Single circularly polarized attosecond pulse generation method and system

Abstract

The present invention belongs to the technical field of attosecond pulses, and specifically relates to a method and system for generating a single circularly polarized attosecond pulse, comprising the following steps: a laser pulse generating device emits a laser pulse I and emits a laser pulse II after a delay setting time; laser pulse I and laser pulse II are focused by an incident laser focusing device, laser pulse I and laser pulse II are focused and transmitted in a colinear manner, and are formed into a synthetic laser through a beat frequency effect, the synthetic laser comprising a main pulse and a tail pulse connected to each other; after the synthetic laser is reflected by a plasma target, the reflected laser is passed through a filtering device to obtain a single attosecond pulse, and the present invention can not only effectively improve the intensity and energy conversion efficiency of the generated attosecond pulse, but also ensure the high ellipticity of the single attosecond pulse. The restriction on the pulse width of the driving laser for generating a single attosecond pulse is relaxed, so that a driving laser pulse with a longer pulse width can also stably generate a single attosecond pulse with high ellipticity, and the experimental feasibility is higher.

Description

Technical Field

[0001] The present invention belongs to the technical field of attosecond pulses, and in particular relates to a method and system for generating a single circularly polarized attosecond pulse. Background Art

[0002] Since the birth of laser in 1960, laser technology has developed rapidly and is now widely used in scientific research, material processing, medical treatment and other fields. With the development of technology, the width of laser pulse output by laser has also increased from nanoseconds (1 nanosecond = 10 -9 seconds) to picoseconds (1 picosecond = 10 -12 seconds) level, shortened to the current femtosecond (1 femtosecond = 10 -15 Seconds) level.

[0003] Attosecond (1 attosecond = 10 -18 The pulse width of attoseconds (0.1675 Å) has become a unique tool for detecting and controlling ultrafast microscopic phenomena due to its unprecedented spatial and temporal resolution. Its applications include ultrafast biomedical imaging, atomic and molecular detection processes, and diagnosis in inertial confinement fusion experiments. A single circularly polarized attosecond pulse can not only be used for clear real-time imaging and precise control of ultrafast phenomena within the attosecond time scale, but also as a chiral light source, it can be used to identify the chirality of matter, study the magnetism of matter, and detect the spin dynamics of microscopic particles. It has broad application prospects in cutting-edge scientific fields such as physics, chemistry, and biology.

[0004] In 2011, Ji et al. proposed a scheme to generate a single circularly polarized attosecond pulse by driving a near-critical density plasma with a few-cycle circularly polarized laser. As the laser ponderomotive force gradually increases, the plasma target surface can be continuously and stably pushed into the target, and strong electrostatic energy is accumulated. When the laser ponderomotive force weakens, the electrostatic energy is converted into the kinetic energy of the electrons on the target surface. Since the driving laser is a few-cycle laser, the electrons on the target surface will perform a single violent compression on the incident laser when they rebound, thereby generating a single attosecond pulse.

[0005] In 2020, Wang et al. found that when a few-cycle circularly polarized laser is incident on an ultra-thin foil, 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, ultra-thin electron layer. When the electron layer begins to rebound, it becomes a high-quality relativistic electron mirror, and this electron mirror propagates against the tail of the laser pulse. Therefore, the laser can be effectively reflected and compressed on this electron mirror. Although the electron layer actually continues to oscillate multiple times, because the amplitude of each oscillation is inconsistent, a single attosecond pulse can be obtained by appropriate filtering and adjusting parameter conditions.

[0006] However, the attosecond pulses generated by these two methods are difficult to meet the requirements of high intensity and high ellipticity at the same time, and the specific parameter matching conditions are unclear. Specifically, when driven by a circularly polarized laser pulse with a short pulse width, the rebound speed of the target surface will be faster, but because there is a phase difference of π / 2 between the two transverse orthogonal components of the circularly polarized laser, the energy difference between the p component and the s component of the falling edge of the short-pulse laser is large, resulting in a smaller ellipticity of the generated attosecond pulse. If driven by a long-pulse laser pulse, the rebound speed of the target surface is too slow, which not only leads to low harmonic radiation efficiency, but also because the compression time of the target surface to the laser is short, the ellipticity of the synthesized attosecond pulse will be small.

