurring the profile of the particle due to the chromatic aberration of the imaging system. However, the blurring effect of the edge profile is not obvious, and the spatial resolution requirement is very high. SUMMARY

[0003] Therefore, the purpose of the present application is to overcome the defects in the prior art and provide a metal periodic grating, an electron beam pulse width characterization device, a method and a microscope thereof. The device can efficiently and quickly realize accurate measurement of the duration of ultra-short electron pulses at a lower laser pulse energy. The device of the present application uses a new grating combined with an optimized electron beam manipulation method to achieve higher time resolution and better experimental repeatability. In addition, the present application also provides an in-situ measurement method, which allows direct diagnosis of the characteristics of the electron pulse at the sample position, thereby providing more accurate and reliable data for atomic-level dynamic structure research.

[0004] Before the content of the present application is described, the terms used herein are defined as follows:

[0005] The term "UEM" refers to: ultrafast electron microscopy, English name.

[0006] The term "periodic grating" refers to: an optical element in which the refractive index varies according to a certain periodic law in a medium; in the present application, such periodic variation is mainly the periodic variation of the surface structure; in order to realize the phase velocity matching of the electron beam with the surface near field of the metal periodic grating, and then enhance the interaction between the free electron and the light field, according to the wavelength, the incident angle of the pump laser and the electron beam velocity, the length calculated by the phase matching condition required by the inverse Smith-Pursell effect is used to periodically etch the grating groove.

[0007] The term "grating period" refers to: the distance between two grating lines or channels, denoted by d.

[0008] The term "BBO crystal" refers to: barium metaborate crystal.

[0009] The term "LBO crystal" refers to: lithium triborate crystal.

[0010] The term "CCD" refers to: charge-coupled device, English name.

[0011] The term "CMOS" refers to: complementary metal oxide semiconductor, English name.

[0012] To achieve the above-mentioned purpose, the first aspect of the present application provides a metal periodic grating made of a metal material, wherein:

[0013] The metal material section of the metal periodic grating has a smooth surface, the smooth surface forms nanochannels arranged in a grating period, and the nanochannels also have a smooth surface;

[0014] Preferably, the number of grating periods is 10-100, more preferably 20-50, and most preferably 50;

[0015] Preferably, the grating period is 100-500 nm, more preferably 150-400 nm, and most preferably 240.5 nm;

[0016] Preferably, the metal material is selected from one or more of silver, gold, copper, more preferably silver or gold, and most preferably silver; and / or

[0017] Preferably, the purity of the metal material is ≥ 99%, more preferably ≥ 99.9%, and most preferably ≥ 99.99%.

[0018] The second aspect of the present application provides a device for characterizing the pulse width of an ultra-short electron beam, the device comprising: the metal periodic grating of the first aspect, a light-emitting electron gun, an ultrafast laser module, and a camera, the ultrafast laser module comprising: an ultrafast laser, a laser frequency conversion element, and a delay device; wherein:

[0019] The ultrafast laser module is used to convert the laser generated by the ultrafast laser into pump laser and probe laser;

[0020] The light-emitting electron gun is used to convert the probe laser into a pulsed light electron beam.

[0021] The device according to the second aspect of the present application, wherein,

[0022] The ultrafast laser is a titanium-doped sapphire femtosecond laser or a ytterbium-doped femtosecond laser;

[0023] The laser frequency conversion element is a laser frequency conversion element based on a BBO crystal or a LBO crystal, preferably selected from one or more of a second harmonic device, a third harmonic device, and a fourth harmonic device; and / or

[0024] The delay device comprises: an electrically controlled displacement stage and a delay device mirror;

[0025] Preferably, the delay device mirror is located on the electrically controlled displacement stage; and / or

[0026] Preferably, the number of delay device mirrors is 2-6, preferably 2-4, and most preferably 2.

[0027] The device according to the second aspect of the present application, wherein,

[0028] The ultrafast laser module further comprises a beam splitting cube and an ultrafast laser module mirror; and / or

[0029] The camera is selected from one or more of the following: a scintillator fiber coupled CCD camera, a scintillator fiber coupled CMOS camera, a direct electron detection camera.

[0030] A third aspect of the present application provides a method for preparing the metal periodic grating of the first aspect, the method comprising the steps of:

[0031] 1) polishing the section surface of the metal material section;

[0032] 2) flipping the metal material section polished in step 1) by 90°, and etching nanochannels arranged in a grating period on the section surface of the metal material section;

[0033] 3) rotating the metal material section obtained in step 2) by another 90°, and polishing the etched nanochannels to obtain the metal periodic grating.

