A free electron laser system based on planar metallic gratings

Abstract

The application discloses a free electron laser system based on a plane metal grating, which is composed of an electron gun, a metal grating and a collector.The system generates periodic electron bunch strings through the electron gun, and when the electron bunch strings pass over the metal grating in parallel, the resonant mode of the metal grating is excited.Because the electron bunch strings are periodic, when the frequency of the electron bunch strings is consistent with the frequency of the resonant mode, the energy of the resonant mode is outputted in the form of Smith-Purcell super radiation, and the intensity of the coherent radiation will be much greater than the intensity of the ordinary spontaneous radiation generated by the continuous electron beam.In addition, when the resonant mode is near the continuous domain bound state, the Smith-Purcell super radiation will be further enhanced.The application utilizes the periodic electron bunch strings to excite the metal grating to generate coherent spontaneous radiation, and has the characteristics of low cost and strong practicability.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electromagnetic technology, and more particularly to a free electron laser system based on planar metal grating. BACKGROUND

[0002] THz wave is between microwave and infrared light wave, and is a transition zone from macroscopic electronics to microscopic photonics, with high penetration and high bandwidth, and its technical research has great application value for national economy and national security. Recently, with the high attention of the international community, THz technology has made important applications in the fields of electronic communication, biomedicine, material science, and astronomy, but compact THz radiation source has been a bottleneck problem restricting its development. It is of great importance to develop high-power, high-frequency, and integrated THz radiation sources by using new materials, new structures, and new processes, for promoting the development of THz science and technology and its application.

[0003] At present, the THz radiation sources reported internationally are mainly realized based on photonics or electronics. The THz waves generated by using photoconduction, photorectification, and photodifference frequency etc. photonics technology have good directivity and coherence, but have problems such as low energy conversion efficiency. The vacuum electronic device based on Smith-Purcell effect converts electron energy into radiation energy with vacuum as the transmission medium, has the advantages of high efficiency and room temperature operation, and is one of the main THz radiation sources. However, this effect usually has non-coherent characteristics, which limits the application of vacuum electronic devices. Therefore, it is necessary to realize the coherent and high-power vacuum electronic device for the practicalization of such radiation source. SUMMARY

[0004] Technical problem: The technical problem to be solved by the present application is to provide a free electron laser system based on planar metal grating. The system generates coherent spontaneous radiation through the interaction of periodic electron bunch string and metal grating, and can obtain higher radiation intensity and radiation efficiency. The system has simple structure, small volume, low processing difficulty, and is easy to realize high-density silicon substrate integration. The system uses metal grating to solve the problems of charge accumulation and heat dissipation of dielectric grating when interacting with electrons; and uses high-quality factor resonance near the continuous domain bound state to solve the problem of low radiation intensity of conventional Smith-Purcell.

[0005] Technical scheme: The free electron laser system based on planar metal grating of the present application comprises an electron gun, a planar metal grating and a collecting electrode arranged in sequence; the electron gun and the collecting electrode are arranged on the same horizontal plane, and the planar metal grating is arranged between the electron gun and the collecting electrode, and the height is slightly lower than the periodic electron bunch string emitted by the electron gun; the periodic electron bunch string passes above the planar metal grating and finally enters the collecting electrode.

[0006] The period electron bunch string is a period electron bunch string with a rectangular cross section, and the electron bunch string is sheet-shaped along the length direction, and the current density is in the order of 100 amperes per square meter.

[0007] The planar metal grating is a low-duty cycle metal grating with a duty cycle of 0.1 or less, and has good conductivity.

[0008] When the period electron bunch string sweeps above the planar metal grating, the resonant mode of the planar metal grating is excited, and the energy of the resonant mode is output in the form of Smith-Purcell super radiation, and high-intensity Smith-Purcell super radiation is simultaneously generated in the space above and below the planar metal grating.

