Harmonic radiation enhancement system driven by circularly polarized laser

Abstract

The utility model discloses a harmonic radiation enhancement system driven by circularly polarized laser, comprising a circularly polarized laser pulse generation chamber used for outputting circularly polarized light pulses; the light beam focusing device is used for focusing and outputting the circularly polarized light pulse; the capacitor plasma target is used for receiving the focused circularly polarized light pulse and generating a single attosecond pulse; the reflected laser focusing device is used for focusing and outputting the reflected laser pulse with the single attosecond pulse output by the capacitor plasma target; and the filtering device is used for filtering low-frequency stray light and outputting a single attosecond pulse. The laser driving device is applied to the field of ultrafast optics, can effectively improve the radiation efficiency of harmonic waves so as to obtain attosecond pulses with high brightness, is still applicable in a relatively wide parameter interval range, and is higher in radiation efficiency compared with an existing circular polarization laser driving scheme.

Description

Technical Field

[0001] The utility model relates to the field of ultrafast optics, and particularly to a harmonic radiation enhancement system driven by circularly polarized laser. Background Technique

[0002] In 1960, Maiman developed the world's first solid-state laser using ruby. Subsequently, the development of laser technologies such as Q-switching and mode-locking enabled the study of ultrafast dynamic processes to enter the femtosecond (1 fs = 10 -15 s) level. Humans were able to conduct scientific research and technological innovation at the atomic and molecular levels, strongly promoting the rapid development of fields such as physics, chemistry, biology, and medicine, and giving rise to new scientific frontiers. Femtochemistry is a typical example. Scientist Zewail successfully captured the process of molecular chemical bond breaking and formation using femtosecond lasers, fundamentally changing people's understanding of chemical reaction processes, creating the possibility of controllable chemical reactions, and triggering major revolutions in the entire field of chemistry and related disciplines. However, in the following 15 years, due to the fact that the duration of laser pulses was close to several femtosecond oscillation periods of their carrier waves, the resolution ability of ultrafast spectroscopy stagnated again. In 1985, the chirped pulse amplification technology (Chirped Pulse Amplification, abbreviated as CPA) proposed by Mourou and Strickland achieved a significant increase in the peak energy and peak power of femtosecond lasers. Subsequently, the titanium-sapphire lasers developed by Moulton et al. and the Kerr-lens mode locking technology (Kerr-lens mode locking, abbreviated as KLM) proposed by Sibbett et al. were also important utility models during the same period. These three utility models gave rise to the second-generation femtosecond technology based on solid-state laser media (mainly titanium-sapphire). Compared with the first-generation technology, the peak and average powers of femtosecond laser pulses can be increased by several orders of magnitude. After the emergence of CPA technology, laser technology has developed rapidly again. The proposal of optical parametric chirped pulse amplification technology (Optical Parametric Chirped Pulse Amplification, abbreviated as OPCPA) has further increased the laser intensity to 10 17 -10 18 W / cm 2 , at which time the interaction between laser and matter enters the relativistic regime. To date, the peak focused intensity of laser pulses achieved in the laboratory is as high as 10 23 W / cm 2 . Under such high-intensity laser irradiation, matter is instantly completely ionized into a plasma state. At this time, the interaction between laser and matter transforms into the interaction between strong relativistic laser and plasma. This interaction process provides a new way to obtain new radiation sources.

[0003] The rapid development of ultrafast laser technology has greatly improved the resolution of pump-probe techniques. Nowadays, laser pulses with femtosecond pulse widths are used as probes to diagnose ultrafast phenomena at the atomic and molecular levels. However, to further capture transient phenomena at the electronic level, explore microscopic processes within the atomic scale, and further enable microscopic control, laser pulses with higher time resolution are required - attosecond pulses (1 attosecond = 10 -18 seconds). Isolated attosecond pulses can be used for clear real-time imaging within the attosecond time scale and precise control of ultrafast phenomena. The generation and regulation of their characteristics such as amplitude, polarization, and wavelength have become a research hotspot at home and abroad and are also one of the key issues in attosecond science research.

