Method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source

Abstract

This invention belongs to the field of nonlinear optics and extreme ultraviolet coherent light source technology, specifically relating to a method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source. The method involves: constructing a reverse-rotating bicolor circularly polarized laser field as the driving light source; this reverse-rotating bicolor circularly polarized laser field is a superimposed field of reverse-rotating circular polarization composed of a fundamental frequency light field and a second harmonic light field; in a vacuum environment, the reverse-rotating bicolor circularly polarized laser field is coincident in time and space and focused into a medium to generate high-order harmonic radiation; by adjusting the relative intensity of the fundamental frequency light field and the second harmonic light field, a quasi-monochromatic circularly polarized extreme ultraviolet light source is generated; the medium is an atomic, ionic, or unarranged molecular system. The method of this invention is simple to implement experimentally, and the resulting circularly polarized extreme ultraviolet light source has high monochromaticity and high polarization purity, solving the problem of previous methods for generating quasi-monochromatic extreme ultraviolet light sources that relied on complex media and could not simultaneously achieve monochromaticity and circular polarization state.

Description

Technical Field

[0001] This invention belongs to the field of nonlinear optics and extreme ultraviolet coherent light source technology, specifically relating to a method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source. Background Technology

[0002] Higher harmonics (HHMs) possess excellent temporal and spatial coherence, covering a spectral range from the ultraviolet to the soft X-band. Furthermore, the experimental setup required to generate HHMs is relatively simple, making it one of the main technical approaches for obtaining ultrashort pulses and ultraviolet radiation with pulse widths on the femtosecond to attosecond scales. In numerous applications, such as circular dichroism attosecond transient absorption and photoelectron circular dichroism experiments, quasi-monochromatic circularly polarized extreme ultraviolet (EUV) sources are particularly important.

[0003] However, previous methods for obtaining quasi-monochromatic light sources using higher harmonics typically require the fabrication of metal droplets or laser ablation plumes, placing high demands on the experimental environment and equipment. In addition, another method for generating quasi-monochromatic higher harmonics involves optimizing the phase-matching conditions of specific-order higher harmonics using a multicolor light field. However, the monochromaticity of higher harmonics generated by this method is relatively low. More importantly, previous research primarily focused on the generation efficiency and monochromaticity of higher harmonics, only yielding linearly polarized or near-linearly polarized quasi-monochromatic higher harmonics, and still struggling to generate circularly polarized quasi-monochromatic extreme ultraviolet pulses. Summary of the Invention

[0004] To address the aforementioned deficiencies or improvement needs of existing technologies, this invention provides a method for generating a quasi-monochromatic circularly polarized extreme ultraviolet (EUV) light source based on higher harmonics. This invention achieves selective control of the higher harmonic frequency components and polarization states by constructing a reverse-rotating dual-color circularly polarized driving field with specific symmetry and an interacting medium, combined with a resonance enhancement mechanism in the medium's energy level structure. This results in a highly monochromatic circularly polarized EUV ultrashort pulse light source.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows.

[0006] This invention provides a method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source, comprising the following steps: A reverse-rotating bicolor circularly polarized laser field is constructed as the driving light source; the reverse-rotating bicolor circularly polarized laser field is a superimposed field of reverse-rotating circular polarization composed of a fundamental frequency light field and a second harmonic light field; in a vacuum environment, the reverse-rotating bicolor circularly polarized laser field is superimposed in time and space and focused into a medium to generate high-order harmonic radiation; by adjusting the relative intensity of the fundamental frequency light field and the second harmonic light field, a quasi-monochromatic circularly polarized extreme ultraviolet light source is generated; the medium is an atomic, ionic, or unarranged molecular system.

