Laser pretreatment method for nonlinear optical crystal

Abstract

The present application relates to the technical field of laser pretreatment, and in particular to a laser pretreatment method for a nonlinear optical crystal. In the present application, a nonlinear optical crystal is irradiated with laser having a photon energy of Eg / 2-Eg, Eg being the band gap of the crystal. Compared with laser pretreatment based on three-photon absorption, the laser pretreatment method based on two-photon absorption of the present application has greatly improved efficiency. Upon comparison of the embodiments, the comparative examples and the prior art, the results show that in the present application, laser of 266 nm which is much weaker than 355 nm is used, and under the condition that the laser power density is only 0.13 GW / cm2, the zero probability bulk damage threshold is increased by 120% compared with that before pretreatment. Moreover, the KDP crystal laser pretreatment based on two-photon absorption of the present application not only has higher pretreatment efficiency than the laser pretreatment based on three-photon absorption, but also has greater potential in increasing the bulk damage threshold of the crystal than the laser pretreatment based on three-photon absorption.

Description

A laser pretreatment method for nonlinear optical crystals

[0001] This application claims priority to the Chinese patent application filed with the Patent Office of China on August 29, 2023, with application number CN202311103942.8 and invention name “A laser pretreatment method for nonlinear optical crystals”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application belongs to the field of laser preprocessing technology, and specifically relates to a laser preprocessing method for nonlinear optical crystals. Background Art

[0003] Nonlinear artificial crystals are essential key optical components in lasers and large-scale high-power laser devices. Potassium dihydrogen phosphate (KDP) nonlinear optical crystals, and other similar crystals with similar lattice structures, such as deuterated potassium dihydrogen phosphate (DKDP) with different deuterium substitution rates, ammonium dihydrogen phosphate (ADP), etc., are important nonlinear artificial crystals that are currently widely used. These crystals are usually used as frequency doubling elements or electro-optical switching elements in laser devices and need to be frequently exposed to strong laser irradiation. When the laser intensity I (unit is W / cm 2 ) is greater than a certain threshold (body damage threshold I th ) after the laser will cause irreversible damage inside the crystal, causing the nonlinear artificial crystal to lose its basic functions, and thus the laser to lose its function. Therefore, the ability of KDP and similar crystals to resist strong laser damage, that is, the body damage threshold I th The size is one of its very important performance indicators.

[0004] In order to improve the ability of KDP and similar crystals to resist strong laser damage, people have developed a technology called laser pretreatment in the past 40 years. The basic method of this technology is to assume that the body damage threshold of untreated KDP and similar crystals is I th_i If the laser intensity is greater than I th_i Direct irradiation of the crystal with laser of intensity less than I th_i Laser, for example, with an intensity of 0.5 times I th_i The crystal is irradiated with a certain number of pulses of laser light, and then the crystal is irradiated with a laser light with an intensity of I th_i When the laser irradiates the crystal, the crystal will no longer be damaged. This method improves the laser body damage threshold I th The method is called laser pretreatment technology for KDP and similar crystals. Taking DKDP crystal as an example, the I of crystals grown by the current highest level crystal growth process is th_i About 4J / cm2 / 3ns. After pre-treatment with a laser of 355nm wavelength, its I th Can be increased to about 8J / cm 2 / 3ns. In practice, in order to obtain the best pretreatment effect, through the optimization of laser parameters, people have found that for lasers with a wavelength of 355nm, the pretreatment effect has a strong dependence on the pretreatment laser intensity. The stronger the laser, the better the pretreatment effect. However, the laser cannot be too strong, otherwise it will directly damage the crystal. The laser intensity with the best pretreatment effect is about 5GW / cm 2 .

[0005] "Offline sub-nanosecond laser pretreatment technology for large-aperture deuterated potassium dihydrogen phosphate crystals" (Liu Zhichao et al., Acta Physica Sin., Vol. 70, No. 7 (2021), 074208) discloses a laser pretreatment technology for KDP / DKDP crystals using a tripled frequency laser (wavelength of 355 nm) output by an Nd:YAG laser. The pretreatment experimental parameters are: the pulse width of the pulsed laser acting on the DKDP crystal to be pretreated is 0.5 nanoseconds, the spot diameter is 0.68 mm, the wavelength is 355 nm, and the maximum pretreatment laser flux is 2 J / cm 2 , maximum laser power density 4GW / cm 2 ,Sub-nanosecond laser pretreatment can increase the zero-probability body damage threshold of DKDP crystal by about 1 times.