[0007] In summary, although researchers have proposed theoretical solutions to obtain single circularly polarized attosecond pulses based on the interaction between relativistic lasers and plasmas, there are still many problems that need to be solved. First, the driving light sources of these theoretical solutions are limited to few-cycle laser pulses, which puts relatively high demands on the experiment. Second, it is currently difficult to simultaneously ensure high intensity and high ellipticity of the generated attosecond pulses. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a single circularly polarized attosecond pulse generation method and system which can generate high-intensity and high-ellipticity attosecond pulses under the premise of low pulse width limitation requirements.

[0009] The present invention provides a method for generating a single circularly polarized attosecond pulse, comprising the following steps:

[0010] The laser pulse generating device emits laser pulse I and emits laser pulse II after a delay setting time, and the laser pulse I and the laser pulse II are circularly polarized lasers rotating in the same direction;

[0011] Laser pulse I and laser pulse II are focused by an incident laser focusing device, laser pulse I and laser pulse II are focused and transmitted in a co-linear manner, and are formed into a synthetic laser through a beat frequency effect, and the synthetic laser includes a main pulse and a tail pulse connected to each other;

[0012] The synthetic laser is directed toward the plasma target. The rising edge of the main pulse of the synthetic laser compresses the target surface of the plasma target violently. When the falling edge of the main pulse acts, the target surface rebounds at high speed. During the rebound process, the reflected part of the main pulse is distorted. After the target surface of the plasma target has compressed the main pulse at high speed, it continues to compress the tail pulse connected to the main pulse, and the tail pulse is also reflected from the target surface.

[0013] The reflected laser passes through a filtering device to obtain a single attosecond pulse.

[0014] Furthermore, the laser pulse generating device emits laser pulse I and emits laser pulse II after a delay of a set time, comprising:

[0015] The pulse width and frequency of the laser pulse I and the laser pulse II satisfy: τ2 / 6<|π / (ω2-ω1)|<τ2, wherein |π / (ω2-ω1)| is the width of the main pulse determined by the beat frequency in the synthetic laser, ω1 is the frequency of the laser pulse I, ω2 is the frequency of the laser pulse II, and τ2 is the pulse width of the laser pulse II.

[0016] Furthermore, the synthetic laser includes a main pulse and a tail pulse connected to each other including:

[0017] The pulse width of the main pulse is not longer than the pulse width of the laser pulse II; the intensity of the main pulse is not lower than the intensities of the laser pulse I and the laser pulse II.

[0018] Furthermore, the synthetic laser is directed toward the plasma target, comprising: the synthetic laser is incident along the axis of the plasma target.

[0019] Furthermore, the laser pulse generating device emits laser pulse I and emits laser pulse II after a delay of a set time, comprising:

[0020] The wavelength of the laser pulse I is 1 μm, the pulse width is 33 fs, and the peak intensity is 9.85×10 19 W / cm 2 , spot radius is 8-15μm;

[0021] The wavelength of the laser pulse II is 0.8 μm, the pulse width is 16 fs, and the peak intensity is 1.08×10 21 W / cm 2 , spot radius is 8-15μm;

[0022] The delay setting time is 4.95 fs.

[0023] Furthermore, the synthetic laser directed toward the plasma target comprises:

[0024] The plasma target is a flat target with a thickness of not less than 1 μm and a density of 8.92×10 21 cm -3 .

[0025] Furthermore, the reflected laser is passed through a filtering device to obtain a single attosecond pulse, which includes:

[0026] The filtering device is a tunable filter in the extreme ultraviolet band, and the filtering range can be tuned to 30-100nm.

[0027] The present invention also provides a single circularly polarized attosecond pulse generation system, comprising a laser pulse generation device, an incident laser focusing device, a plasma target and a filtering device;

[0028] The laser pulse generating device comprises two laser pulse emitters, which are used to generate laser pulse I and laser pulse II respectively;

[0029] The incident laser focusing device is arranged on the optical path of laser pulse I and laser pulse II, and is used to focus laser pulse I and laser pulse II and transmit them collinearly along the axis of the plasma target;

[0030] The filtering device is arranged on the reflected light path of the plasma target and is used for filtering the reflected laser to obtain attosecond pulses.

