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Abstract
In this paper, we report the damage and damage growth in potassium dihydrogen phosphate and its deuterated analog crystals. A time-resolved shadow imaging system was used to investigate the damage behavior in the bulk and on the rear surface. The damage images show differences in the damage sizes of the crystals with different deuterization rates. Theoretical simulations demonstrated that this may be due to differences in the crystallographic defects. The experimental results showed that the development of crystal damage was not only manifested as the expansion of damage on the rear surface of the crystal but also as an increase in pin-point density and size within the crystal. Crystals with higher deuterization rates had higher probability of the increasing of initial damage size, rather than the increasing of pin-point density.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Potassium dihydrogen phosphate (KDP) and its deuterated analog (DKDP) have been incorporated into various laser systems for harmonic generation and electrooptic switching [1]. Their rapidly growth rate (10–20 mm/day) and large crystal sizes (up to 55 cm) have positioned them as crucial optical materials in high-energy laser systems [2–5], such as the National Ignition Facility (NIF) in the USA, the Laser Mega Joule (LMJ) in France, and the SG-III Laser Facility in China. Their irreplaceability in laser systems has motivated continued interest in their susceptibility to laser-induced damage at high laser fluences [6].
With the development of high-power lasers, optical components require increased damage threshold [7]. It has been widely accepted that bulk damage in crystal is generated by defect cluster or foreign particles, which cause additional absorption [8,9]. Bulk damage to a crystal appears as a series of pin-points [10,11]. This damage increases the laser near-field modulation, which increases the damage risk of the downstream optical elements. Analyzing the damage mechanism of crystals and studying their damage development are of practical significance for facility operation and maintenance [12,13].
For fused silica, the laser-induced damage mainly occurred on the rear surface. The growth of the damaged area increased exponentially with the accumulation of laser irradiation [14,15]. However, for KDP/DKDP crystals, laser-induced damage can occur not only on the rear surface but also inside the crystal, resulting in a series of pin-point damage along the laser transmission path [16,17]. The study of the crystal damage behavior and damage growth under laser irradiation accumulation is a complex task. Therefore, it is necessary to consider the damage growth inside the crystal.
In a high-power solid-state laser system, SHG(Second Harmonic Generation), THG(Third Harmonic Generation), and optoelectronic switching crystals exhibited deuterization rates of 0, 70, and 99%, respectively. However, the damage and damage growth behaviors, and the correlation of these behaviors with the deuterization rate have not been systematically investigated.
In this study, we conducted experimental research and theoretical analysis to investigate the damage growth behavior of crystals with different deuterization rates in engineering applications. Through a theoretical analysis of the thermal diffusion and stress wave transmission behavior of crystals with different deuterization rates, we determined the key factors that contribute to the differences observed in the damage and damage growth behavior of these crystals. The experimental results recorded the bulk and rear surface damage features using pump-probe imaging. This study provides a reference for operation and maintenance strategies of crystals at different engineering positions.
2. Experimental set-up and results
In this study, we built a pump-probe system to experimentally investigate crystal damage behavior. All the crystals used in the experiments were grown in the Z direction and in the defined crystallographic direction (θ = 90°) using the rapid growth method. As shown in Fig. 1, a Q-switched Nd:YAG laser with THG at a wavelength of 355 nm was used as the pump source in the experiment to induce crystal damage. A wavelength of 532 nm was used for probe laser, for green light is more sensitive to the CCD in this experiment. The full width at half maximum of the Gaussian pump pulse was 12 ns. The initial beam diameter is 12 mm. After focusing using a lens with a focal length of 100 mm, the beam diameter at the rear surface was 1 mm. In the experiment, the focus was located in the air behind the crystal to prevent filamentous damage.’/ The average laser fluence at the target was incrementally varied from 6 to 12 J/cm2 with a step of 3 J/cm2. An energy meter and charge-coupled device were used to measure the sampling pump light in real time. After the laser shot, the bulk damage and rear surface damage were imaged and recorded from both the probe direction and rear surface.
