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Rare earth doped LiYF4 single crystalline films grown by liquid phase epitaxy for the fabrication of planar waveguide lasers
Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Rare earth doped LiYF4 single crystalline films grown by liquid phase epitaxy for the fabrication of planar waveguide lasers Florent Starecki, Western Bolaños, Gurvan Brasse n, Abdelmjid Benayad, Magali Morales, Jean-Louis Doualan, Alain Braud, Richard Moncorgé, Patrice Camy Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP), UMR 6252 CEA-CNRS-Ensicaen, Université de Caen, 6 Boulevard du Maréchal Juin, 14050 Caen cedex 4, France
art ic l e i nf o
Keywords: A2. Czochralski method A2. Single crystal growth A3. Liquid phase epitaxy B1. Fluorides B1. Rare earth ions B3. Laser waveguide
a b s t r a c t This article reports the epitaxial growth process and the characterizations of thulium doped or praseodymium doped LiYF4 layers grown on pure LiYF4 substrates for the development of planar waveguide lasers. After having presented the key points of the growth process to achieve high quality crystalline layers, microstructural, chemical and optical characterizations of the epilayers are reported for various dopants compositions. & 2014 Elsevier B.V. All rights reserved.
1. Introduction The development of miniaturized optoelectronic devices is a technologic challenge for the current decade to improve the performance and the capacity of various systems and to initiate new functionalities of the photonic devices. Integrated optics is a pertinent approach to implement various functionalities on the same substrate in order to develop very promising technologies such as labs on chips elements, as it has been realized years ago in the microelectronic domain. Waveguide lasers researches have been deeply investigating since the first report of guided laser operation in 1961 by Snitzer et al. [1] who used a glass core rod surrounded by a lower refractive index material to guide the laser beam. The first holmium doped YAG waveguide laser was reported in 1972 by Van der Ziel et al. [2]. Crystalline waveguides bring the well known high laser potential of the crystals together with a confined structure which lead to compact, low threshold lasers. Concerning fluoride crystalline waveguides, quite a few results have been reported up to now. Laser operation on liquid phase epitaxy layers has been demonstrated first on homoepitaxial Nd3 þ :LiYF4 waveguides [3,4]. The purpose of this article is to expose the liquid phase epitaxy (LPE) technique as a very pertinent method to synthesize rareearth (RE) doped fluoride crystalline layers, in order to build
n
Corresponding author. E-mail address: [email protected] (G. Brasse).
compact and efficient waveguide lasers. The principles of the technique as well as the various steps of the experimental protocol are first presented. In the second part of this article, the characterizations of the elaborated epilayers and their related experimental setups are exposed and discussed. Finally, very good optical performances of the laser waveguides based on RE-doped LiYF4 (YLF) layers are highlighted.
2. Liquid phase epitaxy of RE-doped YLF The liquid phase epitaxy technique is a well suitable method for the fabrication of thick single crystalline films epitaxied on the similar natured single crystalline substrate, also called homoepitaxy. Indeed, this method allows the growth of very high optical quality epilayers around 80–100 mm thickness, easily and quite quickly. The steps of the experimental process for such realization are the following: (i) first it is necessary to grow bulk YLF crystals by the Czochralski method to prepare the oriented substrates for the epitaxial growth process, (ii) the second step is the epitaxial growth of the rare earth doped YLF film on the substrate, and (iii) preparation of the as grown layer is then required to achieve an optical waveguide with good optical characteristics suitable for laser purpose operation. 2.1. Preparation of the YLF substrates High quality pure YLF bulk single crystals were grown by the Czochralski technique. The starting materials are LiF (Alfa Aesar
http://dx.doi.org/10.1016/j.jcrysgro.2014.01.039 0022-0248 & 2014 Elsevier B.V. All rights reserved.
Please cite this article as: F. Starecki, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.039i
F. Starecki et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
Puratronic, 2N), yttrium and rare earth oxides (Alfa Aesar REacton), following the molar ratio 52% LiF and 48% YF3, according to the related “LiF–YF3” phase diagram [5]. Oxide precursors were then fluorinated, using an excess of ammonium hydrogen difluoride solution (NH4HF2) under heating at 250 1C, following the chemical equation: Y2 O3 þ 6 NH4 HF2 -2 YF3 þ 6 NH4 F þ 3 H 2 O ; T1 ¼ 250 1C The so obtained raw materials were annealed at 650 1C under a controlled argon atmosphere to stay free from residual NH4F and water. The drawing of the bulk crystals could then be performed under an oxygen free Ar–CF4 atmosphere using a c-oriented seed, the pulling rate being fixed at 1 mm/h. The as-grown crystals were also oriented following the (001) direction. By this way, the faces of the substrates, which have to be polished for the epitaxial growth, were perpendicular to this orientation. Thus it would be possible to choose either the π or s polarization for the optical pumping and laser operations. Typically, 30 11 2.5 mm3 substrates were needed for the LPE process, so that the bulk crystals grown by the Czochralski technique were around 60 mm height and 30–40 mm in diameter. Finally, the substrates had to be very well and fine polished on both the faces and a particular attention had to be paid concerning the planarity of these faces, as well as their surface roughness.
