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High-resolution Fourier spectroscopy as a tool for studying quality of rare-earth-doped crystals
JOURNAL OF RARE EARTHS, Vol. 32, No. 3, Mar. 2014, P. 230
High-resolution Fourier spectroscopy as a tool for studying quality of rare-earth-doped crystals Marina N. Popova* (Institute of Spectroscopy, Russian Academy of Sciences, 5, Fizicheskaya Str., 142190, Troitsk, Moscow, Russia) Received 9 June 2013; revised 9 September 2013
Abstract: Three examples were considered of the use of high resolution Fourier-transform optical spectroscopy for studying quality of rare-earth-doped crystals. The first example was connected with defects present in crystals grown by flux techniques. The second example dealt with detection of stresses and deformations in rare-earth-containing crystals, by registering splitting of spectral lines. The third example showed that a very small amount (at the level of ppm) of different RE ions present in a crystal could be determined using high-resolution spectroscopy. This work was carried out by the author’s group in collaboration with several institutions in Russia and abroad. Keywords: Fourier-transform optical spectroscopy; high resolution; absorption spectra; rare earths
Optical spectra of rare-earth (RE) ions in crystals are, mainly, due to parity forbidden f-f transitions that become allowed due to odd components of the crystal field. The 4f-shell is well shielded from a crystalline environment by the filled 5s and 5p shells, and the crystal field acts as a weak perturbation to the energy levels of a free RE ion. Due to slightly different environments experienced by different RE ions in a real crystal, spectral lines from an ensemble of RE ions broaden. However, this inhomogeneous broadening is small in many cases (0.001–0.1 cm–1, see, e.g., Refs. [1,2]), which delivers a possibility to study the presence of point defects, strains, other (unwanted) RE ions in a sample. The f-f spectra of RE ions extend from the far infrared to ultraviolet range, the most intense optical transitions allowed for a free ion lie in the infrared range. Registering such high-resolution spectra with a Fourier spectrometer rather than with a grating one gives a tremendous gain in sensitivity, resolution, and precision of the wavenumber scale[3]. Three examples of the use of high-resolution Fourier spectroscopy for determing the quality of crystals are considered. The first example is connected with defects present in multifunctional nonlinear laser crystals from the family of rare-earth borates with the structure of the natural mineral huntite. The method can be used, in particular, for a rapid detection of a molybdenum impurity that diminishes a transparency in the ultraviolet (UV) spectral region and for improvement of growth technique. The second example deals with detection of stresses and deformations in rare earth containing crystals, by registering splitting of spectral lines. This method may be
useful for controlling the quality of crystals for, e.g., optical quantum memories. The third example shows that a very small amount (at the level of ppm) of different RE ions present in a crystal can be determined using high-resolution spectroscopy.
1 Experimental Polarized optical absorption spectra in a spectral range of 1000–25000 cm–1 were measured using a high-resolution Fourier spectrometer Bruker IFS 125 HR. The method of Fourier spectroscopy is beyond comparison for taking broadband high-resolution spectra with a precise wavenumber scale. The resolution for the absorption measurements was up to 0.005 cm–1 and was chosen to reproduce correctly the narrowest details in the spectra. The spectra were registered in the α (k||c, E, H ⊥ c), σ (k ⊥ c, E ⊥ c), and π (k ⊥ c, E||c) polarizations. For low-temperature experiments, we used either a closed-cycle Cryomech ST403 cryostat or a helium-vapor cryostat.
2 Aluminum borates RAl3(BO3)4 Aluminum borate crystals, especially YAl3(BO3)4 (YAB), are important nonlinear optical materials with a wide band gap[4–6]. Doped with the Nd3+ or Yb3+ ions, they are efficiently used in self-frequency doubling and self-frequency summing lasers[7–12]. YAB crystals codoped with Yb3+ and Tm3+ are promising for up-conversion lasers[13,14]. Undoped YAB crystals are considered for the fourth harmonic generation of the Nd:YAG laser radiation.
