Epitaxial Growth of Lattice Matched KY1-x-yGdxLuy(WO4)2 Thin Films on KY(WO4)2 Substrates for Waveguiding Applications Western Bolan˜os,† Joan J. Carvajal,*,† Maria Cinta Pujol,† Xavier Mateos,† Gine´s Lifante,‡ Magdalena Aguilo´,† and Francesc Dı´az†
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 8 3525–3531
Fı´sica i Cristal · lografı`a de Materials i Nanomaterials, UniVersitat RoVira i Virgili, Campus Sescelades, c/Marcel · lı` Domingo s/n, 43007 Tarragona, Spain, and AdVanced Materials for Integrated Guided Optics (AMIGO), Departamento de Fı´sica de Materiales, UniVersidad Auto´noma de Madrid, c/Francisco Toma´s y Valiente No. 7. Ctra. Colmenar Viejo, km. 15, 28049, Cantoblanco, Madrid, Spain ReceiVed March 3, 2009; ReVised Manuscript ReceiVed May 22, 2009
ABSTRACT: The main purpose of this work was the determination of the composition of a KY1-x-yGdxLuy(WO4)2 layer grown by liquid phase epitaxy (LPE) technique on KY(WO4)2 substrate having enough refractive index contrast and appropriate lattice mismatch (fhkl) to act as planar waveguides. To achieve this we have grown KY1-x-yGdxLuy(WO4)2 single crystals by the flux method and characterized them in terms of chemical composition, structure and refractive indices. A high quality epitaxial layer of KY0.58Gd0.22Lu0.20(WO4)2 with a thickness of 10 µm was grown on a b oriented KY(WO4)2 substrate by LPE. At λ ) 632.8 nm this planar waveguide can support five and four guided modes along the Ng and Nm directions, respectively and five TM modes along Np direction.
1. Introduction Monoclinic potassium double tungstates KRE(WO4)2 (RE ) Y, Gd, Lu) belonging to the C2/c space group have been shown to be excellent laser hosts for active lanthanide ions due to the following advantages: (i) very high values of absorption and emission cross sections;1 and (ii) relatively large ion separation allowing doping levels with minimum quenching effects.2 Additionally they display a high value of third order nonlinearity,3 which allows frequency conversion phenomena such as efficient Raman conversion for ps/ns laser pulses.4 The crystal growth and physical properties of these compounds have been intensively studied.5-7 The laser operation of Yb3+, Tm3+ and Er3+ ions in bulk monoclinic potassium double tungstates KY(WO4)2 (KYW), KLu(WO4)2 (KLuW) and KGd(WO4)2 (KGdW), to name a few, has also been reported.8-12 As thin films, two main applications have been proved until now for these materials: thin disk lasers and waveguides. The concept of thin-disk laser has been demonstrated in 5 and 10 atom % Yb-doped KYW and 5 atom % Yb-doped KGdW.13 Recently, we used a 50 µm thick 32 atom % Ybdoped KLuW epilayer grown on KLuW substrate to generate 9 W of laser near 1030 nm in the cw regime with a 77% efficiency in a single pass configuration and in the absence of thermal roll-off in the power dependence up to the maximum applied pump levels used,14,15 simplifying in this way the configuration for thin-disk lasers previously reported.16 The increasing interest for on chip integrated waveguides based on these materials is due to their refractive indices (∼2.0-2.1), which provide a rather high refractive index contrast with air in surface structures or with a low refractive index cover layer in buried structures, thus enabling high integration density and excellent light confinement. Waveguide lasers based on potassium double tungstates have been reported for Yb:KYW/KYW and Tm:KYW/KYW epitaxies.17,18 More * To whom correspondence should be addressed. E-mail: joanjosep.carvajal@ urv.cat. † Universitat Rovira i Virgili. ‡ Universidad Auto´noma de Madrid.
recently, Gardillou et al. developed single mode channel waveguide structures using as confining layer the KLu0.253 Gd0.13Yb0.017Y0.60(WO4)2 compound grown on KYW substrates.19 All these achievements in optical waveguides and active components for integrated optics (IO) mentioned above concerned only double tungstates doped with active lanthanide ions, including the passive waveguide proposed by Romanyuk et al. in Tb3+-doped KYW epilayers grown on KYW substrates by means of low temperature LPE.20 However, to the best of our knowledge, a detailed study on the development of passive optical waveguides based on monoclinic potassium double tungstates not substituted with active lanthanide ions in the visible and near-IR region of the electromagnetic spectrum has not been reported. Since such passive optical components are also crucial to develop IO components, their development is also necessary. In this article we determined the best composition of KY1-x-yGdxLuy(WO4)2 layer to be grown on KYW substrates that exhibited a contrast in refractive index of about 10-3 for passive optical waveguiding and an appropriate lattice mismatch, which is essential to obtain layers of these compounds on KYW substrates free of cracks. Once this composition was established, a high quality epitaxial layer of this compound was grown on a b oriented KYW substrate by liquid phase epitaxy. The obtained epitaxy exhibited good crystalline quality, and their fundamental guiding properties have been measured.
