ARTICLE pubs.acs.org/crystal
Epitaxial Growth of NaGd0.935Yb0.065(WO4)2 Layers on Lattice Matched Tetragonal Double Tungstate Substrates for Ultrafast Thin Disk Lasers Jose M. Cano-Torres, Xiumei Han, Fatima Esteban-Betegon, Ana Ruiz, M. Dolores Serrano, Concepcion Cascales, and Carlos Zaldo* Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, c/Sor Juana Ines de la Cruz 3, 28049 Madrid, Spain ABSTRACT: Liquid-phase epitaxial growth of Yb3þ doped NaGd(WO4)2 crystalline layers has been obtained on Y modified NaGd(WO4)2 Czochralski grown single crystal substrates. It is shown that the lattice parameter of the layer and substrate can be equalized by proper selection of the Yb and Y compositions. The epitaxial layers obtained are highly transparent, with thicknesses in the range 1070 μm, and with crystallographic quality close to that of the used substrates. The spectroscopic properties of Yb3þ in the layer are anisotropic and similar to those of Yb-doped NaGd(WO4)2 single crystals. The layers are envisaged as optical gain media for the design of ultrafast modelocked thin disk lasers.
1. INTRODUCTION Sodium-based double tungstate (DW) single crystals with tetragonal (I4) structure and nominal formula NaT3þ(WO4)2 (T3þ = Y3þ or trivalent lanthanides) have shown remarkable capabilities as optical gain media for mode-locked ultrafast (fs) lasers. Yb:NaY(WO4)2 crystal supported 67 fs laser pulses at 1035 nm directly from the optical resonator, these pulses were further compressed to 53 fs pulses with an average power of 91 mW at 96 MHz of repetition rate.1 To the best of our knowledge, shorter laser pulses have been demonstrated only in Yb:Ca4YO(BO3)3 and Yb:CaGdAlO4 crystals with 42 fs2 and 47 fs,3 respectively. In all these crystals Yb3þ exhibits broad optical absorption and emission bands due to the presence of shared crystallographic sites for Naþ/Ca2þ and the trivalent cations. Such crystals are often termed “disordered” in contrast to “ordered” crystals with an unique kind of crystallographic environment for the trivalent lasant ion. The relevance of this type of crystallographic disorder in the operation of mode-locked ultrafast laser is out of doubt from the recent comparison of the performance of Tm:Ho:KY(WO4)2 ordered crystal and that obtained in Tm:Ho:NaY(WO4)2 disordered crystal using the same experimental setup. While the ordered crystal supported 570 fs Fourier transform limited pulses,4 in the disordered one the pulse duration limit was reduced to 191 fs.5 The power operation of these laser crystals is limited by the moderate thermal conductivity of corresponding oxides. To enhance power operation, free-standing thin disks are used in “thin disk” laser designs.6 Alternatively, layers of laser active media supported on a substrate and grown by liquid phase epitaxy (LPE) are also considered for such type of lasers.7 The combination of thin disk laser designs with ultrafast mode-locked laser operation r 2011 American Chemical Society
requires the epitaxial growth of disordered crystals. Several disordered crystal structures can be selected for this purpose, namely CaLa4(SiO4)3O (mp ≈ 2170 °C),8 CaGdAlO4 (mp ≈ 1700 °C),3 Ca4YO(BO3)3 (mp ≈ 1480 °C),2 or the presently considered NaT3þ(WO4)2 family, among others. The advantages of the latter family of crystals include the lower melting temperature (1200 °C for NaY(WO4)2, 1250 °C for NaLa(WO4)2, and 1260 °C for NaGd(WO4)2); the already developed technology to dissolve tungstate compounds in Na2W2O7 or Na2WO4 autofluxes; their high miscibility, which allows us to tailor the physical properties; and the tetragonal crystal structure, which confers uniaxial character to the optical properties and therefore simplifies crystal orientation procedures. Optical applications require a high crystal quality and transparency. Therefore, the density of light scattering centers in the optical layer must be minimized. Such centers are often associated with stress induced upon after-growth cooling because of differences in unit-cell lattice parameters and thermal expansion coefficients of the layer and substrate materials. Doping a crystal with a lasant ion up to several atomic percents leads to lattice modifications that may induce such defects. The purpose of the present work is to study strategies to compensate these crystallographic modifications in DW crystals and to determine a viable layer/ substrate combination for the development of Yb-doped layers. In this respect, it must be considered that NaT(WO4)2 compounds with T = Y, La, and Gd are transparent crystals that melt congruently and therefore they can be grown by the Received: December 29, 2010 Revised: February 20, 2011 Published: March 09, 2011 1807
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Crystal Growth & Design Czochralski (Cz) method. These crystals are used as passive media to host the active laser ion, Yb3þ. Contrary, NaLu(WO4)2, another optically passive bulk crystal, melts with decomposition, therefore its growth is only possible by the top seeded solution growth (TSSG) method using a flux. Here, we shall limit to substrates grown by the Cz method, which eases the substrate preparation to a large extent and favors the technological applications.
