A high-power polychromatic terahertz radiation generation device based on beat frequency laser accumulation

Abstract

The application provides a high-power multicolor terahertz radiation generation device based on beat laser accumulation, which comprises an injector, an electron beam modulation system and a radiation system arranged in sequence; and a femtosecond laser system comprising a femtosecond laser, a first light splitting plate and a first mirror arranged in sequence and parallel to the electron beam transmission axis, the first light splitting plate splits the femtosecond Gaussian laser pulses from the femtosecond laser into first and second path femtosecond Gaussian lasers, the first light splitting plate, a pulse broadening system and the injector are arranged in sequence, and the first mirror, a laser accumulation system and the electron beam modulation system are arranged in sequence; the laser accumulation system generates beat accumulation laser with multiple terahertz cycles after receiving the second path femtosecond Gaussian laser. The device of the application can realize the center frequency of the multicolor terahertz radiation being adjustable, the number of the multicolor being variable, the frequency and time interval being controllable, the layout being simple, the tunability being strong and the flexibility being relatively high.

Description

Technical Field

[0001] The present invention belongs to the field of terahertz optical instruments, and in particular relates to a high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking. Background Art

[0002] Terahertz radiation refers to the frequency band between microwaves and visible light in the electromagnetic spectrum, typically ranging from 0.1 terahertz to tens of terahertz. With continuous breakthroughs in related technologies, terahertz science has developed into a cutting-edge, multidisciplinary research field. High-power multicolor terahertz radiation (i.e., radiation simultaneously containing multiple discrete terahertz frequencies) has demonstrated significant application value in multispectral terahertz imaging, differential absorption lidar, atomic multi-level excitation, and molecular fingerprinting due to its unique advantages.

[0003] On the other hand, high-power two-color terahertz radiation with controllable pulse temporal spacing provides an important research tool for exploring ultrafast dynamics in material science. This type of radiation can not only be used to study key scientific issues such as insulator-metal phase transitions, semiconductor carrier dynamics (impact ionization, valley-to-valley scattering, etc.), and the effective mass anisotropy of hot electrons, but can also reveal ultrafast dynamics in ferroelectrics and coherent oscillations of collective electron excitations in condensed matter systems. Notably, combining terahertz pump-probe technology with terahertz scanning tunneling microscopy promises to provide exclusive access to ultrafast dynamics in single molecules and nanostructures with atomic resolution. However, while existing high-power terahertz sources can achieve millijoule-level pulse energies, they suffer from common bottlenecks such as fixed radiation frequency, difficulty in generating multiple colors, and insufficient precision in controlling time-frequency parameters.

[0004] Free-electron lasers (FELs) couple relativistic electron beams with electromagnetic fields, generating high-power, narrowband radiation with continuously tunable frequency across a wide frequency range from terahertz to X-rays. Based on undulator resonance modulation or electron beam property manipulation, this technology has successfully achieved dual-color and multi-color radiation output in the X-ray band, supporting advanced scientific experiments such as resonant inelastic X-ray scattering, X-ray pump-X-ray detection experiments, and high-precision spectral analysis. These breakthroughs provide important technical insights for similar applications in the terahertz band.

[0005] Currently, the generation of multicolor terahertz radiation relies primarily on the nonlinear effects of optical crystals or the resonance modulation of undulators, but these methods suffer from significant drawbacks such as low radiation power and limited time-frequency parameter adjustment. Using an external seed laser to manipulate the properties of an electron beam promises the potential for highly stable and repeatable multicolor terahertz free electron lasers, providing an ideal radiation source for experiments such as terahertz pumping and detection. However, due to key technical bottlenecks in terahertz seed laser sources, there is currently no seed laser capable of directly modulating electron beams with multiple terahertz cycles, a major technical obstacle hindering the development of this field. Summary of the Invention

[0006] The present invention aims to provide a high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking to generate high-power multi-color terahertz radiation with adjustable center frequency, variable multi-color quantity, and controllable frequency and time interval.

[0007] To achieve the above-mentioned objectives, the present invention provides a high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking, comprising an injector, an electron beam modulation system, and a radiation system, which are sequentially arranged along the transmission axis of the electron beam and located on the same horizontal plane; and a femtosecond laser system, which comprises a femtosecond laser, a first beam splitter, and a first reflector, which are sequentially arranged along the optical path and parallel to the transmission axis of the electron beam, wherein the first beam splitter is configured to split the femtosecond Gaussian laser pulse from the femtosecond laser into a first path of femtosecond Gaussian laser and a second path of femtosecond Gaussian laser, the first beam splitter, the pulse stretching system, and the injector are sequentially arranged along the optical path of the first path of femtosecond Gaussian laser, and the first reflector, the laser stacking system, and the electron beam modulation system are sequentially arranged along the optical path of the second path of femtosecond Gaussian laser; the laser stacking system generates a beat-frequency stacking laser with multiple terahertz cycles after receiving the second path of femtosecond Gaussian laser.

[0008] The pulse stretching system generates a pulse stretching laser after receiving the first femtosecond Gaussian laser reflected by the first beam splitter; the injector generates an electron beam when receiving the pulse stretching laser; the electron beam modulation system uses the beat frequency stacking laser to achieve energy modulation and density modulation of the electron beam to obtain an electron bunch train with multiple terahertz periods; the radiation system uses the electron bunch train to generate high-power multi-color terahertz radiation.

[0009] The femtosecond laser is configured to generate Gaussian laser pulses with an adjustable half-width of 30 to 200 femtoseconds; the pulse stretching system receives the femtosecond Gaussian laser pulses and stretches their pulse width to the picosecond level; the injector is a photocathode injector, comprising a photocathode electron gun and a downstream S-band or X-band acceleration structure, and the energy of the electron beam at the injector outlet is adjustable between 100 and 150 MeV.

