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The structural stability of nonlinear optical material KTiOPO4 (KTP) under laser propagation remains a challenge for its device application for a long time. Here a nano-precipitation recrystallization phenomenon under 632.8 nm laser irradiation was found from KTP crystals via combined techniques of transmission electron microscopy (TEM), ex situ scanning electron microscopy (SEM) and X-ray diffraction (XRD). The morphologies of precipitates include spheres, triangles and tetragons, which were formed on the (100), (010) and (001) surfaces of KTP crystals, respectively. It was revealed that different facets of the KTP crystal exhibited significant discrepancy in the stability under laser irradiation due to different atomic stacking structures. Hence, it is believed that our research findings might have new implications and inspirations in constructing KTP devices with better performance.
© 2015 Optical Society of America
1. Introduction
Recently, KTiOPO4 (KTP) single crystals have attracted tremendous attention because of their nonlinear-optical and electro-optical properties and thereby promising applications for second harmonic generation (SHG) and optical parametric oscillation (OPO) devices [1,2 ], as well as electro-optic modulators of diode-pumped solid-state lasers [1–4 ]. The main advantages of KTP crystals which make it useful in the fields of frequency doubling (Nd:YAG laser) [3] and frequency conversion [4] include their high nonlinear-optical coefficients and optical damage threshold, wide acceptance angles, high thermal and mechanical stability, as well as low dielectric constants [5].
However, the main issues required to be urgently solved is to improve the structural damage during the process of laser transmitting through high-power solid-state laser device made up of KTP crystals with the enhancement of the laser output flux. For example, femto-second laser could easily change the refractive index and induce the formation of undesirable color centers in KTP medium, which puzzles the researchers of this fields [6–8 ]. Although many studies on the structural damage of nonlinear-optical materials are available from literature reports, most of them are not consistent with each other. Crystalline temperature was raised up while KTP worked as a SHG and the unexpected “dark track” would appear [9]. Besides, interference between the incident light field and the electric field of electron plasma could generate periodic nanostructures inside silica glass by a beam of femto-second Ti:sapphire laser [10]. In addition, successive laser irradiation would also cause catastrophic damage inside silicon windows. Moreover, periodic nano-ripple structures were observed on magnesium surface after laser irradiation due to the interference between surface plasmons and incident laser light [11]. Briefly, laser irradiation could damage different types of materials via fast melting, resolidification and weakening/debonding the crystalline networks [12–14 ]. Till now, neither the effective solutions nor clear mechanisms have been proposed for this laser-induced damaging process. One difficulty is the rapid heating of the KTP surface, which would cause undesired point defects, dislocations or even the phase transition inside KTP crystals. At the same time, the currently available characterization methods regarding to this issue mainly include SEM [15], XRD and HRXRD [16], which unfortunately can hardly record the quickly occurred damage process and also suffer a lot from the disturbance from measurement parameters. Hence, the accuracy of current analysis seems quite controversial.
Herein, an intriguing nano-precipitation recrystallization process modulated by exposed facets of KTP crystals was reported, which provides a novel entry point for understanding the damage mechanism of KTP crystals under laser irradiation. Ex situ electron microscopy is exploited to study the structural evolution process in this work. Typically, precipitates with spherical, triangular and tetragonal shapes were formed onside the exposed surfaces of (100), (010) and (001) facets, respectively. Further, the atomic stacking sequence along different crystal orientations was found to be responsible for this facet-selective damaging behavior.
