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Luminescence of the Cr3+ R-lines in pure and MgO co-doped near stoichiometric LiNbO3:Cr crystals
Chemical Physics Letters 369 (2003) 519–524 www.elsevier.com/locate/cplett
Luminescence of the Cr3þ R-lines in pure and MgO co-doped near stoichiometric LiNbO3:Cr crystals T.P.J. Han a
a,*
, F. Jaque a, V. Berm udez b, E. Dieguez
b
Department of Physics, University of Strathclyde, John Anderson Building, 107 Rottenrow, Glasgow G4 0NG, Scotland, UK b Departamento de Fısica de Materials, Universidad Aut onoma de Madrid, Cantoblanco 28049, Madrid, Spain Received 30 August 2002; in final form 17 November 2002
Abstract The photoluminescence property of pure and MgO co-doped near stoichiometric LiNbO3 :Cr crystals has been investigated. It has been found that the presence of low Mg ion content in the crystals induces the coexistence of two types of Cr3þ sites; Cr3þ ion substituting Liþ and Nb5þ sites. In MgO co-doped samples, four narrow emission bands have been found and three of them are correlated with the broad absorption bands associated to the Cr3þ ions located in Nb5þ site (½CrNb centres). The observation of the zero-phonon line at 13 540 cm1 only in pure LiNbO3 :Cr crystals of near stoichiometric composition suggests it could be used as a probe to monitor the composition of LiNbO3 crystals. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Lithium niobate, LiNbO3 , is an important material for applications in electro-optics and integrated optical devices. However, it suffers from photorefractive effect at moderately low optical excitation power. In order to increase the resistance of this material to this Ôoptical damageÕ in nonlinear application, such as amplification and laser action or second harmonic generation, the LiNbO3 crystals, generally in congruent composition where the Li/Nb ratio is 0.946, are usually intentionally doped with a small but significant amount of MgO, 5–6 mol% [1]. The presence of
*
Corresponding author. E-mail address: [email protected] (T.P.J. Han).
these cations is an additional source of disorder within the host lattice and could affect the properties of the optically active dopant ions. Cr3þ ion has been demonstrated as a useful optical and paramagnetic probe to study the location of the doped cation in LiNbO3 crystals [2–6]. The optical absorption spectrum of LiNbO3 :Cr crystal consists of two broad bands ascribed to the vibronic radiative transitions 4 A2 ! 4 T1 and 4 A2 ! 4 T2 . In addition to these broad bands a narrow line is also observed which can be associated to the 4 A2 ! 2 E zero-phonon transition (R-lines), so called because it involves the zero vibrational levels of both initial and final states. In LiNbO3 the oxygen octahedrons cations are not perfect. There are differences in the bond length for the Liþ –O2 and Nb5þ –O2 octahedrons [7] and there is a large lattice relaxation that occurs
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(02)02028-6
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when doping with Cr3þ ions. Because of the different charge states of the dopant ions compared with those of the Liþ and Nb5þ , the electrostatic interaction most probably forces the oxygen ligands to relax inwardly for ½CrLi centres and outwardly for ½CrNb centres. Congruent LiNbO3 crystals doped with Cr3þ ions present an interesting optical effect when the crystals are co-doped with MgO. Below a MgO concentration of 4.5 mol% the crystals present a uniform green colour. Whereas above this value, even just slightly above, the crystal becomes pink in colour [2]. This chromatic effect has also been observed in congruent samples co-doped with the other divalent cation such as ZnO [3]. It has been shown in an early work on near stoichiometric composition LiNbO3 :Cr samples, where the Li/Nb ratio is 1, that a much reduced MgO concentration, 2 mol%, is sufficient to show this change in colour [4]. The role of the Mg2þ ions has been associated with its position in the host lattice. In congruent LiNbO3 , cong-LiNbO3 , crystals the two accepted cation substitution models; Liþ -vacancy and Nb5þ -vacancy models give a total concentration of Liþ vacancy sites ð½VLi Þ and antisites (Nb5þ substituting a Liþ site), [NbLi of 4.6 mol%. This concentration value matches the threshold level observed for the inhibition of the optical damage and the colour change observed in MgO or ZnO co-doped cong-LiNbO3 :Cr crystals [5–8]. A simple model, based on the generally accepted view that Mg2þ ions are preferentially entering into Liþ sites and the coexistence of two types of Cr centres, is able to satisfy most of the experimental data observed to-date [9]. The model suggested that below a MgO content of 4.6 mol% there are sufficient number of free Liþ sites, vacancy or antisite, for the Cr3þ ions to form the so called ½CrLi centres. Thus the two broad bands recorded in the absorption spectrum are associated with the 4 A2 ! 4 T2 and 4 A2 ! 4 T1 vibronic transitions of the ½CrLi centres. To-date three major centres have been identified as c-, unperturbed ½CrLi centre; a-, ½CrLi centre perturbed by ½NbLi ; and b-, ½CrLi centre perturbed by ½VLi . Therefore the broad absorption bands observed should be considered only as the general feature of an envelop of a number of distinct ½CrLi centres [5]. For MgO
concentration above the threshold all the Liþ sites, vacancy and antisites, are filled and intrinsic Liþ and Nb5þ sites are substituted by Cr3þ ions forming ½CrLi and ½CrNb centres. The two types of ½CrLi centres formed below and above the threshold level are indistinguishable. The presence of the ½CrNb centres reflects the changes observed in the absorption spectrum above the threshold [9]. In the case of the R-lines, spectroscopic results to-date are more confusing. This is in part due to the formation of many different sites as a consequence of quality of the samples and the majority of the defect sites have crystal field strength close to the boundary of strong and weak crystal field regime. As a consequence, difficulties in the observation and assignment of the R-lines can arise between different research groups. The relatively new development of stoichiometric or near stoichiometric crystals having much reduced intrinsic defects and spectral linewidth enables a much better opportunity to resolve the many questions involving the R-lines. In this work photoluminescence spectroscopy and time-resolved spectroscopy in near stoichiometric LiNbO3 :Cr samples, nsto-LiNbO3 :Cr, pure and co-doped with MgO have been investigated.