[0007] This invention provides a method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source based on high-order harmonics. The method of this invention uses a reverse-rotating dual-color circularly polarized laser field ( ω 0 + 2ω 0 The driving medium is used to generate higher harmonics, and a dual-color circularly polarized laser field with reverse rotation is used as the driving medium. ω 0 + 2ω 0 This drives atoms, ions, or unaligned molecules to produce high-order harmonic radiation. When the energy of the driving photon matches the energy level difference between the ground state and the excited state of the medium, the multiphoton resonance absorption process, which depends on the energy level difference between the ground state and the excited state, is significantly enhanced, thus forming a quasi-monochromatic high-order harmonic output dominated by specific frequency components in the harmonic spectrum. Simultaneously, because the reverse-rotation two-color circularly polarized driving field and the medium system satisfy specific symmetry conditions—that is, the reverse-rotation two-color circularly polarized driving field has triple rotational symmetry, i.e., C3 symmetry; and the atomic, ion, or unaligned molecular system has isotropic characteristics, i.e., C… Symmetry. When a light field and matter interact in accordance with the aforementioned symmetry, three identical electron trajectories will exist within each optical cycle of the fundamental frequency light. These trajectories radiate higher harmonics with the same photon energy at time intervals of T / 3. The harmonics radiated within each optical cycle coherently superimpose, ultimately suppressing the 3n-th order harmonic. The left-hand and right-hand circularly polarized components of different order harmonics are separated from each other in the spectrum. Therefore, the method of this invention can obtain a quasi-monochromatic circularly polarized extreme ultraviolet light source.

[0008] Preferably, the method for constructing a reverse-rotating dual-color circularly polarized laser field is: The output pulse is split into a first pulse and a second pulse by a beam splitter. The energy of the first pulse is lower than that of the second pulse. The first pulse is used as the fundamental frequency light field, and the second pulse is converted into a second harmonic by a nonlinear crystal to form a second harmonic light field, thus forming a two-color laser pulse and constructing a two-color driving light field. The two-color driving light field is a superposition field of the fundamental frequency light field and the second harmonic light field. The two-color driving light field is subjected to energy adjustment and polarization state control by a half-wave plate, a Brewster angle polarizer, and a quarter-wave plate, respectively, so that the fundamental frequency light field and the second harmonic light field of the two-color driving light field are converted into circular polarization states before entering the action region, and the rotation directions are opposite, thus constructing a reverse rotating two-color circularly polarized laser field.

[0009] More preferably, the dual-color laser pulses are frequency combinations of... ω 0 and 2ω 0 Two-color laser pulses; a reverse-rotating two-color circularly polarized laser field has a frequency combination of... ω0 and 2ω 0 A reverse-rotating dual-color circularly polarized laser field; ω 0 Indicates the frequency of the fundamental frequency optical field; 2ω 0 This indicates the frequency of the second harmonic optical field.

[0010] The preferred method for ensuring that the reverse-rotated two-color circularly polarized laser fields coincide in time and space is: Two beams of a reverse-rotating dichromatic circularly polarized laser field are spatially combined using a dichroic mirror. By adjusting the optical path difference between the two beams, the reverse-rotating dichromatic circularly polarized laser fields are made to coincide in time and space, forming a combined reverse-rotating dichromatic circularly polarized laser beam.

[0011] A further preferred embodiment of the time and space coincidence of the reverse-rotating dual-color circularly polarized laser field is that the two beams of the reverse-rotating dual-color circularly polarized laser field achieve pulse synchronization in time, coaxial propagation in space, and coincide at the same focal point.

[0012] A further preferred method for determining the circular polarization conditions of a counter-rotating two-color circularly polarized driving field coinciding at the same focal point is: By adjusting the polarization states of the two beams of the reverse-rotating dual-color circularly polarized laser field, the 3n-order harmonic signal is suppressed, so that the reverse-rotating dual-color circularly polarized driving field coinciding at the same focal point satisfies the circular polarization condition.

[0013] In this invention, experimental observations determine that the opposing rotating bicolor circularly polarized driving fields coinciding at the same focal point satisfy the circular polarization condition by suppressing the 3n-order harmonics. Here, the circular polarization of the driving field is adjusted at the focal point to ensure the output of circularly polarized ultraviolet harmonics.

