A prediction method for laser-induced periodic structures of ultraviolet optical elements

Abstract

The present invention provides a prediction method for laser-induced periodic structures of ultraviolet optical elements, and belongs to the field of engineering optical technology. In order to solve the problem that the formation theory and process technology of laser-induced microstructures of ultraviolet optical elements in the prior art are still imperfect, it is necessary to study the laser-induced periodic structures of ultraviolet optical elements through a large number of exploratory experiments. Through theoretical analysis of the formation mechanism of laser-induced periodic structures of ultraviolet optical elements, a component processing surface model under high-power laser irradiation is established by setting an equal-density ion point group, and a two-dimensionally distributed Gaussian femtosecond laser model is used. Based on Maxwell's equations and Newton-Lorentz equations, the motion behavior law of plasma in the process of interaction between short-pulse lasers and the surface of ultraviolet optical elements is studied, and the formation of laser-induced periodic structures of ultraviolet optical elements is simulated. The method of the present invention can accurately simulate the formation of laser-induced periodic structures of ultraviolet optical elements.

Description

Technical Field

[0001] The invention relates to the technical field of engineering optics, and in particular to a method for predicting a laser-induced periodic structure of an ultraviolet optical element. Background Art

[0002] Under certain circumstances, the interaction between short-pulse lasers and materials can produce a striped microstructure (LIPSS) with a certain periodicity on the surface of the material. LIPSS has been widely recognized by the world as a unique structure that can be obtained by laser micro-nano manufacturing. It can be widely prepared on metals, semiconductors, ceramics, electrolytes and polymers. It has been widely used in optics, optoelectronics, electrochemistry, biology and other fields, and has great practical value. Preparing high-quality LIPSS microstructures on the surfaces of different materials to improve the performance of the materials has also become a research hotspot in recent years. However, due to the incompleteness of the current research on the formation theory and process technology of LIPSS, the application of LIPSS microstructures in various fields is limited.

[0003] Ultraviolet optical elements are an important part of modern optical elements. According to their functions, they can be divided into energy regulation, laser pumping, optical frequency conversion, spectroscopy and polarization. However, due to the low laser damage threshold, poor light transmission performance and high absorption rate of current ultraviolet optical elements in the process of use, they still cannot meet the urgent needs of current industrial production and scientific research. The preparation of LIPSS microstructures on the surface of ultraviolet optical elements by short-pulse laser processing methods such as femtosecond and picosecond lasers to improve the optical properties of ultraviolet optical elements is a good way to solve current problems and has become a research hotspot in the field of optical material processing. For a certain specific optical material, laser processing parameters (pulse width, laser operating flux and laser wavelength) play a decisive role in the successful preparation of high-quality LIPSS microstructures. Due to the imperfect formation theory and process technology of LIPSS microstructures, a large number of exploratory experiments are needed to obtain ideal laser processing parameters to prepare LIPSS microstructures on the processing surface of ultraviolet optical elements. Since the preparation of high-quality LIPSS on the surface of optical components is jointly affected by a variety of laser processing parameters, it is undoubtedly time-consuming and laborious to successfully prepare high-quality LIPSS on the surface of different types of UV optical components (especially expensive high-purity UV optical components) by relying only on a large number of exploratory experiments. At the same time, some UV optical materials such as potassium dihydrogen phosphate (KDP) and large-caliber fused quartz optical components are expensive. A large number of exploratory experiments will consume a large number of optical components and generate huge economic costs. Therefore, a prediction method for laser-induced periodic structures of UV optical components is urgently needed to assist exploratory experiments in quickly obtaining optimized laser processing parameters, thereby saving manpower, material and financial resources. Summary of the invention

[0004] The technical problems to be solved by the present invention are:

[0005] The existing technology for the formation theory and process technology of laser-induced microstructures of ultraviolet optical elements is still imperfect, and a large number of exploratory experiments are needed to study the laser-induced periodic structures of ultraviolet optical elements.

