Method for improving the resistance of a nonlinear artificial crystal against damage caused by a powerful laser

Abstract

The present invention, using a power supply to connect a nonlinear artificial crystal to an electrical source under certain atmospheric conditions, can effectively reduce the density of point damage in the nonlinear artificial crystal by means of the "electrical connection method," thereby optimizing the resistance of the nonlinear artificial crystal against damage caused by a powerful laser, particularly the effect being more pronounced for defects with a higher damage threshold. The electrical connection treatment is expected to further reduce the probability of damage caused by an ultraviolet nanosecond pulse-width laser flux with a wavelength of 355 nm or 351 nm that exceeds 8 J / cm² and improve the crystal's resistance to damage. Based on laser pretreatment, this novel method for improving crystal resistance to damage offers a significant economic advantage.

Description

Title of the invention: METHOD FOR IMPROVING THE RESISTANCE OF A NONLINEAR ARTIFICIAL CRYSTAL AGAINST DAMAGE CAUSED BY A POWERFUL LASER FIELD OF INVENTION

[0001] The present invention belongs to the technical field of nonlinear artificial crystals, in particular relates to a method for improving the resistance of a nonlinear artificial crystal against damage caused by a powerful laser. TECHNICAL BACKGROUND

[0002] Nonlinear artificial crystals are indispensable as key optical materials in lasers and large, powerful laser devices. The nonlinear artificial crystal of potassium dihydrogen phosphate (KDP), and other classes of crystals with similar lattice structures, such as deuterated potassium dihydrogen phosphate (DKDP) and ammonium dihydrogen phosphate (ADP), which have different deuteration ratios, are important nonlinear artificial crystals that have been widely used in commercial applications. These crystals, often used in laser devices such as frequency multipliers or electro-optical switches, are always exposed to irradiation from a powerful laser.When the intensity of the laser beam I (in W / cm²) projected onto the crystal exceeds a certain threshold (Ith), the laser beam causes irreversible damage within the crystal, primarily in the form of point damage distributed discretely within a space. An important physical quantity for describing point damage is the damage probability (P), measured using the Ionl method by a small-aperture laser (the beam area is approximately 1 mm²). This is defined as the probability of point damage detected by projecting a pulsed laser beam of the same intensity onto a crystal at different positions. The damage probability is positively correlated with the number of point damages (i.e., the damage density p) per unit volume in the crystal. To ensure proper laser operation, the lower the p, the better.The higher the p-value, the greater the dissipation of laser energy, the more severe the impact of the scattered laser beam on the laser components, and the more pronounced the modulation of the transmitted beam. Therefore, reducing the probability of point damage at a given laser intensity is crucial in research aimed at improving crystal performance.

[0003] In order to improve the resistance capacity of KDP and its equivalents against damage caused by the powerful laser, two types of technologies were developed during Over the last 40 years, thermal annealing and laser pretreatment have been key techniques. Thermal annealing involves slowly raising the crystal temperature to over 100°C and maintaining that temperature for a certain period, then slowly reducing the crystal temperature to room temperature. Laser pretreatment involves the following: assuming the damage threshold of untreated KDP and its equivalents is 1 thJ, if a laser with an intensity greater than 1 thJ is used to irradiate the crystal, it will cause irreversible damage. However, a laser with an intensity slightly less than 1 thJ, such as a laser with an intensity of 0.5 times 1 thJ, irradiates the crystal with a number of pulses, followed by a second irradiation of 1 thJ. At this point, the crystal will no longer be damaged, thus increasing the crystal's damage threshold.

[0004] Although these two crystal processing technologies can improve the resistance of crystals to damage caused by the powerful laser to some extent by reducing internal crystal defects, bottlenecks exist with respect to performance improvement. After complete optimization of these two methods, the current crystal resistance performance index equivalent to the KDP against damage can only reach a certain level; this necessitates a new method to continuously improve resistance performance. Furthermore, the economic cost of laser pretreatment is also high, and more economical methods are needed. SUMMARY OF THE INVENTION

[0005] The objective of the present invention is to provide a method for improving the resistance of a nonlinear artificial crystal against damage caused by a powerful laser. The present invention can effectively lower the density of point damage in the nonlinear artificial crystal and reduce the probability of crystal damage by means of the "electrical branching method," thereby optimizing the resistance of the nonlinear artificial crystal against damage caused by a powerful laser, particularly against defects with a higher damage threshold, and the improvement effect is more pronounced.