[0014] In this invention, when the energy of the driving photon matches the energy level difference between the ground state and the excited state of the medium, the multiphoton resonance absorption process is enhanced, resulting in a significant enhancement of specific order harmonics and thus forming quasi-monochromatic harmonic output. Based on the symmetry selection rule for the interaction between the reverse-rotation bicolor circularly polarized driving field and atomic, ionic, or unaligned molecular systems, the 3n-order harmonics are suppressed. By adjusting the polarization states of the two driving beams, the 3n-order harmonic signal can be suppressed, thereby ensuring that the driving field at the focal point satisfies the circular polarization condition and achieving stable output of circularly polarized extreme ultraviolet harmonics.

[0015] A further preferred method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source is: In a vacuum environment, a combined, counter-rotating, two-color circularly polarized laser beam is focused into a medium through a mirror and interacts with atoms, ions, or unaligned molecular systems in the medium to generate high-order harmonic radiation. By changing the frequency combination of the two beams in the counter-rotating, two-color circularly polarized driving field and selecting a medium whose energy level matches the frequency combination of the two beams, specific order harmonics with selective enhancement can be obtained in the multiphoton resonance enhancement process, thereby realizing the output of quasi-monochromatic circularly polarized extreme ultraviolet light sources with different output wavelengths.

[0016] More preferably, the frequency combination of the two beams of light in the reverse-rotating dual-color circularly polarized driving field is 800nm ​​and 400nm, the medium matching the energy level of the two beams of light with the frequency combination is an unarranged carbon dioxide molecular system, and the output wavelength of the quasi-monochromatic circularly polarized extreme ultraviolet light source is 114.3nm.

[0017] In this invention, depending on the actual situation, different combinations of frequencies can be selected to match the energy level conditions of the reverse-rotating dual-color circularly polarized light driving field, thereby generating a quasi-monochromatic circularly polarized extreme ultraviolet light source that meets the actual wavelength requirements. For example, when the frequency combination of the two beams of the reverse-rotating dual-color circularly polarized driving field is 800 nm and 400 nm, and the medium matching the energy level of the two beams with the frequency combination is an unaligned carbon dioxide molecular system, the output wavelength of the quasi-monochromatic circularly polarized extreme ultraviolet light source is 114.3 nm. The monochromaticity of the quasi-monochromatic circularly polarized extreme ultraviolet light source is 0.93, and the ellipticity is 0.92, which can be regarded as a quasi-monochromatic circularly polarized extreme ultraviolet ultrashort pulse light source.

[0018] The beneficial effects of this invention are: 1. This invention uses a reverse-rotating two-color circularly polarized laser field as the driving field. Based on the matching relationship between the driving field frequency and the energy level structure of the medium, as well as the selection rule of higher harmonic radiation, the reverse-rotating two-color circularly polarized laser field drives atoms, ions, or unaligned molecular systems to achieve selective enhancement of specific order harmonics. This results in a quasi-monochromatic circularly polarized extreme ultraviolet ultrashort pulse light source dominated by specific frequency components. This solves the problem that previous methods of generating quasi-monochromatic extreme ultraviolet light sources relied on complex media and could not simultaneously achieve monochromaticity and circular polarization.

[0019] 2. The method of the present invention is simple to implement experimentally, and the resulting circularly polarized extreme ultraviolet light source has the advantages of high monochromaticity and high polarization purity.

[0020] 3. The method of the present invention is not limited to a specific driving medium or a fixed output wavelength. By selecting different wavelength combinations of reverse rotating dual-color circularly polarized driving fields and matching the medium with the corresponding energy level structure, quasi-monochromatic circularly polarized extreme ultraviolet light output of different wavelengths can be achieved to meet the needs of different application scenarios. Attached Figure Description

[0021] Figure 1This is a flowchart illustrating the process of generating a quasi-monochromatic circularly polarized ultraviolet light source.

[0022] Figure 2 This is a schematic diagram of the experimental optical path and apparatus for the interaction of a reverse-rotating two-color circularly polarized laser field with a gas medium in a high-order harmonic experiment.

[0023] Figure 3 It is a typical high-harmonic experimental spectrum produced by the interaction of a counter-rotating two-color circularly polarized laser field with unaligned carbon dioxide molecules.