2.2. The LPE setup and principles A schematic figure of the LPE setup is described in Fig. 1. The experimental LPE mount can be divided into two main areas: the upper zone, which is isolated from the under zone by an electrovalve. The upper zone is an exchange zone that allows the change of samples and substrates without contaminating the melt located in the under zone when the electrovalve is closed. The vitreous carbide crucible, containing the 160 g load, is hosted in a one end saddled silica tube, which constitutes the under zone and is disposed in the heating zone of a tubular furnace. As explained before, the growth of very high quality fluoride crystals, especially YLF, requires a very pure oxygen free atmosphere. The furnace presents a dual heating zone that ensures a homogeneous temperature on the whole height of the melt in the crucible and the temperature regulation is controlled at 0.1 1C.
Fig. 1. Experimental setup of the LPE experiment.
The raw materials that compose the melt are prepared according to the experimental procedure explained above, except the molar ratio here which is fixed at 27% YF3 and 73% LiF. The fluorinated raw materials are then introduced in the epitaxial growth chamber to be degassed under heating. The charge is superheated to ensure a good homogenization of the melt and an additional stirring of the LiF–YF3 mixture is done. The temperature is then slowly decreased to the crystallization temperature and stabilized. Once the substrate is installed in the exchange area and is ready to be dipped in the melt, it is necessary to first have a very good primary vacuum in the exchange area before opening the electrovalve. After this stage, it is possible to dip the substrate into the melt to allow the epitaxial growth, without risking pollution of the atmosphere. As it moves down, the substrate is progressively thermalized before being dipped in the melted bath. The dipping duration varies between 20 min and 1 h as a function of the desired layer thickness, as well as a function of the temperature and viscosity of the bath. A smooth cooling ramp is finally applied to the sample after extraction to avoid cracks as shown in Ref. [6]. After extraction of the sample out of the furnace, a white deposit is observed on the surface as shown in Fig. 2 and a mechanical polishing step is required to obtain a transparent, free of solvent, epitaxial crystalline layer. For laser experiments, the side faces of the substrates have to be very well polished, as well as to present a rigorous parallelism. A top polishing of the layer is also required to decrease the roughness and to reduce the layer thickness to the desired value. 2.3. Discussion on the fabrication process Epitaxial growth could occur for various melt compositions according to the works of Douysset et al. [4]. However, it has been demonstrated that the preferential compositions to promote such an epitaxial growth lay between 71% mol and 79% mol of LiF, with respectively 29% mol and 21% mol of YF3. Furthermore, LiF presents the benefit to be the solute, as well as the solvent in this chemical system. The first LPE experiments have been realized with a molar ratio of 79% of LiF to work at lower temperature. It appears that for this ratio, the equilibrium temperature is difficult to reach: the temperature range between an uncontrolled and harmful pyramidal-like crystallization and dissolution of the substrate is very thin. By considering these difficulties, the molar ratio of the composition has been progressively adjusted until it reaches the right conditions to allow a good epitaxial growth quality of the RE doped YLF layer, with a
Fig. 2. (a) Raw sample after LPE growth and (b) waveguide sample after preparation and polishing.
Please cite this article as: F. Starecki, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.039i
F. Starecki et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
quite good reproducibility. In this manner, we have found a good compromise by fixing the composition of the melt to 73% mol of LiF and 27% mol of YF3, with a saturation temperature of around 760 1C. The LiF/YF3 ratio is indeed a key parameter which determines the growth temperature of the RE doped YLF epilayer. This growth temperature needs to be the closest as possible to the saturation temperature to allow a slow growth rate of the crystalline film, which was suitable to achieve thick films presenting a good crystallinity and a good optical quality, without cracks, inclusions or stresses. In addition, it was necessary to take into account the progressive vaporization of LiF during the experiment, which could run for few days and cause a progressive low drift of the growth temperature. Moreover, Lu3 þ or Gd3 þ ions, which are non-luminescent ions, were added to the composition. These codopants are substituting Y3 þ ions in the crystalline lattice and are chosen in order to increase the refractive index difference between the epilayers and the substrates without perturbating the desired luminescence properties. Furthermore, due to the diameter difference between these codopant ions and the substituted Y3 þ ions, a lattice distortion can appear as a function of their concentration; the lattice mismatch has been characterized by x-ray diffraction and is discussed hereafter in this article. Finally, it has to be noted that only [001] substrates were used for all the experiments considered in this paper, in order to allow the use of the spectroscopic properties following either the π or s polarization.