[0031] Furthermore, the laser pulse frequencies emitted by the two laser pulse emitters are tunable, the peak intensity is adjustable, and the filtering range of the filtering device is tunable.

[0032] Furthermore, a reflected laser focusing device is provided between the plasma target and the filter device. The reflected laser focusing device is provided on the reflected light path of the plasma target and is used for focusing the reflected laser and emitting it to the filter device.

[0033] The beneficial effect of the present invention is that the present invention uses two circularly polarized lasers rotating in the same direction, which are transmitted in a colinear manner after being delayed for a set time, and are synthesized into a synthetic laser through the beat frequency effect. The synthetic laser interacts with the plasma target, and the synthetic laser is rapidly compressed in a near-critical density plasma. Compared with the monochromatic circularly polarized laser scheme of the same energy, the main pulse in the synthetic laser produced by the present invention has a steeper laser falling edge, so the plasma target surface can be driven to perform a more intense single compression, so a single attosecond pulse can be generated more efficiently; after the target surface of the plasma target has compressed the main pulse at high speed, it will continue to compress the tail pulse connected to the main pulse; the tail pulse makes the laser energy in the two polarization directions compressed by the rebounding electron layer as equal as possible, thereby ensuring the high ellipticity of the attosecond pulse. Therefore, the present invention can not only effectively improve the intensity and energy conversion efficiency of the generated attosecond pulse, but also ensure the high ellipticity of a single attosecond pulse. In addition, a driving laser pulse with a longer pulse width can be used to generate a single attosecond pulse with high ellipticity, which relaxes the restriction on the driving laser pulse width to generate a single attosecond pulse, and the experimental feasibility is higher. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Attached Figure 1 It is a structural schematic diagram of the present invention;

[0035] Attached Figure 2 A schematic diagram of generating a single circularly polarized attosecond pulse according to the present invention;

[0036] Attached Figure 3 It is a comparison diagram of the main physical processes of the specific embodiment of the present invention and the comparative example;

[0037] Attached Figure 4 It is a comparison diagram of the reflected laser pulse and the generated attosecond pulse parameters of the specific embodiment of the present invention and the comparative example;

[0038] Attached Figure 5 A diagram showing the dependence of the energy conversion efficiency, intensity and ellipticity of attosecond pulses generated by the specific embodiments of the present invention and the comparative examples on the delay time and energy ratio of the driving laser;

[0039] Attached Figure 6 Characteristics of attosecond pulses generated on the surface of a near-critical density plasma target with a pre-plasma.

[0040] In the figure, 1-laser pulse generating device; 2-incident laser focusing device; 3-plasma target; 4-reflected laser focusing device; 5-filtering device. DETAILED DESCRIPTION

[0041] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] It should be noted that all directional indications in the embodiments of the present invention (such as up, down, left, right, front, back, etc.) are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0043] In addition, in the present invention, descriptions such as "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" or "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0044] In the present invention, unless otherwise clearly specified and limited, the terms "connection", "fixation", etc. should be understood in a broad sense. For example, "fixation" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, an electrical connection, a physical connection, or a wireless communication connection; it can be a direct connection, or an indirect connection through an intermediate medium, or it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0045] In addition, the technical solutions between the various embodiments of the present invention can be combined with each other, but it must be based on the fact that ordinary technicians in the field can implement it. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0046] As attached Figure 1-6 As shown, the present invention provides a method for generating a single circularly polarized attosecond pulse, comprising the following steps:

[0047] The laser pulse generating device 1 emits laser pulse I and then emits laser pulse II after a delay of a set time, wherein laser pulse I and laser pulse II are circularly polarized lasers rotating in the same direction, and the set time has a value range of 0-20fs, preferably 4.95fs. By setting a certain delay time, the high intensity parts of laser pulse I and laser pulse II can overlap, which is beneficial to the increase of the peak value of the main pulse;

