Method for manufacturing a deep-ultraviolet frequency conversion element based on a quartz crystal and related products

Abstract

The application discloses a quartz crystal-based deep ultraviolet frequency conversion element preparation method and related products, and belongs to the technical field of optoelectronic device preparation. The preparation method comprises the following steps: adding a Bessel beam shaping module composed of a small-base-angle pyramidal lens and a convex lens in the light path of a standard femtosecond laser processing system to form a femtosecond laser Bessel beam shaping system; adjusting the axial spacing between the small-base-angle pyramidal lens and the convex lens so that the femtosecond laser Bessel beam shaping processing system can produce refractive index modification marks with a target longitudinal depth and a target line width; and using a femtosecond laser Bessel beam direct writing technology to prepare a large-size periodic phase grating structure composed of refractive index modification marks on a quartz crystal to be processed, thereby obtaining a deep ultraviolet frequency conversion element. The application can prepare refractive index modification marks with a long longitudinal depth and a narrow line width on a quartz crystal in accordance with preset requirements, and further obtain a large-size and high-power deep ultraviolet frequency conversion element.

Description

Technical Field

[0001] This invention relates to the field of optoelectronic device fabrication technology, and more specifically to a method for fabricating deep ultraviolet frequency conversion elements based on quartz crystals and related products. Background Technology

[0002] Currently, the engineering application of deep ultraviolet frequency conversion devices still faces multiple constraints, with low conversion efficiency being the most direct performance bottleneck. This means that to obtain high-power deep ultraviolet laser output, high-power pump light must be injected into the nonlinear medium, and high-power pump light inevitably results in a large spot size. However, existing deep ultraviolet frequency conversion devices generally employ small-aperture structures, which cannot effectively receive large spots, leading to low pump light utilization and thus limiting the improvement of conversion efficiency. If a large spot is forcibly focused to match a small-aperture device, the focused beam has extremely high power density at the focal point, which can easily damage the crystal and optical components. This contradiction puts the system in a dilemma—high-power pump light cannot be effectively utilized by existing devices, while low-power pump light cannot achieve high-power output.

[0003] Meanwhile, at the device fabrication level, existing APP frequency conversion devices also face structural process bottlenecks. To achieve phase matching in the deep ultraviolet band, a periodic phase grating composed of refractive index-modified grooves needs to be fabricated inside a nonlinear crystal using femtosecond laser direct writing technology. However, existing technologies struggle to simultaneously meet the requirements of long depth, narrow linewidth, and uniform arrangement of the grooves. Specifically, traditional femtosecond laser processing often uses Gaussian beams, whose focal depth after focusing is limited by the Rayleigh length, which is only about tens of micrometers. Chinese patent CN110068979A, applied for by Shandong University, proposes the APP phase matching principle and fabricates a frequency conversion device, but the entire patent does not address the optimization of groove depth and linewidth. Technical literature (Shao, Liang, Zhang, et al. Spatial frequency manipulation of a quartz crystal for phase-matched second-harmonic vacuum ultraviolet generation. Laser & Photonics Review, 2023, 17(10): 2300-244.) shows that although the groove depth can be increased from 40 μm to 120 μm by shaping the Gaussian beam into a Bessel beam, this value is still insufficient for large-size high-power devices. Meanwhile, the narrow linewidth requires the laser energy to be highly localized in the lateral dimension to form fine grating periods on the micrometer or even submicrometer scale. This is inherently contradictory to the need to increase laser energy to increase groove depth: excessively high energy density can easily cause material damage or lead to lateral expansion of the modified region, destroying the narrow linewidth characteristics. To ensure efficient frequency conversion, the grooves need to be arranged precisely and uniformly, which also places extremely high demands on the motion accuracy and stability of the processing platform.

[0004] In summary, existing literature has only completed the fabrication of smaller-sized structures and has not addressed the synergistic design of groove depth and linewidth. Explaining how to simultaneously achieve synergistic optimization of long depth and narrow linewidth structures in the same device remains a technical challenge that has not yet been solved by those skilled in the art. Summary of the Invention

[0005] In view of the above problems, the present invention is proposed to provide a method for preparing a deep ultraviolet frequency conversion element based on quartz crystal and related products that overcomes or at least partially solves the above problems.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] In a first aspect, embodiments of the present invention provide a method for fabricating a deep ultraviolet frequency conversion element based on a quartz crystal, comprising the following steps: In a femtosecond laser processing system, a Bessel beam shaping module consisting of a small-base-angle conical lens and a convex lens is added to form a femtosecond laser Bessel beam shaping processing system that generates femtosecond laser Bessel beams. The axial distance between the small-base-angle conical lens and the convex lens is adjusted so that the femtosecond laser Bessel beam shaping processing system can generate refractive index modification marks with target depth and target linewidth on the quartz crystal to be processed; the base angle of the small-base-angle conical lens, that is, the angle formed between the generatrix of the cone and the bottom plane, is 1°~5°. A large-size periodic phase grating structure composed of refractive index modified grooves of target depth and target linewidth was prepared on the quartz crystal to be processed using femtosecond laser Bessel beam direct writing technology, thus obtaining a deep ultraviolet frequency conversion element.