[0006] The present invention adopts the following technical solutions to solve the above technical problems:

[0007] The present invention provides a method for predicting a laser-induced periodic structure of an ultraviolet optical element, comprising the following steps:

[0008] Step 1: determine the type of ground state electrons ionized on the surface of the ultraviolet optical element under high power laser irradiation conditions;

[0009] Step 2: Calculate the ground state electron density of ionization on the surface of the ultraviolet optical element according to the lattice structure of the element;

[0010] Step 3, by setting up an ion point group of equal density to establish a processing surface model of an ultraviolet optical element under high-power laser irradiation; each ion point contains two positive and negative ions of equal charge; when the atom is not ionized, the positive ion represents the nucleus and the negative ion represents the electron outside the nucleus; when the atom is ionized, the positive ion represents the cation and the negative ion represents the multiple free electrons generated by the ionization;

[0011] Select the charge ratio of the ion points in the model according to the calculation accuracy requirements, and determine the positive and negative ion densities and the positive and negative ion charges in the model according to the charge ratio and the ground state electron density of ionization obtained in step 2;

[0012] The expression of the power ratio is:

[0013] N=n e / n (1)

[0014] Among them, n e is the ground state electron density of ionization on the surface of the component, and n is the positive and negative ion density in the model;

[0015] Step 4: establishing a laser model in the ultraviolet optical element processing surface model, wherein the laser model is a model of the laser electric field component, and solving the laser model to obtain the laser electric field intensity;

[0016] Step 5, determine the time step of the force on the ion point;

[0017] Step 6: Take the interaction time between the single-point laser and the ultraviolet optical element as the threshold; obtain the electric field intensity of the ion point group, and combine it with the laser electric field intensity obtained in step 4, calculate the electromagnetic field intensity, electric field force and movement speed of each positive and negative ion, calculate the next position of each positive and negative ion according to the determined time step, electric field force and movement speed, perform iterative calculations in sequence, and stop when the cumulative movement time reaches the threshold, so as to obtain the laser-induced periodic structure morphology of the ultraviolet optical element;

[0018] Step 7: Extract and analyze the laser-induced periodic structure of the ultraviolet optical element to determine the microstructure morphology formed by single-point laser processing.

[0019] Furthermore, the ultraviolet optical element is one of KDP crystal optical element, fused quartz, GDP, ADP or BK7 glass.

[0020] Furthermore, the electron density n in step 2 e The calculation formula is:

[0021]

[0022] Wherein, m is the number of ground state electrons that are ionized in a single lattice structure of the ultraviolet optical element, and V is the volume of a single lattice structure of the ultraviolet optical element.

[0023] Furthermore, in the step three, PIC software is used to establish a processing surface model of the ultraviolet optical element under high-power laser irradiation through programming.

[0024] Furthermore, in step 3, the positive and negative ion charges are determined according to the following formula:

[0025] Q=N×e(3)

[0026] Among them, Q is the charge of positive and negative ions.

[0027] Furthermore, the laser model in step 4 is:

[0028]

[0029]

[0030]

[0031] Among them, E max is the maximum value of the laser electric field intensity, ω(x) is the spot radius of the incident high-power laser, R(x) is the curvature radius of the incident high-power laser, is the phase factor of the incident high-power laser, ω 0 is the beam waist diameter of the incident high-power laser, η 0is the wave impedance in vacuum, n 2 is the refractive index of the medium, I max is the peak intensity of the incident high-power laser.

[0032] Furthermore, in step 5, the time step of the force on the ion point is determined according to the Courant condition, and the calculation formula is:

[0033]

[0034] Where c is the speed of light, dx and dy are the X and Y dimensions of the calculation area occupied by the ion point, respectively.

[0035] Furthermore, in step six, the electric field strength of the ion point group is obtained by Coulomb's law, and it is combined with the laser electric field strength obtained in step four. By solving Maxwell's equations, the electromagnetic field strength and the electric field force at each positive and negative ion are obtained.