[0048] Laser pulse I and laser pulse II are focused by an incident laser focusing device, and laser pulse I and laser pulse II are co-linearly transmitted after focusing, and form a synthetic laser through the beat frequency effect, wherein the synthetic laser includes a main pulse and a tail pulse connected to each other, and after the beat frequency effect, the pulse width of the main pulse is shorter than the pulse width of laser pulse II, and the intensity of the main pulse is greater than the intensity of laser pulse I and laser pulse II, at this time, the falling edge of the synthetic laser will become steeper than the falling edge of laser pulse I and laser pulse II, and a single oscillation more violent than laser pulse I and laser pulse II can be formed after the synthetic pulse is shot at the target surface of the plasma target 3; and the tail pulse is the subsequent part of laser pulse I, and its intensity is weaker than that of the main pulse, and no other effective target surface oscillation will be caused except the oscillation caused by the main pulse;

[0049] The synthesized laser is shot toward the plasma target 3 and rebounds to form a reflected laser, wherein the main pulse of the synthesized laser compresses the target surface of the plasma target 3 at high speed. After the peak position of the main pulse reaches the target surface, the falling edge of the main pulse continues to shoot toward the target surface, and the target surface rebounds at high speed. During the rebound process, the reflected part of the main pulse is distorted by the relativistic Doppler effect. The distortion of the waveform means that high-frequency components are introduced, which can effectively improve the intensity and energy conversion efficiency of the generated attosecond pulses. After the target surface of the plasma target 3 has compressed the main pulse at high speed, it will continue to compress the tail pulse connected to the main pulse, and the tail pulse will also be reflected from the target surface; the tail pulse makes the laser energy in the two polarization directions compressed by the rebounding electron layer as equal as possible, thereby ensuring the high ellipticity of the attosecond pulse.

[0050] The reflected laser passes through the filter device 5 to filter out the parts with lower frequencies and possible non-physical high-frequency noise on both sides, leaving the parts with high intensity and belonging to the XUV band, thereby obtaining a single attosecond pulse with high intensity and high ellipticity.

[0051] The present invention uses two circularly polarized lasers rotating in the same direction, which are transmitted in a colinear manner after being delayed for a set time, and are synthesized into a synthetic laser through the beat frequency effect. The synthetic laser interacts with the plasma target 3, and the synthetic laser is rapidly compressed in a near-critical density plasma. Compared with the monochromatic circularly polarized laser scheme of the same energy, the main pulse in the synthetic laser produced by the present invention has a steeper laser falling edge, so the plasma target 3 surface can be driven to perform a more intense single compression, so a single attosecond pulse can be generated more efficiently; after the target surface of the plasma target has compressed the main pulse at high speed, it will continue to compress the tail pulse connected to the main pulse, and the tail pulse will also be reflected from the target surface; the tail pulse makes the laser energy in the two polarization directions compressed by the rebounding electron layer as equal as possible, thereby ensuring the high ellipticity of the attosecond pulse. Therefore, the present invention can not only effectively improve the intensity and energy conversion efficiency of the generated attosecond pulse, but also ensure the high ellipticity of a single attosecond pulse. In addition, a driving laser pulse with a longer pulse width can be used to generate a single attosecond pulse with high ellipticity, which relaxes the restriction on the driving laser pulse width for generating a single attosecond pulse, and the experimental feasibility is higher.

[0052] In one embodiment, the laser pulse generating device 1 emits laser pulse I and then emits laser pulse II after delaying a set time, including:

[0053] The pulse width and frequency of the laser pulse I and the laser pulse II satisfy: τ2 / 6<|π / (ω2-ω1)|<τ2, where |π / (ω2-ω1)| is the width of the main pulse determined by the beat frequency in the synthetic laser, ω1 is the frequency of the laser pulse I, ω2 is the frequency of the laser pulse II, and τ2 is the pulse width of the laser pulse II. On this basis, by adjusting the appropriate delay time, energy ratio, initial phase difference, pulse width and other conditions of the two-color laser, the driving laser can be modulated into a synthetic laser of the main pulse carrying a tail pulse. After a single attosecond pulse is generated at the falling edge of the main pulse, the tail pulse makes the laser energy in the two polarization directions compressed by the rebounding electron layer as equal as possible, thereby ensuring the ellipticity of the generated attosecond pulse, which means that a longer driving laser pulse can be used to generate a single attosecond pulse with high ellipticity, relaxing the restriction on the driving laser pulse width for generating a single attosecond pulse, and the experimental feasibility is higher.

