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Abstract
LiB3O5 (LBO) crystal has a very high bulk laser damage threshold. Laser damage often occurs on the surfaces with a large number of processing defects during application. In this paper, the surface laser damage threshold, damage growth threshold, and damage growth curve of LBO crystal and fused silica under the same processing process have been comparatively studied by using a 355 nm pulsed laser. The surface laser damage performance of LBO crystal has been comprehensive evaluated. The results show that the laser damage threshold and damage growth threshold of LBO are about twice that of fused silica, and the damage growth coefficient is about 0.7 times that of fused silica. The detection and analysis of impurity defects and photothermal weak absorption defects show that the subsurface defects of LBO crystal are less than that of fused silica. Laser damage morphologies show that the damage process is related to strongly bonded chemical structure and anisotropic physical characteristics of LBO crystal. These characteristics together determine the high threshold damage performance of LBO crystal. The results of this study are of great guidance for the application of LBO crystal in high-power laser systems.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Lithium triborate, LiB3O5 (LBO), is an excellent nonlinear optical material [1,2]. It has an attractive nonlinear coefficient, a relatively wide acceptance angle and a negligible walk-off angle. Its transparency range spreads from mid-IR (2600 nm) to deep ultraviolet (160 nm). In addition, it has a good chemical stability and an outstanding laser damage resistance [1,3]. Therefore, LBO crystal has been widely used in research and application fields such as Nd: YAG laser di- and triplet frequency devices with high laser output [4,5], optical parametric oscillators and optical parametric amplifiers [6–8]. Based on LBO crystal, many high-power and high-intensity large-scale laser facilities have been built in the past decade [4–8].
Although the laser damage threshold of LBO crystal is very high, with the rapid development of high-power laser devices and the extreme pursuit of laser output capability, LBO crystal is subjected to increasingly high laser fluence and laser damage is unavoidable. Previous studies have shown that the nanosecond laser damage threshold of LBO crystal materials is the highest among inorganic crystal materials, about 1.5-1.8 times that of fused silica [3,9]. The bulk material has a fatigue effect, and the damage threshold decreases to half of the original value with multiple nanosecond 1064 nm laser irradiation [10]. Even at fluences well below the laser damage threshold, laser damage can occur on the surface of LBO crystal with long-term high heavy frequency laser operation [11,12]. The damage may be caused by subsurface defects resulting from the surface processing of LBO crystal. Based on Mie scattering theory, the literature analyzed the enhancement effect of polished particles remaining in the polished layer of LBO crystal on the incident electric field [13]. We experimentally studied the influence of bulk defects on the laser damage properties of fused silica, and showed that the surface damage threshold of fused silica is much lower than the bulk damage threshold [14].
In the last two decades, there have been a large number of research papers on laser damage due to processing defects of optical components, mostly for fused silica optics [15–21]. Two major types of defects are considered responsible for the surface laser damage of optical components: one is highly absorptive contaminants (e.g., Ce, Fe, etc) in the Beilby layer coming from polishing process [15–17]; the other is subsurface damage (SSD) created during grinding and/or polishing of brittle material surfaces [17–21]. The LBO crystal processing process is similar to that of fused silica, and the same problem exists. However, there have been rare studies on the effect of subsurface defects on the surface laser damage properties of LBO crystal so far. Although they are both transparent and brittle materials, the material properties of LBO crystal and fused silica are very different. Subsurface defects may significantly vary with the same process. Therefore, the influence of subsurface defects on the laser damage performance of optical elements is also different. Thus, it is of great interest to study the surface damage performance and the detailed subsurface defect characteristics of LBO crystals.
In this paper, the surface laser damage performance of LBO crystal has been evaluated more comprehensively by comparing the surface damage characteristics of LBO crystal and fused silica optics. The essential mechanism of the high damage threshold of LBO crystal has been revealed by the analysis of subsurface defects and material intrinsic characteristics.
2. Experiments
2.1 Sample preparation
LBO crystal is produced by Fuzhou Core Optronics Company, China. All samples are cut at θ = 42.4° and φ= 90° with the dimensions of 15 mm × 15 mm ×5 mm, as a type-II sum frequency generation (SFG) module. Fused silica come from China Building Materials Academy. The size of fused silica samples is 50 mm × 50 mm × 5 mm. Using CeO2 as the main polishing agent, the sample surfaces of both components were finely polished by Fuzhou Core Optronics Company with the conventional chemical-mechanical polishing method. Surface roughness of all samples is below 1 nm measured by profile meter.