[0036] Furthermore, in step six, the movement speed of each positive and negative ion is obtained by solving the Newton-Lorentz motion equation according to the electric field force on each positive and negative ion.

[0037] Furthermore, Origin mathematical software is used to extract the laser-induced periodic structure of the ultraviolet optical element.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] The invention discloses a prediction method for a laser-induced periodic structure of an ultraviolet optical element. The method theoretically analyzes the formation mechanism of the laser-induced periodic structure, establishes a processing surface model of the ultraviolet optical element under high-power laser irradiation by setting an ion point group of equal density, and uses a two-dimensional distributed Gaussian laser model to simulate laser irradiation. The motion behavior law of plasma in the process of interaction between a short-pulse laser and the surface of the ultraviolet optical element is studied based on Maxwell's equations and Newton-Lorentz equations. The formation of the laser-induced periodic structure of the ultraviolet optical element is accurately simulated.

[0040] The present invention can be used to assist and guide the experiment of preparing laser-induced periodic structures on the surface of materials. The optimized process parameter combination in the process of preparing laser-induced periodic structures is obtained through simulation methods, which greatly reduces the time and expensive sample materials required for the experiment of preparing laser-induced periodic structures. At the same time, the present invention can explore the influencing factors of the characteristic parameters of periodic structures from an atomic perspective, provide new insights into the formation mechanism of periodic structures, and help promote the further development and application of laser-induced periodic structures in various fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1is a flow chart of a method for predicting a laser-induced periodic structure of an ultraviolet optical element in an embodiment of the present invention;

[0042] Figure 2 is a prediction model of laser-induced periodic structure of ultraviolet optical element in an embodiment of the present invention;

[0043] Figure 3 The processing morphology of the laser-induced periodic structure of the ultraviolet optical element after binarization processing in the embodiment of the present invention;

[0044] Figure 4 This is the result of extracting information of laser-induced periodic structure of ultraviolet optical element in an embodiment of the present invention. DETAILED DESCRIPTION

[0045] In the description of the present invention, it should be noted that the terms "first", "second", and "third" mentioned in the embodiments of the present invention are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first", "second", and "third" may explicitly or implicitly include one or more of the features.

[0046] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0047] This embodiment uses KDP optical elements as research objects, and uses the method of the present invention to induce a periodic structure of LIPSS microstructure on the surface of the KDP optical element by femtosecond laser; Figure 1 As shown, the following steps are included:

[0048] Step 1: Determine the type of ground state electrons that are ionized on the surface of ultraviolet optical elements under high-power laser irradiation conditions.

[0049] Existing studies have shown that the hydrogen bond energy in KDP crystals is extremely low (≈0.042eV). Under high-power laser irradiation or high temperature conditions (T>373K), it is easy to break and produce free hydrogen atoms. The lone pair electrons (ground state electrons) in the free hydrogen atoms are unstable and are easy to ionize and transition to the conduction band to become free electrons. Existing studies believe that only when the laser operating flux is slightly greater than the laser damage threshold of the material can the LIPSS microstructure be successfully prepared on the surface of the material. Therefore, the laser operating flux required to successfully prepare the LIPSS microstructure on the surface of the material is not too high. According to existing reports, the ground state electrons that are ionized when preparing the LIPSS microstructure on the surface of the KDP optical element are the lone pair electrons of the free hydrogen atoms; therefore, it is determined that the type of ground state electrons ionized on the surface of the KDP optical element under femtosecond laser irradiation is the lone pair electrons of the free hydrogen atoms on the surface of the element.

[0050] Step 2: Calculate the ground state electron density of ionization on the surface of the ultraviolet optical element based on the lattice structure of the element.