[0024] Figure 4 The results are experimental measurements and reconstructions of the polarization state of a 114.3 nm light source obtained by the interaction of a counter-rotating dual-color circularly polarized laser field with unaligned carbon dioxide molecules.

[0025] Figure 5 It is the absorption spectrum of carbon dioxide molecules in the wavelength range of 104 nm to 170 nm. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0027] This invention provides a method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source, comprising the following steps: Constructing a dual-color driven optical field: The output pulse provided by the femtosecond laser system is split into two beams. The beam with relatively lower energy is used as the fundamental frequency pulse, and the beam with relatively higher energy is converted into a second harmonic through a nonlinear crystal, serving as a second harmonic optical field, thus forming a frequency-driven optical field with frequencies of... ω 0 and 2ω 0 A two-color driving light field is constructed using two-color laser pulses. The two-color driving light field is a superposition field of the fundamental frequency light field and the second harmonic light field.

[0028] Energy and polarization state control: The fundamental frequency light field and the second harmonic light field in the dual-color driving light field are respectively subjected to energy adjustment and polarization control through a half-wave plate, a Brewster angle polarizer and a quarter-wave plate, so that the two beams are converted into circular polarization states before entering the action region, and the rotation directions are opposite, thus constructing a reverse rotating dual-color circularly polarized laser field.

[0029] Spatiotemporal coincidence control: Two beams of light are spatially combined using a dichroic mirror, and the optical path difference between the two beams is adjusted using a high-precision displacement device to ensure that the two beams are time-synchronized and spatially coincident at the focal point, forming a combined, counter-rotating, two-color circularly polarized laser. Specifically, the temporal and spatial coincidence of the counter-rotating, two-color circularly polarized laser field is achieved as follows: the two beams of the counter-rotating, two-color circularly polarized laser field achieve pulse synchronization in time, coaxial propagation in space, and coincidence at the same focal point.

[0030] Before focusing on the medium, ensure that the opposing rotating two-color circularly polarized driving field coinciding at the same focal point satisfies the circular polarization condition. The method for determining the circular polarization condition of the opposing rotating two-color circularly polarized driving field at the same focal point is as follows: by adjusting the polarization states of the two beams of the opposing rotating two-color circularly polarized laser field, the 3n-order harmonic signal is suppressed, thus ensuring that the opposing rotating two-color circularly polarized driving field coinciding at the same focal point satisfies the circular polarization condition.

[0031] Focusing and generation of higher harmonics: The reverse-rotating dual-color circularly polarized laser beam, after being combined, is focused at the gas nozzle by a reflector and interacts with atoms, ions, or unaligned molecular systems in a vacuum environment. By changing the frequency combination of the two beams of the reverse-rotating dual-color circularly polarized driving field and selecting a medium that matches the energy level of the two beams with the frequency combination, specific order harmonics with selective enhancement are obtained through a multiphoton resonance enhancement process, thereby realizing the output of quasi-monochromatic circularly polarized extreme ultraviolet light sources with different output wavelengths.

[0032] In the above process, when the energy of the driving photon matches the energy level difference between the ground state and the excited state of the medium, the multiphoton resonance absorption process is enhanced, resulting in a significant enhancement of specific order harmonics and thus forming quasi-monochromatic harmonic output. According to the symmetry selection rule for the interaction between the reverse-rotational bicolor circularly polarized driving field and atomic, ionic, or unaligned molecular systems, the 3n-order harmonics in the generated harmonics are suppressed. By adjusting the polarization states of the two driving beams, the 3n-order harmonic signal can be suppressed, thereby ensuring that the driving field at the focal point satisfies the circular polarization condition and achieving stable output of circularly polarized extreme ultraviolet harmonics.

[0033] The technical solution of the present invention will be further described below through specific embodiments. Unless otherwise specified, the methods described in the following embodiments are conventional methods; the reagents and materials described are commercially available unless otherwise specified.