Foundation item: Project supported in part by the Russian Foundation for Basic Research (13-02-01091) * Corresponding author: Marina N. Popova (E-mail: [email protected]; Tel.: +7 495 851 02 34) DOI: 10.1016/S1002-0721(14)60067-3
Marina N. Popova, High-resolution Fourier spectroscopy as a tool for studying quality of rare-earth-doped crystals
Rare-earth aluminum borates melt incongruently, so the single crystals can be grown only by a flux method. The first aluminium borates were obtained by Ballman in 1962 using the K2SO4-3MoO3 and PbF2-3B2O3 fluxes[15]. Later, Leonyuk et al. have suggested the K2Mo3O10 flux[16] and succeeded in growing crystals up to 30 mm in size. Recently, various RAl3(BO3)4 crystals of good optical quality and big size were obtained in the group of Bezmaternykh in the Kirenskii Institute of Physics, using a new Bi2Mo3O12-Li2MoO4 flux[17]. Because of the flux growth method, it is very difficult to control growth defects, such as twinning, uncontrollable impurities from the solvent, etc. Such defects diminish an application potential of the considered crystals. For example, the molybdenum impurity restricts the use of YAB crystals in the UV spectral range[18]. Therefore, the problem of impurity control at different growth techniques is highly topical. Recently, we have shown that the measurement of the fine structure of the zero-phonon 0(2F7/2)→0'(2F5/2) absorption line of Yb3+ doped into a RAB crystal is a sensitive method for detecting uncontrollable impurities entering the crystal during the growth by a flux technique[19–21]. Using crystals specially grown with a flux containing excess molybdenum or bismuth components, we were able to identify satellites accompanying the 0–0 Yb3+ line in R1–xYbxAl3(BO3)4, (R=Y, Tm, Lu) crystals. Fig. 1 displays the high resolution spectra in the region near the 0–0 line of the Yb3+ ions which were doped in similar concentrations into YAB crystals grown independently in four different laboratories using different fluxes. The majority of the crystals were grown in the Kirensky Institute of Physics using a bismuth-lithiummolybdenum oxide solvent. The second group of YAB:Yb crystals was grown from flux based on potassium trimolybdate solvent. These crystals with different concentrations of Yb were grown in three different laboratories, namely in the University of Verona, in the Laboratory of Solid State Chemistry in Paris (LCMCP), and in the Moscow State University. The last group of YAB:Yb crystals was grown in LCMCP using LaB3O6
Fig. 1 Zero-phonon line 0(2F7/2)→0'(2F5/2) of Yb3+ in YAB single crystals doped with Yb3+ in similar concentrations but grown by different solvents (T=3.6 K[21])
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as the base for flux. From the relative intensities of the satellite lines related to Mo impurities, it was determined that the concentration of molybdenum in YAB crystals grown with the K2Mo3O10-based flux is more than 300-fold higher than in crystals grown with the Bi2Mo3O12-based flux. As for the crystals grown from the flux based on LaB3O6, they obviously do not contain impurities like molybdenum and bismuth. The lanthanum is the only possible impurity in such crystals. It enters the crystal lattice substituting up to 20% of yttrium ions which results in ytterbium line broadening and shift[21]. In summary, our spectroscopic method can be used for a rapid analysis of the quality of crystals for UV lasers and for improvement of a flux solution technique used to grow laser crystals and crystals for the fourth harmonic generation of the Nd-YAG laser.
3 Detection of stress and deformations Internal stress and deformations in a crystal may arise during the growth and annealing process, they also depend crucially on the mode of mounting a sample for measurements. In particular, a crystal on a cold finger of a cryostat is usually firmly pressed to the finger’s surface or/and glued to it to ensure a good thermal contact. A strong stress that arises in this case can be detected via RE spectral line broadening and, especially, splitting. Fig. 2 shows as an example a drastic distortion of a spectral profile for a singlet-doublet optical transition in LiYF4: Tm3+ when a thin sample is glued to a mount along its perimeter.
Fig. 2 Measured at T=4.3 K (1)–(3) and calculated (4) line shapes for the singlet-doublet transition Γ2(3H6)→Γ34(3H5) of Tm3+ in LiYF4 (non-Kramers ion in S4 symmetry position) (1) 7LiYF4:Ho (0.1 at.%), Tm (0.5 ppm) bulk sample (8.8 mm); (2) 7Li0.936Li0.07YF4:Tm (0.1 at.%) sample (90 μm) placed free into a helium vapor in the helium-vapor cryostat; (3) the sample (2) but glued along its perimeter to a mount (The stress in the glued sample was estimated to be in the range of 8–28 MPa[22])
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Internal stress and deformations can be induced also by intrinsic defects such as impurity ions introduced as dopants. In the latter case, one deals with randomly distributed lattice strains. Random lattice deformations shift the frequency of a singlet-singlet optical transition and, as the deformation tensor components with opposite signs are equally probable, an inhomogeneously broadened spectral line of such transition has a symmetric shape. It is different with an optical transition involving at least one degenerate (non-Kramers) electronic energy level. As in the case of the Jahn-Teller effect, there always exist deformations that lift the degeneracy. Provided that the coupling of the degenerate electronic level with the low-symmetry deformations dominates over its coupling with totally symmetric deformations, which cause level shifts, a specific deep dip appears at the center of the inhomogeneously broadened line. Curve (1) in Fig. 2 demonstrates the line shape of this kind. Recently, we have observed such dips in high-resolution optical spectra of a number of crystals containing RE ions in tetragonal[22], trigonal[23], and cubic[24,25] positions. In the latter case, not only non-Kramers but also Kramers ions can serve as probes of lattice strains. Figs. 3 and 4 present line shapes of the absorption line for the doublet – quadruplet transition 2F7/2(Γ6)→2F5/2(Γ8) of Yb3+ in Cs2NaYF6 and Cs2NaScF6 crystals, respectively, doped with Yb3+ (Kramers ion in Oh symmetry position). Observed line shapes were modeled on the basis of a theory carefully elaborated by Boris Malkin[22,25]. Solid lines in Figs. 2–4 represent calculated absorption line shapes. They reproduce the experimentally measured
