A device and method for generating high-order harmonics based on a four-band aperture mirror

Abstract

This invention provides a high-order harmonic generation device based on four-aperture mirrors, comprising: a driving laser module, a first aperture mirror, a second aperture mirror, a first beam collector, a focusing mirror, a gas target, a third aperture mirror, a second beam collector, a fourth aperture mirror, a third beam collector, a metal film, a vacuum cavity, and a vacuum module. The first aperture mirror, the second aperture mirror, the focusing mirror, the gas target, the third aperture mirror, the fourth aperture mirror, and the metal film are disposed within the vacuum cavity. Through the synergistic effect of the four aperture mirrors, ring shaping, two-stage spatial filtering and efficient separation of the driving laser, as well as lossless transmission of high-order harmonics are achieved. This not only successfully attenuates the hundred-watt-level driving laser by more than a thousand times, solving the problem of optical component damage under high power; but also ensures high-throughput output of high-order harmonics while achieving efficient separation. The generated harmonic spectrum has a wide range and a small divergence angle, making it ideal as a light source for time-resolved pump-probe experiments.

Description

Technical Field

[0001] This invention relates to the fields of ultrafast laser and attosecond science and technology, specifically to high-order harmonic generation technology, and more specifically to a high-order harmonic generation device and method based on a four-band aperture mirror. Background Technology

[0002] The generation of higher harmonics is a key technology for obtaining coherent light sources in the extreme ultraviolet to soft X-ray bands, and is a core tool for cutting-edge scientific research such as time-resolved and angle-resolved photoelectron spectroscopy and attosecond transient absorption spectroscopy. However, the generation of higher harmonics is a high-order nonlinear process with extremely low conversion efficiency (typically 10⁻⁻⁶). 7 ~10⁻ 8 (Scale). Traditional high-order harmonic generation schemes use driving lasers with low repetition frequency (kHz level) and high single-pulse energy (mJ level), and the average harmonic power generated is usually only in the μW level, which severely limits its application range.

[0003] To improve high-order harmonic power, research on high-order harmonic generation has gradually shifted towards using driving lasers with high repetition rates (hundreds of kHz to MHz) and high average power (hundreds of watts). However, this approach presents a key challenge: the generated high-order harmonics propagate collinearly with the high-power driving laser, necessitating efficient separation of the weak harmonic signals from the powerful background laser. Currently, the most common separation method is using metal films; however, when the average power of the driving laser reaches the hundreds of watts, even after initial attenuation, the residual driving laser power is still sufficient to melt or damage the metal film instantaneously. To address this, the industry has proposed pre-attenuation methods such as introducing grazing incidence plates, diffraction gratings, or microchannel plates in front of the metal film. However, these methods often result in significant losses of high-order harmonic power, contradicting the initial goal of increasing output power.

[0004] To maximize the preservation of harmonic power while separating the driving laser, a dual-aperture mirror scheme has emerged. In this scheme, the first aperture mirror shapes the driving light into a ring beam, and the second aperture mirror is positioned at the image plane of the focusing lens to reflect the ring driving light, while allowing higher harmonics to pass through its central aperture without loss. However, although this scheme can preserve harmonic power to a high degree, it still has the following inherent limitations:

[0005] 1) The diffraction effect of the aperture mirror will produce a significant Poisson bright spot, resulting in a still high residual laser power, which poses a threat to downstream components;

[0006] 2) The system has poor flexibility. The second aperture lens must be placed on the image plane, which makes it difficult to adapt to focusing lenses with different focal lengths or meet the optical path matching requirements in pump-probe experiments.

[0007] 3) Under high power conditions, the deformation of the light spot caused by gas ionization will further degrade the separation effect and make the system performance unstable.

[0008] Therefore, there is an urgent need in the field for a new device and method that can more efficiently attenuate high-intensity driven lasers and possess greater system flexibility and robustness. It should be noted that this background information is only used to introduce relevant information about the present invention to help understand the technical solutions of the present invention, but does not mean that the relevant information is necessarily prior art. In the absence of evidence that the relevant information was disclosed before the filing date of this invention, the relevant information should not be considered prior art. Summary of the Invention

[0009] Therefore, the purpose of this invention is to overcome the shortcomings of the prior art and provide a high-order harmonic generation device and method based on a four-strip-aperture mirror. This objective is achieved through the following technical solution:

[0010] According to a first aspect of the present invention, a high-harmonic generation device based on a four-aperture mirror is provided. The device includes: a driving laser module, a first aperture mirror, a second aperture mirror, a first beam collector, a focusing mirror, a gas target, a third aperture mirror, a second beam collector, a fourth aperture mirror, a third beam collector, a metal film, a vacuum cavity, and a vacuum module. The first aperture mirror, the second aperture mirror, the focusing mirror, the gas target, the third aperture mirror, the fourth aperture mirror, and the metal film are disposed within the vacuum cavity. The driving laser module provides a femtosecond driving light source with high repetition rate and high average power. The first aperture mirror is a plane mirror with a central first through-hole, disposed in the incident light path of the driving laser, used to split the incident driving laser beam into transmitted pump light and reflected ring-shaped driving laser. The second aperture mirror is a plane mirror with a central second through-hole, disposed in the reflected light path of the first aperture mirror, used to perform spatial filtering on the ring-shaped driving laser to eliminate Poisson spots generated by ring-shaped driving laser diffraction and generate a ring-like driving laser. The first beam collector is used for… The system comprises: a second aperture mirror for collecting the driving laser transmitted or reflected into the vacuum; a focusing mirror positioned after the second aperture mirror for focusing the filtered, ring-shaped driving laser onto a gas target; a gas target positioned at the focal point of the focusing mirror for providing the rare gas required for high-order harmonic generation; a third aperture mirror, a plane mirror with a central third through-hole, positioned at the image plane of the focusing mirror for reflecting the driving laser out of the vacuum cavity and allowing high-order harmonics and some diffracted laser to pass collinearly through the third through-hole; a second beam collector for collecting the driving laser reflected into the vacuum from the third aperture mirror; a fourth aperture mirror, a plane mirror with a central fourth through-hole, positioned downstream of the transmission path of the third aperture mirror for filtering the beam passing through the third through-hole to attenuate the diffracted laser in the high-order harmonics; a third beam collector for collecting the driving laser reflected into the vacuum from the fourth aperture mirror; a metal film positioned after the fourth aperture mirror for filtering out the remaining laser in the high-order harmonics after passing through the fourth aperture mirror; and a vacuum module for evacuating the vacuum cavity to provide a vacuum environment.

