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The effects of γ-irradiation on potassium dihydrogen phosphate crystals containing arsenic impurities are investigated with different optical diagnostics, including UV-VIS absorption spectroscopy, photo-thermal common-path interferometer and photoluminescence spectroscopy. The optical absorption spectra indicate that a new broad absorption band near 260 nm appears after γ-irradiation. It is found that the intensity of absorption band increases with the increasing irradiation dose and arsenic impurity concentration. The simulation of radiation defects show that this absorption is assigned to the formation of AsO44- centers due to arsenic ions substituting for phosphorus ions. Laser-induced damage threshold test is conducted by using 355 nm nanosecond laser pulses. The correlations between arsenic impurity concentration and laser induced damage threshold are presented. The results indicate that the damage performance of the material decreases with the increasing arsenic impurity concentration. Possible mechanisms of the irradiation-induced defects formation under γ-irradiation of KDP crystals are discussed.
© 2014 Optical Society of America
1. Introduction
Potassium dihydrogen phosphate (KDP) is a preferred and technologically important optical material used in large-aperture high power laser devices and various commercially laser systems with excellent electro-optical and nonlinear optical properties [1–3]. However, the lower laser-induced damage threshold of this material is a significant problem to limit the high power output laser system [4, 5]. The formation of localized damage sites was attributed to either a mass of impurity nanoparticles introduced during growth or intrinsic defects in crystals [6–8]. And it is difficult to identify the accurate nature of these defects due to extremely small size. Moreover, it is a complex physical mechanism of these defects that interact with γ-ray and other ionizing radiations [9–12].
A lot of intrinsic defects in KDP crystals irradiated using γ-ray were reliable identified by optical absorption and electron paramagnetic resonance (EPR) spectroscopy [13, 14], e.g., hydrogen atoms [15], oxygen vacancies [16], self-trapped holes, and holes trapped adjacent to hydrogen vacancies [17]. But most of these defects in pure crystals are hardly detected at room temperature [13]. On the other hand, the defect centers formed in KDP crystals by metal impurity ions (Fe2+, Fe3+, Ca2+, Al3+, Co2+) and other ions (As5+, Cr5+, Si4+) incorporated in the crystal lattice are investigated under γ-ray irradiation [18–20]. The previous results indicated that radiation-induced new absorption peaks appeared in the UV spectral range is because that the impurity ions substitute the normal lattice sites and trap the electrons or holes at impurity sites to form new defects. Therefore, γ-ray irradiation may lead to many changes in the physical and chemistry properties during the exposure to gamma irradiation. Leuchenko et al. reported that the change of electrical dc conductivity with γ-ray irradiation is attributed to decrease of the concentration of L-defects in KDP crystals and disappearance of irradiation-induced defects under thermal, laser, and ionizing irradiation annealing in γ-irradiated KDP crystals containing arsenic ions [11, 21]. Garces et al. reported that the undesired optical absorption bands in the 200~300 nm region are due to Fe ions substituting phosphorus ions and then forming the (FeO4)2- defect centers after X-ray irradiation [22]. Similar process has been reported in the transformation of Cr ions substituting phosphorus ions by Alybakov et al. [23]. The above-mentioned experimental results suggest that the presence of impurity ions can give rise to the formation of defect centers under ion irradiation. Up to now, the radiation-induced changes in KDP crystals are still very complicated and not clear. The formation of defects can give rise to changes in the crystal performance, including laser damage resistance and UV absorption. Thus, understanding the irradiation-induced defects in KDP crystals has been a subject of great interest and importance in improving material performance.
