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Abstract
Bulk damage of deuterated potassium dihydrogen phosphate (DKDP, KD2PO4) crystal can be induced by intense laser irradiation, severely restricting the output of high- power laser energy and the usage of DKDP crystals in high-power laser systems. In this paper, laser-induced damage threshold (LIDT) and damage growth characteristics of DKDP crystal under 355 nm laser irradiation were systematically studied. The bulk laser-induced damage (LID) density of the crystal increased exponentially with the increase of the laser irradiation fluence. LID closely relates to the growth defects called precursors. Laser conditioning could effectively enhance the LID resistance by modifying the precursors. The LID density of DKDP crystals decreases after laser conditioning, and the increasing conditioning fluence can reduce the LID density. Damage growth after the initial LID by 1-on-1 measurement is different from that by R-on-1 measurement since the process of R-on-1 measurement leading to initial LID is equal to the laser conditioning procedure. The study in this paper provides a reference for improving the application of DKDP crystal in high-power laser systems.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
DKDP crystals are one of the non-linear optical and electro-optic crystal materials with superior performance as low half-wave voltage, large non-linear electro-optic coefficient, wide light transmission band, and relatively high laser-induced damage threshold (LIDT). Due to these properties and their large scales exceeding 500 mm [1], DKDP crystals become the only non-linear optical crystal materials applied in the high-power laser for the inertial confinement fusion (ICF) [2–5]. However, the laser-induced damage (LID) of DKDP greatly restricts the output power of pulsed laser and the working life of components severely [6,7]. The researchers of the National Ignition Facility (NIF) at LLNL put forward the steady output fluence of 8 J/cm2 at third harmonic generation (351 nm) leading to the demand for the improvement of the LIDT of DKDP crystals [8]. The same laser damage phenomena and the demand for high quality DKDP crystals also exist in other ICF laser facilities, including Laser Megajoule (LMJ) in France and Shen Guang III (SG-III) in China [9,10].
Many parametric studies of crystal growth have been carried out to improve the laser damage resistance of DKDP crystals, which included the growth rate, the temperature of the growth solution, impurities in the solution, filtration, and others [11–14]. Some aspects of improvement in crystal growth procedure such as improved salt purity and continuous filtration, have been confirmed that could enhance the laser damage resistance of DKDP crystals. In addition, laser conditioning by pre-exposure to subthreshold laser pulse can act as an effective means to improve the laser damage resistance for KDP and DKDP crystals [15,16]. However, the nature and chemical composition of defects related to laser damage initiation precursors are still unclear [17,18], although some characteristics of the damage initiating defect structures have been concluded. They are generally thought to have diameters ∼100 nm, and there are two distinct populations of damage precursors for longer wavelengths (such as at the fundamental of an Nd: YAG laser at 1064 nm) and shorter wavelengths (second and third harmonics at 532 nm and 355 nm), respectively [19,20]. However, the detailed effect of defects on LIDT still needs further exploration. Even though substantial improvements in DKDP growth technology to reduce defects over the past several years [21–24], the best LIDT in the bulk of the best current DKDP material is at laser fluence estimated to be more than an order of magnitude lower than the intrinsic breakdown threshold of the pure material [17]. The relationship between defect and LID leading to the defect-induced damage mechanism is still not well explained [12]. On the other hand, damage growth in DKDP crystals used in large-aperture laser systems is a significant issue that would determine component lifetime and therefore cost of operation [25,26]. Negres et al. [27] had studied the structure of KDP single crystal irradiated with 355 nm laser pulses. The results showed that decomposition of KDP at surface damage sites occurred while bulk damage pinpoints didn’t when irradiated with fluences above the laser-induced breakdown threshold. The results may explain the growth behaviors of laser damage pinpoints for KDP and DKDP crystals. Thus, investigating the LID characteristic and damage growth behaviors has great significance for the optimization of the usage of DKDP crystals.
In this paper, the initial LID characteristics and damage growth characteristics of a 70% DKDP crystal under 355nm laser irradiation were systematically studied. The DKDP samples were prepared from different regions in the boule of the pristine crystal. Moreover, different processes of laser conditioning were employed for exploring the effect of laser conditioning on the initial LID and damage growth characteristics of the 70% DKDP crystal. The results can provide a reference for revealing the relationship between defects and LID of DKDP crystals. It is beneficial for improving the application of DKDP crystals in high-power laser systems.