2. Experimental method Nsto-LiNbO3 crystals doped with 0.1 mol% of Cr2 O3 and with 2 mol% of MgO in the melt were grown in air by the top seeded solution growth (TSSG) method using a Kþ flux at a pull rate of 0.3 mm/h. The nsto-LiNbO3 :Cr crystal boule shows a homogeneous green colour whereas the nsto-LiNbO3 :Cr:MgO crystal shows a uniform pink colour along the full length of the boule. Optical absorption spectra in the temperature range between room temperature and 100 K were recorded with an AVIV 14DS spectrometer. Optical absorption spectra at 10 K were performed using a Varian Carey-SE spectrophotometer. Continuous wave (cw) luminescence emission was performed using the resonant line of an argon-ion laser. The emitted light was dispersed by a SPEX 500M monochromator and detected with a Hamamastu R-2949 photomultiplier. All low temperature mea-
T.P.J. Han et al. / Chemical Physics Letters 369 (2003) 519–524
surements (20–300 K) were obtained using a temperature controlled closed-cycle helium cryorefrigerator. Time-resolved luminescence spectra were obtained by excitation with the second harmonic of a Nd:YAG pulse laser at 532 nm with a pulse width of 5 ns.
3. Experimental results and discussion
Optical Density
Fig. 1 shows the unpolarized absorption spectrum of a nsto-LiNbO3 :Cr and a nsto-LiNbO3 : Cr:MgO sample. Essentially, the addition of MgO has the effect of red shifting the two broad bands, assigned to the 4 A2 ! 4 T2 and 4 A2 ! 4 T1 vibronic transitions, to lower energy. In addition, a signif-
Energy (cm-1)
Optical Density
(a)
(b)
Energy (cm-1)
Fig. 1. Optical absorption spectra of pure and co-doped MgO near stoichiometric LiNbO3 :Cr samples at (a) RT, (b) 10 K. Inset shows the magnified view about the 14 000 cm1 region. (Dotted line is LiNbO3 :Cr and solid line is LiNbO3 :Cr:MgO.)
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icant change in the relative intensity of these two broad bands is observed. This change in the absorption feature is responsible for the colour change detected, from green (no MgO) to pink (codoped with MgO), in these crystals. In congLiNbO3 :Cr:MgO samples this change has been observed for a concentration above 4.5 mol% [9] whereas in nsto-LiNbO3 :Cr:MgO this change is at a much lower level of 2 mol% [4]. The absorption spectra of Fig. 1 resemble those reported for congruent samples, hence the assignment of the broad bands and narrow lines follows that previously used for congruent crystals. It is important to note that the R-lines ð4 A2 ! 2 EÞ assigned to the c-½CrLi and the b-½CrLi centres are the only R-lines clearly observed in the absorption spectra of cong- or nsto-samples (see inset in Fig. 1). The R-lines of the other two major ½CrLi centres and the ½CrNb centres have never been observed in absorption. A more careful inspection of the absorption spectrum corresponding to MgO co-doped samples reveals dips on the broad bands which could be connected with Fano antiresonance as a result of interaction between the sharp transitions (2 E, 2 T1 and 2 T2 ) and the broad vibronic transitions of the excited levels 4 T2 and 4 T1 [10]. The emission spectra of the R-lines are only observable at temperatures below 100 K. Fig. 2a shows the emission spectra under 514.5 nm (19 436 cm1 ) argon line laser excitation at 20 K for the pure and MgO co-doped nsto-LiNbO3 :Cr3þ samples. In pure sample the spectrum consists of three narrow emission bands centred at 13 690, 13 620 and 13 540 cm1 . These emission lines have been assigned to the R-lines of b- and a-centres [11] and a zero-phonon line associated with the 4 T2 excited level, respectively [12]. The zero-phonon line has only been observed in pure nsto-samples. In fact, the closer the LiNbO3 is to the stoichiometric composition the greater is the ratio between this zero-phonon line and the R-lines of the a- and b-centres [12]. The emission spectrum of the codoped nsto-sample reveals four new narrow bands, at 13 510, 13 550, 13 565 and 13 642 cm1 associated with the addition of MgO as well as the R-lines ascribed to the a- and b-centres. In general the linewidth of the a- and b-centres seems to have increased slightly. The peak position of the
T.P.J. Han et al. / Chemical Physics Letters 369 (2003) 519–524
Luminescence Intensity (au)
522
Energy (cm-1)
Luminescence Intensity (au)
(a)
(b)
Energy (cm-1)
Fig. 2. Luminescence spectra under (cw) 514.5 nm argon line laser excitation of pure and co-doped MgO near stoichiometric LiNbO3 :Cr samples (a) 20 K, (b) 60 K. (*, The emission line at 13 642 cm1 , see text for its tentative assignment.)