[0006] In order to achieve the above objective, the present invention provides the technical solution as follows.

[0007] A method for improving the resistance of a nonlinear artificial crystal against damage caused by a powerful laser according to the present invention includes a step of using a regulated DC power supply to connect a nonlinear artificial crystal to an electrical source.

[0008] Preferably, the electrical connection voltage is 0.5-100 kV.

[0009] Preferably, the electrical connection time is 3 ~48h.

[0010] Preferably, the temperature of the nonlinear artificial crystal is 20-25 °C during electrical connection.

[0011] Preferably, the nonlinear artificial crystal includes a deuterated potassium dihydrogen phosphate or a nonlinear optical crystal equivalent to the crystal structure of the deuterated potassium dihydrogen phosphate.

[0012] Preferably, the nonlinear artificial crystal includes potassium dihydrogen phosphate, deuterated potassium dihydrogen phosphate or ammonia dihydrogen phosphate.

[0013] Preferably, the electrical connection is made under atmospheric pressure or vacuum conditions.

[0014] Preferably, the nonlinear artificial crystal is a nonlinear artificial crystal treated during a thermal annealing.

[0015] The present invention relates to a method for improving the resistance of a nonlinear artificial crystal against damage caused by a powerful laser, which includes a step of using a regulated DC power supply to connect a nonlinear artificial crystal to an electrical source. Under the action of a powerful laser, the nonlinear artificial crystal will form damage within the crystal. The most common damage morphology is a pit of micro-explosions that varies in size from a few microns to tens of microns, often called pinpoint damage. After the laser intensity exceeds the crystal's damage threshold, the density of pinpoint damage (pp) within the crystal varies from 10A-2 to 10A6 / mm3 due to the difference in density of the damage precursors within each crystal.The higher the density of point damage, the more serious the hazard posed by lasers using crystals. Developing technology to improve the damage resistance of nonlinear artificial crystals requires extensive research into the crystal damage mechanism. While some knowledge exists regarding the physical mechanism of point damage in KDP crystals under the action of a powerful laser, it is not complete. It is generally believed that the formation of point damage in KDP crystals under the action of a powerful laser is attributed to the presence of damage precursors approximately 100 nanometers in size within the crystal. These precursors contain point defects at a concentration of about 10 Ź⁹ / cm³. For a specific precursor, the higher the concentration of point defects, the lower the damage threshold.During the study of the damage mechanism of KDP crystals, one becomes bogged down in the specific types of point damage in precursors with a size of 100 nanometers for too long. The present invention adopts an electronic paramagnetic method. to study the point defects of DKDP and equivalent crystals, and obtains new and in-depth knowledge of the physical mechanism of crystal damage caused by strong laser irradiation. The most significant development is the deduction that the specific shape of point defects is a hydrogen (deuterium) gap and a hydrogen (deuterium) free space, based on the results of electron paramagnetic resonance experiments. Based on this knowledge, the present invention proposes and verifies experimentally a new method for reducing the concentration of point damage in precursors with a size of 100 nanometers and improving the crystal's resistance to damage.The present invention proposes a novel crystal treatment method, the "electrical branching method." The hydrogen (deuterium) interstices and hydrogen (deuterium) free spaces in precursors with a size of 100 nanometers are both point defects possessing electrical properties. If a constant electric field of a certain intensity is applied to the hydrogen (deuterium) interstices and hydrogen (deuterium) free spaces, under the influence of a sufficiently strong electric field force, the hydrogen (deuterium) interstices with positive electrical properties will move from the damage precursor in the direction along the electric field, and the hydrogen (deuterium) free spaces with negative electrical properties will move from the damage precursor in the direction against the electric field.Once their walking distances significantly exceed the size of the precursor, the defect concentration in the precursor decreases, thereby increasing the damage threshold of a single precursor, and thus effectively reducing the density of point damage formed at a given laser intensity. The method of the present invention for improving the resistance of a crystal against damage caused by a powerful laser by means of electrical connection to the crystal is completely different from the traditional thermal annealing and laser pretreatment method. The process provided by the present invention demonstrates the ability to significantly reduce the probability of point damage, thereby optimizing the resistance of the nonlinear artificial crystal against damage caused by a powerful laser, particularly against defects with a higher damage threshold, and the improvement effect is more evident. BRIEF DESCRIPTIONS OF THE FIGURES

[0016] [Fig.1] is a schematic illustration of the position size of the crystalline sample used for the DKDP crystal electrical branching experiment in the embodiment of the present invention.

