DOI: 10.1021/cg901086e
Growth of High-Usage Pure and Nd3þ-Doped La2CaB10O19 Crystals for Optical Applications
2010, Vol. 10 1574–1577
Jianxiu Zhang,† Yang Wu,†,‡ Guochun Zhang,† Yanlei Zu,†,‡ Peizhen Fu,† and Yicheng Wu*,† †
Beijing Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China, and ‡Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
Received September 7, 2009; Revised Manuscript Received December 23, 2009
ABSTRACT: A series of pure and Nd3þ-doped La2CaB10O19 (LCB) crystals have been grown along different crystallographic directions, [001], [010], [100], [101], ∼[(1,-1,0], and ∼[(1,1,0], respectively, by the top-seeded solution growth (TSSG) method. The effect of seed orientations on the morphologies and the key limiting factors for obtaining high-usage LCB crystals were carefully analyzed. Using the improved method, we grew successfully the high-usage pure and Nd3þ-doped LCB crystals with high quality and sizes up to 55 35 20 mm3, which were larger, and their morphologies were much more suitable for optical applications than those of the available literature.
1. Introduction La2CaB10O19 (LCB) is a new nonlinear optical (NLO) crystal, which exhibits a second-harmonic generation (SHG) powder efficiency 2 times larger than that of KDP.1 The crystal growth and phase-matched (PM) SHG properties of LCB were studied in 2002,2 and subsequently, a second harmonic conversion efficiency of 25% was obtained at a fundamental peak-power intensity of 6 GW/cm2 for the LCB crystal.3 In 2006, UV-induced the NLO effects in LCB were investigated by Kityk4 and Reshak et al.5 In 2007, the electronic and optical properties were calculated by using the FP-LAPW method.6 Recently, Brenier et al. found that Nd3þ-doped LCB might be a good candidate for selffrequency doubling (SFD) due to its good NLO properties, wide transparency range, and chemical stability, and he thereafter performed a number of spectra calculations and laser experiments in LCB. These include a study on the evidence of SFD from two inequivalent Nd3þ centers in LCB,7 roomtemperature spectroscopy and SFD from the 4F3/2 f 4I13/2 laser channel,8 and the generation of simultaneous two-frequency lasing at 1051 and 1068 nm.9 Despite the encouraging results, high-usage LCB crystals grown with large sizes and good optical quality are still a considerable issue in its further assessment and optical applications. But unfortunately the LCB crystals with desirable morphologies are so far still difficult to obtain due to the strong anisotropic and layer growth habits, the very different growth ratio along the þb and -b directions, the high viscosity of the solution, and so on, although much effort has been done to improve the crystal growth technologies in the past few years in our group.10-12 In this paper, we reported on the growth of high-usage LCB crystals with large sizes and good optical quality by improved techniques. The morphologies of these crystals are much more suitable for optical applications than those obtained earlier. The effect of seed orientations on the morphologies and the key limiting factors for obtaining high-usage LCB crystals *Corresponding author. Fax: þ86-10-8524-3709. E-mail: zjx@mail. ipc.ac.cn. pubs.acs.org/crystal
Published on Web 02/22/2010
were carefully analyzed, which has guiding significance for subsequent studies on other borate crystals, since the growth method and the influencing factors of them are similar. 2. Experimental Section LCB crystals have been grown along different directions by the top-seeded solution growth (TSSG) method. All the experiments were done in a vertical cylindrical electric furnace having nickelchrome alloy wire heating and equipped with a Pt-Rh/Pt thermocouple based Al-708P controller as described in earlier literature.12 The solutions were prepared in a cylindrical Pt crucible with a diameter of 80 mm and a height of 100 mm by melting the appropriate quantities of La2O3, CaCO3, Li2CO3, and H3BO3. The homogenization of the solutions was achieved by maintaining the solution at about 30 °C above the expected saturation temperature for 1-2 days. The saturation temperature was determined by a tentative seed method. A seed with [001] orientations was placed into the melt at a temperature of 15 °C above the saturation temperature for about half an hour to dissolve the rough surfaces. The growing crystals were rotated at 4.5 rpm. The rate of cooling was 1 °C/day during the first ten days, and then the solution temperature was decreased at a rate of 2-4 °C/day in subsequent times. In the same