[0006] However, due to the limited laser energy output by the pulsed laser, in order to achieve the 4-5 GW / cm required for pretreatment, 2 The laser intensity can only reduce the laser spot diameter to less than 1mm (the diameter reported in the above literature is 0.68mm). Considering the maximum repetition frequency of the laser pulse, the current laser pretreatment can only process about 20cm2 of crystal surface area per hour in terms of processing efficiency. 2 .

[0007] In summary, the current laser pretreatment technology for KDP and similar crystals has the problems of low processing efficiency and high processing cost.

[0008] Summary of the Invention

[0009] The purpose of this application is to provide a laser pretreatment method for nonlinear optical crystals based on two-photon absorption. The laser pretreatment method provided in this application can achieve a significant and efficient improvement in the damage threshold of the nonlinear optical crystal laser body and reduce the cost of pretreatment.

[0010] In order to achieve the above objectives, this application provides the following technical solutions:

[0011] The present application provides a laser pretreatment method for a nonlinear optical crystal based on two-photon absorption, comprising the following steps:

[0012] The nonlinear optical crystal is irradiated with laser, wherein the photon energy of the laser is Eg / 2 to Eg, and Eg is the band gap width of the nonlinear optical crystal.

[0013] Preferably, the wavelength of the laser is 165-305 nm.

[0014] Preferably, the wavelength of the laser is 266 nm.

[0015] Preferably, the laser is generated by a laser, and the laser is a solid laser or an excimer laser.

[0016] Preferably, the nonlinear optical crystal includes potassium dihydrogen phosphate, deuterated potassium dihydrogen phosphate or ammonium dihydrogen phosphate.

[0017] Preferably, the pulse width of the laser is 0.3 to 40 ns or 100 fs to 100 ps.

[0018] Preferably, the spot diameter of the laser is 0.05 to 5 mm.

[0019] Preferably, the power density of the laser is 0.2 to 1.5 times the bulk damage threshold of the nonlinear optical crystal, and the bulk damage threshold of the nonlinear optical crystal is the bulk damage threshold corresponding to the wavelength of the pre-processing laser.

[0020] Preferably, the number of laser pulses is 90-95% of the saturation pulse number.

[0021] Preferably, the nonlinear optical crystal is deuterated potassium dihydrogen phosphate; the laser has a wavelength of 266 nm, a pulse width of 10 ns, a spot diameter of 1 mm, and a power density of 0.13 GW / cm 2 .

[0022] The present application provides a laser pretreatment method for nonlinear optical crystals based on two-photon absorption, comprising the following steps: irradiating the nonlinear optical crystal with a laser, wherein the laser photon energy is between Eg / 2 and Eg, where Eg is the bandgap of the nonlinear optical crystal. Because the prior art uses a laser with a wavelength of 355 nm for laser pretreatment, the valence band electrons of the nonlinear optical crystal must simultaneously absorb three photons to be ionized into the conduction band, a third-order nonlinear process. However, the present application uses laser photon energies between Eg / 2 and Eg for pretreatment, so the valence band electrons of the nonlinear optical crystal only need to simultaneously absorb two photons to be ionized into the conduction band, a second-order nonlinear process. Because the probability of two-photon absorption is thousands to tens of thousands times higher than that of three-photon absorption, the present application uses laser photon energies between Eg / 2 and Eg, i.e., the laser pretreatment method based on two-photon absorption, significantly improving the efficiency of existing laser pretreatment methods based on three-photon absorption. The results of the examples and comparative examples show that the present invention uses a long-wave laser of 266 nm, which is much weaker (more than 10 times) than 355 nm, for pretreatment. 2 Under the conditions of , the zero probability volume damage threshold of DKDP crystal after pretreatment is 4.7J / cm 2 , which is 2.3J / cm higher than the zero-probability body damage threshold before pretreatment. 2 , an increase of 120%. At the same time, to achieve the same pretreatment effect (body damage threshold increased by about 1 times) as in the existing technology, 4 to 5 GW / cm 2 The laser power density of this application is significantly reduced, thereby improving processing efficiency and reducing pretreatment costs. In addition, the KDP crystal laser pretreatment based on two-photon absorption in this application not only has higher pretreatment efficiency than three-photon absorption laser pretreatment, but also has greater potential for increasing the crystal's bulk damage threshold than three-photon absorption laser pretreatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG1 is a schematic diagram of a process for laser pretreatment of a DKDP crystal based on two-photon absorption according to an embodiment of the present application;