2. Experimental Section 2.1. Bulk Crystal Growth. Nonsubstituted and Gd, Lu and (Gd, Lu) substituted KYW crystals were obtained by means of the high temperature top-seeded solution growth (TSSG) method associated to a slow cooling of the solution using K2W2O7 as solvent.8 In all cases the molar composition of the solution for the growth of bulk crystals was composed of 12 mol % solute and 88 mol % solvent. With this composition we can guarantee that small variations of temperature lead only to small changes in the supersaturation as the solubility curve demonstrates.21 The reagents used to prepare the solutions were powders of Y2O3, Gd2O3, Lu2O3, WO3, and K2CO3 analytical grade of purity
10.1021/cg900259x CCC: $40.75 2009 American Chemical Society Published on Web 06/04/2009
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Table 1. Crystal Growth Conditions and Results Obtained for KY1-x-yGdxLuy(WO4)2 Single Crystalsa A I
II
III IV
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
100 100 100 100 100 75 50 25 80 75 72 68 65 61 57 65
0 0 0 0 0 25 50 75 0 0 3 7 10 14 18 5
0 0 0 0 0 0 0 0 20 25 25 25 25 25 25 30
9.249 13.258 12.598 8.020 9.015 1.056 1.596 1.298 1.232 1.752 1.175 1.669 1.480 1.954 1.078 1.396
58 38 37 43 43 13 12.6 13.6 13.7 11.2 15.1 21.4 17.5 24.5 10.3 22.3
0.10 0.12 0.13 0.15 0.15 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.05 0.10
20.55 25.92 23.39 20.79 21.06 9.47 11.41 12.90 12.15 12.43 11.95 13.91 11.01 13.21 11.89 14.57
10.05 11.94 12.94 10.82 11.84 6.03 6.62 8.85 5.92 6.55 5.55 6.03 6.47 6.07 7.66 7.78
9.00 10.74 14.92 10.40 10.82 4.86 4.83 3.13 5.22 5.79 4.51 4.94 4.90 6.89 4.22 4.81
483 317 280 285 286 130 126 136 137 112 151 214 175 245 206 223
19.1 41.8 45.0 28.1 31.5 8.1 12.7 9.5 9.0 15.6 7.8 7.8 8.5 8.0 5.2 6.3
none none none none none a few a few some none a few a few none none none none some
1.00 1.00 1.00 1.00 1.00 1.05 1.06 1.36 1.07 1.01 1.05 0.99 1.06 1.06 1.03 1.07
0 0 0 0 0 0.84 0.93 0.88 0 0 1.37 1.17 0.99 1.04 1.18 0.97
0 0 0 0 0 0 0 0 0.72 0.97 0.82 0.96 0.84 0.83 0.81 0.84
KY(WO4)2 KY(WO4)2 KY(WO4)2 KY(WO4)2 KY(WO4)2 KY0.79Gd0.21(WO4)2 KY0.53Gd0.47(WO4)2 KY0.34Gd0.66(WO4)2 KY0.85Lu0.15(WO4)2 KY0.76Lu0.24(WO4)2 KY0.75Gd0.04Lu0.21(WO4)2 KY0.68Gd0.08Lu0.24(WO4)2 KY0.69Gd0.10Lu0.21(WO4)2 KY0.65Gd0.15Lu0.20(WO4)2 KY0.58Gd0.22Lu0.20 (WO4)2 KY0.70Gd0.05Lu0.25(WO4)2
a . A: Set of crystals. B: Yttrium content in solution in relation with the global content of rare earths (mol %). C: Gadolinium content in solution in relation with the global content of rare earths (mol %). D: Lutetium content in solution in relation with the global content of rare earths (mol %). E: Crystal mass (g). F: Cooling interval (K). G: Cooling rate (K/h). H, I, J: crystal dimension along c, a*, and b crystallographic directions respectively (mm). K: Growth time (h). L: Growth rate × 10-3 (g/h). M: Observed visual defects in a qualitative scale, some < a few < none. N: Distribution coefficient of Y; KY3+. O: Distribution coefficient of Gd; KGd3+. P: Distribution coefficient of Lu; KLu3+. Q: Stoichiometry of the crystals.