[0017] [Fig.2] is a schematic illustration of the structure of the branching container electrical used for the electrical connection to the DKDP crystal in the embodiment of the present invention.

[0018] In [Fig. 2], 1- container body; 2- ceramic sheet; 3- clamp; 4- crystal artificial non-linear; 5-first metallic electrode; 6-second metallic electrode; 7-second link station; 8-first link station; 9-temperature sensor.

[0019] [Fig.3] is an optical path diagram used to measure the curve of probability of damage to the DKDP crystal caused by a small-aperture triple-frequency laser.

[0020] In [Fig. 3], 10- laser; 11- CW diode laser; 15- energy meter; 16- spectroscope; 17-convex lens; 18-DKDP sample to be observed.

[0021] [Fig.4] shows comparative experimental results between the curves of probability of damage to the DKDP crystal which makes an electrical connection and which does not make an electrical connection in example 1. Description of the invention

[0022] A method for improving the resistance of a nonlinear artificial crystal against damage caused by a powerful laser according to the present invention includes a step of using a regulated DC power supply to connect a nonlinear artificial crystal to an electrical source.

[0023] In the present invention, unless otherwise indicated, all raw materials / components intended for preparation are commercially available products well known to those skilled in the art.

[0024] In the present invention, the artificial nonlinear crystal preferably includes a deuterated potassium dihydrogen phosphate or a nonlinear optical crystal equivalent to the crystal structure of deuterated potassium dihydrogen phosphate, and further preferably includes potassium dihydrogen phosphate (KDP), deuterated potassium dihydrogen phosphate (DKDP), or ammonia dihydrogen phosphate (ADP). In the present invention, the deuteration of DKDP is preferably 5-98%. In specific embodiments of the present invention, DKDP is used as an example to describe the "electrical connection method" of the present invention in detail. The deuteration 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.

[0025] In the present invention, the nonlinear artificial crystal is preferably a nonlinear artificial crystal treated by thermal annealing. In specific embodiments of the present invention, the dimensions of the nonlinear artificial crystal are 1 cm x 1 cm x 5 cm. In the present invention, before electrical connection, the blank of the artificially grown and thermally annealed nonlinear artificial crystal is preferably cut according to the requirements application of the crystal and according to a specific direction and size. In the specific embodiments of the present invention, the artificially grown non-linear artificial crystal blank, treated by the thermal annealing process, is cut twice into two pairing classes, i.e., a crystal with a size of 5 cm x 5 cm x 1 cm is first obtained, then it is cut further.

[0026] In the present invention, the surface of the nonlinear artificial crystal preferably has optically high flatness. In the present invention, before making the electrical connection, the nonlinear artificial crystal is preferably polished, and in specific embodiments of the present invention, the polishing treatment is preferably carried out in the form of fly cutting.