2. EXPERIMENTAL DETAILS 2.1. Growth Procedures. Single crystals of NaT(WO4)2, T = La, Gd, or Y, were grown in a Cz growth system with automatic control of the crystal diameter by weighing the melt. Na2CO3 (99.5% Alfa Aesar), WO3 (99.8% Aldrich) and T2O3 (T = La, Gd or Yb, 99.99%, acquired through Shanghai Zimei International Co LTD) products were mixed in stoichiometric compositions in a 75 mL Pt crucible. The DW phase was first synthesized by heating to 750 °C for 18 h, grounding the products and by a second heating cycle at 850 °C for 24 h. The phase purity was assessed by powder X-ray diffraction (XRD) analyses. Crystals for substrates were pulled using [100] oriented crystal seeds of the corresponding compounds. Typically, the pulling and rotation rates were 12 mm/h and 515 rpm, respectively. After growth the crystals were cooled to room temperature at 10 °C/h. The crystals obtained were pseudocylinders with ∼25 mm of diameter, ∼50 mm length and two well-developed (001) faces in the laterals. Substrates with typical dimensions of ∼20 10 2 mm3 (length width thickness) were sliced from these crystals. Two substrate orientations were selected, a-cut substrates to grow on the (100) crystal plane and c-cut substrates to grow on the (001) crystal plane. The melting temperature of the compounds to be grown and the compound/flux mixtures for LPE processes were monitored by differential scanning calorimetry (DSC) by using a Setaram Setsys Evolution 1700 equipment. Typically, heating/cooling cycles were made in 99.99% Ar atmosphere at a rate of 10 °C/h. Pt crucibles with a lid were used to hold 4060 mg of sample. LPE experiments were conducted in vertical tubular furnaces with SiC resistive heating elements. The target Yb composition of the layer was selected as 10 at % because previous experiences in bulk crystals show that this is an optimal concentration for laser operation.1,9 Ybdoped NaT(WO4)2 compounds used as solute were mixed with Na2W2O7 flux in 75 mL Pt crucibles. The thermal gradient in the liquid phase from just below the melt surface to the crucible bottom was 0.3 °C/cm. The solute/flux composition ratio was selected accordingly to the saturation curves described later. The melt saturation was controlled by monitoring the growth/dissolution of a seed in contact with the melt. The saturation temperature (Ts) is considered as the temperature for which the seed is in thermodynamic equilibrium with the melt. To proceed with the growth of the doped layer the substrate was vertically introduced in the furnace at room temperature and set 5 mm above the surface of the compound/flux mixture. The furnace was heated up to 20 °C above the melting temperature of the mixture and hold at this temperature during 4 h for melt homogenization. Afterward, the temperature was lowered to reach Ts, the substrate height was set only 1 mm above the melt surface, and this temperature was held for 1 h more. Finally, the substrate was immersed in the melt at a rate of 0.1 mm/min. Melt supersaturation was induced by decreasing the melt temperature at a given rate, typically 0.5 °C/h, while the substrate rotates at 1040 rpm. After the layer growth, the substrate was pulled out of the melt and the furnace was cooled at a 5 °C/h rate during the first 100 °C and later at 10 °C/h until room temperature. Substrates and grown layers were polished with a Logitech LP50 equipment using alumina powders. We used the part of the substrate that was not immersed in the melt as reference to completely remove the grown layer from one face. Later, this polished face was used as reference to polish the layer grown on the other side until a uniform transparency
