Terahertz-driven femtosecond electronic pulse chopping and compressing device

Abstract

The invention relates to a terahertz-driven femtosecond electronic pulse chopping and compressing device. The terahertz-driven femtosecond electronic pulse chopping and compressing device comprises a laser light source, a beam splitter, an ultraviolet laser pulse generating device, a single-period terahertz pulse generating device, an electron gun, an electron focusing device, an electron beam deflection device, an electronic pulse chopping device, an electronic pulse length measuring device, a fluorescent screen and a camera. Laser pulses generated by the same laser light source are divided into three paths, one beam of laser enters the ultraviolet laser pulse generation device and is used for generating electron beams, the other two beams of laser enter the single-period terahertz pulse generation device, the generated two paths of terahertz pulses enter the electron beam deflection device and the pulse length measurement device, and the electron beam deflection device and the pulse length measurement device are used for measuring the pulse length. The method is used for electronic modulation. The electronic pulse is excited by ultraviolet light, the electronic focusing device focuses the electronic pulse on the electron beam deflection device, and deflected electrons are chopped and compressed and are collected by the detection system. The time resolution capability of the method can reach 50 femtoseconds, and the method has good time synchronization and can be widely applied to the research process of ultrafast dynamics.

Description

Technical Field

[0001] The invention belongs to the research field of ultrafast electron beam dynamics, and in particular relates to a terahertz-driven femtosecond electron pulse chopping and compression device. Background Art

[0002] Ultrafast electron diffraction technology has been widely used in ultrafast dynamics research such as chemical reactions, structural phase transitions, and charge density waves because it can capture changes in the atomic structure of matter. However, during the transmission of the electron beam, due to the existence of the space charge effect, the pulse width of the electron beam will increase rapidly, which greatly limits the further improvement of the time resolution. In order to overcome this problem, a variety of technologies have been proposed to shorten the electron beam pulse width, such as reducing the electron beam density, shortening the electron beam propagation distance, and increasing the electron energy, etc., to reduce the impact of the space charge force effect on the electron beam pulse width. However, these methods often have limitations and cannot fundamentally solve the problems caused by the space charge effect.

[0003] Although the MeV electron gun driven by the RF cavity can produce electron beams with higher energy, the phase jitter problem of microwave components still limits the improvement of time resolution. Pulse recompression technologies, such as RF compression technology and terahertz speed compression technology, are used to compensate for the pulse broadening caused by the space charge effect. Among them, although the RF compression cavity can compensate for the dispersion of the electron pulse and make the electron beam focused in time, the electron arrival time of its RF cavity has large jitter and cannot meet the requirements of high-precision time resolution.

[0004] In recent years, single-cycle terahertz pulses have been used to compress electron pulses due to their unique properties, such as higher compression field gradients and smaller jitter between electron beams and terahertz pulses. However, existing terahertz-driven electron pulse compression devices often fail to effectively chop electron pulses while achieving electron pulse compression, thus limiting the generation and application of ultrashort electron beams. Summary of the invention

[0005] In view of the shortcomings of the prior art, the present invention proposes a terahertz-driven femtosecond electron pulse chopping and compression device, which aims to overcome the problems of space charge effect and phase jitter, and can reduce the jitter of electron arrival time while obtaining ultrashort electron beams, realize high-quality ultrashort electron beam generation, and improve the time resolution of the system.

[0006] The technical solution provided by the present invention is:

[0007] A terahertz-driven femtosecond electronic pulse chopping and compression device, characterized in that it comprises: a laser light source 1, a beam splitter 2, an ultraviolet laser pulse generating device 3, a single-cycle terahertz pulse generating device 4, an electron gun 5, an electron focusing device 9, an electron beam deflecting device 13, an electronic pulse chopping device 14, a fluorescent screen 19 and a camera 20;

[0008] The laser pulse emitted by the laser light source 1 is divided into two paths by a beam splitter, one path enters the ultraviolet laser pulse generating device 3 to generate an ultraviolet laser pulse (21) to excite the electron gun (5) to generate an electron beam (23); the other path enters the single-cycle terahertz pulse generating device 4 to generate a single-cycle terahertz pulse 22 for modulating the deflection and compression of the electron beam (23);

[0009] The electron gun 5 comprises a cathode 7 and an anode 8, wherein the cathode (7) receives an ultraviolet laser pulse (21) to emit an electron beam (23);

[0010] The electron focusing device 9 is installed between the electron gun 5 and the sample chamber 12, and is used to focus the electron beam (23);

[0011] An electron beam deflection device 13 and an electron pulse chopping device 14 are sequentially placed in the sample chamber 12 along the emission direction of the electron beam 23. The electron beam deflection device (13) comprises a terahertz antenna (18) and a two-dimensional precision translation stage (16), and is used to modulate the energy distribution of the electron beam (23) through the single-cycle terahertz pulse 22; and the electron pulse chopping device (14) is a slit arranged in a vertical direction, and is used to cut off the unmodulated electron beam portion.

