Spectroscopy of multisites Cr3+ and Nd3+ in yttrium aluminum perovskite

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CHEMICAL PHYSICS LETTERS

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Spectroscopy of multisites Cr3+ and Nd3+ in yttrium aluminum perovskite J.A. Mares Divison of Solid State Physics, Institute of Physics, Academy of Sciences of the Czech Republic, CukrovarnicM 10, 162 00 Praha 6, Czech Republic

G. Boulon,

A. Brenier,

L. Lou ’

Laboratoire Physico-Chimie des Materiaux Luminescents, Vniversite Lyon I, VRA 442 CNRS, 43 Boulevard du 1 I Novembre 1918, 69622 Villeurbanne, France

S.R. Rotman Ben Gurion University of the Negev, Department ofElectrical and Computer Engineering, P.O. Box 653. 84105 Beer-Sheva, Israel

Received 8 September 1993; in final form 16 November 1993

Selectively excited Cr3+ and Nd3+ emission spectra and excitation spectra of YA103:Nd, Cr crystals are presented. Up to eight Nd3+ and three Cr3+ multisites are observed in these crystals. C?+ +Nd’+ energy transfer is observed between various multisites starting from *E Cr3+ energy levels. Cr ‘+ donor decay can be adequately fitted according to the standard Inokuti-Hirayama theory but a spatial correlation between Cr’+ donors and Nd”+ acceptors results in a better tit of Cr’+ fluorescence decay curves.

1. Introduction Neodymium-doped yttrium aluminium perovskite YA103 (YAP) is one of the most important solid-state laser crystals [ l-51. To increase the pumping efficiency of this laser crystal, various sensitizers such as Cr3+ or Ce3+ ions are added with the Nd3+ activator [ 1,4,5]. Cr3+ or Ce3+ absorption bands are broad, overlapping many narrow Nd3+ absorption lines; due to the Cr3+ +Nd3+ or Ce3++Nd3+ energy transfer and the good overlapping of the pumping flashlamp spectra with Cr3+ or Ce3+ absorption bands, the pumping efficiency of Cr3+- or Ce3+-codoped YA103: Nd crystals increases. To understand and to improve the energytransfer processes in YA103 : Nd crystals, spectro-

’ On leave from the University of Science and Technology of China, 230026 Hefei, Anhui, People’s Republic of China.

scopic studies of the Nd3+ dopant and the codopants are important. In comparison with the Cr3+- (or Ce3+-) codoped Y3A150,2:Nd crystals which were studied in detail [ 6-l 11, there are only a few studies of YAlO, : Nd or YA103:Nd, Cr crystals [ 1,12,13]. The presence of Nd3+ or Cr3+ multisites and Cr3++Nd3+ energy transfer between some of them was observed in Y3A150,2 : Nd, Cr crystals, while the presence of Cr3+ pairs was discovered by van der Ziel in YA103 : Cr [ 141. Our first results on YA103: Cr crystals have shown that Cr3+ multisites are probably present in YA103 [ 91. Spectroscopic data on YA103:Nd or YA103 : Nd, Cr crystals have not been precisely measured; for this reason we are performing high-resolution and time-resolved spectroscopy experiments on these crystals. With the new experimental data, we are able to understand the optical and energy-transfer processes in Nd3+- and Cr 3+-doped YA103 crystals. In this pa-

0009-2614/94/% 07.00 0 1994 Elsevier Science B.V. All rights reserved. SSDI OOOOS-2614(93)E1362-K

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per, we report the spectroscopic ergy-transfer processes obtained and time-resolved spectroscopies YA103 : Nd, Cr crystals.

CHEMICAL PHYSICS LETTERS

properties and enby high-resolution of YAIOs : Nd and

2. Experimental All the measured crystals were prepared by Preciosa a.s., Division Monokrystaly, Palacktho 175, Tumov, Czech Republic. The measurements were carried out on several samples containing different amounts of Nd and Cr. Our detailed studies have shown [ 15 ] that all crystals including nominally undoped ones contain traces of Nd, Cr, Ce, etc. (around or below 0.02 at%). Singly Cr’+-doped crystals contain x0.25 at% Cr and doubly Cr3+-, Nd3+-doped crystals contain z 0.5-1.0 at% Nd and ~0.03-0.2 at% Cr. Fluorescence of Cr3+ and Nd3+ impurity ions in YA103 crystals was excited by a Lumonics dye laser pumped by a XeCl Lumonics excimer laser. The resolution of the dye laser was equal to 0.1 cm-‘. The emission spectra were recorded with a Jobin-Yvon monochromator (dispersion 0.8 nm/cm) equipped with an AsGa photomultiplier. The fluorescence decay curves were stored with a Canberra multichannel analyzer.