In this work, the evolution of radiation-induced defects has been investigated with γ-ray irradiation at different doses. Optical absorption of irradiated KDP crystals has been studied using UV-Vis absorption spectroscopy and photo-thermal common-path interferometer. The influence of arsenic impurities on the laser damage performance has been also discussed. Although the correlation between the concentration of arsenic impurity and the UV optical absorption has been studied and it can be concluded that the KDP with sufficient purity can withstand the irradiation of neutrons and γ-rays [20]. But there is no data about the KDP crystals with low arsenic concentration less than ~4.8 ppm. In this work, we focus on the low concentration of arsenic impurities. In addition, the irradiation-induced As-related defect model has been established and the simulated optical absorption results are consistent with the experimental results. A method to detect the arsenic impurities and their distribution in KDP crystals is also presented by γ-ray irradiation combined with UV light illumination.
2. Experimental and computational model
2.1 Samples and experimental parameters
The KDP samples were conventionally grown and cut to type ΙΙ frequency conversion orientation and polished on all sides. All the samples were 30 × 30 × 4 mm3. Firstly, an inductively coupled plasma mass spectrometry (ICPMS) was utilized to detect the concentration of impurities in KDP crystals. This is a sensitive and straightforward analytical method that is capable of detecting impurities concentration. Secondly, the KDP crystals were irradiated at room temperature with 60Co γ-ray to the dose of 1 kGy, 10 kGy and 100 kGy at the dose rate of 5.834 Gys−1. After that, a Lambda 950 (PerkinElmer) spectrophotometer was used to measure optical absorption of the KDP crystals in the range of 200 to 800 nm before and after irradiation. The baseline correction procedure was executed prior to each measurement of samples to exclude the absorption of background and environment. Then the spectra of original KDP samples with different arsenic concentrations were collected. The spectra of the original samples are same, so one of them is shown in Fig. 1. Finally, the absorption spectra of γ-ray irradiated KDP samples are obtained. Photo-thermal absorption measurements were performed with a pump light at 355 nm. The CW pump laser is focused to a beam with diameter of 20 μm. The probe beam is a 2.0~2.5 mW He-Ne laser as probe light focused to a spot with diameter of 6 μm passing through the heated area. The photoluminescence (PL) spectra were carried out on a LS 45 Luminescence Spectrometer, with a 20 kW Xe discharge lamp for 8 μs duration used as an excitation source and a gated photomultiplier used as a detector. Finally, the damage threshold was measured by R: 1 test procedure with a 355 nm Nd: YAG laser. An image of the irradiated area was obtained by a CCD microscope before and after each shot in order to observe laser-induced scatter centers of KDP crystals.
Fig. 1 Absorption spectra of KDP crystals before and after γ-irradiation with different irradiation doses for same arsenic concentration (3.154 mg/kg) (a) and with different arsenic concentrations for same irradiation dose (105 Gy) (b). The inset of (a) and (b) show the irradiation-induced absorption intensity at 260 nm as a function of the irradiation dose and arsenic concentration, respectively. The black square in (a) and (b) represent the logarithm of irradiation dose and arsenic concentration, respectively. Red dot lines were used to guide the eyes.
2.2 Computational model
A study of arsenic substituting for phosphorus is presented using an ab initio method. This method is based on density-functional theory (DFT) and ultra-soft pseudo-potential [24] was used to describe the ion-electron interaction. The exchange-correlation potential was described by the Perdew-Burke-Ernzerhof (PBE) under generalized gradient approximation [25]. The cut-off kinetic energy of the plane-wave function is set to be 680 eV, yielding a convergence for the total energy better than 1 meV/atom [26]. According to the Monkhorst-Pack scheme [27], convergence tests for (2 × 2 × 2), (4 × 4 × 4), and (5 × 5 × 5) divisions along the reciprocal-lattice directions in the primitive unit cell of pure KDP have been performed. It is found that the total energy converges better than 0.1 meV per atom if a 4 × 4 × 4 k-point grid was used. For the KDP supercell, an appropriately scaled grid was used. This method can yield well-converged results for the total energy. A supercell which consists of eight KH2PO4 formula units (64 atoms) was used to represent a perfect crystal, and a model with desired defect was generated via an arsenic ion substituting for a phosphate ion. Using the conjugate gradient techniques, the supercell is fully relaxed.