b-centre remains the same for the two samples whereas the peak position for the a-centre is blue shifted slightly. The intensity of the line associated with the b-centre has also reduced significantly for the nsto-LiNbO3 :Cr:MgO sample. The two new R-line type narrow bands centred at 13 510 and 13 565 cm1 are in close proximity to the R-lines ascribed to the ½CrNb centres in cong-LiNbO3 : Cr:MgO sample (above the threshold of 4.5 mol%) [2,9] which has been confirmed by paramagnetic spectroscopy (EPR and ENDOR) [13,14]. The isotropic characteristic found in the EPR band for this centre in congruent sample was explained by considering that the close proximity of the charge compensating Mg2þ ion shifting the Cr3þ ions to a more central position in the Li-octahedral leads to a more cubic field.
The Liþ position in the crystal lattice is a weak crystal-field site, hence the lowest excited energy level corresponds to the 4 T2 and no emission from the 2 E excited level (R-lines) is expected. This is the case for the c-½CrLi centre. However, if the Dq/B parameter is close to the crossover point in the Sugano and Tanabe diagram, where the energy levels are plotted against the crystal field strength, the 2 E excited level can be activated thermally at high temperatures resulting in R-line emission. Fig. 2b displayed the emission spectra recorded at 60 K for pure and MgO co-doped LiNbO3 :Cr samples. As can be seen, in both samples the R-lines emission ascribed to the unperturbed c ½CrLi centre are now observed. Fig. 3a shows the time-resolved spectroscopy under 532 nm excitation (second harmonic of Nd:YAG pulse laser) of a pure nsto-LiNbO3 :Cr sample at 20 K. Curve 1 was obtained without any delay and curve 2 with a 200 ls delay. The small signal to noise ratio is due to pulse to pulse fluctuation of the laser output power and the fact that the data have not been averaged over a number of pulses. As it is expected, curve 1 reproduces the emission spectrum under cw excitation showing the narrow emission bands associated with the aand b-centres and the zero-phonon line attributed to the 4 T2 excited level. However, in the spectrum obtained using a 200 ls delay only the emission from the R-line of the a- and b-centres remains. This result is in accordance with the fluorescence decay-time of the 2 E (R-lines) and the 4 T2 energy level having 1 ms and 10 ls, respectively [9]. Fig. 3b shows the time-resolved spectra for the nsto-LiNbO3 :Cr:MgO sample under the same condition as that described above for Fig. 3a. The three peaks ascribed to R-line emission stay unchanged. This confirmed that the early assignment of the emission line centred at 13 510 cm1 is associated with the 2 E energy level [2]. Unfortunately the much weaker emission intensity of the narrow lines centred at 13 550, 13 565 and the new line at 13 642 cm1 are unable to be resolved in this experiment because of the small signal to noise ratio. It is important to note that the relative intensity ratio of the ½CrNb lines with the ½CrLi lines, in particular the a-centre, is much larger for the pulsed measurement than the continuous wave
Energy (cm-1) Fig. 4. Emission spectra of near stoichiometric LiNbO3 :Cr sample co-doped with MgO at different temperatures. Excitation wavelength is 514.5 nm. (*, The emission line at 13 642 cm1 , see text for its tentative assignment.)