[0024] FIG2 is a schematic diagram of an optical path system for a DKDP crystal quadruple frequency laser pretreatment experiment based on two-photon absorption provided in an embodiment of the present application;

[0025] FIG3 is a diagram showing a volume damage probability curve of a crystal measured by a small-aperture laser and a schematic diagram of an optical path system for triple frequency laser pretreatment;

[0026] FIG4 is an electron paramagnetic resonance spectrum of the DKDP crystal before and after laser pretreatment in Example 1;

[0027] FIG5 is a graph showing the measurement results of the DKDP crystal damage probability curves before and after laser pretreatment in Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0028] The present application provides a laser pretreatment method for a nonlinear optical crystal based on two-photon absorption, comprising the following steps:

[0029] The nonlinear optical crystal is irradiated with laser, wherein the photon energy of the laser is Eg / 2 to Eg, and Eg is the band gap width of the nonlinear optical crystal.

[0030] In this application, unless otherwise specified, all preparation raw materials / components are commercially available products well known to those skilled in the art.

[0031] Based on the physical phenomenon of two-photon absorption of crystals, the present application performs laser pretreatment on nonlinear optical crystals, including potassium dihydrogen phosphate (KDP) nonlinear optical crystals and similar crystals with similar KDP lattice structures; similar crystals preferably include deuterated potassium dihydrogen phosphate (DKDP) or ammonium dihydrogen phosphate (ADP), etc., which can significantly and efficiently improve the laser body damage threshold of these crystals.

[0032] This application has no special requirements for the preparation method of the nonlinear optical crystal. Nonlinear optical artificial crystals obtained by artificial growth and annealing, which are well known to those skilled in the art, can be used. In this application, before the irradiation, this application preferably cuts and polishes the nonlinear optical crystal in sequence. This application has no special requirements for the specific implementation of the cutting and surface polishing. The cutting is preferably carried out in the direction and size required for the use of the nonlinear optical crystal. The surface of the nonlinear optical crystal obtained after the surface polishing preferably achieves optical grade smoothness.

[0033] In the present application, the nonlinear optical crystal preferably includes potassium dihydrogen phosphate (KDP), deuterated potassium dihydrogen phosphate (DKDP), or ammonium dihydrogen phosphate (ADP). In the present application, the deuterium substitution rate of the DKDP is preferably 5 to 98%. In the specific embodiments of the present application, the laser pretreatment method of the present application is described in detail using DKDP as an example. The deuterium substitution rate of the DKDP crystal is 70%. The DKDP crystal is an insulator with a large bandgap (Eg). The bandgap of the DKDP crystal is 7.5 to 9 eV.

[0034] In the present application, the laser is generated by a laser, which is preferably a solid-state laser or an excimer laser, more preferably a solid-state laser. Compared to excimer lasers, solid-state lasers are more environmentally friendly. In the present application, solid-state lasers are preferably used for laser pretreatment of nonlinear optical crystals. The treatment process is environmentally friendly and suitable for widespread industrial application. In a specific embodiment of the present application, the laser used for the laser pretreatment is specifically an Nd:YAG laser. The pulse width of the Nd:YAG laser is nanosecond to sub-nanosecond, and it can efficiently output quadrupled frequency laser light.

[0035] In this application, the laser photon energy (hν) is in the range of Eg / 2 to Eg. Laser photons with photon energies of Eg / 2 to Eg can ionize valence band electrons in the nonlinear optical crystal into the conduction band of the crystal through the two-photon absorption process of the nonlinear optical crystal. By subjecting the nonlinear optical crystal to a certain number of laser pulses at a laser intensity below its bulk damage threshold, a significant and efficient improvement in the bulk damage threshold of the nonlinear optical crystal can be achieved.

