Infrared photoluminescence of Nd multicenters in KTiOPO crystals

Transcription

1 Journal of Luminescence 79 (1998) Infrared photoluminescence of Nd multicenters in KTiOPO crystals C. Zaldo *, M. Rico, M.J. Martín, J. Massons, M. Aguiló, F.Dı az Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientı&ficas, Cantoblanco, Madrid, Spain Laboratori de Fı&sica Aplicada i Cristal.lografı&a, Universitat Rovira i Virgili, Tarragona, Spain Received 27 October 1997; received in revised form 12 February 1998; accepted 8 March 1998 Abstract The infrared photoluminiscence and optical absorption of Nd in KTP single crystals and in those co-doped with Al, Na and W have been studied. Characteristic emission band sets in and nm corresponding to the and transitions, respectively, have been observed. The excitation spectra of these bands show the splitting of the multiplet in two sublevels. The excitation spectrum of Nd in KTP only shows the contribution of one type of Nd center, the energy difference between the sublevels of this center is 183 cm.nd in the two slightly different titanium lattice sites is likely contributing to this spectrum. In crystals co-doped with Al the splitting is larger, namely 202 cm but still a single center is observed. The co-doping with Na or W induces the presence of additional new centers as well as new emission bands. It is inferred that Na Nd and W Nd pairs have been formed. W enhances the optical absorption cross section of Nd in KTP Elsevier Science B.V. All rights reserved. PACS: 4270.a Keywords: KTiOPO ; Neodymium photoluminescence 1. Introduction Bulk KTiOPO crystals have reached an outstanding position as a non linear optical medium for frequency doubling Nd-YAG lasers [1]. The possibility of developing self frequency doubling in KTP is hampered by the low admittance of rare-earth (RE) oxides in the melt and by the low distribution coefficients of RE ions in the crystals grown in melted fluxes, typically K " The highest Nd concentration achieved in the crystal is * Corresponding author. Fax: ; cezaldo@icmm.csic.es. only [Nd]"138 ppm ( cm ) [2]. Due to this situation, the optical absorption of Nd has been reported only recently in KTP crystals grown in a W-rich flux and most of the absorption bands were too weak to be observed at room temperature [3]. Therefore, it is neccesary to explore methods to enhance the incorporation of RE ions to the KTP lattice as well as to improve the characterization of the optical properties of the crystals obtained. Nd has been recently diffused into the KTP surface, providing a method to achieve higher neodymium concentration, [Nd]*10 cm [4]. The purpose of this work is to study the influence of co-dopants on the optical properties of the Nd /98/$ see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S ( 9 8 )

2 128 C. Zaldo et al. / Journal of Luminescence 79 (1998) Table 1 Molar composition of the K P O and tungsten rich fluxes. Impurity molar concentrations obtained by ICP technique and the corresponding Nd distribution coefficient obtained Crystal Composition of thek O Na O P O TiO Al O WO #Nd O flux (mol %) [Nd] (ppm) (cm ) [co-dopant] (ppm) K 10 KTP : Nd # KTP(W) : Nd # KTP : Na : Nd # KTP : Al : Nd # incorporated to the lattice providing basic spectroscopic information of Nd in KTP, which may help in the understanding of the spectroscopy of Nd ions diffused in KTP. The work reports on the photoluminescence of Nd in KTP for the first time to our knowledge. 2. Crystal growth In the present work K P O and tungsten rich fluxes have been used to obtain KTP single crystals. Details of the crystal growth procedures have been reported previously [2,5]. By using the top seeded solution growth slow cooling technique, KTP crystals free of inclusions were obtained. Samples obtained by this method may reach dimensions in the 1 5 cm range, as required for some applications, but the sample sizes used in this work have limited dimensions in the 1 5 mm range due to the slow growth rate of KTP (typically in the range mm/day). The composition of the solutions used for growth are indicated in Table 1. The neodymium concentrations used in the solution were the maximum values allowed to obtain the KTP phase [2]. The distribution coefficient of RE ions in KTP increases when the impurity ionic radii (in the A range) are closer to the Ti ionic radium (namely r "0.68 A ) [2]. It is thus assumed that Nd and other RE ions substitute host titanium in spite of the large lattice and charge mismatches expected. With the hypothesis to compensate the possible local lattice distortion, we have grown Nd doped KTP crystals co-doped with additional non optically active impurities. For this purpose we have selected W and Al which replace Ti having equal or lower ionic radius (r "0.68 A, r "0.66 A and r "0.5 A ) and different charge states. On the other hand, we have selected Na that replaces K also with a lower radii (r " 0.95 A (r "1.33 A ). 3. Experimental The impurity concentration in the crystal has been determined by inductively coupled plasma (ICP) using a Jovin Yvon JY-38 equipment. Table 1 shows the impurity concentrations and the Nd distribution coefficients obtained. It is worth noting that the co-doping ions are efficiently incorporated to the KTP lattice. The present results do not support the previous suggestion about an increase of the Nd distribution coefficient induced by W [3]. Only Al, with the smallest ionic radii induces an slight increase of the Nd distribution coefficient. The optical absorption of Nd was studied at 4.2 K by using a Varian 5E spectrophotometer. Due to the low concentration of Nd in the KTP crystals grown in self fluxes and to the limited size of the samples available (about 1 mm thick), it was not possible to determine the whole optical absorption spectrum expected, but Table 2 summarizes the positions of the most intense bands observed. Fig. 1 shows a comparison of the I P # S transitions of Nd in KTP and KTP(W) samples. In the spectral region displayed, KTP : Nd crystals show a well resolved and narrow band at