(99.99%) that were mixed in the appropriate amounts in cylindrical platinum crucibles. All the experiments were carried out in a vertical tubular furnace. The mixtures were homogenized by maintaining the solutions at about 50 K above the expected saturation temperature, Ts, for about 5-6 h. Afterward, Ts was accurately determined by placing the seed in contact with the center of the surface of the solution and controlling the growth or dissolution of the crystal seed with an accuracy of up to 10 µm with a micrometer comparer until neither growth nor dissolution was observed. Once Ts was determined, the seed was placed in contact with the surface of the solution and we applied a controlled cooling program of about 0.05-0.15 K/h to the system solution-seed until a final temperature between 10 and 30 K below Ts, depending on the experiment. The seed was rotated at 42 rpm. When the growth ramp was over, the grown crystal was slowly removed from the solution and placed slightly above the surface of the solution while the furnace was cooled down slowly (20 K/h) to room temperature. Four sets of crystals have been grown in this work: I.nonsubstituted KYW crystals; II.Gd-substituted KYW crystals, KY1-xGdx(WO4)2; III.Lu-substituted KYW crystals, KY1-yLuy(WO4)2; and IV.Gd/Lu-substituted KYW crystals, KY1-x-yGdxLuy(WO4)2. In set I, 50 mm in diameter and 50 mm in height platinum crucible were filled with 300 g of solution. The axial temperature gradient in these experiments was 0.83 K/cm. For experiments in sets II, III and IV, platinum cylindrical crucibles of 30 mm of inner diameter and 60 mm height were used to prepare ∼60 g of solution by mixing the appropriate amounts of solute and solvent. The axial temperature gradient in the experiments was ∼0.5 K/cm. Table 1 summarizes the growth conditions used in these experiments. 2.2. Y, Gd and Lu Concentration Analysis. We refer in general as KY1-x-yGdxLuy(WO4)2 to crystals obtained in the four sets, i.e. nonsubstituted, Gd-substituted, Lu-substituted and (Gd, Lu)-substituted KYW crystals. The chemical composition of each KY1-x-yGdxLuy (WO4)2 single crystal, as well as that of the epitaxial layer obtained from LPE, was determined by electron probe microanalysis (EPMA). A CAMECA SX50 electron microprobe was used in the analyses operating at 20 mA of beam current and 20 kV of accelerating voltage. Nonsubstituted KYW, KGdW and KLuW single crystals grown by us were used as standards for the determination of Y, Gd and Lu, respectively, as well as for the determination of K, W and O with aims of minimizing the matrix effects on the samples thanks to its similar chemical composition. Gadolinium and lutetium were analyzed with a lithium fluoride (LiF) crystal, used as a diffracter crystal, tungsten and yttrium with a thallium phthalate acid (TAP) crystal, and finally potassium and oxygen were analyzed with a pentaerythritol (PET) crystal and a multilayered pseudocrystal (PC1) consisting of several alternating layers of W and Si with 2d ) 60 Å, respectively. For each element, the measuring time was 10 s using the KR, lines for oxygen and potassium, LR lines for yttrium, gadolinium and lutetium and the MR line for tungsten.
2.3. Structural Characterization by X-ray Powder Diffraction. The lattice parameters of the KY1-x-yGdxLuy(WO4)2 single crystals were determined at room temperature by X-ray powder diffraction (XRD). Data were collected on a Siemens D5000 X-ray diffractometer with Bragg-Brentano parafocusing geometry and a θ-θ configuration. The Cu KR radiation was used in these experiments, and the 2θ range was from 10° to 70°, with a step scan (ss) of 0.02° and a scan time (st) of 16 s. 2.4. Optical Characterization. As monoclinic double tungstates are biaxial crystals, the optical characterization comprised the determination of the orientation of the dielectric frame and the measurement of the three refractive indices for E parallel to Ng, Nm and Np at λ ) 632.8 nm. The orientation of the optical ellipsoid was performed by measuring the angle, κ, between the c crystallographic axis and the Ng principal optical axis by using two crossed Glan-Taylor polarizers. The refractive indices were measured along the three principal optical axes using a prism-film coupler Metricon 201022 using two plates cut from every crystal studied in this paper. One plate was cut perpendicular to the b crystallographic direction, which allowed the measurement of ng and nm indices. The other plate was cut perpendicular to the Ng principal optical direction, which allowed the measurement of np. 2.5. Epitaxial Growth and Characterization. An epilayer with the composition KY0.58Gd0.22Lu0.20(WO4)2 was grown on a b oriented KYW substrate by means of liquid phase epitaxy (LPE), in a vertical tubular furnace with a wide zone of uniform temperature to obtain an almost zero temperature gradient inside of the solution. The solvent used was, again, K2W2O7. The solution was prepared at the molar ratio solute/solvent 7/93 because we observed that with this composition of the solution we have better control over the supersaturation degree of the solution.21 We mixed the corresponding amounts of the initial reagents (Y2O3, Gd2O3, Lu2O3, WO3, and K2CO3) to prepare 70 g of solution in a cylindrical platinum crucible, and we homogenized it as described before in section 2.1. The saturation temperature was determined with a thick KYW crystal seed in the same way as explained before. Before being placed inside the vertical furnace, the substrate was carefully cleaned by immersing it in a mixture of HNO3:H2O (1:1 in volume) during 5 min, followed by an immersion in distilled water during 5 min and finally immersing it in acetone for 5 min and in ethanol for 5 min more. Then, the substrate was introduced slowly in the furnace, it was kept above the surface of the solution for 1 h, and, after reaching the thermal equilibrium, it was introduced during 5 min inside of the solution at a temperature Td (dissolution temperature) of 1 K above the saturation temperature determined previously, so that the outer layer of the crystal was dissolved. The epitaxial growth was then carried out during 2 h at a constant temperature, Tg (growth temperature) 3 K below Ts. Finally, the epitaxy was removed from the solution, and kept 3 mm above the surface of the solution while the
Growth of KY1-x-yGdxLuy(WO4)2 Thin Films furnace was cooled to room temperature at 15 K/h to avoid thermal stresses between the substrate and the epilayer that could result in cracks. The thickness of the obtained epilayer was measured by taking extended profiles along different zones in both faces (including substrate and epilayer) with an optical confocal microscope Sensofar PLµ 2300. The quality of the interface substrate/layer was observed by means of environmental scanning electron microscopy (ESEM) FEI QUANTA 600 operating at 20 kV, and the composition of the epilayer was determined from EPMA measurements, in a similar way as those performed on bulk crystals. However, in this case we scanned several points across the epitaxial layer and the substrate to detect if diffusion of the ions from the epilayer to the substrate occurred. The homogeneity of composition along the whole thickness of the epilayer was also measured by EPMA. 2.6. Planar Waveguide Fabrication and Characterization. We used the epitaxy grown in the previous section to fabricate a planar waveguide. To do that we first removed the epilayer grown on the (01j0) face of the substrate, and then, the epilayer grown on the (010) face of the substrate was polished with alumina powders to have a highly transparent layer with a thickness of 10 µm. Finally, the lateral faces were cut in such a way that they were oriented along Ng and Nm optical principal axes, and the four lateral faces of the epitaxy were polished to a high quality. The optical characterization of the planar waveguide was divided in three steps. First, we recorded the dark-mode spectra of the sample for TE and TM polarizations at λ ) 632.8 nm and room temperature, using the Metricon 2010 prism-film coupling system. From these spectra we were able to calculate how many guided modes the waveguide could support. Second, we visualized, with a CCD camera, the different guided modes that the waveguide can support by coupling the light of a He-Ne laser at 632.8 nm at the input and output ends of the waveguide with long working distance microscope objectives (40× and 63×, respectively). Third, we estimated the upper limit of the waveguide losses by imaging the scattered light intensity of a He-Ne laser coupled into the epilayer along the waveguide with a CCD camera.