[0027] In the present invention, the electrical connection is preferably made in the crystal electrical connection container. [Fig. 2] is a schematic illustration of the structure of the crystal electrical connection container used in the embodiment of the present invention. The crystal electrical connection container comprises: a container body, a ceramic sheet disposed on the base surface inside the container body, a crystal clamp and a parallel-plate metal electrode disposed in the container body, and a sealing cover. The sealing cover includes a temperature sensor and a metal electrode retaining piece. The crystal electrical connection container is made of stainless steel.In the present invention, the nonlinear artificial crystal is preferably clamped between a pair of metal electrodes connected to an electrical source, and the clamped crystal is placed in a stainless steel electrical connection container. In the present invention, the air in the crystal electrical connection container is first pumped out by a vacuum pump before electrical connection, so that the vacuum inside the container reaches 10A-3Pa. Then, dry nitrogen is infused into the container, and subsequently, the crystal electrical connection container is placed in a thermostatic box, and the temperature of the thermostatic box is precisely controlled to 0.1°C during electrical connection.In the present invention, dry nitrogen is infused inside the container body to ensure that the container body is filled with an insulating atmosphere, so as to exert a sufficiently high electric field intensity on the crystal. In the present invention, the power source employs a regulated DC power supply. In the present invention, a high-voltage DC power source is used to continuously supply a stable DC voltage to the crystal. In the present invention, the voltage of the power connection is preferably 0.5–100 kV, and more preferably 0.8 kV, 0.95 kV, or 10 kV. The power connection time is preferably 3–48 hours, and more preferably 3, 8, or 48 hours. In the specific embodiment of the present... In this invention, the electrical connection voltage and connection time are related to the fault parameters of the nonlinear artificial crystal. Furthermore, the higher the electrical connection voltage, the shorter the processing time requiring electrical connection. However, under a specific electrical connection voltage, as the connection time increases, a saturation effect appears, meaning that the resistance to damage does not improve. In the specific embodiment of the present invention, when the nonlinear artificial crystal is preferably 1 cm thick, the electrical connection voltage is preferably 0.5 x 0.3 V ~ 10 x 0.5 V. In the present invention, the temperature of the nonlinear artificial crystal is preferably 20 ~ 25 °C during electrical connection. The shielding gas is preferably nitrogen.The pressure of the protective gas is preferably atmospheric pressure.

[0028] In the present invention, the formation of point damage in the nonlinear artificial crystal including KDP (DKDP) crystals under the action of a powerful laser is attributed to the existence of discretely distributed defects contained within the artificially grown crystal; these are also commonly called damage precursors. The defect precursor is approximately 100 nm in size and contains a point defect with a concentration of approximately 10 Ź⁹ / cm³, which is related to the hydrogen element within it. The specific form of the point defects is a hydrogen (deuterium) gap and a hydrogen (deuterium) free space. Both hydrogen (deuterium) gaps and hydrogen (deuterium) free spaces are point defects having electrical properties.If a constant electric field of a certain intensity is applied to the interstices and free spaces of hydrogen (deuterium) particles, under the influence of a sufficiently strong electric field force, hydrogen (deuterium) interstices with positive electrical properties will move away from the damage precursor in the direction of the electric field, and hydrogen (deuterium) free spaces with negative electrical properties will move away from the damage precursor in the direction of the electric field. Once their walking distances significantly exceed the size of the precursor, the concentration of defects in the precursor decreases, thereby increasing the damage threshold of a single damage precursor and effectively reducing the density of point damage formed at a given laser intensity.

[0029] The hydrogen (deuterium) gap and the hydrogen (deuterium) free space are electrically opposed; there is an attraction between them, and for them to separate, the driving electric field must reach a certain intensity. The lower the concentration of hydrogen point defects in the precursor, the higher the corresponding damage threshold, and the greater the intensity of the electric field required by the precursor. A fault with a lower concentration of point faults is less likely to displace point faults. This may explain why electrical connection treatment is more effective for faults with high damage thresholds.

[0030] The present invention adopts the "electrical connection method" as a new process for treating KDP and equivalent crystals, which considerably reduces the probability of damage to the crystals in the hope of producing better experimental results.

[0031] Currently, laser pretreatment technology is most widely applied to KDP / DKDP crystals using a triple-frequency laser beam (wavelength 355 nm) emitted by a sub-nanosecond Nd:YAG laser. The latest pretreatment results were published in the Journal of Physics (2021) (Zhichao Liu, et al., Offline sub-nanosecond laser pretreatment technology for large-caliber deuterated potassium dihydrogen phosphate crystals. Journal of Physics, 2021.). The experimental parameters described by the authors are as follows: the pulse width of the pulsed laser acting on the DKDP crystal to be pretreated is approximately 0.5 nanoseconds, the spot diameter is 0.68 mm, the wavelength is 355 nm, the maximum pretreatment laser flux is 2 J / cm2 and the maximum laser power density is 4 GW / cm2.Sub-nanosecond laser pretreatment can increase the zero probability damage threshold of DKDP crystals by approximately 1-fold (Shaotao Sun, Zhengping Wang, Xinguang Xu, Effect of annealing on damage to DKDP crystals. Collection of abstracts of academic papers from the academic conference, namely the Fourth National Congress of the Chinese Society of Crystallography, 2008), reaching approximately 8 J / cm2.