[0010] The pulse stretching system includes a first grating pair as a dispersion element, a second reflector and a third reflector for reflecting the pulse stretched laser from the first grating pair back to the first grating pair to eliminate spatial dispersion. The pulse stretching system is used to output the pulse stretched laser after eliminating spatial dispersion.

[0011] The pulse stretching system further includes a fourth reflecting mirror, which reflects the pulse stretching laser to the injector to generate an electron beam with a picosecond time length.

[0012] The laser stacking system includes a second grating pair, a fifth reflector and a sixth reflector located on the other side of the second grating pair that receives the second path of femtosecond Gaussian laser, an optical isolation component located on the same side of the second grating pair that receives the second path of femtosecond Gaussian laser, a splitting system for splitting light to obtain multiple paths of laser light, and a movable reflector located on each path of laser light.

[0013] The optical isolation component comprises at least an optical isolator; and / or

[0014] Each laser beam maintains equal intensity; and / or

[0015] The movable reflector moves with a precision of micrometer level, thereby adjusting the relative time delay between multiple laser beams, and finally forming a beat frequency stacked laser beam with multiple different terahertz frequencies on the envelope.

[0016] The electron beam modulation system includes a modulation segment undulator, a first magnetic compressor, and a ninth reflector, which are arranged in sequence along the common propagation direction of the electron beam and the beat-frequency stacking laser; the modulation segment undulator is a planar undulator, which is configured to receive the electron beam and the beat-frequency stacking laser, so that the electron beam and the beat-frequency stacking laser interact with each other so that the electron beam produces energy modulation with multiple different terahertz periods; the first magnetic compressor converts the energy modulation of the electron beam into density modulation, so that the electron beam forms a micro-cluster structure with multiple different terahertz frequencies in the longitudinal direction, forming an electron beam cluster with multiple terahertz period information; the ninth reflector leads the beat-frequency stacking laser after interacting with the electron beam out of the electron beam modulation system through reflection.

[0017] The radiation system comprises a first radiation section undulator, a second magnetic compressor, a second radiation section undulator, a bending magnet and a beam dump arranged in sequence along the transmission direction of the electron bunch string; the first radiation section undulator and the second radiation section undulator are both planar undulators with a period of 20 cm and a continuously adjustable magnetic gap of 50-160 mm; the second magnetic compressor time delays the electron bunch string after the first radiation section undulator, so that the electron bunch string is separated from the radiation field, thereby realizing controllable time spacing between the terahertz radiation generated by the first radiation section undulator and the terahertz radiation generated by the second radiation section undulator; the bending magnet separates the electron bunch string after the second radiation section undulator from the radiation field, and the electron bunch string is guided into the beam dump through the bending magnet.

[0018] The first radiation section undulator and the second radiation section undulator resonate at the same wavelength, so that the electron bunch string directly generates polychromatic terahertz radiation; or the first radiation section undulator and the second radiation section undulator resonate at different wavelengths respectively, and the second magnetic compressor realizes controllable time spacing of terahertz radiation pulses, so that the electron bunch string generates two different frequency terahertz radiations with a larger frequency difference.

[0019] The high-power polychromatic terahertz radiation generation device based on beat laser accumulation of the application realizes adjustable center frequency, variable polychromatic quantity, controllable frequency and time spacing of polychromatic terahertz radiation by adjusting beat laser accumulation and modulation of the electron beam, and has simple layout, strong tunability and flexibility. The beat accumulation laser obtained by sub-laser accumulation has multiple different terahertz frequencies on the envelope, and each terahertz frequency can be continuously adjusted by the relative time delay between the sub-lasers. This beat accumulation laser modulates the energy of the electron beam in the modulation section undulator and converts the energy modulation into density modulation after the first magnetic compressor, so that the electron beam forms a micro-cluster structure with multiple different terahertz frequencies in the longitudinal direction, and the electron beam becomes an electron bunch string. Subsequently, based on the principle of high-gain free electron laser radiation, the electron bunch string can generate polychromatic terahertz radiation with a continuously tunable center frequency from 0.1 to 30 terahertz and a peak power of hundreds of megawatts in the radiation section undulator.

[0020] In addition, a light splitting system using a light splitting plate and a movable mirror is arranged on the sub-laser light path, which can further increase the number of polychromatic colors and control the polychromatic spacing. The number and spacing of the polychromatic colors in the radiation spectrum can be precisely controlled by the number of sub-laser accumulation and the relative time delay. The time spacing of the radiation pulse can also be precisely controlled by separating the electron bunch string from the radiation field through the second magnetic compressor.

[0021] Therefore, the high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking of the present invention can generate high-power multi-color terahertz radiation with a continuously adjustable center frequency between 0.1 and 30 terahertz, a variable number of multi-colors, and controllable frequency and time intervals. It can be used to carry out a series of advanced scientific experiments that require the use of high-power multi-color terahertz radiation, such as atomic multi-energy level excitation, molecular fingerprint recognition, and terahertz pump-terahertz detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a structural schematic diagram of a high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking of the present invention.

[0023] Figure 2 It is a schematic diagram of the pulse stretching system in the present invention.

[0024] Figure 3 It is a schematic diagram of the laser deposition system in the present invention.

[0025] Figure 4 It is a schematic diagram of the electron beam modulation system in the present invention.

[0026] Figure 5 It is a schematic diagram of the radiation system in the present invention.

[0027] Figure 6A This is a distribution diagram of sub-laser intensities and time intervals required for amplifying three-color terahertz radiation when the high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking of the present invention uses four sub-lasers for stacking.

