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Large single crystals of a nonlinear optical material LaxYyScz(BO3)4 have been obtained by High Temperature Top-Seeded Solution Growth (HTTSSG). This material is very interesting due to its easy growth procedure, its non hygroscopic properties, a suitable hardness to be reliably cut and polished, a large transparency wavelength range and also good nonlinear properties with potential for UV generation. However, the crystals show inhomogeneities and growth imperfections which can be observed by light scattering and by Schlieren photography method. Chemical etching is used to reveal defects. Observed striations can be related to the growth of rhombohedral facets.
©2011 Optical Society of America
1. Introduction
Among nonlinear crystals, borates have been extensively studied for about 30 years. The most prominent members of the borate family are BBO, LBO, LTB, KABO as well as CLBO [1–7]. In general, these crystals have many favorable properties but present some important disadvantages. Among them, hygroscopic behavior, homogeneity issues [1], limited UV transparency, inherent limits of nonlinear optical (NLO) properties, appear to various degrees for one crystal or another and compromises have to be made. In recent years, new borate crystals have aroused a large interest in the NLO community. Among them, LayYxScz(BO3)4 (LYSB) belonging to R32 space group symmetry [8–10], presenting interesting NLO properties, has appropriate birefringence, large nonlinear coefficients, favorable UV transparency. It is relatively easy to grow and to polish. Moreover, it is non hygroscopic. Between crossed polarizer, LYSB crystals appear homogeneous. However, diffraction phenomena appear as transmitted beams are scattered along certain preferential directions. Additional UV absorption problems lead us to analyze the crystal quality in more details. Several samples from LYSB crystals cut along the X, Y, Z directions, have been studied in this paper. Schlieren photography [11] and chemical etching [12] are suitable processes to evaluate inhomogeneities in crystals. Schlieren photography followed by chemical etching allowed to correlate observed defects and rhombohedral facetting observed in as-grown crystals.
2. Crystal growth and experimental procedures
2.1. Growth
A large single crystal of LYSB with a weight of 93.4 g has been grown at Cristal Laser and LCMCP. The crystal was grown from flux along the [001] direction, without any pulling Fig. 1 . The flux composition was similar to the one described in Ref [9]. To grow the non-centrosymetric phase the composition of the melt was the following:LYSB + 2.5(Li6B4O9) + 0.145 Sc2O3 + 0,315 Y2O3 with LYSB = La0.72Y0.57Sc2.71 (BO3)4.The synthesis of LYSB powder has been previously made by solid state reaction of starting materials at stoechhiometric composition. Polishing of the oriented samples has been performed at LMOPS. The samples studied have parallelepiped shape, they have been cut along the main axes with dimensions of 7 x 5 x 6 mm3 along X, Y, Z directions respectively. Cell parameters have been measured by X-ray diffraction on powder and found to be equal to A = 9.8185 Å and C = 7.9893Å (hexagonal cell). The cell volume is 667.006 Å3. Our values are very close to those reported in the literature, 9.774Å and 7.946Å [8,9] or 9.805Å and 7.980Å [10], respectively. These values are slightly different probably due to a shift in composition of LYSB. Indeed by ICP AES (Inductively Coupled Plasma Atomic Emission Spectroscopy) seven as-grown crystals have been analyzed; the calculated formula for LYSB is averaged to La0.826Y0.334Sc2.840(BO3)4. From A and C we have calculated α the angle between two edges of the rhombohedral facets Fig. 2(a) .
The orthogonal axes X, Y and Z are respectively oriented along the hexagonal vector B, along the projection of the rhombohedral vector a onto the AB plane, and along the hexagonal vector C. Figures 2(b) and 2(c), show a model of the rhombohedral cell viewed along various directions and indicate the angles theoretically calculated with respect to the Z axis as observed in a plane perpendicular to viewing direction. Figure 2(d) shows the angle defined by the intersection of the XY plan and rhombohedral facets.
Fig. 1 As grown crystal from LCMCP 8.3g in crucible (diameter Ф = 40mm) and from Cristal Laser 93 g. in Ф = 80 mm crucible.