[0009] The Smith-Purcell super radiation is coherent radiation, and the radiation frequency is the frequency of the resonant mode of the grating, which is determined by the structural parameters of the metal grating, that is, the relationship f>c / [2(1-ρ)P] is required to be met, wherein c is the speed of light in vacuum, P is the period of the planar metal grating, and ρ is the duty cycle.

[0010] The resonant mode is a resonant mode near the symmetrically protected continuous domain bound state, and the corresponding radiation direction is near the normal direction of the grating.

[0011] Beneficial effects: The application provides a free electron laser system based on a planar metal grating. The metal material is adopted to avoid the problems of loss and heat dissipation existing in the dielectric material. The low-duty cycle metal grating is adopted to effectively generate the TM1 waveguide mode in addition to the TEM mode, so that it is possible to generate the symmetrically protected continuous domain bound state in the excited resonant mode, and the radiation intensity is effectively enhanced. The electron gun adopts the form of a period electron bunch string, and the clustered electron bunch string enables the resonant mode energy to be output in the form of coherent radiation, and the radiation intensity is further enhanced. The application has the advantages of simple structure, low processing difficulty, easy realization of miniaturization and integration of the free electron laser system, and consideration of high power, and has important significance for the development of compact THz radiation sources. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 is a schematic diagram of a free electron laser system based on a planar metal grating of the application;

[0013] Figure 2 is a Smith-Purcell super radiation field distribution diagram under a black star resonant mode;

[0014] Figure 3 is a time domain diagram of the magnetic induction intensity y component of the free space above the center of the grating under the black star resonant mode;

[0015] Figure 4is the time-domain graph of the y-component of magnetic induction intensity of a resonant mode far from the continuous domain bound state;

[0016] Figure 5 is the time-domain graph of the y-component of magnetic induction intensity of a non-resonant mode;

[0017] Figure 6 is the f-k x0 graph of the -1 order reflection coefficient obtained by simulation.

[0018] There are: an electron gun 1, a collector 2, a periodic electron bunch string 3, and a planar metal grating 4. DETAILED DESCRIPTION

[0019] The free electron laser system based on a planar metal grating comprises an electron gun, a metal grating, and a collector arranged in sequence.

[0020] The electron gun and the collector are arranged on the same horizontal plane, and the metal grating is arranged between the electron gun and the collector and is slightly lower than the electron bunch string generated by the electron gun.

[0021] The electron gun periodically emits an electron bunch string, which passes above the metal grating and finally enters the collector. When the periodic electron bunch string passes above the metal grating in parallel, the resonant mode of the metal grating is excited. Since the electron bunch string is periodically clustered, coherent radiation dominates, which effectively enhances the radiation intensity. The energy of the resonant mode is output in the form of Smith-Purcell super radiation, and stronger energy than the ordinary spontaneous radiation generated by a continuous electron beam can be obtained. The frequency of the periodic electron bunch string is consistent with the resonant frequency, which depends on the structural parameters of the metal grating.

[0022] Further, the electron gun can generate a periodic electron bunch string with a rectangular cross section. The electron bunch string is in the shape of a sheet, and the required current density is small, and the electron energy can be set as required.

[0023] Further, the metal grating is a low-duty-cycle metal grating, and the duty cycle can be set as required, which aims to generate other waveguide modes in addition to the TEM mode. The metal grating has good conductivity, and the periodic rectangular metal structure is surrounded by a dielectric.

[0024] The metal grating, due to the characteristics of the good conductor metal material, has no electromagnetic field inside and the surface tangential electric field is zero, and the electromagnetic field caused by the surrounding medium is spatially distributed abruptly. By reasonably selecting the parameters of the metal grating, the medium region forms a waveguide structure between the two metal sheets. Since the TM1 waveguide mode is odd symmetric along the direction of electron motion, it is not matched with the external radiation mode perpendicular to the direction of electron motion, and the coupling is prohibited, forming a symmetric protection type continuous domain bound state, so that the metal grating generates a resonant mode with extremely high quality factor. At the same time, under this resonant mode, the conversion efficiency of electron energy to radiation energy is effectively improved.