[0036] In the present application, the wavelength of the laser is preferably 165 to 305 nm, more preferably 175 to 295 nm, and even more preferably 205 to 275 nm. In a specific embodiment of the present application, the wavelength of the laser is 266 nm. The present application uses a quadrupled frequency laser with a wavelength of 266 nm and a photon energy of 4.66 eV. The photon energy is between 3.75 and 7.5 eV, which meets the photon energy requirements for laser pretreatment of DKDP crystals based on two-photon absorption.

[0037] In the present application, the pulse width of the laser is preferably 0.3 to 40 ns or 100 fs to 100 ps, ​​more preferably 5 to 40 ns or 150 fs to 80 ps, ​​and even more preferably 15 to 30 ns or 500 fs to 50 ps. The spot diameter of the laser is preferably 0.05 to 5 mm, more preferably 0.1 to 5 mm, and even more preferably 1 to 5 mm. The power density of the laser is preferably 0.2 to 1.5 times the bulk damage threshold of the nonlinear optical crystal, more preferably 0.3 to 1.5 times the bulk damage threshold of the nonlinear optical crystal, even more preferably 0.4 to 1.5 times the bulk damage threshold of the nonlinear optical crystal, and most preferably 0.5 to 1.5 times the bulk damage threshold of the nonlinear optical crystal. The bulk damage threshold of the nonlinear optical crystal is the bulk damage threshold corresponding to the wavelength of the pretreatment laser. In the present application, the body damage threshold of the nonlinear optical crystal to be pretreated varies with the change of the pretreatment laser wavelength. In the present application, the power density of the laser is preferably 0.2 to 1.5 times the body damage threshold of the nonlinear optical crystal. The body damage threshold mentioned in the body damage threshold means the body damage threshold of the nonlinear optical crystal corresponding to the wavelength of the pretreatment laser.

[0038] In a specific embodiment of the present application, the power density of the laser is preferably 0.565 times the volume damage threshold of the nonlinear optical crystal. In the present application, the volume damage threshold of the nonlinear optical crystal is the maximum laser power density (in W / cm2) when the nonlinear optical crystal is irradiated with laser to produce irreversible damage before pretreatment. 2 ). The present application preferably evaluates and calculates the nonlinear optical crystal to be pretreated before performing the laser pretreatment to obtain its body damage threshold. The number of laser pulses is preferably 90 to 95% of the saturation pulse number, more preferably 95%. In the present application, during the laser pretreatment, as the number of laser pulses increases, the body damage threshold of the nonlinear optical crystal gradually increases, but after the number of laser pulses reaches a certain value, the body damage threshold of the nonlinear optical crystal no longer increases and reaches saturation when the number of laser pulses is further increased. The present application defines the number of laser pulses at which the body damage threshold of the nonlinear optical crystal no longer increases with the increase in the number of laser pulses during the laser pretreatment as the saturation pulse number.

[0039] In a specific embodiment of the present application, the nonlinear optical crystal is deuterated potassium dihydrogen phosphate; the wavelength of the laser is 266 nm, the pulse width is 10 ns, the spot diameter is 1 mm, and the power density is 0.13 GW / cm 2 , the number of pulses is 5. The number of pulses is the number of laser pulses irradiated to the same position of the nonlinear optical crystal. The number of pulses is 5, which means that the same position of the nonlinear optical crystal is irradiated with 5 laser pulses.

[0040] In a specific embodiment of the present application, a schematic diagram of the laser pretreatment process of the nonlinear optical crystal is shown in FIG1 , and a schematic diagram of the optical path system is shown in FIG2 . As shown in FIG2 , the optical path system used in the laser pretreatment system provided by the present application is composed of a laser and an optical path transmission and adjustment system, wherein: the laser is an Nd:YAG laser; the optical path transmission and adjustment system comprises, in sequence, an elevation mirror, a beam splitter, a 532 high-reflection mirror, a triple beam reduction system, a quadruple frequency crystal, a prism, a convex lens, and a reflector. In a specific embodiment of the present application, as shown in FIG2 , the 266nm ultraviolet laser output by the Nd:YAG laser is focused by a long-focal-length converging lens and incident on the nonlinear optical crystal to be pretreated. Usually, the area of ​​the crystal to be processed is much larger than the laser spot. The present application preferably uses laser spot scanning to uniformly pretreat the entire crystal.