3. LiYF4 waveguides for laser purpose 3.1. Epilayers characterization 3.1.1. Observation of the epilayers through optical microscopy An observation of the epilayers was first realized by optical microscopy as shown in Fig. 3. In this case, a codoped Gd3 þ and Tm3 þ –YLF layer grown on a pure YLF substrate could be observed thanks to the refractive index contrast which allowed us to estimate the thickness of the layer around 20 μm Furthermore, it is important to note that there was no LiF inclusion migrating to
3
the outer surface of the epilayer and that the interface between the epilayer and the substrate was very clean, showing a very good homogeneity without too strong roughness. Indeed, the substrates used for the epitaxial growth present a 20 nm roughness and the sectional view of the sample shown in Fig. 3 allows us to note a roughness of the interface lower than the micrometer range. These preliminary observations confirmed that the epitaxial growth process has proceeded well, leading to a transparent layer presenting characteristics suitable for waveguiding applications. 3.1.2. Determination of the active ions concentrations The absorption spectra realized with a Perkin Elmer spectrophotometer were used to determine the thulium and praseodymium concentrations, by the aid of absorption cross sections measured for both Tm3 þ :LiYF4 and Pr3 þ :LiYF4 as references. For thulium and praseodymium, the only scanning direction allowed in this configuration was along the c axis, perpendicularly to the surface of the epilayers, as shown in Fig. 3(b). This means we could only have access to the s-polarized absorption spectra. By this way, the probe beam gets across the two epilayers grown on both faces of the substrates, which are equivalent to a thickness of a few tens of micrometers. Moreover, it is noticeable that the absorption cross sections lineshapes of the epilayers are less well defined than their bulk counterparts and present a higher signal-to-noise ratio, due to the low thickness as shown in Refs. [7,8]. Nevertheless, these experimental results show that the absorption cross sections of the epitaxially grown layers are sensibly equivalent to these measured on bulk samples and is not significantly affected by the introduction of Gd3 þ and Lu3 þ ions as codopants in the considered molar ratio ( 5% mol), compared to the bulk crystals Tm3 þ : YLF and Pr3 þ :YLF free of Gd3 þ and Lu3 þ [7,8]. It is thus well founded to assume that the active ions absorption cross sections does not depend on the gadolinium and lutetium concentrations; the optical density corresponding to a known thickness has been measured. From these measurements, the number of optically active ions per cm3 has been obtained, as well as the corresponding segregation coefficients. All these data are reported in Table 1 and it can be noted that the Pr3 þ segregation coefficient in the YLF matrix is estimated to 0.22 which confirms the bibliographic data [9] and the Tm3 þ segregation coefficient in the YLF matrix has been measured, as expected, close to the unity. 3.1.3. Determination of the passive ions concentrations To estimate the gadolinium and lutetium concentrations, Energy Dispersive X-ray analysis (EDX) has been done. Lutetium and gadolinium are both heavy elements, so an EDX analysis seems to be a reliable technique to estimate their respective concentrations. After scanning several zones of each sample the results have been averaged in order to obtain the Gd/Y and Lu/Y atomic ratios which are reported in Table 1. 3.1.4. Lattice mismatch characterization by x-ray diffraction Crystal quality as well as cell parameters of the {001}-oriented 5% Gd3 þ –5% Lu3 þ –1.5% Pr3 þ :YLF film was characterized using respectively high-resolution x-ray diffraction rocking curve (RC) Table 1 Measurements of the dopants concentrations in the epilayers.
Fig. 3. (a) Observation by optical microscopy of a codoped Gd3 þ –Tm3 þ :LiYF4 epilayer grown on a LiYF4 substrate and (b) description of the method used for the measurement of the absorption cross section on the epilayers.
Melt
Percent at bath
Segregation coefficient
Percent at layer
Gd Lu Pr Tm
5 5 1.5 7
0.703 0.94 0.22 6.15
3.52 4.7 0.33 0.88
Please cite this article as: F. Starecki, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.039i
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F. Starecki et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 4. {004} Reciprocal space mapping representation.
and symmetric and skew symmetric reflections Reciprocal Space Mapping (RSM). These x-ray measurements were performed using a triple-axis Ge 220 detecting optics on a PANalytical X'Pert Pro MRD diffractometer equipped with an incident four-bounce Ge 220 monochromator (λ CuKα1 ¼1.54056 Å). The symmetric {004} RSM, shown in Fig. 4, exhibits two contributions: the strongest one, which is located at a 2θ angle of 33.32551, is due to the YLF substrate, while the smallest one is related to the film contribution. By optimizing this latter on {004} ϕ-scans, two single crystalline domains have been observed at 1801 each, other in ϕ, at 2θ angles of 33.86121 and 33.82621, corresponding to c lattice parameters of 10.5803 Å and 10.5909 Å, respectively. Compared to the value of 10.7437 Å obtained for the YLF substrate, such domain c-axis reveals an extension of the film in the substrate plane. An identical {004} rocking curve full width at half maximum (FWHM) of 76.96″ is measured for the two film domains, a value smaller than the best ones of 81.4″ of the literature for YLF:Ce single crystals [10]. This FWHM value attests the good single crystalline quality of the film, though the two domains are at 0.381 from the [001] substrate direction. Using skew symmetric reflection, the {103} RSM have allowed the determination of the a-cell parameters of the film, which were measured to 5.322 Å and 5.3333 Å correspondingly for the two domains, as well as for the substrate with a value of 5.1552 Å, where it appears as a direct evidence of a film under extension stress in the sample plane. Assuming a biaxial stress state of the film and bulk cell parameters of the substrate and using the stiffness constants from Ref. [11], an extension stress of 5 GPa along the film plane has been estimated. 3.2. Characterization of the optical properties of the waveguides 3.2.1. Measurement of the refractive index of the epilayers The refractive indices have been measured at five wavelengths by the M-lines technique, using a Metricon 2012/M commercial setup. For both TE and TM modes propagating into the planar waveguide, the refractive index step between the substrate and the layer is always higher than 1.2 10 3 and is reported in Fig. 5 for TM modes. As expected, the refractive index contrast is larger for higher doping concentrations. The uncertainty of these measurements have been estimated to be of the order of 7 10 4. Given the thickness and the refractive indices evolution as a function of the wavelength, the number of modes at a given wavelength could be calculated and thus the required thickness to have a single mode propagation could be estimated. As an example, the TM1 mode of the codoped Tm3 þ –Gd3 þ waveguide vanishes if the thickness is lower than 14 μm at a wavelength of 1.877 μm.
