Open Access
Add to BibSonomyAbstract
For rapid grown DKDP crystals, homogeneity should be paid more attention than traditional grown ones because their deuterated levels will be affected more significantly by the large supersaturation, which give harmful impact on optical property as well as the related nonlinear optical applications. In this paper, from the points of deuterated level and transmittance, the homogeneity of a point-seed rapid grown DKDP crystal with sizes of 65 × 65 × 113 mm3 is investigated. The results show that the maximum discrepancy of deuterium content is 5.4%, and the ultra-violet transmittances of the dislocation zone and its surrounding areas are ~30% inferior to other regions.
© 2014 Optical Society of America
1. Introduction
As an important high power nonlinear optical (NLO) material, DKDP crystal has been used for multiple frequency conversion processes such as Second Harmonic Generation (SHG), Third Harmonic Generation (THG), and Non-Critical Phase-Matched (NCPM) Fourth Harmonic Generation (FHG) of Nd:glass laser [1–8]. Presently two methods have been adopted for DKDP crystal growth: traditional growth method and point-seed rapid growth method. Although the growth period was reduced by 85% with the point-seed rapid growth method, the large supersaturation during the growth process will aggravate the inhomogeneity of the deuterium distribution, which would arouse the discrepancy of phase-matching condition in nonlinear optical applications. In this paper, the homogeneity of deuterium content of a point-seed rapid grown DKDP crystal (65 × 65 × 113 mm3 in size) was studied with Raman spectroscopy. Meanwhile, the transmission properties distribution all over the whole crystal was examined. We believe this work may contribute to the preparation of DKDP optical components with high quality and large aperture.
2. Crystal growth and sample preparation
The DKDP crystal was grown with point-seed rapid growth method and the growth period was 20 days. The solution was prepared with a deuterium concentration of 75%. It was filtered with a polysulfide filter with a pore diameter of 0.1 micrometer (µm). Crystallization was performed in the temperature range of 318 - 325 K and the growth rate of the DKDP crystal was 5.6 millimeter per day (mm/d). The crystal was rotated in the forward-stop-backward mode with a speed of 77 revolutions per minute (r/min) [9]. The size of the as-grown crystal was 65 × 65 × 113 mm3, no macroscopic defect was observed, as shown in Fig. 1.
The Crystal was cut into 10 samples: the 1 ~5# samples were selected in the (001) plane from the bottom part of the boule, the other 5 samples were chosen from the (110) diagonal plane of the boule. Their concrete locations were demonstrated in Fig. 2.All of the samples were processed into 10 × 10 × 9 mm3 rectangular blocks, with the 9 mm edge along [110] direction, i.e. θ = 90°, ϕ = 45° [7]. The other two edges are along [001] and [10] directions, respectively, as shown in the top-left inset of Fig. 2. As (110) surface is the type-I NCPM FHG direction, they are polished for the measurement of transmission property [10]. The Raman spectrum is measured in the X(ZZ)Y geometry, it indicates that both the incidence of excitation light and the detecting of Raman scattering are along Z direction, so (001) parallel surface are also polished to increase transmittance.
Fig. 2 Cutting schematic diagram of DKDP samples from the as-grown boule. Top-left inset: processing directions of individual DKDP sample.
3. Deuterium content analysis
In addition to the obvious reduction of the Raman peak intensity, deuterium doping also resulting in Raman peak position shift, and the linear relationship between deuteration level and Raman peak position shift has been reported [11]. By Raman spectroscopy measurement of the 10 samples, the deuterium distribution difference in the studied crystal space can be derived. Such parameter is particularly important for short wavelength NLO conversions, such as NCPM FHG of Nd:glass laser, because the deuterated inhomogeneity will cause the obvious changes of refractive indexes in ultra-violet (UV) region, which affect phase-matching temperature and lead to the output instability and efficiency decreasing.
The Raman spectra were recorded by a high resolution Raman spectrometer (LabRAM HR 800, HORIBA Jobin Yvon, Japan) with excitation wavelength of 632.81 nm. In order to achieve high accuracy, the spectral resolution is set to be 0.1 cm−1 with a scanning interval of 1 cm−1.