[0025] Further, the metal grating is a rectangular periodic metal grating, and the electromagnetic field excited by the interaction between the periodic electron group string and the metal grating near the continuous bound state is composed of a series of different spatial harmonics of diffraction modes. The-1 order diffraction mode can be used as a propagation mode, and the energy of the resonant mode is output in the form of-1 order Smith-Purcell super radiation. The Smith-Purcell super radiation is coherent radiation, and the radiation frequency is the frequency of the resonant mode of the metal grating, which is determined by the structural parameters of the metal grating, that is, the relationship f > c / [2(1-ρ)P] needs to be satisfied, where c is the speed of light in vacuum, P is the period of the planar metal grating, and ρ is the duty cycle.

[0026] It is worth noting that the Smith-Purcell radiation relationship is λ = P(1 / β-cosθ) / (-n), where P is the period of the metal grating, and λ is the radiation wavelength. For the symmetric protection type continuous domain bound state, the-1 order radiation angle θ is 90 degrees. Therefore, the ratio of the electron speed v to the speed of light is β = P / λ. Since the electron speed is synchronized with the phase speed of the 0th order diffraction mode, the wave number k = ω / v = 2π / P. In addition, the reflection coefficient of the-1 order diffraction mode can be used to reflect the resonance strength of the metal grating. The two are positively correlated, and the higher the reflection coefficient, the stronger the corresponding resonant mode. The reflection coefficient of the-1 order diffraction mode can be obtained by using the field matching method based on the waveguide array theory.

[0027] The field matching method based on the waveguide array theory is a commonly used method for analyzing the diffraction characteristics of gratings. The system is divided into three regions, separated by the upper and lower surfaces of the metal grating. Write the mode expressions corresponding to the three regions, and match the analysis by the field continuity between the metal grating and the medium and the boundary conditions of the metal grating, to obtain the strict solution of the reflection coefficient of each order diffraction mode.

[0028] The application will be further described below in conjunction with the drawings and specific embodiments.

[0029] One embodiment of the application is a free electron laser system based on a planar metal grating, as shown in Figure 1As shown, it comprises an electron gun 1, a collector 2, a periodic electron bunch train 3, and a planar metallic grating 4.

[0030] The electron gun 1 and the collector 2 are arranged in pairs and located in the same horizontal plane. The planar metallic grating 4 is placed between the electron gun 1 and the collector 2. The upper surface of the planar metallic grating 4 is slightly lower than the periodic electron bunch train 3 generated by the electron gun 1.

[0031] The planar metallic grating 4 is a low-duty-cycle periodic metallic grating, and each periodic unit is located at the same height. The planar metallic grating 4 has good electrical conductivity.

[0032] The periodic electron bunch train 3 generated by the electron gun 1 has a cross-section in the shape of a rectangular sheet, in which the thickness in the z direction is much smaller than the width in the y direction. The movement direction of the periodic electron bunch train 3 is the x direction. The z direction is along the height direction of the planar metallic grating 4 and is perpendicular to the period. The y direction is the out-of-plane direction toward the in-plane direction.

[0033] The periodic electron bunch train 3 generated by the electron gun 1 passes through the upper side of the planar metallic grating 4 in parallel and interacts with it, thereby exciting the resonant mode of the planar metallic grating 4 near the symmetric protected continuous domain bound state. The resonant mode effectively enhances the radiation intensity. Since the electron bunch train is periodically clustered, the Smith-Purcell radiation behaves as coherent spontaneous radiation and is finally output in the form of super radiation.

[0034] In this embodiment, the periodic electron bunch train 3 is generated by the electron gun 1, the electron energy is 319.73 keV, the thickness of the electron bunch train in the z direction is 20 μm, and the current density is 100 × [1 + sin(2πft)] A / m2, in which the frequency f of the periodic electron bunch train 3 is the resonant mode frequency. The distance between the periodic electron bunch train 3 and the planar metallic grating 4 is 20 μm.