approach, a series of pure and Nd3þ-doped (5%, 8%, 10%, and 17% in the melt) LCB crystals with seed orientations along other directions, [010], [100], [101], ∼[(1,-1,0], and ∼[(1,1,0], were also obtained. Figure 1(a-f) shows the growth morphologies of LCB crystals in Pt crucibles with various seed orientations. Figure 1(1, 3-5, 7-9, 12, 13) shows the as-grown pure and (2) 5%, (6, 10) 8%, and (11) 10% Nd3þ-doped LCB crystals, and furthermore, their dimensions, Nd concentration in the melt, weight, optical quality, and utilization ratio were listed in Table 1. The Nd concentrations in LCB crystals were determined by the inductively coupled plasma atomic emission spectroscopy (ICPAES) method. The segregation coefficients of Nd ions in LCB were calculated. The unit-cell parameters of the pure and Nd3þ-doped LCB were derived by powder X-ray measurements. Figure 2a shows the unit-cell parameters a, b, and c and the cell volume V, and Figure 2b shows the Nd concentration in crystals and segregation coefficients as a function of the Nd content x = 0, 5, 8, 10, and 17% in the melt. From Figure 2a we can see that the unit-cell parameters for the pure LCB are in good agreement with previously reported data,1 and all three a, b, and c parameters decrease with increasing Nd concentration. The a parameter lies between 11.043 and 10.985 A˚, b lies between 6.563 and 6.5548 A˚, and c varies from 9.129 to 9.083 A˚. r 2010 American Chemical Society
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The overall variation of the unit-cell volume is approximately 0.8%. From Figure 2b, we learned that the measured Nd concentrations in LCB crystals are 2.2, 4.0, 5.1, and 9.2 atom %, corresponding to 5, 8, 10, and 17% in melts. So the calculated segregation coefficients are 0.45, 0.50, 0.51, and 0.54, respectively.
3. Results and Discussions
Figure 1. (a-f) growth morphologies of LCB crystals with various seed orientations in Pt crucibles; (1, 3-5, 7-9, 12, 13) the as-grown pure and (2) 5%, (6, 10) 8%, and (11) 10% Nd3þ-doped LCB crystals.
3.1. Utilization Ratio of LCB Crystals. The utilization ratio of LCB crystals is mainly determined by the phase matching (PM) directions, growth morphology, and optical quality. For monoclinic LCB crystals, the three refractive indices along the optical indicatrix axis (Ng, Nm, Np) are different and they do not coincide with the crystallographic axes (a, b, c). Ng and Nm are located in the ac plane, while Np is parallel to b and the principal axis with maximum refractive index, Ng, was located at about 44.47° from the -c axis, and therefore the value of the Nm orientations is 43° with respect to the a crystallographic axis11. The theoretical calculations suggested that the PM-I(Type I) angles for 800-1064 nm fundamental frequencies are corresponding to 46.69-36.59°, and the PM-II(Type II) angle for the 1064 nm fundamental frequency is 55.38°. Figure 3 shows the orientations relationship of the PM-I, PM-II, and optical indicatrix axes (Ng, Nm, Np), the crystallographic axes (a, b, c), and the real LCB crystals. From Figure 3 we can see that the PM directions are almost parallel to the -c axis; that is, for obtaining specimens long enough along the PM directions, crystals along the c axis must be as thick as possible. In addition, we also learned that the larger the area of the (001) faces is, the higher the utilization ratio would be. Namely, we expect that the (001) faces are well developed and that the (111) and (202) faces, which are obliquely intersected by (001) faces disappear during the crystal growth. 3.2. Morphologies of LCB. X-ray power diffraction showed that the crystalline structure of LCB is not changed
Table 1. Sizes, Nd Concentration in Melts, Weight, Optical Quality, and Utilization Ratio of LCB Crystals Grown along Different Directions seed orientations [001] [010] [100] [101] ∼[(1,-1,0]
∼[(1,1,0]
[(1,-1,0]b [(1,-1,0]a [(1,-1,0]a≈10 þb [(1,1,0]g10 þb [(1,1,0]3-10
crystals 1 2 3 4 5 6 7 10 12 8 9 11 13
Nd (atom %) in melts 0 5 0 0 0 8 0 8 0 0 0 10 0
sizes (mm) 30 28 4 22 20 3 47 22 5 38 25 5 45 40 4 30 16 8 32 17 8 55 35 20 50 30 25 40 25 9 38 38 8 55 30 15 45 28 18
weight (g) 16 13 15 14 12 25 28 72 60 24 29 43 39
optical quality poor good poor poor poor poor good excellent good poor good excellent excellent
utilization ratio low low low low low low low high high low low high high
Figure 2. (a) Unit-cell parameters a, b, and c and cell volume V, (b) Nd concentration in crystals and segregation coefficients as a function of the Nd content x = 0, 5, 8, 10, and 17% in the melt.