[0054] In one embodiment, the synthetic laser includes a main pulse and a tail pulse connected to each other including:

[0055] The pulse width of the main pulse is shorter than the pulse width of laser pulse II; the intensity of the main pulse is greater than the intensities of laser pulse I and laser pulse II, so that the generated attosecond pulse has higher intensity and higher energy conversion efficiency.

[0056] In one embodiment, the synthetic laser is directed toward the plasma target 3, which includes: the synthetic laser is incident along the axis of the plasma target 3. Since the synthetic laser is a circularly polarized laser, the smaller the incident angle is, the higher the ellipticity of the generated attosecond pulse is. Therefore, it is preferred that the synthetic laser is incident along the axis of the plasma target 3, and at this time, the incident angle of the synthetic laser is zero, which effectively improves the ellipticity of the attosecond pulse.

[0057] In one embodiment, the laser pulse generating device emits laser pulse I and emits laser pulse II after a delay of a set time, comprising:

[0058] The wavelength of the laser pulse I is 1 μm, the pulse width is 33 fs, and the peak intensity is 9.85×10 19 W / cm 2 , spot radius is 8-15μm;

[0059] The wavelength of the laser pulse II is 0.8 μm, the pulse width is 16 fs, and the peak intensity is 1.08×10 21 W / cm 2 , spot radius is 8-15μm;

[0060] The delay setting time is 4.95 fs.

[0061] In this embodiment, the synthetic laser is directed toward the plasma target 3:

[0062] The plasma target 3 is a flat target with a thickness of not less than 1 μm and a density of 8.92×10 21 cm -3 . Ensure that the flat target is not penetrated.

[0063] In this embodiment, the reflected laser passes through the filter device 5 to obtain a single attosecond pulse:

[0064] The filtering device 5 is a tunable filter in the extreme ultraviolet band, and the filtering range can be tuned to 30-100nm.

[0065] The present invention also provides a single circularly polarized attosecond pulse generation system, comprising a laser pulse generation device 1, an incident laser focusing device 2, a plasma target 3 and a filtering device 5;

[0066] The laser pulse generating device comprises two laser pulse emitters, which are used to generate laser pulse I and laser pulse II respectively, and the two laser pulse emitters can delay the emission of laser pulse I and laser pulse II by a set time by controlling their emission time;

[0067] The incident laser focusing device is arranged on the optical path of the laser pulse I and the laser pulse II, and is used to focus the laser pulse I and the laser pulse II and transmit them collinearly along the axis of the plasma target 3;

[0068] The filtering device 5 is arranged on the reflected light path of the plasma target 3 and is used to filter the reflected laser to obtain attosecond pulses.

[0069] In one of the embodiments, the laser pulse frequencies emitted by the two laser pulse emitters are tunable, the peak intensity is adjustable, and the filtering range of the filtering device 5 is tunable, which facilitates parameter adjustment and simplifies experimental operations.

[0070] In one of the embodiments, a reflected laser focusing device 4 is further provided between the plasma target 3 and the filtering device 5. The reflected laser focusing device 4 is provided on the reflected light path of the plasma target 3 and is used to focus the reflected laser and emit it to the filtering device 5, so as to facilitate the arrangement of the light path.