Fig. 3 Measured (symbols) and simulated (solid curves (1) and (2) correspond to single ion and dimer centers, respectively) absorption line shapes for the doublet-quadruplet transition 2F7/2(Γ6)→2F5/2(Γ8) of Yb3+ in Cs2NaYF6:Yb3+ samples with the ytterbium concentration: (a) 0.01 at.% and (b) 1 at.% (spectra were taken with the resolution 0.05 and 0.1 cm–1, respectively, at T=3.5 K. Inset shows the spectral envelope calculated for a perfect sample (without random strains) containing odd and even Yb isotopes with natural abundances[25])
JOURNAL OF RARE EARTHS, Vol. 32, No. 3, Mar. 2014
Fig. 4 Measured (symbols) and simulated (solid curves) absorption line shapes for the transition 2F7/2(Γ6)→2F5/2(Γ8) in Cs2NaScF6:Yb3+ with the ytterbium concentrations: (a) 0.1 at.%, and (b) 2 at.%, T=3.5 K. Pay attention to that the wavenumber scales in (a) and (b) differ by an order of magnitude
ones rather well. Such modeling enabled to evaluate quantitatively intrinsic strains present in doped crystals. They are dependent on the impurity concentration and on the difference in ionic radii between impurity and host ions. Because the differences between the ionic radii of Yb3+ (0.0868 nm), and Y3+ (0.09 nm)[30] are small, perturbations of the lattice by the impurity ions in Cs2NaYF6 can be neglected and the strains in weakly doped crystals can be associated mainly with intrinsic lattice defects. Deformation splittings differ by less than 30% while the Yb3+ concentrations vary by two orders of magnitude (compare Figs. 3(a) and 3(b)). On the contrary, a pronounced effect caused by lattice deformations due to the impurity RE ions was found in the scandium eplasolites doped with the Yb3+ ions, where the difference between the ionic radii of the host Sc3+ (0.0745 nm) and the Yb3+ (0.0868 nm) impurity ions is rather large. In these scandium crystals the deformation splitting increases markedly with the concentration of the impurity Yb3+ ions, in particular, it changes fivefold in Cs2NaScF6 crystals when the nominal ytterbium concentration c increases twenty times (see Fig. 4). An interesting example of an altered line shape due to random lattice deformations of another type was given by Chaminade, Macfarlane, et al. for Pr3+:CsCdBr3 [26]. Crystals of rare-earth-doped quasi-one-dimensional double bromides CsCdBr3 have a property to incorporate R3+ ions in pairs, even at low concentrations of a rare earth. This makes them a promising material for up-conversion lasers. The structure of CsCdBr3 consists of linear chains of face-sharing [CdBr6]4+ octahedra along the c axis. The positional symmetry for Cd2+ is D3d. R3+ ions substitute for Cd2+, forming centers with different mechanisms of charge compensation. The main center consists of two R3+ ions placed in the chain on each side of an adjacent
Marina N. Popova, High-resolution Fourier spectroscopy as a tool for studying quality of rare-earth-doped crystals
cadmium vacancy. Both R3+ ions in such center are equivalent; their positional symmetry lowers from D3d to C3v. In the spectrum of the singlet-doublet 3H4(Γ1)→ 1 D2(Γ3) transition of Pr3+ in CsCdBr3 measured with a single frequency dye laser at a resolution of 1 MHz a peculiar absorption line shape was registered (see the left part of Fig. 5). The observed doublet structure was attributed to a combined effect of hyperfine coupling and randomly distributed non-axial crystal strains that originate from local misalignments of the crystal c-axis[26,27]. The crystals used in this study were grown by Chaminade et al. in the University of Bordeaux, France. Interestingly, several years later Chaminade et al. succeeded in growing a crystal of the same compound having much better quality which was demonstrated by a perfectly resolved hyperfine structure in our high-resolution spectra taken with a Fourier spectrometer[28,29] (right part of Fig. 5). Small residual local misalignments of the crystal c-axis resulted in a central dip observed in this spectrum. Modification of the spectral line shape induced by local strains may be important for a number of modern applications of rare-earth-doped crystals, e.g., for applications in quantum information storage and signal processing[31].
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Fig. 6 Absorption line corresponding to the lowest frequency transition in the 4I15/2→4I13/2 optical multiplet of Er3+ in LiYF4:Ho (0.1%) at 5 K unregistrated at different spectral resolutions of a Fourier spectrometer. A low-intensity structure in the left icon is due to hyperfine interactions in the 167Er isotope. The next icon demonstrates the instrumental function of the Fourier spectrometer
4 Detection of other (unwanted) RE ions at the ppm level Even ultra-high purity rare-earth compounds available at the market contain parts per million (ppm) of other rare earth elements. These trace impurities can be reliably detected using high-resolution spectroscopy. Fig. 6 illustrates this statement. It presents the narrowest line in the spectrum of the erbium ion substituting for yttrium in the LiYF4 crystal. At a resolution 0.005 cm–1 (the left icon) the true line width of 0.01 cm–1 can be measured. Using the calibration curve (Fig. 7), one finds 3±0.5 ppm for the erbium concentration in the crystal. It is evident
Fig. 7 Calibration curve for the lowest frequency transition in the 4I15/2→4I13/2 optical multiplet of Er3+ in LiYF4 at 5 K
from Fig. 6 that the maximum sensitivity is of about an order of magnitude better. In the case of record narrow lines (0.001 cm–1) of a trace impurity, it can be detected at the level of several tens of ppb. If a resolved spectral interval exceeds the line width, the line broadens but its amplitude diminishes, so that the integral is conserved. This reduces a sensitivity of impurity detection by a spectral method. At a further decay of resolution the line disappears completely (the right icon of Fig. 6).
