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Fast energy transfer from donor pairs to single acceptors in a Cr3+, Nd3+-doped YAlO3 crystal
Optical Materials 13 (1999) 63±65
Fast energy transfer from donor pairs to single acceptors in a Cr3, Nd3-doped YAlO3 crystal S.R. Rotman
a,*
, E. Luria a, E. Herman a, A. Shohan a, J.A. Mares b, G. Boulon c, A. Brenier c, L. Lou c
a b
Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev, 84105 Beer-Sheva, Israel Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, Prague 6, 16253 Czech Republic c Laboratoire de Physico-Chimie des Materiaux Luminescents, Universite Lyon I, Villeurbanne, France
Abstract Energy transfer between chromium ion Cr3 pairs and single Nd3 neodymium ions in YAP is reported. Evidence of clusters of two chromium Cr3 ions and a neodymium Nd3 ion physically colocating next to each other in the crystal is presented. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Nd3 -doped yttrium aluminum perovskite (YAlO3 or YAP) is one of the most important solid-state laser materials [1±3]. One can add Cr3 to this crystal to produce an ecient donor±acceptor combination in which the chromium absorbs the pump radiation and the neodymium emits at the desired wavelength [3±7] in the near IR (1.06 lm). In this way, more ecient laser materials will be possible due to energy transfer processes. Recently, we have studied and evaluated the non-radiative energy transfer between Cr3 and Nd3 in various crystals [4,7±11]. By performing low-temperature site selection spectroscopy, we were able to selectively excite luminescent impurity ions which exist in sites which are subtly dierent from the main site in which such ions occur; these * Corresponding author. Fax: 011 972 7 6472949; e-mail: [email protected].
sites are called multisites. The energy transfer properties of such sites are quite dierent than the main site, often exhibiting a very fast initial transfer between donors and acceptors. We have modeled this type of transfer in several ways, as has been reviewed in Ref. [9]. Beside the pairing that may occur between Cr3 donors and Nd3 acceptors, Cr3 ions themselves often enter oxide crystals in pairs, colocating next to one another. This can have a major eect on the spectroscopic properties of the Cr3 ions; in YAP at 4 K, the single-ion chromium R-line emissions occur at 723 and 725 nm, while the emission from the N-lines from Cr3 pairs occurs at 731 and 732 nm [4]. This is an enormous gap, which enables us to study the energy transfer properties of this center to Nd3 ions. 2. Experimental procedure The measured crystals were prepared by the company Preciosa, Crytur, Palackeho 175, 51119
0925-3467/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 9 ) 0 0 0 1 2 - 9
64
S.R. Rotman et al. / Optical Materials 13 (1999) 63±65
Turnov, Czech Republic. The singly doped crystal used for this study contained approximately 1.1% at.% Cr3 (referred to as Cr:YAP); the doubly doped crystal had approximately 0.3 at.% Nd3 and 1.1 at.% Cr3 (Cr,Nd:YAP). The crystals were excited at 4 K by a Lumonics dye laser pumped by a XeCl Lumonics excimer laser at approximately 570 nm. The emission spectra were recorded with a Jobin±Yvon monochromator (dispersion 0.8 nm/cm) equipped with an AsGA photomultiplier. The ¯uorescence decay curves were stored with a Canberra multichannel analyzer. 3. Experimental results Fig. 1 shows a sample 732 nm emission decay curve obtained from the singly-doped Cr3 crystal which has been excited at 570 nm. This curve was ®t by two exponential components. At short times (the ®rst 3 ms), the emission decays with a lifetime of approximately 1.1 ms. At later times (from 10 ms and onward), the lifetime is approximately 42.6 ms. If we compare these results to those of the regular R-line emission of a Cr3 -doped YAG crystal, we ®nd that the lifetime of 42.6 ms in our crystal is similar to that of the R-line emission (of
Fig. 2. Decay of the emission of Cr3 ions in Nd, Cr-doped YAP at 732 nm (the N-lines).
around 50 ms). However, the 1.1 ms lifetime is unique for the N-line emission. Fig. 2 gives a similar Cr3 732 nm emission decay curve when the Cr,Nd:YAP crystal is excited at 570 nm. Once again the emission exhibits two ¯uorescence lifetimes: at short times, the lifetime is approximately 0.47 ms; at longer times, the lifetime is 13.5 ms. 4. Discussion
Fig. 1. Decay of the emission of Cr3 ions in Cr:YAP at 732 nm (the N-lines).
