Method for improving the anti-strong laser damage performance of nonlinear intraocular lenses

Abstract

The present application belongs to the technical field of nonlinear artificial crystal, and particularly relates to a method for improving the strong laser damage resistance of a nonlinear artificial crystal. The present application can effectively reduce the density of the needle point damage in the structure of the nonlinear artificial crystal by the method of electrification, so as to optimize the strong laser damage resistance of the nonlinear artificial crystal, and the effect is more obvious for the defects with a higher damage threshold. The method of electrification provided by the present application is expected to further reduce the probability of damage after the wavelength of 355nm or 351nm ultraviolet nanosecond pulse width laser flux is greater than 8J / cm 2 The method of electrification provided by the present application is expected to further reduce the probability of damage after the wavelength of 355nm or 351nm ultraviolet nanosecond pulse width laser flux is greater than 8J / cm On the basis of the laser pretreatment, the electrification treatment provided by the present application is a new comprehensive method for improving the damage resistance of the crystal, and has great economic advantages.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nonlinear artificial crystal, and particularly relates to a method for improving the strong laser damage resistance of a nonlinear artificial crystal. BACKGROUND

[0002] Nonlinear artificial crystals are essential key optical materials in lasers and large-scale high-power laser devices. Potassium dihydrogen phosphate (KDP for short) nonlinear crystals and other similar crystals with similar crystal lattice structures, such as deuterated potassium dihydrogen phosphate (DKDP for short) with different deuterium contents, ammonium dihydrogen phosphate (ADP for short) and the like, are important nonlinear artificial crystals that are widely used in commercial applications. These crystals are usually used as frequency doubling elements or electro-optical switching elements in laser devices and are always exposed to strong laser irradiation. When the laser intensity I (unit: W / cm 2 ) irradiated on the crystal is greater than a certain threshold value (I th ), irreversible damage will be formed in the crystal, and the main damage form is spatially discrete needle-shaped body damage. An important physical quantity for quantitatively describing the needle-shaped body damage is the damage probability (P) measured by the 1on1 method with a small-diameter laser (beam area of about 1mm 2 ). The definition is that the crystal is irradiated at different positions by pulse laser with the same intensity, and the probability of occurrence of needle-shaped body damage is detected. The damage probability and the number of needle-shaped body damage in a unit volume in the crystal (i.e., damage number density ρ) are positively correlated. In order to ensure that the laser can work normally, the lower the ρ value, the better. The higher the ρ value, the stronger the dissipation of laser energy, the more serious the influence of scattered laser on the components of the laser, and the more obvious the modulation of the transmitted light beam. Therefore, reducing the needle-shaped damage probability under a certain laser intensity is a very important research content for improving the performance of the crystal.

[0003] In order to improve the strong laser damage resistance of KDP and its similar crystals that have been grown, two types of technologies, namely, thermal annealing and laser pretreatment, have been developed in the past 40 years. The thermal annealing technology is to slowly raise the temperature of the crystal to more than 100℃, maintain the sample temperature for a certain period of time, and then slowly reduce the temperature of the crystal to room temperature. The basic method of the laser pretreatment technology is that the damage threshold of an untreated KDP and its similar crystals is I th_i , and if the crystal is directly irradiated by laser with an intensity greater than I th_i , irreversible bulk damage will be caused in the crystal. However, the crystal can be irradiated by laser with an intensity slightly less than I th_i , such as laser with an intensity of 0.5 times I th_i , for a certain number of pulses, and then irradiated by laser with an intensity of I th_iWhen the laser irradiates the crystal, the crystal no longer suffers from damage, thereby improving the damage threshold of the crystal.

[0004] Although the two crystal processing techniques can improve the strong laser damage resistance of the crystal to a certain extent by improving the defects in the crystal, the performance improvement has bottlenecks, and after comprehensive optimization of the two methods, the strong laser damage performance index of the KDP crystal can only reach a certain level, and a new method is needed to continue to improve. In addition, the economic cost of laser pretreatment is also very high, and a more economical method is needed. SUMMARY

[0005] The purpose of the present application is to provide a method for improving the strong laser damage resistance of a nonlinear artificial crystal. The present application can effectively reduce the density of needle-shaped damage in the structure of a nonlinear artificial crystal by using the "power-on method", thereby reducing the damage probability of the crystal and optimizing the strong laser damage resistance of the nonlinear artificial crystal. The improvement effect is more obvious for defects with higher damage threshold.