3. Results and Discussion 3.1. Bulk Crystal Growth. All the growth experiments were carried out by using b oriented seeds of nonsubstituted KYW. Nonsubstituted KYW crystals obtained in set I were crack-free, while some of the crystals of sets II, III and IV substituted with large concentrations of Gd and/or Lu showed some minor defects like small inclusions and/or small cracks mainly near the area where the crystal seed was located. However, we got pieces of all crystals grown in these series free of cracks and inclusions and large enough to carry out their optical characterization. Table 1 summarizes the results obtained in the different growth experiments. Set I shows the results for growth experiments of nonsubstituted KYW crystals. These crystals were grown to be used as crystalline substrates in the epitaxial growth experiments we planned. For this reason we were interested in growing large crystals free of defects in the fastest possible way. Our results show that large high-quality nonsubstituted KYW crystals free of defects can be grown at cooling rates of 0.15 K/h, the fastest cooling rate used in all the crystal growth experiments performed in this paper. The crystals obtained at this cooling rate are not the biggest obtained in the experiments we performed (bigger crystals were obtained when applying cooling rates of the order of 0.12 or 0.13 K/h); however, the surface area that these crystals provide in the plane perpendicular to the b crystallographic direction, that is the plane of interest for us to undertake the epitaxial growth experiments, is large enough. Furthermore, the dimensions and weight of the different nonsubstituted KYW single crystals grown at this cooling rate are similar, which provides a good reproducibility for the experiments undertaken at these growth conditions.
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Crystals in sets II, III and IV had to be grown at a cooling rate of 0.10K/h since faster cooling rates caused an increased number of inclusions and/or cracks in the single crystals obtained. In set IV, for the largest Gd2O3 concentration used in this study, we had to apply a lower cooling rate of 0.05 K/h to obtain crystals free of inclusions and cracks and with quality enough for further characterization from the solution containing 57% Y2O3, 18% Gd2O3, and 25% Lu2O3. Dividing the crystal mass (column E) and the growth time (column K) we obtained the growth rate (column L). Figure 1 shows some of the single crystals obtained in these experiments. KYW crystals partially substituted only with one rare earth ion (either Gd3+ or Lu3+) (see crystals in sets II and III) tended to grow at a lower growth rate when compared to nonsubstituted KYW crystals when the same cooling rate was applied. This tendency was also observed in crystals grown in set IV, in which Y3+ was substituted simultaneously by Gd3+ and Lu3+. It also seems that the fact that 3 ions are in competition to occupy the same structural site in the crystals makes the crystals grow at a slower growth rate when compared to KYW crystals only substituted with one rare earth ion, although the difference in growth rate is small in some cases. The composition of each KY1-x-yGdxLuy(WO4)2 single crystal was measured by EPMA, and the distribution coefficient, KRE3+, of yttrium, gadolinium and lutetium in the KYW matrix was calculated from each concentration measured in the crystal and the initial concentration introduced in the solution of growth using the following expression: KRE3+ ) {[RE3+]/([Y3+] + [Gd3+] + [Lu3+])}crystal/{[RE3+]/ ([Y3+] + [Gd3+] + [Lu3+])}solution, where RE ) Y, Gd or Lu. The results obtained are also summarized in Table 1. The distribution coefficients of these ions also increased as the concentration in solution increased, getting values close to 1 for the largest concentrations of Gd3+ and Lu3+ studied in this work. However, if we compare the KY0.79Gd0.21(WO4)2 and KY0.76Lu0.24(WO4)2 crystals, grown in solutions containing 75 mol % Y2O3 and 25 mol % of Gd2O3 and Lu2O3, respectively, considering only the content of the solution on rare earth oxides, we observe that KLu3+ > KGd3+, producing crystals richer in Lu3+ than in Gd3+. This tendency might be explained in terms of the sizes of the ions. Lu3+ is the smallest of the three ions considered, and it will have a greater mobility in the solution, allowing it to be incorporated in a larger quantity into the crystals. When Gd3+ and Lu3+ are present simultaneously in the crystal, we observe that KGd3+ > KLu3+, and KGd3+ tends to be larger than unity, indicating that the content of Gd3+ in the crystal is larger than that introduced initially in the solution. On the contrary, KLu3+ tends to be less than unity in these crystals, which indicates that its content in the crystal is always less than that introduced initially in the solution. This observation might be explained in terms of the difference of ionic radii among Y3+ (1.019 Å), Gd3+ (1.053 Å) and Lu3+ (0.977 Å).23 Here, there exists a competition mechanism for the incorporation of Y3+, Gd3+ and Lu3+ in the same structural sites of the crystal. Then, the ion with an ionic radius more similar to that of Y3+ would be incorporated easily into the crystal structure. This would also explain the slowest growth rate observed in these crystals substituted with Gd3+ and Lu3+. Despite this competition mechanism, if we take a look at set IV in Table 1, it seems that the concentration of Lu3+ tends to stabilize at a value of 21 atom % to respect Y3+ and Gd3+, and the increase of the Gd2O3 concentration in the solution seems
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Figure 1. As grown single crystals: (a) KY(WO4)2, (b) KY0.34Gd0.66(WO4)2, (c) KY0.85Lu0.15(WO4)2 and (d) KY0.58Gd0.22Lu0.20(WO4)2.