[0008] Further, adjusting the axial distance between the small-base-angle conical lens and the convex lens specifically includes the following steps: Obtain the optical parameters of the femtosecond laser Bessel beam shaping system, including the femtosecond laser wavelength λ, the base angle α of the small base angle cone lens, the refractive index n of the small base angle cone lens, the focal length f of the convex lens, and the numerical aperture NA0 of the microscope objective. Based on the aforementioned optical parameters and diffraction optics theory, a quantitative mapping relationship is established between the axial distance d between the small-base-angle conical lens and the convex lens, the radius w(d) of the central spot of the Bessel beam, and the non-diffraction distance Z(d) of the Bessel beam: ; ; Where R is the radius of the parallel beam incident on the small-base-angle conical lens, and K w and K z These are the correction coefficients determined through system calibration; Based on the radius w(d) of the central spot of the Bessel beam and the non-diffraction distance Z(d) of the Bessel beam, a structure is constructed that includes the target depth D. target and target line width W target The optimization objective function J(d) is:

[0009] The axial spacing d at which the value of the objective function J(d) is minimized by using the Bayesian optimization algorithm is used as the final axial distance between the small-base-angle conical lens and the convex lens.

[0010] Furthermore, the quartz crystal to be processed is fixed on the platform of the six-dimensional electric displacement stage, and can move horizontally along the x and y directions with the platform.

[0011] Furthermore, the six-dimensional electric displacement stage includes a three-layer stacked precision displacement mechanism and a three-point support precision attitude adjustment mechanism. The three-layer stacked precision displacement mechanism is an XYZ three-axis serial sliding structure with a single-axis positioning accuracy of ≤±50nm and a repeatability of ≤±20nm, used to drive the processing stage to complete the full-area scanning and wide-range focusing adjustment of the substrate to be processed. The three-point support precision attitude adjustment mechanism is a three-degree-of-freedom rotational parallel structure, fixedly installed on the three-layer stacked precision displacement mechanism, used to correct the flatness of the processing surface and compensate for processing errors caused by substrate warping.

[0012] Furthermore, the above method also includes optical polishing of the two end faces perpendicular to the y-direction of the quartz crystal after the periodic phase grating structure is fabricated.

[0013] Secondly, embodiments of the present invention provide a deep ultraviolet frequency conversion element based on a quartz crystal, prepared according to the deep ultraviolet frequency conversion element preparation method described in any one of the first aspects of the present invention, comprising a quartz crystal, wherein the light transmission direction of the quartz crystal is along the crystal... y Oriented, and its interior is provided with crystal-oriented features. y A large-size periodic phase grating structure composed of long, deep, narrow linewidth refractive index modified grooves arranged in a directional pattern.

[0014] Preferably, the depth of a single scribe line in the large-size periodic phase grating structure is 300 μm and the line width is 2.1 μm.

[0015] Preferably, the periodic phase grating structure in the large-size periodic phase grating structure adopts a linear array and a sinusoidal array.

[0016] Preferably, the duty cycle of a single period in the large-size periodic phase grating structure is 0.5.

[0017] Preferably, the number of periods in the large-size periodic phase grating structure is ≥500.

[0018] Thirdly, embodiments of the present invention provide a deep ultraviolet frequency converter based on a quartz crystal, comprising an ultraviolet pulsed laser, a half-wave plate, a focusing lens, a deep ultraviolet frequency conversion element, and a beam splitter arranged sequentially along the beam transmission direction, wherein the deep ultraviolet frequency conversion element is prepared based on the deep ultraviolet frequency conversion element preparation method described in any one of the first aspects of the present invention.

[0019] Preferably, the beam splitter includes a CaF2 beam splitter.