[0071] The specific working principle of the present invention is:

[0072] The present invention uses the relativistic Doppler frequency shift technology to generate an ultra-short (attosecond level) ultra-strong (relativistic intensity) single circularly polarized pulse. When the circularly polarized laser (main pulse) with relativistic intensity is incident on the plasma target 3, the target surface will be Here, E L is the instantaneous laser electric field acting on the plasma target 3 surface, B Lis the instantaneous laser magnetic field acting on the plasma target 3 surface, and v is the instantaneous motion speed of the plasma target 3 surface. When the plasma target 3 surface moves toward the laser at nearly the speed of light, a relativistic Doppler frequency shift will be introduced in the frequency domain of the reflected laser. The specific process is that due to the relativistic Doppler effect, the energy and wavelength of the laser will be strongly compressed during the reflection process of the target surface moving toward it. At this time, the waveform of the reflected laser will be distorted, that is, a high-frequency signal is introduced. After the required frequency band is intercepted by the filter, attosecond pulses can be synthesized. In addition, there is also a continuous weak deformation at the tail of the reflected laser, which means that after the target surface compresses the falling edge of the main pulse, it will still continue to compress the tail pulse slightly, so the ellipticity of the generated attosecond pulse can be guaranteed without generating additional strong attosecond pulses.

[0073] The present invention provides a specific embodiment and two comparative examples for effect verification, including:

[0074] The specific embodiment is as follows:

[0075] The wavelength of laser pulse I is 1 μm and the peak intensity is 9.85×10 19 W / cm 2 , the spot radius is 8-15μm, and the pulse width is τ1=10T1=33fs;

[0076] The wavelength of laser pulse II is 0.8 μm and the peak intensity is 1.08×10 21 W / cm 2 , spot radius is 8-15μm; pulse width is τ2=6T2=4.8T1=16fs;

[0077] The delay time of laser pulse I and laser pulse II is Δd=1.5T1, and the energy ratio of laser pulse II / (laser pulse I+laser pulse II) is W=0.84; Figure 3-4 Marked as (c).

[0078] Comparative Example 1 has a wavelength of 1 μm, a pulse width of τ1=10T1=33 fs, and a peak intensity of 6.16×10 20 W / cm 2 , monochromatic circularly polarized laser with a spot radius of 8-15μm; Figure 3-4 Marked as (a).

[0079] Comparative Example 2 is a wavelength of 0.8 μm, a pulse width of τ2 = 6T2 = 4.8T1 = 16 fs, and a peak intensity of 1.28 × 10 21 W / cm 2 , monochromatic circularly polarized laser with a spot radius of 8-15μm; Figure 3-4 Marked as (b).

[0080] The total laser energy in Comparative Example 1, Comparative Example 2 and the specific embodiment of the present invention is consistent.

[0081] Figure 3 (a-Ⅰ), (b-Ⅰ) and (c-Ⅰ) are the time-space evolution diagrams of the target surface electrons of Comparative Example 1, Comparative Example 2 and the specific embodiment, respectively, and the illustration is the electron density distribution at the maximum propulsion displacement of the target surface, where n c =1.11×10 21 cm -3 is the normalization coefficient of electron density; (a-Ⅱ), (b-Ⅱ) and (c-Ⅱ) are the longitudinal force distribution (electrostatic force F) of typical electrons on the target surface of comparative example 1, comparative example 2 and specific embodiment, respectively. s , Lorentz force F l , the combined force F x ) and the normalized velocity β x =v x / c; (a-Ⅲ), (b-Ⅲ) and (c-Ⅲ) are the longitudinal Lorentz factors of the radiation time of the comparative example 1, the comparative example 2 and the specific embodiment respectively

[0082] from Figure 3 It can be seen that, compared with two monochromatic circularly polarized sub-pulses at the same energy, the driving field synthesized by the present invention under appropriate delay and energy ratio can push the target surface of the plasma target 3 to a deeper position, thereby accumulating stronger electrostatic energy, and therefore the rebound speed of the electrons on the target surface at the moment of radiation will also be accelerated.

[0083] Figure 4 (a-Ⅰ), (b-Ⅰ) and (c-Ⅰ) are respectively the reflected laser waveforms of Comparative Example 1, Comparative Example 2 and the specific embodiment, wherein E r =3.21×10 12 V / m is the normalization coefficient of the laser electric field; (a-Ⅱ), (b-Ⅱ) and (c-Ⅱ) are the reflected laser spectra of comparative example 1, comparative example 2 and specific embodiment, respectively, where I r =1.37×10 18 W / cm 2 is the normalization coefficient of laser intensity; (a-Ⅲ), (b-Ⅲ) and (c-Ⅲ) are respectively the spatial distributions of the attosecond pulses synthesized after the reflected laser filtering in comparative example 1, comparative example 2 and the specific embodiment; (a-Ⅳ), (b-Ⅳ) and (c-Ⅳ) are respectively the ellipticity of the attosecond pulses synthesized after the reflected laser filtering in comparative example 1, comparative example 2 and the specific embodiment, and e is the corresponding ellipticity.

