A terahertz-driven femtosecond electronic pulse chopping and compression device

Abstract

The present invention relates to a terahertz-driven femtosecond electron pulse chopping and compression device, comprising a laser light source, a beam splitter, an ultraviolet laser pulse generator, a single-cycle terahertz pulse generator, an electron gun, an electron focusing device, an electron beam deflection device, an electron pulse chopping device, an electron pulse length measurement device, a fluorescent screen, and a camera. Laser pulses generated by the same laser light source are split into three paths: one laser beam enters the ultraviolet laser pulse generator for electron beam generation, and the other two beams are incident on the single-cycle terahertz pulse generator. The two generated terahertz pulses are incident on the electron beam deflection device and the pulse length measurement device for electron modulation. The electron pulses are excited by ultraviolet light, and the electron focusing device focuses them on the electron beam deflection device. The deflected electrons are chopped and compressed before being collected by a detection system. The present invention has a time resolution of up to 50 femtoseconds and good time synchronization, making it widely applicable to the study of ultrafast dynamics.

Description

Technical Field

[0001] The present 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 temporal resolution. 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, to reduce the influence 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 MeV electron guns driven by radio frequency cavities can produce electron beams with higher energies, phase jitter in microwave components still limits improvements in temporal resolution. Pulse recompression technologies, such as radio frequency compression and terahertz velocity compression, are used to compensate for pulse broadening caused by space charge effects. While radio frequency compression cavities can compensate for the dispersion of electron pulses, allowing for temporal focusing of the electron beam, the arrival time jitter of the electrons in the RF cavity is significant, making it incapable of meeting the requirements for high-precision temporal resolution.

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

[0005] In response to the shortcomings of the existing technology, the present invention proposes a terahertz-driven femtosecond electron pulse chopping and compression device, which aims to overcome problems such as space charge effect and phase jitter, and can reduce the jitter of electron arrival time while obtaining ultrashort electron beams, thereby achieving high-quality ultrashort electron beam generation and improving the system's time resolution.

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

[0007] A terahertz-driven femtosecond electron pulse chopping and compression device is characterized in that it includes: 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 electron 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 ultraviolet laser pulses (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) includes a terahertz antenna (18) and a two-dimensional precision translation stage (16), which is used to modulate the energy distribution of the electron beam (23) through the single-cycle terahertz pulse 22; the electron pulse chopping device (14) is a slit arranged in a vertical direction, which is used to cut off the unmodulated electron beam part.

[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 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 spatiotemporal 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 doubled frequency crystal and a quadrupled frequency crystal arranged in sequence along the direction of laser pulse emission; the doubled frequency crystal is a type I phase-matched BBO crystal with a thickness of 2 mm and a cutting angle of 23.4°; the quadrupled 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 consists of a terahertz antenna and a two-dimensional precision translation 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 an 18-fold field enhancement at the center of the terahertz antenna; the terahertz antenna is arranged on a two-dimensional precision translation stage, and by controlling the movement of the two-dimensional precision translation 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 translation 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 consists of a terahertz antenna and a six-dimensional precision translation stage; the terahertz antenna is exponential in shape, which can realize ultra-wideband terahertz coupling, greatly reducing the group velocity dispersion of the incident single-cycle terahertz pulse due to the coupling angle and impedance mismatch, and maintaining 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 an 18-fold field enhancement at the center of the terahertz antenna; the terahertz antenna is arranged on a six-dimensional precision translation stage, and by controlling the movement of the six-dimensional precision translation 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 translation stage can translate up and down a distance of 0 to 10mm, left and right a distance of 0 to 50mm, and front and back a distance of 0 to 30mm.

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

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

[0023] The laser pulses emitted by the laser source are split into two beams by a first beam splitter. One of these beams is frequency quadrupled to generate an ultraviolet laser pulse, which is used to excite the photocathode in the electron gun to produce an electron beam. The other laser beam is split into two beams of equal energy, each used to generate two single-cycle terahertz pulses. After DC acceleration and focusing, the electron beam enters the sample chamber. At the electron beam deflection device, the electron beam is deflected by the edge bending electric field of the terahertz antenna, causing some electrons at the upper end of the beam to be decelerated and others at the lower end to be accelerated, while the energies of the electrons at the front and rear ends remain unchanged. Subsequently, upon entering the center of the terahertz antenna, the electron beam is modulated by the vertically polarized single-cycle terahertz electric field. The electrons with different energies in the upper and lower portions are distributed at different longitudinal positions after vertical deflection. At this time, electrons at the front of the middle beam are decelerated and those at the rear are accelerated, while the energies of the electrons at the top and bottom ends remain unchanged. An electron pulse chopper removes the unmodulated portion of the electron beam vertically, generating 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 a terahertz antenna design, which can produce a significant field enhancement effect in the terahertz band while maintaining the single-cycle characteristics of the incident terahertz, greatly improving the modulation efficiency and chopping accuracy of the electron beam, thereby enabling the present invention to 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 optimal emission characteristics of the cathode.