2.2 Laser damage test method
The layout of laser damage experiment is shown in Fig. 1. The output energy of the tripled Nd: YAG laser can reach up to 500 mJ. The output energy is adjusted by using an energy attenuator with an accuracy of 10 mJ. Telescope system is applied to filter high frequency modulation and reduce the diameter of beam. An uncoated fused silica pickoff wedge reflects two beams for the diagnostic systems. A calibrated pyroelectric detector measures pulse energy proportional to the energy of target surface. A beam profiler placed at the same optical distance as target surface provides beam size and spatial profile information. A fast photodiode allows measurement of the temporal profile. The spot shape and pulse shape are shown in Fig. 2. The spatial beam distribution is an approximate flat-topped Gaussian with a diameter of ∼1.22 mm. Its modulation is about 2.1. The pulse waveform is a single longitudinal mode with about 9.5 ns (FWHM). An optical microscope with a resolution of about 3 µm is placed behind the sample for online observation of the damaged region on the rear surface of the sample.
The initial damage threshold and damage growth threshold is obtained by R on 1 procedure [22], i.e., for the same location, starting with a fluence J well below the initial damage threshold, hitting one shot and raising the fluence j once until laser damage occurred, recording the fluence as J + (n-1)j, with n being the number of shots. For most of the test data, 20 mJ was set as the energy test step. Only the energy test step for damage growth threshold of fused silica is 10 mJ. The uncertainty of the energy test system is ±5.6%. Ten points are tested for each sample. The damage probabilities are calculated for each energy separately, and the damage probability curves are derived for different irradiation fluences.
3. Laser damage results
The laser damage characteristics of optics mainly include the initial damage threshold, the damage growth threshold, and the damage growth rate. The laser damage performance of LBO crystal was analyzed mainly from the parameters below.
3.1 Laser damage threshold and damage growth threshold
The initial laser damage to transparent media generally occurs on the rear surface due to the Fresnel reflection, which results in a field strength about 1.4 times higher at the medium/air interface than at the air/media interface [23]. However, when the laser fluence reaches the threshold fluence for excited Brillouin scattering, it leads to more damage on the front surface of the transparent medium [24]. Only the rear surface damage was found during the laser damage test of fused silica. However, during the laser damage experiment of LBO crystal, the damage may occur on the rear surface, or on the front surface. In addition, the damage may occur simultaneously on both the front and back sides. No bulk damage was found. We only took the data of rear surface damage for the sake of uniform comparison.
The damage thresholds and damage growth thresholds of LBO and fused silica are shown in Fig. 3, where red is LBO crystal, green is fused silica, hollow is the initial damage threshold, and solid is the damage growth threshold. The zero probability damage thresholds of LBO crystal and fused silica are about 22 J/cm2 and 12 J/cm2, respectively. The corresponding zero probability damage growth thresholds are about 6 J/cm2 and 3 J/cm2, respectively. The laser damage resistance of LBO crystal is almost two times that of fused silica. From the damage probability curves, it can be seen that the initial damage probability curves of the LBO crystal and fused silica optics have a similar variation trend. This indicates that the subsurface defect density caused by processing for these two materials is similar.
Fig. 3. Laser damage thresholds (DT) and damage growth thresholds (DGT) of LBO crystal and fused silica (FS). LBO-DGT: Damage growth thresholds of LBO crystal-LBO-DT: Laser damage thresholds of LBO crystal; FS-DGT: Damage growth thresholds of fused silica-FS-DT: Laser damage thresholds of fused silica.
We explain the above phenomenon by processing technology of optics. In order to improve the resistance of optics to laser damage, low threshold defects are controlled. When one defect is removed, another defect with a slightly higher threshold is revealed. In fact, it is very difficult to completely remove low threshold defects, but it is controlled as little as possible. It can be irradiated to cause damage only when the irradiation area is large enough. Therefore, under the same irradiation flux, some irradiated areas will be damaged, and some irradiated areas will not be damaged, which is the damage probability. The discretization and diversity of optics defects result in the damage probability distribution shown in the figure. In general, the zero probability damage thresholds represent the laser damage threshold of optics, and 100% probability damage thresholds represent the highest threshold that can be obtained by the current processing method, which is the best state that can be achieved after removing all defects.
The above explanation can also explain the damage growth probability curve. The initial damage pit used to study the damage growth characteristics was obtained by Ron1 test with a diameter of less than 100µm. The damage growth threshold of fused silica is very concentrated (3J/cm2∼ 7J/cm2), while LBO covers from 6J/cm2 to 26J/cm2. This implies a more complex mechanism of laser damage growth triggered by the initial damage of LBO crystal.