As Hydrogen- Deuterium exchange took place at the surface, so the focus point was selected to be inside the crystal with the transmission direction along Z axis. The Raman spectrum in the 100 to 2000 cm−1 range was recorded. Comparing with the KDP Raman spectrum, the 914 cm−1 sharp Raman peak of KDP crystal split into two weak peaks in present study, and the main peak moves toward low wave number direction as deuterated level raises [11, 12].
The results of 1~5# samples are shown in Fig. 3.From Fig. 1 it can be seen that these samples are selected from the (001) bottom plane. Figure 3(a) demonstrates that in samples 1-3#, the peak position of the Raman line of interest remains at 891.4 cm−1, which indicates that at the boundary of the crystal surface the deuterated level is the same. However, the shifted Raman peak of sample 4# (from central of the crystal, see Fig. 2) located at 889.4 cm−1, which is on the left side comparing with the peak position 891.4 cm−1 of other several samples, as shown in Fig. 3(b). As studied before [10] the Raman peak shift linearly as deuterated level changes, 1 cm−1 shift variation of PO4 vibration correspond to 2.68% deuterated-level difference [11]. So a discrepancy of 2 cm−1 means that the reducing of supersaturation in the process of crystal growth has led a 5.4% dropping of deuterated level from the point seed to the crystal boundary. According to the reference [13], it will take ~3 × 10−4 variation for the refractive index of ordinary light (no), and ~7 × 10−4 variation for the refractive index of extra-ordinary light (ne) in visible and UV regions. Consequently, for a 10 mm length crystal, the induced wave front variations are 6 ~14 λ if the wavelength λ is 0.5 μm. Besides, the PM angles will also take on obvious variations. Based on the results of Zhu et al [14], they are estimated to be 0.9°, 1.3° for the type-I and type-II SHG of 1053 nm, and 0.05°, 0.15° for the type-I and type-II THG of 1053 nm. For NCPM FHG of 1053 nm, our experiment has confirmed that 1% difference of deuterium content may cause 0.25 °C deviation of NCPM temperature, thus the 5.4% difference will lead a 1.35 °C deviation on NCPM temperature. The above analyzing indicates that 5.4% difference of deuterium content may affect the Non-linear Optics (NLO) applications of DKDP crystals negatively, and resulting in output energy decreasing as well as low conversion efficiency.
Selected samples from the prismatic face region (2#, 9#, 10#) and non-prismatic face region (4#, 6#, 7#, 8#) are measured to estimate the deuterated distribution difference in and out the prismatic region. As shown in Fig. 2, 4#, 6#, 7# and 8# sample were taken from the similar region (same in XY coordinates) but different height (Z coordinate). In Fig. 4(a), the Raman peak shift rightwards from 4# (889.4 cm−1) to 8# (890.6 cm−1), and the difference between the two samples is 1.2 cm−1 (equivalent to 3.2% deuterated level). Samples 2#, 9# and 10# are taken along the prismatic face. In Fig. 4(b), the Ramen peak of samples 2#, 9# and 10# are identical. These results agree well with the fact that the entire prismatic face forms with the same supersaturation while the non-prismatic are formed at different growth period with different supersaturation level.
4. Transmission spectrum
As introduced in the part 2, the 1-10# samples mentioned above are also polished at their (110) parallel surfaces for the measurement of transmittance, the light transmission direction is θ = 90°, ϕ = 45°. Along this direction the thickness of each sample is strictly controlled to be 9 mm to avoid measuring discrepancy caused by the transmission length. With a U-3500 spectrophotometer (HITACHI Inc., Japan) the transmission property was measured, as shown in Fig. 5.The relative characteristics are also summarized in Table 1.Since the change of the refractive index among different samples is at a level of 10−4, and crystal processing conditions are identical, the difference of transmittance can be attributed to the intrinsic absorption. The inset of Fig. 5 indicates that at the FHG wavelength (263 nm), the transmittances T of samples 6~10# are 75% above, which are much higher than those of other five samples. Samples 1~5# are cut from the bottom of the crystal boule (Fig. 1) which is close to the support shelf. Previously, many literatures have pointed out that impurity ions incorporated in the material and intrinsic defects form lattice imperfections will lead to the reduction of UV transmission [15–18]. For the present situation, the main reason inducing the decreasing of UV transmittance may be the crystal’s downward growth is inhibited by the support shelf, and the intense expansibility forms a rich dislocation area at this region [19]. On the other hand, since organic impurity ions mainly have an impact on transmittance in fundamental wavelength region [20, 21], when they gather near the cylinder walls under the high speed rotation of the solution, the decreasing of visible and infrared transmittance for 8~10# samples are induced.