[0027] 3. The present invention simultaneously implements electron pulse chopping and compression technology, realizes an ultrashort electron 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 electron 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 electronic 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 provide a clear and complete description of the technical solution of the present invention in conjunction with the embodiments of the present invention and the accompanying drawings. Obviously, the described embodiments are not intended to limit the present invention.

[0036] The terahertz driven femtosecond electronic pulse chopping and compression device of 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 generator 3, a single-cycle terahertz pulse generator 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 1030 nm and a pulse width of 1.5 ps. The laser pulse is split into two paths by the first beam splitter 2-1. One path of the laser enters the ultraviolet laser pulse generator 3 and, after passing through the beam reduction lens, the doubler crystal, and the quadrupler crystal, generates an ultraviolet laser pulse 21, which is used to excite the photocathode in the electron gun 5 to generate an electron beam 23. The other path of the laser is split into two paths by the second beam splitter 2-2 and respectively incident on two single-cycle terahertz pulse generators 4. By means of optical nonlinear difference frequency, a single-cycle terahertz pulse 22 with a central frequency of approximately 0.18 THz is generated. The two single-cycle terahertz pulses 22 are respectively incident on the electron beam deflection device 13 and the electron pulse length measurement device 15.

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

[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 translation stage 16. The terahertz antenna 18 is exponentially shaped, which can achieve ultra-wideband terahertz coupling, significantly reducing the group velocity dispersion of the incident single-cycle terahertz pulse 22 due to the coupling angle and impedance mismatch, and maintaining 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, some electrons at the upper end of the electron beam 23 are decelerated, and some electrons at the lower end are accelerated, while 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. The electrons with different energies in the upper and lower parts will be distributed at different longitudinal positions after vertical deflection. At this time, the electrons at the front end of the middle part of the electron beam 23 are decelerated, and the electrons at the rear end 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. 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 chopper 14, and the portion of the electron beam that is not energy modulated is cut off 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 ultrashort electron beam pulse width measurement results described above represent only preferred embodiments of the present invention and are not intended to limit its scope. Various variations are possible. In other words, any simple, equivalent variations and modifications made in accordance with the claims and description of this application fall within the scope of protection of the patent. Anything not fully described in this invention represents conventional technology.

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 split into two paths by a beam splitter. One path enters an ultraviolet laser pulse generating device (3) to generate an ultraviolet laser pulse (21) to excite an electron gun (5) to generate an electron beam (23); the other path enters a 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) by the single-cycle terahertz pulse (22); 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; 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).

2. The terahertz-driven femtosecond electronic pulse chopping and compression device according to claim 1, 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).

3. The terahertz-driven femtosecond electronic pulse chopping and compression device according to any one of claims 1 or 2, characterized in that: 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.

4. The terahertz-driven femtosecond electronic pulse chopping and compression device according to any one of claims 1 or 2, 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, comprising a cathode electrode and a cathode window plated with a metal film, and the anode (8) is grounded.

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

6. The terahertz-driven femtosecond electronic pulse chopping and compression device according to any one of claims 1 or 2, characterized in that: The electron focusing device (9) comprises 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.

7. The terahertz-driven femtosecond electronic pulse chopping and compression device according to any one of claims 1 or 2, characterized in that: The electronic pulse chopping device (14) is a slit with a width of 50 μm.

8. The terahertz-driven femtosecond electronic pulse chopping and compression device according to any one of claims 1 or 2, 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) caused by coupling angle and impedance mismatch, and maintain its single-cycle characteristics; The spacing between the terahertz antennas (18) is 50 μm, a single-cycle terahertz pulse (22) is focused in a sub-wavelength-order 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 arranged on a two-dimensional precision displacement stage (16), and the electron beam (23) can be made to pass through different positions of the terahertz antenna (18) by controlling the movement of the two-dimensional precision displacement stage (16).

9. The terahertz-driven femtosecond electronic pulse chopping and compression device according to claim 8, characterized in that: The two-dimensional precision displacement 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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