[0004] Currently, the shortest pulse width of the experimentally reported isolated attosecond pulse is about 650 attoseconds, which is synthesized from high-order harmonics generated by the interaction of femtosecond laser pulses with noble gases. As an effective radiation source for synthesizing attosecond pulses, the spectrum of high-order harmonics can be broadened to the extreme ultraviolet to X-ray frequency bands and can be generated through the highly nonlinear interaction of intense lasers with matter. However, although isolated or even single attosecond pulses have been successfully generated based on high-order harmonics generated by noble gases, due to the limitation of the ionization threshold of the gas medium, the intensity of the driving laser cannot exceed 10 16 W / cm 2 , so the intensity of the attosecond pulses generated by this method will also be limited, and there are also problems such as difficult phase matching and low harmonic radiation efficiency (10 -9 ~10 -5 ). Compared with the gas medium, the plasma material, which is theoretically composed entirely of electrons and ions, can withstand relativistic intense laser irradiation. Therefore, using it as a medium can make full use of the ultra-high power density of the laser to greatly improve the harmonic radiation efficiency and thus increase the intensity of the attosecond pulses. Especially for dense plasmas, high-order harmonics are generated within the skin depth of the nanoscale and also have the advantage of self-phase locking.

[0005] The principle of relativistic Doppler frequency shift can be used to obtain ultrashort (attosecond scale) and ultra-intense (relativistic intensity) isolated or even single laser pulses. When a linearly polarized laser with relativistic intensity is incident perpendicularly on a plasma target, the target surface will oscillate under the action of the laser Lorentz force , where E Lis the instantaneous laser electric field acting on the plasma target surface, with an amplitude of E0, a time envelope of f(t), and an oscillation frequency of ω0. Whenever the plasma target surface moves towards the laser at nearly the speed of light, a relativistic Doppler frequency shift is introduced in the frequency domain of the reflected laser. The specific process is as follows: Due to the relativistic Doppler effect, the energy and wavelength of the laser are strongly compressed during the reflection from the target surface moving towards it. This results in the distortion of the waveform of the reflected laser, that is, high-frequency signals (higher harmonics) are introduced. After intercepting the required frequency band through a filter, attosecond pulses can be synthesized.

[0006] In the method of synthesizing attosecond pulses through relativistic Doppler frequency shift technology, the most crucial thing is that the target surface oscillates violently. And to generate such a rapid oscillation, there must be at least a rapid oscillation term of the order of the laser fundamental frequency in the Lorentz force exerted on the target surface by the driving laser. For linearly polarized laser, the laser Lorentz force v×B L ∝f 2 (t)(1 + cos(2ω0t)), where cos(2ω0t) is such a rapid oscillation term. This means that after the incident of this type of driving laser on the target surface, the target surface will oscillate at twice the frequency of the laser. For circularly polarized laser, the Lorentz force it provides to the target surface under normal incidence Obviously, compared with the linearly polarized laser, its Lorentz force lacks the previous rapid oscillation term cos(2ω0t). Therefore, under the drive of the circularly polarized laser incident normally, the movement trajectory of the target surface will be like the laser envelope, smoothly advancing into the target and then slowly being pulled out of the target, which results in the speed of the target surface movement not being fast enough, the Doppler effect being very weak, and thus suppressing the generation of higher harmonics. Due to this fundamental difficulty, for a long time, there has been little research on high-order harmonic radiation driven by circularly polarized laser.

[0007] This difficulty was overcome in 2011. Ji et al. found through Particle-in-Cell (PIC) simulation studies that single attosecond pulses can be generated by irradiating near-critical density plasmas with few-cycle circularly polarized lasers. They utilized the ponderomotive force of the incident laser during the rising edge stage to continuously accumulate a large amount of electrostatic energy into the target, and when the laser weakened to below the charge separation field, they began to violently compress the laser through the electron restoring force, thereby generating high-intensity isolated attosecond pulses. However, this scheme requires the plasma density to be near the critical density, which poses extremely harsh requirements for experimental conditions. In addition, the radiation efficiency of harmonics drops sharply with the increase in plasma density, and it is not a robust system. Therefore, researchers had to start from other characteristics of the plasma to find new solutions. In December 2020, Wang et al. found that when a few-cycle circularly polarized laser is normally incident on an ultrathin target, the ponderomotive force of the laser field is sufficient to push all the electrons on the target surface out of the ion layer, thereby generating a huge charge separation field and forming a dense and ultrathin electron layer. When the electron layer starts to rebound, it becomes a high-quality relativistic electron mirror, and this electron mirror propagates against the tail of the laser pulse. Thus, the laser can be effectively reflected on this electron mirror and compressed to radiate high-order harmonics. Although it will cause the tail electron layer to oscillate continuously for many times, since the amplitude of each oscillation is inconsistent, isolated or even single attosecond pulses can be obtained by appropriate filtering and adjusting parameter conditions. However, the harmonic radiation efficiency obtained by this method is not high (0.07%), so the intensity of the attosecond pulses obtained is relatively low.