Fig. 5. TM modes refractive index measurement of several epilayers.
3.2.2. Measurement of the optical losses Several methods exist to determine the waveguide optical losses, but the easiest way is to directly measure the transmitted power. The determination of the coupled power in the waveguide is then a key issue for doing rigorous loss measurements. By this way, an He–Ne laser beam was injected in the epilayer through a 5 microscope objective. This ensures the numeral apertures compatibility, making thus a 32 μm spot diameter at the focus point. By using roughly 40 μm thick layers, we can assume that there is a 100% coupling efficiency. So taking into account the Fresnel reflections, the internal optical losses obtained for various compositions are reported in Table 2. At the sight of these results it can be noticed that the tested waveguides exhibit low propagation losses, which is a key point for efficient laser oscillations.
4. Conclusions and perspectives In summary, liquid phase epitaxy appears to be a very promising method for the realization of high optical quality RE-doped fluoride planar waveguides. Various growth parameters have been studied to develop the LPE technique like the growth temperature, the melt composition, the dopants concentrations, the durations of the draw stages, as well as the substrates synthesis by the Czochralski method and their preparation. By this way, it has been demonstrated that it is possible to achieve single crystal epilayer of few tens of micrometers thickness with very low optical losses and very good crystallinity. It has been also proved that the codoping of the epilayer with non-active RE3 þ ions like Gd3 þ and Lu3 þ in the considered ratios increases the refractive index of the layer, without modifying its spectroscopic properties. In addition, the Tm3 þ doped or Pr3 þ doped planar waveguides achieved have been characterized in a plano–plano laser cavity and very good CW laser performances were obtained. Indeed a maximum output power of 560 mW at 1.877 μm, with a 76% slope efficiency related to the absorbed pump power for Tm3 þ doped YLF waveguide on one hand [7], and laser output powers of 25 mW and 12 mW respectively at 639.4 and 604.2 nm for Pr3 þ doped YLF waveguide on the other [8]. The laser based on this technology could be potentially developed for innovative applications, such as surgical microprobes or LIDAR applications for Tm3 þ :YLF or embedded microprojectors for Pr3 þ :YLF. The next stage is to functionalize these waveguides by designing and engineering various 2-D microstructures. Other kinds of improvements can also arise as possible issues, like the
Please cite this article as: F. Starecki, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.039i
F. Starecki et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Table 2 Measurements of the optical losses in the waveguides having various chemical compositions. Substitution rates
Optical path (cm)
Losses (dB/cm)
0.33% Pr 3.5% Gd–0.33% Pr 4.7% Lu–3.5% Gd–0.33% Pr 6.1% Tm–3.5% Gd 0.88% Tm–3.5% Gd
5.15 9 9.25 6.01 7.97
2.96 0.65 0.35 0.11 0.26
optimization of the RE concentrations, the engineering of dopant gradient or the coatings of mirrors on the end-faces. By considering the lattice parameters of YLF, another perspective is to grow epitaxial RE3 þ doped YLF layers on CaF2 or LiCaAlF6 substrates, which seems to be an interesting alternative to increase the refractive index contrast, without addition of gadolinium or lutetium ions that can distort the crystalline lattice. Acknowledgments The authors wish to acknowledge the support from the French National Research Agency (ANR) within the framework of the FLUOLASE research program. Thanks are expressed to V. Nazabal for the M-lines measurements, F. Lemarié for the EDX analysis, and D. Chateigner for the fruitful discussions concerning XRD characterizations.
