Method for improving strong-laser-damage resistance of nonlinear artificial crystal

Abstract

The present invention relates to a method for improving strong-laser-damage resistance of a nonlinear artificial crystal, comprising using a direct-current regulated power supply to energize a nonlinear artificial crystal under a certain atmosphere condition. By means of an energization method, the density of pinpoint damage in the structure of a nonlinear artificial crystal can be effectively reduced, thereby optimizing strong-laser-damage resistance of the nonlinear artificial crystal; and the effect is more obvious particularly for the defects having higher damage thresholds. The provided energization treatment is expected to further reduce the damage probability when the fluence of an ultraviolet nanosecond pulse width laser having a wavelength of 355 nm or 351 nm exceeds 8 J / cm2, thereby improving the damage resistance of a crystal. On the basis of laser pretreatment, further performing energization treatment is a new comprehensive method for improving the damage resistance of a crystal, having huge economic advantages.

Description

A method to improve the resistance of nonlinear artificial crystals to strong laser damage Technical Field

[0001] This invention belongs to the field of nonlinear artificial crystal technology, specifically relating to a method for improving the resistance of nonlinear artificial crystals to strong laser damage. Background Technology

[0002] Nonlinear artificial crystals are essential optical materials in lasers and large-scale high-power laser devices. Potassium dihydrogen phosphate (KDP) nonlinear crystals, and other similar crystals with similar lattice structures, such as deuterated potassium dihydrogen phosphate (DKDP) with different deuteration rates and ammonium dihydrogen phosphate (ADP), are currently important nonlinear artificial crystals with widespread commercial applications. These crystals are typically used as frequency doubling elements or electro-optic switching elements in laser devices and are always exposed to high-power laser radiation. The laser intensity I (unit: W / cm²) irradiating the crystal is... 2 After exceeding a certain threshold (I) th Laser light will cause irreversible damage inside the crystal, the most prominent damage form being spatially discrete needle-like structures. An important physical quantity for quantitatively describing this needle-like damage is the 1-on-1 method, using a small-aperture laser (beam area approximately 1 mm²). 2 The damage probability (P) is defined as the probability of pinpoint damage occurring when a crystal is irradiated at different locations with a pulsed laser of the same intensity. The damage probability is positively correlated with the number of pinpoint damages per unit volume within the crystal (i.e., the damage number density ρ). To ensure proper laser operation, ρ should be as low as possible. Higher ρ values ​​result in stronger laser energy dissipation, more severe effects of scattered laser light on the laser's components, and more pronounced modulation of the transmitted beam. Therefore, reducing the probability of pinpoint damage under a given laser intensity is a crucial research area for improving crystal performance.

[0003] To improve the resistance of grown KDP and similar crystals to strong laser damage, two techniques have been developed over the past 40 years: thermal annealing and laser pretreatment. Thermal annealing involves slowly raising the crystal temperature to above 100°C, holding the sample at that temperature for a certain period, and then slowly lowering the crystal temperature back to room temperature. The basic principle of laser pretreatment is to assume that the damage threshold of untreated KDP and similar crystals is I. th_i If the laser intensity is greater than I th_i Direct laser irradiation of a crystal will cause irreversible bulk damage to the crystal's interior. However, lasers with an intensity slightly lower than I can be used instead. th_i Lasers, for example, with an intensity of 0.5 times that of I... th_iThe crystal is irradiated with a certain number of laser pulses, followed by an intensity of I. th_i When a crystal is irradiated with a laser, the crystal will no longer be damaged, thereby increasing the damage threshold of the crystal.

[0004] While both crystal processing techniques can improve the crystal's resistance to high-intensity laser damage to some extent by addressing internal defects, there are limitations to performance enhancement. Even after comprehensive optimization of these two methods, the performance of KDP-type crystals against high-intensity laser damage can only reach a certain level, requiring new methods for further improvement. Furthermore, laser pretreatment is very costly, necessitating more economical approaches.

[0005] Summary of the Invention

[0006] The purpose of this invention is to provide a method for improving the resistance of nonlinear artificial crystals to strong laser damage. This invention can effectively reduce the density of pinpoint damage in the structure of nonlinear artificial crystals by using the "electrical method", thereby reducing the probability of crystal damage and optimizing the resistance of nonlinear artificial crystals to strong laser damage. In particular, the improvement effect is more obvious for defects with higher damage thresholds.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] This invention provides a method for improving the resistance of nonlinear artificial crystals to strong laser damage, comprising the following steps:

[0009] A regulated DC power supply is used to energize the nonlinear artificial crystal.

[0010] Preferably, the voltage applied is 0.5 to 100 kV.