[0034] According to the method of the third aspect of the present application, the etching method in step 2) is focused ion beam etching or electron beam exposure.

[0035] A fourth aspect of the present application provides a characterization method of ultra-short electron beam pulse width, which uses the device of the second aspect and preferably comprises the following steps:

[0036] a) setting the grating period d of the metal grating to satisfy the following formula (A);

[0037] d = λ / (β -1 -cosθ) formula (A);

[0038] wherein λ is the wavelength of the incident laser light, θ is the angle of incidence of the laser light, i.e. the angle between the electron beam and the light beam, and β is the ratio of the electron velocity to the speed of light in vacuum;

[0039] b) the ultrafast laser generates ultrafast laser light, which is split into two paths by the beam splitting cube, one path of laser light sequentially passes through the ultrafast laser module mirror and the four-fold frequency device, is converted into probe laser light, and hits the light-emitting electron gun to generate a pulsed light electron beam that parallelly sweeps across the surface of the metal periodic grating, and the other path of laser light passes through the two-fold frequency device, is converted into pump laser light, and hits the surface of the metal periodic grating after passing through the delay device;

[0040] c) adjusting the power and / or polarization direction of the pump laser light so that the pulsed light electron beam is broadened when passing through the metal periodic grating.

[0041] According to the characterization method of the fourth aspect of the present application, wherein the step c) further comprises: adjusting different delay times by a delay device, capturing pictures of the electron beam transverse broadening under different time delays by the camera, extracting the size of the electron beam transverse broadening intensity from each picture, drawing a curve of the change of the electron beam transverse broadening with time delay, and obtaining the electron beam longitudinal pulse width by measuring the curve of the change of the electron beam transverse broadening with time delay.

[0042] The fifth aspect of the present application provides an ultrafast electron microscope, which comprises the metal periodic grating of the first aspect or the device for characterizing ultra-short electron beam pulse width of the second aspect.

[0043] The present application utilizes the metal periodic grating, and through the interaction between the pulsed electron beam passing through the surface of the grating and the grating near field induced by the pump laser, the electron beam is transversely broadened, and the pulse width of the pulsed electron beam is accurately characterized by using the broadening effect.

[0044] The technical problem to be solved by the present application: In order to solve the technical problems existing in the background art, the present application provides a device for characterizing electron beam pulse width in an ultrafast transmission electron microscope and a use method.

[0045] According to a specific embodiment of the present application, the present application uses a silver metal grating, so that the pulsed electron beam passing through the surface of the grating interacts with the grating near field induced by the pump laser, the electron beam is transversely broadened, and the pulse width of the pulsed electron beam is accurately characterized by using the broadening effect.

[0046] The device for characterizing electron beam pulse width in an ultrafast transmission electron microscope of the present application comprises a silver periodic grating, a light-emitting electron gun, an ultrafast laser system, and a camera; wherein the ultrafast laser system comprises an ultrafast laser, a laser frequency conversion element, and a delay device, which is used to convert the laser generated by the ultrafast laser into pump laser and probe laser with required parameters, the light-emitting electron gun is used to convert the probe laser into a pulsed light electron beam; the grating is made of a piece of silver metal (purity 99.99%) and is etched by an ion beam system. In order to obtain a smooth surface, the ion beam is first used to polish the section of the silver sheet. Then the silver sheet is turned over by 90°, and the periodic matrix etching mode is set to etch the nanochannel arranged periodically on the section of the silver sheet at one time. Finally, the silver sheet is rotated by 90° again, and the surface of the etched groove of the silver sheet is polished; the above pulsed light electron beam will gain or lose energy when passing through the silver periodic grating irradiated by the pump laser, so as to make the pulsed light electron beam transversely broadened; the inventor finds that the electron beam transversely broadened by the pump laser can be used to characterize the pulse width of the pulsed light electron beam, and the present application is obtained. Figure 1The delay adjuster adjusts different delay times, a series of pictures of the electron beam transverse broadening under different time delays are collected by the camera, the size of the electron beam transverse width is extracted from each picture, and a curve of the change of the electron beam transverse width with time delay is drawn.

[0047] The method for characterizing the ultra-short electron beam pulse width of the application comprises the following steps:

[0048] 1) In order to meet the phase synchronization condition, the grating period depends on the inverse Smith-Purcell effect, this synchronization condition is consistent with the synchronization condition of the Smith-Purcell effect, and can be expressed as d = λ / (β -1 -cosθ), wherein λ is the wavelength of the incident light, d is the grating period, θ is the included angle between the electron beam and the light beam, and β is the ratio of the electron speed to the speed of light in vacuum, and the value is 0.695 for an electron beam with an energy of 200 keV.