Figure 2. Lattice parameters as a function gadolinium and lutetium content in the KY1-x-yGdxLuy(WO4)2.
to affect more the concentration of Y3+ in the crystals than the concentration of Lu3+. 3.2. Structural Characterization: Crystallographic Lattice Parameters and Theoretical Lattice Mismatch. The lattice parameters of the crystals were calculated from the X-ray powder diffraction data and were refined with the Fullprof software,24 which is based on the Rietveld method.25 Figure 2 shows the variation of lattice parameters and volume of the KY1-x-yGdxLuy(WO4)2 crystals with respect to those of nonsubstituted KYW crystals (a ) 10.631(3) Å, b ) 10.345(4) Å, c ) 7.555(2) Å and β ) 130.752(1)°). Lattice parameters corresponding to crystals substituted only with gadolinium are
larger than those of KYW. This was expected since the ionic radius of Gd3+ is larger than that of Y3+, as it has been seen before. As expected, the lattice parameters of KYW crystals substituted only with Lu3+ are smaller than those of KYW, as the ionic radius of Lu3+ is smaller than that of Y3+ (see section 3.1). When Gd3+ was also introduced in the solution together with Lu3+, the lattice parameters increased as the concentration of gadolinium increased. There are some chemical compositions of the crystals for which one or more lattice parameters are very similar to those of KYW. Those are the compositions that should
Growth of KY1-x-yGdxLuy(WO4)2 Thin Films
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Figure 3. Lattice mismatch of (010) face for KY1-x-yGdxLuy(WO4)2 crystals.
be used when we plan to grow an epitaxial layer of that material on KYW substrates. The lattice parameters of these crystals were calculated to determine the theoretical lattice mismatch that a supposed thin layer of these materials would suffer when grown on a KYW substrate. In previous works we have shown that the (010) face in this family of crystals is an appropriate plane to obtain high quality epitaxial layers of Yb:KYW on KYW substrates.26 The lattice mismatch on the (010) face between a hypothetical epilayer and the KYW substrate was calculated following the expression f(010) ) (SL(010) - SS(010))/SS(010), where SS(hkl) and SL(hkl) are the areas defined by the periodicity vectors of the substrate and the layer, respectively. Figure 3 shows the variation of lattice mismatch as a function of gadolinium and lutetium content for the twelve crystals reported here. For all crystals, the calculated lattice mismatch was around 10-3. If we pay attention to its sign, it is positive when KYW is substituted only with gadolinium and tends to increase from 1.96 × 10-3 (21% Gd in crystal) to 8.39 × 10-3 (79% Gd in crystal) as the gadolinium content increases. For all the crystals containing Lu, it has been found to be negative and tends to zero when the gadolinium content in the crystal increased from 0% to 22%. The crystal with the composition KY0.58 Gd0.22Lu0.20(WO4)2 has the lattice parameters nearest to those of KYW, therefore, it has the smallest lattice mismatch, -0.53 × 10-3, from all the chemical compositions studied in this paper, and it can be assumed to have a lattice mismatch almost zero. This composition was the chosen one to grow epitaxial layers of these materials by LPE on b oriented KYW substrate, as will be described in section 3.4. 3.3. Optical Characterization. Using the cross polarizers method we measured the angle κ between the c crystallographic axis and the Ng principal optical axis, which we used to describe the orientation of the optical ellipsoid according to Figure 4. Table 2 summarizes the evolution of the angle κ with the content of Gd and Lu in the crystals. The κ angle increased as the gadolinium content increases in crystals substituted only with gadolinium. This means that the optical frame turned clockwise with respect to the crystallographic frame when the content of Gd3+ in the crystals increased when compared to that of KYW. On the contrary, when Y3+ was only substituted with Lu, κ decreased by ∼1.5°, which means that the optical frame turned counterclockwise with respect to the crystallographic frame when compared to that of KYW. When Gd3+ and Lu3+ were both present in the crystals, a clear tendency for the behavior of κ could not be extracted since its value decreased for low concentration of Gd3+ in the crystals, but it increased for concentration of Gd3+ above 20%. The refractive indices measured at λ ) 632.8 nm, and corresponding to the three principal optical directions of every sample studied, are also summarized in Table 2. For the nonsubstituted KYW, KGdW, and KLuW crystals, ranked in