[0020] As can be seen from the above technical solution, compared with the prior art, the present invention discloses a method for preparing a deep ultraviolet frequency conversion element based on quartz crystal and related products, which has the following beneficial effects: This invention employs a small-base-angle conical lens and a convex lens as core components for femtosecond laser Bessel beam shaping. By incorporating these shaping core components into the femtosecond laser processing optical path system, a femtosecond laser Bessel beam shaping processing system is constructed. Within the allowable range of the numerical aperture of the microscope objective, the diameter of the annular beam incident on the microscope objective can be changed by adjusting the axial spacing between the small-base-angle conical lens and the convex lens, thereby altering the size of the central spot of the Bessel beam. If the groove linewidth does not meet expectations, the axial spacing is increased to increase the effective numerical aperture of the system, reducing the spot width and thus narrowing the groove linewidth; if the groove depth is insufficient, the spacing is reduced to decrease the effective numerical aperture, extending the Bessel beam's non-diffraction distance and thus increasing the groove depth. This invention establishes a quantitative mapping relationship between the axial spacing d between the small-base-angle conical lens and the convex lens and the radius w(d) of the central spot of the Bessel beam and the non-diffraction distance Z(d) of the Bessel beam, and constructs the target depth D. target and target line width W target The objective function is optimized to obtain the axial spacing that meets the requirements for machining narrow linewidths and deep longitudinal grooves. Furthermore, this invention achieves precise groove arrangement through a high-precision three-dimensional motion control platform and its supporting motion control system. Before machining, the scanning path is programmed and planned using control software, setting the starting coordinates, length, and spacing of each groove, allowing the laser to scan line by line along a preset trajectory within the crystal. During machining, the position synchronization output function is used to link the laser trigger pulse with the platform encoder feedback in real time, ensuring precise synchronization between laser emission and platform displacement, resulting in an accuracy of the starting position and length of each groove better than ±50 nm. The refractive index modified grooves are uniformly arranged within the material, and the error of the array of 800 grooves is less than 0.5 micrometers.

[0021] The core of the large-size deep ultraviolet frequency conversion element provided by this invention lies in its large-size light-transmitting structure, which can directly receive the large light spot generated by high-power pump light, thereby improving laser utilization. While increasing the output power of deep ultraviolet lasers, it avoids the optical damage risk caused by forced focusing. This invention effectively solves the dilemma of traditional small-size structural devices being unable to efficiently utilize large light spots, providing a practical device solution for high-power deep ultraviolet frequency conversion technology.

[0022] This invention not only employs a linear array structure but also uses a sinusoidal linear array as the structural form of a periodic phase grating. Compared to traditional linear array gratings, the smooth contour of the sinusoidal linear array grating can effectively suppress the generation of higher-order harmonics and reduce beam scattering loss, thereby improving the beam quality and signal-to-noise ratio of the output deep ultraviolet laser. This provides a structural guarantee for the efficient and high-quality output of high-power deep ultraviolet lasers.

[0023] The deep ultraviolet frequency converter of the present invention has the advantages of high power carrying capacity, high conversion efficiency and simple fabrication process due to its large-size periodic phase grating structure composed of long depth, narrow linewidth and highly uniform refractive index modified grooves. It can be widely used in advanced industrial manufacturing and basic scientific research. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of a large-size, high-power deep ultraviolet frequency converter based on a quartz crystal, provided in Embodiment 1 of the present invention. Figure 2 A top view of a large-size, high-power deep ultraviolet frequency conversion element based on a quartz crystal, provided in Embodiment 1 of the present invention; Figure 3 A front view of a large-size, high-power deep ultraviolet frequency conversion element based on a quartz crystal, provided in Embodiment 1 of the present invention; Figure 4 The left view of a large-size, high-power deep ultraviolet frequency conversion element based on a quartz crystal provided in Embodiment 1 of the present invention; Figure 5 This is a schematic diagram of a large-size, high-power deep ultraviolet frequency converter based on a quartz crystal, provided in Embodiment 2 of the present invention. Figure 6 This is a top view of a large-size, high-power deep ultraviolet frequency conversion element based on a quartz crystal, provided in Embodiment 2 of the present invention. Figure 7 A front view of a large-size, high-power deep ultraviolet frequency conversion element based on a quartz crystal, provided in Embodiment 2 of the present invention; Figure 8 Left view of a large-size, high-power deep ultraviolet frequency conversion element based on a quartz crystal, provided in Embodiment 2 of the present invention; Among them, 1-femtosecond laser-induced etched structure, 2-quartz crystal, 3-ultraviolet pulsed laser, 4-half-wave plate, 5-focusing lens, 6-CaF2 beam splitter. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] Terminology Explanation: Frequency doubling: A nonlinear frequency conversion process that increases the frequency of fundamental light by two times.

[0028] Duty cycle: The ratio of the width of the refractive index modification region within a single period to the overall width of the single period.