[0011] Preferably, the energizing time is 3 to 48 hours.

[0012] Preferably, during the energizing process, the temperature of the nonlinear artificial crystal is 20–25°C.

[0013] Preferably, the nonlinear artificial crystal comprises a nonlinear optical crystal with a potassium deuterated phosphate or a similar crystal structure.

[0014] Preferably, the nonlinear artificial crystal comprises potassium dihydrogen phosphate, deuterated potassium dihydrogen phosphate, or ammonium dihydrogen phosphate.

[0015] Preferably, the energization is performed under normal pressure or vacuum conditions.

[0016] Preferably, the nonlinear artificial crystal is a nonlinear artificial crystal that has undergone thermal annealing treatment.

[0017] This invention provides a method for improving the resistance of nonlinear artificial crystals to high-intensity laser damage, comprising the following steps: energizing the nonlinear artificial crystal with a constant DC power supply. Under the influence of a high-intensity laser, the nonlinear artificial crystal will develop volume damage within the crystal. The most common morphology of volume damage is a hollow micro-explosion crater with a size of several micrometers to tens of micrometers; this type of damage is commonly referred to as pinpoint damage. When the laser intensity exceeds the damage threshold of the crystal, due to the different densities of damage precursors within each crystal, the density of pinpoint damage (ρ) within the crystal increases. p ) will be in 10 -2 -10 6 / mm 3 The density of needle-like damage varies. The higher the density of needle-like damage, the greater the damage to laser devices using crystals. Therefore, the development of technologies to improve the laser damage resistance of nonlinear artificial crystals requires in-depth research into crystal damage mechanisms. While there is some understanding of the physical mechanism of needle-like damage in KDP crystals under strong laser irradiation, it is not complete. It is generally believed that the formation of needle-like damage in KDP crystals under strong laser irradiation is due to the presence of damage precursors of approximately 100 nm in size within the crystal, with a concentration of approximately 10%. 19 / cm 3Point defects, in a specific precursor, have a higher concentration, resulting in a lower damage threshold. A long-standing challenge in the study of KDP crystal damage mechanisms is identifying the specific types of point defects in 100-nanometer-sized precursors. This invention utilizes electron paramagnetic resonance (EPR) technology to study point defects in DKDP and similar crystals, gaining a new and deeper understanding of the physical mechanism of high-intensity laser irradiation damage. The most significant advancement is the deduction, based on EPR experimental results, that the specific point defect types are interstitial hydrogen (deuterium) and hydrogen (deuterium) vacancies. Based on this understanding, this invention proposes and experimentally verifies a new method for reducing the point defect concentration in 100-nanometer precursors to improve crystal damage resistance. This invention proposes a novel crystal processing method, the "electrification method," where interstitial hydrogen (deuterium) and hydrogen (deuterium) vacancies in 100-nanometer precursors are electrically charged point defects. If a steady electric field of a certain strength is applied to them, under the action of a sufficiently strong electric field force, positively charged interstitial hydrogen (deuterium) will migrate from the damage precursor along the direction of the electric field, while negatively charged hydrogen (deuterium) vacancies will migrate from the damage precursor against the direction of the electric field. When their migration distance significantly exceeds the size of the precursor, the concentration of point defects in the precursor will decrease, thereby increasing the damage threshold of a single damage precursor and effectively reducing the density of pinpoint damage formed under a certain laser intensity. The method of improving the crystal's resistance to strong laser damage by energizing the crystal in this invention is completely different from the traditional annealing and laser pretreatment methods. The method provided by this invention shows a significant ability to reduce the probability of pinpoint damage, thereby optimizing the resistance of nonlinear artificial crystals to strong laser damage, especially for defects with higher damage thresholds, the effect is more obvious. Attached Figure Description

[0018] Figure 1 is a schematic diagram of the crystal sample position and size used in the DKDP crystal energization experiment in an embodiment of the present invention;

[0019] Figure 2 is a schematic diagram of the energizing tank structure for energizing DKDP crystals in an embodiment of the present invention;

[0020] In Figure 2: 1 is the tank, 2 is the ceramic plate, 3 is the clamp, 4 is the nonlinear artificial crystal, 5 is the first metal electrode, 6 is the second metal electrode, 7 is the second terminal, 8 is the first terminal, and 9 is the temperature sensor.

[0021] Figure 3 is a photograph of the energizing tank used for energizing the DKDP crystal in an embodiment of the present invention;

[0022] Figure 4 shows the optical path diagram for measuring the damage probability curve of a small-aperture frequency-harmonic laser DKDP crystal.