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of the layer was obtained. Face parallelism during polishing was controlled by using an optical autocollimator. 2.2. Structural and Morphological Characterizations. The determination of the room temperature lattice parameters of the grown single crystals was made by Rietveld analyses10 of the XRD scans obtained using powdered crystals. Specific compositions of polycrystalline materials synthesized by solid state reaction have been also analyzed by the same procedure. For this purpose XRD θθ scans were acquired in a Bruker D8 Advance diffractometer working in BraggBrentano geometry with a Cu (KR1, λ= 1.540560 Å) anode and a Lynxeye X-ray fast detector. The scanning range was 2θ = 1090° with steps of 0.04° and a counting time of 2 s for each step. Using the same above XRD methods, thermal expansion coefficients (R) were calculated from the evolution of the crystal lattice parameters with temperature (25700 °C). In this case XRD data were collected using a Panalytical X’Pert PRO MPD diffractometer system, with a PW3050/60 goniometer in θθ scan configuration and a X’Celerator detector, equipped with an Anton Paar HTK-1200 high temperature chamber. The samples were disposed in alumina holder disks. The temperature range analyzed is large enough to adequately describe the behavior of crystals under cooling from the growth temperature. The layer crystalline microstructure was studied by rocking scans in a Bruker D8 four circles texture diffractometer also equipped with similar Cu anode. 200 and 004 reflections were selected to characterize layers on a-cut and c-cut substrates, respectively. After polishing, the layers were examined with a Sensofar PLμ 2300 interferometric microscope. Extended scans of the layer and the substrate allowed to determine the layer boundaries, the layer thickness and the surface flatness of the substrate and layer. 2.3. Spectroscopic Studies. Spectroscopic characterization of the Yb3þ was performed at room temperature. Optical absorption spectra were collected with a Cary 5E spectrophotometer. Photoluminescence was excited with a Quanta Ray MOPO-HF laser system at λ= 940 nm and the λ ≈ 1.03 μm luminescence was dispersed in a SPEX 340 E (f = 34 cm) Cerny-Turner spectrometer incorporating a 600 L/mm holographic grating. The light intensity was detected with a Hamamatsu H917075 photomultiplier and recorded with a lock-in amplifier. Lifetime measurements were excited with the same MOPO system delivering pulses shorter than 5 ns and the photomultiplier signal recorded by a Tektronix TDS520 (500 MHz) oscilloscope.
3. EXPERIMENTAL RESULTS 3.1. Design and Growth of Substrates. A first consideration in the design of substrates for epitaxial growth concerns crystal lattice parameters. Values reported for undoped NaT(WO4)2 crystals have shown that the a and c lattice parameters depend on the ionic radius of the T3þ cation, being a = 5.3575(7) Å and c = 11.671(2) Å for NaLa(WO4)2,11 a = 5.2440(5) Å and c = 11.3794(14) Å for NaGd(WO4)2,9 and a = 5.2014(4) Å and c = 11.2740(12) Å for NaY(WO4)2,12 i.e., the lattice parameters decrease with smaller ionic radius of the eightcoordinated T ion, namely 1.16 Å for La3þ, 1.053 Å for Gd3þ, and 1.019 Å for Y3þ.13 Doping with Yb3þ, having one of the smallest ionic radius, 0.985 Å, also induces a reduction of the lattice parameters of NaGd1xYbx(WO4)2 crystals,9 toward those of NaYb(WO4)2 single crystal, a = 5.18 Å and c = 11.19 Å.14 We propose that this lattice shrinkage induced in the layer can be induced also in the substrate by using NaT1yT0 y(WO4)2 compositions where passive T0 ion has smaller ionic radius than T. The point is how to relate the layer (x) and substrate (y) compositions. To solve this, we have synthesized several NaLaT0 (WO4)2, T0 = Gd, Y, and Yb, and NaGdT0 (WO4)2, T0 = Y and Yb, compositions and we have determined their crystal lattice parameters. Figure 1 shows the results obtained. Within the uncertainty of the data, the lattice parameters of a NaTT0 (WO4)2 compounds follow a linear dependence between 1808
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Figure 1. Evolution of the lattice parameters of NaTT0 (WO4)2 compositions. (a) a-parameter. (b) c-parameter. The symbols are the experimental results for NaT(WO4)2 compositions with T = La (squares), Gd (triangles), Y (circles) and Yb (stars) and for mixed TT0 compositions with T0 = Gd (red symbols), Y (green symbols) and Yb (blue symbols) and T with the above-defined symbol shapes. The lines show the linear dependences.