3. Results
3.1 ICPMS measurement
ICP-MS is a good chemical analysis technique used for elemental determinations. Compared with secondary ion mass spectrometry, ICP-MS has better sensitivity and precision. As detected by ICP-MS analysis, the impurity concentrations of arsenic element in three groups of samples were 3.154, 2.805, and 0.089 mg/kg, respectively. Other potentially impurities such as Si and Cr were not detected in all three samples. The detectability limit of element is 0.0002 mg/kg.
3.2 Absorption spectra
Figure 1(a) shows the UV-Vis absorption spectra of the KDP crystals after γ-irradiation with different irradiation doses for same arsenic concentration (3.154 mg/kg) at room temperature. It is obvious from Fig. 1(a) that the KDP crystals have a widely absorption band in the range of 500~700 nm before irradiation. This band may be attributed to color centers created during crystal growth [18, 19]. After γ-ray irradiation, the samples show an overall absorption increase within 200~360 nm spectral range, while the absorption band in the range of 500~700 nm disappears. In addition, a new absorption band at 260 nm appears which is similar to the results in [11] and [20, 21]. The intensity of the absorption band at 260 nm increases with the increasing irradiation dose. The inset shows the irradiation-induced absorption intensity at 260 nm as a function of the irradiation dose.
Figure 1(b) shows the UV-Vis absorption spectra after γ-irradiation with same dose 100 kGy for the samples of different arsenic concentrations at room temperature. The inset shows the corresponding correlation between irradiation-induced absorption peak intensity at 260 nm and arsenic concentration. The absorption intensity increases with the increase of arsenic concentration.
The irradiation-induced absorption at 260 nm may be due to the presence of arsenic impurity element [20]. The interpretation of this absorption band is that arsenic ions incorporate into the anionic sub-lattice and form (AsO4)3- radicals, and then substitute isomorphically for the (PO4)3- groups in the crystal structure. Under irradiation, these radicals turn into color centers and result in appearance of absorption peak at 260 nm [20, 21]. Accordingly, γ-irradiation presumably leads to the following processes:
Interaction between irradiation with KDP crystal gives rise to high concentration of free electrons , which are unstable at room temperature. Subsequently, these successive electrons are captured by the arsenate ion AsO43- which is the isomorphic substitute of the phosphate ion PO43- in the crystal lattice of KDP, and the paramagnetic centers AsO44- form, which causes optical absorption in the UV spectral region, while the AsO32- species is believed to have puniness absorption [20]. As is shown in [20, 21], the paramagnetic centers AsO44- and AsO32- are associated with two broad optical absorption bands at 260 nm and 350 nm, respectively. The concentration of AsO44- radical gradually reduce when the temperature increase from 110 to 150 °C, while AsO32- radical appear after temperature higher than 140 °C [21].Our experiments reveal that the intense absorption band arises at 260 nm after irradiation, which is similar to the previous results in [11, 20]. This radiation-induced absorption band is associated with the presence of arsenic impurities, which easily enter the crystal lattice and substitute isomorphically for phosphorus ions during crystal growing. For this impurity, the arsenic free radicals such as AsO44- and AsO32- easily form due to electron capture or loss in the process of irradiation. Thus it is worth to understand the formation of these defect centers using an ab initio method.
A tetragonal supercell which consists of eight KH2PO4 formula units was used to represent a perfect crystal with lattice vectors ,, [26]. The lattice constant is a = 7.430 Å and c = 6.970 Å. The defect center was generated via an arsenic ion substituting for a phosphorus ion and formed an AsO4 unit with an interstitial oxygen ion as shown in Fig. 2.
Figure 3 shows the optical property for the perfect crystal and defect structure induced by an interstitial oxygen atom near the AsO4 unit for the neutral charge state. Compared with perfect crystal, two apparent absorption peaks are observed at 190 nm and 260 nm for defect model in Fig. 3. For absorption peak at 260 nm, it accords with the previous experimental results [11, 20, 21] and results in this work, while the absorption band at 190 nm is outside the spectral testing range in our experiment. Other defect structures, including the hydrogen vacancy, the hydrogen interstitial and oxygen vacancy near the AsO4 units in the neutral state, were also investigated and the results indicated that there is no absorption band at 260 nm for these defects.