[0008] In summary, although researchers have proposed many theoretical schemes for obtaining isolated or even single attosecond pulses based on the interaction between circularly polarized lasers and plasmas, there are still many problems that need to be solved. First, the intensity of the attosecond pulses obtained through the above theoretical schemes is relatively low; second, the previous scheme has too high requirements for experimental conditions and is difficult to implement. Therefore, designing a new type of circularly polarized laser-driven harmonic radiation enhancement system and deeply studying the generation of single attosecond pulses by the interaction between circularly polarized lasers and plasmas can not only provide theoretical support and technical reference for obtaining single attosecond pulses with high ellipticity and high energy conversion efficiency, but also have important significance and potential application value in research fields such as attosecond science, new X-ray radiation sources, and ultrafast science. Summary of the Invention

[0009] In the theoretical scheme of the interaction between circularly polarized laser and plasma in the above-mentioned existing technologies, the intensity of the attosecond pulses obtained is relatively low, and some of the schemes have strict requirements for experimental conditions. The present utility model provides a harmonic radiation enhancement system driven by circularly polarized laser. Using circularly polarized pulses as the driving laser and vertically irradiating a capacitor plasma target as the generating device, it can effectively generate single attosecond pulses with a harmonic radiation efficiency as high as 0.53%.

[0010] To achieve the above object, the present utility model provides a harmonic radiation enhancement system driven by circularly polarized laser, comprising:

[0011] A circularly polarized laser pulse generation chamber for outputting circularly polarized light pulses;

[0012] A beam focusing device located on the output optical path of the circularly polarized laser pulse generation chamber for focusing and outputting the circularly polarized light pulses;

[0013] A capacitor plasma target located on the output optical path of the beam focusing device for receiving the focused circularly polarized light pulses and generating single attosecond pulses;

[0014] A reflected laser focusing device located on the reflected optical path of the capacitor plasma target for focusing and outputting the reflected laser pulses with single attosecond pulses output by the capacitor plasma target;

[0015] A filtering device located on the output optical path of the reflected laser focusing device for filtering out low-frequency stray light and outputting single attosecond pulses.

[0016] In one embodiment, the capacitor plasma target includes a first plasma target and a second plasma target;

[0017] The first plasma target is located on the output optical path of the beam focusing device, and the first plasma target is perpendicular to the output optical path of the beam focusing device;

[0018] The second plasma target is located behind the first plasma target, and the first plasma target and the second plasma target are parallel to each other;

[0019] The first plasma target is thinner than the second plasma target.

[0020] In one embodiment, the wavelength of the circularly polarized light pulse is 0.8 μm, the duration is 8 fs, the peak intensity is 4.81×10 21 W / cm 2 , and the spot radius is 8 μm to 15 μm.

[0021] In one embodiment, the thickness of the first plasma target is 0.008 μm, and the thickness of the second plasma target is 0.4 μm.

[0022] In one embodiment, the distance between the first plasma target and the second plasma target is 0.24 μm.

[0023] In one embodiment, both the first plasma target and the second plasma target are made of carbon nanotube films.

[0024] In one embodiment, the densities of both the first plasma target and the second plasma target are 4.35×10 29 cm -3 .

[0025] In one embodiment, the filtering device is a tunable filter in the extreme ultraviolet band.

[0026] Compared with the prior art, the present utility model has the following beneficial technical effects:

[0027] The present utility model uses a capacitor plasma target as the target type. Instead of driving the high-frequency oscillation of the target with circularly polarized light, the front thin target is broken near the peak value of the circularly polarized light pulse. All the electrons in it are pushed out under the drive of the laser and obtain sufficient energy to enter the rear thick target. Then, by adjusting the target distance of the capacitor plasma target, when the electrons of the first plasma target just enter the second plasma target, the electrons of the second plasma target are pulled out by the strong electrostatic force between the two targets, forming a relativistic electron sheet that accelerates in the reflection direction. This electron sheet quickly compresses the falling edge of the driving light once and synchronously radiates a single attosecond pulse, which can effectively improve the radiation efficiency of harmonics, thereby increasing the intensity of the obtained attosecond pulse. Moreover, it is still applicable within a relatively wide parameter range and is also beneficial to the current experimental conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present utility model or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present utility model. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on the structures shown in these drawings.