[0041] In the present application, the selection of laser photon energy is the key to achieving a substantial and efficient improvement in the laser body damage threshold of a nonlinear optical crystal. In the prior art, when a tripled frequency laser with a wavelength of 355nm is used for laser pretreatment, the valence band electrons of the nonlinear optical crystal must absorb three photons at the same time before they can be ionized to the conduction band, which is a third-order nonlinear process. The present application uses a quadruple frequency laser with a wavelength of 266nm for laser pretreatment. The valence band electrons of the nonlinear optical crystal only need to absorb two photons at the same time to be ionized to the conduction band, which is a second-order nonlinear process. The probability of two-photon absorption is thousands to tens of thousands times higher than that of three-photon absorption. Therefore, the laser pretreatment technology based on two-photon absorption in the present application will greatly improve the efficiency of the existing laser pretreatment based on three-photon absorption.

[0042] In order to further illustrate the present application, the technical solutions provided in the present application are described in detail below in conjunction with the embodiments, but they should not be understood as limiting the scope of protection of the present application.

[0043] Example 1

[0044] According to the laser pretreatment flow chart described in FIG1 , laser pretreatment is performed on the DKDP crystal (deuterium substitution rate is 70%) using the optical path diagram shown in FIG2 . The specific processing steps include:

[0045] Take a piece of artificially grown and thermally annealed DKDP crystal and cut it into the direction and size required by the application. The cut crystal is then polished to achieve an optical-grade finish.

[0046] The 266nm ultraviolet laser output by the Nd:YAG laser is focused by a long focal length converging lens and incident on the DKDP crystal to be pretreated. The entire crystal is uniformly pretreated by laser spot scanning. The laser parameters during the pretreatment process are: wavelength: 266nm, spot diameter: 1mm, power density: 0.13GW / cm 2 , the number of pulses is 5 (the same position of the DKDP crystal receives 5 laser pulses), pulse width: 10ns, flux: 1.3J / cm 2 .

[0047] Example 1: The number of intrinsic "point defects" in DKDP crystals before and after laser pretreatment was measured using an electron paramagnetic resonance spectrometer. The spectrometer's microwave frequency was 9.4 GHz, and the sample temperature during measurement was 30 K. The obtained electron paramagnetic spectrum is shown in Figure 4. The black curve in Figure 4 is the spectrum of the crystal without laser pretreatment, and the red curve is the spectrum of the crystal after laser pretreatment. As can be seen from Figure 4, the hole-type "point defects" [H2PO4], which are directly related to the laser body damage threshold, are significantly higher than those of the vacancy-type "point defects" [H2PO4]0 The amount of α-glucanase decreased to only 60% after laser pretreatment.

[0048] Comparative Example 1

[0049] According to the laser pretreatment flow chart described in FIG1 , the DKDP crystal is laser pretreated using the optical path diagram shown in FIG3 . The optical path diagram shown in FIG3 is used for both damage probability curve measurement and 355 nm laser pretreatment. The specific processing steps include:

[0050] Take a piece of artificially grown and thermally annealed DKDP crystal and cut it into the direction and size required by the application. The cut crystal is then polished to achieve an optical-grade finish.

[0051] The 355nm ultraviolet laser output by the Nd:YAG laser is focused by a long focal length converging lens and incident on the DKDP crystal to be pretreated. The entire crystal is uniformly pretreated by laser spot scanning. The laser parameters during the pretreatment process are: wavelength: 355nm, spot diameter: 1mm, power density: 0.1GW / cm 2 , the number of pulses is 5 (the same position of the DKDP crystal receives 5 laser pulses), pulse width: 10ns, flux: 1.0J / cm 2 .