[0023] Figure 5 shows the comparative experimental results of the DKDP crystal damage probability curves after no power and after power-on treatment in Example 1. Detailed Implementation

[0024] This invention provides a method for improving the resistance of nonlinear artificial crystals to strong laser damage, comprising the following steps:

[0025] A constant DC power supply is used to energize the nonlinear artificial crystal.

[0026] In this invention, unless otherwise specified, all raw materials / components used in the preparation are commercially available products well known to those skilled in the art.

[0027] In this invention, the nonlinear artificial crystal preferably comprises a nonlinear optical crystal with a potassium dihydrogen phosphate (KDP) or a similar crystal structure, more preferably potassium dihydrogen phosphate (DKDP), or ammonium dihydrogen phosphate (ADP). In this invention, the deuteration rate of the DKDP is preferably 5-98%. In a specific embodiment of this invention, the "electro-current method" of this invention is described in detail using DKDP as an example. The deuteration rate of the DKDP crystal is 70%. The DKDP crystal is an insulator with a large band gap (Eg). The band gap of the DKDP crystal is 7.5-9 eV.

[0028] In this invention, the nonlinear artificial crystal is preferably a nonlinear artificial crystal that has undergone thermal annealing. In a specific embodiment of this invention, the size of the nonlinear artificial crystal is 1cm × 1cm × 5cm. Before applying the current, the nonlinear artificial crystal blank, preferably artificially grown and thermally annealed, is cut into crystals according to specific orientations and sizes based on the application requirements. In a specific embodiment of this invention, the artificially grown and thermally annealed nonlinear artificial crystal blank is subjected to type II matching cutting to first obtain a crystal with a size of 5cm × 5cm × 1cm, and then further cut.

[0029] In this invention, the surface of the nonlinear artificial crystal preferably has an optical-grade finish. Before applying the current, the nonlinear artificial crystal is preferably polished; in a specific embodiment of this invention, the polishing process is preferably fly-cut.

[0030] In this invention, the energizing is preferably performed in a crystal energizing container. A schematic diagram of the crystal energizing container used in this embodiment is shown in Figure 2. The crystal energizing container includes: a container body, a ceramic plate disposed on the bottom surface inside the container body, a crystal clamp and parallel plate metal electrodes disposed within the container body, and a sealing cover; the sealing cover is equipped with a temperature sensor and metal electrode fixing parts. The container body is made of stainless steel. Preferably, the nonlinear artificial crystal is clamped between a pair of parallel energizing metal electrodes, and the clamped crystal is placed in the stainless steel energizing container. Before energizing, the air in the crystal energizing container is first extracted using a vacuum pump to achieve a vacuum degree of 10⁻⁶ inside the container. -3 Pa, and then dry nitrogen gas is introduced into the canister. Subsequently, the crystal energized canister is placed in a constant temperature chamber, with a temperature control accuracy of 0.1℃ during energization. This invention introduces dry nitrogen gas into the canister to ensure an insulating atmosphere, thereby applying a sufficiently high electric field strength to the crystal. In this invention, the power supply is a DC regulated power supply. This invention uses a DC high-voltage power supply to continuously provide a stable DC voltage to the crystal. In this invention, the energizing voltage is preferably 0.5–100 kV, more preferably 0.8 kV, 0.95 kV, or 10 kV. The energizing time is preferably 3–48 h, more preferably 3 h, 8 h, or 48 h. In specific embodiments of this invention, the energizing voltage and energizing time are related to the defect parameters of the nonlinear artificial crystal. Higher energizing voltage results in shorter energizing time. However, at a specific energizing voltage, a saturation effect occurs with increasing energizing time, meaning the damage performance no longer improves. In a specific embodiment of the present invention, when the thickness of the nonlinear artificial crystal is preferably 1 cm, the voltage applied is preferably 0.5 × 10⁻⁶. 3 V~10 5 V. In this invention, during the energizing process, the temperature of the nonlinear artificial crystal is preferably 20-25°C. The protective gas is preferably nitrogen; the pressure of the protective gas is preferably atmospheric pressure.

[0031] In this invention, the needle-like body damage occurring in nonlinear artificial crystals, including KDP (DKDP) crystals, under high-intensity laser irradiation is due to the presence of discretely distributed defects within the artificially grown crystal, commonly referred to as damage precursors. The damage precursors are approximately 100 nm in size and contain approximately 10% hydrogen-related concentrations. 19 / cm 3The point defects are specifically interstitial hydrogen (deuterium) and hydrogen (deuterium) vacancies. Both interstitial hydrogen (deuterium) and hydrogen (deuterium) vacancies are electrically charged point defects. If a steady electric field of a certain strength is applied to them, under the influence of a sufficiently strong electric field, the positively charged interstitial hydrogen (deuterium) will migrate from the damage precursor along the direction of the electric field, while the negatively charged hydrogen (deuterium) vacancies will migrate from the damage precursor against the direction of the electric field. When their migration distance significantly exceeds the size of the precursor, the concentration of point defects in the precursor will decrease, thereby increasing the damage threshold of a single damage precursor and effectively reducing the density of needle-like damage formed under a certain laser intensity.

