- Email: [email protected]
Optical spectroscopy of neodymium-doped calcium barium niobate ferroelectric crystals
ARTICLE IN PRESS Journal of Luminescence 129 (2009) 1658–1660
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Optical spectroscopy of neodymium-doped calcium barium niobate ferroelectric crystals P. Molina a, E. Martı´n Rodrı´guez a, D. Jaque a, L.E. Bausa´ a, J. Garcı´a Sole´ a,, Huaijin Zhang b, Wenlan Gao b, Jiyang Wang b, Minhua Jiang b a b
´noma de Madrid, 28049 Madrid, Spain Dpto. de Fı´sica de Materiales, Facultad de Ciencias, Universidad Auto State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China
a r t i c l e in f o
a b s t r a c t
Available online 27 May 2009
In this work, the optical absorption, luminescence and Raman spectra of a single Ca0.28Ba0.72Nb2O6:Nd3+ ferroelectric crystal have been measured and compared to those obtained for the self-frequency solidstate laser converter Sr0.61Ba0.39Nb2O6:Nd3+ crystal. The calcium–niobate system displays a much higher transition temperature (150 1C) than the strontium–niobate one (80 1C) and so it appears as an excellent candidate for a self-frequency converter solid-state laser, more stable to pumping radiation that than based on the strontium–niobate one. & 2009 Elsevier B.V. All rights reserved.
Keywords: Optical spectroscopy Calcium barium niobate Nd3+
1. Introduction
2. Experimental results and discussion
Strontium barium niobate ferroelectric crystals, SrxBa1xNb2O6 (SBN), belonging to the family of tetragonal tungsten bronze (TTB) structures, have become very relevant to develop multifunctional SBN:Nd3+ solid-state lasers. These multi-functions include different conversion processes [1,2] (in which the pump and fundamental laser radiations are mixed for different geometries), broad laser tunability in the near-infrared and visible spectral regions [3], and bistable laser output [4]. Some of these functions, in particular those involving frequency conversions, are associated with the SBN ferroelectric phase (needle-like ferroelectric domains). However, because of its relative low Curie temperature, the SBN system can undergo the phase transition to the paraelectric phase even by means of pump-induced thermal loading, and so missing the above-mentioned frequency conversion properties [4]. Calcium barium niobate, CaxBa1xNb2O6 (CBN), is a recently discovered family of crystals similar to the SBN one but with higher Curie temperatures [5]. Thus, it is expected that doping CBN crystals with Nd3+ ions will lead to selffrequency converter solid-state lasers more stable to pump than those based on SBN. In this work, the optical spectroscopy (absorption, luminescence and Raman) of a nearly congruent (X ¼ 0.28) single CBN:Nd3+ crystal has been investigated and compared to that of a congruent (x ¼ 0.6) SBN: Nd3+ one, both crystals having the same Nd content.
Single CBN (x ¼ 0.28) and SBN (X ¼ 0.6) crystals doped with 1% of Nd3+ ions (referred to Nb5+) were grown by the Czochralski technique. Details of growth [6] and optical experimental techniques [7] have been already reported and are omitted here for the sake of brevity. As for SBN, the as-grown CBN:Nd3+ crystal displays a non-periodic size distribution of ferroelectric domains with alternating reversed polarization [8]. The ferro-to-paraelectric phase-transition temperature was measured by Rayleigh scattering experiments for both SBN(x ¼ 0.6):Nd and CBN(x ¼ 0.28):Nd crystals under same experimental conditions. In these experiments, the light intensity scattered by the ferroelectric domain walls (illuminating the crystal with a 2 mW He–Ne CW laser) was measured as a function of sample temperature, as in Ref. [7]. As expected, a much lower transition temperature was obtained for SBN (80 1C) than for CBN:Nd (150 1C). Fig. 1 shows the room-temperature-polarized (beam perpendicular to the ferroelectric z-axis, s; E?z and p; EJz) absorption spectra of Nd3+ in CBN, calibrated in cross-section units. For this calibration, we estimated the Nd concentration (r ¼ 1.63 1020 cm3) using the unit cell parameters, a ¼ b ¼ 12.432 A˚ and c ¼ 3.957 A˚, recently reported for this tetragonal tungsten bronze crystal [6]. These absorption spectra are very similar to those previously reported for SBN:Nd3+ [9]. They consist of several groups of lines corresponding to transitions from the 4I9/2 ground state to different 2S+1LJ states (indicated in the Figure) within the 4f3 electronic configuration of the Nd3+ ion. The s-polarized absorption spectrum coincides with obtained for a polarization (beam parallel to z-axis and so E?z, here omitted for simplicity), indicating that optical transitions are of electric dipole (ED)
Corresponding author.