5 Conclusions
Fig. 5 Absorption line shapes for the 3H4(Γ1) )→1D2(Γ3) (left part, experimental trace[26]) and 3H4(Γ1) )→ 3F3(Γ3) (right part[29]) transitions of Pr 3+ in CsCdBr3 (see text)
In this presentation, the use of high resolution Fourier-transform optical spectroscopy for studying quality of rare-earth-doped crystals was considered. It was shown, in particular, that in the case of crystals grown by a flux method impurities that entered the crystal lattice during the growth process manifest themselves by spectral satellites in the vicinity of main lines corresponding
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to the transitions between crystal-field levels of a rareearth ion. High-resolution spectroscopy revealed genuine spectral shapes and could be used for an express analysis of the quality of crystals for UV lasers and for improvement of a flux solution technique used to grow laser crystals and crystals for the fourth harmonic generation of the Nd-YAG laser. Another example concerned detection of strains and deformations in rare-earth-doped crystals, especially those designed for the use in quantum informatics devices, by analysing specific line shapes of transitions involving at least one level with a non-Kramers degeneracy. If complemented by a theoretical modeling of the observed spectral shapes, this analysis delivered quantitative information on lattice strains. The third example dealt with a detection of trace impurities in crystals using high-resolution spectroscopy. For impurities possessing the narrowest spectral lines, sensitivity of the spectral method approached the ppb level. Acknowledgements: I am grateful to B.Z. Malkin for useful discussions. I thank my coauthors, K.N. Boldyrev, P.O. Petit, B. Viana, M. Bettinelli, P. Deren, P. Loiseau, G. Aka, B.Z. Malkin, M.V. Vanyunin, S.A. Klimin, E.P. Chukalina, D.S. Pytalev, E. Antic-Fidancev, P. Porcher who made a decisive contribution to various parts of the research described here, J.P. Chaminade, S.L. Korableva, L.N. Bezmaternykh, V.L. Temerov, I. Gudim, N.I. Leonyuk and N.M. Khaidukov are kindly acknowledged for the crystal growth.
References: [1] Agladze N I, Popova M N, Zhizhin G N, Egorov V J, Petrova M A. Isotope structure in optical spectra of LiYF4:Ho3+. Phys. Rev. Lett., 1991, 66: 477. [2] Macfarlane R M, Meltzer R S, Malkin B Z. Optical measurement of the isotope shifts and hyperfine and superhyperfine interactions of Nd in the solid state. Phys. Rev. B, 1998, 58: 5692. [3] Bell R J. Introductory Fourier Transform Spectroscopy. New York: Academic Press, 1972. [4] Filimonov A A, Leonyuk N I, Meissner I B, Timchenko T I, Rez I S. Nonlinear optical properties of isomorphic family of crystals with yttrium-aluminium borate (YAB) structure. Cryst. Res. Technol., 1974, 9: 63. [5] Nikogosyan D N. Nonlinear Optical Crystals: A Complete Survey. Berlin: Springer, 2005. 427. [6] Leonyuk N I, Leonyuk L I. Growth and characterization of RM3(BO3)4 crystals. Prog. Cryst. Growth Charact. Mater., 1995, 31: 179. [7] Dorozhkin I M, Kuratev I I, Leonyuk N I, Timchenko T I, Shestakov A V. Generation of second optical harmonic in NYAB crystals, new active-nonlinear medium. JTP Lett., 1981, 7: 1297. [8] Jaque D. Self-frequency-sum mixing in Nd doped nonlinear crystals for laser generation in the three fundamental colours: The NYAB case. J. Alloys Compd., 2001,
JOURNAL OF RARE EARTHS, Vol. 32, No. 3, Mar. 2014 323-324: 204. [9] Brenier A A, Tu C, Zhu Z, Wu B. Red-green-blue generation from a lone dual-wavelengthGdAl3(BO3)4 :Nd3+ laser. Appl. Phys. Lett., 2004, 84: 2034. [10] Wang P, Dawes J, Dekker P, Piper J. Highly efficient diodepumped ytterbium-doped yttrium aluminum borate laser. Opt. Commun., 2000, 174: 467. [11] Jiang H D, Li J, Wang J Y, Hu X B, Liu H, Teng B, Zhang C Q, Dekker P, Wang P. Growth of Yb:YAl3(BO3)4 crystals and their optical and self-frequency-doubling properties. J. Cryst. Growth, 2001, 233: 248. [12] Burns P, Dawes J, Dekker P, Piper J, Li J, Wang J. Coupled-cavity, single-frequency, tunable cw Yb-YAB yellow microchip laser. Opt. Commun., 2002, 207: 315. [13] Dominiak-Dzik G, Ryba-Romanowski W, Lisiecki R, Foldvári I, Beregi E. YAl3(BO3)4:Yb&Tm nonlinear crystal: up- and down-conversion phenomena and excited state relaxations. Opt. Mater., 2009, 31: 989. [14] Aleksandrovsky A S, Gudim I A, Krylov A S, Malakhovskii A S, Temerov V L. Upconversion luminescence of YAl3(BO3)4:(Yb3+,Tm3+) crystals. J. Alloys Compd., 2010, 496: L18. [15] Ballman A A. A new series of synthetic borates isostructural with the carbonate mineral huntite. Am. Mineral., 1962, 47: 1380. [16] Leonyuk N I, Semenova T D, Timchenko T I, Sheftal N N. Synthesis of potassium trimolybdate, new flux for YAlborate. Vestnik Moskovskogo Universiteta, ser. Geologiya (in Russ.), 1972, No 2, 112. [17] Bezmaternykh L N, Kharlamova S A, Temerov V L. Flux crystallization of trigonal GdFe3(BO3)4 competing with the crystallization of a-Fe2O3. Crystallogr. Rep., 2004, 49: 855. [18] Yu X S, Yue Y C, Yao J Y, Hu Z G. YAl3(BO3)4 crystal growth and characterization. J. Cryst. Growth, 2010, 312: 3029. [19] Popova M N, Boldyrev K N, Petit P O, Viana B, Bezmaternykh L N. High-resolution spectroscopy of YbAl3(BO3)4 stoichiometric nonlinear laser crystals. J. Phys.: Condens. Matter, 2008, 20: 455210. [20] Boldyrev K N, Popova M N, Bezmaternykh L N, Bettinelli M. Nonequivalent Yb3+ centers in Y1–xYbxAl3(BO3)4 laser crystals. Quantum Electron., 2011, 41: 120. [21] Boldyrev K N, Popova M N, Bettinelli M, Temerov V L, Gudim I A, Bezmaternykh L N, Loiseau P, Aka G, Leonyuk N I. Quality of the rare earth aluminium borate crystals for laser applications, probed by high-resolution spectroscopy of the Yb3+ ion. Opt. Mater., 2012, 34: 1885. [22] Klimin S A, Pytalev D S, Popova M N, Malkin B Z, Vanyunin M V, Korableva S L. High-resolution optical spectroscopy of Tm ions in LiYF4: crystal-field energies, hyperfine and deformation splittings, and the isotopic structure. Phys. Rev. B: Condens. Matter, 2010, 81: 045113. [23] Boldyrev K, Deren P, Popova M. High-resolution optical spectroscopy of LaAlO3 single crystals doped with Ho3+, Pr3+, and Tm3+. International Conference on Rare Earth Materials. Advances in Synthesis, Studies and Applications, Wroclaw, Poland, 26-28 April, 2013, Book of Ab-