Fig. 1. Experimental system based on the pump-probe imaging system; H: 1/2 wave plate; P: polarizer; and M: mirror.
Another Q-switched Nd:YAG laser with an SHG was used as the probe light to observe the crystal inside the image after the laser shot. A movable mirror was used to illuminate the rear surface of the crystal after the laser shot, and the rear surface damage was subsequently recorded.
In the experiment, the crystal target was set to a z-cut 3 cm × 1 cm × 1 cm cuboid, and crystal damage growth was investigated using the S-on-1 method. For the probe imaging system, the magnification of the microscope was 20×, and its maximum spatial resolution was 1 µm. After each shot, a beam of illuminating light was introduced into the optical path through a movable mirror to observe and record the rear surface damage. For the rear-surface imaging system, the magnification of the microscope was 10×, and its maximum spatial resolution was 1.5 µm.
Figure 2 illustrates the bulk damage of crystals with different deuterization rates. Experimental results indicate that bulk damage in crystals is the main manifestation. Generally, it appears as a series of pin-points in the crystal. As shown in Fig. 3, for KDP crystals, the damage growth was attributed to the increase of the density of pin-points. The pin-points gradually connected into pieces with the accumulation of laser shots. With the increase of deuterization rates, the performance of damage growth processes differs. For 99% DKDP crystal, the damage growth is mainly caused by the growth of the crack. The origin crack around the pin-point grows rapidly under 355 nm laser irradiation, and causes severe tearing in the crystal. For 70% DKDP crystals, both types of damage can be observed.
The bulk damage density in the crystal is shown in Fig. 4. It was obtained experimentally with the variation of laser fluence at a single shot. Bulk damage in the three crystals exhibited different characteristics. For KDP crystals, the pin-point damage density in the crystals can be fitted using an exponentially increasing function. With an increase in the deuterization rate of the crystal, the pin-point damage densities of the two types of DKDP crystals were all lower than those of the KDP crystal. In addition, the damage density did not increase with the growth rate of the laser fluence.
Figure 5 shows the bulk damage size at an average fluence of 10 J/cm2. The pin-point damage size in the DKDP crystal increased significantly. The damage growth of in crystal is mainly the increasing of initial damage size, rather than the increase of pin-point number. KDP and DKDP crystals exhibited different bulk damage behaviors. KDP crystals exhibited a higher damage density but a smaller damage size. In contrast, DKDP crystals exhibited low damage density but generally the damage size exceeded 50 microns.
The measurements of the damage growth on the rear surface of the crystal are shown in Fig. 6. The experimental results obtained in this study correspond to the previous results regarding bulk damage inside the crystal. In the KDP crystal, the rear-surface damage stopped growing after several shots. This is because the bulk damage inside the crystal grows faster and results in a decrease in energy when irradiated on the rear surface. For the 99% DKDP crystals, the growth of the bulk damage density in vivo was slower than that of the KDP crystals. The laser energy is mainly released on the rear surface of the crystal and the damage increases rapidly. After several shots, cracks developed in the interior of the crystals, and the damage increased slowly on the rear surface. For the 70% DKDP crystal, the growth rate of the rear surface damage was higher than that of the bulk. The observed damage growth on the rear surface of the crystals falls within the range observed in the two aforementioned crystals.
As shown in Fig. 7, we analyzed the rear surface damage growth at 7/cm2 and 10 J/cm2. The 99% DKDP crystal can be observed in the 6th shot in Fig. 7(a) and the fifth shot in Fig. 7(b), where the damage diameter of the rear surface exhibits an evident step change. This step in the development of crystal back-surface damage is caused by crack propagation. Owing to the brittleness of crystalline materials, cracks caused by laser-induced damage may cause severe damage growth at a shot. This damage–growth phenomenon differs from that of fused silica. In crystals, when the damage grows to a certain size, subsequent damage development may cause disastrous consequences.