[0035] Figure 6 At the black star shown in the middle, the period of the planar metallic grating 4 is 600 μm, the grating duty cycle is 0.05, the grating thickness is 1284 μm, the relative dielectric constant of the surrounding medium is 1, the frequency of the grating resonant mode is 401.82 GHz, and the corresponding Smith-Purcell radiation direction is 88.6 degrees.

[0036] Figure 6 The f-k diagram of the -1 order reflection coefficient obtained by the field matching method based on the waveguide array theory is shown in the right. x0Simulation results. The reflection coefficient of the -1 order is positively correlated with the resonance mode intensity of the planar metallic grating 4. The black star is the resonance mode near the symmetrically protected continuum state. The straight line represents the electron beam with an electron energy of 319.73 keV. The black star is the intersection of the resonance mode of the planar metallic grating 4 and the electron beam. The horizontal axis k x0 represents the propagation constant of the resonance mode along the x direction at this time, which satisfies k x0 = 2.04π / P, where P is the period of the planar metallic grating 4; and the vertical axis f is the frequency of the resonance mode, which is determined by the structural parameters of the planar metallic grating 4, and can be obtained from the horizontal axis as f = k x0 βc / (2π) = 1.02βc / P, where β is the ratio of the electron velocity to the speed of light in vacuum c, which is determined by the electron energy.

[0037] It is worth noting that the group velocity of the resonance mode closer to the symmetrically protected continuum state is closer to 0, so that the radiation intensity of the resonance mode is stronger, and the radiation angle is closer to 90 degrees. Figure 2 The black star is the super-radiation field distribution diagram obtained by simulation, and the radiation angle is consistent with the theoretical calculation. Figure 3 Figure 4 Figure 5 are the magnetic induction intensity y component time domain diagrams of the free space above the grating center under the black star, the resonance mode (f = 401.47 GHz, k x0 = 2.08π / P) farther away from the continuum state, and the ordinary super-radiation (f = 391.82 GHz, k x0 = 2.04π / P) under the same grating structural parameters. By comparison, it can be seen that the super-radiation intensity under the resonance mode is much greater than that under the ordinary super-radiation; the closer to the continuum state, the stronger the super-radiation intensity.​​

Claims

1. A planar metallic grating-based free electron laser system, characterized by, It includes an electron gun (1), a planar metal grating (4), and a collector (2) arranged in sequence; the electron gun (1) and the collector (2) are placed on the same horizontal plane, and the planar metal grating (4) is placed between the electron gun (1) and the collector (2), with a height slightly lower than the periodic electron clusters (3) emitted by the electron gun (1); the periodic electron clusters (3) pass over the planar metal grating (4) and finally enter the collector (2); When the periodic electron cluster (3) passes over the planar metal grating (4), it excites the resonant mode of the planar metal grating (4). The energy of the resonant mode is output in the form of Smith-Purcell superradiation. High-intensity Smith-Purcell superradiation is generated simultaneously in the space above and below the planar metal grating (4). The Smith-Purcell superradiation is coherent radiation, and the radiation frequency is the frequency of the grating resonant mode. This frequency is determined by the structural parameters of the metal grating, i.e., it must satisfy the relationship f>c / [2(1-ρ)P], where c is the speed of light in vacuum, P is the period of the planar metal grating, and ρ is the duty cycle.

2. The planar metal grating-based free electron laser system of claim 1, wherein, The periodic electron string (3) has a rectangular cross-section and is a thin sheet along its length. The current density is on the order of hundreds of amperes per square meter.

3. The planar metal grating-based free electron laser system of claim 1, wherein, The planar metal grating (4) is a low duty cycle metal grating of less than 0.1 and has good electrical conductivity.

4. A free-electron laser system based on a planar metal grating according to claim 1, characterized in that, The resonant mode is a resonant mode near the bound state of the symmetric protected continuous domain, and its corresponding radiation direction is near the grating normal.

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