Figure 4. Dielectric ellipsoid of the KY1-x-yGdxLuy(WO4)2 crystals. Table 2. Variation of K Angle and Refractive Indices of KY1-x-yGdxLuy(WO4)2 Crystalsa KY1-xGdxLuy(WO4)2
κ (deg)
ng
nm
np
KY(WO4)2 KY0.79Gd0.21(WO4)2 KY0.53Gd0.47(WO4)2 KY0.34Gd0.66(WO4)2 KGd(WO4)2 KY0.85Lu0.15(WO4)2 KY0.76Lu0.24(WO4)2 KLu(WO4)2 KY0.75Gd0.04Lu0.21(WO4)2 KY0.68Gd0.08Lu0.24(WO4)2 KY0.69Gd0.10Lu0.21(WO4)2 KY0.65Gd0.15Lu0.20(WO4)2 KY0.58Gd0.22Lu0.20(WO4)2 KY0.70Gd0.05Lu0.25(WO4)2
18.5 18.9 19.5 20.0 21.5 17.1 17.2 18.5 17.5 16.0 14.5 14.0 15.0 16.5
2.0853 2.0852 2.0851 2.0845 2.0841 2.0856 2.0951 2.1131 2.0912 2.0916 2.0914 2.0909 2.0908 2.0919
2.0405 2.0415 2.0432 2.0447 2.0465 2.0415 2.0457 2.0580 2.0438 2.0442 2.0441 2.0447 2.0448 2.0445
1.9976 2.0023 2.0060 2.0151 2.0165 2.0013 2.0057 2.0198 2.0034 2.0044 2.0048 2.0056 2.0060 2.0049
a Also included are the corresponding values for nonsubstituted KGdW and KLuW crystals at λ ) 632.8 nm and at room temperature.
order of increasing values of refractive indices we have np,mKYW < np,mKGdW < np,mKLuW and ngKGdW < ngKYW < ngKLuW. As would be expected from this tendency, when KYW crystals were substituted only with gadolinium, np,m refractive indices increased as the content of Gd3+ increased in the crystals, while ng decreased. When Lu3+ was only substituting yttrium in the crystals and we did not introduce Gd3+, the three refractive indices of the crystals also increased with the content of Lu3+. Finally, for crystals substituted with gadolinium and lutetium np and nm tended to increase as the content of Gd3+ increased in the crystals. However, ng showed a more random behavior although it tended to decrease as the content of Gd increased in the crystals 3.4. Epitaxial Growth and Characterization. Based on the results of the determination of the lattice parameters, the chemical composition of the crystal that presented the smallest lattice mismatch (-0.53 × 10-3) in the a-c plane with the KYW substrate was that formed by KY0.58Gd0.22Lu0.20(WO4)2; this lattice mismatch is practically zero. The contrasts for the ng, nm and np refractive indices between the nonsubstituted KYW crystal and a crystal with this composition are 2.62 × 10-3, 2.12 × 10-3 and 4.21 × 10-3, respectively. This contrast of refractive indices allowed observation of the guiding effect in a waveguide of KY0.58Gd0.22Lu0.20(WO4)2 grown on a nonsubstituted KYW substrate. So, in order to obtain a planar waveguide based on this crystal, we grew an epitaxial layer of this composition deposited on a b oriented substrate of nonsubstituted KYW by the LPE technique under the experimental
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Figure 5. (a) Scheme of the atomic percentage of Y, Gd and Lu across the substrate and the epitaxial layer calculated from the EPMA results and (b) ESEM picture of a KY0.59Gd0.19Lu0.22(WO4)2/KY(WO4)2 interface taken using backscattered electrons.
Figure 6. Photograph of the planar waveguide KY0.59Gd0.19Lu0.22(WO4)2/ KYW epitaxy.
conditions described in section 2.5. We obtained a high quality and crack-free epitaxial layer, with a thickness of about 60-70 µm. The composition of the grown layer was determined by means of EPMA measurements. For this purpose we cut a cross section of the sample containing substrate and epilayer and we scanned with the microprobe across the substrate and the epilayer. This analysis showed that the layer was composed of 59 atom % of yttrium, 19 atom % of gadolinium and 22 atom % of lutetium. This composition is slightly different from that determined in the bulk crystal since the growth conditions are different, and were modified to allow the growth process to proceed in softer conditions, closer to the equilibrium conditions. We have also to take into account that the epilayer was grown in a solution with a different composition, 7% solute and 93% solvent, from that used to grow the bulk crystals, 12% solute and 88% solvent.