[0006] In order to achieve the above purpose, the present application provides the following technical scheme:

[0007] The present application provides a method for improving the strong laser damage resistance of a nonlinear artificial crystal, comprising the following steps:

[0008] A stabilized DC power supply is used to power on the nonlinear artificial crystal.

[0009] Preferably, the voltage of the power-on is 0.5-100kV.

[0010] Preferably, the power-on time is 3-48h.

[0011] Preferably, the temperature of the nonlinear artificial crystal during the power-on process is 20-25℃.

[0012] Preferably, the nonlinear artificial crystal includes deuterated potassium dihydrogen phosphate or a nonlinear optical crystal with a crystal structure similar to that of deuterated potassium dihydrogen phosphate.

[0013] Preferably, the nonlinear artificial crystal includes potassium dihydrogen phosphate, deuterated potassium dihydrogen phosphate, or ammonium dihydrogen phosphate.

[0014] Preferably, the power-on is carried out under normal pressure or vacuum conditions.

[0015] Preferably, the nonlinear artificial crystal is a nonlinear artificial crystal that has been subjected to heat annealing treatment.

[0016] The application provides a method for improving the strong laser damage resistance of a nonlinear artificial crystal, comprising the following steps: applying a steady constant direct current power supply to the nonlinear artificial crystal. When the nonlinear artificial crystal is subjected to strong laser, a bulk damage will be formed in the crystal. The most common damage morphology of the bulk damage is a hollow micro-explosion pit with a size of several microns to tens of microns, and this damage is usually referred to as a pinpoint damage. When the laser intensity exceeds the damage threshold of the crystal, due to the difference in the density of the damage precursors in the crystal, the density (ρ p ) of the pinpoint damage in the crystal will change between 10 -2 -10 6 / mm 3 . The higher the density of the pinpoint damage is, the greater the harm to the laser device using the crystal is. The development of the technology for improving the strong laser damage resistance of the nonlinear artificial crystal needs in-depth research on the damage mechanism of the crystal. The physical mechanism of the pinpoint bulk damage of the KDP crystal under the action of strong laser is known to a certain extent, but is not complete. It is generally believed that the KDP crystal forms the pinpoint damage under the action of strong laser because of the existence of a damage precursor with a size of about 100 nm in the crystal, and the damage precursor contains a concentration of about 10 19 / cm 3The higher the concentration of point defects of a specific precursor is, the lower the damage threshold is. In the study of KDP crystal damage mechanism, the specific type of point defects in the precursor with a size of hundreds of nanometers has long been a difficult problem. The present application uses electron paramagnetic technology to study the point defects of DKDP and its similar crystals, and obtains new and in-depth understanding of the physical mechanism of crystal strong laser irradiation damage. The most important progress is that the specific type of point defects is interstitial hydrogen (deuterium) and hydrogen (deuterium) vacancy through the inference of the experimental results of electron paramagnetic resonance. According to this understanding, the present application proposes and verifies through experiments a new method for reducing the concentration of point defects in the precursor with a size of hundreds of nanometers and improving the damage resistance of the crystal. The present application proposes a new crystal processing method "power-on method". The interstitial hydrogen (deuterium) and hydrogen (deuterium) vacancy in the precursor with a size of hundreds of nanometers are both point defects with electric properties. If a certain intensity of steady electric field is applied to them, the interstitial hydrogen (deuterium) with positive electric property will migrate from the damaged precursor in the direction of the electric field under the action of the electric field force, and the hydrogen (deuterium) vacancy with negative electric property will migrate from the damaged precursor in the direction opposite to the electric field. When the migration distance of them significantly exceeds the size of the precursor, the concentration of point defects in the precursor will be reduced, so that the damage threshold of a single damaged precursor is improved, thereby effectively reducing the density of the needle-shaped damage formed under a certain laser intensity. The method for improving the strong laser damage of the crystal by applying power to the crystal is completely different from the traditional annealing method and laser pretreatment method. The method provided by the present application shows the ability to significantly reduce the probability of needle-shaped damage, thereby optimizing the strong laser damage resistance of the nonlinear artificial crystal, and the effect is more obvious for defects with a higher damage threshold. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 The figure is a schematic diagram of the position and size of the crystal sample used in the power-on experiment of the DKDP crystal in the embodiment of the present application.