[0011] Preferably, the driving laser is a femtosecond pulsed laser with a repetition frequency greater than or equal to 100 kHz, an average power greater than or equal to 100 W, and a pulse width less than or equal to 400 fs.

[0012] Preferably, the incident angles of the first, second, third, and fourth aperture mirrors are 45°, the angles of the first, second, third, and fourth through holes are 45°, each through hole is cylindrical or conical, and each aperture mirror surface is coated with a high-reflectivity broadband dielectric film with a reflectivity greater than or equal to 99.9%, wherein the typical reflection band is 900-1100nm.

[0013] Preferably, the optical path between the first aperture mirror and the second aperture mirror is set to match the optical path between the pump light and the probe light in the pump experiment, and the aperture size of the first through hole and the second through hole is determined according to the power of the pump light probe experiment.

[0014] Preferably, the second aperture lens is positioned at the object plane of the focusing lens, and the third aperture lens is positioned at the image plane of the focusing lens. The ratio of the aperture diameters of the second and third apertures is equal to the imaging magnification of the focusing lens. The positional relationship between the second aperture lens, the third aperture lens, and the focusing lens satisfies the following constraints:

[0015]

[0016]

[0017] in, This indicates the distance between the second aperture lens and the focusing lens. This indicates the distance between the third-aperture lens and the focusing lens. The focal length of the focusing lens. The diameter of the third through hole. The diameter of the second through hole, is the magnification factor of the focusing lens.

[0018] Preferably, the diameter of the fourth through hole is larger than that of the third through hole so that higher harmonics can pass through the fourth through hole without damage. The fourth through hole mirror is mounted on a triaxial displacement stage and its position can be adjusted as needed.

[0019] Preferably, the focusing lens has a transmission or reflection structure, and the focusing lens surface type is one or more of the following: spherical convex lens, concave mirror, off-axis parabolic mirror, ellipsoidal concave mirror, preferably a spherical convex lens or a concave mirror.

[0020] Preferably, the gas target is one or more of the following: a continuous gas pool, a gas nozzle with a gas collection port, or a semi-infinite gas target; the rare gas is one or more of the following: helium, neon, argon, krypton, or xenon, preferably argon.

[0021] Preferably, the metal film is one or more of the following: Al, Zr, Ti, Zn, Mo, Sn, with a thickness of 100~300nm; the first beam collector, the second beam collector, and the third beam collector are one or more of the following: an absorption cone-shaped water-cooled collector, a water-cooled baffle, and an air-cooled baffle.

[0022] Preferably, the diameter of the first through hole is 2mm, and the distance between the first and second aperture lenses is 2030mm; the diameter of the second through hole is 2mm, and the distance between the third aperture lens and the focusing lens is 1289mm; the focal length of the focusing lens is 400mm, and the distance between the focusing lens and the third aperture lens is 585mm; the diameter of the third through hole is 1mm, and the distance between the third and fourth aperture lenses is 600mm; the diameter of the fourth through hole is 4mm, and the distance between the fourth aperture lens 6 and the metal film 7 is 70mm.

[0023] According to a second aspect of the present invention, a method for generating higher harmonics using the apparatus described in the first aspect of the present invention is provided, comprising the following steps: S1, generating a driving laser using a driving laser module; S2, splitting the driving laser beam into a pump beam and a ring driving laser using a first aperture mirror; S3, spatially filtering the ring driving laser beam using a second aperture mirror to eliminate Poisson spots generated by ring driving laser diffraction and generate a ring-like driving laser, and focusing the filtered ring-like driving laser onto a gas target to generate higher harmonics; S4, performing primary separation using a third aperture mirror to reflect the driving laser, so that the higher harmonics and part of the diffracted laser pass collinearly through its central aperture; S5, performing secondary separation using a fourth aperture mirror to attenuate the collinearly propagating diffracted laser; S6, filtering out the remaining residual driving laser in the higher harmonics using a metal film to obtain a pure higher harmonic beam. Compared with the prior art, the advantages of the present invention are:

[0024] 1. High-efficiency attenuation: Through the two-stage separation design of the third and fourth aperture mirrors, the attenuation capability of the driving laser reaches 10. - With a capacity on the order of 3, it can handle high-power lasers of over 100W while effectively protecting the metal film. It not only successfully attenuates driving lasers in the hundreds of watts range by more than a thousand times, but also solves the problem of damage to optical components under high power.

[0025] 2. Lossless transmission: The apertures of the third and fourth aperture mirrors are designed to ensure 100% passage of higher harmonics (with a divergence half-angle typically of 1~3 mrad), guaranteeing the output efficiency of higher harmonics. In other words, while achieving efficient separation, high-throughput output of higher harmonics is ensured.