3. Discussions
3.1 Initial bulk damage
In the current research on crystal damage theory, it is understood that the initial damage in crystals is caused by the laser energy deposition induced by the defects in the crystals. In comparison with KDP, DKDP had a larger initial pin-point damage size in the crystal (as shown in Fig. 5). To explain this phenomenon, we conducted theoretical calculations on dehydration and impurity defects in KDP/DKDP crystals based on first principles [18,19]. In theoretical analysis, An isolated defect is simulated via a repeated supercell which consists of eight KH2PO4/KD2PO4 formula units and contains 64 atoms. The cut-off energy of the plane-wave basis function was set to be 680 eV, yielding a convergence for the total energy that was better than 1 meV per atom. We performed convergence tests for 2 × 2 × 2, 4 × 4 × 4, 5 × 5 × 5 divisions along the reciprocal lattice directions in the primitive unit cell and a 4 × 4 × 4 K point is better. The calculations were based on density-functional theory with the CASTEP implementation, and the total energy ultrasoft pseudopotential method was used. The complex refractive index of a crystal with a defective structure was calculated. The spectral line of the extinction coefficient K was obtained using the imaginary part of the complex refractive index.
The total density of states obtained by calculating the structure of ideal KDP crystals using the PBE (Periodic Boundary Embedding) method is shown in Fig. 8(a). The band gap value of ideal KDP crystals was 5.630 eV. Crystal dehydration causes a common defect observed in KDP crystals. As the dehydration of the KDP crystals increased, a KDP polycrystalline structure containing various intermediate products [mKH2PO4-nH2O] and a polymer (KPO3)n was formed. This process reduced the optical absorption bandgaps, as shown in Figs. 8(b)–(e). Considering the crystal cell structure, the band gap varied with the presence of H2Os, as summarized in Table 1.
Fig. 8. Partial density of KDP crystals with dehydration defects: (a) pure KDP, (b) containing K2H 2P2O7, (c) containing K3H2P3O10, (d) containing K4H2P4O13, and (e) containing (KPO3)n. The corresponding calculated extinction coefficients: (f) pure KDP, (g) containing K2H2P2O7, (h) containing K3H2P3O10, (i) containing K4H2P4O13, and (j) containing (KPO3)n.
Table 1. Band gap variance with KDP structure
The optical absorption spectrum tended to redshift and the extinction coefficient increased with increasing dehydration (as shown in figure(f-j)). As the thermal dehydration process progressed, the extinction coefficient spectrum also exhibited a trend of moving redshift. When a water molecule was removed from the KDP crystal to form a polycrystalline structure containing K2H2P2O7, the overall variation in the extinction coefficient spectrum was small; however, a slight redshift was observed. When two water molecules were removed from the KDP crystal, resulting in a polycrystalline structure containing K3H2P3O10, the redshift phenomenon in its extinction coefficient spectrum became more severe. As the dehydration of the KDP crystal progressed, when the polycrystalline structure containing K4H2P4O13 was present, the extinction coefficient spectrum exhibited a significant redshift, and the extinction coefficient was no longer 0 at 3.5 eV (355 nm). This indicated that light absorption occurred in this region, that is, weak light absorption occurs in this region. When four water molecules were removed from the KDP crystal to form a polycrystalline structure containing polymer (KPO3)n, the extinction coefficient in the range of 3.5 eV (355 nm) was no longer 0, and the maximum absorption coefficient could reach 2%.
The calculated results for the DKDP crystals are shown in Fig. 9(a). The bands gaps varied with the presence of D2Os, as summarized in Table 2.
Fig. 9. Partial density of DKDP crystals with dehydration defects:(a) pure KDP, (b) containing K2D 2P2O7, (c) containing K3D2P3O10, (d) containing K4D2P4O13, and (e) containing (KPO3)n. The corresponding calculated extinction coefficients: (f) pure KDP, (g) containing K2D2P2O7, (h) containing K3D2P3O10, (i) containing K4D2P4O13, and (j) containing (KPO3)n.