[0029] Figure 1 Schematic diagram of the first implementation manner of the harmonic radiation enhancement system in the embodiment of the present utility model;

[0030] Figure 2 Schematic diagram of the second implementation manner of the harmonic radiation enhancement system in the embodiment of the present utility model;

[0031] Figure 3 This is a schematic diagram of the main physical process of the harmonic radiation enhancement system driven by circularly polarized laser in the embodiment of the present invention. Among them: (a) is a schematic diagram of the electron density of the first plasma target and the electron density of the second plasma target at the moment when the charging is completed; (b) is a schematic diagram of the electron density of the first plasma target and the electron density of the second plasma target at the moment when the discharging is completed; (c) is a schematic diagram of the longitudinal Lorentz factor at the moment when the charging is completed; (d) is a schematic diagram of the longitudinal Lorentz factor at the moment when the discharging is completed.

[0032] Figure 4 This is a schematic diagram of the relationship between the intensity and ellipticity of attosecond pulses and the distance between two targets of the capacitor plasma target in the embodiment of the present invention. Among them: (a) is a schematic diagram of the harmonic radiation efficiency and ellipticity of a single attosecond pulse and the radiation efficiency of the best plasma single target obtained when the distance between the two foils changes; (b) is a schematic diagram of the normalized intensity spectrum of the electric field component of the obtained attosecond pulse; (c) is a schematic diagram of the normalized intensity spectrum of the reflected laser obtained.

[0033] Figure 5 This is a schematic diagram of the relationship between the intensity of attosecond pulses and the pulse width of the driving circularly polarized laser in the embodiment of the present invention. Among them: (a) is an image of the change of the intensity of attosecond pulses obtained by driving the capacitor plasma target with circularly polarized light with a pulse width of 1λ with respect to the target distance; (b) is an image of the change of the intensity of attosecond pulses obtained by driving the capacitor plasma target with circularly polarized light with a pulse width of 2λ with respect to the target distance; (c) is an image of the change of the intensity of attosecond pulses obtained by driving the capacitor plasma target with circularly polarized light with a pulse width of 4λ with respect to the target distance.

[0034] Reference numerals in the drawings: circularly polarized laser pulse generation chamber 1, beam focusing device 2, capacitor plasma target 3, first plasma target 301, second plasma target 302, reflected laser focusing device 4, semi-transparent and semi-reflective mirror 401, focusing lens 402, filtering device 5.

[0035] The realization, functional features and advantages of the purpose of the present invention will be further described with reference to the embodiments and the accompanying drawings. Detailed implementation manners

[0036] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0037] It should be noted that all the directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present utility model are only used to explain the relative positional relationship and motion conditions between components in a certain specific posture (as shown in the attached drawings). If the specific posture changes, the directional indications will also change accordingly.

[0038] In addition, in the present utility model, the descriptions such as "first" and "second" are only for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include at least one such feature. In the description of the present utility model, the meaning of "a plurality" is at least two, such as two, three, etc., unless otherwise specifically defined.

[0039] In addition, the technical solutions between various embodiments of the present utility model can be combined with each other, but it must be based on the fact that those of ordinary skill in the art can implement it. When the combination of technical solutions results in contradictions or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the protection scope required by the present utility model.

[0040] Currently, in the theoretical schemes for generating isolated or even single attosecond pulses by the interaction between circularly polarized laser and plasma, the intensity of the obtained attosecond pulses is relatively low, and some schemes have strict requirements for experimental conditions. The main reason is that as the driving light, the Lorentz force exerted by the circularly polarized light on the target surface lacks high-frequency oscillation terms. The motion trajectory of the target surface will be like the laser envelope, smoothly advancing into the target and then slowly being pulled out of the target, which leads to the insufficiently fast speed of the target surface motion and the weak Doppler effect, thus suppressing the generation of high-order harmonics. Based on this, this embodiment discloses a harmonic radiation enhancement system driven by circularly polarized laser (hereinafter referred to as "harmonic radiation enhancement system").