[0034] Example 1 This embodiment uses a reverse-rotating dual-color circularly polarized driving field with a wavelength combination of 800nm ​​+ 400nm to drive unaligned carbon dioxide molecules, generating quasi-monochromatic circularly polarized extreme ultraviolet light. The experimental optical path diagram is shown below. Figure 2 As shown. In this embodiment, the specific steps for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source include:

[0035] Step S1: Construct a reverse-rotating dual-color circularly polarized laser field (800nm ​​+ 400nm) as the driving medium to generate a driving source for higher harmonics. Here, 800nm ​​is the frequency of the fundamental light field; 400nm is the frequency of the second harmonic light field.

[0036] Specifically, a frequency-doubled light field is generated by using the laser output pulse as the fundamental frequency light field and then performing frequency second harmonics. For example, a commercial Ti:sapphire laser system manufactured by Coherent Systems, Inc. can be used as the light source system. This system can output femtosecond pulses with a center wavelength of 800 nm, a pulse width of approximately 35 fs, a repetition rate of 1 kHz, and a maximum single pulse energy of 7 mJ.

[0037] A schematic diagram of the experimental optical path and apparatus for the interaction of a counter-rotating, two-color circularly polarized laser field with a gas medium in high-order harmonics, as shown below. Figure 2 As shown. Figure 2 In the diagram, Delay is an optical delay line used to precisely adjust the optical path difference between two beams; HP1 is a 400nm zero-order half-wave plate; HP2 is an 800nm ​​zero-order half-wave plate; QP1 is a 400nm zero-order quarter-wave plate; QP2 is an 800nm ​​zero-order quarter-wave plate; DM1 and DM2 are both dichroic mirrors that reflect 400nm light and transmit 800nm ​​light; BBO is... β Barium borate crystal is a nonlinear optical crystal; CM is a silver-plated mirror; Gas jet is a gas nozzle; Slit is a slit; Grating is a grating; MCP is a microchannel plate.

[0038] like Figure 2 The output pulse of the light source system is split into two beams by a beam splitter. The lower-energy beam serves as the fundamental frequency light field, while the higher-energy beam passes through... β The barium borate crystal is converted into a 400nm second harmonic wave, which serves as a frequency-doubled optical field.

[0039] The fundamental frequency light field passes sequentially through an 800nm ​​zero-order half-wave plate and an 800nm ​​Brewster angle polarizer. Its pulse energy can be adjusted by rotating the 800nm ​​zero-order half-wave plate. Subsequently, the fundamental frequency light field passes sequentially through an 800nm ​​zero-order half-wave plate (HP2) and an 800nm ​​zero-order quarter-wave plate (QP2). The polarization state of the fundamental frequency light field can be controlled by adjusting the angles of these two plates. The use of the Brewster angle polarizer ensures that the fundamental frequency light field remains linearly polarized before incident on HP2, facilitating subsequent control of the fundamental frequency light's polarization state.

[0040] The frequency-doubled optical field is also adjusted by a combination of a 400nm half-wave plate and a 400nm Brewster angle polarizer to regulate the pulse energy and ensure the linear polarization state. Then, the polarization state of the frequency-doubled optical field is adjusted by a 400nm zero-order half-wave plate (HP1) and a 400nm zero-order quarter-wave plate (QP1).

[0041] The fundamental and second-order light fields are re-beamed through a dichroic mirror that reflects 400nm and transmits 800nm. The optical path difference between the fundamental and second-order light fields is precisely controlled by a high-precision motorized stage to ensure temporal synchronization and spatial coincidence at the focal point. During the experiment, it is essential to ensure that the fundamental and second-order light fields remain coincident in both time and space.

[0042] Step S2: In a vacuum device, a reverse-rotating dual-color circularly polarized driving field with wavelength combination of 800nm+400nm is focused in an unarranged carbon dioxide molecule system. The relative intensity of the fundamental frequency light field and the second-harmonic frequency light field is adjusted to generate a quasi-monochromatic circularly polarized extreme ultraviolet light source with a wavelength of 114.3nm.