Marina N. Popova, High-resolution Fourier spectroscopy as a tool for studying quality of rare-earth-doped crystals stracts P7. [24] Popova M, Pytalev D, Khaidukov N. Signatures of random lattice deformations in high-resolution optical spectra of holmium doped cubic hexafluoroelpasolites. International Conference on Rare Earth Materials. Advances in Synthesis, Studies and Applications, Wroclaw, Poland, 26-28 April, 2013, Book of Abstracts. P53. [25] Malkin B Z, Pytalev D S, Popova M N, Baibekov E I, Falin M L, Gerasimov K I, Khaidukov N M. Random lattice deformations in rare earth doped cubic hexafluoroelpasolites: high-resolution optical spectroscopy and theoretical studies. Phys. Rev. B: Condens. Matter, 2012, 86: 134110. [26] Chaminade J P, Macfarlane R M, Ramaz F, Vial J C. High resolution laser spectroscopy of rare earth doped CsCdBr3. J. Lumin., 1991, 48&49: 531. [27] Ramaz F, Vial J C, Macfarlane R M. Measurement of the angular distribution of the crystallographic c-axis in a
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crystal of CsCdBr3:Pr using hole-burning Stark spectroscopy. Europhys. Lett., 1993, 22: 217. [28] Chukalina E P, Popova M N, Antic-Fidancev E, Chaminade J P. Hyperfine structure in optical spectra of CsCdBr3:Pr3+. Phys. Lett. A, 1999, 258: 375. [29] Popova M N, Chukalina E P, Malkin B Z, Antic-Fidancev E, Porcher P, Chaminade J-P. High resolution infrared absorption spectra, crystal field levels, and relaxation processes in CsCdBr3:Pr3+. Phys. Rev., B: Condens. Matter, 2001, 63: 075103. [30] Shannon R D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr., Sect. A: Found. Crystallogr., 1976, 32: 751. [31] Thiel C W, Bottger T, Cone R L. Rare-earth-doped materials for applications in quantum information storage and signal processing. J. Lumin., 2011, 131: 353.
High-resolution Fourier spectroscopy as a tool for studying quality of rare-earth-doped crystals Marina N. Popova* (Institute of Spectroscopy, Russian Academy of Sciences, 5, Fizicheskaya Str., 142190, Troitsk, Moscow, Russia) Received 9 June 2013; revised 9 September 2013
Abstract: Three examples were considered of the use of high resolution Fourier-transform optical spectroscopy for studying quality of rare-earth-doped crystals. The first example was connected with defects present in crystals grown by flux techniques. The second example dealt with detection of stresses and deformations in rare-earth-containing crystals, by registering splitting of spectral lines. The third example showed that a very small amount (at the level of ppm) of different RE ions present in a crystal could be determined using high-resolution spectroscopy. This work was carried out by the author’s group in collaboration with several institutions in Russia and abroad. Keywords: Fourier-transform optical spectroscopy; high resolution; absorption spectra; rare earths
Optical spectra of rare-earth (RE) ions in crystals are, mainly, due to parity forbidden f-f transitions that become allowed due to odd components of the crystal field. The 4f-shell is well shielded from a crystalline environment by the filled 5s and 5p shells, and the crystal field acts as a weak perturbation to the energy levels of a free RE ion. Due to slightly different environments experienced by different RE ions in a real crystal, spectral lines from an ensemble of RE ions broaden. However, this inhomogeneous broadening is small in many cases (0.001–0.1 cm–1, see, e.g., Refs. [1,2]), which delivers a possibility to study the presence of point defects, strains, other (unwanted) RE ions in a sample. The f-f spectra of RE ions extend from the far infrared to ultraviolet range, the most intense optical transitions allowed for a free ion lie in the infrared range. Registering such high-resolution spectra with a Fourier spectrometer rather than with a grating one gives a tremendous gain in sensitivity, resolution, and precision of the wavenumber scale[3]. Three examples of the use of high-resolution Fourier spectroscopy for determing the quality of crystals are considered. The first example is connected with defects present in multifunctional nonlinear laser crystals from the family of rare-earth borates with the structure of the natural mineral huntite. The method can be used, in particular, for a rapid detection of a molybdenum impurity that diminishes a transparency in the ultraviolet (UV) spectral region and for improvement of growth technique. The second example deals with detection of stresses and deformations in rare earth containing crystals, by registering splitting of spectral lines. This method may be
useful for controlling the quality of crystals for, e.g., optical quantum memories. The third example shows that a very small amount (at the level of ppm) of different RE ions present in a crystal can be determined using high-resolution spectroscopy.