[0029] Example 1 Embodiment 1 of this invention discloses a method for fabricating a deep ultraviolet frequency conversion element based on quartz crystal, specifically including the following steps: (1) A standard femtosecond laser processing system optical path is built using components such as a femtosecond laser, shutter, microscope objective and six-dimensional electric displacement platform; (2) A beam shaping module is constructed using a small-base-angle conical lens and a convex lens as core beam shaping optical elements. This module is added to the optical path of a standard femtosecond laser processing system to receive and act on the input femtosecond laser Gaussian beam, forming a femtosecond laser Bessel beam shaping processing system. The beam wavefront is modulated by its conical structure to change the beam propagation characteristics. Finally, a high-magnification microscope objective (magnification of 50 and numerical aperture of 0.67) converges and outputs a Bessel beam with a long focal depth and makes it perpendicular to the quartz crystal sample.

[0030] The specific femtosecond laser Bessel beam shaping system includes, along the beam propagation direction, a laser, a shutter, an attenuator, a polarizing beam splitter, a first steering mirror, a small-base-angle conical lens, a convex lens, a second steering mirror, a microscope objective, and a six-dimensional motorized stage. The steering mirror is used to guide the laser to be accurately input into the next optical component.

[0031] By adjusting the axial spacing between the small-base-angle conical lens and the convex lens, the femtosecond laser Bessel beam shaping system can produce refractive index modification marks with target depth and linewidth on the quartz crystal to be processed. In practice, the axial spacing that meets the preset target requirements can be solved by establishing a target optimization function. The specific steps are as follows: Obtain the optical parameters of the femtosecond laser Bessel beam shaping system, including the femtosecond laser wavelength λ, the base angle α of the small base angle cone lens, the refractive index n of the small base angle cone lens, the focal length f of the convex lens, and the numerical aperture NA0 of the microscope objective. Based on the aforementioned optical parameters and diffraction optics theory, a quantitative mapping relationship is established between the axial distance d between the small-base-angle conical lens and the convex lens, the radius w(d) of the central spot of the Bessel beam, and the non-diffraction distance Z(d) of the Bessel beam: ; ; Where R is the radius of the parallel beam incident on the small-base-angle conical lens, and K w and K z These are the correction coefficients determined through system calibration; Based on the radius w(d) of the central spot of the Bessel beam and the non-diffraction distance Z(d) of the Bessel beam, a structure is constructed that includes the target depth D. target and target line width W target The optimization objective function J(d) is:

[0032] The axial spacing d at which the value of the objective function J(d) is minimized by using the Bayesian optimization algorithm is used as the final axial distance between the small-base-angle conical lens and the convex lens.

[0033] (3) Place the polished quartz crystal sample to be processed on a six-dimensional electric displacement platform and level it: First, locate the sample surface using a high-magnification microscope objective and a CCD camera, and then use a computer-controlled platform to move along the surface. xy The sample surface was moved along an axis, and the changes in sharpness at different positions on the microscope focal plane were observed. Finally, the required rotation angle along a certain axis was calculated using the positional relationship when the sample was sharp at different positions, and adjustments were made until the sample surface at any position within the movement range could be clearly focused on the same focal plane. Femtosecond laser processing parameters were set (femtosecond laser pulse width of 400 fs, wavelength of 1030 nm, repetition rate of 2.5 MHz, single pulse energy of 2.25 μJ, and femtosecond laser scanning speed of 10 mm / s). A linear periodic array program was designed on a computer, and a six-dimensional electric displacement platform was controlled along the axis. x Move at a constant speed, and after scanning each straight line, move along... y By performing a fixed-period step (the theoretical calculated value of the phase-matching period corresponding to a wavelength of 355 nm is 0.7 μm, but to ensure the uniformity of the periodic structure, the width of the refractive index modified region is adjusted so that the width of both the modified and unmodified regions within a single period is three times the ideal calculated value, i.e., the period is 2.1 μm), a femtosecond laser Bessel beam is used to process a periodic phase grating structure with 800 periods along a predetermined path inside the quartz crystal, resulting in a deep ultraviolet frequency conversion element based on the quartz crystal (such as...). Figures 2-4 (As shown).

[0034] The six-dimensional electric displacement stage used in this invention includes a three-layer stacked precision displacement mechanism and a three-point supported precision attitude adjustment mechanism. The three-layer stacked precision displacement mechanism is a three-axis series sliding structure with a single-axis positioning accuracy of ≤±50 nm and a repeatability of ≤±20 nm. It is used to drive the processing stage to complete the full-area scanning and wide-range focusing adjustment of the substrate to be processed. The three-point supported precision attitude adjustment mechanism is a three-degree-of-freedom rotational parallel structure, which is fixedly installed on the three-layer stacked precision displacement mechanism. It is used to correct the flatness of the processing surface and compensate for the processing errors caused by substrate warping.

[0035] Furthermore, after obtaining the deep ultraviolet frequency conversion element, the ultraviolet pulsed laser, a half-wave plate, an optical fiber coupling system, a quartz crystal-based deep ultraviolet frequency conversion element, an optical fiber coupling receiver, and a filter are sequentially assembled and integrated along the beam propagation direction to form a deep ultraviolet frequency converter, such as... Figure 1 As shown, the half-wave plate, focusing lens, quartz crystal-based deep ultraviolet frequency conversion element, and CaF2 beam splitter are placed in a nitrogen atmosphere.