[0051] Since a single lattice of KDP crystal contains two KH 2 PO 4 molecules, so the number of hydrogen atoms contained in a single lattice in the KDP crystal is m = 4, and the KDP lattice volume is Therefore, the ground state electron density n that is ionized on the surface of the KDP element under femtosecond laser irradiation is e It can be obtained by the following formula:

[0052]

[0053] Here, m is the number of ground state electrons that are ionized in a single lattice structure of the KDP element, and V is the volume of a single lattice structure of the KDP element.

[0054] Step 3, by setting up an ion point group of equal density to establish a processing surface model of an ultraviolet optical element under high-power laser irradiation; each ion point contains two positive and negative ions of equal charge; when the atom is not ionized, the positive ion represents the nucleus and the negative ion represents the electron outside the nucleus; when the atom is ionized, the positive ion represents the cation and the negative ion represents the multiple free electrons generated by the ionization;

[0055] Determine the charge ratio of the ion points in the model according to the accuracy requirements, and determine the positive and negative ion densities and the positive and negative ion charges in the model according to the charge ratio and the ground state electron density of ionization obtained in step 2;

[0056] The expression of the power ratio is:

[0057] N=n e / n (2)

[0058] Among them, n e is the ground state electron density of ionization occurring on the surface of the component, and n is the positive and negative ion density in the model.

[0059] At present, existing research at home and abroad believes that the formation of LIPSS microstructure originates from the Coulomb explosion behavior caused by the generation of plasma. Coulomb explosion behavior is a phenomenon in which strong electromagnetic fields couple electron excitation energy to atomic motion. The Coulomb force of particles with the same charge can break the bonds (forces) that maintain the solid. When the ground state electrons on the surface of the KDP optical element are ionized into free electrons, the remaining positively charged atomic nuclei explode and detach from the material surface due to the electromagnetic force of like charges repelling each other. Therefore, the LIPSS microstructure of the surface of the KDP optical element is related to the position information of the free electrons (plasma) on the surface of the element under femtosecond laser irradiation, that is, the position information of the free electrons mainly determines the LIPSS morphological characteristics of the surface of the element during the processing process.

[0060] like Figure 2 As shown, a two-dimensional coordinate system is constructed, and a UV optical element processing surface model with a calculation area width of 15 μm and a height of 30 μm is constructed.

[0061] According to the ground state electron density n determined in step 2, the ionized surface of the KDP optical element processed under femtosecond laser irradiation e =1.11×10 28 m -3 ; In order to reasonably simulate the electron ionization process of the material surface during the actual femtosecond laser processing, the charge ratio N of the ion point in the model is set to 1000. According to formula (2), the positive and negative ion densities in the model are determined to be n = 1.11 × 10 25 m -3 .

[0062] Calculate the charge of each positive and negative ion in the ion point group; the charge of the positive ions and negative ions in the model are shown in equations (3) and (4), respectively:

[0063] Q 1 =1000×e=1.6×10 -18 C (3)

[0064] Q 2 =1000×e=1.6×10 -18 C (4)

[0065] Where e is the charge carried by an electron, e = 1.6 × 10 -19 C, Q 1 is the charge carried by the positive ion, Q 2 The charge carried by negative ions.

[0066] Step 4: Figure 2 As shown, a laser model is established in the ultraviolet optical element processing surface model, the laser model is a model of the laser electric field component, and the laser model is solved to obtain the laser electric field intensity.

[0067] The femtosecond laser used is a TEM electromagnetic wave. The magnetic field component and the electric field component of the TEM electromagnetic wave are perpendicular to each other and have completely consistent distribution characteristics. Therefore, only the electric field component is selected for research, and a Gaussian femtosecond laser model with two-dimensional distribution is established as shown in formulas (5) to (7).

[0068]

[0069]

[0070]

[0071] Among them, E max The maximum value of the laser electric field intensity, ω(x) is the spot radius of the incident high-power laser, R(x) is the curvature radius of the incident high-power laser, is the phase factor of the incident high-power laser, ω 0 is the beam waist diameter of the incident high-power laser, η 0 is the wave impedance in vacuum, n 2 is the refractive index of the medium, I max is the peak intensity of the incident high-power laser, and λ is the laser wavelength.