[0026] 3. High flexibility: The first aperture mirror can be used to adjust the optical path of the pump light; the position of the fourth aperture mirror can be flexibly arranged according to the structure of the vacuum chamber, reducing the stringent requirements on the mechanical design of the vacuum chamber of the device.

[0027] 4. High robustness: Even if the light spot is deformed due to factors such as gas ionization, the secondary filtering of the fourth aperture mirror can still provide effective supplementary separation capability, resulting in high device stability. The device maintains stable high performance even in real-world environments with non-ideal object-image relationships and ionization effects.

[0028] 5. High practicality: The generated harmonic spectrum has a wide range and a small divergence angle, making it very suitable as an ideal light source for time-resolved pump-probe experiments. Attached Figure Description

[0029] The embodiments of the present invention will be further described below with reference to the accompanying drawings, wherein:

[0030] Figure 1 This is a schematic diagram of the system framework of a high-order harmonic generation device based on a four-strip-aperture mirror according to an embodiment of the present invention;

[0031] Figure 2 This is a schematic diagram of the optical path of a high-order harmonic generation device based on a four-strip-aperture mirror using a transmission-type focusing optical path according to an embodiment of the present invention.

[0032] Figure 3 This is a schematic diagram of the optical path of a high-order harmonic generation device based on a four-band aperture mirror with a reflective focusing optical path structure according to an embodiment of the present invention.

[0033] Figure 4 This is a schematic diagram illustrating the simulated evolution of the laser spot during propagation according to an embodiment of the present invention.

[0034] Figure 5 This is a simulated light intensity distribution diagram of the driving laser in front of and behind each aperture mirror according to an embodiment of the present invention.

[0035] Figure 6 The image shows a high-order harmonic spectrum obtained experimentally according to an embodiment of the present invention. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the invention.

[0037] As mentioned in the background section, existing high-harmonic generation schemes using dual-aperture mirrors have the following inherent limitations: 1) The diffraction effect of the aperture mirror will produce a significant Poisson bright spot, resulting in a still high residual laser power, which poses a threat to downstream components; 2) The system has poor flexibility, as the second aperture mirror must be placed on the image plane, making it difficult to adapt to focusing mirrors with different focal lengths or meet the optical path matching requirements in pump-probe experiments; 3) Under high power conditions, the spot deformation caused by gas ionization will further degrade the separation effect, making the system performance unstable.

[0038] In view of this, the present invention proposes a novel scheme for generating higher harmonics. This scheme employs a four-aperture mirror combination. Through the synergistic effect of the four aperture mirrors, ring shaping of the driving laser, two-stage spatial filtering and efficient separation, and lossless transmission of higher harmonics are achieved. In summary, as... Figure 1 As shown, the high-harmonic generation device based on a four-aperture mirror of the present invention includes: a driving laser module, a first aperture mirror, a second aperture mirror, a first beam collector, a focusing mirror, a gas target, a third aperture mirror, a second beam collector, a fourth aperture mirror, a third beam collector, a metal film, a vacuum cavity, and a vacuum module. The first aperture mirror, the second aperture mirror, the focusing mirror, the gas target, the third aperture mirror, the fourth aperture mirror, and the metal film are disposed within the vacuum cavity. The driving laser module provides a femtosecond driving light source with high repetition rate and high average power. The first aperture mirror is a plane mirror with a first through-hole in the center, disposed in the incident light path of the driving laser, used to split the incident driving laser beam into transmitted pump light and reflected ring driving laser. The second aperture mirror is a plane mirror with a second through-hole in the center, disposed in the reflected light path of the first aperture mirror, used to perform spatial filtering on the ring driving laser to eliminate the Poisson spot generated by ring driving laser diffraction and generate a ring-like driving laser. The first beam collector collects light from the second aperture mirror. The system consists of a third and fourth beam collector. The first beam collector receives the driving laser reflected from the third beam collector into the vacuum cavity. The second beam collector receives the driving laser reflected from the third beam collector into the vacuum cavity. The third beam collector receives the driving laser reflected from the fourth beam collector into the vacuum cavity. The fourth beam collector receives the driving laser reflected from the fourth beam collector into the vacuum cavity. The fifth beam collector receives the driving laser reflected from the fourth beam collector into the vacuum cavity. The sixth beam collector receives the driving laser reflected from the fourth beam collector into the vacuum cavity. The seventh beam collector receives the driving laser reflected from the fourth beam collector into the vacuum cavity. The fifth beam collector receives the driving laser received from the fourth beam collector into the vacuum cavity. The sixth beam collector receives the driving laser received from the fourth beam collector into the vacuum cavity. The seventh beam collector receives the driving laser received from the fourth beam collector into the vacuum cavity. The eighth beam collector receives the driving laser received from the fourth beam collector into the vacuum cavity. The ninth beam collector receives the driving laser reflected from the fourth beam collector into the vacuum cavity. The tenth ...

[0039] To better understand the present invention, the main components of the device of the present invention will be described in detail below.

[0040] The high-repetition-rate, high-power driving laser generated by the driving laser module of this invention is a high-repetition-rate, high-average-power driving laser output from a Yb fiber femtosecond amplified laser. Typical parameters are: center wavelength 1030 nm, repetition rate ≥ 100 kHz, average power ≥ 100 W, pulse width ≤ 400 fs, and output Gaussian spot. According to one embodiment of this invention, the laser parameters used to generate the high-repetition-rate, high-power driving laser are: center wavelength 1030 nm, repetition rate 500 kHz, average power 208 W, pulse width 170 fs, incident spot is Gaussian, and diameter 10 mm (1 / e²). To stabilize the beam direction under high power, this invention sets the laser output to first pass through a beam direction locking system (…). Figure 1 (Not shown in the text).