[0036] The refractive index modified markings prepared using the femtosecond laser processing method of this invention can achieve a length of 1 mm along the laser incident direction. The width of the refractive index modified markings on the crystal surface can reach 1 μm. The processing error of the periodic array is ≤0.5 μm.

[0037] The deep ultraviolet frequency converter constructed in this embodiment, such as Figure 1 Specifically, it includes: z A quartz crystal material platform is used, where the light transmission direction of the crystal is along the crystal. y Towards; z The interior of the quartz crystal is designed with a lateral line. y A periodic phase grating structure 1 composed of refractive index-modified grooves induced by femtosecond lasers; optical components: ultraviolet pulsed laser 3, half-wave plate 4, focusing lens 5, CaF2 beam splitter 6.

[0038] In this embodiment, the width ratio of the laser-processed region to the unprocessed region within a single cycle of the periodic phase grating structure is close to 1:1, i.e., the duty cycle is 0.5. The closer the duty cycle is to 0.5, the higher the conversion efficiency. When the duty cycle reaches 0.5 within a single cycle, the conversion efficiency is the highest. In other words, with a fixed input laser power, the closer the duty cycle is to 0.5 within a single cycle, the greater the output laser power. A duty cycle of 0.5 can ensure the maximum laser output power.

[0039] At room temperature, the period of APP phase matching during nonlinear frequency conversion is:

[0040] Where Λ is the quasi-phase matching period size, λ 2ω It is the wavelength of the emitted frequency-doubled light. n ω and n 2ω These are the refractive index values ​​of the material under fundamental and second-harmonic light conditions, respectively; the dispersion equation is used to calculate the refractive index of a quartz crystal at a certain incident fundamental light wavelength λ0. z The refractive index n0 of the direction is as follows:

[0041] For a specific wavelength of laser, this invention theoretically calculates the corresponding period width and adjusts the duty cycle within the periodic structural unit to 0.5, thereby fabricating a large-size micro / nano structure with hundreds or thousands of periods along the transmission direction of the laser of a specific wavelength, thus achieving efficient frequency conversion.

[0042] In practical applications, in order to ensure the uniformity and stability of the prepared periodic phase structure, and considering the theoretical period width requirements of APP phase matching technology and the current functionality of femtosecond laser direct writing technology, the period is usually set to an even multiple of the ideal period, that is, the widths of the laser-modified region and the unmodified region are odd multiples of the widths of the corresponding regions in the ideal period.

[0043] In this embodiment, to improve the frequency conversion effect, along y The number of periods in the large-size periodic phase grating structure processed by the femtosecond laser is 800.

[0044] In this embodiment, the femtosecond laser travels along the quartz crystal. z The incident fundamental frequency light and the emitted frequency-doubled laser light travel along the quartz crystal through an incident refractive index-modified groove structure. y Transmission.

[0045] Furthermore, the component has a size of 6 ( x ) 3 ( y ) 3 ( z ) mm 3 tangential direction z Cut, light transmission direction along y Towards.

[0046] In this embodiment, the microscope objective has a magnification of 50 and a numerical aperture of 0.67.

[0047] In this embodiment, the femtosecond laser pulse width is 400 fs, the wavelength is 1030 nm, the repetition frequency is 2.5 MHz, the single pulse energy is 2.25 μJ, and the femtosecond laser scanning speed is 10 mm / s.

[0048] In this embodiment, the working principle of the deep ultraviolet frequency converter is as follows: an ultraviolet pulsed laser is used as a pump source to generate linearly polarized laser. The polarization direction of the pulsed laser can be adjusted by rotating a half-wave plate to meet the phase matching condition. Then, the laser is coupled into a quartz crystal deep ultraviolet frequency conversion element containing a periodic phase grating structure through a focusing lens. Subsequently, the pump light and signal light (such as frequency doubling light) are separated by a CaF2 beam splitter, thereby realizing the frequency conversion function of deep ultraviolet laser.

[0049] Example 2 Embodiment 2 of the present invention discloses another method for fabricating a deep ultraviolet frequency conversion element based on quartz crystal, specifically including the following steps: (1) A standard femtosecond laser processing system optical path is built using components such as a femtosecond laser, shutter, microscope objective and six-dimensional electric displacement platform.