Luminescence Intensity (au)
(a)
(b)
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Luminescence Intensity (au)
Luminescence Intensity (au)
T.P.J. Han et al. / Chemical Physics Letters 369 (2003) 519–524
Energy (cm-1)
Fig. 3. Time-resolved spectroscopy spectra under pulsed excitation at 532 nm at 20 K (a) pure near stoichiometric LiNbO3 :Cr sample, (b) near stoichiometric LiNbO3 :Cr sample co-doped with MgO. (*, The emission line at 13 642 cm1 , see text for its tentative assignment.)
excitation (see Fig. 2a). The nature of this observation is unknown at present and is the subject of our current investigation of this material. The temperature dependence of the R-lines emission intensity is presented in Fig. 4 for the nsto-LiNbO3 :Cr:MgO sample. As it can be observed at 20 K, the R-lines ascribed to the a- and b-centres are well defined together with the two R-lines assigned to the ½CrNb centres. The emission intensity of the lines associated with the ½CrNb , a- and b-centres decreases with increasing temperature and at 60 K the R-lines associated with the c-centre are clearly resolvable. An Arrhenius plot of the intensity ratio of R2 =R1 vs reciprocal temperature verified the assignment and energy
splitting of the 2 E state of early report [11]. However, this is based on the assumption that the ½CrLi centres observed in the nsto-sample are the same as those observed in the cong-sample. This is not true in all cases. The observed shift of the peak position of the a-½CrLi centre as described above, Fig. 2, indicates the linewidth of this centre which is inhomogeneously broadened in cong-samples and the observed linewidth in the nsto-sample represents only a subset of centres of similar crystal field symmetry. The model for the a-centre is a ½CrLi distorted by a nearby antisite, i.e., ½CrLi –½NbLi , the substitution of the ½NbLi by ½MgLi in the codoped samples could form ½CrLi –½MgLi type of centres which have a slightly different distortion, hence the observed shift of the peak position. The R-lines associated with the ½CrNb centres, 13 510, 13 550 and 13 565 cm1 have no paired relationship with any other R-line. This is in contradiction to early reports [9,11] where the R-lines at 13 510 and 13 565 cm1 were assigned to be the energy splitting of the 2 E energy level of the Nb centre. The narrow emission line centred at 13 642 cm1 is close (22 cm1 ) to the line associated with the a½CrLi centre. Detailed inspection of the emission spectra of the pure nsto- and co-doped congsamples also revealed a small peak in close proximity to this position. Its absence in cong-samples
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could be because it is obscured by the broadened linewidth of the nearby R-line of the a-½CrLi centre, see Fig. 2. The presence of this emission line in all LiNbO3 :Cr crystals, whether they are co-doped with MgO or not, suggests that the defect centre associated with this emission line is a Cr3þ in a Li site in origin. The position of this line being so close to the line associated with the a-½CrLi centre could be interpreted either as coincident or it is actually related to the a-½CrLi centre, i.e., similar to the a-½CrLi centre but with a slightly different local perturbation, hence the shift in energy. However, there are insufficient data at this point to confirm this hypothesis.
the only R-lines clearly observed in the absorption spectra of both congruent or nsto-samples.
Acknowledgements F.J., V.B. and E.D. would like to acknowledge the support of the Ministerio de Educacion yCultura (Spain). T.P.J.H. would like to acknowledge the financial support of EPSRC. We would like to acknowledge Dr. C. Zaldo (CSIC) for the help with the low temperature absorption measurement.
References 4. Conclusion The Cr3þ ion defect centres observed in nstoLiNbO3 :Cr and in nsto-LiNbO3 :Cr:MgO crystals are surprisingly similar to those observed for their respective counterparts in congruent composition. However, the much reduced intrinsic defect and narrower spectral linewidth in nsto-samples enable this study to resolve a previously obscured ½CrLi centre which is tentatively ascribed to a centre related to the a-½CrLi centre. It is the first time the shift of the R-line ascribed to the a-centre is observed in MgO co-doped crystals enabling the verification of the MgO effect on one of the ½CrLi centres. The appearance of the zero-phonon line at 13 540 cm1 only in pure nsto-LiNbO3 :Cr suggests it could be used as a probe to monitor the composition of LiNbO3 crystals. Three R-lines ascribed to ½CrNb centres associated with the addition of MgO above the critical dopant level are observed. In nsto-crystals this level is found to be at 2 mol% as compared to 4.6 mol% for cong-composition. In contrast to early reports there is no evidence to suggest the pairing of these R-lines with each other or any other Rlines. Somewhat surprisingly the R-lines ð4 A2 ! 2 EÞ assigned to the c-½CrLi and b-½CrLi centres are
