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Procedía
Physics Procedía 8 (2010) 151-156
www.elsevier.com/locate/procedia
VI Encuentro Franco-Español de Química y Física del Estado Sólido VIème Rencontre Franco-Espagnole sur la Chimie et la Physique de l'État Solide
Monoclinic double tungstate lattice matched epitaxial layers for integrated optics applications
Western Bolaños*, Joan J. Carvajal, Maria Cinta Pujol, Xavier Mateos, Magdalena
Aguiló and Francesc Díaz
Física i Cristal-lografia de Materials i Nanomaterials (FiCMA-FiCNA), Universität Rovira i Virgili (URV), Campus Sescelades, c/Marcel-lí
Domingo s/n, 43007 Tarragona, Spain
Abstract
In this work we report on the activation of KY1-x-yGdxLuy(WO4)2 lattice matched epitaxial layers with Er3+ and Tm3+ ions. The epilayers posses low enough lattice mismatch as well as high enough refractive index contrast with respect to the KY(WO4)2 substrates, which are key parameters for the fabrication of high quality optical waveguides. A structural study revealed that there are two types of lattice matched KY1-x-yGdxLuy(WO4)2 compositions, one with a negative lattice mismatch, - 0.53 x 10-3, corresponding to KY0.58Gd0.22Lu0.20(WO4)2 , and the second one with a positive lattice mismatch, 0.55 x 10-3, which corresponds to KY075Gd018Lu0 07(WO4)2. However, the refractive indices corresponding to the first one were greater than those of the second composition. We then chose the composition with a negative lattice mismatch for doping with Er3+ and Tm3+ active ions, both with percentages of 1% at. and 3% at. Epitaxial growth of the active lattice matched layers on b oriented KY(WO4)2 substrates was obtained without loosing structural quality compared with the passive compositions. Waveguiding demonstration was observed by recording the dark mode spectra at different wavelengths.
© 2010 Published by Elsevier Ltd.
Kewywords: single crystal growth; epitaxial growth; dielectric waveguides, optical waveguides, planar waveguide lasers.
1. Introduction
Major advances in the field of integrated optics are based mainly in LiNbO3 and Si technologies. However, it has been demonstrated the potential of monoclinic potassium double tungstates, KRE(WO4)2 or shortened KREW (RE = Y, Gd, Lu), to be used in photonics as passive/active devices [1-3]. These demonstrations were related with the
* Corresponding author. Tel.: +34- 977-558-790; Fax: +34-977-559-563 Email address: westernlike.bolanos@urv.cat
1875-3892 © 2010 Published by Elsevier Ltd. doi:10.1016/j.phpro.2010.10.026
fabrication and characterization of waveguides which were obtained by liquid phase epitaxial growth of the active/passive layer on a substrate of the same family. Prior to the development of KREW waveguides, there has been an intensive study of the optical, thermal and mechanical properties of KREW as bulk (see for example [4-7]). KREW are attractive for active applications -lasing, mainly- because of its large values of absorption and emission cross sections (~ 10-20 cm2) for the active lanthanide ions (such as Yb3+, Tm3+, Er3+ and Ho3+) and also for their large ion to ion separation, which allows high doping levels of these active ions without reduction of the efficiency by quenching of luminescence.
Belonging to the C2/c space group, KREW's are monoclinic crystals (a ^ b ^ c; P > 90°). Their optical anisotropy is manifested by the presence of three main optical directions Ng, Nm and Np , mutually orthogonal. Ng and Nm optical directions lie in the a-c crystallographic plane whereas Np is parallel to the b crystallographic direction. Ng is located at an angle K respect to the c crystallographic axis. There are three different refractive indices, ng, nm, and np, measured along Ng, Nm and Np optical directions, respectively, with values ng > nm, > np. Among the family of stoichiometric monoclinic double tungstates, the KY(WO4)2 crystal presents the lowest refractive indices along the three main optical directions. In other words, if we want to fabricate a waveguide with these materials, we should set the KY(WO4)2 as substrate and the guiding layer should be a KY(WO4)2 thin film partially substituted with Gd and Lu, since the introduction of these ions into the KY(WO4)2 matrix, besides not introducing any transition involving absorption or emission processes in the entire transparency window of these materials, would lead to an increase in the refractive indices with respect to those of the substrate.