Table 1. Lattice Mismatch between NaT(WO4)2 and NaT0 (WO4)2 Compositions T0 T
ΔpTT0 (Å)
Gd
Y
Yb
La
Δa
0.1135
0.1561
0.1775
Δc
0.2916
0.3970
0.4810
Δa
0.0426
0.0640
Δc
0.1054
0.1894
Gd
those of NaT(WO4)2 and NaT0 (WO4)2 compounds. The small departures of the experimental results with respect to this linear law is most likely due to the change of the actual crystal composition with regards to the nominal one, note the DW crystals grown by the Cz method are Na deficient, with an excess of T ions and that the segregation coefficient of Yb is lower than 1.9,12 Figure 1 provides a graphical method to determine lattice matched compositions between the NaT1xYbx(WO4)2 optically active layer and the NaT1yT0 y(WO4)2 passive substrate. Once the desired Yb composition of the layer is known, the T0 composition of the lattice matched substrate can be determined as scheduled in Figure 1. More precisely, the evolution of the lattice parameter (p) of NaT1yT0 y(WO4)2 substrates can be written as pTT0 = ΔpTT0 y þ pT, where ΔpTT0 = pT pT0 is the difference of lattice parameters of the pure compounds assumed that the radius of T3þ is larger than the radius of T0 3þ; therefore, in our case the substrate composition can be obtained as y = (ΔpTYb/ΔpTT0 )x. Table 1 summarizes the ΔpTT0 values for several crystal compositions. For instance, for a NaLa0.9Yb0.1(WO4)2 layer the substrate target compositions to match the a parameter could be either NaLa0.844Gd0.156(WO4)2 or NaLa0.886Y0.114(WO4)2. Similarly, for a NaGd0.9Yb0.1(WO4)2
layer, the substrate composition with lattice matched a parameter is NaGd0.83Y0.15(WO4)2 . It is worth remarking that for c-cut substrates only the a parameters of the layer and the substrate need to be matched, but for a-cut substrates, generally a and c parameters can not be matched simultaneously for given layer and substrate compositions, compare panels a and b in Figure 1. A second consideration for the application of the epitaxies as thin disk lasers is that the refractive index difference between the layer and the substrate must be as close as possible to zero in order to do not modify the beam propagation. From this point of view the NaGd1xYbx(WO4)2 / NaGd1yYy(WO4)2 system seems most favorable than those produced using La because the refractive indexes of NaGd(WO4)29 and NaY(WO4)21 vary by less than 0.005 and the Yb doping does not modify these indexes significantly.9 By the contrary, the difference between the indexes of NaLa(WO4)215and any one of the two above-mentioned crystals is about 0.020.03. For this reason, Gd-based DW is in principle preferred for our epitaxial growth purposes. To produce substrates lattice-matched to NaT0.9Yb0.1(WO4)2 layers, we have grown by the Cz method the Y-modified transparent substrates summarized in Table 2. The NaLa0.88Y0.12(WO4)2 crystals obtained exhibited a relatively large density of macroscopic defects, mainly microbubbles, nevertheless several substrates were sliced in order to study the epitaxy on this crystal. On the other hand, NaGd0.88Y0.12(WO4)2 crystals were free of defects and provided substrates more suitable for the epitaxial growth. The composition of these latter crystals was analyzed by wavelength dispersive X-ray fluorescence technique accordingly to procedures already described.16 The actual composition of the crystal was Na0.937Gd0.923Y0.105(W0.991O4)2, i.e., despite of the addition of 1 wt % Na2W2O7 to the melt, its volatilization induces a Naþ deficiency in the crystal, which is compensated by an extra incorporation of Gd and Y in the crystal sites shared with Na, and charge compensation is obtained by a slight deficiency of W6þ. The lattice parameters of this crystal substrate are included in Table 2. 