E-mail address: [email protected] (J. Garcı´a Sole´). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.04.097
ARTICLE IN PRESS P. Molina et al. / Journal of Luminescence 129 (2009) 1658–1660
1659
3.4 3.2
σ
3.0
π
2
2.8
G7/2:4G5/2
2.6
4S
4 3/2: F7/2
σabs (·10-20 cm-2)
2.4 2.2 2.0 1.8
7H
1.6
4 9/2: F5/2
1.4 1.2 1.0 2
0.8
G3/2:4G7/2 4F 3/2
0.6 0.4
4F 9/2
4G 9/2
0.2 0.0 400
500
600
700 800 Wavelength (nm)
900
1000
1100
Fig. 1. Room-temperature-polarized optical absorption spectra of Nd3+ in CBN. Inset shows the spectra in the wavelength region for diode pumping.
SBN:Nd CBN:Nd
Emitted intensity (Arb. units)
λexc = 870 nm
1050
1060
1070 1080 Wavelength (nm)
1090
1100
Fig. 2. 10 K polarized luminescence spectra of Nd3+ in CBN and SBN.
nature. In addition, it should be noted that practically all the features are observed in both s and p configurations, so that electronic transitions are of ED allowed under both polarizations. The inset shows the absorption spectrum in the spectral region for diode pumping (750–900 nm). Peak cross sections, at around 800 nm, are found to be 1.4 1020 and 1.6 1020 cm2 for s and p configurations, respectively. These values are similar to those displayed by the SBN:Nd3+ system, 1.3 1020 and 1.6 1020 cm2 s and p configurations, respectively. Fig. 2 shows the 10 K 4F3/2-4I11/2 emission spectra of Nd3+ in CBN and SBN, measured under the same experimental conditions (excitation wavelength 870 nm and a-polarized emission) for the
sake of comparison. Additional structure (see particularly the highest energy emission line) is found in the emission lines of CBN:Nd3+ with respect to those of SBN:Nd3+. This additional structure, which is not resolved in SBN:Nd3+, very likely indicates the presence of non-equivalent Nd3+ centers. This fact could be due to the double site location of Nd3+ ions in CBN (and very likely in SBN too) or to different environments, very likely due to a different ways of local charge compensation. Nevertheless, it is inferred in Fig. 2 that the individual emission lines of Nd3+ are narrower in CBN, this fact obviously related to a less disordered structure. The luminescence decaytime curves from the 4F3/2 state were measured at 10 K for both crystals. In both cases exponential decays were observed, the lifetime being lower in CBN (189 ms) than in SBN (214 ms). Raman experiments were performed for both SBN:Nd3+ and CBN:Nd3+ crystals at room temperature (i.e. at the ferroelectric phase) in X(Z,U)X and Z(X,U)Z configurations (see Fig. 3). In this notation the figures outside (within) the brackets represent the incoming and out coming directions (incoming and out coming polarization directions; U ¼ unpolarized) of the excitation and emission light beams. In spite of the large number of active Raman modes, the number of those experimentally resolved is significantly smaller due to the broadening that occur as a result of lattice disorder and hot bands [10]. Nevertheless, the Raman spectra of TB crystals are dominated by three main broad bands, peaking at around 250, 650 and 860 cm1. In accordance with the quite similar TTB crystalline structure proposed for both crystals [6,11,12] the Raman spectra are also quite similar, except for the slight differences in the 250 cm1 band. Moreover the bands are somehow narrower in CBN:Nd3+, this fact in accordance with the less disordered structure of this crystal. In summary, CBN:Nd3+ displays similar spectroscopic features to those found for SBN:Nd3+. This fact makes the former crystal as a very promising candidate for a multi-self-frequency converter
ARTICLE IN PRESS 1660
P. Molina et al. / Journal of Luminescence 129 (2009) 1658–1660
Acknowledgements
X(Z,U)X
SBN:Nd CBN:Nd
This work has been supported by the Spanish Ministerio de Educacio´n y Ciencia under Project MAT2007-64686. E. Martı´n Rodrı´guez acknowledges the financial support from the FPU Program of the Spanish MICINN under Grant AP2006-02795. The Chinese laboratory has been supported by the National Natural Science Foundation of China (nos. 50721002 and 50590401), and the Grand for State Key Program of China (2004CB619002).