2. Experiment
The 70% DKDP crystal was grown by a conventional method using continuous flow filtration technology [23]. A specific cut in type-II phase matching was shown in Fig. 1. The samples were collected from different growth regions in the boule of the as-grown DKDP crystal, and separately named 1#, 2#, 3#, and 4#. The samples are in the size of 40×40×10 mm3, and their surfaces were polished by the single point diamond turning device. The 4# sample was divided into 5 sections, and they are named as no conditioning, low energy conditioning (L), medium energy conditioning (M), high energy conditioning (H), and high energy multi-pulses conditioning (HP), respectively. Specific sample treatments were shown in Table 1. In addition, low, medium and high energy in conditioning are defined as that their maximum conditioning energy are 20%, 50% and 80% of LIDT by R-on-1 measurement, respectively. In this work, the fluence of L was 1.3 J/cm2. The fluence gradient of M was 1.3 J/cm2, 1.9 J/cm2, 2.6 J/cm2 and 3.4 J/cm2, and that of H and HP was 1.3 J/cm2, 1.9 J/cm2, 2.6 J/cm2, 3.4 J/cm2, 4.1 J/cm2 and 5.0 J/cm2. Five pulses were applied in the HP configuration. The areas of conditioned regions were about 30×30 mm2. All samples were processed by raster scanning and hexagonal spot splicing. The schematic diagram was shown in Fig. 2.
Table 1. Sample information of DKDP crystals for LID properties
Nd: YAG pulsed laser with 355nm and 1Hz was adopted in this work. The pulse duration was about 5.8 ns. And the output laser beam had a single longitudinal mode. The Schematic for the measurement of LIDT was shown in Fig. 3. In addition, a coaxial transmission of 532 nm continuous laser was used as the probe light. Meanwhile, an off-axis long working distance microscope with high depth-of-field was placed on the side of the crystal to receive the scattering light from a 532 nm laser, thus realizing real-time monitoring of the LID state. The beam diameter, the beam modulation factor and the area of the effective laser spot were about 1.58 mm, 1.7 and 1.25 mm2, respectively. The pulse duration, the beam diameter and other bell-shaped parameters are defined as the 1/e2 of them. The polarization state with respect to the crystal orientation is e polarization state. The LID density was measured by a “blade” shaped laser with a size of about 6 mm×0.5 mm. The crystal was illuminated by the “blade” laser along the LIDT test direction, and the CCD was placed on the side of the crystal for observation. By moving the crystal at a certain interval such as 0.5mm, and then observing the number of damage pinpoints in the crystal at each moving interval, and counting the number of damage points in the whole damage threshold test area, the damage point density under this condition could be calculated.
3. Results and Discussion
3.1 Initial damage characteristics
3.1.1 LIDT by 1-on-1
The laser-induced damage measurements in 1-on-1 method for DKDP samples in different growth regions and the sample dealt with different laser conditioning processes by 355 nm pulsed- laser were obtained. As shown in Fig. 4 and Table 2, the LIDTs (In our essay, the LIDT is described as the parameter of T) of DKDP crystals in different growth regions by 1-on-1 measurement are ranked as T4#>T2#>T3#>T1#. 1# locates next to the regeneration region (polycrystalline region) of traditional DKDP crystal, and many defects such as dislocations, inclusions and grain boundaries exist [28], leading to the lowest LIDT of 0.8 J/cm2. Since sample 4# is located in the pyramidal section, with relatively better crystallinity and fewer defects, resulting in the highest LIDT of 2.2 J/cm2, still far from the theoretical LIDT of DKDP crystal [29]. 2# and 3# regions are near to the (100) face. In the growth process of DKDP crystal with the traditional method, tapering may appear due to the adsorption of impurity ions [30], therefore the T2# and T3# are lower than T4#. While under different laser conditioning protocols, the LIDTs are ranked as: THP (5.1 J/cm2)> TH (4.4 J/cm2)> TM (4.3 J/cm2)> TL (3.2 J/cm2)> Tnon-conditioning (2.2 J/cm2). Table 2 also tells the truth that under laser conditioning the increasing rate of LIDT of DKDP crystals by 1-on-1 measurement is at least 47%. The increasing rate of LIDT is defined as (TX-T4#)/T4#, in which T is defined as the laser-induced damage threshold, and X represents the different conditioning such as the L, M, H, and HP. Considering the measuring error, the increasing rate may fluctuate to some extent. For HP, the increasing rate can even reach 135%. This net gain in fluence is close to the results reported by Adams, where an ∼1.5X improvement in damage fluence was realized after conditioning with 500 ps pulses to 5 J/cm2 [31]. The results indicate the fact that laser conditioning can effectively improve the LIDTs of DKDP crystals.