[0028] Figure 6B This is a distribution diagram of sub-laser intensities and time intervals required for amplifying two-color terahertz radiation when the high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking of the present invention uses four sub-lasers for stacking.

[0029] Figure 7A and Figure 7B This is a numerical simulation result diagram of the three-color terahertz radiation generated by the high-power multi-color terahertz radiation generating device based on beat frequency laser stacking of the present invention through the radiation segment undulator when four beams of sub-lasers are stacked. The radiation frequency spacing can be precisely controlled by adjusting the relative time delay of the sub-lasers.

[0030] Figure 7C and Figure 7D This is a numerical simulation result diagram of the six-color terahertz radiation generated by the high-power multi-color terahertz radiation generating device based on beat frequency laser stacking of the present invention through the radiation segment undulator after increasing the number of sub-laser stacking. The radiation frequency spacing is also adjustable.

[0031] Figure 8A and Figure 8BThis is a numerical simulation result diagram of the two-color terahertz radiation generated by the first radiation segment undulator and the second radiation segment undulator of the high-power multi-color terahertz radiation generating device based on beat frequency laser stacking of the present invention when the first radiation segment undulator and the second radiation segment undulator resonate at different wavelengths respectively. DETAILED DESCRIPTION

[0032] The present invention will be further described below with reference to the embodiments and drawings, but the scope of protection of the present invention should not be limited thereto.

[0033] Figure 1 1 is a connection diagram of a high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking according to an embodiment of the present invention. Figure 2 yes Figure 1 The pulse stretching system diagram is shown in Figure 1. Figure 3 yes Figure 1 The schematic diagram of the laser deposition system is shown in Figure 1. Figure 4 yes Figure 1 The schematic diagram of the electron beam modulation system is shown in FIG. Figure 5 yes Figure 1 Schematic diagram of the radiation system shown. Figure 6A and Figure 6B This is a numerically simulated intensity distribution diagram of the high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking of the present invention when four beams of sub-lasers are stacked. Figures 7A-7D This is a numerical simulation result diagram of the multi-color terahertz radiation generated by the high-power multi-color terahertz radiation generating device based on beat frequency laser stacking of the present invention through the radiation segment undulator when multiple beams of sub-lasers are stacked. The radiation frequency spacing can be precisely controlled by adjusting the relative time delay of the sub-lasers. Figure 8A and Figure 8B This figure shows the numerical simulation results of the two-color terahertz radiation generated by the high-power multicolor terahertz radiation generator based on beat-frequency laser stacking, when the first and second radiation-segment undulators resonate at different wavelengths. This allows for a larger radiation frequency spacing, and the radiation pulse temporal spacing can be precisely controlled via a second magnetic compressor. The high-power multicolor terahertz radiation generator of the present invention is based on beat-frequency laser stacking technology.

[0034] like Figure 1As shown, the high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking of the present invention includes an injector 1, an electron beam modulation system 2, and a radiation system 3, which are arranged in sequence along the transmission axis of the electron beam and located at the same horizontal plane; the laser pulses of the injector 1 and the electron beam modulation system 2 are provided by a femtosecond laser system, and the femtosecond laser system includes a femtosecond laser 4, a first beam splitter 5, and a first reflector 6, which are arranged in sequence along the optical path and parallel to the transmission axis of the electron beam. The first beam splitter 5 is configured to split the femtosecond Gaussian laser pulse from the femtosecond laser 4 into a first femtosecond Gaussian laser 41 and a second femtosecond Gaussian laser 42. The first beam splitter 5, the pulse stretching system 7, and the injector 1 are arranged in sequence along the optical path of the first femtosecond Gaussian laser 41, and the first reflector 6, the laser stacking system 8, and the electron beam modulation system 2 are arranged in sequence along the optical path of the second femtosecond Gaussian laser 42. In the present invention, the configuration of the pulse stretching system 7 and the femtosecond laser 4 is consistent with the prior art, the difference being the newly added laser stacking system 8 for generating beat-stacked laser 45 with multiple terahertz cycles.

[0035] like Figure 1 As shown, the pulse stretching system 7 generates a pulse-stretched laser 43 upon receiving the first femtosecond Gaussian laser beam 41 reflected by the first beam splitter 5. The injector 1 generates an electron beam upon receiving the pulse-stretched laser beam 43. The laser stacking system 8 generates a beat-stacked laser beam 45 with multiple terahertz periods upon receiving the second femtosecond Gaussian laser beam 42 reflected by the first reflector 6. The electron beam modulation system 2 uses the beat-stacked laser beam 45 to modulate the energy and density of the electron beam, generating an electron bunch train with multiple terahertz periods. The radiation system 3 uses the electron bunch train to generate high-power multi-color terahertz radiation. High power refers to peak powers of hundreds of megawatts. The center frequency of the multi-color terahertz radiation can be continuously tuned from 0.1 to 30 terahertz.

[0036] Specifically, the femtosecond laser 4 is configured to generate Gaussian laser pulses with a half-width of 30 to 200 femtoseconds and a central wavelength of 800 nanometers, and the energy of the Gaussian laser pulses is It can be expressed as:

[0037] ,

[0038] in, is the amplitude of the laser, is the vertical time coordinate, is the initial half-maximum width of the laser pulse, is the central angular frequency of the laser.

[0039] The first beam splitter 5 splits the femtosecond Gaussian laser pulse into a first path of femtosecond Gaussian laser light 41 and a second path of femtosecond Gaussian laser light 42 .

[0040] The pulse stretching system 7 is used to stretch the first femtosecond Gaussian laser 41 to produce a pulse-stretched laser 43. In this embodiment, the pulse width of the stretched pulse-stretched laser 43 reaches the ps level, resulting in an electron beam with a length of the ps level. The driving laser has the same time scale.