Fig. 3 Comparison of optical absorption spectra calculated for crystal containing defects and perfect crystal.
The formation of the radiation-induced absorbing centers may be due to the following reason: arsenic ion is easy to substitute for isomorphically phosphorus ions and enters the anionic sub-lattice occupying the normal lattice position during crystal growing. Under irradiation, the arsenic ion AsO43- transforms into the defect center AsO44- as a result of free electron capture. A portion of AsO44- radicals will decompose into an AsO32- radical and an O2- ion. Additionally, a free electron produced by γ-irradiation may be captured by an OH- radical and thus form an OH2- radical. The OH2- radical is unstable and directly decomposes into a neutral hydrogen atom and an O2- ion. The resulted hydrogen atom in turn is ejected from its original site, and subsequent the [HPO4]- radical is formed by trapping the hole on the oxygen atom closest to the newly formed the H vacancy. The oxygen ion decomposed from OH2- easily moves to the interstitial site nearest to an AsO44- unit and thus causes the optical absorption at 260 nm. The electron paramagnetic resonance (EPR) experiments [18, 20, 28] provided direct spectroscopic evidence of these centers, including the [HPO4]- hole centers, the hydrogen interstitial H0 electron centers, the paramagnetic center AsO44- and AsO32-. Among them, the free electron, atom H0 and [HPO4]- hole centers are unstable at room temperature [13], while the paramagnetic centers of AsO44- and AsO32- are stable for many days. Thus, arsenic impurity defects resulted from arsenic impurities play a major role in the optical absorption property of KDP.
3.3 Photo-thermal absorption measurement
The principle of photo-thermal common-path interferometer is to measure the absorption of the material with the heating effect [29]. The pump beam energy absorbed and not lost by subsequent emission results in sample heating. The heating results in temperature change as well as the change of spatial refractive index of the sample which is related to temperature. The deflection of the transmitted probe beam which is due to modulated refractive index gradient is measured by a position sensor. This photo-thermal spectroscopic technique has a high sensitivity for the weak optical absorption. Optical–thermal absorption is measured in scanning mode with the area of 9 mm2 for each crystal. Because the distribution of impurities in crystal bulk is not uniform, the average absorption coefficient was used to characterize the optical–thermal absorption of irradiated samples. The sensitivity of photo-thermal spectroscopy is about in the order of 0.01 ppm. The absorption coefficient after γ-irradiation with same dose of 100 kGy for three samples is 9244, 7720 and 2724 ppm, corresponding to the arsenic concentration of 3.154, 2.805 and 0.089 mg/kg, respectively. This result shows that the average absorption coefficient is small because of the low arsenic concentration. Usually, absorption and subsequent heating are due to two-photon absorption process which is resulted from the impurities incorporated into the crystal [30]. Irradiation generates impurity defects, a portion of which may eventually evolve into a band gap state that often lies within the band gap of KDP. Such a state may be an electronic defect center in the crystal and acts as a heating source. The dimensions of the heating source is in the order of tens of nanometers or smaller because large size of impurities are filtered using a constant filtration process during growth. The simulation result [8] also showed that optical absorption could be enhanced by such nanometer sized particles or clusters of defects.
3.4 Irradiation image
Figure 4 show the microscopic images of scattering centers in the KDP crystals before and after irradiation. As for the pristine KDP, no scatter center is observed in Fig. 4(a). After γ-irradiation, Figs. 4(b) shows the presence of irradiation-induced light emitting “particles” inside the KDP crystal. The γ-irradiated samples are illuminated with Nd: YAG laser at 355 nm. The image of pinpoint damage sites resulted from a single pulse at 5.25 J/cm2 is also presented in Fig. 4(c). As is known in [6, 31], laser damage sites occur in crystal bulk as a few or a series of pinpoints which consists of a core and a surrounding deformed zone. Compared with the laser damage site, the γ-irradiation induced scatter site consists of defect clusters like nebulae as shown in Fig. 4(b). This result suggests that the formation of new defect species may be due to the interaction between the lattice or impurity and the γ-rays.
