Spectral properties of Yb 3+ ions in LiNbO 3 single crystals: influences of other rare-earth ions, OH ions, and -irradiation

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1 Journal of Alloys and Compounds xxx (4) xxx xxx Spectral properties of Yb + ions in LiNbO single crystals: influences of other rare-earth ions, OH ions, and -irradiation Taiju Tsuboi a,, Sławomir M. Kaczmarek b, Georges Boulon c a Faculty of Engineering, Kyoto Sangyo University, Kamigamo, Kita-ku, Kyoto 6-8, Japan b Institute of Physics, Technical University of Szczecin, Al. Piastów 48, 7-1 Szczecin, Poland c Physical Chemistry of Luminescent Materials, Claude Bernard /Lyon 1 University, UMR CNRS 6, Bat. A. Kastler, 1 rue Ampère, 696 Villeurbanne, France Abstract Absorption spectra have been studied in 19 1 nm region at various temperatures from 16 to 9 K for Yb + -doped and Yb + /Nd + -, Yb + /Er + - and Yb + /Pr + -co-doped LiNbO single crystals before and after -irradiation with a dose of Gy. Intense 4 and nm absorption bands were observed after -irradiation, which are due to the creation of oxygen vacancy (F-type color center) and Nb 4+ polaron, respectively. Different change was observed in the 87 nm OH absorption band intensity among the various rare-earth doped crystals. These are interpreted by discrepancy of ionic radii between substituting rare earth dopant ion and Li + or Nb + host ion. The observed temperature dependence of the hot bands is understood by electronic transition from the thermally populated F 7/ Stark levels to the excited F / level. The position of the Yb + F 7/ F / first resonant line was observed to be a slightly different among the co-doped crystals. This is due to the perturbation of Yb + by co-doped rare earth ion which is located at the neighborhood of theyb +. 4 Published by Elsevier B.V. Keywords: Spectral properties; Rare-earth ions; LiNbO single crystals 1. Introduction Among many kinds of oxide crystals doped with Yb + ions, Ca (PO 4 ) F and Sr (PO 4 ) F are selected as the favorable hosts for laser action, while LiNbO (LN) is suggested as the most unfavorable one [1]. The LN crystal, however, is one of the useful optical materials because of its nonlinearity. Montoya et al. [] obtained not only the 16 nm Yb + laser action with high efficiency of about 47% but also self-frequency doubled laser action using Yb + /Mg + -co-doped LN crystal. Jones et al. [] achieved lasing with a slope efficiency of about 16% using Yb + /Ti-co-doped LN waveguide. Co-doping is one of the methods to improve the laser efficiency of Yb + -doped LN since the presence of additional impurity ions changes the ligand field of Yb + in LN lattice and changes the electronic states of Yb +. Therefore we are interesting in the spectral properties of Yb + : LN crystals co-doped with rare-earth ions. Corresponding author. Tel.: ; fax: address: tsuboi@cc.kyoto-su.ac.jp (T. Tsuboi). No systematic study has been undertaken on the influence of rare-earth ions to Yb + ions in LN crystal. In this paper we investigate the absorption spectra of Yb + :LN crystals co-doped with Pr +,Nd + and Er + ions. It is interesting to investigate: (1) what kind of influence do the co-doped rare earth ions gives to Yb + and () whether its influence depends on ionic radius of the co-doped rare earth ions. For this purpose, we check whether the positions of Yb + absorption bands are different among various co-doped crystals. The ionic crystals usually contain OH ions when they are doped with impurity ions such as transition-metal ion and rare-earth ions. It is interesting to study whether the presence of OH ions changes among various co-doped rare earth ions. We have also an interest on the effect of -ray irradiation on Yb + ions. It was reported that Yb + ions are created in Yb + -doped CaF crystal by -ray irradiation, giving rise to absorption bands in 8 nm range [4]. No report, however, has been made on a question whether such an Yb + Yb + conversion occurs in LN crystal. Here attention is paid to clarify whether the effect of -ray irradiation on Yb + depends on ionic radius and concentration of the co-doped rare earth ions /$ see front matter 4 Published by Elsevier B.V. doi:1.116/j.jallcom.4..4