[0043] Specifically, the counter-rotating dual-color circularly polarized laser field is focused by a concave mirror, achieving a peak intensity of [insert value here]. At this time, it interacts with unaligned carbon dioxide molecules, producing higher harmonic radiation. By adjusting the relative pulse energy of the fundamental and harmonic fields, the relative intensity of different orders in the harmonic spectrum was optimized. In the experiment, the seventh harmonic was significantly enhanced, forming an output peak dominated by a single frequency component in the harmonic spectrum. The experimental results are as follows: Figure 3 As shown. Furthermore, in this embodiment of the invention, a higher harmonic polarization measurement device (rotating gold mirror group) is used to characterize the polarization state of the seventh harmonic, reconstructing the ellipsoid of the seventh harmonic, as shown in the figure. Figure 4 As shown.

[0044] In this embodiment, a quasi-monochromatic circularly polarized extreme ultraviolet light source with a wavelength of 114.3 nm was obtained, achieving a monochromaticity of 0.93 and an ellipticity of 0.92. Figure 3 and Figure 4 As shown, it can be regarded as a quasi-monochromatic circularly polarized extreme ultraviolet ultrashort pulse light source.

[0045] The following explains the principle of using a reverse-rotating dual-color circularly polarized laser field to drive unaligned carbon dioxide molecules to generate a 114.3nm quasi-monochromatic circularly polarized extreme ultraviolet light source.

[0046] When the frequency of the driving optical field is tuned to resonate with a specific electronic transition channel, the probability of an electron transitioning to an excited state through multiphoton absorption increases significantly. At this point, the electron can effectively transition to a specific excited state through multiphoton resonant absorption. The total energy of the absorbed photons must satisfy the energy difference required for the electron transition: nω = E. excited -E ground Where nω represents the total photon energy required for an electron to transition from the ground state to the excited state; E excited E represents the energy of an electron in an excited state. groundThis represents the energy of an electron in its ground state. Based on the crucial role of excited states in the generation of higher harmonics, the frequency of the driving optical field can be tuned to resonate with a specific electron transition channel, thereby significantly enhancing the multiphoton absorption process and significantly enhancing specific orders of higher harmonics, producing quasi-monochromatic extreme ultraviolet higher harmonics. The absorption spectrum of carbon dioxide molecules in the 104 nm–170 nm wavelength range is shown below. Figure 5 As shown in the reference: C. Kuo, Y. Chen, S. Wang, S. Li, J. Nee. The Photoabsorption Spectrum of CO2 at 104-170 nm. Chinese Journal of Physics, 2004, 42(1):65–73.). This molecule has abundant excited states (3pπ states) near the 114 nm wavelength. Electrons can be resonantly excited to the 3pπ state by absorbing three 800 nm photons and two 400 nm photons, and then re-transition back to the ground state in the subsequent evolution process, radiating higher harmonics with a wavelength of 114 nm. By adjusting the relative pulse energy of the fundamental frequency light field and the harmonic light field, and changing the relative number of fundamental frequency photons and harmonic photons, the single-photon radiation transition to the ground state photon channel can be optimized, and a high monochromatic extreme ultraviolet light source can be obtained.

[0047] In one embodiment, the wavelength combination of the driving laser and the interaction medium can be selected according to actual application requirements. Based on the target extreme ultraviolet (EUV) output wavelength, an atomic, ionic, or unaligned molecular system with a corresponding energy level structure is selected as the interaction medium, ensuring that the driving photon energy and the energy level difference between the ground state and excited state of the medium satisfy the matching condition. Preferably, the medium contains specific excited states to enhance relevant transition channels, thereby facilitating the selective enhancement of specific harmonics and achieving quasi-monochromatic circularly polarized EUV output at the desired wavelength.

[0048] In one embodiment, the output efficiency and monochromaticity of quasi-monochromatic circularly polarized extreme ultraviolet light can be improved by adjusting the relative intensity of the fundamental frequency light field and its harmonics. Since the driving field consists of the fundamental frequency component and its harmonics, adjusting the relative pulse energy of the two beams can change the relative ratio of photons of the two frequencies, thereby affecting the contribution weights of different transition channels. By optimizing the relative intensity, the radiation efficiency of the target harmonic can be enhanced, and the dominance and polarization purity of the spectral lines can be improved.