3. Results of measurements Nd3+ excitation spectra of one line from the 4I9,2 +4G5,2 transition for four selective emission wavelengths (4F3,2(a)-+41g,2(1) ) are presented in fig. 1. Up to five excitation peaks are observed for &,=875.85 nm (fig. lc) but the peak position of the main Nd3+ site (Nd3+ instead of Y3+ in a dodecahedra1 site) is located at i1,,=589.6 nm. The other peak positions should be due to Nd3+ multisites. The peak positions from different excitation spectra are not the same and generally up to eight peaks can be resolved. We note that Nd3+ multisites in YAlO, arise even if the content Nd is low ( x 0.02 at%); since the number of such sites would normally be small if we assume a random distribution of ions, this indicates that Nd3+ ions are preferentially close 106

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either to other impurity ions (e.g. to Cr3+) or are close to lattice imperfections. Selectively excited Cr3+ emission spectra in a YAlO, crystal are presented in fig. 2. These spectra were excited into different wavelengths of the heterogeneously broadened Cr3+ Rz line (4A,-+ *E transition). The excitation wavelengths and emission peak positions are summarized in table 1. From table 1 and from the Cr3+ emission spectra, we can determine that the peak positions of the main Cr3+ site 724.04nm (R, line) andApx721.5 nm areatAfl= ( R2 line). The other Cr3+ sites are on either side of the main Cr’+ site and their peak positions for Rr linesare:A$” (R1)x723.9nmand1$‘) (R,)s724.3 nm. These results confirm our previous findings [ 91 where in the Cr3+ emission spectra at 4.2 K under A,= 532 nm, one Cr3+ main site and two additional Cr3+ multisites are observed. Detailed experiments of Cr3++Nd3+ energy transfer in the YA103 : Cr, (Nd) crystal are presented in fig. 3. Several Nd3+ emission wavelengths were monitored and the excitation wavelength was scanned around the Rr and R2 Cr3+ lines at low temperature. The major Cr 3+ site was clearly monitored (see fig. 3b) and its peak positions are ng = 721.5 nm (Rq line) and Jr” ~723.96 nm (R, line). By examining the other Cr3+ excitation lines we can resolve two additional Cr3+ multisites for Ai ’ ) = 723.8 nm and AI*’ = 724.3 nm for the R, line. Another important aspect of rig. 3 is that it shows evidence for Cr3+ +Nd3+ energy transfer between Cr3+ and Nd3+ multisites. This transfer is observed for all Cr3+ multisites to all Nd3+ multisites (see the Nd3+ emission wavelengths). Cr3+ and Nd3+ fluorescence decays were excited not only at 532 nm (the second harmonic of the YAG: Nd laser) or at 570 nm into the broad 4A2+4T2 Cr3+ absorption band but also when pumped directly into the Cr3+ ‘E energy level (the 4A2-+2E transition). The results of Cr3+ and Nd3+ fluorescence decay studies are presented in figs. 4 and 5. In figs. 4a and 5b there is evidence of Cr3++Nd3+ energy transfer for the samples with Nd traces and with Nd doping ions. There is also energy transfer between Cr3+ multisites (see figs. 4b and 5a). Cr3+ fluorescence decay consists of two components (fast and slow) for Cr3+-+Cr3+ transfer between multisites. In a future paper, we will show that the fast ini-

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Fig. 1. Selectively excited Nd’+ emission spectrum and Nd3+ excitation spectra of a YA103: Cr (Nd traces) crystal at T= 10 K. (a) Emission spectrum forl,=577.65 nm; (b)-(e) excitation spectra for (b) R.,=875.7 nm, (c) &,=875.85 nm, (d) 1,=875.947 nm and (e) A.,,,= 876.05; arrows in part (a) indicate the emission wavelengths.