2 T. Tsuboi et al. / Journal of Alloys and Compounds xxx (4) xxx xxx. Experimental procedure Single crystals were grown by the Czochralski method from a congruent melt at the Institute of Electronic Materials Technology, Warsaw. The conditions of growth were described elsewhere []. We studied the following seven LN crystals. Of these crystals, two crystals are not irradiated with -rays. The first non- -rayed LN crystal contains about: 1 wt.% Yb + (called #4 crystal), and the second crystal contains.8 wt.% Yb + and.1 wt.% Pr + (#8 crystal). The # and #7 crystals which were -rayed are called #4 and #8 crystals, respectively. The other -rayed and co-doped LN crystals contain. wt.% Yb + (1.9 1 ions/cm ) and. wt.% Nd + ( ions/cm ) (#9),.8 wt.% Yb + and. wt.% Pr + (#1), and. wt.% Yb + and 1 wt.% Er + (#11). The -ray irradiation was undertaken at room temperature from 6 Co source (with dose of Gy). Unpolarized absorption spectra were measured with a Cary-E spectrophotometer in a spectral range of 19 1 nm at 16 K and with a Lambda-9 of Perkin-Elmer at room temperature. Photoluminescece measurements were carried out using a SS-9 Edinburgh Inc. spectrophotometer at the Institute of Optoelectronics, MUT, Poland.. Experimental results.1. Low temperature absorption spectra Fig. 1 shows the absorption spectra of the #, 4, 7, 8, 9, 1 and 11 crystals at 17 K in spectral range of 86 1 nm where Yb + absorption bands appear. The spectra consist of intense and sharp absorption bands due to the F 7/ F / electronic transition and weak bands due to vibronic transitions. Such vibronic bands are observed in various Yb + -doped oxides [6 8]. The intense bands are observed at 918., 9 and about 98. nm. In the #9 crystal, ad- absorption coefficient (cm -1 ) K #4 Yb #8 Pr/Yb # Yb γ #7 Pr/Yb γ #9 Nd/Yb γ #1 Pr/Yb γ #11 Er/Yb γ wavelength (nm) Fig. 1. Near infrared absorption spectra of LiNbO crystals #, #4, #7, #8, #9, #1 and #11 at 17 K. absorption coefficient (cm -1 ) 1 1 RT #7 Pr/Yb γ # Yb γ #4 Yb #8 Pr/Yb #9 Nd/Yb γ #1 Pr/Yb γ #11 Er/Yb γ wavelength (nm) Fig.. Near infrared absorption spectra of LiNbO crystals #, #4, #7, #8, #9, #1 and #11 crystals at room temperature (88 K). ditional bands are observed at 87.6 and 888. nm. These are attributed to the 4 I 9/ 4 F / electronic transition of Nd +. Similarly additional bands are observed at 969. and 974. nm in the #11 crystal, which are due to the 4 I 1/ 4 F 11/ transition of Er +, while in the #7, #8 and #1 crystals very weak bands are observed at 987.8, 119, 1 and 17 nm, which are due to the 4 H 4 4 G 4 transition of Pr +. It is observed that the peak position of the sharp line at the lowest energy is different among the crystals. The single-doped crystals # and #4 have a peak at nm, the Pr + -co-doped crystals #7, #8 and #1 have a peak at nm, while the Nd + - and Er + -co-doped crystals #9 and #11 have peaks at 98. and nm, respectively. The Er + -co-doped crystal seems to have the same peak position as the single-doped crystals. These positions are slightly different from each other nm for the former crystal and nm for the latter ones. Fig. shows the absorption spectra of the #, 4, 7, 8, 9, 1 and 11 crystals at 88 K in spectral range of nm. Weak additional bands are observed at 14, 14 and 16 nm in the absorption spectrum