[0072] According to the KDP optical element surface laser damage threshold measurement data published by the French Atomic Energy Association (CEA): the wavelength is 800nm ​​and the laser flux is 68TW / cm 2 , with a pulse width of 55fs. Existing studies at home and abroad believe that only when the operating laser flux is slightly greater than the laser damage threshold of the material can the LIPSS microstructure be successfully prepared on the surface of the material. Therefore, the operating laser flux of the femtosecond laser model is set to 70TW / cm 2 , wavelength is 800nm, pulse width is 55fs. According to the formula Determine the peak intensity I of the incident high-power laser max , and set the parameters: beam waist radius ω 0 is 3.55μm, and the medium refractive index n 2 is 1.49, η 0 The vacuum wave impedance is 377Ω; the laser model is solved to obtain the laser electric field intensity.

[0073] Step 5: Determine the time step of the force on the ion point.

[0074] In the electromagnetic wave model, the time step satisfies the Courant-Friedrichs-Lewy condition (CFL condition), that is:

[0075]

[0076] Where c is the speed of light, dx and dy are the dimensions of the calculation area occupied by the ion point in the X and Y directions respectively; the time step Δt = 0.05fs of the force applied to the ion point is determined according to formula (8).

[0077] Step six, taking the interaction time of the single-point laser and the ultraviolet optical element as the threshold; obtain the electric field strength of the ion point group, and combine it with the laser electric field strength obtained in step four, calculate the electromagnetic field strength, electric field force and movement speed of each positive and negative ion, calculate the next position of each positive and negative ion according to the determined time step, electric field force and movement speed, perform iterative calculations in sequence, and stop when the cumulative movement time reaches the threshold, to obtain the laser-induced periodic structural morphology of the ultraviolet optical element.

[0078] With reference to existing research, the single laser-material interaction time is set to 300fs. The electric field intensity of the ion point group under high-power laser irradiation is obtained by Coulomb's law, and it is combined with the laser electric field intensity obtained in step four. By solving the Maxwell equations, the electromagnetic field intensity at each positive and negative ion in the calculation area is obtained, and the electric field force on each positive and negative ion is obtained. According to the electric field force on each positive and negative ion, the movement speed of each positive and negative ion is obtained by solving the Newton-Lorentz motion equation. The position of each positive and negative ion at the next moment is calculated according to the determined time step, the electric field force on each positive and negative ion, and the movement speed. It is iterated until the cumulative movement time reaches the threshold of 300fs. Finally, the position of each positive and negative ion at different times under high-power laser irradiation can be obtained. In order to further study the surface morphology of the laser-induced periodic structure of the KDP optical element, the position image of each positive and negative ion in the model is binarized. As Figure 3 Shown is a binary image of the laser-induced periodic structure processing morphology of the KDP optical element at 130 fs.

[0079] Step 7: Extract and analyze the laser-induced periodic structure of the ultraviolet optical element to determine the microstructure morphology formed by single-point laser processing.

[0080] The Origin mathematical software was used to extract information from the predicted processing morphology of the laser-induced periodic structure of the KDP optical element after binarization, and the following results were obtained: Figure 4 The laser-induced periodic structure processing morphology of the KDP optical element is shown. By analyzing the structure processing morphology, the LIPSS microstructure morphology formed by single-point laser processing is determined, which can be further used to predict the LIPSS microstructure morphology formed by laser scanning processing.

[0081] This embodiment uses KDP optical elements as an example for research, and the method of the present invention can be extended to the prediction of laser-induced periodic structures of ultraviolet optical elements such as fused quartz, GDP, ADP, BK7 glass, etc.

[0082] Although the present invention is disclosed as above, the protection scope of the present invention is not limited thereto. Those skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention, and these changes and modifications will fall within the protection scope of the present invention.