3.2 Laser damage growth
To evaluate the service life of an optics, the damage growth coefficient of the optics is an important evaluation factor. The results of the literature [25] show that the damage crater size increases exponentially with the number of shots, $S = {S_0}{e^{\alpha N}}$, S0 is the initial damage crater size, N is the number of shots, and α is the damage growth factor. We carried out the experiment on the variation of damage pit size with increasing laser shots, and studied the damage growth performance of LBO crystal and fused silica irradiated by 355 nm laser. The results are shown in Fig. 4, with LBO crystal in red and fused silica in green. LBO-12.4, LBO-18.6, LBO-24.8, LBO-29.7 represent the diameter of LBO crystal damage pit varying with laser shots under the irradiation fluence of 12.4mJ/cm2, 18.6 mJ/cm2, 24.8 mJ/cm2 and 29.7 mJ/cm2, respectively. FS-6.4, FS-11.4, FS-16.6, FS-22.8 represent the diameter of fused silica damage pit varying with laser shots under the irradiation fluence of 6.4mJ/cm2, 11.4 mJ/cm2, 16.6 mJ/cm2 and 22.8 mJ/cm2, respectively. The marked points in the figure are the experimental data, and the curves are obtained by fitting the experimental data using the above equation. It can be seen that the fitted curves of the exponential law are in good agreement with our experimental results.
Fig. 4. The increasing of surface damage pit size of LBO crystal and fused silica with laser shots under different 355nm laser irradiation fluence (J/cm2).
Because there are many differences in damage threshold and damage growth threshold between fused silica and LBO, it is difficult to make the same fluence of the two materials when doing the laser damage growth curve. Comparing the two sets of data with similar fluence, LBO-12.4 and FS-11.4, it can be seen that the damage growth of LBO crystal is much lower than that of fused silica, even with higher irradiation fluence. A more intuitive comparison is the damage growth coefficient of the material under different laser irradiation fluxes, as shown in Fig. 5, where the LBO crystal is red and the fused quartz is green. The laser damage growth coefficients of two types of materials both increase with the laser irradiation fluence. The laser damage growth coefficient of fused silica under the same fluence is larger, indicating that the damage growth rate of fused silica is faster.
Fig. 5. The increasing of laser damage growth coefficient of LBO crystal and fused silica with average fluence.
4. Analysis and discussion
We analyze the causes of the damage differences between LBO crystal and fused silica by analyzing their subsurface defects and damage morphology.
4.1 Subsurface layer impurity defects
Time-of-flight secondary ion mass spectrometry (TOF-SIMS) [17] is an extremely high-resolution measurement technique to measure ion mass. The sample surface was excited with primary ions to hit a small number of secondary ions. The ion masses were determined from the different flight times of the secondary ions to the detector due to their different masses. Figure 6 shows the depth change of positive secondary ions for the two materials generated by the bombardment of energetic particles. From the figure, it can be seen that the subsurface defects of the two optical elements differ greatly under the same processing method. The sputtering yields (Si+) of fused silica are very high with the same incident ion bombardment and are uniform on the surface and within the body. In contrast, the sputtering yields (Li + and B+) of the LBO crystal are very low and decrease rapidly with depth. This is because the bombardment of energetic particles breaks the silicon-oxygen bond in fused silica more easily, causing the outermost atoms to be stripped away and the sputtering yield to be high. A large amount of Si + are detected. However, the chemical bonds of the LBO crystal atoms are so strong that it is difficult for the energetic particles to break the chemical bonds in the crystal state. The detected Li + and B + are mostly ions formed by hydrolysis. It can be seen from the Fig. 6 that the hydrolyzed layer of LBO is very thin. We speculate that it is introduced in the polishing process. The effect of the thin hydrolyzed layer on laser damage has not been found in this paper.
It can also be seen from the figure that the subsurface impurities of the two materials are similar, but their content and distribution are very different. The fused silica contains more impurity components with deeper impurity distribution. There is still a considerable amount of K + at the depth of 300 s of etching time. However, the impurity content of the LBO crystal was almost 0 at the depth of 100s of etching time. It can be seen that the polishing process leaves a very shallow deposition layer on the surface of the LBO crystal. Due to the strong bonding and lack of suitable lattice site in LBO crystal [1], it is difficult to have other impurities embedded in the subsurface layer. This may be one of the important reasons for its higher damage threshold compared to the fused silica component.