[1] G. Zhong, J. Jian, Z. Wu, in: Proc. 11th Int. Quantum Electro. Conference, Optical Society of American, Washington, DC, 1980, p. 631. [2] E. Camarillo, J. Tocho, I. Vergara, E. Dieguez, J. GarcıaSole, F. Jaque, Phys. Rev. B 45 (1992) 4600. [3] G.A. Torchia, J.A. Sanz-Garcıa, J. Diaz-Caro, F. Jaque, T.P.J. Han, Chem. Phys. Lett. 288 (1998) 65. [4] G. Torchia, J.A. Sanz-Garcıa, F.J. Lopez, D. Bravo, J. Garcıa-Sole, F. Jaque, H.G. Gallagher, T.P.J. Han, J. Phys. Condens. Matter 10 (1998) L341. [5] K.L. Sweeney, L.E. Halliburton, D.A. Bryan, R.R. Rice, R. Gerson, H.E. Tomaschke, J. Appl. Phys. 57 (1985) 1036. [6] S.C. Abrahams, P. Marsh, Acta Crystallogr. B 42 (1986) 1846. [7] J.R. Carruthers, G.E. Peterson, M. Grasso, P.M. Bridenbaugh, J. Appl. Phys. 42 (1971) 1846. [8] N. Iyi, K. Kitmaura, F. Izumi, J.K. Yamamoto, T. Hayashi, H. Asano, S. Kimura, J. Solid State Chem. 101 (1992) 340. [9] J. Diaz-Caro, J. Garcıa-Sole, D. Bravo, J.M. Sanz-Garcıa, F.J. Lopez, F. Jaque, Phys. Rev. B 54 (1996) 27. [10] M. Voda, J. Garcıa-Sole, F. Jaque, I. Vergara, A. Kaminskii, B. Mill, A. Butashin, Phys. Rev. B 49 (1994) 3755. [11] P.I. Macfarlane, K. Holliday, J.F.H. Nicholls, B. Henderson, J. Phys. Condens. Matter 7 (1995) 9643. [12] G.M. Salley, S.A. Basun, A.A. Kaplyanskii, R.S. Meltzer, K. Polgar, U. Happek, J. Lumin. 87 (2000) 1133. [13] A. Martın, F.J. Lopez, F. Agull o-Lopez, J. Phys. Condens. Matter 4 (1992) 847. [14] G. Corradi, H. Soethe, J.M. Spaeth, K. Polgar, Ferroelectrics 125 (1992) 295.
Luminescence of the Cr3þ R-lines in pure and MgO co-doped near stoichiometric LiNbO3:Cr crystals T.P.J. Han a
a,*
, F. Jaque a, V. Berm udez b, E. Dieguez
b
Department of Physics, University of Strathclyde, John Anderson Building, 107 Rottenrow, Glasgow G4 0NG, Scotland, UK b Departamento de Fısica de Materials, Universidad Aut onoma de Madrid, Cantoblanco 28049, Madrid, Spain Received 30 August 2002; in final form 17 November 2002
Abstract The photoluminescence property of pure and MgO co-doped near stoichiometric LiNbO3 :Cr crystals has been investigated. It has been found that the presence of low Mg ion content in the crystals induces the coexistence of two types of Cr3þ sites; Cr3þ ion substituting Liþ and Nb5þ sites. In MgO co-doped samples, four narrow emission bands have been found and three of them are correlated with the broad absorption bands associated to the Cr3þ ions located in Nb5þ site (½CrNb centres). The observation of the zero-phonon line at 13 540 cm1 only in pure LiNbO3 :Cr crystals of near stoichiometric composition suggests it could be used as a probe to monitor the composition of LiNbO3 crystals. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Lithium niobate, LiNbO3 , is an important material for applications in electro-optics and integrated optical devices. However, it suffers from photorefractive effect at moderately low optical excitation power. In order to increase the resistance of this material to this Ôoptical damageÕ in nonlinear application, such as amplification and laser action or second harmonic generation, the LiNbO3 crystals, generally in congruent composition where the Li/Nb ratio is 0.946, are usually intentionally doped with a small but significant amount of MgO, 5–6 mol% [1]. The presence of
*
Corresponding author. E-mail address: [email protected] (T.P.J. Han).
these cations is an additional source of disorder within the host lattice and could affect the properties of the optically active dopant ions. Cr3þ ion has been demonstrated as a useful optical and paramagnetic probe to study the location of the doped cation in LiNbO3 crystals [2–6]. The optical absorption spectrum of LiNbO3 :Cr crystal consists of two broad bands ascribed to the vibronic radiative transitions 4 A2 ! 4 T1 and 4 A2 ! 4 T2 . In addition to these broad bands a narrow line is also observed which can be associated to the 4 A2 ! 2 E zero-phonon transition (R-lines), so called because it involves the zero vibrational levels of both initial and final states. In LiNbO3 the oxygen octahedrons cations are not perfect. There are differences in the bond length for the Liþ –O2 and Nb5þ –O2 octahedrons [7] and there is a large lattice relaxation that occurs
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(02)02028-6