Emitted intensity (Arb. units)
References
Z(X,U)Z
0
200
400
600
[1] J.J. Romero, D. Jaque, J. Garcı´a Sole´, A.A. Kaminskii, Appl. Phys. Lett. 78 (2001) 1961. [2] P. Molina, M.O. Ramı´rez, L.E. Bausa´, Adv. Funct. Mater. 18 (2008) 709. [3] M.O. Ramı´rez, J.J. Romero, P. Molina, L.E. Bausa´, Appl. Phys. B 81 (2005) 827. [4] M.O. Ramı´rez, D. Jaque, L.E. Bausa´, J. Garcı´a Sole´, A.A. Kaminskii, Phys. Rev. Lett. 95 (2005) 267401. [5] Y.J. Qi, C.J. Lu, J. Zhu, X.B. Chen, H.L. Song, H.J. Zhang, X.G. Xu, Appl. Phys. Lett. 87 (2005) 82904. [6] Hualong Song, Huaijin Zhang, Xiangang Xu, Xiaobo Hu, Xiufeng Cheng, Jiyans Wang, Minhua Jiang, Mater. Res. Bull. 40 (2005) 643. ˜ o, P. Molina, M.O. Ramirez, D. Jaque, L.E. Bausa´, C. Zaldo, L. Ivleva, M. [7] U. Caldin Bettinelli, J. Garcı´a Sole´, Ferroelectrics 363 (2008) 150. [8] C.J. Lu, C.J. Nie, X.F. Duan, J.Q. Li, H.J. Zhang, J.Y. Wang, Appl. Phys. Lett. 88 (2006) 201906. [9] J.J. Romero, Ph.D. Thesis, Multifrequency Laser Action in Nd:SBN crystals, Universidad Auto´noma de Madrid (2002). [10] R.E. Wilde, J. Raman Spectrosc. 22 (1991) 321. [11] M. Eber, M. Burianek, P. Held, J. Stade, S. Bulut, C. Wickleder, M. Mu¨hlberg, Cryst. Res. Technol. 38 (6) (2003) 457. [12] S. Podlozhenov, H.A. Graetsch, J. Schneider, M. Ulex, M. Wo¨hlecke, K. Betzler, Acta Crystallogr. B 62 (2006) 960.
SBN:Nd CBN:Nd
800
Raman shift
1000
1200
1400
(cm-1)
Fig. 3. Room-temperature Raman spectra of CBN:Nd3+ and SBN:Nd3+ crystals for two different configurations (see text).
solid-state laser but with a better stability to pump radiation than SBN:Nd3+. Non-linear frequency conversion and laser oscillation experiments are now underway.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Optical spectroscopy of neodymium-doped calcium barium niobate ferroelectric crystals P. Molina a, E. Martı´n Rodrı´guez a, D. Jaque a, L.E. Bausa´ a, J. Garcı´a Sole´ a,, Huaijin Zhang b, Wenlan Gao b, Jiyang Wang b, Minhua Jiang b a b
´noma de Madrid, 28049 Madrid, Spain Dpto. de Fı´sica de Materiales, Facultad de Ciencias, Universidad Auto State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China
a r t i c l e in f o
a b s t r a c t
Available online 27 May 2009
In this work, the optical absorption, luminescence and Raman spectra of a single Ca0.28Ba0.72Nb2O6:Nd3+ ferroelectric crystal have been measured and compared to those obtained for the self-frequency solidstate laser converter Sr0.61Ba0.39Nb2O6:Nd3+ crystal. The calcium–niobate system displays a much higher transition temperature (150 1C) than the strontium–niobate one (80 1C) and so it appears as an excellent candidate for a self-frequency converter solid-state laser, more stable to pumping radiation that than based on the strontium–niobate one. & 2009 Elsevier B.V. All rights reserved.
Keywords: Optical spectroscopy Calcium barium niobate Nd3+
1. Introduction
2. Experimental results and discussion
Strontium barium niobate ferroelectric crystals, SrxBa1xNb2O6 (SBN), belonging to the family of tetragonal tungsten bronze (TTB) structures, have become very relevant to develop multifunctional SBN:Nd3+ solid-state lasers. These multi-functions include different conversion processes [1,2] (in which the pump and fundamental laser radiations are mixed for different geometries), broad laser tunability in the near-infrared and visible spectral regions [3], and bistable laser output [4]. Some of these functions, in particular those involving frequency conversions, are associated with the SBN ferroelectric phase (needle-like ferroelectric domains). However, because of its relative low Curie temperature, the SBN system can undergo the phase transition to the paraelectric phase even by means of pump-induced thermal loading, and so missing the above-mentioned frequency conversion properties [4]. Calcium barium niobate, CaxBa1xNb2O6 (CBN), is a recently discovered family of crystals similar to the SBN one but with higher Curie temperatures [5]. Thus, it is expected that doping CBN crystals with Nd3+ ions will lead to selffrequency converter solid-state lasers more stable to pump than those based on SBN. In this work, the optical spectroscopy (absorption, luminescence and Raman) of a nearly congruent (X ¼ 0.28) single CBN:Nd3+ crystal has been investigated and compared to that of a congruent (x ¼ 0.6) SBN: Nd3+ one, both crystals having the same Nd content.