4.2 Photothermal weak absorption of subsurface layers
Photothermal deflection technique is an effective method to detect the photothermal weak absorption of transparent optical materials. Huang et al. have reported that the weak photothermal absorption has a good correlation with the laser damage performance of the optical elements [26]. When the inherent absorption of optical material is determined, the higher the weak absorption of optical surface indicates that the more absorption defects are introduced during the processing process, and thus the lower the damage threshold is. For different materials, such as LBO crystal and fused silica, their inherent absorption at the 355 nm laser is different. Figure 7 shows the weak absorption distribution of the two materials, respectively. The figures show that the inherent absorption of LBO crystal is much higher than that of fused silica. The average surface weak absorption values of the LBO crystal and fused silica are 29.6 ppm and 13.6 ppm, respectively. The weak absorption on the surface of LBO crystal was very uniform, with the weak absorption value concentrated at 27 to 32 ppm. However, the weak absorption value on the surface of fused silica was discrete from 8 ppm to 36 ppm. This indicates that the photothermal weak absorption induced by processing of the LBO crystal surface is much less than that of fused silica. Fewer processing defects on the surface of LBO crystal may be another reason for its higher damage threshold.
4.3 Damage morphology
The initial damage morphology and multiple sub-irradiation damage morphology of the LBO crystal and the fused silica are shown in Fig. 8. From Fig. 8(a) and Fig. 8(b), it can be seen that the initial damage morphology of both materials is very different. The initial damage pits of fused silica are close to circular in shape, although the surrounding structure is irregular [27], while the initial damage pits of LBO crystal are generally butterfly-shaped. This is related to the micro-structure and thermo-mechanic properties of both materials. The fused silica has an isotropic structure, while the LBO crystal is highly anisotropic in the three axes (Its space group is Pna21.The thermal expansion coefficients are ${\alpha _a} = 101 \times {10^{ - 6}}^\circ {C^{ - 1}},{\alpha _b} = 31 \times {10^{ - 6}}^\circ {C^{ - 1}},{\alpha _c} ={-} 71 \times {10^{ - 6}}^\circ {C^{ - 1}}$, respectively) [28]. Krupych et al. [29] show that cracks formed by laser-induced damage are always perpendicular to the direction of the minimum of the expansion coefficient. Point defects on the surface of LBO crystal absorb laser energy, form local high temperatures, and then generate thermal stress. When the stress exceeds the fracture strength of the material, combined with the cutting direction of the LBO crystal (θ = 42.2° and φ = 90°), the anisotropy of the thermal expansion of the crystal will lead to the butterfly-like damage pit in Fig. 8(a)). When the laser irradiates the damage pit of LBO crystal again, the damage site that triggers the damage growth can no longer be regarded as one point structure, but a body structure formed by many points. Although the damage of each point still follows the anisotropic characteristic, the damage growth of the whole damage pit is the superposition of these point damages. Therefore, with the increase of damage pits, this anisotropy tends to disappear. Figure 8(c) and 8(d) show the multiple irradiation damage morphology of the LBO crystal and fused silica, respectively. The final morphology of multiple irradiation damaged structures of LBO is similar to fused silica.
Fig. 8. Laser damage morphology of LBO crystal and fused silica, a) the initial damage morphology of LBO, b) the initial damage morphology of fused silica, c) the multiple irradiation damage morphology of LBO, d) the multiple irradiation damage morphology of fused silica.
5. Conclusion
In this paper, the UV laser damage performance of LBO crystal and fused silica surface under the same processing process was comparatively studied, and the characterization and analysis of subsurface defects of LBO crystal and fused silica were carried out. The results show that the UV laser damage performance of LBO crystal is much greater than that of fused silica. The laser damage threshold and damage growth threshold of LBO crystal are about twice that of fused silica, and the damage growth coefficient is about 0.7 times that of fused silica. The subsurface layer of LBO crystal is thinner compared to fused silica, with few impurity present in a very thin surface layer other than the hydrolytic ion component. The intrinsic absorption of the LBO crystal material is larger resulting in a larger surface weak absorption. The uniformity of the weak absorption distribution of the LBO crystal surface suggests that the photothermal weak absorption caused by processing defects is small and uniform. The defect analyses can explain well why the LBO crystal has a high laser damage performance. In addition, the UV laser damage morphologies on the surface of the LBO crystal and fused silica are also analyzed based on the thermal expansion properties. The initial damage morphology, with butterfly-shaped of LBO crystal and nearly circular of fused silica, are explained reasonably. In summary, the surface damage properties of LBO crystal is shown comprehensively and the effect of subsurface defects on the damage of LBO crystal is also given in this paper. It is an important guideline for the application of LBO crystal in high-power laser devices.
Funding
National Natural Science Foundation of China (62005258, 62175222).
Acknowledgments
The authors acknowledge the support from the Institute of Research Center of Laser Fusion.
Disclosures
The authors declare no conflict of interest.
Data availability
The data that support the findings of this study are, available from the corresponding author upon reasonable request.
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