Fig. 4. LID curves of 70% DKDP crystal in 1-on-1 measurement (a) different growth regions; (b) under different laser conditioning processes. The dot is the damage probability and the solid line is the linear fitting curve. The term “multi-pulses” solely apply to the conditioning, but not to the 1-on-1 procedure.
Table 2. The LIDTs of 70% DKDP crystal by 1-on-1 measurement under different growth regions and different laser conditioning
3.1.2 LIDT by R-on-1
The results of laser-induced damage measurements in R-on-1 method for DKDP samples in different growth regions and the sample dealt with different laser conditioning cases by 355 nm pulsed- laser were shown in Fig. 5 and Table 3. For different growth regions of the DKDP crystal, the LIDTs by R-on-1 measurement are ranked as T4#> T2#> T3#> T1#, which greatly agrees with the results in 1-on-1 measurement. Meanwhile, as shown in Table 3, the LIDTs of DKDP crystals can be improved by at least 5.6% in R-on-1 measurement due to the effect of laser conditioning. The different conditioning cases on DKDP crystal led to the rank of LIDT by R-on-1 measurement as THP (8.2 J/cm2) ≈ TH (8.2 J/cm2) ≈ TM (7.9 J/cm2) ≈ TL (8.0 J/cm2) > Tnone conditioning (4#, 7.5 J/cm2), which show the fact that the conditioning energy affects little in LIDT by R-on-1 measurement since the R-on-1 measurement is equal to a laser conditioning procedure which improves the LID resistance.
Table 3. LIDTs of DKDP crystal by R-on-1 measurement under different conditioning
3.1.3 LID density
The LIDT by 1-on-1 and R-on-1 measurements are sampling tests. Under intense laser irradiation, dense damage pinpoints appear in the bulk of DKDP crystal. Compared with “sampling” LIDT results (1-on-1 and R-on-1 test results), the LID density, which is closely related to the quantity and density of defects in the bulk, can better reflect the LID performance of DKDP crystal. All the LID density results are shown in Fig. 6.
Fig. 6. Curves of LID density vs. laser fluence of DKDP crystals under different growth regions and different conditioning cases (a) Raw data for different growth regions; (b) Raw data for different conditioning processes.
Figure 6 shows that the LID densities of all DKDP crystals exponentially increase as the laser fluence rises up. The different defects distribution results in different LID density curves of samples in different growth regions. When the laser fluence is 2.2 J/cm2, the initial LID appears, with damage density of sample 1# about 0.01/mm3, as laser fluence increases to 4.2 J/cm2, the LID density rises to 30.39/mm3, four orders of magnitude over the former. Then when the laser fluence of samples 2#, 3#, 4# increase, their LID density increases from 0.03/mm3, 0.07/mm3, 0.06/mm3 to 5.56/mm3, 1.8/mm3, 3.72/mm3, separately. Therefore, the DKDP crystal quality of different growth regions decrease with the rank as 4#, 2#, 3#, 1#, which agrees well with the above sampling LIDT results. Moreover, the experimental data can also certify that the laser conditioning can obviously enhance the LID resistance. In addition, with the increase of laser conditioning fluence, the LID density under the same damage testing fluence continually decreases, indicating that the defects in crystals can be modified by laser conditioning [32]. The subthreshold fluence of the laser conditioning may heat the inclusion defects to the temperature sufficient to lead to crystalline rearrangements and a subsequent significant decrease in absorption. Another mechanism is the annihilation of point defects by providing electrons or holes that modify their electronic structure, thus making them less absorbing at the testing laser irradiation [33–35].