[0018] Figure 2 The figure is a schematic diagram of the structure of the power-on tank used for the power-on of the DKDP crystal in the embodiment of the present application.

[0019] Figure 2 In the figure, 1 is a tank body, 2 is a ceramic sheet, 3 is a clamp, 4 is a nonlinear artificial crystal, 5 is a first metal electrode, 6 is a second metal electrode, 7 is a second terminal post, 8 is a first terminal post, and 9 is a temperature sensor.

[0020] Figure 3 The figure is a photograph of the power-on tank used for the power-on of the DKDP crystal in the embodiment of the present application.

[0021] Figure 4 The figure is a diagram of the light path for measuring the damage probability curve of the small-diameter three-frequency laser DKDP crystal.

[0022] Figure 5 The comparative experimental results of the damage probability curves of DKDP crystals before and after the non-energized and energized treatments of Example 1. DETAILED DESCRIPTION

[0023] The application provides a method for improving the strong laser damage resistance of a nonlinear artificial crystal, which comprises the following steps:

[0024] The nonlinear artificial crystal is energized by using a steady constant direct current power supply.

[0025] In the application, all the preparation raw materials / components are commercially available products known to those skilled in the art, unless otherwise specified.

[0026] In the application, the nonlinear artificial crystal preferably comprises a deuterated potassium dihydrogen phosphate or a nonlinear optical crystal with a crystal structure similar to that of the deuterated potassium dihydrogen phosphate, and more preferably comprises potassium dihydrogen phosphate (KDP), deuterated potassium dihydrogen phosphate (DKDP) or ammonium dihydrogen phosphate (ADP). In the application, the deuteration rate of the DKDP is preferably 5-98%. In the specific embodiments of the application, the "energization method" of the application is described in detail by taking DKDP as an example. The deuteration rate of the DKDP crystal is 70%. The DKDP crystal is an insulator and has a large band gap (Eg). The band gap of the DKDP crystal is 7.5-9 eV.

[0027] In the application, the nonlinear artificial crystal is preferably a nonlinear artificial crystal after heat annealing treatment. In the specific embodiments of the application, the size of the nonlinear artificial crystal is 1 cm x 1 cm x 5 cm. Before the energization, the artificial grown and heat annealed nonlinear artificial crystal blank is cut according to the application requirements of the crystal in a specific direction and size. In the specific embodiments of the application, the artificial grown and heat annealed nonlinear artificial crystal blank is cut in a type II matching manner to obtain a crystal with a size of 5 cm x 5 cm x 1 cm, and then the crystal is further cut.

[0028] In the application, the surface of the nonlinear artificial crystal preferably has an optical finish. Before the energization, the nonlinear artificial crystal is preferably polished, and in the specific embodiments of the application, the polishing is preferably fly cutting.

[0029] In the application, the energization is preferably performed in a crystal energization tank. The structure of the crystal energization tank used in the embodiments of the application is shown in Figure 2The crystal energizing tank comprises a tank body, ceramic sheets arranged on the inner bottom surface of the tank body, a crystal clamp and parallel plate metal electrodes arranged in the tank body, and a sealing cover; the sealing cover is provided with a temperature sensor and a metal electrode fixing part. The tank body material of the crystal energizing tank is stainless steel. The nonlinear artificial crystal is clamped between a pair of parallel energized metal electrodes, and the clamped crystal is placed in the stainless steel energized tank. In the present application, the air in the crystal energizing tank is first pumped out by a vacuum pump before energization, so that the vacuum degree in the tank body reaches 10 - 3 Pa, then dry nitrogen is filled into the tank body, and then the crystal energizing tank is placed in a thermostat, and the temperature control accuracy of the thermostat during energization is 0.1℃. The dry nitrogen filled into the tank body in the present application ensures that the atmosphere in the tank body is an insulating atmosphere, so as to apply a high enough electric field strength to the crystal. In the present application, the power supply is a direct current stabilized power supply. A direct current high voltage power supply is used to continuously provide a stable direct current voltage to the crystal. In the present application, the voltage during energization is preferably 0.5-100kV, more preferably 0.8kV, 0.95kV or 10kV. The energization time is preferably 3-48h, more preferably 3h, 8h or 48h. In the specific embodiments of the present application, the voltage during energization and the energization time are related to the defect parameters of the nonlinear artificial crystal. At the same time, the higher the voltage during energization, the shorter the required energization treatment time. However, under a specific energization voltage, with the increase of the energization time, saturation effect will occur, that is, the damage performance no longer improves. In the specific embodiments of the present application, when the thickness of the nonlinear artificial crystal is preferably 1cm, the voltage during energization is preferably 0.5x10 3 V-10 5 V. In the present application, the temperature of the nonlinear artificial crystal during energization is preferably 20-25℃. The protective gas is preferably nitrogen; the pressure of the protective gas is preferably normal pressure.