[0012] Furthermore, it also includes an electron pulse length measuring device, which is arranged in the sample chamber 12 and is used to measure the pulse width of the electron beam (23) after being compressed and chopped by the electron beam deflection device (13) and the electron pulse chopping device (14).

[0013] Furthermore, it also comprises a fluorescent screen (19) and a camera (20), which are arranged at the rear end of the sample chamber (12) and are used to capture the temporal and spatial distribution information of the electron beam (23).

[0014] Furthermore, the single-cycle terahertz pulse generating device includes a transmission grating, a lens, a half-wave plate and a lithium niobate crystal arranged in sequence along the laser pulse emission direction, and generates a single-cycle terahertz pulse in the lithium niobate crystal by optical nonlinear difference frequency.

[0015] Furthermore, the electron gun is a photocathode DC high-voltage electron gun, and a negative high voltage of 0 to 125 kV can be applied to the electrode; the cathode is a back-illuminated photocathode, including a cathode electrode and a cathode window coated with a metal film; and the anode is grounded.

[0016] Furthermore, the ultraviolet laser pulse generating device includes a beam reduction lens, a double frequency crystal and a quadruple frequency crystal which are arranged in sequence along the direction of laser pulse emission; the double frequency crystal is a type I phase-matched BBO crystal with a thickness of 2 mm and a cutting angle of 23.4°; the quadruple frequency crystal is a type I phase-matched BBO crystal with a thickness of 1 mm and a cutting angle of 50°.

[0017] Furthermore, the electron focusing device includes a flexible bellows and a magnetic lens; the magnetic lens is composed of a focusing copper coil and a metal shell covering the focusing copper coil.

[0018] Furthermore, the electronic pulse chopping device is a slit with a width of 50 μm.

[0019] Furthermore, the electron beam deflection device is composed of a terahertz antenna and a two-dimensional precision displacement stage; the terahertz antenna is exponential in shape, which can realize ultra-wideband terahertz coupling, greatly reduce the group velocity dispersion of the incident single-cycle terahertz pulse due to the coupling angle and impedance mismatch, and maintain its single-cycle characteristics; the spacing between the terahertz antennas is 50μm, and the single-cycle terahertz pulse is focused in a sub-wavelength space, the electric field is superimposed and enhanced at the center of the terahertz antenna, and the magnetic field is offset at the center of the terahertz antenna, thereby generating 18 times the field enhancement at the center of the terahertz antenna; the terahertz antenna is arranged on a two-dimensional precision displacement stage, and by controlling the movement of the two-dimensional precision displacement stage, the electron beam can pass through different positions of the terahertz antenna; relative to the position of the electron beam, the two-dimensional precision displacement stage can translate up and down a distance of 0 to 10mm and translate left and right a distance of 0 to 10mm.

[0020] Furthermore, the electronic pulse length measuring device is composed of a terahertz antenna and a six-dimensional precision displacement stage; the terahertz antenna is exponential in shape, which can realize ultra-wideband terahertz coupling, greatly reduce the group velocity dispersion of the incident single-cycle terahertz pulse due to the coupling angle and impedance mismatch, and maintain its single-cycle characteristics; the spacing of the terahertz antenna is 50μm, and the single-cycle terahertz pulse is focused in a sub-wavelength space, the electric field is superimposed and enhanced at the center of the terahertz antenna, and the magnetic field is offset at the center of the terahertz antenna, thereby generating 18 times the field enhancement at the center of the terahertz antenna; the terahertz antenna is arranged on a six-dimensional precision displacement stage, and by controlling the movement of the six-dimensional precision displacement stage, the electron beam can pass through different positions of the terahertz antenna; relative to the position of the electron beam, the six-dimensional precision displacement stage can translate up and down a distance of 0 to 10mm, translate left and right a distance of 0 to 50mm, and translate forward and backward a distance of 0 to 30mm.

[0021] Furthermore, a vacuum flange window is provided on the electron gun chamber for receiving ultraviolet laser pulses to enter the electron gun chamber; and a vacuum flange window is provided on the sample chamber for receiving terahertz pulses to enter the sample chamber.