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Fig. 2. C?+ A,,=721.42

selectively excited emission spectra (in the spectral range of Rr line) of a YAlO,:Cr nm, (b) ;1,.=721.52 nm, (c) ,X.,=721.6 nm and (d) 1,=721.68 nm.

Table 1 Excitation wavelengths and Cr3+ emission peaks in a YAlOs: Cr (Nd traces) crystal at T= 6 K (according to fig. 2) A, (nm) ( R2 line)

A,, (nm)

721.42 721.52 721.60 721.68

723.88 724.03 723.90 723.94

(R, line)

724.07 724.14 724.20 724.08

724.32 724.38 724.80 724.31

tial Cr3+ decay cannot be completely attributed to chromium-neodymium transfer. Fig. 5b presents evidence for Cr3++Nd3+ energy transfer, the Nd3+ fluorescence decay consists of two components (i) the Nd3+ decay with lifetimes TZ 0.19 ms and (ii) the long nonexponential part with lifetime r>r(Cr3) s 54 ms. The fast Nd3+ decay is from Nd3+ ions which have received energy from Cr3+ ions quickly (within a few microseconds). The slow decay is evidence that Cr3++Nd3+ energy transfer is from the *E Cr’+ level to some of the Nd’+ energy levels. 108

(Nd traces)

crystal at Tz 6 K for (a)

4. Discussion YA103 has a perovskite-like orthorhombic structure with Y3+ occupying a polyhedral site (YOrz) while A13+ ions are at sites with a nearly cubic structure (AlO6 octahedron) [ 161. An important feature of the YA103 structure is its compactness and a tendency to create pairs (both Nd3+ and Cr3+ [ 13,141). Lupei et al. [ 13 ] have characterized the Nd3+ pairs by the pair of numbers (ZV,Ri) where N is the number of Y3+ positions at a distance Ri from a given site at the origin. A similar calculation was also carried out by Stadler and Bass [ 141 so we can characterize Y3+ positions by (2, 3.642 A), (2, 3.731 A), (2, 3.792 A), etc. The same type of analysis should be valid for Cr3+ ions but these calculations were not carried out. These sites and distances between Cr3+ or Nd3+ sites in the YA103 crystal show that there is the possibility of different types of pairs and multisites with various crystal fields at each of the individual multisites. Our spectroscopic studies have shown that in YA103:Nd or YA103: Cr crystals are probably eight

Volume 2 17, number

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Fig. 3. (a) Nd’+ emission spectrum and (b)-(g) Nd’+ excitation spectra of a YA103:Cr spectrum excited with &=724.32 nm and (b)-(g) excitation spectra for (b) A.,,,--875.65 (e) &=875.85 nm, (f) &=875.95 nm and (g) 1,,=876.05 nm.

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Fig. 4. Fluorescence decays of a YAIOp:Cr (Nd traces) crystal at Tx6 K. (a) Nd3+ decay for &,,,=875.65 nm excited with ~,,=721.52nm(R,line); (b)Cr3+decayfor1,=724.017nm (R, line) excited with 1.,=721.46 nm.

Nd3+ nonequivalent centres (multisites) and three Cr3+ nonequivalent centres (multisites) together with Cr3+ pairs [9,14]. Both Nd’+ and Cr3+ multisites are observed even for low concentrations (traces of Nd or Cr). The arising of Cr3+, Nd3+ or Cr3+-Nd3+ pairs of multisites is caused due to variously distant ions at the sites characterized by (ZV, Rj).Due to the shorter distances between ions in the YA103 crystal in comparison with the Y3A150r2 one, multisites arising due to cation inversion are not as significant as in Y3A150i2. Relatively short distances between ions result in efficient dipole-dipole interaction in this crystal [ 13 1. It was observed that Cr3++Nd3+ energy transfer is from the ‘E Cr3+ energy level and that this transfer is observed between almost all Cr3+ and Nd3+ multisites. If we assume that the energy transfer is due to a dipole-dipole mechanism this is evidence that distances between Nd3+ and Cr3+ ions are short. 110

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tIms1

Fig. 5. Fluorescence decays of YAlO,: Cr (Nd traces) and YAIOB:Nd, Cr crystals at Ts6 K. (a) C?+ decay for 1,=724.398 nm (R, line) excited with 1,=721.6 nm (Rr line) in a YAlOs:Cr (Nd traces) crystal; (b) Nd”+ decay for ,I.,=8758 nm excited with 1,=721.5 nm (R, line) in a YAIOp:Nd, Cr crystal.