[0052] In Example 1 and Comparative Example 1, the DKDP crystals before and after laser pretreatment were tested according to the optical path diagram of the small-aperture laser measuring crystal body damage probability curve shown in Figure 3. The test results are shown in Figure 5. Figure 5 shows the damage probability curves of the DKDP crystals before and after laser pretreatment obtained in Example 1 and Comparative Example 1. It can be concluded from Figure 5 that the zero probability body damage threshold of the DKDP crystal before laser pretreatment is 2.3 J / cm 2 The zero probability body damage threshold of the DKDP crystal after the quadrupled frequency pretreatment of Example 1 is 4.7 J / cm 2 The zero probability body damage threshold of the DKDP crystal after the triple frequency pretreatment of comparative example 1 is 2.7J / cm 2By comparing the results of Example 1 and Comparative Example 1, it can be seen that: when the laser pulse width is the same, the laser power (intensity) is almost the same, and the laser wavelength is different, the zero-probability body damage threshold of the 266nm laser pretreatment based on two-photon absorption in Example 1 is increased by 120%, while the zero-probability body damage threshold of the 355nm laser pretreatment based on three-photon absorption in Comparative Example 1 is only increased by 20%. Figure 5 proves the effectiveness of the efficient laser pretreatment method for nonlinear optical crystals based on two-photon absorption provided by the present application. At the same time, this embodiment uses a laser wavelength of 266nm, which is much weaker than 355nm (more than 10 times), for pretreatment. When the laser power density is only 0.13GW / cm 2 Under the conditions of 4.7 J / cm, the zero probability volume damage threshold of DKDP crystal after pretreatment was observed to be 4.7 J / cm 2 , which is 2.3J / cm higher than the zero-probability body damage threshold before pretreatment. 2 , increased by 120%. Compared with comparative example 1, the pretreatment method provided in Example 1 of the present application has a significant effect on improving the body damage threshold of the crystal. Moreover, the pretreatment effect achieved in the prior art is the same (the body damage threshold is increased by about 1 times). The present application uses a laser with a wavelength of 266nm and a laser power density of 0.13GW / cm 2 ) requires only a few tenths of the wavelength of 355nm laser (4-5GW / cm 2 ), thereby improving laser pretreatment efficiency and reducing pretreatment costs. Finally, the present application's two-photon absorption-based KDP crystal laser pretreatment not only boasts higher pretreatment efficiency than three-photon absorption laser pretreatment, but also has greater potential for increasing the crystal's bulk damage threshold. This potential lies in the ability to achieve a higher conduction band electron density when using two-photon absorption laser pretreatment.

[0053] Although the above embodiment provides a detailed description of the present application, it is only a part of the embodiments of the present application, not all of the embodiments. Other embodiments can be obtained based on this embodiment without creativity, and these embodiments all fall within the scope of protection of the present application.

Claims

1. A laser pretreatment method for nonlinear optical crystals, characterized in that: The following steps are involved: The nonlinear optical crystal is irradiated with laser, the photon energy of the laser is Eg / 2-Eg, and Eg is the bandgap width of the nonlinear optical crystal.

2. The laser pretreatment method according to claim 1, characterized in that: The wavelength of the laser is 165-305 nm.

3. The laser preprocessing method according to claim 2, characterized in that: The wavelength of the laser is 266 nm.

4. The laser pretreatment method according to claim 1 or 2, characterized in that: The laser is generated by a laser, and the laser is a solid laser or an excimer laser.

5. The laser preprocessing method according to claim 1, characterized in that: The nonlinear optical crystal includes potassium dihydrogen phosphate, deuterated potassium dihydrogen phosphate or ammonium dihydrogen phosphate.

6. The laser preprocessing method according to claim 1, characterized in that: The pulse width of the laser is 0.3-40ns or 100fs-100ps.

7. The laser preprocessing method according to claim 1, characterized in that: The spot diameter of the laser is 0.05-5 mm.

8. The laser pretreatment method according to claim 1 or 5, characterized in that: The power density of the laser is 0.2 to 1.5 times the volume damage threshold of the nonlinear optical crystal, and the volume damage threshold of the nonlinear optical crystal is the volume damage threshold corresponding to the wavelength of the preprocessing laser.

9. The laser pretreatment method according to claim 1 or 5, characterized in that: The number of laser pulses is 90-95% of the saturation pulse number.

10. The laser preprocessing method according to claim 1, characterized in that: The nonlinear optical crystal is deuterated potassium dihydrogen phosphate; the wavelength of the laser is 266nm, the pulse width is 10ns, the spot diameter is 1mm, and the power density is 0.13GW / cm 2 .

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