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when doping with Cr3þ ions. Because of the different charge states of the dopant ions compared with those of the Liþ and Nb5þ , the electrostatic interaction most probably forces the oxygen ligands to relax inwardly for ½CrLi centres and outwardly for ½CrNb centres. Congruent LiNbO3 crystals doped with Cr3þ ions present an interesting optical effect when the crystals are co-doped with MgO. Below a MgO concentration of 4.5 mol% the crystals present a uniform green colour. Whereas above this value, even just slightly above, the crystal becomes pink in colour [2]. This chromatic effect has also been observed in congruent samples co-doped with the other divalent cation such as ZnO [3]. It has been shown in an early work on near stoichiometric composition LiNbO3 :Cr samples, where the Li/Nb ratio is 1, that a much reduced MgO concentration, 2 mol%, is sufficient to show this change in colour [4]. The role of the Mg2þ ions has been associated with its position in the host lattice. In congruent LiNbO3 , cong-LiNbO3 , crystals the two accepted cation substitution models; Liþ -vacancy and Nb5þ -vacancy models give a total concentration of Liþ vacancy sites ð½VLi Þ and antisites (Nb5þ substituting a Liþ site), [NbLi of 4.6 mol%. This concentration value matches the threshold level observed for the inhibition of the optical damage and the colour change observed in MgO or ZnO co-doped cong-LiNbO3 :Cr crystals [5–8]. A simple model, based on the generally accepted view that Mg2þ ions are preferentially entering into Liþ sites and the coexistence of two types of Cr centres, is able to satisfy most of the experimental data observed to-date [9]. The model suggested that below a MgO content of 4.6 mol% there are sufficient number of free Liþ sites, vacancy or antisite, for the Cr3þ ions to form the so called ½CrLi centres. Thus the two broad bands recorded in the absorption spectrum are associated with the 4 A2 ! 4 T2 and 4 A2 ! 4 T1 vibronic transitions of the ½CrLi centres. To-date three major centres have been identified as c-, unperturbed ½CrLi centre; a-, ½CrLi centre perturbed by ½NbLi ; and b-, ½CrLi centre perturbed by ½VLi . Therefore the broad absorption bands observed should be considered only as the general feature of an envelop of a number of distinct ½CrLi centres [5]. For MgO
concentration above the threshold all the Liþ sites, vacancy and antisites, are filled and intrinsic Liþ and Nb5þ sites are substituted by Cr3þ ions forming ½CrLi and ½CrNb centres. The two types of ½CrLi centres formed below and above the threshold level are indistinguishable. The presence of the ½CrNb centres reflects the changes observed in the absorption spectrum above the threshold [9]. In the case of the R-lines, spectroscopic results to-date are more confusing. This is in part due to the formation of many different sites as a consequence of quality of the samples and the majority of the defect sites have crystal field strength close to the boundary of strong and weak crystal field regime. As a consequence, difficulties in the observation and assignment of the R-lines can arise between different research groups. The relatively new development of stoichiometric or near stoichiometric crystals having much reduced intrinsic defects and spectral linewidth enables a much better opportunity to resolve the many questions involving the R-lines. In this work photoluminescence spectroscopy and time-resolved spectroscopy in near stoichiometric LiNbO3 :Cr samples, nsto-LiNbO3 :Cr, pure and co-doped with MgO have been investigated.
2. Experimental method Nsto-LiNbO3 crystals doped with 0.1 mol% of Cr2 O3 and with 2 mol% of MgO in the melt were grown in air by the top seeded solution growth (TSSG) method using a Kþ flux at a pull rate of 0.3 mm/h. The nsto-LiNbO3 :Cr crystal boule shows a homogeneous green colour whereas the nsto-LiNbO3 :Cr:MgO crystal shows a uniform pink colour along the full length of the boule. Optical absorption spectra in the temperature range between room temperature and 100 K were recorded with an AVIV 14DS spectrometer. Optical absorption spectra at 10 K were performed using a Varian Carey-SE spectrophotometer. Continuous wave (cw) luminescence emission was performed using the resonant line of an argon-ion laser. The emitted light was dispersed by a SPEX 500M monochromator and detected with a Hamamastu R-2949 photomultiplier. All low temperature mea-
T.P.J. Han et al. / Chemical Physics Letters 369 (2003) 519–524
surements (20–300 K) were obtained using a temperature controlled closed-cycle helium cryorefrigerator. Time-resolved luminescence spectra were obtained by excitation with the second harmonic of a Nd:YAG pulse laser at 532 nm with a pulse width of 5 ns.
3. Experimental results and discussion
Optical Density
Fig. 1 shows the unpolarized absorption spectrum of a nsto-LiNbO3 :Cr and a nsto-LiNbO3 : Cr:MgO sample. Essentially, the addition of MgO has the effect of red shifting the two broad bands, assigned to the 4 A2 ! 4 T2 and 4 A2 ! 4 T1 vibronic transitions, to lower energy. In addition, a signif-
Energy (cm-1)
Optical Density
(a)
(b)
Energy (cm-1)
Fig. 1. Optical absorption spectra of pure and co-doped MgO near stoichiometric LiNbO3 :Cr samples at (a) RT, (b) 10 K. Inset shows the magnified view about the 14 000 cm1 region. (Dotted line is LiNbO3 :Cr and solid line is LiNbO3 :Cr:MgO.)