Table 2. Band gap variance with DKDP structure
The calculated extinction coefficients in DKDP are shown in figure(f-j). When a D2O molecule was removed from the DKDP crystal, resulting in a polycrystalline structure containing K2D2P2O7, the overall variation in the extinction coefficient spectrum was small; however, a slight redshift was observed. When two D2Os were removed from the DKDP crystal, resulting in a polycrystalline structure containing K3H2P3O10, the redshift phenomenon in its extinction coefficient spectrum became more significant. As the dehydration process of the DKDP crystal progressed, the three D2Os were lost, and a polycrystalline structure containing K4D2P4O13 was produced. It was observed that the extinction coefficient spectrum exhibited a significant redshift, with a maximum extinction coefficient of 21% at 3.5 eV (355 nm), indicating strong light absorption in this region.
We also conducted a theoretical analysis of the presence of iron impurity defects in the crystals, as shown in Fig. 10. Similarly, the extinction coefficient was used to analyze the effect of Fe-impurity defects on crystal damage. Theoretical simulations demonstrated that the absorption caused by Fe impurities in the DKDP crystals was stronger than that in the KDP crystals.
Fig. 10. The calculated extinction coefficients: (a) pure KDP, (b) pure DKDP (c) KDP with iron mixed defects,(d) DKDP with iron mixed defects
Comparing the simulation results of dehydration between KDP and DKDP, for both dehydration and vacancy defects, the band gap of the DKDP crystal decreased the most. In comparison with KDP crystals, the presence of these two defects in DKDP crystals caused greater band changes and higher extinction coefficients that induced laser energy deposition and higher laser deposition rates in DKDP crystals with defects, resulting in a larger initial damage size for DKDP crystals than for KDP crystals. D–O bonds have weaker deformation resistance than H–O bonds [20,21], defects in the DKDP crystal are more likely to cause collapse damage to the crystal skeleton structure than defects in KDP, resulting in an unstable crystal cell structure and reduced resistance to crystal damage.
Defects in DKDP crystals have a higher induced extinction coefficient and weaker bond strength compared to defects in KDP crystals. In comparison with KDP crystals, the initial pin-point damage formed in DKDP crystals has a larger scale, as shown in Fig. 2. Figure 4 shows the difference in the initial damage density inside the crystal with crystal deuterization rate variation. In the process of rapid crystal growth, it is easier to control the internal ion impurities owing to the differences in preparation compared to pure water, which results in fewer defects in the crystal and a lower initial damage density in experiments.
3.2 Damage growth
Laser-induced damage and damage growth in crystals with different deuterization rates were investigated using the s-on-1 method. It was observed that the origin damage occurred both inside the crystal and on the rear surface. The damage inside the crystal has a significant effect on the growth of damage on the rear surface. Severe bulk damage can cause stagnation of damage growth on the rear surface of the crystal. At this point, the initial pinpoints inside the crystal are connected to form large voids. These large hollow structures significantly affect the use of the crystals and cause great difficulties in their maintenance.
Damage to the crystal body mainly originates from defects during the crystal growth process. In the experiment, it was observed that the initial damage to the crystal grew along the crack direction of the pinpoints. The expansion of the cracks caused the initial pinpoint to form a large void. This further increased the deposition rate of the incident laser energy in the damaged crystal. In addition, a decrease in laser energy deposition on the back surface of the crystal occurred, and damage to the rear surface stopped.
The subsequent damage growth behavior was observed and summarized. As shown in Fig. 11, we recorded the transient and final bulk damage inside the KDP crystal to investigate its damage growth characteristics.
Fig. 11. Transient and final images of the damage after the 4th, 5th, 6th shots inside KDP crystals, under a laser fluence of 7 J/cm2.
As the laser irradiation increased, a large number of pinpoints accumulated in the crystal body. This resulted in the formation of a large number of damaged cavities in the crystal. In the subsequent irradiation of the laser pulse, the surrounding voids caused by these pinpoints led to increased laser energy deposition. This resulted in the phase transition of the material and the formation of “dark zones” on the transient image [22,23].