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by Nd3þ doping and still belongs to the space group C2. So the effect on the growth morphologies of the Nd ions in LCB could be neglected. According to the crystal structure of LCB, we can find the main strong periodic bond chains (PBCs), [110], [110], [010], and [001], which determine the growth morphologies of LCB. Table 2 listed the (hkl) faces on the grown crystals, their interplanar dhkl, the PBCs parallel to the (hkl) faces, and the observed faces of crystals grown along different directions. From Table 2 we can see that the (001) faces are parallel to three strong PBCs, [110], [1,-1,0], and [010], which are so-called F (flat) faces. However, the other faces are only parallel to one strong PBC. For example, (1,-1,L) (L: 0, 1, or -1) faces are parallel to the strong PBC [110], (11L) (L: 0, 1, or -1) faces are parallel to the strong PBC [1,-1,0], and (20L) (L: 0, 1, or -1) faces are parallel to the strong PBC [010], which are referred to as S faces. Generally, F faces of all crystals are well developed, since there are no strong PBCs running across these faces, and therefore, they have a slow growth rate. However, S faces only appeared in certain asgrown crystals because only one strong PBC parallels them and the growth ratios of these S faces are easily disturbed by dynamics factors of crystal growth. Figure 4a shows the predicted hexagonal shape of LCB crystals. We can see that the (001) faces are well developed and the (1,-1,L), (11L) and (20L) (L: 0, 1, or -1) faces also appeared. The morphology of the as-grown crystal shown in Figure 1(1) is in good agreement with the hexagonal shape. Above we mentioned that S faces, (1,-1,L), (11L), and (20L) (L: 0, 1, or -1), are only parallel to one strong PBC, and therefore their growth ratios are easily disturbed by dynamics factors of crystal growth. In other words, the
development of S faces of LCB crystals varies greatly with little difference in the dynamics factors, such as flow pattern of the solution, mass energy transfer, or supersaturations σ generated by the instability of the temperature controllers. Figure 4b shows the other predicted rhombic shape of LCB crystals. We can see that the S faces, (201) and (-201), disappeared although they were parallel to the strong PBC [010]. That is because the components of the strong PBCs, [110] and [1,-1,0], are running across these faces, and this made them grow quickly. The morphologies of the as-grown crystals shown in Figure 1(2, 8-13) are in agreement with the rhombic shape. Our experiments showed that only crystals with seed orientations along the c axis could exhibit the hexagonal shape, and crystals grown along other directions exhibit the rhombic shape or it is variant. Following the experiments, we learned that high usage LCB crystals with large sizes and good optical quality are difficult to obtain (see Figure 1(1-9) and Table 1). The main reasons are as follows. (1) Layer growth along the c axis: the growth ratio of (001) slices is the slowest, and this made crystals flat and a low utilization ratio. From Table 1 we can see that the thicknesses along the c axis of crystals shown in Figure 1(1-5) are less than 6 mm. (2) Polarized growth along the b axis: the þb direction grows more quickly than the -b direction, maybe because the þb direction always ends with edges or faces whereas the -b direction ends with the vertices of the BO4 tetrahedrons. This is clearly not good because weight unbalance of the seed side could make the growing crystal fall into the solution, or the þb axis could scrape the inside wall of the Pt crucible when crystals rotate (Figure 1(d, e, 5, 6)). (3) Some unexpected faces are developed: as above-mentioned, we hope the faces obliquely cross the (001) faces, such as (111), (1,1,-1), and (202) always disappear, but in most cases, as shown in Figure 1(1-8), these unexpected faces are more or less developed, which reduces the area of the (001) faces and therefore lowers the utilization ratio.
Figure 3. Orientational relationship of the PM-I, PM-II, and optical indicatrix axes (X, Y, Z), the crystallographic axes (a, b, c), and the real LCB crystals.
Figure 4. (a) Hexagonal and (b) rhombic shapes of LCB crystals.