[0030] In the present application, the reason why the nonlinear artificial crystal including KDP (DKDP) crystal is damaged in the form of needle point under the action of strong laser is that the artificially grown crystal contains discrete distributed defects, which are usually also called damage precursors. The size of the damage precursor is about 100nm, and the internal hydrogen element related concentration is about 10 19 / cm 3Point defects, specifically interstitial hydrogen (deuterium) and hydrogen (deuterium) vacancy. Both interstitial hydrogen (deuterium) and hydrogen (deuterium) vacancy are electrically charged point defects. If a steady electric field of a certain strength is applied to them, under the action of a strong enough electric field force, interstitial hydrogen (deuterium) with positive electrical charge will migrate from the damage precursor in the direction of the electric field, and hydrogen (deuterium) vacancy with negative electrical charge will migrate from the damage precursor against the direction of the electric field. When the migration distance of them significantly exceeds the size of the precursor, the concentration of point defects in the precursor will decrease, and the damage threshold of a single damage precursor will be improved, thereby effectively reducing the density of pinhole-like damage formed under a certain laser intensity.

[0031] The electrical properties of interstitial hydrogen (deuterium) and hydrogen (deuterium) vacancy are opposite, and there is an attractive force between them. In order 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, and the lower the concentration of point defects in the precursor, the lower the electric field strength required to migrate the point defects out of the precursor, which can explain why the treatment effect of the power-on method is more significant for defects with high damage threshold.

[0032] The present application adopts the power-on method as a new treatment method for KDP and its similar crystals, which can significantly reduce the probability of crystal damage and is expected to produce better experimental results.

[0033] The most widely used KDP / DKDP crystal laser pre-treatment technology at present is carried out by using a third harmonic laser (wavelength 355 nm) output by a nanosecond Nd:YAG laser. The latest reported pre-treatment experimental results are published in the 2021 issue of the Physical Review (Liu Z C, et al., Offline nanosecond laser pre-treatment technology for large-aperture deuterated potassium dihydrogen phosphate crystals. Physical Review, 2021.), and the authors describe the experimental parameters as follows: the pulse width of the pulsed laser acting on the DKDP crystal to be pre-treated is about 0.5 nanoseconds, the spot diameter is 0.68 mm, the wavelength is 355 nm, the maximum laser flux is 2 J / cm 2 , and the maximum laser power density is 4 GW / cm 2 . Nanosecond laser pre-treatment can increase the zero-probability damage threshold of DKDP crystal by about 1 times (Sun S T, Wang Z P, Xu X G, Study on the effect of annealing on DKDP crystal damage. Abstracts of the Fourth National Congress of the Chinese Crystal Society and Academic Conference, 2008.) (see Figure 11 in document 2), to about 8 J / cm 2 .

[0034] The power-on method provided by the present application is expected to further reduce the laser flux to more than 8 J / cm 2The damage probability after that is reduced. The damage resistance of the crystal is improved. Therefore, on the basis of the laser pre-treatment, the power-on treatment provided by the present application is a new method for improving the damage resistance of the crystal.

[0035] On the other hand, the current price of the laser pre-treatment equipment is about 3 million yuan, while the price of the power-on treatment equipment provided by the present application is about 100,000 yuan, which has a great economic advantage.

[0036] In order to further illustrate the present application, the technical solutions provided by the present application are described in detail below in conjunction with examples, but they should not be understood as limiting the scope of protection of the present application.