of all crystals. It is noted that the single-doped crystal # shows much higher absorption intensity for the 14 nm band than the other crystals. The 14 nm band consists of two bands with peak at 14 and 11 nm. The 14, 14 and 16 nm bands appear above about, 8 and 17 K and their intensities increase with increasing temperature. Thus they are called hot bands. These hot bands have been observed by other scientists (see e.g. [9 11]). In Fig., the absorption intensities of these bands observed in crystal #1 are plotted against temperature. Fig. 4 shows the OH absorption band observed in various single and co-doped LN crystals. The measurement was undertaken at the same condition for these crystals. Much intense OH band is observed in not only Pr + -single doped crystal but also Pr + /Yb + -co-doped crystals. Of the

3 T. Tsuboi et al. / Journal of Alloys and Compounds xxx (4) xxx xxx absorption intensity (arb. units) #1 1 1 temperature (K) 14nm 14nm 16nm Fig.. Temperature dependence of the intensities of the 14 nm (open circle), 14 (closed circle) and 16 nm (triangle) hot bands observed in the #1 crystal. Solid lines are the best fit to the experimental points (see text). Pr + /Yb + -co-doped crystals, crystal with higher Pr + concentration shows more intense band than crystal with lower Pr + concentration. When compared with the non-doped LN crystal, only the Er + single doped crystal shows a red shift in the OH band, while the other crystals show a blue shift... Effect of γ-ray irradiation Oxygen vacancies are present in the LN lattice for charge compensation, because trivalent rare earth ion is substituted for Li + ion and Nb + ion. The -irradiation introduces electrons to the oxygen sites and F-type color centers are formed. At the same time Nb 4+ polarons are created as an effect of Compton secondary electron capture. Fig. shows the change of the absorption spectra between before and after -ray irradiation for non-doped, Yb + -doped, and Yb + /Nd + -co-doped LN crystals. A broad band is observed for all the crystals in a 7 nm region. The band has a peak at about 4 nm and a shoulder at about nm. Such a double structure band was previously observed and the 4 and nm bands have been attributed to F-type color center and Nb 4+ polaron, respectively [1]. We could not observe any absorption band due to Yb +. This indicates that electrons created in LN host crystal by -ray irradiation are not trapped by Yb + ions. Same is true for co-doped rare earth ions such as Pr + and Nd +. The intensity of the 4 and nm bands strongly depends on the -ray dose. The intensity increases with increasing the dose, but the increase stops above about 1 6 Gy. Of the various co-doped crystals, Er + co-doped crystal shows a much smaller increase in the 4 and nm bands than the other co-doped crystals. In Yb + /Nd + -co-doped crystal, a decrease of the OH absorption band at about 87 nm is observed after -ray irradiation when compared with non-irradiated and non-doped LN crystal. We also measured the absorption spectra of -rayed Yb + /Er + - and Yb + /Pr + -co-doped LN crystals, and observed a decrease of the OH absorption band in Yb + /Pr + -co-doped crystal as observed in Yb + /Nd + -co-doped crystal. Absorption coefficient [1/cm], 1,8 1,6 1,4 1, 1,,8,6,4, LN - LN:Yb (1wt.%) - LN:Er (.at.%) 4 - LN:Pr (1at.%) - LN:Er,Yb (1wt.%,.wt.%) 6 - LN:Nd,Yb (.wt.%,.wt.%) 7 - LN:Pr,Yb (.1wt.%,.8wt.%) 8 - LN:Pr,Yb (.wt.%,.8wt.%), Wavelength [nm] Fig. 4. The OH absorption band of various LiNbO (LN) crystals at room temperature. Non-doped LN (curve 1), LN:Yb + (1 wt.%) (), LN:Er + (. wt.%) (), LN:Pr + (1 wt.%) (4), LN:Er +,Yb + (1 and. wt.%) (), LN:Nd +,Yb + (. and. wt.%) (6), LN:Pr +,Yb + (.1 and.8 wt.%) (7), and LN:Pr +,Yb + (. and.8 wt.%) (8).