[0041] Refer to Figure 1 , the harmonic radiation enhancement system in this embodiment includes a circularly polarized laser pulse generation chamber 1, a beam focusing device 2, a capacitor plasma target 3, a reflected laser focusing device 4, and a filtering device 5. Among them, the circularly polarized laser pulse generation chamber 1 is used to output circularly polarized light pulses; the beam focusing device 2 is located on the output optical path of the circularly polarized laser pulse generation chamber 1 and is used to focus the circularly polarized light pulses and output them; the capacitor plasma target 3 is located on the output optical path of the beam focusing device 2 and is used to receive the focused circularly polarized light pulses and generate single attosecond pulses; the reflected laser focusing device 4 is located on the reflected optical path of the capacitor plasma target 3 and is used to focus and output the reflected laser pulses with single attosecond pulses output by the capacitor plasma target 3; the filtering device 5 is located on the output optical path of the reflected laser focusing device 4 and is used to filter out low-frequency stray light and output single attosecond pulses.

[0042] In the specific implementation process, the wavelength of the circularly polarized light pulse is 0.8 μm, the duration is 8 fs, and the peak intensity is 4.81×10 21 W / cm 2 , and the spot radius is 8 μm to 15 μm.

[0043] In this embodiment, the capacitor plasma target 3 includes a first plasma target 301 and a second plasma target 302. The first plasma target 301 is located on the output optical path of the beam focusing device 2, and the first plasma target 301 is perpendicular to the output optical path of the beam focusing device 2. The second plasma target 302 is located behind the first plasma target 301, and the first plasma target 301 and the second plasma target 302 are parallel to each other, and the first plasma target 301 is thinner than the second plasma target 302. The circularly polarized light pulse focused by the beam focusing device 2 is vertically incident on the first plasma target 301. The first plasma target 301 is broken near the peak of the circularly polarized light pulse. All the electrons in it are pushed out under the drive of the laser and obtain enough energy to enter the second plasma target 302. Through the design of the target spacing between the first plasma target 301 and the second plasma target 302, when the electrons of the first plasma target 301 just enter the second plasma target 302, the electrons of the second plasma target 302 are pulled out by the strong electrostatic force between the two targets, forming a relativistic electron sheet that accelerates in the reflection direction. This electron sheet performs a single and rapid compression on the falling edge of the circularly polarized light pulse, thereby synchronously radiating a single attosecond pulse, which can effectively improve the radiation efficiency of harmonics, thereby increasing the intensity of the obtained attosecond pulse, and it is still applicable within a relatively wide parameter range, which is more conducive to the current experimental conditions.

[0044] In the specific implementation process, both the first plasma target 301 and the second plasma target 302 are made of carbon nanotube films, and the density of both the first plasma target 301 and the second plasma target 302 is 4.35×10 29 per cm 3 . Among them, the thickness of the first plasma target 301 is 0.008 μm, the thickness of the second plasma target 302 is 0.4 μm, and the distance between the first plasma target 301 and the second plasma target 302 is 0.24 μm.

[0045] In this embodiment, the normalized parameters of the harmonic radiation enhancement system are unified as follows: λ0 = 0.8 μm, T0 = λ0 / c, ω0 = 2πλ0 / c, E r = m e cω0 / q e , The normalized parameter of the force is (m e ω0c)-1 , where λ0 is the wavelength of the circularly polarized light pulse, T0 is the period of the circularly polarized light pulse, c is the speed of light in vacuum, ω0 is the frequency of the circularly polarized light pulse, E r is the normalized electric field strength, I r is the normalized optical field strength, n c is the critical plasma density, m e and -q e are the mass and charge of the elementary charge respectively, and ε0 is the permittivity of vacuum.

[0046] In this embodiment, the beam focusing device 2 is composed of multiple reflectors and / or off-axis paraboloids, so that the circularly polarized laser pulse output from the circularly polarized laser pulse generation chamber 1 is reflected and focused multiple times and then perpendicularly incident on the first plasma target 301, that is Figure 1 as shown. It should be noted that in the specific application process, the beam focusing device 2 can also be composed of multiple reflectors and focusing lenses.

