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Luminescence of trivalent rare earth ions in the yttrium aluminium borate non-linear laser crystal
Journal of Luminescence 102–103 (2003) 216–219
Luminescence of trivalent rare earth ions in the yttrium aluminium borate non-linear laser crystal E. Cavallia, A. Speghinib, M. Bettinellib, M.O. Ram!ırezc, J.J. Romeroc, L.E. Bausa! c, J. Garc!ıa Sole! c,* a
Dipartimento di Chimica GIAF, Universita" di Parma and INFM, UdR Parma, Parco Area delle Scienze 17/A, I-43100 Parma, Italy b Dipartimento Scientifico e Tecnologico, Universita" di Verona and INSTM, UdR Verona, Ca’ Vignal, Strada Le Grazie 15, I-37134 Verona, Italy c ! Departamento de F!ısica de Materiales, C-IV, Universidad Autonoma de Madrid, Cantoblanco, E-28049 Madrid, Spain
Abstract Single crystals of YAl3(BO3)4 doped with the rare earth ions Sm3+, Eu3+, Tb3+, Dy3+, Tm3+ and Yb3+ were grown using the flux growth technique. Luminescence spectra and decay curves were recorded in the visible range at room and low temperatures. The observed transitions were assigned and analysed on the basis of group theory for the D3 point symmetry. Evidence for the presence of non-equivalent centres has been found for Yb3+ and possibly for Sm3+ and Dy3+ doped crystals. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Yttrium aluminium borate; Rare earth ions; Non-equivalent centres
1. Introduction Yttrium aluminium borate YAl3(BO3)4 (YAB) is a non-linear crystal which has proved to be an excellent host for Nd3+ and Yb3+ ions in order to develop self-frequency doubling and self-frequency-sum solid state lasers [1,2]. These lasers have a number of potential applications in optoelectronics as compact diode pumped solid state lasers for the visible and ultraviolet spectral regions. Thus, the incorporation of other trivalent rare earth (RE) ions into this host matrix appears to be very interesting in order to search for new *Corresponding author. Tel.: +34-913975029; fax: +34913978579. E-mail address: jose.garcia [email protected] (J. Garc!ıa Sol!e).
laser channels which could extend the spectral range to new frequencies. In this respect YAB:Pr3+ crystals have been already grown and the optical spectroscopy of Pr3+ ion investigated [3]. Among the other RE ions, the energy level structure of Er3+ in YAB crystals has been recently published [4], but in general the optical spectroscopy of the entire series of lanthanide ions in this host remains to be studied in a detailed way. In this paper we report on the luminescence of YAB crystals doped with Sm3+, Eu3+, Tb3+, Dy3+, Tm3+ and Yb3+. Luminescence spectra and decay times have been measured at room and low temperature. The low temperature emission lines are in agreement with a D3 local environment for the RE ions, indicating that they are located in the Y3+ lattice sites. Evidence for the appearance
0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 2 ) 0 0 4 9 5 - 7
E. Cavalli et al. / Journal of Luminescence 102–103 (2003) 216–219
217
of non-equivalent centres has been found for the crystals doped with Yb3+ and very likely with Sm3+ and Dy3+.
2. Experimental Emitted Intensity (Arb.Units)
Single crystals of YAB doped with various nominal concentrations of Sm3+, Eu3+, Tb3+, Dy3+, Tm3+ or Yb3+ were grown by spontaneous nucleation from a K2Mo3O10 and B2O3 flux using a procedure previously described [3,6]. The reagents employed were of high purity (99.99% or better for Y2O3 and the RE oxides). The largest crystals were affected by flux inclusions, but crystals of about 2 1 1 mm3 were clear and transparent and could be used for the optical measurements. Luminescence spectra at room temperature (RT) and 10 K and RT decay times were measured using the set-up previously described [7].