[0032] Interstitial hydrogen (deuterium) vacancies and hydrogen (deuterium) vacancies have opposite charges and are attractive to each other. To separate them, the driving electric field must reach a certain strength. The lower the concentration of hydrogen point defects in the precursor, the higher the corresponding damage threshold. Conversely, the lower the concentration of point defects in the precursor, the lower the electric field strength required to migrate point defects. This explains why electrostatic treatment is more effective at treating defects with high damage thresholds.

[0033] This invention employs an energizing method as a novel approach to treating KDP and similar crystals, which significantly reduces the probability of crystal damage and holds promise for producing better experimental results.

[0034] The most widely used laser pretreatment technique for KDP / DKDP crystals currently employs a third-harmonic laser (wavelength 355 nm) output from a sub-nanosecond Nd:YAG laser. The latest reported pretreatment experimental results were published in the 2021 issue of Acta Physica Sinica (Liu Zhichao, et al., Offline sub-nanosecond laser pretreatment technique for large-aperture potassium dihydrogen phosphate crystals. Acta Physica Sinica, 2021.). The authors described the experimental parameters as follows: a pulse width of approximately 0.5 nanoseconds, a spot diameter of 0.68 mm, a wavelength of 355 nm, and a maximum pretreatment laser flux of 2 J / cm². 2 Maximum laser power density 4GW / cm² 2 Sub-nanosecond laser pretreatment can increase the zero-probability bulk damage threshold of DKDP crystals by about 1 time (Sun Shaotao, Wang Zhengping, Xu Xinguang, Study on the Damage Effect of Annealing on DKDP Crystals. Abstracts of Academic Papers of the Fourth National Congress and Academic Conference of the Chinese Crystallographic Society, 2008). (See Figure 11 in Reference 2), reaching approximately 8 J / cm. 2 .

[0035] The current-pass processing method provided by this invention is expected to further reduce laser flux to greater than 8 J / cm. 2The probability of subsequent damage. Improving the crystal's damage resistance. Therefore, based on laser pretreatment, further performing the current-pass treatment provided by this invention is a new method to improve the crystal's damage resistance.

[0036] On the other hand, the current price of laser pretreatment equipment is around 3 million yuan, while the price of the electro-energized processing equipment provided by this invention is around 100,000 yuan, which has a huge economic advantage.

[0037] To further illustrate the present invention, the technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0038] Example 1

[0039] This embodiment uses DKDP crystal as an example. DKDP crystal has a band gap of (7.5-9) eV, resulting in very low electron and hole concentrations at room temperature. Its ability to conduct electricity through electrons and holes is weak, classifying it as an insulator with very low DC conductivity. On the other hand, DKDP crystal contains hydrogen bonds with bond energies less than 1 eV. At room temperature and higher temperatures, it can generate a certain concentration of negatively charged hydrogen vacancies and positively charged interstitial hydrogens. These point defects become conductive charge carriers under the influence of an applied electric field. Therefore, the conductivity of DKDP crystal is ionic.

[0040] This embodiment provides a method for improving the laser damage resistance of DKDP crystals, specifically including the following steps:

[0041] Step 1: Take a piece of artificially grown DKDP crystal blank that has undergone heat annealing, as shown in Figure 1. Cut the crystal according to the specific direction and size required for the application of the crystal (in this embodiment, the crystal was cut using type II matching), to obtain a crystal with a size of 5cm×5cm×1cm.

[0042] Step 2: Further cut the crystal into four initially adjacent cuboids, each measuring 5cm × 1cm × 1cm. Their initial resistance to strong laser damage is essentially the same. The cut crystals are then polished using a specific process (such as fly-cutting) to achieve an optical-grade finish.