[0047] In this embodiment, the reflected laser focusing device 4 can adopt a single off-axis paraboloid, and the filtering device 5 is located on the reflection focusing optical path of the single off-axis paraboloid, that is Figure 1 as shown. The focused circularly polarized laser pulse output by the beam focusing device 2 is perpendicularly incident on the capacitor plasma target 3, and a single attosecond pulse is excited on the capacitor plasma target 3. The reflected laser pulse with the single attosecond pulse shoots out in the opposite direction of the incident direction. Since the radius of the reflected laser pulse is much larger than that of the focused circularly polarized laser pulse output by the beam focusing device 2, the single off-axis paraboloid serving as the reflected laser focusing device 4 is arranged in the non-overlapping area between the reflected laser pulse and the focused circularly polarized laser pulse, so that the reflected laser pulse can be reflected and focused, and at the same time, it will not interfere with the output optical path of the beam focusing device 2.

[0048] It should be noted that in the specific application process, the reflected laser focusing device 4 is not limited to adopting Figure 1 the single off-axis paraboloid shown, and a reflected laser focusing device 4 can also be composed of a semi-transparent and semi-reflective mirror 401 and a focusing lens 402. Specifically, one side of the semi-transparent and semi-reflective mirror 401 is a transmission surface and the other side is a reflection surface. The transmission surface can transmit most of the light, and the reflection surface can reflect most of the light. The semi-transparent and semi-reflective mirror 401 is arranged on the output optical path of the beam focusing device 2, with the transmission surface facing the beam focusing device 2 and the reflection surface facing the capacitor plasma target 3, and the included angle between the semi-transparent and semi-reflective mirror 401 and the output optical path of the beam focusing device 2 is 90°. The focusing lens 402 is arranged on the reflection optical path of the semi-transparent and semi-reflective mirror 401, and the filtering device 5 is located on the focusing optical path of the focusing lens 402, that is Figure 2As shown in the figure. The focused circularly polarized laser pulse output by the beam focusing device 2 passes through the semi-transparent and semi-reflective mirror 401 and is vertically incident on the capacitor plasma target 3, where a single attosecond pulse is excited. The reflected laser pulse carrying the single attosecond pulse is emitted in the opposite direction of the incident direction, reflected by the semi-transparent and semi-reflective mirror 401 and then incident on the focusing lens 402, and then focused by the focusing lens 402 and injected into the filtering device 5.

[0049] In this embodiment, the filtering device 5 adopts a tunable filter in the extreme ultraviolet band, and the filtering range can be tuned to 10 - 100 nm.

[0050] The harmonic radiation enhancement system in this embodiment will be further described below in conjunction with specific simulation examples.

[0051] In this example, the subscripts 1 and 2 respectively represent parts of the first plasma target and the second plasma target in the capacitor plasma target. For example, d1 and d2 respectively represent the thicknesses of the first plasma target and the second plasma target, n1 and n2 respectively represent the electron densities of the first plasma target and the second plasma target, and L represents the distance between the two targets.

[0052] The harmonic radiation enhancement system in this embodiment can effectively generate a single attosecond pulse while ensuring a relatively high energy conversion efficiency (about 1%) through the interaction between circularly polarized laser and capacitor plasma target. The generation principle of the single attosecond pulse is as follows: Through theoretical modeling and numerical simulation calculations, first, the parameter matching conditions required for the capacitor plasma target to be charged under the drive of circularly polarized laser are obtained, so that the electrons in the first plasma target can be completely pushed out near the peak of the circularly polarized laser pulse and obtain enough energy to enter the second plasma target. Then, the target spacing of the capacitor plasma target is adjusted so that when the electrons in the first plasma target just enter the second plasma target, the electrons in the second plasma target are pulled out by the strong electrostatic force between the two targets. In this way, not only can the laser energy be utilized with the highest efficiency, but also it is beneficial to effectively and robustly generate and control a single attosecond pulse.

[0053] Reference Figure 3 is the main physical process of the harmonic radiation enhancement system driven by circularly polarized laser. Figure 3 (a), Figure 3 (b) are respectively the electron density n1 of the first plasma target and the electron density n2 of the second plasma target, the electrostatic field E x and the laser electric field E a distribution maps at the moments when charging and discharging are completed. Figure 3 (c), Figure 3 (d) are the longitudinal Lorentz factors at the corresponding moments, where β x= v x / c is the normalized velocity. From Figure 3 It can be seen that the physical process of the whole system is mainly divided into a charging process and a discharging process. When the circularly polarized laser pulse acts on the first plasma target, almost all the electrons are pushed out to form a relativistic flying mirror. After this electron sheet passes through the second plasma target, it diverges and disappears. This process is similar to the charging process of a capacitor, making the first layer of the target positively charged. When the laser pulse transmitted from the first plasma target acts on the second plasma target, through the combined action of the ponderomotive force and the Coulomb force, a relativistic electron sheet accelerating in the reflection direction can be formed from the second plasma target. When the electron sheet from the second plasma target reaches the first plasma target, it is equivalent to completing the discharging process.