Single CBN (x ¼ 0.28) and SBN (X ¼ 0.6) crystals doped with 1% of Nd3+ ions (referred to Nb5+) were grown by the Czochralski technique. Details of growth [6] and optical experimental techniques [7] have been already reported and are omitted here for the sake of brevity. As for SBN, the as-grown CBN:Nd3+ crystal displays a non-periodic size distribution of ferroelectric domains with alternating reversed polarization [8]. The ferro-to-paraelectric phase-transition temperature was measured by Rayleigh scattering experiments for both SBN(x ¼ 0.6):Nd and CBN(x ¼ 0.28):Nd crystals under same experimental conditions. In these experiments, the light intensity scattered by the ferroelectric domain walls (illuminating the crystal with a 2 mW He–Ne CW laser) was measured as a function of sample temperature, as in Ref. [7]. As expected, a much lower transition temperature was obtained for SBN (80 1C) than for CBN:Nd (150 1C). Fig. 1 shows the room-temperature-polarized (beam perpendicular to the ferroelectric z-axis, s; E?z and p; EJz) absorption spectra of Nd3+ in CBN, calibrated in cross-section units. For this calibration, we estimated the Nd concentration (r ¼ 1.63 1020 cm3) using the unit cell parameters, a ¼ b ¼ 12.432 A˚ and c ¼ 3.957 A˚, recently reported for this tetragonal tungsten bronze crystal [6]. These absorption spectra are very similar to those previously reported for SBN:Nd3+ [9]. They consist of several groups of lines corresponding to transitions from the 4I9/2 ground state to different 2S+1LJ states (indicated in the Figure) within the 4f3 electronic configuration of the Nd3+ ion. The s-polarized absorption spectrum coincides with obtained for a polarization (beam parallel to z-axis and so E?z, here omitted for simplicity), indicating that optical transitions are of electric dipole (ED)
Corresponding author.
E-mail address: [email protected] (J. Garcı´a Sole´). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.04.097
ARTICLE IN PRESS P. Molina et al. / Journal of Luminescence 129 (2009) 1658–1660
1659
3.4 3.2
σ
3.0
π
2
2.8
G7/2:4G5/2
2.6
4S
4 3/2: F7/2
σabs (·10-20 cm-2)
2.4 2.2 2.0 1.8
7H
1.6
4 9/2: F5/2
1.4 1.2 1.0 2
0.8
G3/2:4G7/2 4F 3/2
0.6 0.4
4F 9/2
4G 9/2
0.2 0.0 400
500
600
700 800 Wavelength (nm)
900
1000
1100
Fig. 1. Room-temperature-polarized optical absorption spectra of Nd3+ in CBN. Inset shows the spectra in the wavelength region for diode pumping.
SBN:Nd CBN:Nd
Emitted intensity (Arb. units)
λexc = 870 nm
1050
1060
1070 1080 Wavelength (nm)
1090
1100
Fig. 2. 10 K polarized luminescence spectra of Nd3+ in CBN and SBN.