The results from the N-lines of the Cr:YAP crystal in Fig. 1 are quite dramatic. The longer lifetime appears to represent the time decay of the single Cr3 ions. Indeed, Ref. [4] shows that there is emission in that area from the vibronic sidebands of the R-lines overlapping the N-lines. On the other hand, the 1.1 ms emission from the Nlines is dramatically dierent from that of the Rlines. The Cr3 pairing has a major eect on the emission lifetime just as it did on the emission wavelength [4,5]. The results from the N-lines of the Nd,Cr:YAP crystals in Fig. 2 are also indicative of the eects of pairing in the crystal. We interpret the 13.5 ms emission to be from the sidebands of the R-lines;
S.R. Rotman et al. / Optical Materials 13 (1999) 63±65
the 0.47 ns emission is from the N-lines. Both decay curves are exponential, as seen by the straight lines on the semi-log plot of emission vs. time. This indicates that the standard Inokuti±Hirayama model [9,12±14] does not apply. This model assumes that the Nd3 and Cr3 centers are randomly distributed throughout the crystal; since closer donor±acceptor pairs will transfer faster than ones further apart, we would expect a nonexponential decay representing the sum of the dierent dierence between the pairs. In our measurement, this has not occurred. Both the Nline and the R-line emissions decay exponentially, faster than in the singly-doped crystal. We propose that this indicates that the Nd3 ions are pairing both with the isolated Cr3 ions and with the Cr3 pairs. If the distance between the Nd3 ion and the Cr3 pairs were ®xed (and not randomly distributed), then the transfer rate to the Nd3 ions would be a constant for all Nd3 ±Cr3 interactions, and an exponential decay would ensue. The same is true of such pairing between Nd3 and single Cr3 ions. While evidence of such pairing has been reported for the single Cr3 and Nd3 ions, this is the ®rst such report for the chromium pairs. We note that the reduction of the emission lifetime of the chromium pair went from 1.1 to 0.47 ms, a factor of a little more than 2.3. The single chromium ion went from 42.7 to 13.5 ms, a factor of 3. The rate of the decay n(r) for dipole± dipole interaction is equal to [9] 6
n r 1=Td r0 =r ; where Td is the time constant of the donor without the center, r is the distance between the donor and the center, and r0 is the critical distance. The critical distance represents the degree of interaction between the donor and the acceptor; if it equals r, then the donor is equally likely to transfer its energy to an acceptor as it is to spontaneously emit. If we consider a donor which can either emit spontaneously (with rate 1/TD ) or can transfer to an acceptor at a distance r1 away (with a rate (1/TD ) (r0 /r1 )6 ), then the measured rate of deacti-
65
vation will be (1/TD ) (1 + (r0 /r1 )6 ). Based on the measured data for the chromium pair and the single chromium ions, (r0 /r1 ) is approximately equal to 1.05 for energy transfer between chromium pairs and neodymium ions; (r0 /r1 ) is equal to 1.13 for energy transfer between single chromium ions and neodymium. The single Cr3 ions actually have a larger value of r0 /r1 (with a stronger interaction) than the paired Cr3 ions in this crystal.
5. Conclusions The N-line emission of Cr3 in Cr, Nd-doped yttrium aluminum perovskite has been analyzed. The pairing of the Cr3 ions to each other greatly aects the spectral and temporal behavior of the Cr3 emission. The addition of the Nd3 ions seems to cause the pairing of the Cr3 centers with the Nd3 ions, with a consequential enhanced exponential Cr3 decay.
References [1] M.J. Weber, J. Appl. Phys. 44 (1973) 4058. [2] M.J. Weber, T.E. Varitimos, J. Appl. Phys. 42 (1973) 4996. [3] J. Kvapil, B. Perner, J. Kvapil, J. Kubelka, K. Hamal, M. Koselja, Czech J. Phys. B 36 (1986) 751. [4] J.A. Mares, W. Nie, G. Boulon, J. Phys. France 51 (1990) 1655. [5] P.R. Nelson, A. Brenier, C. Pedrini, G. Boulon, J.A. Mares, J. Physique IV, Colloque C7, Suppl. J. Physique III 1 (1991) C7±375. [6] A. Lupei, V. Lupei, S. Georgescu, J. Phys. Cond. Mater. 4 (1992) L221. [7] J.A. Mares, G. Boulon, A. Brenier, L. Lou, S.R. Rotman, Chem. Phys. Lett. 217 (1,2) (1994) 105. [8] E. Luria, S.R. Rotman, J.A. Mares, G. Boulon, A. Brenier, L. Lou, J. Lum. 72±74 (1997) 951. [9] S.R. Rotman, E. Luria, N. Yitzhaki, A. Eyal, Opt. Mater. 5 (1996) 1. [10] S.R. Rotman, Opt. Mater. 8 (1997) 11. [11] S.R. Rotman, E. Luria, J.A. Mares, G. Boulon, A. Brenier, L. Lou, J. Appl. Phys. 80 (2) (1996) 1094. [12] T. Foester, Ann. Physik. 2 (1948) 55. [13] D.L. Dexter, J. Chem. Phys. 21 (1953) 836. [14] M. Inokuti, F. Hirayama, J. Chem. Phys. 43 (1953) 1978.