[0050] (2) A beam shaping module is constructed using a small-base-angle conical lens and a convex lens as core beam shaping optical elements. This module is added to the optical path of a standard femtosecond laser processing system to receive and act on the input femtosecond laser Gaussian beam, forming a femtosecond laser Bessel beam shaping processing system. The beam wavefront is modulated by its conical structure to change the beam propagation characteristics. Finally, a high-magnification microscope objective (magnification of 50 and numerical aperture of 0.67) converges and outputs a Bessel beam with a long focal depth and makes it perpendicular to the quartz crystal sample.

[0051] The specific femtosecond laser Bessel beam shaping system includes, along the beam propagation direction, a laser, a shutter, an attenuator, a polarizing beam splitter, a first steering mirror, a small-base-angle conical lens, a convex lens, a second steering mirror, a microscope objective, and a six-dimensional motorized stage. The steering mirror is used to guide the laser to be accurately input into the next optical component.

[0052] By adjusting the axial spacing between the small-base-angle conical lens and the convex lens, the femtosecond laser Bessel beam shaping system can produce refractive index modification marks with target depth and linewidth on the quartz crystal to be processed. In practice, the axial spacing that meets the preset target requirements can be solved by establishing a target optimization function. The specific steps are as follows: Obtain the optical parameters of the femtosecond laser Bessel beam shaping system, including the femtosecond laser wavelength λ, the base angle α of the small base angle cone lens, the refractive index n of the small base angle cone lens, the focal length f of the convex lens, and the numerical aperture NA0 of the microscope objective. Based on the aforementioned optical parameters and diffraction optics theory, a quantitative mapping relationship is established between the axial distance d between the small-base-angle conical lens and the convex lens, the radius w(d) of the central spot of the Bessel beam, and the non-diffraction distance Z(d) of the Bessel beam: ; ; Where R is the radius of the parallel beam incident on the small-base-angle conical lens, and K w and K z These are the correction coefficients determined through system calibration; Based on the radius w(d) of the central spot of the Bessel beam and the non-diffraction distance Z(d) of the Bessel beam, a structure is constructed that includes the target depth D. target and target line width W target The optimization objective function J(d) is:

[0053] The axial spacing d at which the value of the objective function J(d) is minimized by using the Bayesian optimization algorithm is used as the final axial distance between the small-base-angle conical lens and the convex lens.

[0054] (3) Place the polished quartz crystal sample to be processed on a six-dimensional electric displacement platform and level it: First, locate the sample surface using a high-magnification microscope objective and a CCD camera, and then use a computer-controlled platform to move along the surface. xy The sample surface was moved along an axis, and the changes in sharpness at different positions on the microscope focal plane were observed. Finally, the required rotation angle along a certain axis was calculated using the positional relationships when the sample was sharp at different positions, and adjustments were made until the sample surface at any position within the movement range could be clearly focused on the same focal plane. Femtosecond laser processing parameters were set (femtosecond laser pulse width of 400 fs, wavelength of 1030 nm, repetition rate of 2.5 MHz, single pulse energy of 2.25 μJ, and femtosecond laser scanning speed of 10 mm / s). A sinusoidal periodic array program was designed on a computer, and a six-dimensional electric displacement platform was controlled along the axis. x Moving at a constant speed, and after scanning each sine wave... y By performing a fixed-period step (the theoretical calculated value of the phase-matching period corresponding to a wavelength of 355 nm is 0.7 μm, but to ensure the uniformity of the periodic structure, the width of the refractive index modified region is adjusted so that the width of both the modified and unmodified regions within a single period is three times the ideal calculated value, i.e., the period is 2.1 μm), a femtosecond laser Bessel beam is used to process a periodic phase grating structure with 800 periods along a predetermined path inside the quartz crystal, resulting in a deep ultraviolet frequency conversion element based on the quartz crystal (such as...). Figures 6-8 (As shown).

[0055] (4) Two quartz crystals y End face (i.e., with quartz crystal) y Optical polishing is performed on the two vertical end faces, removing approximately 0.5 mm from each face. The sample is then cleaned with a mixture of ethanol and acetone to obtain the polished surface.

[0056] Furthermore, after obtaining the deep ultraviolet frequency conversion element, the ultraviolet pulsed laser, the half-wave plate, the fiber coupling system, the quartz crystal-based deep ultraviolet frequency conversion element, the fiber coupling receiver, and the filter are sequentially assembled and integrated along the beam transmission direction. The half-wave plate, the focusing lens, the quartz crystal-based deep ultraviolet frequency conversion element, and the CaF2 beam splitter are placed in a nitrogen environment to form a deep ultraviolet frequency converter.

[0057] Forming a deep ultraviolet frequency converter, such as Figure 5 Specifically, it includes: z A quartz crystal material platform is used, where the light transmission direction of the crystal is along the crystal. y Towards; z The interior of the quartz crystal is designed with a lateral line. y A periodic phase grating structure 1 composed of refractive index-modified grooves induced by femtosecond lasers; optical components: ultraviolet pulsed laser 3, half-wave plate 4, focusing lens 5, CaF2 beam splitter 6.