[0032] The electrical branching treatment provided by the present invention is expected to further reduce the probability of damage caused by laser flux exceeding 8 J / cm² and improve the crystal's resistance to damage. Therefore, based on laser pretreatment, a further advancement to the electrical branching treatment provided by the present invention is a novel method for improving the crystal's resistance to damage.

[0033] On the other hand, laser pretreatment equipment currently costs about 3 million yuan, while electrical connection processing equipment provided by the present invention costs about 100,000 yuan, presenting a huge economic advantage.

[0034] In order to further illustrate the present invention, the technical solutions provided by the present invention are described in detail below in combination with embodiments, but they cannot be understood as a limitation on the scope of protection of the present invention. EXAMPLE 1

[0035] In this embodiment, a DKDP crystal is taken as an example. The band gap of the DKDP crystal is (7.5-9) eV, the electron and hole concentration is very low at room temperature, the ability to conduct electricity through the electrons and holes is very low, it belongs to an insulator, and the DC conductivity is very small. On the other hand, the DKDP crystal contains hydrogen bonds with bond energies less than 1 eV, which can produce a certain concentration of negatively monovalent hydrogen free spaces and positively monovalent hydrogen interstices at room temperature and at higher temperatures. These two point defects become charge carriers under the influence of an external electric field, so the conductive characteristics of the DKDP crystals belong to the ionic conductivity category.

[0036] This embodiment provides a method for improving the resistance of the DKDP crystal to laser damage, which specifically comprises the following steps:

[0037] SI: Taking a piece of the DKDP crystal blank artificially grown and treated by the thermal annealing process, as shown in [Fig.1], the crystal is cut according to the specific direction and size according to the application requirements of the crystal (the crystal is cut twice in the form of two pairing classes in this embodiment), in order to obtain a crystal with a size of 5 cm x 5 cm x 1 cm.

[0038] S2: Cut the crystal further into four adjacent pieces of crystal The crystals, each measuring 5cm x 5cm x 5cm, were initially identical in size relative to one another, and their initial resistance to damage from a powerful laser was also identical. The cut crystal was polished using a specific process (e.g., fly-cutting) to achieve optical-grade flatness.

[0039] S3: In this embodiment, the electrical connection treatment to the crystal The process is carried out in the crystal electrical connection container shown in [Fig. 2]. The DKDP crystal, measuring 1 cm x 1 cm x 5 cm and processed in step 2, is stably clamped between a pair of metal electrodes connected to an electrical source, and the clamped crystal is placed in the stainless steel electrical connection container. Before electrical connection, the air in the electrical connection container is pumped out by a vacuum pump until the vacuum inside the container reaches 10 A - 3 Pa. The container is then filled with dry nitrogen at atmospheric pressure. Subsequently, the electrical connection container is placed in a thermostatic box, and the temperature of the thermostatic box is controlled to within 0.1 °C during electrical connection. The temperature of the crystalline sample is 25 °C during the electrical connection. A high-voltage DC power source is used to continuously supply a stable DC voltage to the crystal. Crystal numbered DKDP-1 has an electrical connection voltage of 0.95 kV and a connection time of 48 hours. Crystal numbered DKDP-2 has an electrical connection voltage of 10 kV and a connection time of 3 hours. Crystal numbered DKDP-3 is used as a comparison sample without electrical connection. Crystal numbered DKDP-4 has an electrical connection voltage of 0.8 kV and a connection time of 8 hours.

[0040] S4: The resistance of the DKDP crystal treated by the electrical connection against The damage caused by a laser is measured using a small-aperture laser device (beam area approximately 1 mm²) with a wavelength of 355 nm to obtain the laser damage probability curve. The laser pulse width is 5 ns and the near-field modulation depth of the beam is 2.4. The measurement method is linear, and each laser beam is measured in 10 counts. The optical path diagram of the device is shown in [Fig. 3]. The test laser reaches the DKDP sample to be observed 18 through the lens group, making it possible to plot the damage probability curves by observing the DKDP sample to be observed 18.