Tm3+
Eu
Dy3+
3+
Sm Tb
3. Results and discussion The RT visible luminescence spectra of YAB doped with Sm3+, Eu3+, Tb3+, Dy3+ and Tm3+ are shown in Fig. 1. The spectra contain several emission transitions of technological relevance, e.g. the 4F9/2-6H13/2 (570 nm) and 4F9/2-6H11/2 (660 nm) channels of Dy3+ [9] and the 1G4-3F4 (650 nm) and 1G4-3H5 (790 nm) channels of Tm3+ [10]. Following pulsed excitation, all the RT decays for diluted crystals were found to be perfectly exponential. This indicates that, under the adopted experimental conditions, energy transfer processes are not important in the relaxation of the emitting levels. The RT lifetimes t of the emitting levels for diluted RE ions in doped YAB crystals are listed in Table 1. The 10 K luminescence spectra show bands assigned to transitions between manifolds belonging to the respective 4fn configurations. These bands are composed of well-resolved lines, whose number agrees with the predictions of group theory for the dopant ions substituting Y3+ ions in sites of D3 symmetry, as found in previous
3+
450
500
550
600
650
700
3+
750
800
Wavelength (nm) Fig. 1. Overview of the room temperature luminescence spectra of YAB doped with Sm3+ (0.10%, lexc ¼ 403 nm), Eu3+ (1.0%, lexc ¼ 524 nm), Tb3+ (2.9%, lexc ¼ 486 nm), Dy3+ (3.0%, lexc ¼ 450 nm) and Tm3+ (0.10%, lexc ¼ 355 nm).
studies on Pr3+, Nd3+ and Eu3+ in YAB [3,7,8]. These assignments have been confirmed by the measurement of 10 K polarised emission spectra. As an example, the 4G5/2-6H9/2 polarised emission spectra of YAB:0.10% Sm3+ are shown in Fig. 2. These emission spectra consist of five lines at 646.0 nm (s; p), 647.4 nm (s; p), 648.6 nm ðsÞ; 651.0 nm (s; p) and 654.5 nm ðsÞ; in agreement with the electric dipole inter-Stark transitions E1/2(4G5/2)-E1/2, E1/2, E3/2, E1/2, E3/2(6H9/2), respectively, in the D3 double group. These transitions and their corresponding polarisation character are in good agreement with the replacement of Y3+ lattice ions by the Sm3+ ions in sites of D3 symmetry. The additional features observed in the low temperature spectra are possibly associated to the presence of additional Sm3+ centres.
E. Cavalli et al. / Journal of Luminescence 102–103 (2003) 216–219
218
Table 1 RT lifetimes of several emitting levels of Sm3+, Eu3+, Tb3+, Dy3+ and Tm3+ in YAB Concentration (%)
Emitting level
t (ms)
Sm3+ Eu3+ Tb3+ Dy3+ Tm3+
0.10 1.0 2.9 0.30 0.10
4
1800 1300 1900 510 7 110 30
G5/2 D0 5 D4 4 F9/2 1 D2 1 G4 3 H4 5
All the decays were exponential.
D0→7F4
Emitted Intensity (Arb. Units)
Ion
Eu3+ 5
13500 13750 14000 14250 14500 14750 15000
4
Dy3+ 6 F9/2→ H11/2
14400 14600 14800 15000 15200 15400
G5/2→ H9/2
Emission Intensity (Arb. Units)
4
6
σ
15100 15200 15300 15400 15500 15600 15700
G5/2→ H9/2
4
6
π
15100 15200 15300 15400 15500 15600 15700 Energy (cm-1)
Fig. 2. 10 K 4G5/2-6H9/2 polarised luminescence spectra of YAB:Sm3+ (0.10%).
Fig. 3 shows details of the low temperature emission spectra of Eu3+, Dy3+ and Yb3+ ions in YAB. While in YAB:Eu3+ (1.0%) no additional centres are observed, the dashed arrows marked over the luminescence spectra of YAB:Dy3+ (3.0%) and YAB:Yb3+ (2.0%) strongly suggest the formation of new Dy3+ and Yb3+ centres. The presence of Eu3+ ions in non-equivalent sites has been reported in EuAl3(BO3)4 [5] and YxEu1x(BO3)4 (x ¼ 0:1; 0.50 and 0.9) [8] crystals, through the observation of satellite lines in the
2
Yb3+
F5/2→2 F7/2
9700 9800 9900 10000 10100 10200 10300 Energy (cm-1)
Fig. 3. Details of the 10 K luminescence spectra of YAB doped with 1.0% Eu3+, 3.0% Dy3+ and 2.0% Yb3+, showing the presence of non-equivalent centres in the latter two crystals (arrows indicate the substitutional sites, dashed arrows indicate the new centres).
emission and excitation spectra. At the concentration level under investigation (1.0%, corresponding to x ¼ 0:01), the optical spectra of YAB:Eu3+ only shows features which are attributed to Eu3+ substituting for Y3+ in the regular D3 sites. In the case of Yb3+ the additional bands are relatively strong, and the presence of non-equivalent centres can be confirmed by site selective spectroscopy [11].