[0058] In this embodiment, a 355 nm deep ultraviolet frequency converter was also fabricated. Unlike Embodiment 1, the periodic phase grating structure of the quartz crystal-based deep ultraviolet frequency conversion element in this embodiment is a sinusoidal linear array, rather than a simple linear array. In APP-based phase-matched deep ultraviolet frequency conversion devices, sinusoidal linear arrays offer advantages over linear arrays. First, the refractive index distribution of the sinusoidal linear array grating in the light transmission direction transitions smoothly with a curve, unlike the steep interface of the linear array grating. This smooth transition effectively reduces scattering loss when the beam crosses the interface, while gently constraining and guiding beam propagation, resulting in a better match between the mode field distribution of the fundamental and harmonic frequencies, which is beneficial for improving nonlinear conversion efficiency and transmission power. Furthermore, the steep boundary of the linear array grating easily excites higher-order harmonic components, introducing stray light, while the smooth structure of the sinusoidal linear array grating effectively suppresses and reduces stray light and ghosting effects, improving the output beam quality.

[0059] In this embodiment, to improve the frequency conversion effect, along y The number of periods in the large-size periodic phase grating structure processed by the femtosecond laser is 800.

[0060] In this embodiment, the femtosecond laser travels along the quartz crystal. z The incident fundamental frequency light and the emitted frequency-doubled laser light travel along the quartz crystal through an incident refractive index-modified groove structure. y Transmission.

[0061] Furthermore, the component has a size of 6 ( x ) 3 ( y ) 3 ( z ) mm 3 tangential direction z Cut, light transmission direction along y Towards.

[0062] In this embodiment, the microscope objective has a magnification of 50 and a numerical aperture of 0.67.

[0063] In this embodiment, the femtosecond laser pulse width is 400 fs, the wavelength is 1030 nm, the repetition frequency is 2.5 MHz, the single pulse energy is 2.25 μJ, and the femtosecond laser scanning speed is 10 mm / s.

[0064] Similar to Embodiment 1, the working principle of the deep ultraviolet frequency converter in this embodiment is as follows: an ultraviolet pulsed laser is used as the pump source to generate linearly polarized laser light. The polarization direction of the pulsed laser light can be adjusted by rotating a half-wave plate to meet the phase matching condition. Then, the laser light is coupled into a quartz crystal deep ultraviolet frequency conversion element containing a periodic phase grating structure through a focusing lens. Subsequently, the emitted pump light and signal light (such as frequency-doubled light) are separated by a CaF2 beam splitter, thus realizing the frequency conversion function of deep ultraviolet laser light. Since the periodic phase grating in the deep ultraviolet frequency conversion element in this embodiment adopts a sinusoidal linear array, the frequency conversion device of Embodiment 2 has a higher conversion efficiency than the frequency conversion device of Embodiment 1, and thus can output a higher power deep ultraviolet laser light.

[0065] In this embodiment, a comparative experiment is conducted using two frequency conversion elements prepared in comparative examples and the frequency doubling structure element prepared in Example 2: Comparative Example 1 differs from Example 2 in that a frequency conversion element with the same period is prepared using a femtosecond laser Gaussian beam and the frequency converter is assembled in the same way. Comparative Example 2 differs from Example 2 in that the periodic phase grating structure of the quartz crystal frequency conversion element has a duty cycle of 0.7 in a single period, and the frequency converter is assembled in accordance with Example 2.

[0066] In Comparative Example 1, the longitudinal length of the groove prepared by the femtosecond laser Gaussian beam is too short, resulting in a significant reduction in frequency doubling conversion efficiency and output power; in Comparative Example 2, the frequency doubling signal is also significantly weakened because the duty cycle of a single period in the periodic structure is far from 0.5.

[0067] Experiments show that this invention innovatively combines femtosecond laser direct writing technology with femtosecond laser Bessel beam shaping technology, and applies it to the fabrication of deep ultraviolet frequency converters in quartz crystals. The shaped femtosecond laser Bessel beam can be used to process refractive index modified notched structures with a longitudinal length of several hundred micrometers and a lateral width of several micrometers. Furthermore, combined with a periodic array processing procedure, it is possible to fabricate large-scale three-dimensional micro / nano structures with hundreds or thousands of periods. The large-size, high-power deep ultraviolet frequency converter fabricated in quartz crystals using this femtosecond laser processing method can achieve efficient output of frequency-doubled lasers in the deep ultraviolet band, demonstrating the significant role of this method in the field of femtosecond laser processing.