1 Experimental Polarized optical absorption spectra in a spectral range of 1000–25000 cm–1 were measured using a high-resolution Fourier spectrometer Bruker IFS 125 HR. The method of Fourier spectroscopy is beyond comparison for taking broadband high-resolution spectra with a precise wavenumber scale. The resolution for the absorption measurements was up to 0.005 cm–1 and was chosen to reproduce correctly the narrowest details in the spectra. The spectra were registered in the α (k||c, E, H ⊥ c), σ (k ⊥ c, E ⊥ c), and π (k ⊥ c, E||c) polarizations. For low-temperature experiments, we used either a closed-cycle Cryomech ST403 cryostat or a helium-vapor cryostat.
2 Aluminum borates RAl3(BO3)4 Aluminum borate crystals, especially YAl3(BO3)4 (YAB), are important nonlinear optical materials with a wide band gap[4–6]. Doped with the Nd3+ or Yb3+ ions, they are efficiently used in self-frequency doubling and self-frequency summing lasers[7–12]. YAB crystals codoped with Yb3+ and Tm3+ are promising for up-conversion lasers[13,14]. Undoped YAB crystals are considered for the fourth harmonic generation of the Nd:YAG laser radiation.
Foundation item: Project supported in part by the Russian Foundation for Basic Research (13-02-01091) * Corresponding author: Marina N. Popova (E-mail: [email protected]; Tel.: +7 495 851 02 34) DOI: 10.1016/S1002-0721(14)60067-3
Marina N. Popova, High-resolution Fourier spectroscopy as a tool for studying quality of rare-earth-doped crystals
Rare-earth aluminum borates melt incongruently, so the single crystals can be grown only by a flux method. The first aluminium borates were obtained by Ballman in 1962 using the K2SO4-3MoO3 and PbF2-3B2O3 fluxes[15]. Later, Leonyuk et al. have suggested the K2Mo3O10 flux[16] and succeeded in growing crystals up to 30 mm in size. Recently, various RAl3(BO3)4 crystals of good optical quality and big size were obtained in the group of Bezmaternykh in the Kirenskii Institute of Physics, using a new Bi2Mo3O12-Li2MoO4 flux[17]. Because of the flux growth method, it is very difficult to control growth defects, such as twinning, uncontrollable impurities from the solvent, etc. Such defects diminish an application potential of the considered crystals. For example, the molybdenum impurity restricts the use of YAB crystals in the UV spectral range[18]. Therefore, the problem of impurity control at different growth techniques is highly topical. Recently, we have shown that the measurement of the fine structure of the zero-phonon 0(2F7/2)→0'(2F5/2) absorption line of Yb3+ doped into a RAB crystal is a sensitive method for detecting uncontrollable impurities entering the crystal during the growth by a flux technique[19–21]. Using crystals specially grown with a flux containing excess molybdenum or bismuth components, we were able to identify satellites accompanying the 0–0 Yb3+ line in R1–xYbxAl3(BO3)4, (R=Y, Tm, Lu) crystals. Fig. 1 displays the high resolution spectra in the region near the 0–0 line of the Yb3+ ions which were doped in similar concentrations into YAB crystals grown independently in four different laboratories using different fluxes. The majority of the crystals were grown in the Kirensky Institute of Physics using a bismuth-lithiummolybdenum oxide solvent. The second group of YAB:Yb crystals was grown from flux based on potassium trimolybdate solvent. These crystals with different concentrations of Yb were grown in three different laboratories, namely in the University of Verona, in the Laboratory of Solid State Chemistry in Paris (LCMCP), and in the Moscow State University. The last group of YAB:Yb crystals was grown in LCMCP using LaB3O6
Fig. 1 Zero-phonon line 0(2F7/2)→0'(2F5/2) of Yb3+ in YAB single crystals doped with Yb3+ in similar concentrations but grown by different solvents (T=3.6 K[21])
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as the base for flux. From the relative intensities of the satellite lines related to Mo impurities, it was determined that the concentration of molybdenum in YAB crystals grown with the K2Mo3O10-based flux is more than 300-fold higher than in crystals grown with the Bi2Mo3O12-based flux. As for the crystals grown from the flux based on LaB3O6, they obviously do not contain impurities like molybdenum and bismuth. The lanthanum is the only possible impurity in such crystals. It enters the crystal lattice substituting up to 20% of yttrium ions which results in ytterbium line broadening and shift[21]. In summary, our spectroscopic method can be used for a rapid analysis of the quality of crystals for UV lasers and for improvement of a flux solution technique used to grow laser crystals and crystals for the fourth harmonic generation of the Nd-YAG laser.