Fast energy transfer from donor pairs to single acceptors in a Cr3, Nd3-doped YAlO3 crystal S.R. Rotman
a,*
, E. Luria a, E. Herman a, A. Shohan a, J.A. Mares b, G. Boulon c, A. Brenier c, L. Lou c
a b
Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev, 84105 Beer-Sheva, Israel Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, Prague 6, 16253 Czech Republic c Laboratoire de Physico-Chimie des Materiaux Luminescents, Universite Lyon I, Villeurbanne, France
Abstract Energy transfer between chromium ion Cr3 pairs and single Nd3 neodymium ions in YAP is reported. Evidence of clusters of two chromium Cr3 ions and a neodymium Nd3 ion physically colocating next to each other in the crystal is presented. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Nd3 -doped yttrium aluminum perovskite (YAlO3 or YAP) is one of the most important solid-state laser materials [1±3]. One can add Cr3 to this crystal to produce an ecient donor±acceptor combination in which the chromium absorbs the pump radiation and the neodymium emits at the desired wavelength [3±7] in the near IR (1.06 lm). In this way, more ecient laser materials will be possible due to energy transfer processes. Recently, we have studied and evaluated the non-radiative energy transfer between Cr3 and Nd3 in various crystals [4,7±11]. By performing low-temperature site selection spectroscopy, we were able to selectively excite luminescent impurity ions which exist in sites which are subtly dierent from the main site in which such ions occur; these * Corresponding author. Fax: 011 972 7 6472949; e-mail: [email protected].
sites are called multisites. The energy transfer properties of such sites are quite dierent than the main site, often exhibiting a very fast initial transfer between donors and acceptors. We have modeled this type of transfer in several ways, as has been reviewed in Ref. [9]. Beside the pairing that may occur between Cr3 donors and Nd3 acceptors, Cr3 ions themselves often enter oxide crystals in pairs, colocating next to one another. This can have a major eect on the spectroscopic properties of the Cr3 ions; in YAP at 4 K, the single-ion chromium R-line emissions occur at 723 and 725 nm, while the emission from the N-lines from Cr3 pairs occurs at 731 and 732 nm [4]. This is an enormous gap, which enables us to study the energy transfer properties of this center to Nd3 ions. 2. Experimental procedure The measured crystals were prepared by the company Preciosa, Crytur, Palackeho 175, 51119
0925-3467/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 9 ) 0 0 0 1 2 - 9
64
S.R. Rotman et al. / Optical Materials 13 (1999) 63±65
Turnov, Czech Republic. The singly doped crystal used for this study contained approximately 1.1% at.% Cr3 (referred to as Cr:YAP); the doubly doped crystal had approximately 0.3 at.% Nd3 and 1.1 at.% Cr3 (Cr,Nd:YAP). The crystals were excited at 4 K by a Lumonics dye laser pumped by a XeCl Lumonics excimer laser at approximately 570 nm. The emission spectra were recorded with a Jobin±Yvon monochromator (dispersion 0.8 nm/cm) equipped with an AsGA photomultiplier. The ¯uorescence decay curves were stored with a Canberra multichannel analyzer. 3. Experimental results Fig. 1 shows a sample 732 nm emission decay curve obtained from the singly-doped Cr3 crystal which has been excited at 570 nm. This curve was ®t by two exponential components. At short times (the ®rst 3 ms), the emission decays with a lifetime of approximately 1.1 ms. At later times (from 10 ms and onward), the lifetime is approximately 42.6 ms. If we compare these results to those of the regular R-line emission of a Cr3 -doped YAG crystal, we ®nd that the lifetime of 42.6 ms in our crystal is similar to that of the R-line emission (of
Fig. 2. Decay of the emission of Cr3 ions in Nd, Cr-doped YAP at 732 nm (the N-lines).
around 50 ms). However, the 1.1 ms lifetime is unique for the N-line emission. Fig. 2 gives a similar Cr3 732 nm emission decay curve when the Cr,Nd:YAP crystal is excited at 570 nm. Once again the emission exhibits two ¯uorescence lifetimes: at short times, the lifetime is approximately 0.47 ms; at longer times, the lifetime is 13.5 ms. 4. Discussion
Fig. 1. Decay of the emission of Cr3 ions in Cr:YAP at 732 nm (the N-lines).