4. Conclusions In this communication we have reported on the luminescence spectra and decay times of YAB single crystals doped with trivalent RE ions. At the concentration levels under investigation in this study, the RE ions appear to be
E. Cavalli et al. / Journal of Luminescence 102–103 (2003) 216–219
accommodated in the substitutional Y3+ site. This agrees with previous results obtained for Pr3+ and Nd3+ ions [3,7]. On the other hand, for the heavier Sm3+, Dy3+ and especially Yb3+ ions new centres seem to appear, giving rise to additional features in the optical spectra. The nature of these new centres is still an open question, but their presence could be related to smaller size of the heavier RE ions, caused by the lanthanide contraction.
Acknowledgements This work was carried out in the frame of a Spain-Italy Integrated Action cooperative project. The authors gratefully acknowledge Erica Viviani (DST, Univ. Verona) for expert technical assistance. Work partially supported under project number MAT2001-0167.
219
References [1] D. Jaque, J. Capmany, J. Garc!ıa Sol!e, Appl. Phys. Lett. 75 (1999) 325. [2] P. Dekker, J.M. Dawes, J.A. Piper, Y. Liu, J. Wang, Opt. Commun. 195 (2001) 431. [3] M.H. Bartl, K. Gatterer, E. Cavalli, A. Speghini, M. Bettinelli, Spectrochim. Acta Part A 57 (2001) 1981. . ari, E. Beregi, A. Munoz F., R. Sosa, V. Horv!ath, [4] I. Foldv! Opt. Mater. 19 (2002) 241. [5] F. Kellendonk, G. Blasse, J. Chem. Phys. 75 (1981) 561. [6] E.V. Koporulina, N.I. Leonyuk, D. Hansen, K.L. Bray, J. Crystal. Growth 191 (1998) 767. [7] D. Jaque, J. Capmany, Z.D. Luo, J. Garc!ıa Sol!e, J. Phys.: Condens. Matter 9 (1997) 9715. . [8] C. Gorller-Walrand, P. Vandevelde, I. Hendrickx, P. Porcher, J.C. Krupa, G.S.D. King, Inorg. Chim. Acta 143 (1988) 259. . [9] A. Kaminskii, U. Hommerich, D. Temple, J.T. Seo, K. Ueda, S. Bagayev, A. Pavlyulk, Jpn. J. Appl. Phys. 39 (2000) L208. [10] R. Moncorg!e, L.D. Merkle, B. Zandi, MRS Bull. 24 (1999) 21. [11] M.O. Ram!ırez, et al., submitted.
Luminescence of trivalent rare earth ions in the yttrium aluminium borate non-linear laser crystal E. Cavallia, A. Speghinib, M. Bettinellib, M.O. Ram!ırezc, J.J. Romeroc, L.E. Bausa! c, J. Garc!ıa Sole! c,* a
Dipartimento di Chimica GIAF, Universita" di Parma and INFM, UdR Parma, Parco Area delle Scienze 17/A, I-43100 Parma, Italy b Dipartimento Scientifico e Tecnologico, Universita" di Verona and INSTM, UdR Verona, Ca’ Vignal, Strada Le Grazie 15, I-37134 Verona, Italy c ! Departamento de F!ısica de Materiales, C-IV, Universidad Autonoma de Madrid, Cantoblanco, E-28049 Madrid, Spain
Abstract Single crystals of YAl3(BO3)4 doped with the rare earth ions Sm3+, Eu3+, Tb3+, Dy3+, Tm3+ and Yb3+ were grown using the flux growth technique. Luminescence spectra and decay curves were recorded in the visible range at room and low temperatures. The observed transitions were assigned and analysed on the basis of group theory for the D3 point symmetry. Evidence for the presence of non-equivalent centres has been found for Yb3+ and possibly for Sm3+ and Dy3+ doped crystals. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Yttrium aluminium borate; Rare earth ions; Non-equivalent centres
1. Introduction Yttrium aluminium borate YAl3(BO3)4 (YAB) is a non-linear crystal which has proved to be an excellent host for Nd3+ and Yb3+ ions in order to develop self-frequency doubling and self-frequency-sum solid state lasers [1,2]. These lasers have a number of potential applications in optoelectronics as compact diode pumped solid state lasers for the visible and ultraviolet spectral regions. Thus, the incorporation of other trivalent rare earth (RE) ions into this host matrix appears to be very interesting in order to search for new *Corresponding author. Tel.: +34-913975029; fax: +34913978579. E-mail address: jose.garcia [email protected] (J. Garc!ıa Sol!e).