[0022] The working process of the present invention is as follows:

[0023] The laser pulse emitted by the laser light source is divided into two beams by the first beam splitter, one of which generates ultraviolet laser pulses after quadrupling, which is used to excite the photocathode in the electron gun to generate an electron beam. The other laser is divided into two laser beams with the same energy, which are used to generate two single-cycle terahertz pulses respectively. After DC acceleration and focusing, the electron beam enters the sample chamber. In the electron beam deflection device, the electron beam is deflected by the edge bending electric field of the terahertz antenna, that is, part of the electrons at the upper end of the electron beam are decelerated, and part of the electrons at the lower end are accelerated, and the energy of the electrons at the front and rear ends remains unchanged. Subsequently, when the electron beam enters the center of the terahertz antenna, it is modulated by the vertically polarized single-cycle terahertz electric field. The electrons with different energies in the upper and lower parts will be distributed in different longitudinal positions after vertical deflection. At this time, the front electrons in the middle part of the electron beam are decelerated, and the rear electrons are accelerated, while the energy of the electrons at the upper and lower ends remains unchanged. The electron beam part that is not energy modulated is cut off in the vertical direction by the electron pulse chopping device to obtain an ultrashort electron beam. Finally, the cut ultrashort electron beam enters the electron pulse length measurement device for pulse width measurement to confirm the pulse width of the electron beam and verify the feasibility of the terahertz-driven femtosecond electron pulse chopping and compression device.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] 1. The present invention adopts the design of terahertz antenna, which can produce a significant field enhancement effect in the terahertz band while maintaining the single-period characteristics of the incident terahertz, greatly improving the modulation efficiency and chopping accuracy of the electron beam, so that the present invention can obtain a higher quality ultrashort electron beam.

[0026] 2. The cathode electrode and cathode window of the electron gun of the present invention adopt a detachable connection structure. When the performance of the metal film decreases, the cathode window coated with the metal film can be replaced, thereby maintaining the best emission characteristics of the cathode.

[0027] 3. The present invention realizes electronic pulse chopping and compression technology simultaneously, realizes an ultrashort electronic pulse of 50 fs, reduces the arrival time jitter of the electron beam, and has great application potential in ultrafast technology.

[0028] 4. By combining components such as laser light source, ultraviolet laser pulse generator, single-cycle terahertz pulse generator, electron gun, etc., a complete electronic pulse chopping and compression system is constructed to achieve efficient modulation and chopping of electron beams.

[0029] 5. The present invention has small size, simple structure, low cost and can be widely used. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the femtosecond electron synchronization device for terahertz drive electron pulse chopping and compression.

[0031] Figure 2 Schematic diagram of the terahertz antenna structure.

[0032] Figure 3 Schematic diagram of electronic pulse chopping and compression.

[0033] Figure 4 This is the measurement result of the ultrashort electron beam pulse width.

[0034] In the figure: 1-laser light source, 2-1 first beam splitter, 2-2 second beam splitter, 3-ultraviolet laser pulse generating device, 4-single cycle terahertz pulse generating device, 5-electron gun, 6-vacuum flange window, 7-cathode, 8-anode, 9-electron focusing device, 10-flexible bellows, 11-magnetic lens, 12-sample chamber, 13-electron beam deflecting device, 14-electron pulse chopping device, 15-electron pulse length measuring device, 16-two-dimensional precision moving platform, 17-six-dimensional precision moving platform, 18-terahertz antenna, 19-fluorescent screen, 20-camera, 21-ultraviolet laser pulse, 22-single cycle terahertz pulse, 23-electron beam, 24-reflector. DETAILED DESCRIPTION

[0035] The following will be combined with the embodiments of the present invention and the accompanying drawings to clearly and completely describe the technical solution of the present invention. Obviously, the described embodiments are not intended to limit the present invention.

[0036] The femtosecond electronic pulse chopping and compression device driven by terahertz in this embodiment is as follows Figure 1 As shown, it mainly includes components such as a laser light source 1, a beam splitter 2, a reflector 24, an ultraviolet laser pulse generating device 3, a single-cycle terahertz pulse generating device 4, an electron gun 5 and a sample chamber 12.