3 C. Zaldo et al. / Journal of Luminescence 79 (1998) Table 2 Energy position (cm ) of the Nd optical absorption bands resolved at 4.2 K in KTP crystals grown with different co-dopants KTP : 5%Nd KTP : 5%Al : 5%Nd KTP : 5%Na : 5%Nd KTP(W) : 5%Nd Nd Nd W F # S G # G G Fig. 1. Unpolarized optical absorption, measured at 4.2 K, of Nd in KTP crystals grown in self flux and in W-rich flux cm and other minor features at and cm. The later peaks are reliably above those of the oscillatory background noise related to the light interference in the sample faces. The absorption at cm appears more or less intense in all crystals studied in this work (see Table 2), thus we consider that it corresponds to a non perturbed Nd center in KTP. In the KTP(W) crystal additional bands appear between and cm. It is worth remarking that the total integrated absorption in KTP(W) crystals is much higher than in KTP, KTP : Al or KTP : Na crystals despite that the Nd concentration in these crystals does not change too much (see Table 1). It must be concluded that the absorption cross section of some of the Nd centers in KTP(W) crystals are much larger than in the other KTP crystals. This behavior is most likely related to the presence of new Nd centers with a local lattice symmetry lower than that corresponding to the non perturbed Nd-centers in KTP. The local lattice symmetry of the Ti site is C, therefore, it is suggested that the Nd ions incorporated in the Ti lattice sites of KTP have a local symmetry higher than that corresponding to the Ti site of the lattice. Unfortunately, lack of the whole Nd spectrum, and the presence of several centers make the analysis of the multiplets splitting in terms of the local symmetry unreliable. In order to get a better knowledge about the effect of co-doping ions in the spectral properties of Nd, we have studied the Nd photoluminescence