[0049] 2) The corresponding pump laser power is adjusted according to the material of the grating to generate a near field, so that the pulsed light electron beam is broadened when passing through the grating.

[0050] 3) The polarization direction of the pump laser is adjusted to make the pulsed electron beam broadening phenomenon most obvious.

[0051] 4) The pulsed electron beam is adjusted to be parallel to the surface of the grating, and the position of the pump laser on the surface of the grating is adjusted.

[0052] 5) The delay adjuster adjusts different delay times, a series of pictures of the electron beam transverse broadening under different time delays are collected by the camera, the size of the electron beam transverse width is extracted from each picture, and a curve of the change of the electron beam transverse width with time delay is drawn.

[0053] Specifically, the ultrafast laser generates ultrafast laser beams which are split into two beams by a beam splitting cube, one of the beams is converted into ultraviolet probe laser by a frequency conversion device, usually a four-fold frequency conversion device, and is incident on the light-emitting electron gun to generate a pulsed light electron beam which is parallel to the surface of the metal periodic grating, and the other beam is converted into pump laser by a two-fold frequency conversion device and is incident on the surface of the metal periodic grating after passing through the delay adjuster; the power and / or polarization direction of the pump laser are adjusted to make the pulsed light electron beam be broadened when passing through the metal periodic grating.

[0054] The metal periodic grating and the device for characterizing the ultra-short electron beam pulse width of the application can have the following beneficial effects, but are not limited to:

[0055] 1. The application can use the short interaction time of near field and electron beam to significantly improve the precision of the electron beam pulse width of the measurement, and the error is small.

[0056] 2. The application is suitable for most ultra-short pulse electron beam measurement requirements, simple to make, low cost and low operation difficulty.

[0057] 3. The application produces obvious experimental phenomena, and the spatial resolution requirement is not high. BRIEF DESCRIPTION OF DRAWINGS

[0058] Hereinafter, the embodiments of the application will be described in detail with reference to the accompanying drawings, in which:

[0059] Figure 1 The structure schematic diagram of the metal periodic grating and the device for characterizing the ultra-short electron beam pulse width of the application is shown.

[0060] Figure 2 The scanning electron microscope graph of the silver metal grating in Example 1 is shown.

[0061] Figure 3 The electron beam shape real space graph before and after the laser switch measured by the metal grating is shown; wherein, Figure 3 A shows the electron beam shape real space graph when the pump laser is not turned on; Figure 3 B shows the electron beam shape real space graph after the pump laser is turned on.

[0062] Figure 4 The pulse width obtained after Gaussian fitting of the electron beam broadening part is shown.

[0063] BRIEF DESCRIPTION OF DRAWINGS

[0064] 1. Metal periodic grating; 2. Light emitting electron gun; 3. Ultrafast laser module; 4. Camera; 5. Ultrafast laser; 6. Laser frequency conversion element; 7. Delay; 8. Beam splitter cube; 9. Ultrafast laser module mirror. DETAILED DESCRIPTION

[0065] The application will be further described below by specific examples in conjunction with the accompanying drawings, but it should be understood that these examples are only used for more detailed and specific description, and should not be understood as limiting the application in any form.

[0066] This part generally describes the materials used in the experiment of the application and the experimental method. Although many materials and operation methods used to achieve the purpose of the application are known in the art, the application is described as much as possible. The skilled person in the art knows that, if not specifically stated, the materials and operation methods used in the application are known in the art.

[0067] Example 1

[0068] This embodiment is used to illustrate the metal periodic grating and the device for characterizing the ultra-short electron beam pulse width of the present application.

[0069] The device of the present application is shown in Figure 1 The device for characterizing the ultra-short electron beam pulse width of the present application comprises a metal periodic grating 1, a light-emitting electron gun 2, an ultrafast laser module 3 and a camera 4; wherein: the ultrafast laser module comprises an ultrafast laser 5, a laser frequency conversion element 6 and a delay device 7. The device for characterizing the ultra-short electron beam pulse width further comprises a beam splitter cube 8 and an ultrafast laser module mirror 9. The grating is a metal periodic grating. The light-emitting electron gun is used to convert the probe laser into a pulsed light electron beam. The camera is a scintillator fiber coupled CMOS camera. The ultrafast laser module is used to convert the laser generated by the ultrafast laser into pump laser and probe laser. The ultrafast laser is a high repetition frequency ytterbium-doped femtosecond laser of American Spectra-Physics Company.

[0070] The laser frequency conversion element comprises a BBO crystal to realize the frequency doubling, tripling or quadrupling process.