laser channels which could extend the spectral range to new frequencies. In this respect YAB:Pr3+ crystals have been already grown and the optical spectroscopy of Pr3+ ion investigated [3]. Among the other RE ions, the energy level structure of Er3+ in YAB crystals has been recently published [4], but in general the optical spectroscopy of the entire series of lanthanide ions in this host remains to be studied in a detailed way. In this paper we report on the luminescence of YAB crystals doped with Sm3+, Eu3+, Tb3+, Dy3+, Tm3+ and Yb3+. Luminescence spectra and decay times have been measured at room and low temperature. The low temperature emission lines are in agreement with a D3 local environment for the RE ions, indicating that they are located in the Y3+ lattice sites. Evidence for the appearance
0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 2 ) 0 0 4 9 5 - 7
E. Cavalli et al. / Journal of Luminescence 102–103 (2003) 216–219
217
of non-equivalent centres has been found for the crystals doped with Yb3+ and very likely with Sm3+ and Dy3+.
2. Experimental Emitted Intensity (Arb.Units)
Single crystals of YAB doped with various nominal concentrations of Sm3+, Eu3+, Tb3+, Dy3+, Tm3+ or Yb3+ were grown by spontaneous nucleation from a K2Mo3O10 and B2O3 flux using a procedure previously described [3,6]. The reagents employed were of high purity (99.99% or better for Y2O3 and the RE oxides). The largest crystals were affected by flux inclusions, but crystals of about 2 1 1 mm3 were clear and transparent and could be used for the optical measurements. Luminescence spectra at room temperature (RT) and 10 K and RT decay times were measured using the set-up previously described [7].
Tm3+
Eu
Dy3+
3+
Sm Tb
3. Results and discussion The RT visible luminescence spectra of YAB doped with Sm3+, Eu3+, Tb3+, Dy3+ and Tm3+ are shown in Fig. 1. The spectra contain several emission transitions of technological relevance, e.g. the 4F9/2-6H13/2 (570 nm) and 4F9/2-6H11/2 (660 nm) channels of Dy3+ [9] and the 1G4-3F4 (650 nm) and 1G4-3H5 (790 nm) channels of Tm3+ [10]. Following pulsed excitation, all the RT decays for diluted crystals were found to be perfectly exponential. This indicates that, under the adopted experimental conditions, energy transfer processes are not important in the relaxation of the emitting levels. The RT lifetimes t of the emitting levels for diluted RE ions in doped YAB crystals are listed in Table 1. The 10 K luminescence spectra show bands assigned to transitions between manifolds belonging to the respective 4fn configurations. These bands are composed of well-resolved lines, whose number agrees with the predictions of group theory for the dopant ions substituting Y3+ ions in sites of D3 symmetry, as found in previous
3+
450
500
550
600
650
700
3+
750
800
Wavelength (nm) Fig. 1. Overview of the room temperature luminescence spectra of YAB doped with Sm3+ (0.10%, lexc ¼ 403 nm), Eu3+ (1.0%, lexc ¼ 524 nm), Tb3+ (2.9%, lexc ¼ 486 nm), Dy3+ (3.0%, lexc ¼ 450 nm) and Tm3+ (0.10%, lexc ¼ 355 nm).
studies on Pr3+, Nd3+ and Eu3+ in YAB [3,7,8]. These assignments have been confirmed by the measurement of 10 K polarised emission spectra. As an example, the 4G5/2-6H9/2 polarised emission spectra of YAB:0.10% Sm3+ are shown in Fig. 2. These emission spectra consist of five lines at 646.0 nm (s; p), 647.4 nm (s; p), 648.6 nm ðsÞ; 651.0 nm (s; p) and 654.5 nm ðsÞ; in agreement with the electric dipole inter-Stark transitions E1/2(4G5/2)-E1/2, E1/2, E3/2, E1/2, E3/2(6H9/2), respectively, in the D3 double group. These transitions and their corresponding polarisation character are in good agreement with the replacement of Y3+ lattice ions by the Sm3+ ions in sites of D3 symmetry. The additional features observed in the low temperature spectra are possibly associated to the presence of additional Sm3+ centres.
E. Cavalli et al. / Journal of Luminescence 102–103 (2003) 216–219
218
Table 1 RT lifetimes of several emitting levels of Sm3+, Eu3+, Tb3+, Dy3+ and Tm3+ in YAB Concentration (%)
Emitting level
t (ms)
Sm3+ Eu3+ Tb3+ Dy3+ Tm3+
0.10 1.0 2.9 0.30 0.10
4
1800 1300 1900 510 7 110 30
G5/2 D0 5 D4 4 F9/2 1 D2 1 G4 3 H4 5
All the decays were exponential.