[0041] The results of the damage probability curves for the four crystal samples obtained from the electrical connection experiment in Example 1 are shown in [Fig. 4]. The horizontal ordinate in [Fig. 4] represents the average laser flux of the test laser with a pulse width of 5 ns and a wavelength of 355 nm, and the longitudinal coordinate in [Fig. 4] represents the damage probability that occurs to the DKDP crystal at a given laser flux. Overall, the damage probability of the treated crystal during electrical connection is significantly lower than that of the untreated crystal sample at the same laser intensity, showing that applying a regulated DC field with a certain intensity to the DKDP crystal for a certain period of time can considerably reduce the damage probability. Table 1 summarizes the experimental data for the four samples in Example 1.Table 1 shows that, for the first dataset, when the laser flux is approximately 9.6 J / cm², the probability of damage is 90% for sample numbered DKDP-3, which is not treated during electrical connection, while the probability of damage for the three other crystals treated during electrical connection varies from 20% to 30%. For the second dataset, when the laser flux is approximately 11.7 J / cm², the probability of damage is 100% for sample numbered DKDP-3, which is not treated. not during electrical connection, while the probability of damage for the other three crystals processed during electrical connection varies from 40% to 50%. Both datasets show that the probability of damage to the crystal connected to an electrical source decreases by more than 50%.

[0042] Table 1 Experimental results of the change in the probability of damage to the DKDP crystal before and after electrical connection treatment Sample Number Electrical Connection Parameters First Set Second Set Laser Flux (J / cm²) Damage Probability (%) Laser Flux (J / cm²) Damage Probability (%) DKDP-3 without electrical connection 9.7 90 11.8 100 DKDP-1 0.95kV, electrical connection for 48 h 9.4 30 11.9 40 DKDP-2 10kV, electrical connection for 3h 9.4 30 11.3 50 DKDP-4 0.8kV, electrical connection for 8h 9.6 20 11.3 50

[0043] The above results show that electrical connection to the DKDP crystal has the effect of reducing the probability of damage to the crystal and optimizing its resistance against damage caused by a powerful laser, in particular for defects with a higher damage threshold, the effect is even more remarkable.

[0044] From the above embodiments, it can be seen that the present invention provides a method for improving the resistance of a nonlinear artificial crystal against damage caused by a powerful laser. According to the present invention, there is a step of using a power supply to connect a nonlinear artificial crystal to an electrical source in a protective gas atmosphere. The present invention can effectively lower the density of point damage in the nonlinear artificial crystal and reduce the probability of crystal damage by means of the "electrical connection method," thus optimizing the resistance of the nonlinear artificial crystal against damage caused by a powerful laser, particularly against defects with a higher damage threshold, and the improvement effect is more pronounced.The electrical connection treatment provided by the present invention is expected to further reduce the probability of damage caused by laser flux exceeding 8 J / cm² and improve the crystal's resistance to damage. Therefore, based on the laser pretreatment, further progression to the... The electrical connection treatment provided by the present invention is a novel method for improving the resistance of crystal against damage.

[0045] Although the above embodiment shows the present invention in detail, it is only a part of the embodiments of the present invention, instead of all the embodiments, and other embodiments can also be obtained without inventive step according to the embodiment, and these embodiments fall within the scope of protection of the present invention.

Claims

Demands

1. A method for improving the resistance of a nonlinear artificial crystal against damage caused by a powerful laser, characterized in that it includes a step of using a regulated DC power supply to connect a nonlinear artificial crystal to an electrical source.

2. Method according to claim 1, characterized in that a voltage of said electrical connection is 0.5-100 kV.

3. Method according to claim 1, characterized in that a time of said electrical connection is 3~48h.

4. Method according to claim 1, characterized in that a temperature of said nonlinear artificial crystal is 20-25 °C during said electrical connection.

5. Method according to claim 1, characterized in that said nonlinear artificial crystal includes a deuterated potassium dihydrogen phosphate or a nonlinear optical crystal equivalent to the crystal structure of deuterated potassium dihydrogen phosphate.

6. Method according to claim 5, characterized in that said nonlinear artificial crystal includes potassium dihydrogen phosphate, deuterated potassium dihydrogen phosphate or ammonia dihydrogen phosphate.

7. Method according to claim 1, characterized in that said electrical connection is made under atmospheric pressure or vacuum conditions.

8. Method according to claim 1, characterized in that said nonlinear artificial crystal is a nonlinear artificial crystal treated during a thermal annealing.

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