[0084] from Figure 4It can be seen that, compared with Comparative Examples 1 and 2, the driving field in the present invention can cause a higher target surface electron rebound velocity, so that the rebounding electrons can be compressed more violently during the reflection process, which is reflected in the more obvious distortion of the reflected laser waveform, thereby more effectively generating a continuous spectrum. This continuous spectrum also marks the occurrence of a single attosecond pulse radiation. In addition, there is also a continuous slight deformation at the tail of the reflected laser, which means that the target surface is still continuously compressing the tail pulse slightly, which can ensure the ellipticity of the generated attosecond pulse without generating additional strong attosecond pulses. Therefore, when the reflected laser on the target surface in the present invention is filtered by an XUV band filter, a single attosecond pulse with a stronger (c-Ⅲ) and higher ellipticity (c-Ⅳ) can be obtained.

[0085] Figure 5 The pulse components of the laser pulse I and laser pulse II of the two-color laser in (ac) are monochromatic circularly polarized lasers with a wavelength of λ1=1μm and a pulse width of τ1=10T1=33fs and a wavelength of λ2=0.8μm and a pulse width of τ2=6T2=4.8T1=16fs respectively.

[0086] Figure 5 (a), (b), and (c) are the energy conversion efficiency, peak intensity, and ellipticity of attosecond pulses, respectively.

[0087] Figure 5 The sub-pulse components of the two-color laser in (de) are respectively monochromatic circularly polarized lasers with a wavelength of λ1=1μm and a pulse width of τ1=18T1=60fs and a wavelength of λ2=λ1 / 1.1=0.91μm and a pulse width of τ2=12T2=12T1 / 1.1=36.36fs.

[0088] Figure 5 (d), (e), and (f) are the energy conversion efficiency of attosecond pulses, the peak intensity of attosecond pulses, and the ellipticity of attosecond pulses, respectively.

[0089] Figure 5 It can be seen that by changing the delay time and energy ratio of the two-color laser, within a relatively wide parameter range, the method of the present invention can generate a single circularly polarized pulse that satisfies both high intensity and high ellipticity, which is difficult to achieve even for a few-cycle monochromatic circularly polarized laser. It appears that this method can use a driving laser pulse with a longer pulse width to generate a single attosecond pulse with high ellipticity, relaxing the restriction of the driving laser pulse width on the generated single attosecond pulse.

[0090] Figure 6 The characteristics of attosecond pulses generated on the surface of a near-critical density plasma target 3 with pre-plasma;

[0091] Figure 6It can be seen from the figure that when the target surface of the plasma target 3 is attached with an exponentially distributed pre-plasma with a density scale length of L = 0.1λ1 = 0.1μm, the attosecond pulses generated by the system of the invention can still generate single circularly polarized attosecond pulses with high ellipticity and high intensity, and the results are basically consistent with those without the pre-plasma distribution. Therefore, this is also very beneficial for the initial incident laser carrying the pre-pulse. Among them, a p and a s They respectively represent the components of the normalized electric field that generates attosecond pulses along the p-polarization direction and the s-polarization direction. Therefore, the present invention changes the design to replace the near-critical density plasma with a low-density plasma target with different density standard length pre-plasmas. This type of plasma target can also generate a single circularly polarized attosecond pulse under appropriate parameter matching conditions when driven by two-color co-rotating circularly polarized laser pulses of unequal frequencies.

[0092] The contents not described in detail in this specification belong to the prior art known to professional and technical personnel in this field.