[0054] Reference Figure 4 is a schematic diagram of the relationship between the intensity and ellipticity of the attosecond pulse and the distance between the two targets of the capacitor plasma target. Figure 4 (a) shows the harmonic radiation efficiency (CNTC) and ellipticity (CNTE) of the single attosecond pulse obtained and the radiation efficiency (SFTC) of the optimal single plasma target when the distance between the two foils changes; Figure 4 (b) and Figure 4 (c) represent the electric field component of the attosecond pulse and the normalized intensity spectrum of the reflected laser. The solid line and the dashed line respectively represent the capacitor plasma target (CNT) and the single plasma target (SFT); From Figure 4 It can be seen that, compared with the single plasma target in the case of the optimal radiation efficiency, a stronger and higher-ellipticity single attosecond pulse can be obtained by driving the plasma capacitor target with circularly polarized light.

[0055] Reference Figure 5 is a schematic diagram of the relationship between the intensity of the attosecond pulse and the pulse width of the driving circularly polarized laser. Figure 5 (a)- Figure 5 (c) give the images of the change of the attosecond pulse intensity with the target distance obtained by driving the capacitor plasma target with circularly polarized light with pulse widths of 1λ, 2λ, and 4λ respectively; From Figure 5 It can be seen that by changing the pulse width of the circularly polarized light, within a relatively wide parameter range, the harmonic radiation enhancement system in this embodiment can still generate high-intensity single attosecond pulses.

[0056] The above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention. Any equivalent structural transformation made under the inventive concept of the present invention by using the content of the specification and drawings of the present invention, or any direct / indirect application in other related technical fields is included in the patent protection scope of the present invention.

Claims

1. A circularly polarized laser driven harmonic radiation enhancement system, characterized in that: include: A circularly polarized laser pulse generating chamber, used for outputting circularly polarized light pulses; A beam focusing device, located on the output optical path of the circularly polarized laser pulse generating chamber, for focusing and outputting the circularly polarized light pulse; A capacitor plasma target is located on the output optical path of the beam focusing device, and is used to receive the focused circularly polarized light pulse and generate a single attosecond pulse; A reflected laser focusing device, located on the reflected light path of the capacitor plasma target, for focusing and outputting the reflected laser pulse with a single attosecond pulse output by the capacitor plasma target; A filtering device, located on the output optical path of the reflected laser focusing device, for filtering out low-frequency stray light and outputting a single attosecond pulse; The capacitor plasma target includes a first plasma target and a second plasma target; The first plasma target is located on the output light path of the beam focusing device, and the first plasma target is perpendicular to the output light path of the beam focusing device; The second plasma target is located behind the first plasma target, and the first plasma target and the second plasma target are parallel to each other; The first plasma target is thinner than the second plasma target.

2. The circularly polarized laser driven harmonic radiation enhancement system according to claim 1, characterized in that: The circularly polarized light pulse has a wavelength of 0.8 μm, a duration of 8 fs, and a peak intensity of 4.81×10 21 W / cm 2 , the spot radius is 8μm~15μm.

3. The circularly polarized laser driven harmonic radiation enhancement system according to claim 2, characterized in that: The thickness of the first plasma target is 0.008 μm, and the thickness of the second plasma target is 0.4 μm.

4. The circularly polarized laser driven harmonic radiation enhancement system according to claim 3, characterized in that: The distance between the first plasma target and the second plasma target is 0.24 μm.

5. The circularly polarized laser driven harmonic radiation enhancement system according to any one of claims 1 to 4, characterized in that: The first plasma target and the second plasma target are both made of carbon nanotube films.

6. The circularly polarized laser driven harmonic radiation enhancement system according to claim 5, characterized in that: The density of the first plasma target and the second plasma target is 4.35×10 29 cm -3 .

7. The circularly polarized laser driven harmonic radiation enhancement system according to any one of claims 1 to 4, characterized in that: The filtering device is a tunable filter in the extreme ultraviolet band.

Images