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icant change in the relative intensity of these two broad bands is observed. This change in the absorption feature is responsible for the colour change detected, from green (no MgO) to pink (codoped with MgO), in these crystals. In congLiNbO3 :Cr:MgO samples this change has been observed for a concentration above 4.5 mol% [9] whereas in nsto-LiNbO3 :Cr:MgO this change is at a much lower level of 2 mol% [4]. The absorption spectra of Fig. 1 resemble those reported for congruent samples, hence the assignment of the broad bands and narrow lines follows that previously used for congruent crystals. It is important to note that the R-lines ð4 A2 ! 2 EÞ assigned to the c-½CrLi and the b-½CrLi centres are the only R-lines clearly observed in the absorption spectra of cong- or nsto-samples (see inset in Fig. 1). The R-lines of the other two major ½CrLi centres and the ½CrNb centres have never been observed in absorption. A more careful inspection of the absorption spectrum corresponding to MgO co-doped samples reveals dips on the broad bands which could be connected with Fano antiresonance as a result of interaction between the sharp transitions (2 E, 2 T1 and 2 T2 ) and the broad vibronic transitions of the excited levels 4 T2 and 4 T1 [10]. The emission spectra of the R-lines are only observable at temperatures below 100 K. Fig. 2a shows the emission spectra under 514.5 nm (19 436 cm1 ) argon line laser excitation at 20 K for the pure and MgO co-doped nsto-LiNbO3 :Cr3þ samples. In pure sample the spectrum consists of three narrow emission bands centred at 13 690, 13 620 and 13 540 cm1 . These emission lines have been assigned to the R-lines of b- and a-centres [11] and a zero-phonon line associated with the 4 T2 excited level, respectively [12]. The zero-phonon line has only been observed in pure nsto-samples. In fact, the closer the LiNbO3 is to the stoichiometric composition the greater is the ratio between this zero-phonon line and the R-lines of the a- and b-centres [12]. The emission spectrum of the codoped nsto-sample reveals four new narrow bands, at 13 510, 13 550, 13 565 and 13 642 cm1 associated with the addition of MgO as well as the R-lines ascribed to the a- and b-centres. In general the linewidth of the a- and b-centres seems to have increased slightly. The peak position of the
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Energy (cm-1)
Luminescence Intensity (au)
(a)
(b)
Energy (cm-1)
Fig. 2. Luminescence spectra under (cw) 514.5 nm argon line laser excitation of pure and co-doped MgO near stoichiometric LiNbO3 :Cr samples (a) 20 K, (b) 60 K. (*, The emission line at 13 642 cm1 , see text for its tentative assignment.)
b-centre remains the same for the two samples whereas the peak position for the a-centre is blue shifted slightly. The intensity of the line associated with the b-centre has also reduced significantly for the nsto-LiNbO3 :Cr:MgO sample. The two new R-line type narrow bands centred at 13 510 and 13 565 cm1 are in close proximity to the R-lines ascribed to the ½CrNb centres in cong-LiNbO3 : Cr:MgO sample (above the threshold of 4.5 mol%) [2,9] which has been confirmed by paramagnetic spectroscopy (EPR and ENDOR) [13,14]. The isotropic characteristic found in the EPR band for this centre in congruent sample was explained by considering that the close proximity of the charge compensating Mg2þ ion shifting the Cr3þ ions to a more central position in the Li-octahedral leads to a more cubic field.
The Liþ position in the crystal lattice is a weak crystal-field site, hence the lowest excited energy level corresponds to the 4 T2 and no emission from the 2 E excited level (R-lines) is expected. This is the case for the c-½CrLi centre. However, if the Dq/B parameter is close to the crossover point in the Sugano and Tanabe diagram, where the energy levels are plotted against the crystal field strength, the 2 E excited level can be activated thermally at high temperatures resulting in R-line emission. Fig. 2b displayed the emission spectra recorded at 60 K for pure and MgO co-doped LiNbO3 :Cr samples. As can be seen, in both samples the R-lines emission ascribed to the unperturbed c ½CrLi centre are now observed. Fig. 3a shows the time-resolved spectroscopy under 532 nm excitation (second harmonic of Nd:YAG pulse laser) of a pure nsto-LiNbO3 :Cr sample at 20 K. Curve 1 was obtained without any delay and curve 2 with a 200 ls delay. The small signal to noise ratio is due to pulse to pulse fluctuation of the laser output power and the fact that the data have not been averaged over a number of pulses. As it is expected, curve 1 reproduces the emission spectrum under cw excitation showing the narrow emission bands associated with the aand b-centres and the zero-phonon line attributed to the 4 T2 excited level. However, in the spectrum obtained using a 200 ls delay only the emission from the R-line of the a- and b-centres remains. This result is in accordance with the fluorescence decay-time of the 2 E (R-lines) and the 4 T2 energy level having 1 ms and 10 ls, respectively [9]. Fig. 3b shows the time-resolved spectra for the nsto-LiNbO3 :Cr:MgO sample under the same condition as that described above for Fig. 3a. The three peaks ascribed to R-line emission stay unchanged. This confirmed that the early assignment of the emission line centred at 13 510 cm1 is associated with the 2 E energy level [2]. Unfortunately the much weaker emission intensity of the narrow lines centred at 13 550, 13 565 and the new line at 13 642 cm1 are unable to be resolved in this experiment because of the small signal to noise ratio. It is important to note that the relative intensity ratio of the ½CrNb lines with the ½CrLi lines, in particular the a-centre, is much larger for the pulsed measurement than the continuous wave
Energy (cm-1) Fig. 4. Emission spectra of near stoichiometric LiNbO3 :Cr sample co-doped with MgO at different temperatures. Excitation wavelength is 514.5 nm. (*, The emission line at 13 642 cm1 , see text for its tentative assignment.)