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: KY(WO4)2/ KY(WO4)2 and Tm:KY(WO4)2/KY(WO4)2 epitaxies [8,9].
More recently, Gardillou et. al. developed single mode channel waveguide and beam splitter structures using as confining layer the KLuo.253Gdo.i3Ybo.oi7Yo.6o(WO4)2 compound grown on KYW substrates [10], and demonstrated waveguiding of Yb3+ fluorescence on these structures. According to this work, right percentages of Gd, Lu and the active ions as well in the KYW matrix should be carefully chosen so that, in addition to the refractive index contrast enhancement, the lattice mismatch between the guiding layer and the substrate should be minimized in order to avoid cracks and other defects which could increase dramatically the scattering losses of the waveguide devices.
In previous work, we analyzed carefully the lattice parameters and refractive indices of several crystals of KYW with different levels of substitution of Y by Gd and Lu, and established the optimal content of Y, Gd, and Lu in a KYi-x-yGdxLuy(WO4)2 single crystal that theoretically exhibited the lowest lattice mismatch, f(oio) with respect to the b oriented KY(WO4)2 substrate while still showing a large enough refractive index contrast with KYW [3]. We found that KY059Gd021Lu020(WO4)2 presented a negative lattice mismatch of only -0.53 x 10-3, low enough to not generate problems during the crystal growth of epitaxial layers of this compound on KY(WO4)2 substrates that can later affect to the waveguiding properties we are interested in pursue for these thin films. However, it is also possible to obtain a composition with positive lattice mismatch and enough refractive index contrast to demonstrate guided light (depending also on the thickness of the guiding layer). In this work, we show how the Er3+ and Tm3+ doped lattice matched epitaxial layers of KY1-x-yGdxLuy(WO4)2 can be used as active waveguides for future integrated optics applications. We have interest in Er3+ and Tm3+ as active ions for doping the passive layer due to their attractive characteristics for integrated optical applications. On one hand, we are interested in the 1.5 цm transition from Er3+ which matches well with the third window defined for telecommunications. On the other hand, the emission at around 1.9 цm from Tm3+ has applications on optical amplification, surgery and remote sensing.
In this work we extended the range of Er3+ and Tm3+ concentrations in KYj-x-yGdxLuy(WO4)2 epitaxial layers grown on KY(WO4)2 substrates we studied in a previous work [11] in order to get efficient laser operation.
2. Experimental
Some small KYi-x-yGdxLuy(WO4)2 bulk 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 [6], with the aim to investigate the lattice parameters and refractive indices as a function of Y, Gd and Lu content. The molar composition of the solution for the growth of bulk crystals was composed by 12 mol% solute and 88 mol% solvent. The reagents used to prepare the solutions were powders of Y2O3, Gd2O3, Lu2O3, WO3, and K2CO3 with analytical grade of purity (99.99%) that were mixed in the appropriate amounts in cylindrical platinum crucibles. All the experiences were carried out in a vertical tubular furnace by applying a controlled cooling program of about 0.05 -0.10 K/h to the system solution-seed until a final temperature between 10 and 25 K below the saturation temperature, depending on the experiment
KY(WO4)2 bulk crystals with larger dimensions than those described previously were grown to be used as substrates for epitaxial layers. The crystals were grown in similar conditions to those described in the previous paragraph to a final temperature 25 K below the saturation temperature.
Er3+, Tm3+ and undoped lattice matched KYj-x-yGdxLuy(WO4)2 epitaxial layers were grown on b- oriented KY(WO4)2 substrates by Liquid Phase Epitaxial growth. In this case we prepared the solutions at 7 mol% solute and 93 mol% solvent (K2W2O7) since this proportion solute/solvent is suitable for a better control of the supersaturation of the solution [11]. Experimental details concerning the epitaxial growth of double tungstates can be found in [3,].
The chemical composition of the samples was determined by Electron Probe Microanalysis (EPMA) with a CAMECA SX 50 equipment. The measurements were performed at 20 mA of beam current and 20 kV of acceleration voltage. The measurements were integrated for 10 s for measuring oxygen, potassium, yttrium and tungsten, and for 30 s for measuring the gadolinium, lutetium, erbium and thulium.
The lattice parameters of undoped, Er and Tm doped KY1-x-yGdxLuy(WO4)2 bulk crystals were determined by X-ray powder diffraction using a Bruker- AXS D8-Discover diffractometer. Cu Ka radiation was used for these measurements. Data were collected in the range of 26 angles of 5-70°, with an angular step of 0.02° at 16 s per step.