nature. In addition, it should be noted that practically all the features are observed in both s and p configurations, so that electronic transitions are of ED allowed under both polarizations. The inset shows the absorption spectrum in the spectral region for diode pumping (750–900 nm). Peak cross sections, at around 800 nm, are found to be 1.4 1020 and 1.6 1020 cm2 for s and p configurations, respectively. These values are similar to those displayed by the SBN:Nd3+ system, 1.3 1020 and 1.6 1020 cm2 s and p configurations, respectively. Fig. 2 shows the 10 K 4F3/2-4I11/2 emission spectra of Nd3+ in CBN and SBN, measured under the same experimental conditions (excitation wavelength 870 nm and a-polarized emission) for the
sake of comparison. Additional structure (see particularly the highest energy emission line) is found in the emission lines of CBN:Nd3+ with respect to those of SBN:Nd3+. This additional structure, which is not resolved in SBN:Nd3+, very likely indicates the presence of non-equivalent Nd3+ centers. This fact could be due to the double site location of Nd3+ ions in CBN (and very likely in SBN too) or to different environments, very likely due to a different ways of local charge compensation. Nevertheless, it is inferred in Fig. 2 that the individual emission lines of Nd3+ are narrower in CBN, this fact obviously related to a less disordered structure. The luminescence decaytime curves from the 4F3/2 state were measured at 10 K for both crystals. In both cases exponential decays were observed, the lifetime being lower in CBN (189 ms) than in SBN (214 ms). Raman experiments were performed for both SBN:Nd3+ and CBN:Nd3+ crystals at room temperature (i.e. at the ferroelectric phase) in X(Z,U)X and Z(X,U)Z configurations (see Fig. 3). In this notation the figures outside (within) the brackets represent the incoming and out coming directions (incoming and out coming polarization directions; U ¼ unpolarized) of the excitation and emission light beams. In spite of the large number of active Raman modes, the number of those experimentally resolved is significantly smaller due to the broadening that occur as a result of lattice disorder and hot bands [10]. Nevertheless, the Raman spectra of TB crystals are dominated by three main broad bands, peaking at around 250, 650 and 860 cm1. In accordance with the quite similar TTB crystalline structure proposed for both crystals [6,11,12] the Raman spectra are also quite similar, except for the slight differences in the 250 cm1 band. Moreover the bands are somehow narrower in CBN:Nd3+, this fact in accordance with the less disordered structure of this crystal. In summary, CBN:Nd3+ displays similar spectroscopic features to those found for SBN:Nd3+. This fact makes the former crystal as a very promising candidate for a multi-self-frequency converter
ARTICLE IN PRESS 1660
P. Molina et al. / Journal of Luminescence 129 (2009) 1658–1660
Acknowledgements
X(Z,U)X
SBN:Nd CBN:Nd
This work has been supported by the Spanish Ministerio de Educacio´n y Ciencia under Project MAT2007-64686. E. Martı´n Rodrı´guez acknowledges the financial support from the FPU Program of the Spanish MICINN under Grant AP2006-02795. The Chinese laboratory has been supported by the National Natural Science Foundation of China (nos. 50721002 and 50590401), and the Grand for State Key Program of China (2004CB619002).
Emitted intensity (Arb. units)
References
Z(X,U)Z
0
200
400
600
[1] J.J. Romero, D. Jaque, J. Garcı´a Sole´, A.A. Kaminskii, Appl. Phys. Lett. 78 (2001) 1961. [2] P. Molina, M.O. Ramı´rez, L.E. Bausa´, Adv. Funct. Mater. 18 (2008) 709. [3] M.O. Ramı´rez, J.J. Romero, P. Molina, L.E. Bausa´, Appl. Phys. B 81 (2005) 827. [4] M.O. Ramı´rez, D. Jaque, L.E. Bausa´, J. Garcı´a Sole´, A.A. Kaminskii, Phys. Rev. Lett. 95 (2005) 267401. [5] Y.J. Qi, C.J. Lu, J. Zhu, X.B. Chen, H.L. Song, H.J. Zhang, X.G. Xu, Appl. Phys. Lett. 87 (2005) 82904. [6] Hualong Song, Huaijin Zhang, Xiangang Xu, Xiaobo Hu, Xiufeng Cheng, Jiyans Wang, Minhua Jiang, Mater. Res. Bull. 40 (2005) 643. ˜ o, P. Molina, M.O. Ramirez, D. Jaque, L.E. Bausa´, C. Zaldo, L. Ivleva, M. [7] U. Caldin Bettinelli, J. Garcı´a Sole´, Ferroelectrics 363 (2008) 150. [8] C.J. Lu, C.J. Nie, X.F. Duan, J.Q. Li, H.J. Zhang, J.Y. Wang, Appl. Phys. Lett. 88 (2006) 201906. [9] J.J. Romero, Ph.D. Thesis, Multifrequency Laser Action in Nd:SBN crystals, Universidad Auto´noma de Madrid (2002). [10] R.E. Wilde, J. Raman Spectrosc. 22 (1991) 321. [11] M. Eber, M. Burianek, P. Held, J. Stade, S. Bulut, C. Wickleder, M. Mu¨hlberg, Cryst. Res. Technol. 38 (6) (2003) 457. [12] S. Podlozhenov, H.A. Graetsch, J. Schneider, M. Ulex, M. Wo¨hlecke, K. Betzler, Acta Crystallogr. B 62 (2006) 960.
SBN:Nd CBN:Nd
800
Raman shift
1000
1200
1400
(cm-1)
Fig. 3. Room-temperature Raman spectra of CBN:Nd3+ and SBN:Nd3+ crystals for two different configurations (see text).
solid-state laser but with a better stability to pump radiation than SBN:Nd3+. Non-linear frequency conversion and laser oscillation experiments are now underway.