[0041] The driving laser enters the vacuum cavity through an incident window. The vacuum cavity and vacuum pipes are evacuated by a vacuum pump. The vacuum cavity and vacuum pipes are connected by a vacuum gate valve. Since the vacuum cavity is a technology known to those skilled in the art, the vacuum cavity will not be described in detail in this embodiment. In one embodiment of the present invention, the vacuum cavity is a combination of a square vacuum cavity and a circular vacuum cavity, the vacuum pipe is a combination of a cylindrical pipe and a shaped corrugated pipe, and the vacuum pump is a combination of a turbine dry pump and a vortex molecular pump, thereby maintaining the vacuum level of this part at 1×10⁻⁶. -6 mbar~1×10 -8 Within the mbar range. The core optical path of the entire device of this invention, except for the beam collector, is encapsulated in a vacuum cavity (such as a stainless steel cavity) to prevent air from absorbing the high harmonics in the extreme ultraviolet band and to avoid pulse broadening of the driving laser during transmission; the sidewall of the vacuum cavity is provided with multiple entrance and exit ports for the introduction and extraction of the driving laser.

[0042] The first apertureed mirror of this invention has a first through-hole with a diameter of d1 at its center. This mirror is located in the incident light path of a high-power driving laser and is used to split the laser beam, so that part of the laser beam is transmitted through its first through-hole and used as pump light; the other part of the laser beam is reflected by the mirror to form a ring-shaped driving light. By designing the diameter d1 of the first through-hole, the power ratio of transmitted and reflected light can be precisely controlled to meet the needs of different pump-probe experiments. According to one embodiment of the present invention, the diameter of the first through-hole is d1 = 2 mm, and the distance between it and the second apertureed mirror is z1 = 2030 mm, which is used to achieve pump / probe beam splitting, and the power ratio of reflected light to transmitted light is approximately 12:1.

[0043] The second aperture lens of this invention has a second through-hole with a diameter of d2 at its center. This lens is located on the reflected light path of the first aperture lens, with a distance of z1 between them, and is used for primary spatial filtering of the ring-shaped driving light. Due to the diffraction effect of the first aperture lens, the reflected ring light will exhibit a strong Poisson spot at its center after propagating a very short distance. The second aperture diameter d2 of the second aperture lens is designed to allow the ring beam to pass through while blocking the central Poisson spot, thereby filtering out most of the diffracted light and outputting a higher-quality ring-like beam. According to one embodiment of the invention, the diameter of the second through-hole d2 = 2 mm, and the distance between it and the focusing lens z2 = 1289 mm, is used for primary spatial filtering of the ring light to eliminate the Poisson spot.

[0044] The focusing lens of this invention is located after the second aperture lens, with a focal length of f. It is used to efficiently focus the filtered ring-driven laser onto the gas target. It typically employs a spherical convex lens or a concave mirror, with a typical focal length of 100-500 mm, a focused spot diameter of 20-100 μm, and a peak power density at the focal point greater than 1 × 10¹. 4 W / cm². According to one embodiment of the invention, the focal length of the focusing lens is f = 400 mm, the distance between it and the third aperture lens is z3 = 585 mm, and the peak power density at the focal point is approximately 2 × 10⁻⁶ W / cm². 14 W / cm 2 .

[0045] The gas target of this invention is positioned at the focal point of the focusing lens to provide the rare gas required for high-order harmonic generation. The gas target is typically a gas nozzle with a gas collecting port. By injecting a rare gas (such as argon or krypton) into the vacuum chamber, a localized high-density gas region is formed at the laser focal point, while the background gas pressure is maintained in the range of 0.1~2 bar. Here, a high peak power density (~10¹) is achieved. 4 A laser with a power density of W / cm² interacts with a gas to efficiently radiate high-order harmonics in the extreme ultraviolet band through a high-order harmonic generation process. According to one embodiment of the present invention, the gas target structure is a gas nozzle with a gas collecting port, the selected rare gas is argon, and the background pressure is 1 bar.

[0046] The third aperture lens of this invention has a third through-hole with a diameter of d3 at its center. This lens is positioned at the image plane of the focusing lens, with a distance of z3 between it and the focusing lens, such that 1 / z2 + 1 / z3 = 1 / f. According to the principle of geometrical optics imaging, the driving laser stop (i.e., the second aperture lens) before focusing is imaged at this position. Therefore, the ratio of the diameter of the third through-hole to the diameter of the second through-hole should satisfy d3 / d2 = z3 / z2 = M, where M is the magnification factor of the focusing lens, and the diameter d3 should be sufficient for higher harmonics to pass through. However, due to installation errors and lens processing, the diameter of the through-hole may not perfectly satisfy the above ratio. But based on the scheme of this invention, this has little impact on the separation effect of the driving laser, demonstrating the strong robustness of this device and scheme. Most of the ring-shaped driving laser is reflected by the mirror surface of the third aperture lens and exited outside the vacuum cavity. Simultaneously, since the higher harmonics are generated at the focal point and have a very small divergence angle (typically 1~3 mrad), all their energy can continue to be transmitted through the third through-hole with a suitable diameter d3, thereby achieving primary separation. According to one embodiment of the present invention, the diameter of the third through hole is d3=1mm, and the distance between it and the fourth perforated mirror is z4=600mm.