[0041] like Figure 2 As shown, the pulse stretching system 7 includes a first grating pair 71 serving as a dispersive element, a second reflector 72 and a third reflector 73 for reflecting the pulse-stretched laser light 43 from the first grating pair 71 back to the first grating pair 71 to eliminate spatial dispersion, and a fourth reflector 74. The pulse stretching system 7 is configured to output the pulse-stretched laser light 43 after eliminating spatial dispersion. The fourth reflector 74 reflects the pulse-stretched laser light 43 back to the injector 1 to generate an electron beam. Therefore, the first grating pair 71, the second reflector 72, the third reflector 73, the first grating pair 71, and the fourth reflector 74 are arranged sequentially along the optical path of the first femtosecond Gaussian laser light 41.

[0042] In some embodiments with different layouts, the fourth reflector 74 may be omitted.

[0043] The first grating pair 71 introduces dispersion to broaden the first path femtosecond Gaussian laser 41, and its dispersion effect can be expressed as:

[0044] ,

[0045] in, express The phase constant at yes The group delay at It is second-order dispersion or group delay dispersion.

[0046] The energy of the pulse-stretched laser 43 with pulse width stretched to picoseconds It can be expressed as:

[0047] ,

[0048] in, is the amplitude of the laser, is the vertical time coordinate, is the initial half-maximum width of the laser pulse, is the central angular frequency of the laser, is the pulse width after stretching, is the linear chirp parameter.

[0049] Pulse width after stretching , linear chirp parameter .

[0050] The pulse stretching system 7 can adjust the stretching degree of the first femtosecond Gaussian laser 41. Compared with the use of dispersion elements such as prisms, the use of the first grating pair 71 can adjust the stretching degree of the laser. The stretching degree of the laser is mainly adjusted by adjusting the parameters such as the grating constant, the grating spacing, and the laser incident angle. Among them, the main factors affecting the stretching are the second-order dispersion or group delay dispersion in the dispersion effect. ,in is the grating pitch, is the speed of light in vacuum, is the central angular frequency of the laser, is the grating constant, is the laser incident angle.

[0051] In other embodiments, the pulse stretching system 7 may replace the grating pair in the dispersion element with other types of dispersion elements, such as prisms, optical fibers, etc., but the grating pair is more common and can control the stretching of the laser.

[0052] The injector 1 is a photocathode injector, comprising a photocathode electron gun and a downstream S-band or X-band accelerating structure. The photocathode of the photocathode electron gun receives the pulse-stretching laser 43, thereby generating a high-quality electron beam with a duration on the order of picoseconds. The S-band or X-band accelerating structure is configured to accelerate the electron beam such that the electron beam energy at the injector 1 outlet is adjustable between 100 and 150 MeV.

[0053] like Figure 3 As shown, the laser stacking system 8 includes a second grating pair 81, a fifth reflector 82 and a sixth reflector 83 located on the other side of the second grating pair 81 for receiving the second path of femtosecond Gaussian laser 42, an optical isolation component (including a seventh reflector 84, an optical isolator 85, and an eighth reflector 86) located on the same side of the second grating pair 81 for receiving the second path of femtosecond Gaussian laser 42, a spectroscopic system for splitting light to obtain multiple paths of laser light, and a movable reflector located on each path of laser light.

[0054] The optical splitting system includes a second beam splitter 87 for splitting light into a first optical path and a second optical path, a third beam splitter 88 on the first optical path, and a fourth beam splitter 89 on the second optical path. The movable reflectors include a first movable reflector 881 on the first laser path and a second movable reflector 882 on the second laser path, located downstream of the third beam splitter 88, as well as a third movable reflector 891 on the third laser path and a fourth movable reflector 892 on the fourth laser path, located downstream of the fourth beam splitter 89.

[0055] As a result, the second femtosecond Gaussian laser 42 is stretched by the second grating pair 81 to produce a pulse-stretched laser 44. The fifth and sixth reflectors 82 and 83 reflect the pulse-stretched laser 44 back to the second grating pair 81 to eliminate spatial dispersion. The optical isolation assembly transmits the pulse-stretched laser 44 to the second beam splitter 87 and blocks the reflected beat-frequency pile-up laser 45. In some embodiments, by changing the spatial arrangement, the seventh and eighth reflectors 84 and 86 of the optical isolation assembly can be omitted, and the optical isolation assembly can simply include an optical isolator.

[0056] In this embodiment, the second beam splitter 87, the third beam splitter 88, and the fourth beam splitter 89 split the pulse-stretched laser 44 into a first sub-laser 441, a second sub-laser 442, a third sub-laser 443, and a fourth sub-laser 444. The four sub-lasers meet Maintaining equal intensity in each laser beam is beneficial to structural stability. Otherwise, varying intensities will occur across the entire laser pulse width, leading to an unbalanced microclustering structure of the electron beam. Although multi-color terahertz radiation can still be emitted, its radiation intensity will ultimately be weakened.

[0057] The movable mirror can be moved with an accuracy of micrometers, thereby adjusting the relative time delay between multiple lasers, and finally forming a beat frequency stacked laser 45 with multiple different terahertz frequencies on the envelope. It can be expressed as:

[0058] ,

[0059] in, for 、 The time delay between the two laser beams, is the linear chirp parameter.

[0060] Each terahertz frequency can be continuously adjusted by the relative time delay between sub-lasers. Beam lasers can be composed different envelope frequencies. However, only The envelope frequency can become the main frequency component.