Fluorescence decay studies have shown that there is also energy transfer between Cr3+ ions (see figs. 4b and 5a). However, it appears likely that there are anomalous short-range interactions that occur in pairs. New modes for the treatment of fluorescence decays due to such anomalous energy transfer has been developed [ 17 1. Moreover, in YA103, we have found that CT’+Nd3+ pairs arise from the inhomogeneous distribution of Cr3+ and Nd3+ ions; in particular the probability of the close-colocation of Cr3+ donors and Nd3+ acceptors is enhanced beyond that for a random distribution. When combined with the aforementioned fast energy transfer, the temporal behavior of excited-state concentrations of Nd3+ and Cr3+ is complicated. The exact calculations of the decays are in progress now together with an analysis of mul-

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tisites in YAIOS:Cr, Nd. Careful spectroscopy has allowed us to identify specific sites and study the temporal response of both the excited Cr3” donors and Nd3+ acceptors. The conclusions we have reached are similar to those which were arrived at for YjA150i2:Cr, Tm, Ho [18]: (1) The Cr 3+ donor decay can be adequately fitted with the standard Inokuti-Hirayama theory. (2) The specific acceptor (some Nd3+ sites) decay we studied does not fit the standard model. This is true despite changing all possible parameters in the model (r$ and R,,). Evidently, the Nd’+ acceptor does not receive its energy equally from all donors. (3) An assumption of a spatial correlation between the Cr3+ donors and this Nd3+ acceptor does a better job fitting the acceptor. This seems to be characteristic of multisites in crystals, and an important direction for research in the future.

References [l] M.J. Weber, J. Appl. Phys. 44 (1973) 4058. [2] M.J. Weber and T.E. Varitimos, J. Appl. Phys. 42 (1973) 4996.

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[ 3 ] A.A. Kaminski, Lazemye kristaly (Izd. Nauka, Moscow, 1975). [4] J.A. MareS and J. Kvapil, Desk. Gas. Fyz. A 38 (1988) 248 (in Czech). [ 5 ] J. Kvapil, B. Pemer, Jos. Kvapil, J. Kubelka, K. Hamal and M. KoSelja, Czech. J. Phys. B 36 (1986) 751. [ 61 W. Nie, G. Boulon and J. Mares, Chem. Phys. Letters 160 (1989) 597. [ 71 W. Nie, A. Monteil and G. Boulon, Opt. Quantum Electron. 22 (1990) S227. [8] W. Nie, G. Boulon and A. Monteil, J. Phys. France 50 (1989) 3309. [9] J.A. Mares, W. Nie and G. Boulon, J. Phys. France 51 (1990) 1655. [ lo] J.A. Mare;, 2. Khas, W. Nie and G. Boulon, J. Phys. I 1 (1991) 881. [ 111 V. Lupei, L. Lou, G. Boulon and A. Lupei, J. Phys. Condens. Matter 5 (1993) L35. [ 121P.R. Nelson, A. Brennier, C. Pedrini, G. Boulon and J.A. Mares, J. Physique IV, Colloque C7, Suppl. J. Physique III, 1 (1991) c7-375. [ 131 A. Lupei, V. Lupei and S. Georgescu, J. Phys. Condens. Matter 4 (1992) L221. [ 141 J.P. van der Ziel, J. Chem. Phys. 57 (1972) 2442. [ 151J.A. Mares, M. Nikl, J. Chval and G. Boulon, Czech J. Phys. B 43 (1993) 683. [ 161 R. Diehl and G. Brandt, Mater. Res. Bull. 10 (1975) 85. [ 171 S.R. Rotman and F.X. Hartmann, Chem. Phys. Letters 152 (1988) 311. [ 181 S.R. Rotman, J. Appl. Phys. 72 (1992) 224.

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