The deposited laser energy caused damage to grow along these “dark zones”; thus, larger damage cavity areas were formed inside the crystals. Moreover, as the number of damaged cavities in the crystal increased, the development of damage in the crystal grew along the direction of laser injection. This caused more laser energy deposition in bulk of the crystal, resulting in less energy deposition of damage pits on the rear surface of the crystal, and the development of damage on the rear surface of the crystal stagnated, as shown in Fig. 6. In engineering, severe damage in bulk of the crystal results in a decrease in its repairability. In the growth process of crystals, it is necessary to control the number of defects in the crystal strictly.
Figure 12 shows the damage growth behavior from the back surface to the crystal body. In these experimental results, the pinpoint damage density in the DKDP crystal was low, and the damage mainly developed around the rear surface. The initial laser-induced damage formed cracks around the damage pit(as shown in Fig. 6). In subsequent laser irradiations, the damage grew along the crack and formed larger damage pits. Because of the spallation effect and brittleness of the crystal, the size of the damaged area may increase stepwise. With further accumulation of laser irradiation, the crystal damage grew in the direction of the laser irradiation in the crystal.
Fig. 12. Transient images of the damage after the 8th, 9th, 10th shots in (99%) DKDP crystals under a laser fluence of 7 J/cm2.
The main sources of damage in fused silica are material tearing and crack propagation caused by stress waves. However, unlike the crack structure formed during the damage development of fused silica, the damage extends further into the bulk of the crystal, forming a series of damage cavities. When these cavity structures are formed, during the subsequent laser irradiation, laser energy is more likely to be deposited around these cavities, resulting in more significant damage and development of damage within the crystal body. This also causes a decrease in laser energy deposition on the back surface and a slowdown in the development of damage on the back surface of the crystal.
Because the experimental images show that the damage in the crystal develops into a series of voids with a pinpoint connection, it is difficult to determine the pinpoint density to measure the development of damage. We analyzed the damage development behavior by recording the final state of the black area formed on the laser transmission path and its proportion throughout the entire path in the crystal. We analyzed the bulk damage growth at 7 and 10 J/cm2, as shown in Figures 13(a) and 13(b), respectively.
Fig. 13. Damage growth of crystal damage coverage rate under laser fluence of (a) 7 J/cm2 and (b) 10 J/cm2.
The experimental results of damage growth in the crystal body indicated that the damage growth behavior of 99% DKDP crystals was significantly different from that of KDP crystals, and the damage growth behavior of 70% DKDP was slightly different from that of KDP crystals. In the experiment, both KDP and 99% DKDP crystals exhibited stepwise destructive growth. The initial damage density of the KDP crystal was higher than that of the 99% DKDP crystal. The growth rate of KDP crystals under bulk damage was greater than that of 99% DKDP under the accumulation of a few initial shots. However, after irradiation for the 6th round at 7 J/cm2 and the 4th round at 10J/cm2, destructive growth of 99% DKDP crystals was observed. In the 99% DKDP crystal, the damage growth curve became steeper after destructive growth. It has been previously reported that DKDP and KDP crystals undergo a phase transformation at approximately 140 and 170 °C, respectively [24]. Under the same laser fluence irradiation, the lower phase transition temperature in DKDP was one reason for the more significant damage growth.
4. Conclusion
In this study, we observed and recorded the initial damage morphology and damage development behavior of KDP, 70% DKDP, and 99% DKDP crystals. In the experimental results, the development of crystal damage was not only manifested as the expansion of damage on the back surface of the crystal but also as an increase in pin-point density and size within the crystal. We observed that crystals with higher deuterization rates had larger initial pinpoint damage sizes. Theoretical analysis results demonstrated that, compared to KDP crystals, the same defects in DKDP crystals caused a more significant reduction in band gaps, a larger extinction coefficient, more defect-induced laser energy depositions, and a decrease in the ability to resist damage. This study provides a reference for studying the laser-induced damage characteristics of KDP/DKDP crystals under ultraviolet laser irradiation.
Funding
National Natural Science Foundation of China (No. 61622501).
Acknowledgments
The authors acknowledge the Natural National Science Foundation of China
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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