3.2 Damage growth characteristics
As shown in the former results that the LIDT of DKDP crystals can be divided into three classes with their values as low, medium and high. Then 1#, 4#, and HP samples were adopted for representations to investigate the LID growth characteristics. The initial damages were induced under laser irradiation by 1-on-1 and R-on-1 measurements, separately. Then the R-on-1 measurement was implemented for the experiment on LID growth properties. According to the different LIDT of the samples, the laser fluence to induce the initial damage and examining damage growth are in discrepancy.
3.2.1 Initial LID by 1-on-1
For 1# sample, under one laser pulse of 3.0 J/cm2, a certain amount of damage pinpoints occurred in the bulk. Then the laser fluence (100 laser pulses at each laser fluence) increases by a gradient of 0.2 J/cm2, 0.9 J/cm2, 1.8 J/cm2, 3.00 J/cm2, 5.8 J/cm2, 7.8 J/cm2 and 8.8 J/cm2 to get the damage growth characteristics, as shown in Fig. 7(a). New damage pinpoints occurred at the 30th irradiation of 7.8 J/cm2 in the position circled in yellow. There’s no obvious damage growth after the 100th irradiation of 7.8 J/cm2. Repeating the process on tens of pinpoints, the average value 7.7 J/cm2 can be taken as the LID growth threshold. The process of damage growth test of 4# and high energy multi-pulses laser conditioning 4# samples are similar to that of sample 1#, as shown in Fig. 7(b) and (c). The final LID growth threshold of 4# and high energy multi-pulses laser conditioning 4# samples are 9.0 J/cm2 and 10.1 J/cm2, respectively.
Fig. 7. Image of laser-induced initial damage growth by 1-on-1 measurement (a) 1#; (b) 4#; (c) high energy multi-pulses conditioning sample. The texts in the figure describe the phenomenon in the yellow circle.
3.2.2 Initial LID by R-on-1
For 1# sample, the laser fluence (100 laser pulses at each laser fluence) was successively increased from an initial value of 0.2 J/cm2 until the damage was detected with the value of 4.6 J/cm2. The initial LID was obtained by R-on-1 measurement. The laser fluences were set at a gradient of 0.2 J/cm2, 0.6 J/cm2, 1.1 J/cm2, 1.8 J/cm2, 2.8 J/cm2, 3.4 J/cm2, 4.6 J/cm2. After the initial damage sites were induced, the laser fluence (100 laser pulses at each laser fluence) was increased by a gradient of 1.4 J/cm2, 2.8 J/cm2, 4.1 J/cm2, 5.5 J/cm2, 6.6 J/cm2 and 7.8 J/cm2 to get the damage growth characteristics, as shown in Fig. 8(a). New scattering points occurred at the 4th irradiation of 7.9 J/cm2 in the position circled in yellow which are shown in Fig. 8(a). Then there’s no obvious damage growth after the 100th irradiation of 7.9 J/cm2. Repeating the process on tens of pinpoints to reduce errors, the LID growth threshold of the initial damage pinpoint induced by R-on-1 measurement of 1# is 7.6 J/cm2. The process of damage growth test of 4# and high energy multi-pulses laser conditioning 4# samples are similar to that of sample 1#, as shown in Fig. 8(b) and (c). The final LID growth threshold of 4# and high energy multi-pulses laser conditioning 4# samples are 10.5 J/cm2 and 8.5 J/cm2, respectively.
Fig. 8. Image of initial LID growth by R-on-1 measurement (a) 1#; (b) 4#; (c) high energy multi-pulses conditioning sample. The texts in the figure describe the phenomenon in the yellow circle.
3.2.3 Damage morphology
The morphologies of LID sites of 70% DKDP crystals irradiated by 355 nm pulsed laser were collected by a microscope from the perpendicular direction to the laser transmission, and the typical morphologies of bulk damage sites were shown in Fig. 9.
Fig. 9. Typical bulk damage morphologies of different growth regions of 70% DKDP crystal (circled in red).
As shown in Fig. 9, the distribution of damage pinpoints in 1# sample are relatively dense, and the sizes are below 10 µm. The morphologies include dots, single cracks and multi-directional cracks extending around the central cavity. The damage pinpoints appearance in 2# and 3# samples similarly with morphologies include dots, single cracks and multi-directional cracks extending around the central cavity and sizes from ten to tens microns. The quantities of damage pinpoints are lower than that in 1# sample. In addition, cluster damage pinpoints appear in 4# sample. These morphologies depict that many types of defects exist in the pristine growth DKDP crystals, including the low, medium and high damage threshold defects. Meanwhile, the sizes of damage pinpoints induced by low damage threshold defects are smaller than that induced by medium and high damage threshold defects since the laser fluence in inducing high LIDT damage pinpoints is much higher than that in inducing low LIDT damage pinpoints. Under the irradiation of high laser fluence, the size of the damage pinpoint increases obviously.