D0→7F4
Emitted Intensity (Arb. Units)
Ion
Eu3+ 5
13500 13750 14000 14250 14500 14750 15000
4
Dy3+ 6 F9/2→ H11/2
14400 14600 14800 15000 15200 15400
G5/2→ H9/2
Emission Intensity (Arb. Units)
4
6
σ
15100 15200 15300 15400 15500 15600 15700
G5/2→ H9/2
4
6
π
15100 15200 15300 15400 15500 15600 15700 Energy (cm-1)
Fig. 2. 10 K 4G5/2-6H9/2 polarised luminescence spectra of YAB:Sm3+ (0.10%).
Fig. 3 shows details of the low temperature emission spectra of Eu3+, Dy3+ and Yb3+ ions in YAB. While in YAB:Eu3+ (1.0%) no additional centres are observed, the dashed arrows marked over the luminescence spectra of YAB:Dy3+ (3.0%) and YAB:Yb3+ (2.0%) strongly suggest the formation of new Dy3+ and Yb3+ centres. The presence of Eu3+ ions in non-equivalent sites has been reported in EuAl3(BO3)4 [5] and YxEu1x(BO3)4 (x ¼ 0:1; 0.50 and 0.9) [8] crystals, through the observation of satellite lines in the
2
Yb3+
F5/2→2 F7/2
9700 9800 9900 10000 10100 10200 10300 Energy (cm-1)
Fig. 3. Details of the 10 K luminescence spectra of YAB doped with 1.0% Eu3+, 3.0% Dy3+ and 2.0% Yb3+, showing the presence of non-equivalent centres in the latter two crystals (arrows indicate the substitutional sites, dashed arrows indicate the new centres).
emission and excitation spectra. At the concentration level under investigation (1.0%, corresponding to x ¼ 0:01), the optical spectra of YAB:Eu3+ only shows features which are attributed to Eu3+ substituting for Y3+ in the regular D3 sites. In the case of Yb3+ the additional bands are relatively strong, and the presence of non-equivalent centres can be confirmed by site selective spectroscopy [11].
4. Conclusions In this communication we have reported on the luminescence spectra and decay times of YAB single crystals doped with trivalent RE ions. At the concentration levels under investigation in this study, the RE ions appear to be
E. Cavalli et al. / Journal of Luminescence 102–103 (2003) 216–219
accommodated in the substitutional Y3+ site. This agrees with previous results obtained for Pr3+ and Nd3+ ions [3,7]. On the other hand, for the heavier Sm3+, Dy3+ and especially Yb3+ ions new centres seem to appear, giving rise to additional features in the optical spectra. The nature of these new centres is still an open question, but their presence could be related to smaller size of the heavier RE ions, caused by the lanthanide contraction.
Acknowledgements This work was carried out in the frame of a Spain-Italy Integrated Action cooperative project. The authors gratefully acknowledge Erica Viviani (DST, Univ. Verona) for expert technical assistance. Work partially supported under project number MAT2001-0167.
219
References [1] D. Jaque, J. Capmany, J. Garc!ıa Sol!e, Appl. Phys. Lett. 75 (1999) 325. [2] P. Dekker, J.M. Dawes, J.A. Piper, Y. Liu, J. Wang, Opt. Commun. 195 (2001) 431. [3] M.H. Bartl, K. Gatterer, E. Cavalli, A. Speghini, M. Bettinelli, Spectrochim. Acta Part A 57 (2001) 1981. . ari, E. Beregi, A. Munoz F., R. Sosa, V. Horv!ath, [4] I. Foldv! Opt. Mater. 19 (2002) 241. [5] F. Kellendonk, G. Blasse, J. Chem. Phys. 75 (1981) 561. [6] E.V. Koporulina, N.I. Leonyuk, D. Hansen, K.L. Bray, J. Crystal. Growth 191 (1998) 767. [7] D. Jaque, J. Capmany, Z.D. Luo, J. Garc!ıa Sol!e, J. Phys.: Condens. Matter 9 (1997) 9715. . [8] C. Gorller-Walrand, P. Vandevelde, I. Hendrickx, P. Porcher, J.C. Krupa, G.S.D. King, Inorg. Chim. Acta 143 (1988) 259. . [9] A. Kaminskii, U. Hommerich, D. Temple, J.T. Seo, K. Ueda, S. Bagayev, A. Pavlyulk, Jpn. J. Appl. Phys. 39 (2000) L208. [10] R. Moncorg!e, L.D. Merkle, B. Zandi, MRS Bull. 24 (1999) 21. [11] M.O. Ram!ırez, et al., submitted.