4 130 C. Zaldo et al. / Journal of Luminescence 79 (1998) Fig K photoluminescence emissions of Nd in various KTP crystals. (a) KTP crystals, "874.5 nm. (b) KTP crystals co-doped with Al, solid line, "875.6 nm; or co-doped with Na, dashed line, "875.5 nm. (c) KTP crystals grown in W-rich fluxes, "861 nm, chain line; "874.5 nm, solid line and "877 nm, dashed line. in singly doped KTP and in co-doped crystals. To this purpose the neodymium level has been excited with a Ti-sapphire cw-laser. The unpolarized emission has been detected at 77 K with a lock-in amplifier by using a Spex 340E spectrometer (f"34 cm) and a Ge photodiode, model ADC-403 IR, cooled to 77 K. In order to improve the signal to noise ratio a long lock-in integration time (1 5 s) was used, the detector was shielded against cosmic rays and the fluorescence of the Ti-sapphire laser filtered. The resolution of the IR emission spectra is in the range nm. Three main emission band sets were observed in the , and nm spectral regions, corresponding to the radiative transitions from the multiplet to the I, I and I multiplets, respectively [6]. Fig. 2a c show a comparison of the P I photoluminescence emissions of Nd in KTP and those obtained in crystals co-doped with Na, Al and W. In KTP : Nd, the main band peaks at nm and four additional weaker bands are also observed. Table 3 summarizes the emission bands observed. The presence of several emission bands is consistent with the lifting to the I degeneracy by the crystal field. Due to the odd electron number of the Nd configuration, the levels must be at least doubly degenerated, therefore, the I multiplet may split in a maximum of 6 sublevels, five of them have been resolved and most likely the missing one is overlapping some of the emission bands observed. Under our experimental resolution, the position of these bands is independent of the excitation wavelength. In KTP : Al : Nd the emission spectrum observed is quite similar to that described previously, only a slightly larger I level splitting has been found, see Fig. 2b and Table 3. In KTP : Na : Nd, the band position and shape of the emissions depend on the excitation wavelength. The excitation at nm yields an emission spectrum very similar to that observed in KTP : Nd, but the excitation in the nm range yields new emission bands shifted to low energy with regards to KTP : Nd, see Fig. 2b and Table 3. The photoluminescence spectrum of KTP(W) : Nd excited at nm shows the emissions observed in KTP : Nd, moreover, new emission bands are observed by exciting at nm. Both type of emission bands may be observed by exciting the high energy (a) sublevel, i.e. exciting at nm. Fig. 3 shows the emission arising from the radiative transition of Nd. In KTP : Nd six of the 7 possible bands arising from the splitting of the I multiplet have been observed. The presence of perturbed Nd emission bands in the KTP crystals co-doped with Al, Na and in crystals grown in W-rich fluxes is also evident by comparing Fig. 3a c. Fig. 4 shows the excitation spectra of Nd in the different crystals used in this work. The spectra correspond to the excitation of the level. Two excitation bands appear at 861 and nm, see Fig. 4a, this shows that the crystal field symmetry is low enough to partially lift the degeneracy of the

5 C. Zaldo et al. / Journal of Luminescence 79 (1998) Table 3 Spectral positions of the different photoluminescence bands observed at 77 K in Nd doped KTP crystals. The splitting energy E of the sublevels (labeled as a and b) of different centers is included. The symbols *, and & distinguish the photoluminescence properties of different centers coexisting in each matrix I P (nm)/ E (cm ) (nm) (nm) KTP : 5%Nd b Non-perturbed a 874.9/ Nd in KTP KTP : 5%Al : 5%Nd b Non-perturbed a 875.4/ Nd in KTP : Al KTP : 5%Na : 5%Nd b As in KTP : Nd As in KTP : Nd Non-perturbed a 874.4/185 Nd in KTP : Na b Na-perturbed a /174* * * Nd a /175* * * a /177* * * a / * * a /187& & * & a /192& & * & & & & & & & KTP(W) : 5%Nd b As in KTP : Nd As in KTP : Nd Non-perturbed a 874.9/183 Nd in KTP(W) b a /87* * * a /118& * & * & a /170& * & * & a /211& & * & a /231& a /259& a /285& W-perturbed Nd