[0043] Step 3: In this embodiment, the crystal energizing process is performed in the crystal energizing tank shown in Figures 2 and 3. The DKDP crystal, processed in Step 2 and measuring 1cm × 1cm × 5cm, is stably clamped between a pair of energized metal electrodes, and the clamped crystal is placed in a stainless steel energizing tank. Before energizing, the air in the energizing tank is first extracted using a vacuum pump to achieve a vacuum level of 10⁻⁶ inside the tank. -3The pressure was increased to Pa, and then dry nitrogen gas at one atmosphere was introduced into the container. The container was then placed in a constant temperature chamber with a temperature control accuracy of 0.1℃, and the crystal sample temperature was maintained at 25℃ during energization. A DC high-voltage power supply continuously provided a stable DC voltage to the crystals. The crystal numbered DKDP-1 was energized at 0.95kV for 48 hours; the crystal numbered DKDP-2 was energized at 10kV for 3 hours; the crystal numbered DKDP-3 served as a control sample and was not energized; and the crystal numbered DKDP-4 was energized at 0.8kV for 8 hours.

[0044] Step 4: The laser damage resistance of the DKDP crystal after power-on treatment was tested using a small aperture (beam area approximately 1 mm²) with a wavelength of 355 nm. 2 A laser device was used to measure the laser damage probability curve. The laser pulse width was 5 ns, and the near-field modulation index was 2.4. The measurement method was 1-on-1, with 10 laser fluxes measured per laser. The optical path diagram of the device is shown in Figure 4.

[0045] Figure 5 shows the test results of the damage probability curves of the four crystal samples obtained from the energizing experiment in Example 1. In Figure 5, the horizontal axis represents the average laser flux of the test laser with a pulse width of 5 ns and a wavelength of 355 nm, and the vertical axis represents the probability of damage to the DKDP crystal under a certain laser flux. Overall, the damage probability of the energized crystal, under the same laser intensity, is significantly lower than that of the untreated crystal sample, demonstrating that applying a steady DC electric field of a certain intensity to the DKDP crystal for a certain period of time can significantly reduce the probability of damage. Table 1 summarizes the experimental data of the four samples in Example 1. As can be seen from Table 1, for the first set of data, when the laser flux is 9.6 J / cm... 2 Near the target location, the probability of damage to the untreated sample DKDP-3 was 90%, while the damage probability for the other three treated crystals was between 20% and 30%. For the second set of data, when the laser flux was 11.7 J / cm²... 2 Near the sample, sample DKDP-3, which had not undergone electrification, had a 100% probability of damage, while the other three electrified crystals had a damage probability between 40% and 50%. Both sets of data indicate that the damage probability of the crystals decreased by more than 50% after electrification.

[0046] Table 1. Experimental results showing the change in the damage probability of DKDP crystals before and after energization.

[0047] The above results show that energizing DKDP crystals can reduce the probability of crystal damage and optimize their resistance to strong laser damage, especially for defects with higher damage thresholds.

[0048] As shown in the above embodiments, this invention provides a method for improving the laser damage resistance of nonlinear artificial crystals, comprising the following steps: energizing the nonlinear artificial crystal with a power source in a protective gas atmosphere. This invention, through the "energizing method," can effectively reduce the density of pinpoint damage in the nonlinear artificial crystal structure, lower the crystal damage probability, and thus optimize the nonlinear artificial crystal's resistance to strong laser damage. The effect is particularly pronounced for defects with higher damage thresholds. The energizing method provided by this invention is expected to further reduce laser flux to greater than 8 J / cm². 2 The probability of subsequent damage. Improving the crystal's damage resistance. Therefore, based on laser pretreatment, further performing the current-pass treatment provided by this invention is a new comprehensive method to improve the crystal's damage resistance, with significant economic advantages.

[0049] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. Other embodiments can be obtained based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for improving the resistance of nonlinear artificial crystals to strong laser damage, characterized in that, Includes the following steps: A DC regulated power supply is used to energize the nonlinear artificial crystal.

2. The method according to claim 1 or 2, characterized in that, The voltage applied is 0.5–100 kV.

3. The method according to claim 1 or 2, characterized in that, The energizing time is 3 to 48 hours.

4. The method according to claim 1, characterized in that, During the energizing process, the temperature of the nonlinear artificial crystal is 20-25°C.

5. The method according to claim 1, characterized in that, The nonlinear artificial crystal includes potassium deuterated dihydrogen phosphate or a nonlinear optical crystal with a similar crystal structure to potassium deuterated dihydrogen phosphate.

6. The method according to claim 5, characterized in that, The nonlinear artificial lens includes potassium dihydrogen phosphate, deuterated potassium dihydrogen phosphate, or ammonium dihydrogen phosphate.

7. The method according to claim 1, characterized in that, The energization is performed under normal pressure or vacuum conditions.

8. The method according to claim 1 or 5, characterized in that, The nonlinear artificial crystal is a nonlinear artificial crystal that has undergone thermal annealing.

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