[0047] The fourth aperture mirror of this invention has a fourth through-hole at its center, with a diameter of d4. This mirror is located downstream of the third aperture mirror, with a distance of z4 between them. After separation by the third aperture mirror, a small amount of residual driving laser light, due to diffraction and spot deformation, still propagates collinearly with higher harmonics. The function of the fourth aperture mirror is to perform secondary spatial filtering. The diameter d4 of its fourth through-hole is larger than the diameter d3 of the third through-hole, ensuring that higher harmonics can pass through 100% without obstruction. Simultaneously, for the residual driving laser light, since it has already propagated a certain distance and diverged, the larger aperture edge can further block its spatial distribution, thereby significantly attenuating its power. According to one embodiment of the present invention, the diameter of the fourth through-hole is d4 = 4 mm, and the distance between it and the metal film is z5 = 70 mm.

[0048] It should be noted that, theoretically, the second and third aperture lenses should strictly satisfy the object-image relationship 1 / z2 + 1 / z3 = 1 / f, and the aperture ratio d3 / d2 = z3 / z2. However, in this embodiment, due to the limited internal space of the vacuum cavity, it is not necessary to strictly satisfy the above relationship. Only the axial position of the third aperture lens needs to be finely adjusted. When z3 = 585 mm, the transmission power is minimized, thus the imaging relationship has been optimized.

[0049] The metal film of this invention is located after the fourth aperture mirror, with a spacing of z5 from the third aperture mirror. It is used to finally filter out the remaining trace amount of driving laser light after two stages of attenuation, outputting pure high-order harmonics. The material can be selected from Al, Zr, Ti, Zn, Mo, Sn, etc., depending on the wavelength of the high-order harmonics, and the thickness is typically 100~300 nm. According to one embodiment of this invention, the metal film is a 200 nm thick Al film, which has 0 transmittance for low-power 1030 nm driving laser light and 0.2~0.6 transmittance for high-order harmonics of 20~50 eV.

[0050] The multiple beam collectors of this invention are used to collect the driving laser beams transmitted or reflected by each apertured mirror to the outside of the vacuum cavity. Since high-power driving lasers can cause strong thermal absorption, a cone-shaped water-cooled collector is typically used to effectively dissipate heat while collecting the driving laser beams.

[0051] As can be seen, the core of this invention lies in the use of four aperture mirrors and one focusing mirror. All four aperture mirrors are plane mirrors with an incident angle of 45° and a central through-hole angle of 45°, and the through-hole type is conical. Their surfaces are coated with a high-reflectivity broadband dielectric film, possessing a high damage threshold and low dispersion coefficient, a reflection band of 900~1100nm, and a reflectivity ≥99.9%. The third and fourth aperture mirrors are mounted on a three-axis displacement stage, allowing for precise position adjustment. The focusing mirror adopts a transmission structure with a spherical convex lens surface. The device of this invention, by introducing the second and fourth aperture mirrors, constitutes an innovative two-stage spatial filtering mechanism: the second aperture mirror filters out the Poisson spot generated by the first aperture mirror before the imaging optical path, creating ideal beam conditions for the efficient separation of the third aperture mirror; the fourth aperture mirror, after the imaging optical path, removes residual laser light that the third aperture mirror could not completely process, greatly enhancing robustness to high power and spot deformation conditions. Furthermore, the optical path z1 between the first and second aperture mirrors of this invention is adjustable. This design allows the device to flexibly match the optical path of the pump optical path and the probe optical path, greatly improving its applicability and ease of use in time-resolved pump-probe experiments. Simultaneously, the position of the fourth aperture mirror can be flexibly arranged according to the actual mechanical structure of the vacuum cavity, breaking through the strict limitations on the image plane position in traditional dual-mirror schemes.

[0052] To provide a more intuitive understanding of the present invention, the invention will be described below with reference to two specific embodiments.

[0053] Example 1

[0054] Figure 2A schematic diagram of the optical path of a high-harmonic generation device based on four-aperture mirrors with a transmission-type focusing optical path structure is shown. The device includes: a high-repetition-rate, high-power driving laser; a first aperture mirror 1; a second aperture mirror 2; a focusing mirror 3; a gas target 4; a third aperture mirror 5; a fourth aperture mirror 6; a metal film 7; a beam collector 8; a beam collector 9; a beam collector 10; a vacuum cavity; and a vacuum module. The first aperture mirror 1, the second aperture mirror 2, the focusing mirror 3, the gas target 4, the third aperture mirror 5, the fourth aperture mirror 6, and the metal film 7 are located within the vacuum cavity.

[0055] The driving laser enters the vacuum cavity through the entrance window and passes successively through the first aperture mirror 1, the second aperture mirror 2, and the focusing mirror 3, generating high-order harmonics at the gas target 4. These high-order harmonics then pass sequentially through the third aperture mirror 5, the fourth aperture mirror 6, and the metal film 7, thus separating the driving laser in each stage. The driving laser transmitted through the second aperture mirror 2 and reflected by the third aperture mirror 5 and the fourth aperture mirror 6 enters the beam collector outside the vacuum cavity through the exit window.

[0056] A gas target 4 is positioned at the focal point of the focusing lens 3 to provide the rare gas required for the generation of higher harmonics. In this embodiment, the gas target structure is a gas nozzle with a gas collecting port, and the selected rare gas is argon, with a background pressure of 1 bar.

[0057] Metal film 7 is a 200 nm thick Al film, which has a transmittance of 0 for low-power 1030 nm driving laser and a transmittance of 0.2 to 0.6 for high-order harmonics of 20 to 50 eV.