6 132 C. Zaldo et al. / Journal of Luminescence 79 (1998) Fig K photoluminescence emissions of Nd in various KTP crystals. (a) KTP crystals, "874.5 nm. (b) KTP crystals co-doped with Al, solid line, "875.6 nm; or co-doped with Na, dashed line, "875.5 nm. (c) KTP crystals grown in W-rich fluxes, "874.5 nm, solid line, "877 nm, dashed line and "867 nm, chain line. Fig K excitation spectra of the level of Nd in KTP. (a) KTP crystals, "1052 nm. (b) KTP crystals codoped with Al, solid line, "1054 nm; or co-doped with Na, dashed line, "1056 nm. (c) KTP crystals grown in W-rich fluxes, "1340 nm, solid line and "1320 nm, dashed line. level. Hereafter we will name these sublevels a and b for increasing energy. The energy difference, E, between these two sublevels may characterize the distortion of the Nd centers in KTP. Table 3 summarizes the E values obtained for the different Nd centers observed in KTP : Nd and in co-doped crystals. The E values obtained are in the same order of magnitude that those observed for Nd perturbed ions in other lattices, i.e. in LiNbO [7]. Fig. 4a shows that some asymmetry may be observed in the band peaking at nm. This could be related to the presence of Nd in the two titanium sites of the KTP structure. The optical absorption and photoluminescence results presented above for W doped crystals show the coexistence of Nd centers similar to those found in KTP : Nd with new Nd centers characteristic of KTP(W) : Nd crystals, see Fig. 2c and Fig. 3c. From the excitation spectrum shown in Fig. 4c up to seven different new centers may be identified. The (b) sublevels of all of these Nd centers appear overlapped but a clear distinction may be found in the position of the (a) sublevels. The center with smallest E splitting shows an emission slightly shifted to high energy with regards to the other new centers, see Fig. 3c. Within the spectral resolution of our results, the rest of new Nd centers show similar emission spectra. It has been established that W replaces Ti in the two titanium lattice sites and it coexists in several oxidation states, namely W,W and very likely

7 C. Zaldo et al. / Journal of Luminescence 79 (1998) W, as a function of the thermal and optical history of the sample [8]. It seems probable that the new Nd centers correspond to Nd ions perturbed by tungsten in a neighboring titanium site: Nd W and Nd W. The presence of tungsten ions with different oxidation states may be considered as a pausible hypothesis to explain the multiplicity of centers. Co-doping with Al induces a modification of the E splitting of the level. This is most likely due to the change of the KTP lattice parameters as observed in isomorphic compounds [9]. The substitution of potassium by sodium produces a shortening in the lattice parameters of the matrix [9] therefore slightly different centers may be expected in KTP : Na : Nd crystals. Fig. 4b shows the presence of overlapped bands in the nm region corresponding to the (b) sublevel, moreover several well resolved low energy (a) sublevels may also be observed. This shows that several Nd centers with slightly perturbed environments coexist. From the comparison with the cases discussed above it seems likely that one of those centers corresponds to a non perturbed Nd center in a KTP lattice compressed with respect to the undoped KTP, the presence of other centers may be due to the presence of Na in the Nd neighborhood. 4. Conclusions In summary we have characterized the Nd optical absorption and photoluminescence in KTP. It has been shown that W induces new Nd centers, likely Nd W pairs, with an optical absorption cross section higher than of isolated Nd in KTP. Further, Al and Na also create perturbed Nd centers likely due to a lattice distortion induced by the co-dopant ion and to the presence of Nd Na pairs. The Nd photoluminescence can be used as very sensitive tool to monitor KTP lattice distortions. Acknowledgements This work has been supported by CICyT under project TIC The cooperation of Natalia Denisenko in the development of experimental hardware for the photoluminescence technique is gratefully acknowledged. References [1] J.D. Bierlein, H. Vanherzeele, J. Opt. Soc. Am. B 6 (1989) 622. [2] R. Solé, V. Nikolov, I. Koseva, P. Peshev, X. Ruiz, C. Zaldo, M.J. Martín, M. Aguiló, F. Dı az, Chem. Mater. 9 (1997) [3] C. Zaldo, M. Aguiló, F. Dı az, H. Loro, J. Phys.: Condens. Matter 8 (1996) [4] M.J. Martín, C. Zaldo, M.F. da Silva, J.C. Soares, F. Díaz, M. Aguiló, J. Phys: Condens. Mater 9 (1997) L465. [5] R. Solé, X. Ruiz, R. Cabré, Jna. Gavaldà, M. Aguiló, F. Díaz, V. Nikolov, X. Solans, J. Crystal Growth 167 (1996) 681. [6] B. Henderson, G.F. Imbusch, Optical Spectroscopy of Inorganic Solids, Oxford Science Publications, Clarendon Press, Oxford, [7] J.O. Tocho, J.A. Sanz García, F. Jaque, J. García Solé, J. Appl. Phys. 70 (1991) [8] D. Bravo, X. Ruiz, F. Dı az, F.J. Lo pez, Phys. Rev. B 52 (1995) [9] M.E. Hagerman, K.R. Poeppelmeir, Chem. Mater. 7 (1995) 602.