4 4 T. Tsuboi et al. / Journal of Alloys and Compounds xxx (4) xxx xxx 1, 4 gamma 1 7 Gy,8,6 1 - LN - LiNbO - LiNbO :Yb (1%), 4 - LiNbO :Nd, Yb (.%,.%) K [1/cm],4, 1 gamma 1 Gy, -, 1 1 Wavelength [nm] Fig.. Change of the absorption spectra ( k, k: absorption coefficient) between before and after 1 Gy -ray irradiation for LiNbO crystal without any rare-earth ions (curve 1), the #4 crystal (curve ), and the #9 crystal (curve ). Curve 4 shows for the #9 crystal after 1 7 Gy -ray irradiation. 4. Discussion As seen in Fig. 4, we observed the much higher OH absorption band intensity in Pr + -doped crystals and the increase of the intensity with increasing Pr + concentration. Why are the Pr + -doped crystals different from the other crystals? Electron spin resonance study shows that rare-earth ions enter both Li + and/or Nb + lattice sites in LiNbO and the local symmetry is the same C v in the both cases [1]. The Rutherford back-scattering (RBS) study has suggested that they usually enter Li + sites [14 16]. Asan example, the Nd + ions are in Li + octahedrons and located at off-center position along the c-axis. The ionic radii of Yb + and Er + ions are.88 and.881 nm in octahedral coordination, respectively. They are smaller than the ionic radius of the other rare earth ions. Therefore it is suggested that Yb + and Er + ions are substituted for Li + ion with small ionic radius of.74 nm, while Pr + ion with large ionic radius of 1.1 nm is substituted for Nb + ion with much smaller ionic radius of.64 nm [,6]. Such a substitution by much large ion leads to the deformed octahedrons and creation of positively charged defects of (Pr Nb+ + ) + type (where Pr Nb+ + means vacancy created by substitution of Pr + ion for Nb + ion) for charge compensation, giving rise to easy introduction of negatively charged OH ions during the crystal growth. Therefore we understand the increase of OH band in Pr + -doped crystals. It was observed that the 4 and nm band intensity is weaker in Yb + - and Er + -doped crystals than the other crystals. This is understood by the difference of ionic radius of rare earth ion. It is expected that, as the difference of ionic radius between rare earth ion and substituted Li + (or Nb + ) ion becomes large, the oxygen vacancies and color centers, which are responsible for the 4 and nm bands, are more easily created by -irradiation. The ionic radii of Yb + and Er + are more close to those of Li + and Nb + than those of Pr + and Nd +, resulting in weaker 4 and nm bands in Yb + - and Er + -doped crystals. Like this we confirm the influence of difference of ionic radius in the rare earth doped LN crystals. Next we shall consider the Yb + absorption bands observed in 9 18 nm region. Yb + ion has the ground state F 7/ and excited state F /, which split into four and three levels in the C v symmetry crystal field, respectively. Fig. 6 shows the energy level diagram deduced from the absorption measurements. From the hot bands at 14, 14 and 16 nm, it is determined that the first, second and third excited levels are at 41, 46 and 767 cm 1 above the ground level, respectively, which are called levels, and 4, while the three levels of the F / state are called levels, 6 and 7 in order of increasing energy. We observed three emission bands at 14, 14 and 16 nm in all the crystals by excitation with 98 nm light. The peak positions of these emission bands are the same as those of the hot absorption bands. These emission bands are attributed to the transition from the relaxed excited level of the F / state (i.e. the level ). In this way we confirmed the presence of the levels, and 4 at 41, 46 and 767 cm 1 above the ground level 1 in the F 7/ state, respectively. It is suggested that the intensities I i of the hot bands i are described by the following equation. It consists of two parts. One is the probability P i of transition from the levels (i =, and 4) in the F 7/ state to the level in the F / state.