Luminescence Intensity (au)
(a)
(b)
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Fig. 3. Time-resolved spectroscopy spectra under pulsed excitation at 532 nm at 20 K (a) pure near stoichiometric LiNbO3 :Cr sample, (b) near stoichiometric LiNbO3 :Cr sample co-doped with MgO. (*, The emission line at 13 642 cm1 , see text for its tentative assignment.)
excitation (see Fig. 2a). The nature of this observation is unknown at present and is the subject of our current investigation of this material. The temperature dependence of the R-lines emission intensity is presented in Fig. 4 for the nsto-LiNbO3 :Cr:MgO sample. As it can be observed at 20 K, the R-lines ascribed to the a- and b-centres are well defined together with the two R-lines assigned to the ½CrNb centres. The emission intensity of the lines associated with the ½CrNb , a- and b-centres decreases with increasing temperature and at 60 K the R-lines associated with the c-centre are clearly resolvable. An Arrhenius plot of the intensity ratio of R2 =R1 vs reciprocal temperature verified the assignment and energy
splitting of the 2 E state of early report [11]. However, this is based on the assumption that the ½CrLi centres observed in the nsto-sample are the same as those observed in the cong-sample. This is not true in all cases. The observed shift of the peak position of the a-½CrLi centre as described above, Fig. 2, indicates the linewidth of this centre which is inhomogeneously broadened in cong-samples and the observed linewidth in the nsto-sample represents only a subset of centres of similar crystal field symmetry. The model for the a-centre is a ½CrLi distorted by a nearby antisite, i.e., ½CrLi –½NbLi , the substitution of the ½NbLi by ½MgLi in the codoped samples could form ½CrLi –½MgLi type of centres which have a slightly different distortion, hence the observed shift of the peak position. The R-lines associated with the ½CrNb centres, 13 510, 13 550 and 13 565 cm1 have no paired relationship with any other R-line. This is in contradiction to early reports [9,11] where the R-lines at 13 510 and 13 565 cm1 were assigned to be the energy splitting of the 2 E energy level of the Nb centre. The narrow emission line centred at 13 642 cm1 is close (22 cm1 ) to the line associated with the a½CrLi centre. Detailed inspection of the emission spectra of the pure nsto- and co-doped congsamples also revealed a small peak in close proximity to this position. Its absence in cong-samples
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could be because it is obscured by the broadened linewidth of the nearby R-line of the a-½CrLi centre, see Fig. 2. The presence of this emission line in all LiNbO3 :Cr crystals, whether they are co-doped with MgO or not, suggests that the defect centre associated with this emission line is a Cr3þ in a Li site in origin. The position of this line being so close to the line associated with the a-½CrLi centre could be interpreted either as coincident or it is actually related to the a-½CrLi centre, i.e., similar to the a-½CrLi centre but with a slightly different local perturbation, hence the shift in energy. However, there are insufficient data at this point to confirm this hypothesis.
the only R-lines clearly observed in the absorption spectra of both congruent or nsto-samples.
Acknowledgements F.J., V.B. and E.D. would like to acknowledge the support of the Ministerio de Educacion yCultura (Spain). T.P.J.H. would like to acknowledge the financial support of EPSRC. We would like to acknowledge Dr. C. Zaldo (CSIC) for the help with the low temperature absorption measurement.
References 4. Conclusion The Cr3þ ion defect centres observed in nstoLiNbO3 :Cr and in nsto-LiNbO3 :Cr:MgO crystals are surprisingly similar to those observed for their respective counterparts in congruent composition. However, the much reduced intrinsic defect and narrower spectral linewidth in nsto-samples enable this study to resolve a previously obscured ½CrLi centre which is tentatively ascribed to a centre related to the a-½CrLi centre. It is the first time the shift of the R-line ascribed to the a-centre is observed in MgO co-doped crystals enabling the verification of the MgO effect on one of the ½CrLi centres. The appearance of the zero-phonon line at 13 540 cm1 only in pure nsto-LiNbO3 :Cr suggests it could be used as a probe to monitor the composition of LiNbO3 crystals. Three R-lines ascribed to ½CrNb centres associated with the addition of MgO above the critical dopant level are observed. In nsto-crystals this level is found to be at 2 mol% as compared to 4.6 mol% for cong-composition. In contrast to early reports there is no evidence to suggest the pairing of these R-lines with each other or any other Rlines. Somewhat surprisingly the R-lines ð4 A2 ! 2 EÞ assigned to the c-½CrLi and b-½CrLi centres are
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