[0037] The laser light source 1 emits a laser pulse with a central wavelength of 1030nm and a pulse width of 1.5ps. The laser pulse is divided into two paths by the first beam splitter 2-1, wherein one path of laser light enters the ultraviolet laser pulse generator 3, and after the action of the beam reduction lens, the double frequency crystal and the quadruple frequency crystal, an ultraviolet laser pulse 21 is generated, which is used to excite the photocathode in the electron gun 5 to generate an electron beam 23. The other path of laser light is divided into two paths by the second beam splitter 2-2, and is incident on two single-cycle terahertz generators 4 respectively. A single-cycle terahertz pulse 22 with a central frequency of about 0.18THz is generated by optical nonlinear difference frequency. The two single-cycle terahertz pulses 22 are incident on the electron beam deflection device 13 and the electron pulse length measurement device 15 respectively.

[0038] The electron gun 5 includes a vacuum flange window 6, a cathode 7 and an anode 8 arranged opposite to the cathode 7; the cathode 7 is a back-illuminated photocathode, including a cathode electrode and a cathode window coated with a metal film. The anode 8 is grounded. The electron beam 23 is accelerated under the action of the electric field formed between the cathode 7 and the anode 8. The accelerated electron beam 23 enters the sample chamber 12 after being focused by the electron focusing device 9.

[0039] In the sample chamber 12 , the electron beam 23 passes through the electron beam deflection device 13 , the electron pulse chopping device 14 , and the electron pulse length measuring device 15 in sequence.

[0040] like Figure 2 As shown, the electron beam deflection device 13 is composed of a terahertz antenna 18 and a two-dimensional precision displacement stage 16. The terahertz antenna 18 is exponentially shaped, which can realize ultra-wideband terahertz coupling, greatly reduce the group velocity dispersion caused by the coupling angle and impedance mismatch of the incident single-cycle terahertz pulse 22, and maintain its single-cycle characteristics. Before the electron beam 23 enters the electron beam deflection device 13, under the action of the bending electric field at the edge of the terahertz antenna 18, part of the electrons at the upper end of the electron beam 23 are decelerated, and part of the electrons at the lower end are accelerated, and the energy of the electrons at the front and rear ends remains unchanged. Subsequently, when the electron beam 23 enters the center of the terahertz antenna 18, it is modulated by the electric field of the vertically polarized single-cycle terahertz pulse 22, and the electrons with different energies in the upper and lower parts will be distributed in different longitudinal positions after vertical deflection. At this time, the front end electrons of the middle part of the electron beam 23 are decelerated, and the rear end electrons are accelerated, while the energy of the electrons at the upper and lower ends remains unchanged. The spacing between the terahertz antennas 18 is 50 μm, and the single-cycle terahertz pulse 22 is focused in a sub-wavelength space. The electric field is superimposed and enhanced at the center of the terahertz antenna 18, and the magnetic field is reversely offset at the center of the terahertz antenna, thereby generating an 18-fold field enhancement at the center of the terahertz antenna. The modulated electron beam 23 passes through the electronic pulse chopping device 14 to cut off the electron beam portion that is not energy modulated in the vertical direction to obtain an ultrashort electron beam, such as Figure 3 shown.

[0041] The electron pulse length measuring device 15 is also composed of a terahertz antenna 18 and a six-dimensional precision translation stage 17, and is used to measure the pulse width. The pulse width of the electron beam is 50 fs. Figure 4 As shown, the horizontal axis represents time, and the vertical axis represents the normalized electron intensity. Numerical results show that the pulse width of the electrons in this embodiment after pulse chopping and compression is 50 fs.

[0042] A fluorescent screen 19 is also installed at the rear end of the sample chamber 12 , and a camera 20 is placed outside the sample chamber 12 opposite to the fluorescent screen 19 .

[0043] The above-mentioned ultrashort electron beam pulse width measurement results described in this example are only preferred embodiments of the present invention and are not intended to limit the scope of the present invention. The above-mentioned embodiments of the present invention can also be changed in various ways. That is, all simple and equivalent changes and modifications made according to the claims and the contents of the specification of the present invention fall within the scope of protection of the claims of the present invention. The contents not described in detail in the present invention are all conventional technical contents.