[0061] like Figure 6AAs shown, when four sub-laser beams are stacked, the peak times of the first sub-laser 441, the second sub-laser 442, the third sub-laser 443, and the fourth sub-laser 444 are arranged in sequence. Therefore, only the envelope frequencies formed by the first sub-laser 441 and the second sub-laser 442, the second sub-laser 442 and the third sub-laser 443, and the third sub-laser 443 and the fourth sub-laser 444 dominate. The envelope frequencies of other combinations exist only at harmonics of the dominant frequency due to the large relative time delay between the sub-lasers. In other words, only the envelope frequency formed by two sub-laser beams with similar time delays dominates, while the envelope frequency formed by two sub-laser beams with significantly different time delays will have very low frequency components. Moreover, at the harmonic frequencies of the dominant frequency, in the case of fundamental radiation, the harmonic frequencies exceed the gain bandwidth of the radiation-band undulator and are not amplified.

[0062] Furthermore, the optical path of the spectroscopic system can be supplemented with additional beam splitters to increase the total number of sub-laser paths, along with movable mirrors for each additional sub-laser path, to further increase the number of multi-colors and control the inter-color spacing. At the same time, maintaining equal laser intensity in each laser path promotes structural stability. Otherwise, varying intensities across the entire laser pulse width would result in an uneven microclustering structure of the electron beam. While this would still produce multi-color terahertz radiation, its intensity would ultimately be weakened. The intensity of each laser path can be adjusted by using glass slides and polarizers.

[0063] like Figure 4 As shown, the electron beam modulation system 2 includes a modulation section undulator 21 , a first magnetic compressor 22 , and a ninth reflector 23 , which are sequentially arranged along the common propagation direction of the electron beam and the beat frequency stacking laser 45 .

[0064] The modulation-segment undulator 21 is a planar undulator configured to receive the electron beam and the beat-stacked laser 45, allowing the electron beam and the beat-stacked laser 45 to interact with each other to produce energy modulation with multiple different terahertz periods. Specifically, the electron beam and the longitudinally stacked beat-stacked laser 45 are transmitted to the vacuum tube at the magnetic center of the modulation-segment undulator 21, where they undergo sinusoidal periodic motion. The transverse and longitudinal (temporal) positions of the beat-stacked laser 45 and the electron beam within the modulation-segment undulator 21 are synchronized to the greatest possible degree, so that the electron beam produces energy modulation consistent with the envelope frequency characteristics of the beat-stacked laser 45 within its longitudinal distribution.

[0065] In the present invention, the dimensionless energy deviation of the electron beam caused by the interaction between the beat frequency stacking laser 45 and the electron beam in the modulation section undulator 21 is It can be linearized as:

[0066] ,

[0067] wherein, is the dimensionless energy deviation of the electron beam itself, is the modulation amplitude of the laser, is the longitudinal time coordinate, is the central angular frequency of the laser, is the , time delay between the two sub-lasers, is the linear chirp parameter.

[0068] In the present embodiment, the modulation section undulator 21 is a planar undulator with a period of 5 cm. The size of the modulation section undulator 21 can be in the range of 0.5 m to 1 m.

[0069] The root mean square gain bandwidth of the modulation section undulator 21 can be expressed as:

[0070] ,

[0071] wherein, is the Pierce parameter of the free electron laser, is the wave vector of the undulator, is the longitudinal coordinate along the undulator.

[0072] Only the seed laser whose frequency is within the gain bandwidth 6 of the modulation section undulator 21 can be coupled to the electron beam and map its structure onto the electron beam. In the present embodiment, the envelope of the beat stack laser 45 has multiple different terahertz periodic structures, but the carrier wavelength is maintained at 800 nm (i.e. the 800 nm Gaussian laser pulse provided by the femtosecond laser 4). Therefore, this multi-color terahertz modulation is not limited to the gain bandwidth of the modulation section undulator 21. Thus, the present application can make the electron beam have such energy modulation with multiple different terahertz periodic structures by using the modulation section undulator 21 in combination with the beat stack laser 45 as the seed laser, only once modulation is needed.

[0073] The first magnetic compressor 22 converts the energy modulation of the electron beam into density modulation, so that the electron beam forms a micro-cluster structure with multiple different terahertz frequencies in the longitudinal direction, forming an electron beam bunch string with multiple terahertz periodic information. After passing through the first magnetic compressor 22, the longitudinal time (i.e. the periodic time along the longitudinal direction of the electron beam) of the density-modulated electron beam propagating can be expressed as:

[0074] ,

[0075] wherein, is the initial longitudinal time of the electron beam propagating along the electron beam, is the speed of light in vacuum, is the momentum compression factor, is the dimensionless energy deviation of the electron beam caused by the beat-wave packet laser and electron beam interaction, is the root-mean-square slice emittance of the electron beam, is the average energy of the electron beam.

[0076] Due to the modulation process, the electron beam adopts a relativistic electron beam, so the influence of space charge effect on the multi-color terahertz structure is small, which ensures the feasibility of generating and maintaining multiple different terahertz periodic structures in the electron beam bunch string.

[0077] The ninth mirror 23 is used to reflect the beat-wave packet laser 45 after interacting with the electron beam out of the electron beam modulation system 2, preventing the laser from being transmitted together with the electron beam bunch string to the radiation system 3, thereby affecting the electron beam bunch string to generate high-power multi-color terahertz radiation.

[0078] The radiation system 3 generates high-power multi-color terahertz radiation by using the electron beam bunch string. The radiation system 3 at least includes a first radiation section undulator 31. In this embodiment, as shown in Figure 5 The radiation system 3 includes the first radiation section undulator 31, the second magnetic compressor 32, the second radiation section undulator 33, the bending magnet 34, and the beam dump 35 arranged in sequence along the transmission direction of the electron beam bunch string.