Table 1. The relative transmission characteristics of different samples.
5. Conclusion
For rapid grown crystals, homogeneity is a prominent and vital property that will affect various applications. In this paper, we researched the homogeneity of a 65 × 65 × 113 mm3 DKDP crystal grown by point-seed rapid grown method, from the points of deuterium content and transmittance. From Raman spectrum measurement we estimate that the maximum discrepancy of deuterium content from the center to the boundary is 5.4%. It corresponds to the 0.9°, 0.15° discrepancy of PM angle for the type-I SHG and type-II THG of Nd:glass laser, respectively. For NCPM FHG application, it will lead a 1.35 °C variation on NCPM temperature. The transmission spectra of different samples indicate for SHG and THG applications whose related wavelengths are 1064, 532, 355 nm, the uncoated optical component selected from any part of the rapid grown boule can satisfy the condition T > 80%, and the one cut from the central part might possess better transmittance (T > 90%). Comparing with the visible and near infrared regions, the transmittance at deep UV region has taken on more remarkable discrepancy. For FHG application which generate 263 nm laser, the high quality optical component should be selected from the upper or middle part of the crystal boule, a fail to do this might bring a 30% decreasing of transmittance at 263 nm, which produces fatal effect to this NLO process. To improve the crystal utilization for NCPM FHG application, directional growth with (90°, 45°) orientated point-seed may be a solution because it has no limitation of the support shelf at [001] direction, which is helpful to obtain homogeneous component with large aperture.
Now, a DKDP crystal with similar deuterium content and sizes is being grown by traditional growth method for the same measurements, through the comparison a more integrated evaluation about the influence of growth technique on homogeneity is hopeful to be given.
Acknowledgments
This work is supported by the Natural Science Foundation of China (51323002), Program for New Century Excellent Talents in University (NCET-10-0552), Natural Science Foundation for Distinguished Young Scholar of Shandong Province (2012JQ18), and Independent Innovation Foundation of Shandong University (2012JC016, 2012TS215).
References and links
1. Y. S. Liu, W. B. Jones, and J. P. Chernoch, “High-efficiency high-power coherent uv generation at 266 nm in 90° phase-matched deuterated KDP,” Appl. Phys. Lett. 29(1), 32–34 (1976). [CrossRef]
2. D. A. V. Kliner, F. Di Teodoro, J. P. Koplow, S. W. Moore, and A. V. Smith, “Efficient second, third, fourth and fifth harmonic generation of a Yb-doped fiber amplifier,” Opt. Commun. 210(3–6), 393–398 (2002). [CrossRef]
3. J. Reintjes and R. C. Eckardt, “Efficient harmonic generation from 532 to 266 nm in ADP and KD*P,” Appl. Phys. Lett. 30(2), 91–93 (1977). [CrossRef]
4. D. Bruneau, A. M. Tournade, and E. Fabre, “Fourth harmonic generation of a large-aperture Nd:glass laser,” Appl. Opt. 24(22), 3740–3745 (1985). [CrossRef] [PubMed]