3.3 Discussion
The LIDT of DKDP crystal is closely related to the clusters of intrinsic defects or other nano-particle impurities generated during the crystal growth process. The LIDTs of the DKDP crystals by 1-on-1 or R-on-1 measurement show the same trend: T4#> T2#> T3#> T1#. Since 1# sample locates closely to the regeneration zone of DKDP crystal, with relatively poor crystallinity, leading to its lowest LIDT of 0.8 J/cm2 in 1-on-1 measurement and 2.9 J/cm2 in R-on 1 measurement. While 4# sample locates in the pyramidal section, with relatively better crystallinity and fewer defects, its LIDT is the highest both in 1-on-1 measurement and R-on-1 measurement, with the value of 2.2 J/cm2 and 7.5 J/cm2 respectively, 1.5 orders higher over that of T1#. However, the LIDTs of the same sample tested by R-on-1 measurement is higher than that tested by 1-on-1 measurement as the R-on-1 measurement is equal to the laser conditioning procedure, which can enhance the LID resistance by modifying the defects.
For 70% DKDP crystal, when laser fluence is lower than the threshold leading to initial LID, there will not be obvious damage growth, no new damage pinpoints appear as well. Then when laser fluence grows higher than LIDT, new damage pinpoints come out. This may be attributed to the presence of intrinsic defect precursors in DKDP crystals [17], which act as high LIDT defects in this context. As for 1-on-1 measurement, when low energy laser modifies the low threshold precursors, initial damage pinpoints come out. Since the absence of low LID precursors, continued laser irradiation with the same fluence will not lead to damage growth. While increasing the fluence, laser irradiation will modify other high LIDT precursors, and then new damage pinpoints occur, which leads to damage growth. While for R-on-1 induced initial damage procedure, it is similar to the 1-on-1 measurement. What makes difference is the R-on-1 measurement is equal to a laser conditioning procedure. Laser irradiation can not only lead to the disappearance of the micro-liquid inclusion defects by regrowth process [36], but also induce the multiphoton absorption which can perhaps “passivate” the damage-initiating defects by providing electrons or holes that alter their electronic structure [35], thus making them less absorbing at the irradiating laser frequency. These phenomena both can raise the LIDT of the crystals, resulting in the fact that the LID growth threshold of initial damage induced by R-on-1 measurement may be higher than that by 1-on-1 measurement.
4. Conclusions
The initial LID and damage growth characteristics of 70% DKDP crystal by 355 nm laser were comprehensively investigated in this paper. The main conclusions are shown as follows:
- 1) Laser conditioning can effectively enhance the LID resistance. Under laser conditioning, the increasing rate of LIDT of DKDP crystal by 1-on-1 measurement is at least 47%. With high energy multi-pulses conditioning, the increasing rate can even reach 135%. While for R-on-1 measurement, laser conditioning can also improve the LIDT with an increasing rate of at least 5.6%. The bulk LID density of the crystal exponentially increases as the laser fluence rises. Crystallinity inhomogeneity leads to the distinct difference of LID density curves of different growth regions, the LID density agrees well with the sampling LIDT data in 1-on-1 and R-on-1 measurements.
- 2) The initial LID by R-on-1 measurement is equal to a laser conditioning procedure which can effectively improve the LID resistance by modifying defects called precursors, bringing about that the initial laser fluence of inducing LID by R-on-1 measurement is higher than that by 1-on-1 measurement. The defects in 1# sample can be divided into three classes by their LIDT as low, medium, and high LIDT defects, while only the medium and high damage threshold defects exist in other samples. The size of damage pinpoints induced by low LIDT precursors in DKDP crystals are small, while those of medium and high LIDT precursors are larger, mostly with the morphology of radiation-extended multi-directional cracks around the central cavity.
Funding
CAS Special Research Assistant Project; Young scholars Program of Shandong University (grant no 2018WLJH65).
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
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