This can also affect the chemical composition of the epilayer. Finally, in this case the substrate is also playing a major role in determining the composition of the grown epilayer. Despite these inconveniences, the composition of the epilayer was very homogeneous along its thickness as can be seen in Figure 5(a). No diffusion of gadolinium and/or lutetium inside the substrate could be observed from EPMA measurements. Figure 5(b) shows an ESEM picture of the layer/substrate interface after the epilayer was polished down to 10 µm. The picture was taken recording backscattered electrons generated in the sample, that contain information about the composition of the sample, giving a sharp change in contrast at the interface between the substrate and the epilayer, indicating, again, that no diffusion of Gd and/or Lu occurred from the epilayer to the substrate. Any other attempt to grow epilayers with larger lattice mismatches with respect to nonsubstituted KYW crystals resulted in cracked epilayers. 3.5. Waveguide Characterization. The passive planar waveguide we fabricated is shown in Figure 6. With the prism film coupling system we recorded the dark-mode spectrum of the sample at λ ) 632.8 nm at room temperature. From this spectrum we calculated the effective refractive index for the different guided modes that this planar waveguide can support with propagation of the light along the Ng, Nm and Np directions. Table 3 lists the different effective refractive indices, neff, calculated from the expression neff ) nprism · sin θ1, where θ1 is the light angle of incidence on the base of the prism.
Figure 7. Near field intensity distribution. (a) E//Ng direction (TE3 mode), (b) E//Nm direction (TE2 mode) and (c) H//Np direction (TM4 mode).
Growth of KY1-x-yGdxLuy(WO4)2 Thin Films
Crystal Growth & Design, Vol. 9, No. 8, 2009 3531
Table 3. Effective Refractive Indices for the Different Guided Modes along the Ng, Nm and Np Directions Calculated from the Dark-Mode Spectruma direction
polarization
film refractive index
mode order
effective refractive indices
Ng
TE
2.0908
0 1 2 3 4 0 1 2 3 0 1 2 3 4
2.0906 2.0899 2.0888 2.0875 2.0857 2.0444 2.0436 2.0424 2.0408 2.0053 2.0044 2.0031 2.0019 1.9989
Nm
TE
2.0447
Np
TM
2.0057
a Five and four TE guided modes can be supported by this planar waveguide when light propagates parallel to the Ng and Nm directions, respectively, and five TM modes when light propagates parallel to Np direction.
An end-coupling arrangement with a He-Ne laser at 632.8 nm and a CCD camera was performed to investigate the near field intensity distribution of the light carried in the different modes supported by this planar waveguide. The results are shown in Figure 7. In this configuration we can excite the different modes of propagation observed in the dark-mode spectrum. Finally, we could establish an upper limit for the propagation losses of the waveguide by measuring the intensity of the scattered light along the waveguide with a CCD camera. The corresponding intensity decays along the waveguide according to the following equation: IL ) I0 exp(-R · L), where IL is the intensity of the scattered light along the length L, I0 is the light intensity at the input face of the waveguide and R is the loss coefficient. The upper limit was about 1 dB/cm for the three directions investigated: Ng, Nm, and Np. These losses are of the order of magnitude of those determined by Gardillou et al. in similar crystals.19
4. Conclusions KY1-x-yGdxLuy(WO4)2 crystals were grown by the TSSG slow cooling method and characterized structurally and optically, and we determined that one of the compositions with the lowest lattice mismatch in the a-c plane when compared to KYW corresponds to that of KY0.58Gd0.22Lu0.20(WO4)2. The contrast of refractive indices between a crystal with this composition and nonsubstituted KYW crystal, together with this lattice mismatch close to zero, allowed us to fabricate a planar waveguide by growing an epilayer of KY0.59Gd0.19Lu0.22(WO4)2 on a b oriented KYW substrate by the LPE technique. In this planar waveguide the KY0.59Gd0.19Lu0.22(WO4)2 epitaxial layer acts as the guiding layer. This planar waveguide has been characterized, and we could determine that it can support up to five TE guided modes along the Ng direction, four TE guided modes along the Nm direction, and five TM guided modes along the Np direction. The upper limit for the propagation losses in this waveguide was established at ∼1 dB/cm. These epitaxial layers will be used to develop channel waveguides and integrated optical elements and devices based on these materials. Acknowledgment. This work was supported by the Spanish Government under Projects MAT-05-06354-C03-02 and MAT2008-06729-C02-02/NAN and the Catalan Government under Project 2005SGR658. The authors are grateful to the
Serveis Cientifico-Te`cnics of the University of Barcelona for EPMA measurements. W.B. thanks the Catalan Government for the funds provided through the fellowship 2008FI_B 00626. J.J.C. and M.C.P. are supported by the Education and Science Ministry of Spain and European Social Fund under the Ramon y Cajal program, RYC2006-858 and RYC20041453, respectively.