3.2. LPE of Yb-Doped Layers. Solubility curves of NaLa0.90Yb0.10(WO4)2 and NaGd0.90Yb0.10(WO4)2 in the Na2W2O7 flux were determined as a preliminary step for the LPE growth. For this purpose the solidification temperature upon cooling of the melted solute/flux mixture was determined by DSC. Figure 2 shows the results obtained. The slope of these curves increases as the solute concentration decreases. We selected a solute/flux composition corresponding to a high slope in order to ensure that the temperature fluctuations during growth lead to small melt supersaturation changes. This condition is fulfilled when the solute composition is about 58 mol %. 3.2.1. NaLa0.90Yb0.10(WO4)2/NaLa0.88Y0.12(WO4)2. LPE growth of 10 at % Yb-doped NaLa(WO4)2 on NaLa0.88Y0.12(WO4)2 a-cut and c-cut crystal substrates was made using a 4.83/95.17 molar ratio between the NaLa0.90Yb0.10(WO4)2 solute and Na2W2O7 solvent. The nominal saturation temperature (Ts) was determined in the growth system by a thermocouple close to the heating element of the furnace (that is the reason of the different results shown in Figure 2a). Ts was found at 910 °C; above this temperature, the substrate dissolves in the melt. Growth was observed by decreasing 1 °C over a period of 2 h with a substrate rotation rate of 20 rpm. Polishing of these layers showed that the whole layer was polycrystalline independently of the substrate orientation used. One possible reason for that could be the low quality of the substrates available, but also the matching of the thermal expansion coefficients must be considered. Thermal expansion coefficients can be found in the literature for NaT(WO4)2, T = Y9 and Gd,17 but to the best of our knowledge similar experiments have not been reported for NaLa(WO4)2 single crystal. We have calculated the thermal expansion coefficients of NaLa(WO4)2 from the evolution of crystal lattice parameters with temperature as R= Δp(T)/pRTΔT, where pRT is the lattice parameter at room 1809
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Table 2. Details of the Growth of Substrate Crystalsa 1
2
3
4
5
6
7
NaLa0.88Y0.12W
NaLaW[001]
0
1227.7
10
1.8
10
NaGd0.88Y0.12W
NaGdW[100]
1
1233.5
15
1.5
10
NaGd0.88Y0.12W
NaGdW[100]
1
1230.5
8
1.2
7
8
9
10
5.3402(5)
11.627(1)
Na0.937Gd0.923Y0.105W0.991
5.2401(3)
11.3675(8)
(1) Nominal cationic composition. (2) Used seed. (3) Na2W2O7 flux amount in wt% added to the melt. (4) Crystallization temperature in °C. (5) Seed rotation rate in rpm. (6) Crystal pulling rate in mm/h. (7) Crystal cooling rate in °C/h. (8) Measured cationic crystal composition. (9) a lattice parameter in Å. (10) c lattice parameter in Å. a