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References [1] E. Snitzer, Optical maser action of Nd3 þ in a barium crown glass, Phys. Rev. Lett. 7 (12) (1961) 444–446. [2] J.P. Van der Ziel, W.A. Bonner, L. Kopf, L.G. Van Uitert, Coherent emission from Ho3 þ ions in epitaxially grown thin aluminum garnet films, Phys. Lett. A 42 (1972) 105–106. [3] J. Hulliger, P. Rogin, R. Burkhaltler, Waveguides in YLF grown by liquid phase epitaxy, Laser Phys. 8 (3) (1998) 764–768. [4] L. Douysset-Bloch, B. Ferrand, M. Couchaud, L. Fulbert, M.F. Joubert, G. Chadeyron, B. Jacquier, Growth by liquid phase epitaxy and characterization of Nd:YLiF4 layers, J. Alloy. Compd. 275/277 (1998) 67–71. [5] R.E. Thoma, C.F. Weaver, H.A. Freidman, H. Insley, L.A. Harris, H.A. Yakel , Phase equilibria in the system LiF–YF3, J. Phys. Chem. 65 (1961) 1096–1099. [6] P. Rogin, G. Huber, J. Hulliger, LiYF4 liquid phase epitaxy using an inverted slider geometry, J. Cryst. Growth 198/199 (1999) 564–567. [7] W. Bolaños, F. Starecki, A. Benayad, G. Brasse, V. Ménard, J.-L. Doualan, A. Braud, R. Moncorgé, P. Camy, Tm:LiYF4 planar waveguide laser at 1.9pla, Opt. Lett. 37 (19) (2012) 4032–4034. [8] F. Starecki, W. Bolaños, A. Braud, J.-L. Doualan, G. Brasse, A. Benayad, V. Nazabal, B. Xu, R. Moncorgé, P. Camy, Red and orange Pr:LiYF4 planar waveguide laser, Opt. Lett. 38 (4) (2013) 455–458. [9] A. Richter, E. Heumann, G. Huber, V. Ostroumov, W. Seelert, Power scaling of semiconductor laser pumped praseodymium lasers, Opt. Exp. 15 (8) (2007) 5172–5178. [10] Y. Yokota, A. Yamaji, N. Kawaguchi, K. Fukuda, S. Kurosawa, A. Yoshikawa. Growth of Ce:LiYF4 bulk single crystal with high Ce concentration by Cz method and the scintillation properties, Phys. Status Solidi C (Special Issue: Fifth International Conference on Optical, Optoelectronic and Photonic Materials and Applications (ICOOPMA12); see further papers in Phys. Status Solidi A 209 (12) 9 (12) (2012) 2279–2283. [11] P. Blanchfield, G.A. Saunders, The elastic constants and acoustic symmetry of LiYF4, J. Phys. C 12 (22) (1979) 4673–4689.
Please cite this article as: F. Starecki, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.039i
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Rare earth doped LiYF4 single crystalline films grown by liquid phase epitaxy for the fabrication of planar waveguide lasers Florent Starecki, Western Bolaños, Gurvan Brasse n, Abdelmjid Benayad, Magali Morales, Jean-Louis Doualan, Alain Braud, Richard Moncorgé, Patrice Camy Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP), UMR 6252 CEA-CNRS-Ensicaen, Université de Caen, 6 Boulevard du Maréchal Juin, 14050 Caen cedex 4, France
art ic l e i nf o
Keywords: A2. Czochralski method A2. Single crystal growth A3. Liquid phase epitaxy B1. Fluorides B1. Rare earth ions B3. Laser waveguide
a b s t r a c t This article reports the epitaxial growth process and the characterizations of thulium doped or praseodymium doped LiYF4 layers grown on pure LiYF4 substrates for the development of planar waveguide lasers. After having presented the key points of the growth process to achieve high quality crystalline layers, microstructural, chemical and optical characterizations of the epilayers are reported for various dopants compositions. & 2014 Elsevier B.V. All rights reserved.
1. Introduction The development of miniaturized optoelectronic devices is a technologic challenge for the current decade to improve the performance and the capacity of various systems and to initiate new functionalities of the photonic devices. Integrated optics is a pertinent approach to implement various functionalities on the same substrate in order to develop very promising technologies such as labs on chips elements, as it has been realized years ago in the microelectronic domain. Waveguide lasers researches have been deeply investigating since the first report of guided laser operation in 1961 by Snitzer et al. [1] who used a glass core rod surrounded by a lower refractive index material to guide the laser beam. The first holmium doped YAG waveguide laser was reported in 1972 by Van der Ziel et al. [2]. Crystalline waveguides bring the well known high laser potential of the crystals together with a confined structure which lead to compact, low threshold lasers. Concerning fluoride crystalline waveguides, quite a few results have been reported up to now. Laser operation on liquid phase epitaxy layers has been demonstrated first on homoepitaxial Nd3 þ :LiYF4 waveguides [3,4]. The purpose of this article is to expose the liquid phase epitaxy (LPE) technique as a very pertinent method to synthesize rareearth (RE) doped fluoride crystalline layers, in order to build
n
Corresponding author. E-mail address: [email protected] (G. Brasse).
compact and efficient waveguide lasers. The principles of the technique as well as the various steps of the experimental protocol are first presented. In the second part of this article, the characterizations of the elaborated epilayers and their related experimental setups are exposed and discussed. Finally, very good optical performances of the laser waveguides based on RE-doped LiYF4 (YLF) layers are highlighted.
2. Liquid phase epitaxy of RE-doped YLF The liquid phase epitaxy technique is a well suitable method for the fabrication of thick single crystalline films epitaxied on the similar natured single crystalline substrate, also called homoepitaxy. Indeed, this method allows the growth of very high optical quality epilayers around 80–100 mm thickness, easily and quite quickly. The steps of the experimental process for such realization are the following: (i) first it is necessary to grow bulk YLF crystals by the Czochralski method to prepare the oriented substrates for the epitaxial growth process, (ii) the second step is the epitaxial growth of the rare earth doped YLF film on the substrate, and (iii) preparation of the as grown layer is then required to achieve an optical waveguide with good optical characteristics suitable for laser purpose operation. 2.1. Preparation of the YLF substrates High quality pure YLF bulk single crystals were grown by the Czochralski technique. The starting materials are LiF (Alfa Aesar
http://dx.doi.org/10.1016/j.jcrysgro.2014.01.039 0022-0248 & 2014 Elsevier B.V. All rights reserved.