References (1) Kuleshov, N.; Lagatsky, A. A.; Podlipensky, A. V.; Mikhailov, V. P.; Huber, G. Opt. Lett. 1997, 22, 1317–1319. (2) Klevtsov, P. V.; Kozeeva, L. P. SoV. Phys.-Dokl. 1969, 14, 185–187. [translated from Dokl. Akad. Nauk SSSR, 1969, 571-579]. (3) Mochalov, I. V. J. Opt. Technol. 1995, 62, 746. (4) Basiev, T. T.; Sobol, A. A.; Zverev, P. G.; Osiko, V. V.; Powell, R. C. Appl. Opt. 1999, 38, 594–598. (5) Petrov, V.; Pujol, M. C.; Mateos, X.; Silvestre, O.; Rivier, S.; Aguilo´, M.; Sole´, R. M.; Liu, J.; Griebner, U.; Dı´az, F. Laser Photon ReV. 2007, 1, 179–212. (6) Pujol, M. C.; Mateos, X.; Sole´, R.; Massons, J.; Gavalda`, Jna.; Dı´az, F.; Aguilo´, M. Mater. Sci. Forum 2001, 378, 710–717. (7) Sole´, R.; Nikolov, V.; Ruiz, X.; Gavalda`, Jna.; Solans, X.; Aguilo´, M.; Dı´az, F. J. Cryst. Growth 1996, 169, 600–603. (8) Lagatsky, K. A.; Kuleshov, N. V.; Mikhailov, V. P. Opt. Commun. 1999, 165, 71–75. (9) Mateos, X.; Petrov, V.; Aguilo´, M.; Sole´, R.; Gavalda`, Jna.; Massons, J.; Dı´az, F.; Griebner, U. IEEE J. Quantum Electron. 2004, 40, 1056– 1059. (10) Bagaev, S. N.; Vatnik, S. M.; Maiorov, A. P.; Pavlyuk, A. A. Plakushchev. Quantum Electron 2000, 30, 310–314. [translated From KVantoVaya Elektronika, 2000, 30, 310-314. (11) Petrov, V.; Gu¨ell, F.; Massons, J.; Gavalda`, Jna.; Sole´, R.; Aguilo´, M.; Dı´az, F.; Griebner, U. IEEE J. Quantum Electron. 2004, 40, 1244– 1251. (12) Mateos, X.; Petrov, V.; Liu, J.; Pujol, M. C.; Griebner, U.; Aguilo´, M.; Dı´az, F.; Gala´n, M.; Viera, G. IEEE J. Quantum Electron. 2006, 42, 1008–1015. (13) Brunner, F.; Su¨dmeyer, T.; Innerhofer, E.; Morier-Genoud, F.; Paschotta, R.; Kisel, V. E.; Scherbitsky, V. G.; Kuleshov, N. V.; Gao, J.; Kontag, K.; Giesen, A.; Keller, U. Opt. Lett. 2002, 27, 1162–1164. (14) Erhard, S.; Gao, J.; Giesen, A.; Kontag, K.; Lagatsky, A. A.; Abdolvand, A.; Kuleshov, N. V.; Aus der Au, J.; Spuhler, G. J.; Brunner, F.; Paschotta, R.; Keller, U. Lasers and Electro-Optics, 2001. CLEO 01. Technical Digest. Summaries of papers presented at the conference on. 2001, 333. (15) Rivier, S.; Mateos, X.; Silvestre, O.; Petrov, V.; Griebner, U.; Pujol, M. C.; Aguilo´, M.; Dı´az, F.; Vernay, S.; Rytz, D. Opt. Lett. 2008, 33, 735–737. (16) Aus der Au, J.; Sphu¨gler, G. J.; Su¨dmeyer, T.; Paschotta, R.; Ho¨vel, R.; Moser, M.; Erhard, S.; Karzewsky, M.; Giesen, A.; Keller, U. Opt. Lett. 2000, 25, 859–861. (17) Rivier, S.; Mateos, X.; Petrov, V.; Griebner, U.; Romanyuk, Y. E.; Borca, C. N.; Gardillou, F.; Pollnau, M. Opt. Express 2007, 15, 5885– 5892. (18) Romanyuk, Y. E.; Borca, C. N.; Pollnau, M.; Griebner, U.; Rivier, U.; Petrov, V. Opt. Lett. 2006, 31, 53–55. (19) Gardillou, F.; Romanyuk, Y. E.; Borca, C. N.; Salathe´, R. P.; Pollnau, M. Opt. Lett. 2007, 32, 488–490. (20) Romanyuk, Y. E.; Utke, I.; Ehrentraut, D.; Apostolopoulos, V.; Pollnau, M.; Garcı´a-Revilla, S.; Valiente, R. J. Cryst. Growth 2004, 269, 377– 384. (21) Silvestre, O.; Pujol, M. C.; Sole´, R.; Bolan˜os, W.; Carvajal, J. J.; Massons, J.; Aguilo´, M.; Dı´az, F. Mater. Sci. Eng. B 2008, 146, 59– 65. (22) Tien, P. K.; Ulrich, R.; Martin, J. Appl. Phys. Lett. 1969, 14, 291– 294. (23) Shannon, R. D. Acta Crystallogr. 1976, A32, 751–767. (24) Rodrı´guez -Carvajal, J. Short reference guide for the computer program FULLPROF; Laboratorie Leon Brilluin, CEA-CNRS: Gif sur Yvette, 1995. (25) Young, R. A. The RietVeld Method, International Union of Crystallography, monographs on crystallography 5; Oxford University Press: Oxford, 1995. (26) Aznar, A.; Silvestre, O.; Pujol, M. C.; Aguilo´, M.; Dı´az, F. Cryst. Growth Des. 2006, 6, 1781–1787.
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