Figure 2. (a) Solubility curves of NaLa0.90Yb0.10(WO4)2 and (b) NaGd0.90Yb0.10(WO4)2 in Na2W2O7. The lines are a visual help. temperature, Δp(T) = p(T) pRT, the parameter increment for a given temperature with regards to room temperature and ΔT the temperature change with regards to room temperature. Figure 3 shows the results obtained and Table 3 compares the results for the three crystal hosts of interest. In NaLa(WO4)2 a discontinuity of the thermal properties is observed around 450 °C, the thermal expansion coefficients as well as their ratio R3/R1 suddenly increase for higher temperatures. This feature was not observed in the evolution of the thermal expansion of NaY(WO4)2 or NaGd(WO4)2, so it seems likely to us that the transition upon cooling of this thermal discontinuity may introduce significant stress in the layer and eventually produce a severe fracture at the microscopic level. To this process may also contribute the fact that the R3 expansion coefficient of NaLa(WO4)2 crystal is significantly larger than in the other DW crystals and the larger R3/R1 ratio represents larger anisotropy. Therefore, not further efforts were made with this system. 3.2.2. NaGd0.92Yb0.08(WO4)2/Na0.937Gd0.923Y0.105(W0.991O4)2. It was shown in Table 2 that the actual composition (10.5 at % Y) of the Gd/Ybased substrates available was lower that the nominal (12 at % Y). The corresponding solute composition for a good lattice match of the a layer and substrate parameters is 6.6 at % Yb; however, as the distribution coefficient of Yb in NaGd(WO4)2 substrate is 0.8,9 the Yb composition of the solute used for growing was selected as 8 at % Yb. Growth experiences were conducted on a-cut and c-cut Na0.937Gd0.923Y0.105(W0.991O4)2 substrates. The solute/solvent molar ratio was 4.72/95.28 and in all cases the substrate was rotated at 10 rpm
Figure 3. Evolution of the thermal expansion coefficients of NaLa(WO4)2 crystal. (a) R1 (9) and R3 (b) coefficients. (b) Ratio R3/ R1 (().
Table 3. Thermal Expansion Coefficients (r) of NaT(WO4)2 Crystals NaLa(WO4)2 6
R1 ( 10
a
1
°C )
a
NaGd(WO4)2
NaY(WO4)2
8.3, 9.1
7.89
8.4 (2)
R3 ( 106 °C1) R3/R1
19.5, 23.9a 2.35, 2.63
16.0 2.03
18.5(4) 2.2
ref
this work
9
17
High-temperature regimen.
during the growth and at 50 rpm during substrate pulling out of the melt. Table 4 summarizes the results obtained. The nominal saturation temperature for the growth initially was found as Ts= 921 °C, but it increased as different layers were sequentially grown from the same melt. We attribute this to the volatility of the flux as bronze phases resulting of the intercalation of Naþ in WO3, NaxWO3, x < 1, and the subsequent enrichment of the melt in solute. After the introduction of the substrate into the melt, the furnace temperature was reduced by 45 °C and the layer growth proceeded at constant temperature for a period between 0.5 and 2 h. Trials to further decrease the furnace temperature during the growth period produced polycrystalline layers. 1810
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Table 4. LPE Experiences of NaGd0.92Yb0.08(WO4)2 Layers on Na0.937Gd0.923Y0.105(W0.991O4)2 Substratesa
a
experiment order
substrate orientation
Tc (° C)
1
a-cut
916.8
1
transparent
2
a-cut
917
1
transparent
3
a-cut
917.5
1.5
transparent, with fractures
4
c-cut
917.5
1
rough layer surface and partial transparency
5
c-cut
919.5
0.5
rough layer surface but transparent
6
c-cut
921
1
rough layer surface and substrate fractures and slight dissolution
7
c-cut
924
2
very rough layer surface and large substrate fractures and dissolution
tc (h)
comments
Tc is the furnace temperature used for the layer growth. tc is the layer growth duration.