Please cite this article as: F. Starecki, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.039i
F. Starecki et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
Puratronic, 2N), yttrium and rare earth oxides (Alfa Aesar REacton), following the molar ratio 52% LiF and 48% YF3, according to the related “LiF–YF3” phase diagram [5]. Oxide precursors were then fluorinated, using an excess of ammonium hydrogen difluoride solution (NH4HF2) under heating at 250 1C, following the chemical equation: Y2 O3 þ 6 NH4 HF2 -2 YF3 þ 6 NH4 F þ 3 H 2 O ; T1 ¼ 250 1C The so obtained raw materials were annealed at 650 1C under a controlled argon atmosphere to stay free from residual NH4F and water. The drawing of the bulk crystals could then be performed under an oxygen free Ar–CF4 atmosphere using a c-oriented seed, the pulling rate being fixed at 1 mm/h. The as-grown crystals were also oriented following the (001) direction. By this way, the faces of the substrates, which have to be polished for the epitaxial growth, were perpendicular to this orientation. Thus it would be possible to choose either the π or s polarization for the optical pumping and laser operations. Typically, 30 11 2.5 mm3 substrates were needed for the LPE process, so that the bulk crystals grown by the Czochralski technique were around 60 mm height and 30–40 mm in diameter. Finally, the substrates had to be very well and fine polished on both the faces and a particular attention had to be paid concerning the planarity of these faces, as well as their surface roughness.
2.2. The LPE setup and principles A schematic figure of the LPE setup is described in Fig. 1. The experimental LPE mount can be divided into two main areas: the upper zone, which is isolated from the under zone by an electrovalve. The upper zone is an exchange zone that allows the change of samples and substrates without contaminating the melt located in the under zone when the electrovalve is closed. The vitreous carbide crucible, containing the 160 g load, is hosted in a one end saddled silica tube, which constitutes the under zone and is disposed in the heating zone of a tubular furnace. As explained before, the growth of very high quality fluoride crystals, especially YLF, requires a very pure oxygen free atmosphere. The furnace presents a dual heating zone that ensures a homogeneous temperature on the whole height of the melt in the crucible and the temperature regulation is controlled at 0.1 1C.
Fig. 1. Experimental setup of the LPE experiment.
The raw materials that compose the melt are prepared according to the experimental procedure explained above, except the molar ratio here which is fixed at 27% YF3 and 73% LiF. The fluorinated raw materials are then introduced in the epitaxial growth chamber to be degassed under heating. The charge is superheated to ensure a good homogenization of the melt and an additional stirring of the LiF–YF3 mixture is done. The temperature is then slowly decreased to the crystallization temperature and stabilized. Once the substrate is installed in the exchange area and is ready to be dipped in the melt, it is necessary to first have a very good primary vacuum in the exchange area before opening the electrovalve. After this stage, it is possible to dip the substrate into the melt to allow the epitaxial growth, without risking pollution of the atmosphere. As it moves down, the substrate is progressively thermalized before being dipped in the melted bath. The dipping duration varies between 20 min and 1 h as a function of the desired layer thickness, as well as a function of the temperature and viscosity of the bath. A smooth cooling ramp is finally applied to the sample after extraction to avoid cracks as shown in Ref. [6]. After extraction of the sample out of the furnace, a white deposit is observed on the surface as shown in Fig. 2 and a mechanical polishing step is required to obtain a transparent, free of solvent, epitaxial crystalline layer. For laser experiments, the side faces of the substrates have to be very well polished, as well as to present a rigorous parallelism. A top polishing of the layer is also required to decrease the roughness and to reduce the layer thickness to the desired value. 2.3. Discussion on the fabrication process Epitaxial growth could occur for various melt compositions according to the works of Douysset et al. [4]. However, it has been demonstrated that the preferential compositions to promote such an epitaxial growth lay between 71% mol and 79% mol of LiF, with respectively 29% mol and 21% mol of YF3. Furthermore, LiF presents the benefit to be the solute, as well as the solvent in this chemical system. The first LPE experiments have been realized with a molar ratio of 79% of LiF to work at lower temperature. It appears that for this ratio, the equilibrium temperature is difficult to reach: the temperature range between an uncontrolled and harmful pyramidal-like crystallization and dissolution of the substrate is very thin. By considering these difficulties, the molar ratio of the composition has been progressively adjusted until it reaches the right conditions to allow a good epitaxial growth quality of the RE doped YLF layer, with a
Fig. 2. (a) Raw sample after LPE growth and (b) waveguide sample after preparation and polishing.
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quite good reproducibility. In this manner, we have found a good compromise by fixing the composition of the melt to 73% mol of LiF and 27% mol of YF3, with a saturation temperature of around 760 1C. The LiF/YF3 ratio is indeed a key parameter which determines the growth temperature of the RE doped YLF epilayer. This growth temperature needs to be the closest as possible to the saturation temperature to allow a slow growth rate of the crystalline film, which was suitable to achieve thick films presenting a good crystallinity and a good optical quality, without cracks, inclusions or stresses. In addition, it was necessary to take into account the progressive vaporization of LiF during the experiment, which could run for few days and cause a progressive low drift of the growth temperature. Moreover, Lu3 þ or Gd3 þ ions, which are non-luminescent ions, were added to the composition. These codopants are substituting Y3 þ ions in the crystalline lattice and are chosen in order to increase the refractive index difference between the epilayers and the substrates without perturbating the desired luminescence properties. Furthermore, due to the diameter difference between these codopant ions and the substituted Y3 þ ions, a lattice distortion can appear as a function of their concentration; the lattice mismatch has been characterized by x-ray diffraction and is discussed hereafter in this article. Finally, it has to be noted that only [001] substrates were used for all the experiments considered in this paper, in order to allow the use of the spectroscopic properties following either the π or s polarization.