Fig. 4 Comparative images in bulk KDP samples obtained for a) pristine KDP; b) γ-irradiated KDP crystal with 100 kGy; c) laser-irradiated KDP crystal with 5.25 J/cm2.
3.5 PL measurement
The PL spectra before and after γ-irradiation for KDP crystals with different arsenic concentrations are shown in Fig. 5(a). All of the spectra obtained with 215 nm (5.77 eV) excitation have two broad luminescence peaks in the range 300 ~400 nm (3.1 ~4.1 eV) and 550 ~700 nm (1.77 ~2.25 eV), respectively. The PL intensity increases distinctly after γ-irradiation. For analysis, the spectra of each sample have been deconvoluted into Gaussian peaks before and after irradiation as shown in Fig. 5(b). The information of these Gaussian bands is listed in Table 1. The center position of the five Gaussian bands allowed slight shift in optimizing the deconvolution [32]. The spectral analysis shows that the γ-irradiation does not produce new luminescence centers, but the PL intensity is strongly enhanced by irradiation. The Gaussian bands centered at 660 nm (1.88 eV), 600 nm (2.07 eV) and 370 nm (3.35 eV) are originated from the hydrogen defects [33, 34]. The 455 nm (2.73 eV) emission band may be associated with the interstitial oxygen ions [35]. The emission band centered at 330 nm (3.76 eV) may be attributed to the presence of impurities element such as Ce, Ba and Sr due to intense emission in the UV range [28, 36].
Fig. 5 (a) PL emission spectra of KDP crystals before and after irradiation for different arsenic concentrations; (b) represents the PL spectra of one of the samples. Real lines are experimental data, and the dotted line and dash dot line represent the Gaussian components of the signals. The PL spectra are excited at 5.77 eV after γ-irradiation at a dose of 100 kGy for samples.
Table 1. Center Position of Gaussian Bands Obtained for PL Spectra before and after Irradiation.
3.6 Damage performance measurement
The laser-induced damage thresholds have been tested for three groups of samples with different arsenic concentrations after γ-irradiation. A frequency tripled Nd: YAG laser was operated at 355 nm in our experiment. The pulse duration was 8 ns full width at half maximum (FWHM) and the beam profile was near Gaussian with a focused diameter of about 2.6 mm at the sample plane. The modulation of irradiation area is a factor of 2.7 and the laser repetition was 1 Hz. Before damage test, three groups of KDP samples are irradiated by γ-ray at a dose of 100 kGy. The results of bulk damage thresholds tested with R on 1 mode are shown in Fig. 6. Linear curve fit of the damage probability versus fluence extrapolates to onset of damage at 3.34 J /cm2, 4.85 J /cm2 and 5.63 J /cm2, respectively. The lowest arsenic impurity concentration had the highest damage threshold. In other words, the damage threshold of irradiated sample decreases with the increase of arsenic concentration, as shown in Fig. 6. Our experimental results agree with the previous results [11, 20], which shows that the irradiation resistance degradation is attributed to the formation of arsenic-containing free radicals under irradiation.
Fig. 6 R on 1 damage probability vs fluence. The solid line represents linear fitting of the data points in each curve. The error bars represent a typical uncertainty of fluence for each test.