Fig. 2 (a) Representation of rhombohedral and hexagonal structures, (b) rhombohedra cell viewed along X direction, Z axis is vertical, the angles measure the intersection of the YZ plane and edges of rhombohedral facets with respect to the Z axis ; (c) rhombohedral cell viewed along Y direction, Z axis is vertical ; dotted and full lines show the intersection of rhombohedral facets with the XZ plane ; (d) rhombohedral cell viewed along Z direction, the angle is defined by the intersection of the XY plane and the rhombohedral facets.
2.2. Schlieren photography
Schlieren photography is used in order to investigate the optical quality of LYSB samples. In fact, this technique allows us to record local changes in the refractive index of a transparent medium. The result is a 2D picture displaying the inhomogeneities of the studied material. The experimental setup for the Schlieren measurement is shown in Fig. 3 . A white light source collimated to a large beam diameter passes through the sample and a lens projects an image of the crystal onto a screen.
2.3. Chemical etching process
For chemical etching process the samples are immersed into H3PO4 bath (>85%) heated up to 180°C during 1 hour [12,13]. Then the samples are rinsed in distilled hot water and subsequently cooled down to room temperature and finally rinsed with ethanol. The process is repeated 5 times in order to observe the evolution of chemical etching with time. Between each step, microscopy pictures are taken in order to observe the general evolution of each opposite X, Y and Z faces. An optical microscope Olympus BH-2, equipped with a camera, is used to investigate the etched surfaces in a reflected light mode with magnification from x10 to x500.
3. Results and discussion
3.1 Schlieren photography of polished (unetched)samples
Initially the LYSB samples from bigger Cristal Laser bawl (shown in Fig. 1) are observed between crossed polarizer with a binocular microscope; a 4 time magnification does not reveal defects in transmitted light Fig. 4 , and at the same time we have observed surfaces with a phase contrast microscope to control polishing quality. We did not observe defect in reflected light. Nevertheless Schlieren photography [9] brings out striation lines on X cut and Z cut faces. On both faces two sets of lines, are observed, at 65° and 44° with Z axis on X cut sample and + 30° and −30° with X axis on Z cut sample. Angles between different striation lines and main crystallographic directions have been reported on pictures Fig. 5 . The well contrasted parallel lines on X cut surface show a main pseudo periodic step of about 10μm. In Y cut face no stripe can be observed on our 3 samples.
Fig. 4 Crystal between crossed polarizers. (a): PA axis parallel to XZ axis; (b): axis slightly tilted. Sample size is 7x6mm
From cell parameters, in the plane perpendicular to X axis, the angle between one edge of rhombohedral facet and Z direction has been calculated, this angle is equal to 64.86°. In addition, angle between rhombohedral faces, previously described, and the Z axis is equal to 46.81°. These values are very close of these we observed experimentally. Axis and angles are reported on Schlieren picture Fig. 5(a). In the plane perpendicular to Z, the edges of rhombhoedral faces and Z plane, are theoretically at + 30° or −30° from the X axis, in accordance with the observed lines in Schlieren photography Fig. 5(b). Consequently the striations are well connected to the rhombohedra growing facets.Schlieren pictures of the Y cut face are not presented here because no striation is revealed.
3.2 Observation after chemical etching
Chemical etchings are carried out and permit to observe evolution with time of etched facets but also to visualize etch-pits organization and morphology. After each chemical etching step, microscopy pictures are taken with different magnification allow us to study the evolution of chemical etching with time of X cut, Y cut and Z cut faces. Opposite sides are qualified by X- and X + , for the X direction, – and + are not connected with any polarization state of the surface.
Opposite sides perpendicular to X direction reveal immediately the same striations than we can observe by Schlieren photography. These surfaces were etched for 1h and no significant change was observed in further etching steps Fig. 6 . In another way a simple piezo-electric test to measure the polarity of X direction was not concluding.
Fig. 6 Evolution of X cut face with time, X + facet: (a) chemical etching at 1h, and (b) at 5h. The whole surface is reconstructed with 9 microscope pictures.
Facets perpendicular to the Y direction are not polar faces. This type of face has the characteristic to present identical chemical etching rate on the opposite sides. Nevertheless, Y + face Figs. 7(a) -7(e) seem to be slightly more etched than Y- face Figs. 7(f)-7(j).