Figure 4. NaGd0.92Yb0.08(WO4)2 epitaxial layers (l ) grown on (a, c) acut or with (b, d) c-cut Na0.937Gd0.923Y0.105(W0.991O4)2 substrates (s). (a, b) Before and (c, d) after polishing. The squares in the background papers are 1 1 mm2. As-grown layers on a-cut substrates show good transparency although the surface is rough, see Figure 4a. These samples show very good transparency once the first few micrometers are removed by mechanical polishing, see Figure 4c. Most likely, this rough aspect is due to the fast solidification of the liquid adhered to the surface during pulling out of the melt, leading to a thin amorphous layer. The growth on c-cut substrates produces layers with larger surface roughness, see Figure 4b, and they require the removal of more layer material to reach transparency, see Figure 4d. For growth temperatures above 921 °C in experiments 6 and 7 of Table 4, layers with very rough surfaces were obtained and the edges of the substrate were dissolved and even cracked.
3.3. Morphological and Structural Layer Characterization. Layers with NaGd0.92Yb0.08(WO4)2 nominal composition grown on acut Na0.937Gd0.923Y0.105(W0.991O4)2 substrates have a transparent thickness of 70 ( 10 μm for the growth times described in experiences 13 of Table 4. Although the substrates remain free of defects after cooling and polishing, the layers are prone to cleavage along (001) planes, see Figure 4c. This behavior is likely related to the layer stress induced by the different thermal expansion coefficients of the a and c
Figure 5. Comparison of the rocking curves of NaGd0.92Yb0.08(WO4)2 layers grown on Na0.937Gd0.923Y0.105(W0.991O4)2 substrates (lines) with those of the single-crystal substrate (points) for the rocking directions shown in the insets. For each rocking direction, the full ω width at halfmaximum, Δω, are given. (a) c-cut substrates. 004 X-ray reflection, ωca1 (dashed line, Δca1), ωca2 (continuous line, Δca2) for layer rocking and ωca (points, Δs) for substrate rocking. (b) a-cut substrates, 200 X-ray reflection, ωaa (dashed line, Δaa1), ωac (continuous line, Δac) for layer rocking, and ωaa (points, Δs) for substrate rocking.
directions coexisting in the layer/substrate interface. Note that the thermal expansion coefficient of these type of tetragonal double tungstates along the c axis is about twice than that observed along the a axis, see Table 3. Similar layers grown on c-cut substrates have a thinner transparent region, typically only the first 10 μm are transparent, but once polished these layers are free of cleavage. Note that in the latter configuration the substrate/layer interface only contains a axes, therefore the in-plane strain is isotropic, while the strain of the c axis occurs freely perpendicular to the substrate surface. The crystalline quality of the layers was studied by the rocking curves of the 200 and 004 reflections of layers on a-cut and c-cut substrates, respectively. The same substrate reflections were used for reference. Figure 5 shows the results obtained. The samples were rocked in two orthogonal planes as sketched in the insets of Figure 5. From the structural point of view the layer remaining after polishing is single crystalline. In c-cut substrates the full widths at half-maximum (Δω) of the ω scans in the layers are about double than those observed for the substrates (Δω = 0.0600.056° for layers and Δs = 0.025° for crystalline substrates) but still quite low in comparison to polycrystalline materials. 1811
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Figure 6. Comparison of the 300 K polarized 2F7/2f2F5/2 Yb3þ optical absorption (lines) of a NaGd0.935Yb0.065(WO4)2 epitaxial layer grown on an a-cut Na0.937Gd0.923Y0.105(W0.991O4)2 substrate with the absorption cross-section (σA) of Yb3þ in NaGd(WO4)2 (points).