5 T. Tsuboi et al. / Journal of Alloys and Compounds xxx (4) xxx xxx nm 9.8 nm 98. nm Yb + 14 nm 14 nm 16 nm P P P Fig. 6. The Yb + energy level diagram of the F 7/ and F / states in the C v crystal field and the assignment for the observed absorption bands. P,P and P 4 are the transition probabilities from the levels, and 4 to the level, respectively. The other is the population of Yb + ions in the i level. I i P i exp( E i /kt) exp( E i /kt), i = 1, 4 where E i is the energy measured from the level 1. The denominator is given as a normalization factor. The best fit to the experiment was obtained using P :P :P 4 =.74:1.:.6 as shown by solid lines in Fig.. This indicates that the transition probability is different among the hot bands, i.e. the transition has higher transition probability than the other transitions. So far no work has been done on such a fitting for the hot bands in rare-earth doped crystals. The absorption band peak due to the transition from the level1of F 7/ state to the level of F / state is observed at same nm in crystals #, 4, and 11, while at 98. nm in crystal #9 and nm in crystals #7,8 and 1. No effect was observed on the peak position by -irradiation. Such a different position of the 1 transition band is explained as follows. In the co-doped crystals, two kinds of Yb + ions are present, one is Yb + accompanied by nearby rare-earth ion such as Nd +, which is called perturbed Yb +, the other is Yb + which is located far from the rare-earth ion, which is called isolated Yb +. The energy levels of the perturbed Yb + are shifted from those of the isolated Yb +. Such a shift is observed in crystals #9 and #1. However, no shift is observed in crystals #7 and #8. The crystals #7 and #8 are co-doped with Pr + ions of much lower concentration than the crystal #1, i.e. presence of the perturbed Yb + ions is negligible. Therefore the 1 absorption band in crystals F F / 7/ #7 and #8 is observed at the same position as in the crystals # and #4 containing only isolated Yb + ions.. Summary Absorption spectra have been studied on Yb + -doped and Yb + /Nd + -, Yb + /Er + - and Yb + /Pr + -co-doped LiNbO single crystals to investigate the effect of co-doping and -irradiation. Yb + and Yb 4+ ions which are converted from Yb + were not observed by the -irradiation. However, the intense 4 and nm absorption bands were observed after -irradiation, which are due to the creation of oxygen vacancy (F-type color center) and Nb 4+ polaron, respectively. Different change was observed in the 87 nm OH absorption band intensity among the various rare-earth doped crystals. These are interpreted by discrepancy of ionic radii between substituting rare earth dopant ion and Li + or Nb + host ion. The observed temperature dependence of the hot bands is understood by electronic transition from the thermally populated F 7/ Stark levels to the excited F / level. The position of the Yb + F 7/ F / first resonant line was a slightly different among the co-doped crystals. This is due to the perturbation of Yb + by co-doped rare earth ion which is located at the neighborhood of theyb +. References [1] G. Boulon, Opt. Mater. () 8. [] E. Montoya, J. Capmany, L.E. Bausa, T. Kellner, A. Diening, G. Huber, Appl. Phys. Lett. 74 (1999) 11. [] J.K. Jones, J.P. de Sandro, H. Hempstead, A.C. Large, A.C. Tropper, J.S. Wilkinson, Opt. Lett. (199) [4] D.S. McClure, Z. Kiss, J. Chem. Phys. 9 (196) 1. [] S.M. Kaczmarek, I. Pracka, Z. Mierczyk, K. Kopczyñski, R. Piramidowicz, M. Malinowski, J. Kisielewski, A.O. Matkovskii, D.Yu. Sugak, Acta Phys. Pol. A 9 (1996) 411. [6] A. Lorenzo, H. Joffresic, B. Roux, G. Boulon, J. Garcia-Sole, Appl. Phys. Lett. 67 (199) 7. [7] P.H. Haumesser, R. Gaume, B. Viana, E. Antic-Fidancev, D. Vivien, J. Phys. Cond. Matter 1 (1) 47. [8] R. Gaume, P.H. Haumesser, B. Viana, D. Vivien, B. Ferrand, G. Aka, Opt. Mater. 19 () 81. [9] E. Cantelar, J.A. Sanz-Garcia, F. Cusso, J. Cryst. Growth (1999) 196. [1] G. Foulon, M. Ferriol, A. Brenier, M.T. Cohen-Adad, G. Boulon, Chem. Phys. Lett. 4 (199). [11] E. Montoya, A. Lorenzo, L.E. Bausa, J. Phys. Cond. Matter 11 (1999) 11. [1] S.M. Kaczmarek, Ferroelectrics 6 (1) 17. [1] G. Burns, D.F. O Kane, R.S. Title, Phys. Rev. 167 (1968) 14. [14] J. Garcia-Solé, T. Petit, H. Jaffrezic, G. Boulon, Europhys. Lett. 4 (199) 719. [1] A. Lorenzo, H. Jaffrezic, B. Roux, G. Boulon, L.E. Bausa, J. Garcia-Solé, Phy. Rev. B (199) 678. [16] A. Lorenzo, H. Loro, M.C. Terrile, G. Boulon, L.E. Bausa, J. Garcia Sole, Opt. Mater. 8 (1997).