3. LiYF4 waveguides for laser purpose 3.1. Epilayers characterization 3.1.1. Observation of the epilayers through optical microscopy An observation of the epilayers was first realized by optical microscopy as shown in Fig. 3. In this case, a codoped Gd3 þ and Tm3 þ –YLF layer grown on a pure YLF substrate could be observed thanks to the refractive index contrast which allowed us to estimate the thickness of the layer around 20 μm Furthermore, it is important to note that there was no LiF inclusion migrating to
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the outer surface of the epilayer and that the interface between the epilayer and the substrate was very clean, showing a very good homogeneity without too strong roughness. Indeed, the substrates used for the epitaxial growth present a 20 nm roughness and the sectional view of the sample shown in Fig. 3 allows us to note a roughness of the interface lower than the micrometer range. These preliminary observations confirmed that the epitaxial growth process has proceeded well, leading to a transparent layer presenting characteristics suitable for waveguiding applications. 3.1.2. Determination of the active ions concentrations The absorption spectra realized with a Perkin Elmer spectrophotometer were used to determine the thulium and praseodymium concentrations, by the aid of absorption cross sections measured for both Tm3 þ :LiYF4 and Pr3 þ :LiYF4 as references. For thulium and praseodymium, the only scanning direction allowed in this configuration was along the c axis, perpendicularly to the surface of the epilayers, as shown in Fig. 3(b). This means we could only have access to the s-polarized absorption spectra. By this way, the probe beam gets across the two epilayers grown on both faces of the substrates, which are equivalent to a thickness of a few tens of micrometers. Moreover, it is noticeable that the absorption cross sections lineshapes of the epilayers are less well defined than their bulk counterparts and present a higher signal-to-noise ratio, due to the low thickness as shown in Refs. [7,8]. Nevertheless, these experimental results show that the absorption cross sections of the epitaxially grown layers are sensibly equivalent to these measured on bulk samples and is not significantly affected by the introduction of Gd3 þ and Lu3 þ ions as codopants in the considered molar ratio ( 5% mol), compared to the bulk crystals Tm3 þ : YLF and Pr3 þ :YLF free of Gd3 þ and Lu3 þ [7,8]. It is thus well founded to assume that the active ions absorption cross sections does not depend on the gadolinium and lutetium concentrations; the optical density corresponding to a known thickness has been measured. From these measurements, the number of optically active ions per cm3 has been obtained, as well as the corresponding segregation coefficients. All these data are reported in Table 1 and it can be noted that the Pr3 þ segregation coefficient in the YLF matrix is estimated to 0.22 which confirms the bibliographic data [9] and the Tm3 þ segregation coefficient in the YLF matrix has been measured, as expected, close to the unity. 3.1.3. Determination of the passive ions concentrations To estimate the gadolinium and lutetium concentrations, Energy Dispersive X-ray analysis (EDX) has been done. Lutetium and gadolinium are both heavy elements, so an EDX analysis seems to be a reliable technique to estimate their respective concentrations. After scanning several zones of each sample the results have been averaged in order to obtain the Gd/Y and Lu/Y atomic ratios which are reported in Table 1. 3.1.4. Lattice mismatch characterization by x-ray diffraction Crystal quality as well as cell parameters of the {001}-oriented 5% Gd3 þ –5% Lu3 þ –1.5% Pr3 þ :YLF film was characterized using respectively high-resolution x-ray diffraction rocking curve (RC) Table 1 Measurements of the dopants concentrations in the epilayers.
Fig. 3. (a) Observation by optical microscopy of a codoped Gd3 þ –Tm3 þ :LiYF4 epilayer grown on a LiYF4 substrate and (b) description of the method used for the measurement of the absorption cross section on the epilayers.
Melt
Percent at bath
Segregation coefficient
Percent at layer
Gd Lu Pr Tm
5 5 1.5 7
0.703 0.94 0.22 6.15
3.52 4.7 0.33 0.88
Please cite this article as: F. Starecki, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.039i
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Fig. 4. {004} Reciprocal space mapping representation.