[0058] The core of this device consists of four perforated mirrors and one focusing lens. All four perforated mirrors are plane mirrors with an incident angle of 45° and a central through-hole angle of 45°. The through-holes are conical in shape. Their surfaces are coated with a high-reflectivity broadband dielectric film, exhibiting a high damage threshold and low dispersion coefficient. The reflection wavelength range is 900~1100nm, and the reflectivity is ≥99.9%. The third and fourth perforated mirrors (5 and 6) are mounted on a three-axis displacement stage, allowing for precise position adjustment. The focusing lens (3) adopts a transmission structure with a spherical convex lens surface. Its specific parameters are as follows: First perforated mirror 1: First through-hole diameter d1 = 2mm, distance from the second perforated mirror 2 z1 = 2030mm, used for pump / probe beam splitting, with a reflected light to transmitted light power ratio of approximately 12:1. Second perforated mirror 2: Second through-hole diameter d2 = 2mm, distance from the focusing lens 3 z2 = 1289mm, used for primary spatial filtering of the ring light to eliminate Poisson spots. Focusing lens 3: focal length f=400mm, distance from the third aperture lens 5 z3=585mm, peak power density at the focal point is approximately 2×10⁻⁶. 14 W / cm 2Third aperture mirror 5: The diameter of the third through hole is d3=1mm, and the distance between it and the fourth aperture mirror 6 is z4=600mm. Fourth aperture mirror 6: The diameter of the fourth through hole is d4=4mm, and the distance between it and the metal film 7 is z5=70mm.

[0059] In this embodiment, the vacuum chamber is a combination of a square vacuum chamber and a circular vacuum chamber, the vacuum pipe is a combination of a cylindrical pipe and a shaped corrugated pipe, and the vacuum pump is a combination of a turbine dry pump and a vortex molecular pump, thereby maintaining the vacuum level of this part at 1×10⁻⁶ in this embodiment. -6 mbar~1×10 -8 Within the mbar range.

[0060] The laser parameters used to generate high-repetition-rate, high-power driven lasers are: center wavelength 1030 nm, repetition rate 500 kHz, average power 208 W, pulse width 170 fs, incident spot is Gaussian type, diameter 10 mm (1 / e²). To stabilize the beam direction under high power, the laser output first passes through a beam pointing locking system.

[0061] Example 2

[0062] Figure 3 This diagram illustrates the optical path of a high-harmonic generation device based on a four-band aperture mirror with a reflective focusing optical path structure. Embodiment Two modifies the device based on Embodiment One. Figure 3 As can be seen, the device includes: a high-repetition-rate, high-power driven laser, a first aperture mirror 1', a second aperture mirror 2', a focusing mirror 3', a gas target 4', a third aperture mirror 5', a fourth aperture mirror 6', a metal film 7', a beam collector 8', a beam collector 9', a beam collector 10', a plane mirror 11, a vacuum cavity, and a vacuum module. The core difference between this embodiment and Embodiment 1 lies in the focusing component: the focusing mirror 3' uses a concave mirror. To achieve this reflective focusing optical path, the system is configured with one or more plane mirrors 11 to fold and guide the beam. Those skilled in the art will understand that the number and spatial position of the plane mirrors 11 are adjustable. For example, the number of mirrors can be increased to adapt to different vacuum cavity spatial layouts, match the optical path of the pump-probe optical path, or precisely control the beam's directivity. This design, utilizing multiple mirrors to fold the optical path, demonstrates the excellent flexibility of integrating the core four-aperture mirror separation scheme of this invention with a complex beam transmission system.

[0063] Except for the changes in the above-mentioned focusing components and their supporting optical path structures, the configuration parameters of the four perforated mirrors, the gas target, the metal film, and other technical features in this device are the same as those in the first embodiment. Therefore, this embodiment also has all the technical effects described in the first embodiment and can efficiently generate and separate high-order harmonics under high-repetition-rate and high-average-power driving lasers. Any variant that changes the optical path structure by adding or subtracting reflectors based on the core concept of this invention should fall within the protection scope of this invention.

[0064] To intuitively understand the effectiveness of this invention, the inventor simulated the complete transmission process of the driving laser in the structure of the first embodiment through physical optics simulation. The results are shown in Figure 4 and Figure 5 , where Figure 4 is a schematic diagram of the spot evolution of the driving laser during propagation, Figure 5 is the light intensity distribution diagram of the driving laser before and after each perforated mirror. Before being reflected by the first perforated mirror 1 and reaching the metal film 7, the spot propagation evolution and power change process of the driving laser are mainly divided into the following 5 stages:

[0065] Stage 1 (the first perforated mirror 1 → the second perforated mirror 2): The initial incident spot in front of the first perforated mirror 1 is Gaussian (as shown in (a) in Figure 5 ), with an average power of P0. After being reflected by the first perforated mirror 1, it forms a standard annular light (as shown in (e) in Figure 5 ), and the remaining average power of the reflected light is P1 (<P0). It propagates a distance of z1 to the second perforated mirror 2.

[0066] Stage 2 (the second perforated mirror 2 → the focusing mirror 3): Before reaching the second perforated mirror 2, due to diffraction, there is an extremely strong Poisson bright spot at the center of the spot (as shown in (b) in Figure 5 ), and the diameter of this bright spot is about 2 mm. Therefore, aperture filtering with a 2-mm aperture is used to obtain an annular-like spot as shown in (f) in Figure 5 . The remaining average power of the reflected light is P2 (<P1). It propagates a distance of z2 to the focusing mirror 3.

[0067] Stage 3 (the focusing mirror 3 → the third perforated mirror 5): The diffracted spot is focused by the focusing mirror 3, and the average power after focusing is P3 (=P2). It propagates a distance of z3 to the third perforated mirror 5.