The refractive indices of the crystals and the epitaxial layers were measured at different wavelengths by the prism film coupling technique using a METRICON 2010 system. TE polarization allowed the measurement of the refractive indices ng and nm refractive indices whilst TM polarization was used to determine the refractive index np. We also used the prism-film coupling system to observe the guided modes supported by our waveguides by recording the dark- mode spectra along Ng, Nm and Np principal optical directions.
3. Results and discussion
The obtained KYj-x-yGdxLuy(WO4)2 bulk single crystals show maximum dimensions of 11-12 mm along the c crystallographic direction, 6-7 mm along a* crystallographic direction 4-5 mm along b crystallographic direction, with crystal masses of about 1.7 g. Some small inclusions were observed in some KYj-x-yGdxLuy(WO4)2 crystals, but their quality was good enough to perform the characterizations. High quality KY(WO4)2 crystals free of defects were obtained by cooling the solution at 0.12 K/h. The crystal masses were about 11 g. The dimensions along c, a* and b crystallographic directions were 24 mm, 12 mm and 9.5 mm, respectively.
The lattice parameters of the crystals were refined from the X-ray powder diffraction data with the Fullprof software based on the Rietveld method. 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 KY(WO4)2 substrate. The lattice mismatch on the (010) face between a hypothetical epilayer and the KY(WO4)2 substrate was calculated following the expression:
f(oio) = (Si(oio) - Ss(oio))/Ss(0l0),
where Ssfoio) and Sl(oio) are the areas defined by the periodicity vectors of the substrate and the layer, respectively. Figure 1 shows the calculated lattice mismatch as a function of Gd and Lu content.
Figure 1. Lattice mismatch of (010) face for KY1.x.yGdxLuy(WO4)2 crystals.
For all crystals investigated here, the calculated lattice mismatch was around 10-3. The crystals with compositions KY058Gd022Lu020(WO4)2 and KY0.75Gd0.j8Lu0.07(WO4)2. have lattice parameters values close to those of KY(WO4)2, therefore, they have the smallest lattice mismatches values, -0.53x10"3 and 0.55x10"3, respectively, and are marked in red in Figure 1. However the refractive indices of the KY0.58Gd0.22Lu0.20(W O4)2 crystal, with a negative lattice mismatch, were greater than the refractive indices corresponding to the KY075Gd0.i8Lu007(WO4)2 crystal with a positive lattice mismatch.
The refractive indices of each undoped KY1-x-yGdxLuy(WO4)2 bulk crystals were measured along the three principal optical directions by the prism-film coupling technique. Figure 2 shows the variation of the refractive index nm associated to the Nm optical direction measured in these crystals with respect to those of KY(WO4)2. In the figure we marked in red the variation of the refractive index corresponding to the two crystals that show the smallest lattice mismatch with the KY(WO4)2 substrate.
As can be seen in figure 1, the refractive index contrasts with respect to the KY(WO4)2 for the KY058Gd022Lu020(WO4)2 composition (f < 0) were greater than that of the KY0 75Gd0.i8Lu007(WO4)2 composition (f
> 0). This difference is due to the lutetium content in the f < 0 composition that is almost three times the content in f
> 0 composition, which induces an increase of the refractive indices for this crystal.
In spite of such a difference in refractive index contrast between theoretically positive and negative lattice mismatched epitaxial layers, we grew a couple of epitaxial layers with both compositions, under the same experimental conditions, over b- oriented KY(WO4)2 substrates, in order to determine in practice which of these compositions has a better quality. The as-grown epitaxial layers were crack and macroscopic-defect free in both cases. No significant qualitative differences were observed between both compositions. However, we chose the KY0.58Gd0.22Lu0.20(WO4)2 (f < 0) composition for the fabrication of active and passive waveguides since it had the highest refractive index values (with respect to the KY(WO4)2 substrate).
The active and passive films were grown by LPE over KY(WO4)2 substrates by doping the lattice matched film with Er3+ and Tm3+ ions, both with percentages of 1% and 3% without loosing their structural and crystalline quality. Their chemical composition was measured again by EPMA.
In order to determine the refractive index and dark mode spectra, we polished down the epitaxial layers to a thickness of 10 цm.. Table 1 summarizes the results corresponding to the refractive index measurements of these epitaxial layers measured at two different wavelengths.