2. Experimental methods and observation
The KTP crystal was prepared according to Ref [17]. Typically, the KTP crystal was firstly cut into small cubes along the crystal facet of (100), (001) and (010) with a mechanical cutting machine (USA, Allied-5-5000 model), polished mechanically with diamond paste and then cleaned with acetone under ultra-sonication for the subsequent laser irradiation process and SEM characterization (Hitachi S-4800F),as exhibited in Fig. 1 . The three facets were all irradiated for 1 h, 2 h and 3 h, during which ex situ SEM characterizations were performed to record the surface evolution. The laser irradiation process were carried out utilizing a laser confocal Raman micro-spectrometer (InVia Reflex, Renishaw, UK) equipped with an integral microscope (Leica DM LM, Germany). Specifically, a 632.8 nm He-Ne laser, laser power of 300 mW (100% × 300 mW), grating of 1800 1/mm and objective lens of 100 × / 0.85 were used during all experiments. Afterwards, a field-emission scanning electron microscope (FESEM, Hitachi, S-4800) operated at 1.0 kV and a X-ray diffractometer (D8 Advance, Bruker, Germany) working with the Cu-Kα radiation (1.5406 Ǻ) were used to perform the SEM and wide-angle XRD characterization.
Fig. 1 (a) A schematic illustration of the experiment and the photographs of (b) the laser irradiation process as well as (c) the investigated KTP crystals.
3. Surfaces precipitated particles after laser irradiation
Various crystal facets showed extremely different surface morphology after successive irradiation for different time. All the three surfaces precipitated particles after 1 h of irradiation, while the surfaces ascribed to (100), (010) and (001) facets precipitated sphere-like, triangular and tetragonal particles respectively as the irradiation time were extended, as showed in Fig. 2 . As far as we know, it is the first time to report nano-precipitation from KTP surface so far.
Fig. 2 SEM images of the surfaces of KTP crystals with different crystal facets exposed after continuously irradiated by a 632.8 nm laser from 1 h to 3 h. The simulated atomic arrangement of different crystal facets is displayed in the middle.
Before irradiation, the XRD characterization was firstly performed to confirm that the exposed crystallographic plane series was {100} (Fig. 3(a) ), where only the facet (400) and (600) could be detected. The (100) surface of KTP crystals was initiated by a 632.8 nm laser with the time varied from 1 h to 3 h, as shown in Fig. 3. It is obvious that after 1 h of laser irradiation, a few particles with rounded shapes began to precipitate (Fig. 3(a)), and the average size of these particles was about 100 nm. More spheres with larger average sizes of around 150 nm precipitated on the irradiated site after 2 h of irradiation (Fig. 3(b)). Next, high-density spheres gathered and kept separating from each other on the (100) surface after irradiated for 3 h. In addition, it is also worth mentioning that several tetragonal and triangular particles were mixed among these spherical particles, which are both marked by the blue circles in Figs. 3(b) and 3(c), respectively.
Fig. 3 The surface-morphology evolution of the KTP (100) plane after (a) 1 h, (b) 2 h and (c) 3 h of laser irradiation. The corresponding XRD pattern was displayed in (a), where the indexed plane (400) and (600) help to confirm {100} series. The detailed SEM images recorded under higher magnifications were indicated by green, blue and orange squares on the right side.
A similar phenomenon was observed on the (010) facet which were also confirmed by XRD analysis showed in Fig. 4(a) . Unlike the (100) facet mentioned above, the sizes of the precipitated triangular particles varied from 60 to 600 nm after 1 h of laser irradiation (Fig. 4(a)). Afterwards, some of these precipitates began to aggregate together and melt with the neighbor particles (Fig. 4(b)) with increased irradiation time to 2 h. Part of them cannot be distinguished any more. Further, Fig. 4(c) reveals a transition from small precipitation and recrystallization to several merging-damaged surfaces after irradiation for 3 h. The localized merging rather than the precipitation began to act as the dominant behavior to release the thermal energy induced by the irradiation. This was probably because that the (010) facet was more vulnerable under laser irradiation than that of the (100) facet, and as the time went on, there came too much heat accumulated on (010) surface that led to the merging process at last.
Fig. 4 The surface-morphology evolution of the KTP (010) plane after (a) 1 h, (b) 2 h and (c) 3 h of laser irradiation. The corresponding XRD pattern was displayed in (a), where the indexed plane (020), (060) and (080) help to confirm {010} series. The detailed SEM images recorded under higher magnifications were indicated by green, blue and orange squares on the right side.