[0037] Example 1

[0038] In this embodiment, DKDP crystal is taken as an example. The band gap width of DKDP crystal is (7.5-9) eV, the concentration of electrons and holes is very low at room temperature, the ability of conduction through electrons and holes is weak, and it belongs to insulator, and the direct current conductivity is very small. On the other hand, DKDP crystal contains hydrogen bonds with bond energy less than 1 eV, and at room temperature and higher temperature, a certain concentration of negative monovalent hydrogen vacancies and positive monovalent interstitial hydrogen can be generated. Under the action of an external electric field, these two kinds of point defects become conductive carriers, so the conduction characteristics of DKDP crystal belong to ionic conduction.

[0039] The present application provides a method for improving the laser damage resistance of DKDP crystal, which specifically comprises the following steps:

[0040] Step 1: Take a piece of artificially grown DKDP crystal embryo which has been treated by a heat annealing process, as shown in Figure 1 , cut the crystal according to the specific direction and size according to the application requirements of the crystal, (in this embodiment, the crystal is cut according to type II matching), to obtain a crystal with a size of 5 cm x 5 cm x 1 cm.

[0041] Step 2: Further cut the crystal into four pieces of adjacent crystals with an initial position, and the size of each piece is 5 cm x 1 cm x 1 cm, and their initial strong laser damage resistance performance is basically the same. The cut crystal is polished according to a certain process (such as fly cutting) to complete the surface polishing, so that the surface reaches the optical level of smoothness.

[0042] Step 3: In this embodiment, the power-on treatment of the crystal is carried out in a crystal power-on tank as shown in Figure 2 and Figure 3 . The DKDP crystal with a size of 1 cm x 1 cm x 5 cm processed by step 2 is stably clamped between a pair of power-on metal electrodes, and the clamped crystal is placed in a stainless steel power-on tank. Before power-on, the air in the power-on tank is first pumped out by a vacuum pump, so that the vacuum degree inside the tank body reaches 10 -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.

[0043] 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 pulse. The optical path diagram of the device is shown below. Figure 4 As shown.

[0044] The test results of the damage probability curves of the four crystal samples obtained from the energizing experiment in Example 1 are as follows: Figure 5 As shown. 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. Figure 5 The vertical axis represents the probability of damage to the DKDP crystal under a certain laser flux. Overall, the damage probability of the crystal after electrostatic treatment is significantly lower than that of the untreated crystal sample at the same laser intensity, 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 / cm²... 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.

[0045] Table 1. Experimental results showing the change in the damage probability of DKDP crystals before and after energization.

[0046]

[0047] The above results show that the energization of the DKDP crystal has the effect of reducing the probability of crystal damage and optimizing the anti-strong laser damage performance of the crystal, and the effect is more obvious for defects with a higher damage threshold.

[0048] The above examples show that the present application provides a method for improving the anti-laser damage performance of a nonlinear artificial crystal, which comprises the following steps: energizing the nonlinear artificial crystal by using a power supply in a protective gas atmosphere. The present application can effectively reduce the density of the needle-shaped damage in the structure of the nonlinear artificial crystal and reduce the probability of crystal damage, thereby optimizing the anti-strong laser damage performance of the nonlinear artificial crystal, and the effect is more obvious for defects with a higher damage threshold. The energization method provided by the present application is expected to further reduce the probability of damage when the laser flux is greater than 8 J / cm 2 and improve the anti-damage performance of the crystal. Therefore, on the basis of laser pretreatment, further energization treatment provided by the present application is a new comprehensive method for improving the damage performance of the crystal, which has great economic advantages.

[0049] Although the above examples have described the present application in detail, they are only part of the embodiments of the present application, not all the embodiments, and other embodiments can be obtained under the premise of no creativity according to the present embodiments, which all belong to the protection scope of the present application.

Claims

1. A method for improving the anti-strong laser damage performance of a nonlinear intraocular lens, characterized in that, The method comprises the following steps: The nonlinear artificial crystal is electrified by a direct current stabilized power supply; the nonlinear artificial crystal is potassium dihydrogen phosphate, deuterium potassium dihydrogen phosphate or ammonium dihydrogen phosphate; the strong laser damage is a pinhole damage; the voltage of the electrification is 0.5-10 kV.

2. The method of claim 1, wherein, The electrification time is 3-48 h.

3. The method of claim 1, wherein, During the electrification, the temperature of the nonlinear artificial crystal is 20-25 ℃.

4. The method of claim 1, wherein, The electrification is carried out under normal pressure or vacuum condition.

5. The method of claim 1, wherein, The nonlinear artificial crystal is a nonlinear artificial crystal subjected to heat annealing treatment.

Images