Claims

1. A terahertz-driven femtosecond electronic pulse chopping and compression device, characterized in that: include: Laser light source 1, beam splitter 2, ultraviolet laser pulse generating device 3, single cycle terahertz pulse generating device 4, electron gun 5, electron focusing device 9, electron beam deflecting device 13, electron pulse chopping device 14, fluorescent screen 19 and camera 20; The laser pulse emitted by the laser light source 1 is divided into two paths by a beam splitter, one path enters the ultraviolet laser pulse generating device 3 to generate an ultraviolet laser pulse (21) to excite the electron gun (5) to generate an electron beam (23); the other path enters the single-cycle terahertz pulse generating device 4 to generate a single-cycle terahertz pulse 22 for modulating the deflection and compression of the electron beam (23); The electron gun 5 comprises a cathode 7 and an anode 8, wherein the cathode (7) receives an ultraviolet laser pulse (21) to emit an electron beam (23); The electron focusing device 9 is installed between the electron gun 5 and the sample chamber 12, and is used to focus the electron beam (23); An electron beam deflection device 13 and an electron pulse chopping device 14 are sequentially placed in the sample chamber 12 along the emission direction of the electron beam 23. The electron beam deflection device (13) comprises a terahertz antenna (18) and a two-dimensional precision translation stage (16), and is used to modulate the energy distribution of the electron beam (23) through the single-cycle terahertz pulse 22; and the electron pulse chopping device (14) is a slit arranged in a vertical direction, and is used to cut off the unmodulated electron beam portion.

2. The terahertz-driven femtosecond electronic pulse chopping and compression device according to claim 1, characterized in that: It also includes an electron pulse length measuring device, which is arranged in the sample chamber 12 and is used to measure the pulse width of the electron beam (23) after being compressed and chopped by the electron beam deflection device (13) and the electron pulse chopping device (14).

3. The terahertz-driven femtosecond electronic pulse chopping and compression device according to claim 1 or 2, characterized in that: It also includes a fluorescent screen (19) and a camera (20), which are arranged at the rear end of the sample chamber (12) and are used to capture the temporal and spatial distribution information of the electron beam (23).

4. The terahertz-driven femtosecond electronic pulse chopping and compression device according to any one of claims 1 to 3, characterized in that: The single-cycle terahertz pulse generating device comprises a transmission grating, a lens, a half-wave plate and a lithium niobate crystal arranged in sequence along the laser pulse emission direction, and generates a single-cycle terahertz pulse in the lithium niobate crystal by means of optical nonlinear difference frequency.

5. The terahertz-driven femtosecond electronic pulse chopping and compression device according to any one of claims 1 to 3, characterized in that: The electron gun 5 is a photocathode DC high voltage electron gun, and a negative high voltage of 0 to 125 kV can be applied to the electrode; the cathode 7 is a back-illuminated photocathode, including a cathode electrode and a cathode window coated with a metal film, and the anode 8 is grounded.

6. The terahertz-driven femtosecond electronic pulse chopping and compression device according to any one of claims 1 to 3, characterized in that: The ultraviolet laser pulse generating device 3 comprises a beam reduction lens, a double frequency crystal and a quadruple frequency crystal which are sequentially arranged along the direction of laser pulse emission; the double frequency crystal is a type I phase-matched BBO crystal with a thickness of 2 mm and a cutting angle of 23.4°; the quadruple frequency crystal is a type I phase-matched BBO crystal with a thickness of 1 mm and a cutting angle of 50°.

7. The terahertz-driven femtosecond electronic pulse chopping and compression device according to any one of claims 1 to 3, characterized in that: The electron focusing device 9 includes a flexible bellows 10 and a magnetic lens 11; the magnetic lens 11 is composed of a focusing copper coil and a metal shell covering the focusing copper coil.

8. The terahertz-driven femtosecond electronic pulse chopping and compression device according to any one of claims 1 to 3, characterized in that: The electronic pulse chopping device 14 is a slit with a width of 50 μm.

9. The terahertz-driven femtosecond electronic pulse chopping and compression device according to any one of claims 1 to 3, characterized in that: The terahertz antenna 18 is exponentially shaped, which can realize ultra-wideband terahertz coupling, reduce the group velocity dispersion of the incident single-cycle terahertz pulse 22 due to the coupling angle and impedance mismatch, and maintain its single-cycle characteristics; The spacing of the terahertz antenna 18 is 50 μm, and the single-cycle terahertz pulse 22 is focused in a sub-wavelength space, the electric field is superimposed and enhanced at the center of the terahertz antenna 18, and the magnetic field is offset at the center of the terahertz antenna 18, thereby generating an 18-fold field enhancement at the center of the terahertz antenna; The terahertz antenna 18 is disposed on a two-dimensional precision translation stage 16 , and the electron beam 23 can pass through different positions of the terahertz antenna 18 by controlling the movement of the two-dimensional precision translation stage 16 .

10. The terahertz-driven femtosecond electronic pulse chopping and compression device according to claim 9, characterized in that: The two-dimensional precision translation stage 16 can translate up and down relative to the position of the electron beam 23 by a distance of 0 to 10 mm, and translate left and right by a distance of 0 to 10 mm.

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