[0079] The first radiation section undulator 31 and the second radiation section undulator 33 are both planar undulators with a period of 20 cm and a continuously adjustable magnetic gap of 50 to 160 mm, so that the electron beam bunch string performs a wiggling motion therein to generate high-power multi-color terahertz radiation. The natural line width of the first radiation section undulator 31 and the second radiation section undulator 33 can be represented as:

[0080] ,

[0081] wherein, is the radiation wavelength, is the harmonic number, is the number of undulator periods.

[0082] The frequency spacing between the multi-color terahertz radiations must be greater than the natural line width of the radiation section undulators of the radiation system 3 , otherwise, the longitudinal modes cannot be separated, resulting in mode fusion.

[0083] In addition, when the first radiation range undulator 31 and the second radiation range undulator 33 resonate at the same wavelength, the frequency spacing between the dominant polychromatic terahertz radiations must be smaller than the gain bandwidth 6 of the first radiation range undulator 31 and the second radiation range undulator 33. Otherwise, terahertz radiation that deviates too far from the resonant wavelength cannot be amplified.

[0084] The second magnetic compressor 32 time-delays the electron bunch string after being radiated by the first radiation segment undulator 31, so that the electron bunch string is separated from the radiation field, thereby achieving controllable time interval between the terahertz radiation generated by the first radiation segment undulator 31 and the terahertz radiation generated by the second radiation segment undulator 33. Specifically, the second magnetic compressor 32 only affects the electron beam and does not affect the radiation field. By adjusting the time delay length of the electron bunch string, the electron beam and the radiation field are separated. When the electron beam enters the second radiation segment undulator, a new radiation field is generated again, and the time interval is controllable. Finally, a time delay is formed between the first radiation field and the second radiation field. The two groups of terahertz radiation with controllable time interval can be used in a two-color terahertz radiation scheme, in particular, to realize terahertz pumping-terahertz detection experiments.

[0085] The bent iron 34 separates the electron bunch string radiated by the second radiation section undulator 33 from the radiation field, and the high-power multi-color terahertz radiation is transmitted to the end through the beam line for experimental use. The electron bunch string is guided into the beam trash can 35 through the bent iron 34.

[0086] The numerical simulation results of the multi-color terahertz radiation generated by the resonance of the first radiation section undulator 31 and the second radiation section undulator 33 at the same wavelength are as follows: Figure 7A-7B As shown in the figure, it can be seen that when four sub-lasers are stacked, three-color terahertz radiation can be generated, and the radiation frequency spacing can be precisely controlled by adjusting the relative time delay of the sub-lasers. After further increasing the number of sub-laser stacks, six-color terahertz radiation can be generated, and the radiation frequency spacing can also be adjusted, as shown in the figure. Figure 7C-7D As shown in Figure 3, the electron bunch can generate high-power multi-color terahertz radiation after passing through the radiation system 3. However, the closer the radiation frequency is to the gain bandwidth limit of the undulator, the more difficult it is to be amplified by the gain.

[0087] In other embodiments, in order to break through the limitation of the gain bandwidth of the undulator, the first radiation section undulator 31 and the second radiation section undulator 33 can also resonate at different wavelengths respectively, and the time interval of the terahertz radiation pulses can be controlled by the second magnetic compressor 32, so that the electron bunch train can generate two different frequencies of terahertz radiation with a greater frequency difference. Figure 6B As shown, compared to Figure 6A ,exist Figure 6BIn the process, the time interval between the second sub-laser and the third sub-laser is widened by controlling the movable reflector, resulting in only the frequencies one and two, and three and four being dominant. Since the frequency difference between the second sub-laser and the third sub-laser is much larger than the gain bandwidth of the radiation segment undulator, when the first radiation segment undulator 31 and the second radiation segment undulator 33 resonate respectively, only their respective resonant frequencies are amplified, while the other frequency is not amplified, thus achieving high-power two-color terahertz radiation with a frequency difference much larger than the gain bandwidth of the undulator. Figure 6B As shown, in the stacking of four sub-laser beams, only the envelope frequency composed of the first sub-laser 441, the second sub-laser 442, the third sub-laser 443, and the fourth sub-laser 444 is dominant, while the envelope frequencies of other combinations exist only at the harmonics of the dominant frequency due to the large relative time delay between the sub-lasers. Figure 6A It is used to describe the sub-laser intensity and time spacing distribution required for the amplification of three-color terahertz radiation. Figure 6B It is used to describe the sub-laser intensity and time spacing distribution required for the amplification of two-color terahertz radiation.

[0088] like Figure 8A-8B As shown, the electron bunch string carrying the micro-cluster structure of two different terahertz frequencies first generates and amplifies the high-frequency terahertz radiation separately through the first radiation segment undulator 31, and then the second magnetic compressor 32 also delays the electron bunch string after the first radiation, so that the electron bunch string is separated from the radiation field, thereby achieving controllable time spacing of high-power multi-color terahertz radiation pulses. The electron bunch string that radiates high-frequency terahertz radiation still meets the conditions for amplifying low-frequency terahertz radiation. Finally, the electron bunch string generates and amplifies low-frequency terahertz radiation again separately in the second radiation segment undulator 33. Among them, Figure 8A is a spectrum diagram that describes the first frequency being amplified alone, Figure 8B It is a spectrum diagram that describes the second frequency being amplified separately, indicating that when the radiation field is amplified, the frequency beyond the gain bandwidth is not amplified by the undulator. Therefore, the high-power multi-color terahertz radiation generating device based on beat frequency laser stacking of the present invention can achieve a larger radiation frequency spacing when the first radiation segment undulator and the second radiation segment undulator resonate at different wavelengths respectively, and the radiation pulse time spacing can be precisely controlled by the second magnetic compressor.