[0068] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since it corresponds to the method disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0069] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for fabricating a deep ultraviolet frequency conversion element based on quartz crystal, characterized in that, Includes the following steps: In a femtosecond laser processing system, a Bessel beam shaping module consisting of a small-base-angle conical lens and a convex lens is added to form a femtosecond laser Bessel beam shaping processing system that generates a femtosecond laser Bessel beam. Adjusting the axial distance between the small-base-angle conical lens and the convex lens allows the femtosecond laser Bessel beam shaping processing system to produce refractive index modification marks of target depth and linewidth on the quartz crystal to be processed. The base angle of the small-base-angle conical lens, i.e., the angle formed between the generatrix of the cone and the base plane, is 1°. ° ~5 ° ; A large-size periodic phase grating structure composed of refractive index modified grooves of target depth and target linewidth was prepared on the quartz crystal to be processed using femtosecond laser Bessel beam direct writing technology, thus obtaining a deep ultraviolet frequency conversion element.

2. The method for fabricating a deep ultraviolet frequency conversion element as described in claim 1, characterized in that, Adjusting the axial distance between the small-base-angle conical lens and the convex lens specifically includes the following steps: Obtain the optical parameters of the femtosecond laser Bessel beam shaping system, including the femtosecond laser wavelength λ, the base angle α of the small base angle cone lens, the refractive index n of the small base angle cone lens, the focal length f of the convex lens, and the numerical aperture NA0 of the microscope objective. Based on the aforementioned optical parameters and diffraction optics theory, a quantitative mapping relationship is established between the axial distance d between the small-base-angle conical lens and the convex lens, the radius w(d) of the central spot of the Bessel beam, and the non-diffraction distance Z(d) of the Bessel beam: ; ; Where R is the radius of the parallel beam incident on the small-base-angle conical lens, and K w and K z These are the correction coefficients determined through system calibration; Based on the radius w(d) of the central spot of the Bessel beam and the non-diffraction distance Z(d) of the Bessel beam, a structure is constructed that includes the target depth D. target and target line width W target The optimization objective function J(d) is: The axial spacing d at which the value of the objective function J(d) is minimized by using the Bayesian optimization algorithm is used as the final axial distance between the small-base-angle conical lens and the convex lens.

3. The method for fabricating a deep ultraviolet frequency conversion element as described in claim 1, characterized in that, The quartz crystal to be processed is fixed on the platform of the six-dimensional electric displacement stage and can move horizontally along the x and y directions with the platform.

4. The method for fabricating a deep ultraviolet frequency conversion element as described in claim 2, characterized in that, The six-dimensional electric displacement stage includes a three-layer stacked precision displacement mechanism and a three-point supported precision attitude adjustment mechanism. The three-layer stacked precision displacement mechanism is an XYZ three-axis serial sliding structure with a single-axis positioning accuracy of ≤±50 nm and a repeatability of ≤±20 nm, used to drive the processing stage to complete the full-area scanning and wide-range focusing adjustment of the substrate to be processed. The three-point supported precision attitude adjustment mechanism is a three-degree-of-freedom rotational parallel structure, fixedly installed on the three-layer stacked precision displacement mechanism, used to correct the flatness of the processing surface and compensate for processing errors caused by substrate warping.

5. A deep ultraviolet frequency conversion element based on a quartz crystal, prepared according to the deep ultraviolet frequency conversion element preparation method according to any one of claims 1-4, characterized in that, Includes a quartz crystal, wherein the light transmission direction of the quartz crystal is along the crystal... y Oriented, and its interior is provided with crystal-oriented features. y A large-size periodic phase grating structure composed of long, deep, narrow linewidth refractive index modified grooves arranged in a directional pattern.

6. The deep ultraviolet frequency conversion element as described in claim 5, characterized in that, The large-size periodic phase grating structure has a single scribe depth of 300 μm and a linewidth of 2.1 μm.

7. The deep ultraviolet frequency conversion element as described in claim 5, characterized in that, The periodic phase grating structure in the large-size periodic phase grating structure adopts a linear array and a sinusoidal array.

8. The deep ultraviolet frequency conversion element as described in claim 5, characterized in that, The duty cycle of a single period in the large-size periodic phase grating structure is 0.

5.

9. The deep ultraviolet frequency conversion element as described in claim 5, characterized in that, The large-size periodic phase grating structure has a period number ≥ 500.

10. A deep ultraviolet frequency converter based on a quartz crystal, characterized in that, The device includes an ultraviolet pulsed laser, a half-wave plate, a focusing lens, a deep ultraviolet frequency conversion element, and a beam splitter arranged sequentially along the beam transmission direction. The deep ultraviolet frequency conversion element is prepared based on the deep ultraviolet frequency conversion element preparation method according to any one of claims 1-4.

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