3 Detection of stress and deformations Internal stress and deformations in a crystal may arise during the growth and annealing process, they also depend crucially on the mode of mounting a sample for measurements. In particular, a crystal on a cold finger of a cryostat is usually firmly pressed to the finger’s surface or/and glued to it to ensure a good thermal contact. A strong stress that arises in this case can be detected via RE spectral line broadening and, especially, splitting. Fig. 2 shows as an example a drastic distortion of a spectral profile for a singlet-doublet optical transition in LiYF4: Tm3+ when a thin sample is glued to a mount along its perimeter.
Fig. 2 Measured at T=4.3 K (1)–(3) and calculated (4) line shapes for the singlet-doublet transition Γ2(3H6)→Γ34(3H5) of Tm3+ in LiYF4 (non-Kramers ion in S4 symmetry position) (1) 7LiYF4:Ho (0.1 at.%), Tm (0.5 ppm) bulk sample (8.8 mm); (2) 7Li0.936Li0.07YF4:Tm (0.1 at.%) sample (90 μm) placed free into a helium vapor in the helium-vapor cryostat; (3) the sample (2) but glued along its perimeter to a mount (The stress in the glued sample was estimated to be in the range of 8–28 MPa[22])
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Internal stress and deformations can be induced also by intrinsic defects such as impurity ions introduced as dopants. In the latter case, one deals with randomly distributed lattice strains. Random lattice deformations shift the frequency of a singlet-singlet optical transition and, as the deformation tensor components with opposite signs are equally probable, an inhomogeneously broadened spectral line of such transition has a symmetric shape. It is different with an optical transition involving at least one degenerate (non-Kramers) electronic energy level. As in the case of the Jahn-Teller effect, there always exist deformations that lift the degeneracy. Provided that the coupling of the degenerate electronic level with the low-symmetry deformations dominates over its coupling with totally symmetric deformations, which cause level shifts, a specific deep dip appears at the center of the inhomogeneously broadened line. Curve (1) in Fig. 2 demonstrates the line shape of this kind. Recently, we have observed such dips in high-resolution optical spectra of a number of crystals containing RE ions in tetragonal[22], trigonal[23], and cubic[24,25] positions. In the latter case, not only non-Kramers but also Kramers ions can serve as probes of lattice strains. Figs. 3 and 4 present line shapes of the absorption line for the doublet – quadruplet transition 2F7/2(Γ6)→2F5/2(Γ8) of Yb3+ in Cs2NaYF6 and Cs2NaScF6 crystals, respectively, doped with Yb3+ (Kramers ion in Oh symmetry position). Observed line shapes were modeled on the basis of a theory carefully elaborated by Boris Malkin[22,25]. Solid lines in Figs. 2–4 represent calculated absorption line shapes. They reproduce the experimentally measured
Fig. 3 Measured (symbols) and simulated (solid curves (1) and (2) correspond to single ion and dimer centers, respectively) absorption line shapes for the doublet-quadruplet transition 2F7/2(Γ6)→2F5/2(Γ8) of Yb3+ in Cs2NaYF6:Yb3+ samples with the ytterbium concentration: (a) 0.01 at.% and (b) 1 at.% (spectra were taken with the resolution 0.05 and 0.1 cm–1, respectively, at T=3.5 K. Inset shows the spectral envelope calculated for a perfect sample (without random strains) containing odd and even Yb isotopes with natural abundances[25])
JOURNAL OF RARE EARTHS, Vol. 32, No. 3, Mar. 2014
Fig. 4 Measured (symbols) and simulated (solid curves) absorption line shapes for the transition 2F7/2(Γ6)→2F5/2(Γ8) in Cs2NaScF6:Yb3+ with the ytterbium concentrations: (a) 0.1 at.%, and (b) 2 at.%, T=3.5 K. Pay attention to that the wavenumber scales in (a) and (b) differ by an order of magnitude
ones rather well. Such modeling enabled to evaluate quantitatively intrinsic strains present in doped crystals. They are dependent on the impurity concentration and on the difference in ionic radii between impurity and host ions. Because the differences between the ionic radii of Yb3+ (0.0868 nm), and Y3+ (0.09 nm)[30] are small, perturbations of the lattice by the impurity ions in Cs2NaYF6 can be neglected and the strains in weakly doped crystals can be associated mainly with intrinsic lattice defects. Deformation splittings differ by less than 30% while the Yb3+ concentrations vary by two orders of magnitude (compare Figs. 3(a) and 3(b)). On the contrary, a pronounced effect caused by lattice deformations due to the impurity RE ions was found in the scandium eplasolites doped with the Yb3+ ions, where the difference between the ionic radii of the host Sc3+ (0.0745 nm) and the Yb3+ (0.0868 nm) impurity ions is rather large. In these scandium crystals the deformation splitting increases markedly with the concentration of the impurity Yb3+ ions, in particular, it changes fivefold in Cs2NaScF6 crystals when the nominal ytterbium concentration c increases twenty times (see Fig. 4). An interesting example of an altered line shape due to random lattice deformations of another type was given by Chaminade, Macfarlane, et al. for Pr3+:CsCdBr3 [26]. Crystals of rare-earth-doped quasi-one-dimensional double bromides CsCdBr3 have a property to incorporate R3+ ions in pairs, even at low concentrations of a rare earth. This makes them a promising material for up-conversion lasers. The structure of CsCdBr3 consists of linear chains of face-sharing [CdBr6]4+ octahedra along the c axis. The positional symmetry for Cd2+ is D3d. R3+ ions substitute for Cd2+, forming centers with different mechanisms of charge compensation. The main center consists of two R3+ ions placed in the chain on each side of an adjacent