[0089] This invention generates high-power, multi-color terahertz radiation on a single beam transmission line, meeting the requirements of advanced experiments such as high-precision spectral analysis, atomic multi-level excitation, and terahertz pump-and-detection, which require different terahertz radiation frequencies. Compared with other solutions, the device described in this invention offers advantages such as adjustable terahertz center frequency, variable number of multi-colors, controllable frequency and time spacing, high electron beam utilization, and high radiation brightness.

[0090] The high-power multi-color terahertz radiation generator based on beat-frequency laser stacking of the present invention achieves adjustable center frequency, variable number of multi-colors, and controllable frequency and time spacing of multi-color terahertz radiation by adjusting the beat-frequency laser stacking and electron beam modulation. It features a simple layout, strong tunability, and flexibility. Sub-laser stacking produces a beat-frequency stacking laser with multiple different terahertz frequencies on its envelope, and each terahertz frequency can be continuously adjusted by the relative time delay between the sub-lasers. This beat-frequency stacking laser modulates the energy of the electron beam in a modulation-segment undulator and, after passing through a first magnetic compressor, converts the energy modulation into density modulation, thereby forming a microcluster structure with multiple different terahertz frequencies in the longitudinal direction of the electron beam, transforming the electron beam into an electron bunch. Subsequently, based on the principle of high-gain free-electron laser radiation, the electron bunch can generate multi-color terahertz radiation with a center frequency continuously tunable from 0.1 to 30 terahertz and a peak power of hundreds of megawatts through fundamental wave radiation in the radiation-segment undulator.

[0091] Furthermore, a spectroscopic system employing beam splitters and movable mirrors within the sub-laser optical paths can further increase the number of colors and control their spacing. The number and spacing of the colors in the radiation spectrum can be precisely controlled by the number of sub-lasers stacked and their relative time delays. The temporal spacing of the radiation pulses can also be precisely controlled by using a second magnetic compressor to separate the electron bunch train from the radiation field.

[0092] Therefore, the high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking of the present invention can generate high-power multi-color terahertz radiation with a continuously adjustable center frequency between 0.1 and 30 terahertz, a variable number of multi-colors, and controllable frequency and time intervals. It can be used to carry out a series of advanced scientific experiments that require the use of high-power multi-color terahertz radiation, such as atomic multi-energy level excitation, molecular fingerprint recognition, and terahertz pump-terahertz detection.

[0093] Based on the high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking, a high-power multi-color terahertz radiation generating method based on beat-frequency laser stacking is implemented, which includes the following steps:

[0094] Step S1: Provide the high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking as described above, use the femtosecond laser 4 to generate Gaussian laser pulses with an adjustable half-width of 30 to 200 femtoseconds, and use the first beam splitter 5 to split the femtosecond laser pulses into a first femtosecond Gaussian laser 41 and a second femtosecond Gaussian laser 42.

[0095] Step S1 further includes: after receiving the first femtosecond Gaussian laser 41, the pulse stretching system 7 stretches it into a picosecond pulse-stretched laser 43. The injector 1 uses the pulse-stretched laser 43 to generate an electron beam with a picosecond duration in the photocathode electron gun through the photoelectric effect. This electron beam is then immediately accelerated to relativistic energy by the accelerating structure. By varying the accelerating field voltage and phase, the energy of the electron beam can be adjusted between 100 MeV and 150 MeV.

[0096] Step S2 : The second femtosecond Gaussian laser 42 passes through the first reflecting mirror 6 and then generates a beat frequency deposition laser 45 through the laser deposition system 8 .

[0097] The movable mirror in the laser stacking system 8 can be moved with micron-level precision, thereby adjusting the relative time delay between the sub-lasers. This ultimately creates a beat-frequency stacked laser with multiple different terahertz frequencies on the envelope, each of which can be continuously adjusted by the relative time delay between the sub-lasers. Furthermore, adding a beamsplitter and movable mirror to the sub-laser optical paths can further increase the number of colors and control the spacing between them.

[0098] Step S3: Injecting the longitudinally stacked beat-pile laser light 45 into the electron beam modulation system 2. In the planar modulation undulator 21, the electron beam interacts with the beat-pile laser light 45 to produce energy modulation in the electron beam's longitudinal distribution consistent with the envelope frequency characteristics of the beat-pile laser light 45. The ninth reflector 23 reflects the beat-pile laser light 45 out of the electron beam modulation system 2 after interaction with the electron beam, preventing the laser light from being transmitted to the radiation system 3 along with the electron bunch, thereby affecting the generation of high-power multi-color terahertz radiation by the electron bunch.

[0099] The step S3 also includes: converting the energy modulation of the electron beam into density modulation by the first magnetic compressor 22 in the electron beam modulation system 2, so that the electron beam forms a micro-cluster structure with multiple different terahertz frequencies in the longitudinal direction, and the electron beam becomes an electron beam cluster.

[0100] Step S4: The electron bunch train passes through the radiation system 3. When the first radiation segment undulator 31 and the second radiation segment undulator 33 resonate at the same wavelength, high-power multi-color terahertz radiation can be directly generated. When the first radiation segment undulator 31 and the second radiation segment undulator 33 resonate at different wavelengths, the electron bunch train can generate two different colors of high-power terahertz radiation with a greater frequency difference. The second magnetic compressor 32 is used to achieve controllable temporal spacing of the terahertz radiation pulses. Ultimately, the center frequency of the multi-color terahertz radiation is adjustable, the number of multi-colors is variable, and the frequency and temporal spacing are controllable. The layout is simple, the tunability is strong, and it is relatively flexible.

[0101] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the present invention. Various modifications are possible. Any simple, equivalent changes and modifications made in accordance with the claims and description of the present invention are within the scope of protection of the patent claims. Anything not fully described in this invention is conventional technology.