[0068] Stage 4 (the third perforated mirror 5 → the fourth perforated mirror 6): The spot in front of the third perforated mirror 5 is the image of the reflected spot at the second perforated mirror 2, and the center is also a concave hole (as shown in (c) in Figure 5 ). After filtering with a 1-mm aperture, only a faint ring of spots passes through the aperture and continues to propagate (as shown in (g) in Figure 5 ), and its average power is P4 (<<P3). This part of the transmitted light then propagates a distance of z4 to the fourth perforated mirror 6.

[0069] Stage 5 (Fourth perforated mirror 6 → Metal film 7): In front of the fourth perforated mirror 6, due to diffraction, the transmitted light spot gradually evolves into Bessel light, and the diameter of the light spot continues to increase, and the energy is dispersed (as shown in (d) of Figure 5 ). After being reflected by the fourth perforated mirror 6, the power of the transmitted light spot further decreases, and the average power of the residual light spot is P5 (<<P4). This part of the transmitted light then propagates a distance of z5 to the metal film 7, and the power is so small that it will not damage the metal film at all.

[0070] Figure 4 And Figure 5 The simulation of

[0071] Table 1

[0072] power <![CDATA[P0]]> <![CDATA[P1]]> <![CDATA[P2]]> <![CDATA[P3]]> <![CDATA[P4]]> <![CDATA[P5]]> Theoretical value (W) 208 192 187 187 0.8 0.3 Measured value (W) 10 9.39 8.76 8.76 0.035 0.015 Conversion value (W) 208 195.3 182.2 182.2 0.73 0.31

[0073] Thus, it can be seen that the device of the present invention finally attenuates the driving laser of 208 W to 0.3 W, and the attenuation ratio reaches 6.9×10 −4 . The experimental measurement values at low power (10W) are in high agreement with the theoretical values. Even when operating at full power (208W), due to the deformation of the light spot caused by gas ionization, the measured residual power is about 1 W, and the attenuation ratio still reaches 4.8×10 −3 order of magnitude. The residual laser at this power level will not damage the metal film at all, which proves the high efficiency and robustness of the solution of the present invention. Using this device, high-order harmonics with photon energies covering 27 eV to 49 eV (corresponding to the 23rd to 41st orders) can be successfully generated in argon with a background pressure of 1 bar. The measured spatially integrated spectral line diagram (see (b) of Figure 6 ) shows that there is a high-intensity plateau region in the range of 29th to 37th orders, and the signal is significantly stronger than other orders. In Figure 6 (a), by measuring the light spot size, it is obtained that the divergence half-angle of the high-order harmonics is about 1 mrad, and this divergence angle is much smaller than the aperture-limited half-angles of DM3 and DM4 (about 2.5 - 2.7 mrad), which proves that the high-order harmonics achieve 100% lossless transmission during the separation process using the four perforated mirrors of the present invention.

[0074] Compared with the prior art, the solution of the present invention has the following effects:

[0075] 1. High-efficiency attenuation: Through the two-stage separation design of the third and fourth perforated mirrors, the attenuation ability of the driving laser reaches 10 -With a capacity on the order of 3, it can handle high-power lasers of over 100W while effectively protecting the metal film. It successfully attenuated the driving laser in the hundreds of watts range by more than a thousand times, solving the problem of damage to optical components under high power.

[0076] 2. Lossless transmission: The apertures of the third and fourth aperture mirrors are rationally designed to ensure 100% passage of higher harmonics (with a divergence half-angle typically of 1~3 mrad), guaranteeing the output efficiency of higher harmonics. This achieves efficient separation while ensuring high-throughput output of higher harmonics.

[0077] 3. High flexibility: The first aperture mirror can be used to adjust the optical path of the pump light; the position of the fourth aperture mirror can be flexibly arranged according to the structure of the vacuum chamber, reducing the stringent requirements on the mechanical design of the vacuum chamber of the device.

[0078] 4. High robustness: Even if the light spot is deformed due to factors such as gas ionization, the secondary filtering of the fourth aperture mirror can still provide effective supplementary separation capability, resulting in high device stability. The device maintains stable high performance even in real-world environments with non-ideal object-image relationships and ionization effects.

[0079] 5. High practicality: The generated harmonic spectrum has a wide range and a small divergence angle, making it very suitable as an ideal light source for time-resolved pump-probe experiments.

[0080] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A high-order harmonic generation device based on a four-strip-aperture mirror, characterized in that, The device includes: a driving laser module, a first aperture mirror, a second aperture mirror, a first beam collector, a focusing mirror, a gas target, a third aperture mirror, a second beam collector, a fourth aperture mirror, a third beam collector, a metal film, a vacuum chamber, and a vacuum module. The first aperture mirror, the second aperture mirror, the focusing mirror, the gas target, the third aperture mirror, the fourth aperture mirror, and the metal film are disposed within the vacuum chamber, wherein: The driving laser module is used to provide a femtosecond driving light source with high repetition rate and high average power; The first aperture mirror is a plane mirror with a first through hole in the center. It is set in the incident light path of the driving laser and is used to split the incident driving laser into transmitted pump light and reflected ring driving laser. The second aperture mirror is a plane mirror with a second through hole in the center. It is set in the reflected light path of the first aperture mirror and is used to perform spatial filtering on the ring-driven laser to eliminate the Poisson bright spot generated by the diffraction of the ring-driven laser and generate a ring-like driven laser. The first beam collector is used to collect the driving laser light transmitted through or reflected out of the vacuum by the second aperture mirror; The focusing mirror is positioned after the second aperture mirror to focus the filtered ring-shaped driven laser onto the gas target; A gas target is placed at the focal point of the focusing lens to provide the rare gas required for the generation of higher harmonics; The third aperture mirror is a plane mirror with a third through hole in the center. It is set at the image plane of the focusing mirror and is used to reflect the driving laser to the outside of the vacuum cavity and allow high-order harmonics and part of the diffracted laser to be transmitted collinearly through the third through hole. The second beam collector is used to collect the driving laser reflected out of the vacuum by the third aperture mirror; The fourth aperture mirror is a plane mirror with a fourth through hole in the center. It is located downstream of the transmission light path of the third aperture mirror and is used to filter the light beam passing through the third through hole to attenuate the diffracted laser in the higher harmonics. The third beam collector is used to collect the driving laser reflected out of the vacuum by the fourth aperture mirror; The metal film is placed behind the fourth aperture mirror to filter out the residual laser in the higher harmonics after passing through the fourth aperture mirror; The vacuum module is used to evacuate the vacuum chamber to provide a vacuum environment.