[0049] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source, characterized in that, Includes the following steps: A reverse-rotating dual-color circularly polarized laser field is constructed as the driving light source; the reverse-rotating dual-color circularly polarized laser field is a superposition field of reverse-rotating circular polarization composed of the fundamental frequency light field and the second harmonic light field. In a vacuum environment, a reverse-rotating dual-color circularly polarized laser field is superimposed in time and space and focused into a medium to generate high-order harmonic radiation; by adjusting the relative intensity of the fundamental frequency light field and the frequency-doubled light field, a quasi-monochromatic circularly polarized extreme ultraviolet light source is generated; the medium is an atomic, ionic, or unarranged molecular system.

2. The method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source according to claim 1, characterized in that, The method for constructing a reverse-rotating two-color circularly polarized laser field is as follows: The output pulse is split into a first pulse and a second pulse by a beam splitter. The energy of the first pulse is lower than that of the second pulse. The first pulse is used as the fundamental frequency optical field, and the second pulse is converted into a second harmonic by a nonlinear crystal as a second harmonic optical field to form a two-color laser pulse, thus constructing a two-color driving optical field. The energy and polarization state of the dual-color driving light field are adjusted and controlled by a half-wave plate, a Brewster angle polarizer, and a quarter-wave plate, respectively. This causes the fundamental frequency light field and the second harmonic light field of the dual-color driving light field to be converted into circular polarization states before entering the action region, with opposite rotation directions, thus constructing a reverse-rotating dual-color circularly polarized laser field.

3. The method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source according to claim 2, characterized in that, The dual-color laser pulses are frequency combinations of... ω 0 and 2ω 0 Two-color laser pulses; a reverse-rotating two-color circularly polarized laser field has a frequency combination of... ω 0 and 2ω 0 A reverse-rotating dual-color circularly polarized laser field; ω 0 Indicates the frequency of the fundamental frequency optical field; 2ω 0 This indicates the frequency of the second harmonic optical field.

4. The method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source according to claim 1, characterized in that, The method to make the reverse-rotated two-color circularly polarized laser fields coincide in time and space is as follows: Two beams of a reverse-rotating dichromatic circularly polarized laser field are spatially combined using a dichroic mirror. By adjusting the optical path difference between the two beams, the reverse-rotating dichromatic circularly polarized laser fields are made to coincide in time and space, forming a combined reverse-rotating dichromatic circularly polarized laser beam.

5. The method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source according to claim 4, characterized in that, The time and space overlap of the reverse-rotating two-color circularly polarized laser field is as follows: the two beams of the reverse-rotating two-color circularly polarized laser field achieve pulse synchronization in time, achieve coaxial propagation in space, and overlap at the same focal point.

6. The method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source according to claim 5, characterized in that, The method for determining the circular polarization condition of a counter-rotating two-color circularly polarized driving field coinciding at the same focal point is as follows: By adjusting the polarization states of the two beams of the reverse-rotating dual-color circularly polarized laser field, the 3n-order harmonic signal is suppressed, so that the reverse-rotating dual-color circularly polarized driving field coinciding at the same focal point satisfies the circular polarization condition.

7. The method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source according to claim 4, characterized in that, The method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source is: In a vacuum environment, a combined, reverse-rotating, two-color circularly polarized laser beam is focused into a medium through a mirror and interacts with atoms, ions, or unaligned molecular systems in the medium to generate higher harmonic radiation. By changing the frequency combination of the two beams of light in the reverse-rotating dual-color circularly polarized driving field and selecting a medium that matches the energy level of the two beams of light with the frequency combination, selectively enhanced specific order harmonics are obtained through a multiphoton resonance enhancement process, thereby realizing the output of quasi-monochromatic circularly polarized extreme ultraviolet light sources with different output wavelengths.

8. The method for generating a quasi-monochromatic circularly polarized extreme ultraviolet light source according to claim 7, characterized in that, The frequency combination of the two beams of light in the reverse rotating dual-color circularly polarized driving field is 800nm ​​and 400nm. The medium that matches the energy level of the two beams of light with the frequency combination is an unarranged carbon dioxide molecular system. The output wavelength of the quasi-monochromatic circularly polarized extreme ultraviolet light source is 114.3nm.

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