Claims

1. A method for generating a single circularly polarized attosecond pulse, characterized in that: The steps include: The laser pulse generating device emits laser pulse I and emits laser pulse II after a delay setting time, and the laser pulse I and the laser pulse II are circularly polarized lasers rotating in the same direction; Laser pulse I and laser pulse II are focused by an incident laser focusing device, laser pulse I and laser pulse II are focused and transmitted in a co-linear manner, and are formed into a synthetic laser through a beat frequency effect, and the synthetic laser includes a main pulse and a tail pulse connected to each other; The synthetic laser is directed toward the plasma target. The rising edge of the main pulse of the synthetic laser compresses the target surface of the plasma target violently. When the falling edge of the main pulse acts, the target surface rebounds at high speed. During the rebound process, the reflected part of the main pulse is distorted. After the target surface of the plasma target has compressed the main pulse at high speed, it continues to compress the tail pulse connected to the main pulse, and the tail pulse is also reflected from the target surface. The reflected laser passes through a filtering device to obtain a single attosecond pulse.

2. The single circularly polarized attosecond pulse generation method according to claim 1, characterized in that: The laser pulse generating device emits laser pulse I and emits laser pulse II after a delay setting time, comprising: The pulse width and frequency of the laser pulse I and the laser pulse II satisfy: τ2 / 6<|π / (ω2-ω1)|<τ2, wherein |π / (ω2-ω1)| is the width of the main pulse determined by the beat frequency in the synthetic laser, ω1 is the frequency of the laser pulse I, ω2 is the frequency of the laser pulse II, and τ2 is the pulse width of the laser pulse II.

3. The single circularly polarized attosecond pulse generation method according to claim 1, characterized in that: The synthetic laser comprises a main pulse and a tail pulse connected to each other including: The pulse width of the main pulse is not longer than the pulse width of the laser pulse II; the intensity of the main pulse is not lower than the intensities of the laser pulse I and the laser pulse II.

4. The single circularly polarized attosecond pulse generation method according to claim 1, characterized in that: The synthetic laser is directed toward the plasma target, comprising: the synthetic laser is incident along the axis of the plasma target.

5. The method for generating a single circularly polarized attosecond pulse according to any one of claims 1 to 4, characterized in that: The laser pulse generating device emits laser pulse I and emits laser pulse II after a delay setting time, comprising: The wavelength of the laser pulse I is 1 μm, the pulse width is 33 fs, and the peak intensity is 9.85×10 19 W / cm 2 , spot radius is 8-15μm; The wavelength of the laser pulse II is 0.8 μm, the pulse width is 16 fs, and the peak intensity is 1.08×10 21 W / cm 2 , spot radius is 8-15μm; The delay setting time is 4.95 fs.

6. The method for generating a single circularly polarized attosecond pulse according to claim 5, characterized in that: The synthetic laser directed toward the plasma target comprises: The plasma target is a flat target with a thickness of not less than 1 μm and a density of 8.92×10 21 cm -3 .

7. The method for generating a single circularly polarized attosecond pulse according to claim 5, characterized in that: The reflected laser is passed through a filtering device to obtain a single attosecond pulse, which comprises: The filtering device is a tunable filter in the extreme ultraviolet band, and the filtering range can be tuned to 30-100nm.

8. A single circularly polarized attosecond pulse generation system, characterized in that: It includes a laser pulse generating device, an incident laser focusing device, a plasma target and a filtering device; The laser pulse generating device comprises two laser pulse emitters, which are used to generate laser pulse I and laser pulse II respectively; The incident laser focusing device is arranged on the optical path of laser pulse I and laser pulse II, and is used to focus laser pulse I and laser pulse II and transmit them collinearly along the axis of the plasma target; The filtering device is arranged on the reflected light path of the plasma target and is used for filtering the reflected laser to obtain attosecond pulses.

9. The single circularly polarized attosecond pulse generator according to claim 8, characterized in that: The laser pulse frequencies emitted by the two laser pulse emitters are tunable, the peak intensity is adjustable, and the filtering range of the filtering device is tunable.

10. The single circularly polarized attosecond pulse generator according to claim 8, characterized in that: A reflected laser focusing device is also provided between the plasma target and the filter device. The reflected laser focusing device is provided on the reflected light path of the plasma target and is used for focusing the reflected laser and emitting it to the filter device.

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