5. Cleveland Crystals Catalog and their references, http://www.clevelandcrystals.com/KDP.htm.
6. W. F. Hagen and P. C. Magnate, “Efficient Second-Harmonic Generation with Diffraction-Limited and High-Spectral-Radiance Nd-Glass Lasers,” J. Appl. Phys. 40(1), 219–224 (1969). [CrossRef]
7. S. T. Yang, M. A. Henesian, T. L. Weiland, J. L. Vickers, R. L. Luthi, J. P. Bielecki, and P. J. Wegner, “Noncritically phase-matched fourth harmonic generation of Nd:glass lasers in partially deuterated KDP crystals,” Opt. Lett. 36(10), 1824–1826 (2011). [CrossRef] [PubMed]
8. S. G. Demos, R. N. Raman, S. T. Yang, R. A. Negres, K. I. Schaffers, and M. A. Henesian, “Estimation of the transverse stimulated Raman scattering gain coefficient in KDP and DKDP at 2ω, 3ω, 4ω,” Proc. SPIE8190(81900S), 1–9 (2011).
9. S. Ji, S. Zhang, M. Xu, B. Liu, L. Zhu, L. Zhang, X. Xu, Z. Wang, and X. Sun, “Non-critical phase-matching conditions for fourth harmonic generation of DKDP crystal,” Opt. Mater. Express 2(6), 735–739 (2012). [CrossRef]
10. S. Ji, F. Wang, L. Zhu, X. Xu, Z. Wang, and X. Sun, “Non-critical phase-matching fourth harmonic generation of a 1053-nm laser in an ADP crystal,” Sci Rep 3(1605), 1605 (2013). [PubMed]
11. T. Huser, C. W. Hollars, W. J. Siekhaus, J. J. De Yoreo, T. I. Suratwala, and T. A. Land, “Characterization of proton exchange layer profiles in KD2PO4 crystals by micro-Raman spectroscopy,” Appl. Spectrosc. 58(3), 349–351 (2004). [CrossRef] [PubMed]
12. C. E. Barker, R. A. Sacks, B. M. V. Wonterghem, J. A. Caird, J. R. Murray, J. H. Campbell, K. Kyle, R. E. Ehrlich, and N. D. Nielsen, “Transverse stimulated Raman scattering in KDP,” Proc. SPIE 2633, 501–505 (1995). [CrossRef]
13. M. S. Webb, D. Eimerl, and S. P. Velsko, “Wavelength insensitive phase-matched second-harmonic generation in partially deuterated KDP,” J. Opt. Soc. Am. B 9(7), 1118–1127 (1992). [CrossRef]
14. L. Zhu, X. Zhang, M. Xu, B. Liu, S. Ji, L. Zhang, H. Zhou, F. Liu, Z. Wang, and X. Sun, “Refractive indices in the whole transmission range of partially deuterated KDP crystals,” AIP Advances 3(112114), 1–8 (2013).
15. A. A. Chernov, “Morphology and kinetics of crystal growth from aqueous solution,” in Morphology and Growth Unit of Crystal, I. Sunagawa, ed. (Aoba, 1989).
16. H. Klapper, Yu. M. Fishman, and V. G. Lutsau, “Elastic energy and line directions of grown-in dislocations in KDP crystals,” Phys. Status Solidi A 21(1), 115–121 (1974). [CrossRef]
17. N. Zaitseva, L. Carman, I. Smolsky, R. Torres, and M. Yan, “The effect of impurities and supersaturation on the rapid growth of KDP crystals,” J. Cryst. Growth 204(4), 512–524 (1999). [CrossRef]
18. N. Zaitseva and L. Carman, “Rapid growth of KDP-type crystals,” Pro. Crystal Growth and Charact. 43(1), 1–118 (2001). [CrossRef]
19. N. Zaitseva, J. Atherton, R. Rozsa, L. Carman, I. Smolsky, M. Runkel, R. Ryon, and L. James, “Design and benefits of continuous filtration in rapid growth of large KDP and DKDP crystals,” J. Cryst. Growth 197(4), 911–920 (1999). [CrossRef]
20. N. P. Rajesh, K. Meera, K. Srinivasan, P. S. Raghavan, and P. Ramasamy, “Effect of EDTA on the metastable zone width of ADP,” J. Cryst. Growth 213(3–4), 389–394 (2000). [CrossRef]
21. J. Podder, “The study of impurities effect on the growth and nucleation kinetics of potassium dihydrogen phosphate,” J. Cryst. Growth 237–239, 70–75 (2002). [CrossRef]