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layer composition is NaLa0.935Yb0.065(WO4)2 which is very close to the target composition (6.6 at% Yb) pursued for perfect lattice match of the a parameter on the designed Na0.937Gd0.923Y0.105(W0.991O4)2 substrate. It is also remarkable that the absorption spectra of the Yb3þ in the layer is anisotropic as expected for an epitaxial growth. Figure 7 shows the radiative properties of the thickest (70 μm) layer available. The Yb-doped layers show photoluminescence similar to those of single crystals with equivalent Yb composition but the spectral features are less defined as corresponding to the lower crystalline quality already mentioned. The emission obtained for similar layers grown on c-cut substrates was basically equivalent, but the absolute intensity was significantly lower due to the smaller thickness, i.e., 10 μm. The 300 K lifetime of the 2F5/2 Yb3þ multiplet has been measured for layers grown on a-cut and c-cut substrates. The excitation was made at 940 nm and the emission was collected at 1010 nm. The recorded light intensities are in all cases close to single exponential decays and the decay constants determined were 410 and 332 μs for the layers on a-cut (70 μm) and c-cut (10 μm) substrates, respectively. As reference, the Yb3þ radiative lifetime of Yb in NaGd(WO4)2 is 320 μs.18 It must be concluded that measurement of the thicker sample includes some emission reabsorption, whereas for the thin sample, the Yb photoluminescence efficiency is little degraded by this phenomena.
4. CONCLUSIONS The crystallographic lattice parameters of tetragonal NaT1-yT0 y(WO4)2 crystals depend linearly with y composition, this fact allows to compensate changes induced by T0 = Yb doping of the active laser layer through the modification of the substrate composition with the proper amount of a passive ion, either T0 = Gd or Y. This principle has been demonstrated by Liquid Phase Epitaxial growth of NaGd0.935Yb0.065(WO4)2 layers on Na0.937Gd0.923Y0.105(W0.991O4)2 Czochralski-grown substrates. Layers grown with high crystalline quality reproduce the anisotropy of the physical properties of the bulk crystals. The crystalline layers grown on a-cut substrates are prone to cleave along (001) planes, this cleavage is avoided by growing on c-cut substrates although the thickness of the so far achieved epitaxial layer is smaller than in the previous case. The spectroscopic properties of Yb3þ in these layers mirror those observed in bulk single crystal, therefore these layers are promising as active media for thin disk lasers and as optical waveguide lasers. In the latter case after properly engineering of the refractive indexes. ’ AUTHOR INFORMATION
Figure 7. 300 K unpolarized 2F5/2f2F7/2 photoluminescence of the NaGd0.935Yb0.065(WO4)2 epitaxial layer grown on an a-cut Na0.937Gd0.923Y0.105(W0.991O4)2 substrate (line) compared to the photoluminescence of 1 at % Yb-doped NaGd(WO4)2 single crystal (points). Layers on a-cut substrates have a remarkable Δω difference for the ω scans in the ac and aa directions, the former (Δac = 0.082°) is about twice larger than the latter (Δaa = 0.041°). This difference is ascribed to the cleavage in the c plane. It is worth to note that the Δaa of the latter layer is basically equal to that of the substrate for the same rocking direction (Δs = 0.045°).
3.4. Optical and Spectroscopic Layer Characterization. Figure 6 shows the 300 K optical absorption of the layer with 70 μm of thickness in comparison to the absorption cross section of Yb3þ in NaGd(WO4)2.9 From this comparison the Yb concentration in the layer results [Yb] = 4.20 1020 at/cm3, i.e., 6.5 at %. Therefore, the grown
Corresponding Author
*Tel: (34) 913349057. Fax: (34) 913720623. E-mail: cezaldo@ icmm.csic.es.
’ ACKNOWLEDGMENT This work was supported by Project MAT2008-06729C02-01. ’ REFERENCES (1) García-Cortes, A.; Cano-Torres, J. M.; Serrano, M. D.; Cascales, C.; Zaldo, C.; Rivier, S.; Mateos, X.; Griebner, U.; Petrov, V. IEEE J. Quantum Electron. 2007, 43, 758–764. (2) Yoshida, A.; Schmidt, A.; Zhang, H.; Wang, J.; Liu, J.; Fiebig, C.; Paschke, K.; Erbert, G.; Petrov, V.; Griebner, U. Opt. Express 2010, 18, 24325–24330. (3) Zaouter, Y.; Didierjean, J.; Balembois, F.; Leclin, G. L.; Druon, F.; Georges, P.; Petit, J.; Goldner, P.; Viana, B. Opt. Lett. 2006, 31, 119–121. 1812
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