Figure 2. Refractive index nm of KYj.x.yGdxLuy(WO4)2 crystals measured at X = 632.8 nm.
Table 1. Refractive indices of the substrate, passive and active films measured at 632.8 nm and 1523 nm
X = 632.8 nm X = 1523 nm
Material ng nm np ng nm np
KY(WO4)2 (substrate) 2.o853 2.o4o5 1.9976 2.o384 1.9965 1.9587
KYo.59Gdo.l9Luo.22(WO4)2 2.o9o4 2.o446 2.oo57 2.o389 1.9987 1 .96o2
KYo.6oGdo.i8Luo.2iEro.oi(WO4)2 2.o899 2.o446 2.oo57 2.o391 1.9987 1.961o
KYo.58Gdo.l9Luo.2oEro.o3(WO4)2 2.o911 2.o45o 2.oo63 2.o439 2.oo23 1.9659
KYo.59Gdo.i8Luo.22Tmo.oi(WO4)2 2.o9o4 2.o447 2.oo57 2.o393 1.9987 1.961o
KYo.58Gdo.22Luo.„Tmo.o3(WO4)2 2.o9io 2.o445 2.oo57 2.o437 2.oo21 1.9662
As expected, the higher the active ion doping, the higher the refractive index contrast, An, between the substrate and the epitaxial layer. The greatest An's (calculated as Ani = nif - nis, where i = g, m, p; f and p refer to film and substrate, respectively) corresponded to KYo.58Gdo.i9Luo.2oEro.o3(WO4)2 epilayer, where we found, for instance, Ang = 5.5 x 10-3; Anm = 5.8 x 10-3 and Anp = 7.2 x 10- at X = 1523 nm. These refractive index contrasts enabled light to be confined into the epitaxial layer, so in this way, we have fabricated dielectric waveguides based on monoclinic double tungstates.
As evidence of waveguiding, the dark mode spectra reported in figure 3 shows the guided modes supported by our waveguides.
2.06 2.05 204 2.03 2.02 2.06 2.0S 204 2.03 2.02
Effective index Effective index
(a) (b)
Figure 3. Visualization of the guided modes supported by the waveguides along Nm optical direction. Typical dark mode spectra (a) and (b) corresponds to KYo.58Gdo.19Luo.2oEroo3(WO4)2 whilst (c) and (d) corresponds to KYo.58Gdo.22Luo.17Tmoo3(WO4)2. In both cases guided modes are marked with lines beneath the corresponding minima.
The number of TE guided modes supported by the KY0.58Gd0.i9Lu0.20Er0.03(WO4)2 waveguide at X = 632.8 nm decreases from four to only two when the wavelength increases up to 1523 nm. Similar behaviour of the KY0.58Gd0.22Lu0.i7Tm003(WO4)2 waveguides were observed since its refractive index contrasts (Ang = 5.3 x 10-3; Anm = 5.6 x 10-3 and Anp = 7.5 x 10- at X = 1523 nm) were of the same order as those of KYo.58Gdo.19Luo.2oEro.o3(WO4)2
4. Conclusions
We have successfully grown passive and active epitaxial layers of KY1-x.yGdxLuy(WO4)2 doped with Er3+ and Tm3+ keeping a low lattice mismatch and a high refractive index contrast with the KY(WO4)2 substrate. The compositions of our lattice matched layers were KYo.6oGdo18Luo.21Eroo1(WO4)2, KYo.58Gdo.19Luo.2oEroo3(WO4)2, KYo.59Gdo18Luo.22Tmoo1(WO4)2 and KYo.58Gdo.22Luo17Tmo.o3(WO4)2. The ion concentrations were 7.8 x 1o19, 2.2 x 1o2°, 6.4 x 1o19 and 1.7 x 1o2°, respectively. We believe these epitaxial layers have a suitable active ion concentration high enough to allow efficient waveguide laser generation.
Acknowledgments
This work was supported by the Spanish Government under projects MAT 2008-06729-C02-02/NAN and PI09/90527 and the Catalan Government under project 2009SGR235.W. Bolaños thanks also the Catalan Government for the funds provided through the fellowship 2009FI_B 00626. J.J. Carvajal is supported by the by the Research and Innovation Ministry of Spain and European Social Fund under the Ramón y Cajal program, RYC2006-858.
5. References
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