Claims

1. A method for predicting laser-induced periodic structures of ultraviolet optical elements, Features The steps include: Step 1: determine the type of ground state electrons ionized on the surface of the ultraviolet optical element under high power laser irradiation conditions; Step 2: Calculate the ground state electron density of ionization on the surface of the ultraviolet optical element according to the lattice structure of the element; Step 3, by setting up an ion point group of equal density to establish a processing surface model of an ultraviolet optical element under high-power laser irradiation; each ion point contains two positive and negative ions of equal charge; when the atom is not ionized, the positive ion represents the nucleus and the negative ion represents the electron outside the nucleus; when the atom is ionized, the positive ion represents the cation and the negative ion represents the multiple free electrons generated by the ionization; Select the charge ratio of the ion points in the model according to the calculation accuracy requirements, and determine the positive and negative ion densities and the positive and negative ion charges in the model according to the charge ratio and the ground state electron density of ionization obtained in step 2; The expression of the power ratio is: N=n e / n (1) Among them, n e is the ground state electron density of ionization on the surface of the component, and n is the positive and negative ion density in the model; Step 4: establishing a laser model in the ultraviolet optical element processing surface model, wherein the laser model is a model of the laser electric field component, and solving the laser model to obtain the laser electric field intensity; Step 5, determine the time step of the force on the ion point; Step 6: Take the interaction time between the single-point laser and the ultraviolet optical element as the threshold; obtain the electric field intensity of the ion point group, and combine it with the laser electric field intensity obtained in step 4, calculate the electromagnetic field intensity, electric field force and movement speed of each positive and negative ion, calculate the next position of each positive and negative ion according to the determined time step, electric field force and movement speed, perform iterative calculations in sequence, and stop when the cumulative movement time reaches the threshold, so as to obtain the laser-induced periodic structure morphology of the ultraviolet optical element; Step 7: Extract and analyze the laser-induced periodic structure of the ultraviolet optical element to determine the microstructure morphology formed by single-point laser processing.

2. The method according to claim 1, Features The ultraviolet optical element is one of KDP crystal optical element, fused quartz, GDP, ADP or BK7 glass.

3. The method according to claim 1, Features The electron density n in step 2 e The calculation formula is: Wherein, m is the number of ground state electrons that are ionized in a single lattice structure of the ultraviolet optical element, and V is the volume of a single lattice structure of the ultraviolet optical element.

4. The method according to claim 1, Features The step three uses PIC software to establish the processing surface model of the ultraviolet optical element under high-power laser irradiation through programming.

5. The method according to claim 4, Features In step 3, the positive and negative ion charges are determined according to the following formula: Q=N×e (3) Among them, Q is the charge of positive and negative ions.

6. The method according to claim 1, Features The laser model described in step 4 is: Among them, E max is the maximum value of the laser electric field intensity, ω(x) is the spot radius of the incident high-power laser, R(x) is the curvature radius of the incident high-power laser, is the phase factor of the incident high-power laser, ω 0 is the beam waist diameter of the incident high-power laser, η 0 is the wave impedance in vacuum, n 2 is the refractive index of the medium, I max is the peak intensity of the incident high-power laser.

7. The method according to claim 1, Features Step 5: Determine the time step of the force on the ion point based on the Courant condition. The calculation formula is: Where c is the speed of light, dx and dy are the X and Y dimensions of the calculation area occupied by the ion point, respectively.

8. The method according to claim 6, Features In step six, the electric field strength of the ion point group is obtained by Coulomb's law, and it is combined with the laser electric field strength obtained in step four. By solving Maxwell's equations, the electromagnetic field strength and the electric field force at each positive and negative ion are obtained.

9. The method according to claim 8, Features In step six, the movement speed of each positive and negative ion is obtained by solving the Newton-Lorentz motion equation according to the electric field force on each positive and negative ion.

10. The method according to claim 1, Features Origin mathematical software is used to extract the laser-induced periodic structure of ultraviolet optical elements.

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