2. The high-order harmonic generation device based on a four-strip-aperture mirror according to claim 1, characterized in that, The driving laser is a femtosecond pulsed laser with a repetition frequency greater than or equal to 100kHz, an average power greater than or equal to 100W, and a pulse width less than or equal to 400fs.

3. The high-order harmonic generation device based on a four-strip-aperture mirror according to claim 1, characterized in that, The incident angles of the first, second, third, and fourth aperture mirrors are 45°, and the angles of the first, second, third, and fourth through holes are 45°. Each through hole is cylindrical or conical. The surface of each aperture mirror is coated with a high-reflectivity broadband dielectric film with a reflectivity greater than or equal to 99.9%, and the reflection band is 900-1100nm.

4. The high-order harmonic generation device based on a four-strip-aperture mirror according to claim 3, characterized in that, The optical path between the first and second aperture mirrors is set to match the optical path between the pump light and the probe light in the pump experiment, and the aperture sizes of the first and second through holes are determined according to the power of the pump light probe experiment.

5. A high-order harmonic generation device based on a four-strip-aperture mirror according to claim 4, characterized in that, The second aperture lens is positioned at the object plane of the focusing lens, and the third aperture lens is positioned at the image plane of the focusing lens. The ratio of the aperture diameters of the second and third apertures is equal to the imaging magnification of the focusing lens. The positional relationship between the second and third aperture lenses and the focusing lens satisfies the following constraints: in, This indicates the distance between the second aperture lens and the focusing lens. This indicates the distance between the third-aperture lens and the focusing lens. The focal length of the focusing lens. The diameter of the third through hole. The diameter of the second through hole, is the magnification factor of the focusing lens.

6. A high-order harmonic generation device based on a four-strip-aperture mirror according to claim 5, characterized in that, The diameter of the fourth through hole is larger than that of the third through hole so that higher harmonics can pass through the fourth through hole without damage. The fourth through hole and the fourth through hole mirror are mounted on a triaxial displacement stage and their positions can be adjusted as needed.

7. A high-order harmonic generation device based on a four-strip-aperture mirror according to claim 5, characterized in that, The focusing lens has a transmission or reflection structure, and the focusing lens surface type is one or more of the following: spherical convex lens, concave mirror, off-axis parabolic mirror, ellipsoidal concave mirror, preferably a spherical convex lens or a concave mirror.

8. A high-order harmonic generation device based on a four-strip-aperture mirror according to claim 5, characterized in that: The gas target can be one or more of the following: a continuous gas pool, a gas nozzle with a gas collection port, or a semi-infinite gas target; The rare gas is one or more of the following: helium, neon, argon, krypton, xenon, with argon being preferred.

9. A high-order harmonic generation device based on a four-band aperture mirror according to claim 5, characterized in that: The metal film is one or more of the following: Al, Zr, Ti, Zn, Mo, Sn, with a thickness of 100~300nm; The first beam collector, the second beam collector, and the third beam collector are one or more of the following: an absorption cone-shaped water-cooled collector, a water-cooled baffle, and an air-cooled baffle.

10. A high-order harmonic generation device based on a four-aperture mirror according to claim 5, characterized in that: The diameter of the first through hole is 2mm, and the distance between the first and second perforated mirrors is 2030mm. The diameter of the second through hole is 2mm, and the distance between the aperture lens and the focusing lens is 1289mm. The focal length of the focusing lens is 400mm, and the distance between the focusing lens and the third aperture lens is 585mm. The diameter of the third through hole is 1mm, and the distance between the third and fourth perforated mirrors is 600mm. The fourth through hole has a diameter of 4mm, and the distance between the fourth perforated mirror 6 and the metal film 7 is 70mm.

11. A method for generating higher harmonics using the apparatus as described in any one of claims 1-9, characterized in that, Includes the following steps: S1. Use the driving laser module to generate driving laser; S2. The driving laser beam is split into pump light and ring driving laser using the first aperture mirror; S3. Spatial filtering of the ring-driven laser light is performed using the second aperture mirror to eliminate the Poisson bright spot generated by the ring-driven laser diffraction and generate a ring-like driven laser. The filtered ring-like driven laser is then focused onto the gas target to generate higher harmonics. S4. Primary separation is performed using the third aperture mirror, which reflects and drives the laser, causing the higher harmonics and part of the diffracted laser to pass collinearly through its central aperture. S5. Use the fourth aperture mirror to perform secondary separation of the driving laser passing through the third aperture mirror in order to attenuate the collinear propagating diffracted laser. S6. Use a metal membrane to filter out the residual driving laser in the higher harmonics to obtain a pure higher harmonic beam.

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