Table 2. (hkl) Faces on the Grown Crystals, Their Interplanar dhkl, the PBCs Parallel to the (hkl) Faces, and the Observed Faces of Crystals Grown along Different Directions observed faces on crystals in Figure 1 (hkl)
dhkl (A˚)
(001) (110) (110) (200) (111) (11 1) (201) (111) (111) (201)
9.126 5.641 5.641 5.520 4.827 4.827 4.778 4.771 4.771 4.670
[uvw] PBCs [110]
[110] [110]
[010]
1
2
3
4
5
6
7
8
9
10
11
12
13
Y
Y
Y Y Y
Y Y Y
Y Y Y
Y Y Y
Y Y Y Y
Y Y Y
Y Y Y Y
Y Y Y
Y Y Y Y
Y Y Y
Y Y Y Y
Y Y
Y Y
Y Y
Y Y
Y Y
Y Y
Y
Y Y
Y Y
Y Y Y Y Y
[110] [010] [110] [110] [010] [110] [110] [010]
Y Y
Y Y Y Y Y
Y Y
Y Y Y Y Y Y
Y Y
Y Y Y Y Y Y
Y Y
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3.3. Effect of Seed Orientations on the Morphology and Utilization Ratio. Following our experiments, we found that the morphologies of LCB crystals could be greatly changed by selecting seeds with different orientations. From Figure 1 (a, 1-2) and Table 1, we can see that, for the [001] seed, the thicknesses along the c axis of both crystals are too flat and less than 4 mm, since on the one hand, layer growth mechanisms limit the c thickness, and on the other hand, the growth ratio along the c directions is halved by seed. Furthermore, the (001) faces are partly destroyed by the seed located in the middle of them. In a word, it is impossible to grow high usage crystals with [001] seeds. For [010], [100], and [101] seeds shown in Figure 1(b-d, 3-5), the thicknesses along c directions are a little bit improved, but the (111) faces are simultaneously formed and moreover the optical quality is worsened by the defect spreading. For [(1,-1,0] or [(1,-1,0]b (deflection toward b axis) seeds shown in Figure 1(eL, 6), serious weight unbalance of the seed side could make the growing crystal fall into the solution and therefore limit the growing of crystals with large sizes. However, we found that [(1,-1,0]a (deflection toward the a axis) seeds shown in Figure 1(eR, 7) are possible to solve the problems. By selecting this kind of seeds, we obtained crystals with sizes 32 17 8 mm3 and good optical quality shown in Figure 1(7). For the [(1,10]þb g10 (deflection greater than 10° toward the þb axis) seeds shown in Figure 1(fL, 8, 9), the recovery regions of seeds in crystals are somewhat large and greatly worsen the optical quality. But when the deflection of seeds toward the þb axis is about 3-10°, as shown in Figure 1(fR, 11, 13), labeled as [(1,1,0]þb 3-10, the situations are greatly improved. From the above discussions, we learned that, for [(1,-1,0], [(1,-1,0]b, and [(1,1,0]þb g10 seeds corresponding to the as-grown crystals shown in Figure 1(6-9), although the thicknesses along the c directions have been comparatively improved up to 9 mm compared to [001], [010], [100], and [101] seeds, they still cannot meet the demand of actual applications of LCB due to the small sizes or low utilization ratio etc. However, we found that it is possible to obtain desirable crystals by selecting seed orientations along the ∼[(1,-1,0]a and [(1,1,0]þb 3-10 directions. Under this guidance, a series of high-usage pure and Nd-doped LCB crystals with large sizes and good optical quality were obtained successfully. For example, Figure 1(10) and (12) show the Nd-doped (8%) and pure LCB crystals with seed orientations along
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[(1,-1,0]a≈10 (deflection about 10° toward a axis), and the thicknesses along c directions are 20 and 25 mm, respectively. Using seeds with orientations along [(1,1,0]þb 3-10, we also grew high-usage Nd3þ-doped(10%) and pure LCB as shown in Figure 1(11) and (13), and their thicknesses along the c directions are 15 and 18 mm, respectively. 4. Conclusions In summary, we have studied the key limiting factors for obtaining high-usage LCB crystals with large sizes and good optical quality. We find that desirable crystals can be obtained by selecting seeds with orientations along the [110]a≈10 and [110]þb 3-10 directions. A series of high-usage LCB crystals have been successfully grown by the TSSG method, which are larger, and their morphologies are more suitable for further optical applications than those ever grown before. Acknowledgment. This work was supported by the National Natural Science Foundation of China under Grant No. 50590402.
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