and symmetric and skew symmetric reflections Reciprocal Space Mapping (RSM). These x-ray measurements were performed using a triple-axis Ge 220 detecting optics on a PANalytical X'Pert Pro MRD diffractometer equipped with an incident four-bounce Ge 220 monochromator (λ CuKα1 ¼1.54056 Å). The symmetric {004} RSM, shown in Fig. 4, exhibits two contributions: the strongest one, which is located at a 2θ angle of 33.32551, is due to the YLF substrate, while the smallest one is related to the film contribution. By optimizing this latter on {004} ϕ-scans, two single crystalline domains have been observed at 1801 each, other in ϕ, at 2θ angles of 33.86121 and 33.82621, corresponding to c lattice parameters of 10.5803 Å and 10.5909 Å, respectively. Compared to the value of 10.7437 Å obtained for the YLF substrate, such domain c-axis reveals an extension of the film in the substrate plane. An identical {004} rocking curve full width at half maximum (FWHM) of 76.96″ is measured for the two film domains, a value smaller than the best ones of 81.4″ of the literature for YLF:Ce single crystals [10]. This FWHM value attests the good single crystalline quality of the film, though the two domains are at 0.381 from the [001] substrate direction. Using skew symmetric reflection, the {103} RSM have allowed the determination of the a-cell parameters of the film, which were measured to 5.322 Å and 5.3333 Å correspondingly for the two domains, as well as for the substrate with a value of 5.1552 Å, where it appears as a direct evidence of a film under extension stress in the sample plane. Assuming a biaxial stress state of the film and bulk cell parameters of the substrate and using the stiffness constants from Ref. [11], an extension stress of 5 GPa along the film plane has been estimated. 3.2. Characterization of the optical properties of the waveguides 3.2.1. Measurement of the refractive index of the epilayers The refractive indices have been measured at five wavelengths by the M-lines technique, using a Metricon 2012/M commercial setup. For both TE and TM modes propagating into the planar waveguide, the refractive index step between the substrate and the layer is always higher than 1.2 10 3 and is reported in Fig. 5 for TM modes. As expected, the refractive index contrast is larger for higher doping concentrations. The uncertainty of these measurements have been estimated to be of the order of 7 10 4. Given the thickness and the refractive indices evolution as a function of the wavelength, the number of modes at a given wavelength could be calculated and thus the required thickness to have a single mode propagation could be estimated. As an example, the TM1 mode of the codoped Tm3 þ –Gd3 þ waveguide vanishes if the thickness is lower than 14 μm at a wavelength of 1.877 μm.
Fig. 5. TM modes refractive index measurement of several epilayers.
3.2.2. Measurement of the optical losses Several methods exist to determine the waveguide optical losses, but the easiest way is to directly measure the transmitted power. The determination of the coupled power in the waveguide is then a key issue for doing rigorous loss measurements. By this way, an He–Ne laser beam was injected in the epilayer through a 5 microscope objective. This ensures the numeral apertures compatibility, making thus a 32 μm spot diameter at the focus point. By using roughly 40 μm thick layers, we can assume that there is a 100% coupling efficiency. So taking into account the Fresnel reflections, the internal optical losses obtained for various compositions are reported in Table 2. At the sight of these results it can be noticed that the tested waveguides exhibit low propagation losses, which is a key point for efficient laser oscillations.
4. Conclusions and perspectives In summary, liquid phase epitaxy appears to be a very promising method for the realization of high optical quality RE-doped fluoride planar waveguides. Various growth parameters have been studied to develop the LPE technique like the growth temperature, the melt composition, the dopants concentrations, the durations of the draw stages, as well as the substrates synthesis by the Czochralski method and their preparation. By this way, it has been demonstrated that it is possible to achieve single crystal epilayer of few tens of micrometers thickness with very low optical losses and very good crystallinity. It has been also proved that the codoping of the epilayer with non-active RE3 þ ions like Gd3 þ and Lu3 þ in the considered ratios increases the refractive index of the layer, without modifying its spectroscopic properties. In addition, the Tm3 þ doped or Pr3 þ doped planar waveguides achieved have been characterized in a plano–plano laser cavity and very good CW laser performances were obtained. Indeed a maximum output power of 560 mW at 1.877 μm, with a 76% slope efficiency related to the absorbed pump power for Tm3 þ doped YLF waveguide on one hand [7], and laser output powers of 25 mW and 12 mW respectively at 639.4 and 604.2 nm for Pr3 þ doped YLF waveguide on the other [8]. The laser based on this technology could be potentially developed for innovative applications, such as surgical microprobes or LIDAR applications for Tm3 þ :YLF or embedded microprojectors for Pr3 þ :YLF. The next stage is to functionalize these waveguides by designing and engineering various 2-D microstructures. Other kinds of improvements can also arise as possible issues, like the
Please cite this article as: F. Starecki, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.039i
F. Starecki et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Table 2 Measurements of the optical losses in the waveguides having various chemical compositions. Substitution rates
Optical path (cm)
Losses (dB/cm)
0.33% Pr 3.5% Gd–0.33% Pr 4.7% Lu–3.5% Gd–0.33% Pr 6.1% Tm–3.5% Gd 0.88% Tm–3.5% Gd
5.15 9 9.25 6.01 7.97
2.96 0.65 0.35 0.11 0.26
optimization of the RE concentrations, the engineering of dopant gradient or the coatings of mirrors on the end-faces. By considering the lattice parameters of YLF, another perspective is to grow epitaxial RE3 þ doped YLF layers on CaF2 or LiCaAlF6 substrates, which seems to be an interesting alternative to increase the refractive index contrast, without addition of gadolinium or lutetium ions that can distort the crystalline lattice. Acknowledgments The authors wish to acknowledge the support from the French National Research Agency (ANR) within the framework of the FLUOLASE research program. Thanks are expressed to V. Nazabal for the M-lines measurements, F. Lemarié for the EDX analysis, and D. Chateigner for the fruitful discussions concerning XRD characterizations.
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Please cite this article as: F. Starecki, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.039i