4. Discussion
The irradiation effect produced various interesting changes in physical and chemical properties of crystalline materials [37–41]. For KDP crystals, the effect of γ-irradiation on optical absorption is significant. Compared with irradiation-induced impurity defects, irradiation-induced high concentrations of secondary electrons, the H0 atoms as well as [H2PO4]0 and [HPO4]- radicals [15] are unstable at room temperature. The impurity ions of incorporation of bivalent (Fe2+, Mn2+, Ba2+), trivalent (Fe3+, Cr3+) and pentavalent (As5+, Si5+) enter into the structure of KDP crystal and substitute for the ions K+ and P5+. Under γ-irradiation, these growth defects undergo transformations accompanied with change of impurity ion charge. For example, when a Si4+ ion substitutes for a P5+ ion in KDP, a vacancy is needed to compensate the excessive positive charge [16]. This is easily achieved by trapping a hole on an oxygen ion adjacent to the silicon impurity. The optical absorption band at 270 nm is due to Fe ions substituting for phosphorus ions [22]. The impurities which incorporated in the crystals such as Al, Fe, As, Cr, and Si can lead to significant absorption in KDP [20]. Similarly, the anion AsO43- which substitutes for the phosphate ion PO43- in the crystal lattice of KDP is easy to capture electrons and transforms into the paramagnetic center AsO44-, which results in the absorption band at 260 nm as shown in Fig. 1(a). The present results suggest that new damage precursors may be formed during exposure to gamma irradiation. The intensity of irradiation-induced absorption band increases with the increasing irradiation dose. In addition, the intensity of irradiation-induced absorption band at 260 nm increases with the increasing arsenic impurity concentration at the same irradiation dose as shown in Fig. 1(b). The test result of optical–thermal weak absorption also showed the same conclusion.
As discussed above, the absorption band at 260 nm has been assigned to the formation of AsO44- centers. For this kind of defects, simulation could also support our experimental result as can be seen in Fig. 3. Our experimental results show that the arsenic impurity appears to play the most important role in the formation of radiation induced defects, and thus affect damage performance of materials. The results presented in Fig. 6 clearly show that the reduction of the damage threshold is attributed to the increase of arsenic concentration. Figure 7 show that the arsenic impurity concentration has a considerable influence on the damage threshold and the absorption band at 260 nm. The absorption intensity at 260 nm increases with the increasing arsenic concentration, while the damage threshold decreases with the increasing arsenic concentration. Previously, R. A. Negres et al. have summarized that the change in the individual damage threshold of the precursors may be attributed to the following possibilities: (1) the electronic structure of the constituent atomic defects, (2) the size of the precursor and, (3) the density of the constituent defects within the precursor [9]. In this case, higher concentration of arsenic impurity ions may tend to form denser precursor defect clusters. Under γ-ray irradiation, the irradiation-induced defects at the damage precursor position may lead to size increase, and thus lower the damage threshold of the precursors [8, 9]. The model of absorbing nanoparticles in damage initiation predicts that the size of the precursors determines their individual damage threshold, i.e., smaller precursors initiate damage at a higher fluence [8, 42]. Therefore, it is can be explained that the influence on the damage performance of irradiated KDP crystal was due to interaction between irradiation induced defects and individual damage precursors.
Fig. 7 Correlations between damage threshold and absorption intensity at 260 nm and arsenic concentration. The red squares represent the absorption intensity at 260 nm. The blues square represent the damage threshold.
5. Conclusion
The effects of γ-irradiation on the optical absorption and 355 nm laser-induced bulk damage performances of KDP crystals containing arsenic impurity have been studied at room temperature. It is found that the irradiation-induced optical absorption at 260 nm depends on the irradiation dose and arsenic impurity concentration. The simulated results confirm that optical absorption at 260 nm is attributed to the formation of defect centers related to arsenic impurities. Optical thermal absorption also has a strong correlation with arsenic impurities. The results of damage test demonstrate that the arsenic impurity has important influence on the laser damage performance of γ-irradiation KDP crystal. Based on the data, the activation processes is a key factor responsible for the formation and transform of irradiation-induced defects during γ-ray interaction with KDP material.
Acknowledgments
This work was performed under the National Natural Science Foundation of China(61178018 and 61078075) and the Ph.D. Funding Support Program of Education Ministry of China (20110185110007).
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