[0071] The delay device comprises an electrically controlled displacement stage and delay device mirrors, the delay device mirrors are located on the electrically controlled displacement stage, the number of the delay device mirrors is 2, and the delay device mirrors realize the optical path delay.

[0072] The ultrafast laser generates ultrafast laser, which is split into two paths by the beam splitter cube. One path of the laser enters the frequency conversion device through the ultrafast laser module mirror, which is a quadrupling device in this embodiment, and is converted into ultraviolet probe laser, which is incident on the light-emitting electron gun to generate a pulsed light electron beam that slightly passes through the surface of the grating. The other path of the laser passes through a doubling device and is converted into pump laser, which is incident on the surface of the metal periodic grating after passing through the delay device.

[0073] The femtosecond laser generated by the ultrafast laser is converted into pump laser and probe laser with required parameters. The pump laser is adjusted to have a suitable power, and after being focused, is incident on the grating surface, and the laser energy density is 34 mJ / cm 2 . At the same time, the probe laser is adjusted to be incident on the light-emitting electron gun to generate a pulsed light electron beam that slightly passes through the grating surface. The pump laser induces plasmonic near field on the grating surface, and the interaction between the pulsed light electron beam and the plasmonic near field causes the electron beam to be transversely widened. By adjusting the delay time of the delay device, a series of pictures of the electron beam transverse width under different time delays are obtained. The intensity of the electron beam transverse width is extracted from each picture, and a curve of the intensity change with time delay is drawn. By measuring the curve of the electron beam transverse width change with time delay, the longitudinal pulse width of the electron beam can be obtained by fitting the curve width with a Gaussian function.

[0074] Figure 2 This embodiment uses a silver periodic grating, made from a piece of silver metal (99.99% purity) etched using a focused ion beam system. To obtain a smooth surface, the cross-section of the silver sheet is first polished using an ion beam. Then, the silver sheet is rotated 90°, and a matrix etching mode with a period of 240.5 nm is used to etch nanochannels arranged according to the grating period onto the cross-section of the silver sheet in a single pass. Finally, the silver sheet is rotated 90° again, and the surface with etched channels is polished. The grating period is determined by d = λ / (β...). -1 The values ​​are calculated using the formula (λ - cosθ), where λ is the incident light wavelength, d is the grating period, θ is the angle between the electron beam and the light beam, and β is the ratio of the electron velocity to the speed of light in vacuum. In this embodiment, the laser incident angle θ is 68.4°, the laser wavelength λ is 515nm, β is 0.695, and the corresponding period d is 240.5nm.

[0075] Figure 3 This is a comparison diagram of the electron beam shape before and after the switching laser, measured using the grating described above in this embodiment. Figure 3 A shows a real-space diagram of the electron beam shape when the pump laser is not turned on; Figure 3 B shows a real-space diagram of the electron beam shape after the pump laser is turned on, indicating a significant lateral broadening of the electron beam shape before and after the laser is switched on.

[0076] Figure 4 The longitudinal pulse width of the electron beam measured using the aforementioned grating in this embodiment is shown. First, images of the lateral broadening of the electron beam collected at different delays are shown ( Figure 3 As shown in B), the magnitude of the integrated intensity of the laterally broadened portion of the electron beam is extracted, and its variation with time delay is plotted, as follows. Figure 4 The box in the diagram is shown. Next, a Gaussian function is used to fit it (…). Figure 4 As shown in the curve, the longitudinal pulse width of the electron beam can be obtained, described in terms of full width at half maximum (FWHM), which is approximately 487 ± 10 fs.

[0077] While the effects of some embodiments have been shown above, those skilled in the art should understand that, based on the concept of the invention, other embodiments not specifically shown or other technical solutions of the invention not shown in the embodiments can also achieve the same technical effects as those claimed in the summary section:

[0078] 1. This invention can significantly improve the accuracy of the measured electron beam pulse width by taking advantage of the short interaction time between the near field and the electron beam, with small error.

[0079] 2、The application is suitable for most ultra-short pulse electron beam measurement requirements, simple to make, low cost and low operation difficulty.

[0080] 3、The application produces obvious experimental phenomena and has low space resolution requirement.

[0081] Although the application has been described to a certain extent, it is obvious that appropriate changes can be made to each condition without departing from the spirit and scope of the application. It is understood that the application is not limited to the described embodiments, but is subject to the scope of the claims, which includes equivalent replacements of each factor described.