Claims

1. A high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking, characterized in that: The invention relates to an injector (1), an electron beam modulation system (2), and a radiation system (3) which are sequentially arranged along the transmission axis of the electron beam and located at the same horizontal plane; and a femtosecond laser system, which comprises a femtosecond laser (4), a first beam splitter (5), and a first reflector (6) which are sequentially arranged along the optical path and parallel to the transmission axis of the electron beam, wherein the first beam splitter (5) is configured to split the femtosecond Gaussian laser pulse from the femtosecond laser (4) into a first femtosecond Gaussian laser pulse (41) and a second femtosecond Gaussian laser pulse ( 42), the first beam splitter (5), the pulse stretching system (7), and the injector (1) are arranged in sequence along the optical path of the first femtosecond Gaussian laser (41), and the first reflector (6), the laser stacking system (8), and the electron beam modulation system (2) are arranged in sequence along the optical path of the second femtosecond Gaussian laser (42); the laser stacking system (8) generates a beat frequency stacking laser (45) having multiple terahertz cycles after receiving the second femtosecond Gaussian laser (42) reflected by the first reflector (6); The pulse stretching system (7) generates a pulse stretching laser (43) after receiving the first femtosecond Gaussian laser (41) reflected by the first beam splitter (5); the injector (1) generates an electron beam when receiving the pulse stretching laser (43); the electron beam modulation system (2) uses the beat frequency stacking laser (45) to achieve energy modulation and density modulation of the electron beam, thereby obtaining an electron bunch train with multiple terahertz periods; and the radiation system (3) uses the electron bunch train to generate high-power multi-color terahertz radiation.

2. The high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking according to claim 1, characterized in that: The femtosecond laser (4) is configured to generate Gaussian laser pulses with an adjustable half-width of 30 to 200 femtoseconds; the pulse stretching system (7) receives the femtosecond Gaussian laser pulses and stretches their pulse width to the picosecond level; the injector (1) is a photocathode injector, comprising a photocathode electron gun and a downstream S-band or X-band acceleration structure, and the energy of the electron beam at the outlet of the injector (1) is adjustable between 100 and 150 MeV.

3. The high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking according to claim 1, characterized in that: The pulse stretching system (7) includes a first grating pair (71) as a dispersion element, a second reflector (72) and a third reflector (73) for reflecting the pulse stretching laser (43) from the first grating pair (71) back to the first grating pair (71) to eliminate spatial dispersion. The pulse stretching system (7) is used to output the pulse stretching laser (43) after eliminating spatial dispersion.

4. The high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking according to claim 3, characterized in that: The pulse stretching system (7) further includes a fourth reflector (74), which reflects the pulse stretching laser (43) to the injector (1) to generate an electron beam with a picosecond time length.

5. The high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking according to claim 1, characterized in that: The laser stacking system (8) includes a second grating pair (81), a fifth reflector (82) and a sixth reflector (83) located on the other side of the second grating pair (81) that receives the second femtosecond Gaussian laser (42), an optical isolation component located on the same side of the second grating pair (81) that receives the second femtosecond Gaussian laser (42), a splitting system for splitting light to obtain multiple laser paths, and a movable reflector located on each laser path.

6. The high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking according to claim 5, characterized in that: The optical isolation component comprises at least an optical isolator (85); and / or Each laser beam maintains equal intensity; and / or The movable reflector moves with a precision of micrometer level, thereby adjusting the relative time delay between the multiple laser beams, and finally forming a beat frequency stacked laser (45) with multiple different terahertz frequencies on the envelope.

7. The high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking according to claim 1, characterized in that: The electron beam modulation system (2) comprises a modulation section undulator (21), a first magnetic compressor (22), and a ninth reflector (23) arranged in sequence along the common propagation direction of the electron beam and the beat frequency stacking laser (45); The modulation section undulator (21) is a planar undulator, configured to receive the electron beam and the beat frequency stacking laser (45), so that the electron beam interacts with the beat frequency stacking laser to generate energy modulation with multiple different terahertz periods in the electron beam; The first magnetic compressor (22) converts the energy modulation of the electron beam into density modulation, so that the electron beam forms a micro-cluster structure with multiple different terahertz frequencies in the longitudinal direction, forming an electron beam cluster with multiple terahertz period information; The ninth reflecting mirror (23) reflects the beat frequency accumulation laser light after interacting with the electron beam and leads it out of the electron beam modulation system (2).

8. The high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking according to claim 1, characterized in that: The radiation system (3) comprises a first radiation section undulator (31), a second magnetic compressor (32), a second radiation section undulator (33), a bent iron (34), and a beam trash can (35) arranged in sequence along the transmission direction of the electron bunch; The first radiation section undulator (31) and the second radiation section undulator (33) are both planar undulators with a period of 20 cm and a magnetic gap continuously adjustable from 50 to 160 mm; The second magnetic compressor (32) performs a time delay on the electron bunch string radiated by the first radiation section undulator (31), so that the electron bunch string is separated from the radiation field, thereby achieving controllable time interval between the terahertz radiation generated by the first radiation section undulator (31) and the terahertz radiation generated by the second radiation section undulator (33); The bent iron (34) separates the electron bunch string radiated by the second radiation section undulator (33) from the radiation field, and the electron bunch string is introduced into the beam waste bin (35) through the bent iron (34).

9. The high-power multi-color terahertz radiation generating device based on beat-frequency laser stacking according to claim 8, characterized in that: The first radiation segment undulator (31) and the second radiation segment undulator (33) resonate at the same wavelength, so that the electron bunch train directly generates multi-color terahertz radiation; or, the first radiation segment undulator (31) and the second radiation segment undulator (33) resonate at different wavelengths respectively, and the time interval of the terahertz radiation pulse is controllable through the second magnetic compressor (32), so that the electron bunch train generates two different frequencies of terahertz radiation with a greater frequency difference.

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