Marina N. Popova, High-resolution Fourier spectroscopy as a tool for studying quality of rare-earth-doped crystals
cadmium vacancy. Both R3+ ions in such center are equivalent; their positional symmetry lowers from D3d to C3v. In the spectrum of the singlet-doublet 3H4(Γ1)→ 1 D2(Γ3) transition of Pr3+ in CsCdBr3 measured with a single frequency dye laser at a resolution of 1 MHz a peculiar absorption line shape was registered (see the left part of Fig. 5). The observed doublet structure was attributed to a combined effect of hyperfine coupling and randomly distributed non-axial crystal strains that originate from local misalignments of the crystal c-axis[26,27]. The crystals used in this study were grown by Chaminade et al. in the University of Bordeaux, France. Interestingly, several years later Chaminade et al. succeeded in growing a crystal of the same compound having much better quality which was demonstrated by a perfectly resolved hyperfine structure in our high-resolution spectra taken with a Fourier spectrometer[28,29] (right part of Fig. 5). Small residual local misalignments of the crystal c-axis resulted in a central dip observed in this spectrum. Modification of the spectral line shape induced by local strains may be important for a number of modern applications of rare-earth-doped crystals, e.g., for applications in quantum information storage and signal processing[31].
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Fig. 6 Absorption line corresponding to the lowest frequency transition in the 4I15/2→4I13/2 optical multiplet of Er3+ in LiYF4:Ho (0.1%) at 5 K unregistrated at different spectral resolutions of a Fourier spectrometer. A low-intensity structure in the left icon is due to hyperfine interactions in the 167Er isotope. The next icon demonstrates the instrumental function of the Fourier spectrometer
4 Detection of other (unwanted) RE ions at the ppm level Even ultra-high purity rare-earth compounds available at the market contain parts per million (ppm) of other rare earth elements. These trace impurities can be reliably detected using high-resolution spectroscopy. Fig. 6 illustrates this statement. It presents the narrowest line in the spectrum of the erbium ion substituting for yttrium in the LiYF4 crystal. At a resolution 0.005 cm–1 (the left icon) the true line width of 0.01 cm–1 can be measured. Using the calibration curve (Fig. 7), one finds 3±0.5 ppm for the erbium concentration in the crystal. It is evident
Fig. 7 Calibration curve for the lowest frequency transition in the 4I15/2→4I13/2 optical multiplet of Er3+ in LiYF4 at 5 K
from Fig. 6 that the maximum sensitivity is of about an order of magnitude better. In the case of record narrow lines (0.001 cm–1) of a trace impurity, it can be detected at the level of several tens of ppb. If a resolved spectral interval exceeds the line width, the line broadens but its amplitude diminishes, so that the integral is conserved. This reduces a sensitivity of impurity detection by a spectral method. At a further decay of resolution the line disappears completely (the right icon of Fig. 6).
5 Conclusions
Fig. 5 Absorption line shapes for the 3H4(Γ1) )→1D2(Γ3) (left part, experimental trace[26]) and 3H4(Γ1) )→ 3F3(Γ3) (right part[29]) transitions of Pr 3+ in CsCdBr3 (see text)
In this presentation, the use of high resolution Fourier-transform optical spectroscopy for studying quality of rare-earth-doped crystals was considered. It was shown, in particular, that in the case of crystals grown by a flux method impurities that entered the crystal lattice during the growth process manifest themselves by spectral satellites in the vicinity of main lines corresponding
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to the transitions between crystal-field levels of a rareearth ion. High-resolution spectroscopy revealed genuine spectral shapes and could be used for an express analysis of the quality of crystals for UV lasers and for improvement of a flux solution technique used to grow laser crystals and crystals for the fourth harmonic generation of the Nd-YAG laser. Another example concerned detection of strains and deformations in rare-earth-doped crystals, especially those designed for the use in quantum informatics devices, by analysing specific line shapes of transitions involving at least one level with a non-Kramers degeneracy. If complemented by a theoretical modeling of the observed spectral shapes, this analysis delivered quantitative information on lattice strains. The third example dealt with a detection of trace impurities in crystals using high-resolution spectroscopy. For impurities possessing the narrowest spectral lines, sensitivity of the spectral method approached the ppb level. Acknowledgements: I am grateful to B.Z. Malkin for useful discussions. I thank my coauthors, K.N. Boldyrev, P.O. Petit, B. Viana, M. Bettinelli, P. Deren, P. Loiseau, G. Aka, B.Z. Malkin, M.V. Vanyunin, S.A. Klimin, E.P. Chukalina, D.S. Pytalev, E. Antic-Fidancev, P. Porcher who made a decisive contribution to various parts of the research described here, J.P. Chaminade, S.L. Korableva, L.N. Bezmaternykh, V.L. Temerov, I. Gudim, N.I. Leonyuk and N.M. Khaidukov are kindly acknowledged for the crystal growth.
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