Claims

1. An apparatus for characterizing the pulse width of an ultrashort electron beam, comprising: The device comprises a metal periodic grating, a light-emitting electron gun, an ultrafast laser module, and a camera, wherein the ultrafast laser module comprises an ultrafast laser, a laser frequency conversion element, and a delay device; wherein: The metal periodic grating is made of a metal material, wherein the metal material section of the metal periodic grating has a smooth surface, the smooth surface forms nanochannels arranged by grating period, and the nanochannels also have a smooth surface, and the metal periodic grating is arranged such that a pulsed electron beam parallelly sweeps across the surface of the metal periodic grating, and a pump laser hits the surface of the metal periodic grating after passing through the delay device; The ultrafast laser module is used to convert the laser generated by the ultrafast laser into a pump laser and a probe laser; The light-emitting electron gun is used to convert the probe laser into a pulsed light electron beam.

2. The device of claim 1, wherein: The number of periods of the metal periodic grating is 10-100; and / or The period of the metal periodic grating is 100-500 nm; and / or The metal material is selected from one or more of silver, gold, and copper; and / or The purity of the metal material is ≥99%.

3. The device of claim 2, wherein: The number of periods of the metal periodic grating is 20-50; and / or The period of the metal periodic grating is 150-400 nm; and / or The metal material is silver or gold; and / or The purity of the metal material is ≥99.9%.

4. The device of claim 3, wherein: The number of periods of the metal periodic grating is 50; and / or The period of the metal periodic grating is 240.5 nm; and / or The metal material is silver; and / or The purity of the metal material is ≥99.99%.

5. The device of claim 1, wherein: The ultrafast laser is a titanium-doped sapphire femtosecond laser or a ytterbium-doped femtosecond laser; and / or The laser frequency conversion element is a laser frequency conversion element based on a BBO crystal or a LBO crystal; and / or The delay device comprises an electrically controlled displacement stage and a delay device mirror.

6. The device of claim 5, wherein: The laser frequency conversion element is selected from one or more of a second-harmonic device, a third-harmonic device, and a fourth-harmonic device; and / or The delay device mirror is located on the electrically controlled displacement stage; and / or The number of delay device mirrors is 2-6.

7. The device of claim 6, wherein: The number of delay device mirrors is 2-4.

8. The device of claim 7, wherein: The number of delay device mirrors is 2.

9. The device of any one of claims 1-8, wherein: The ultrafast laser module further comprises a beam splitting cube and an ultrafast laser module mirror; and / or The camera is selected from one or more of a scintillator fiber-coupled CCD camera, a scintillator fiber-coupled CMOS camera, and a direct electron detection camera.

10. A method for characterizing the pulse width of an ultrashort electron beam, characterized in that, The characterization method uses the device of any one of claims 1-9.

11. The characterization method of claim 10, wherein: the characterization method uses the device of claim 9, and comprises the following steps: a) setting the grating period d of the metal grating to satisfy the following formula (A); d = λ / (β -1 -cosθ) formula (A); wherein λ is the wavelength of the incident laser light, θ is the incident angle of the laser light, i.e. the included angle between the electron beam and the light beam, and β is the ratio of the electron velocity to the speed of light in vacuum; b) the ultrafast laser generates an ultrafast laser light, which is split into two paths by the beam splitter cube, one path of the laser light sequentially passes through the ultrafast laser module mirror and the four-fold frequency conversion device, is converted into a probe laser, and is incident on the light-emitting electron gun to generate a pulsed light electron beam that is parallel to the surface of the metal periodic grating, and the other path of the laser light passes through the two-fold frequency conversion device, is converted into a pump laser, and is incident on the surface of the metal periodic grating after passing through the delay device; c) adjusting the power and / or polarization direction of the pump laser so that the pulsed light electron beam is broadened when passing through the metal periodic grating.

12. The characterization method of claim 11, wherein, In the step c), the pump laser induces the generation of a plasmonic near field on the surface of the metal periodic grating, and the pulsed light electron beam interacts with the plasmonic near field to cause the electron beam to be transversely broadened, and then the camera captures the electron beam. In the step c), the method further comprises: adjusting different delay times by the delay device, capturing pictures of the transverse broadening of the electron beam at different time delays by the camera, extracting the size of the transverse broadening intensity of the electron beam from each picture, drawing a curve of the change of the transverse broadening intensity of the electron beam with time delay, and obtaining the longitudinal pulse width of the electron beam by measuring the curve of the change of the transverse broadening of the electron beam with time delay.

13. The characterization method of claim 12, wherein, The ultrafast electron microscope comprises the device for characterizing the ultra-short electron beam pulse width according to any one of claims 1 to 9.

14. An ultrafast electron microscope, characterized in that, ​

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