As for the case of (001) surface whose XRD pattern is given in Fig. 5(a) , where the diffraction peaks are not as sharp as those of {100} and {010} plane series, but can also be concluded from the crystallographic structures of bulk KTP in Fig. 1. It is interesting that after continuous irradiation for 1 h, almost all of the precipitates showed regular tetragonal shapes (Fig. 5(a)) with the size ranging from 20 nm to 200 nm randomly. Moreover, when the irradiation time was prolonged to 2 h, the amount of precipitated tetragonal particles greatly multiplied while the sizes of these particles remained similar to the particles obtained at 1h. Eventually, after 3 h of irradiation, the average height of precipitates was heavily reduced due to their merging behavior into the bulk (001) surface, although most of them still maintained tetragonal shapes.
Fig. 5 The surface-morphology evolution of the KTP (001) plane after (a) 1 h, (b) 2 h and (c) 3 h of laser irradiation. The corresponding XRD pattern was displayed in (a), and the detailed SEM images recorded under higher magnifications were indicated by green, blue and orange squares on the right side.
4. Discussions
On one hand, it is suggested that there were two different processes during irradiation: the precipitating process and the melting process. All the three facets underwent the precipitation after irradiation for more than 1 h, while the precipitates began to merge in the case of (010) and (001) facets after irradiated for 3 h. It should be mentioned that the precipitating process was similar to the recrystallization of metal nanoparticles. The generated energy on the KTP crystal induced by the laser irradiation was distributed as Gaussian Function. Typically, as the time went on and the KTP surface was heated up, it precipitated new crystalline nuclei/grain on the surface. Meanwhile, for the case of the melting process, it could be interpreted that the precursor absorbed the laser energy and hence the heat accumulated to a high temperature, attributed to which the explosion occurred, yielding the plasma and melted cores under continuous laser irradiation.
On the other hand, the precipitates of different crystal planes demonstrated various morphologies after laser irradiation, indicating that different facets of KTP crystals might own discrepant abilities in withstanding continuous laser irradiation. Shown in Figs. 6(a), 6(d) and 6(g) are three high-resolution TEM images taken from three different zone axes of the KTP crystal, approximately representing the three facets (100), (010) and (001) respectively. The two sets of intersecting fringes seen in Figs. 6(a) and 6(b) correspond to 1.03 and 0.552 nm, respectively, which may be attributed to the (010) and (101) crystal planes. The 1.281-nm fringes and 0.548- nm seen in Figs. 6(d) and 6(e) correspond to the (100) and (011) crystal plane. The 1.058-nm and 0.6403-nm fringes seen in Figs. 6(g) and 6(h) correspond to the (010) and (200) crystal plane of the KTP. Combining this information with the atomic stacking was shown in Fig. 7 , from which we can see that there are different number of PO4 tetrahedrons and TiO8 octahedrons on the three planes (100) (010) and (001). It is clear that the varied periodic crystal structures especially the atomic arrangement of each crystal facets have made distinct contributions to the shape of the precipitates on the KTP surface.
Fig. 6 HRTEM and SAED petterns obtained close to the (a-c) (100), (d-f) (010) and (g-h) (001) facets of the KTP crystal.
5. Conclusion
Stablity of different planes of KTP crystals after laser irradiation were investigated by HRTEM, SAED and ex situ SEM. We recorded the morphology evolution of the precipitates on different surfaces of the KTP crystal under laser irradiation. Spherical, triangular and tetragonal precipitates were generated on the (100), (010) and (001) facets respectively owing to different atomic arrangement, demonstrating the varied stability of different crystal planes of KTP under laser irradiation. Therefore, it may be quite helpful in future studies on the laser damage of KTP crystals as novel laser frequency doubling devices.
Acknowledgments
This work was supported by the Ministry of Science and Technology of China (973 Project No. 2013CB932901), and the National Natural Foundation of China (Nos. U1330118, 11274066, 51172047, 51102050, 11374170). This project was sponsored by Shanghai Pujiang Program and “Shu Guang” project of Shanghai Municipal Education Commission and Shanghai Education Development Foundation (09SG01). Dr. Wang, Dr. Gao, Prof. Teng for the contribution of this work are equal.
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