Fig. 7 Evolution with time of opposite sides for Y cut faces (Y + and Y- facets). First series of images corresponds to Y + facet: (a) chemical etching at 1h, (b) at 2h, (c) at 3h, (d) at 4h and (e) at 5h. Second series correspond to Y- facet: (f), (g), (h), (i) and (j) respectively for 1h, 2h, 3h, 4h and 5h. The high of each picture is 0.5 mm.
The chemical process etchs Z facets after 3hours and goes on to reveal etch pits and so dislocations. On microscopy pictures Fig. 8 , we can observe that the Z- face is more attacked than Z + face. In fact more numerous dislocations are revealed on Z- face but this is not connected with polar state of the crystal (Z axis is not a polar axis in R32 class of symmetry).
Fig. 8 Evolution with time of opposite sides for Z cut faces (Z- upper and Z + bottom facets). First series of images corresponds to Z- facet: (a) chemical etching at 1h, (b) 2h, (c) 3h, (d) 4h and (e) 5h. The second series corresponds to Z + facet: (f), (g), (h), (i) and (j) respectively for 1h, 2h, 3h, 4h and 5h. The width of each picture is 0.5mm.
3.3 Characteristics of etch pits: organization and morphology
For X + and X- facets, after just 1h, the chemical etching reveals Fig. 9(a) two families of line oriented at + 65° and −46° from Z axis as it is already observed on Schlieren photography. Striation lines show different width and after 3 hours dark stripes alternate with clear stripes showing a differential etching rate on the same face Fig. 9(b). In clear stripes we can see fine lines oriented along the other direction of striation. Etch-pits with arrow shape start to be revealed in dark stripes Fig. 9(c).Trigonal structure can explain microscopy pictures of chemical etching on Y faces. As we have seen Fig. 2(c), the XZ plane cuts rhombohedra showing four edges, parallels two by two which are intersections of XZ plan and (001) and (010) faces. Theoretical angle between the Z axis and this intersection lines is calculated from cell parameters to be equal to 50.9° that is on good agreement with our measurements 51.5° on microscopy pictures Fig. 10(b) . Chemical etching on Y faces reveals etch pits with head of arrow shape Fig. 10(a), the base of the head shows the same angle 51.5° with z axis.For Z cut faces, after etching we reveal the intersection of the XY plan with rhombohedra then etch pits have triangular form with summits pointing Y directions. This shape is according with the 3 order symmetry of rhombohedra. The macroscopic organization of etch-pits Figs. 11(a) -11(b) also reveal the three faces of the rhombohedra (trigonal system: (001), (010) and (100)) as we have seen Fig. 2(d). In addition, triangular etch pits with varied orientations (top-left corner Fig. 11(b)) are present and could arise from structure disorientations or twinning.
Fig. 9 The sample is observed under microscope after chemical etching, X axis is perpendicular to the observation plane, Z axis being vertical. (a) after 1 hour a crossed lines structure is revealed. (b and c) after 3 hours clear stripes show etch pits and fine structures orientated in the other direction, (c) with higher magnification the shape of etch pits is observed.
Fig. 10 (a) Y + face, magnification x200, after 1h numerous etch-pits with head of arrow shape; (b) Y- face, magnification x470, after 4h clear stripes oriented at ± 51.5° from z axis show a fine sub-structure.
Fig. 11 (a) Numerous etch-pits are revealed after 4 hours, (b) the triangular etch-pit shape and the orientation of dislocations are due to the growth of (001), (010), and (001) faces of LYSB crystal.
4. Conclusions
LYSB crystals have been grown and studied by Schlieren photography and chemical etching. Schlieren photography revealed on Z cut and X cut faces striations corresponding to rhombohedra growing facets. Otherwise, chemical etching microscopy pictures show as well relations between rhombohedra growing facets and striations, etch pits morphologies and theirs organizations. Indeed, calculated theoretical angles and observed experimental angles show a good agreement. Dislocations, striations and growth imperfections in the crystal bulk result to fluctuations (temperature, composition, presence of impurities) and arise from the growth of trigonal structure.
Acknowledgments
This work was supported by the French National Research Agency (No. ANR-06-BLAN-0169-01).
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