The results from the N-lines of the Cr:YAP crystal in Fig. 1 are quite dramatic. The longer lifetime appears to represent the time decay of the single Cr3 ions. Indeed, Ref. [4] shows that there is emission in that area from the vibronic sidebands of the R-lines overlapping the N-lines. On the other hand, the 1.1 ms emission from the Nlines is dramatically dierent from that of the Rlines. The Cr3 pairing has a major eect on the emission lifetime just as it did on the emission wavelength [4,5]. The results from the N-lines of the Nd,Cr:YAP crystals in Fig. 2 are also indicative of the eects of pairing in the crystal. We interpret the 13.5 ms emission to be from the sidebands of the R-lines;
S.R. Rotman et al. / Optical Materials 13 (1999) 63±65
the 0.47 ns emission is from the N-lines. Both decay curves are exponential, as seen by the straight lines on the semi-log plot of emission vs. time. This indicates that the standard Inokuti±Hirayama model [9,12±14] does not apply. This model assumes that the Nd3 and Cr3 centers are randomly distributed throughout the crystal; since closer donor±acceptor pairs will transfer faster than ones further apart, we would expect a nonexponential decay representing the sum of the dierent dierence between the pairs. In our measurement, this has not occurred. Both the Nline and the R-line emissions decay exponentially, faster than in the singly-doped crystal. We propose that this indicates that the Nd3 ions are pairing both with the isolated Cr3 ions and with the Cr3 pairs. If the distance between the Nd3 ion and the Cr3 pairs were ®xed (and not randomly distributed), then the transfer rate to the Nd3 ions would be a constant for all Nd3 ±Cr3 interactions, and an exponential decay would ensue. The same is true of such pairing between Nd3 and single Cr3 ions. While evidence of such pairing has been reported for the single Cr3 and Nd3 ions, this is the ®rst such report for the chromium pairs. We note that the reduction of the emission lifetime of the chromium pair went from 1.1 to 0.47 ms, a factor of a little more than 2.3. The single chromium ion went from 42.7 to 13.5 ms, a factor of 3. The rate of the decay n(r) for dipole± dipole interaction is equal to [9] 6
n r 1=Td r0 =r ; where Td is the time constant of the donor without the center, r is the distance between the donor and the center, and r0 is the critical distance. The critical distance represents the degree of interaction between the donor and the acceptor; if it equals r, then the donor is equally likely to transfer its energy to an acceptor as it is to spontaneously emit. If we consider a donor which can either emit spontaneously (with rate 1/TD ) or can transfer to an acceptor at a distance r1 away (with a rate (1/TD ) (r0 /r1 )6 ), then the measured rate of deacti-
65
vation will be (1/TD ) (1 + (r0 /r1 )6 ). Based on the measured data for the chromium pair and the single chromium ions, (r0 /r1 ) is approximately equal to 1.05 for energy transfer between chromium pairs and neodymium ions; (r0 /r1 ) is equal to 1.13 for energy transfer between single chromium ions and neodymium. The single Cr3 ions actually have a larger value of r0 /r1 (with a stronger interaction) than the paired Cr3 ions in this crystal.
5. Conclusions The N-line emission of Cr3 in Cr, Nd-doped yttrium aluminum perovskite has been analyzed. The pairing of the Cr3 ions to each other greatly aects the spectral and temporal behavior of the Cr3 emission. The addition of the Nd3 ions seems to cause the pairing of the Cr3 centers with the Nd3 ions, with a consequential enhanced exponential Cr3 decay.
References [1] M.J. Weber, J. Appl. Phys. 44 (1973) 4058. [2] M.J. Weber, T.E. Varitimos, J. Appl. Phys. 42 (1973) 4996. [3] J. Kvapil, B. Perner, J. Kvapil, J. Kubelka, K. Hamal, M. Koselja, Czech J. Phys. B 36 (1986) 751. [4] J.A. Mares, W. Nie, G. Boulon, J. Phys. France 51 (1990) 1655. [5] P.R. Nelson, A. Brenier, C. Pedrini, G. Boulon, J.A. Mares, J. Physique IV, Colloque C7, Suppl. J. Physique III 1 (1991) C7±375. [6] A. Lupei, V. Lupei, S. Georgescu, J. Phys. Cond. Mater. 4 (1992) L221. [7] J.A. Mares, G. Boulon, A. Brenier, L. Lou, S.R. Rotman, Chem. Phys. Lett. 217 (1,2) (1994) 105. [8] E. Luria, S.R. Rotman, J.A. Mares, G. Boulon, A. Brenier, L. Lou, J. Lum. 72±74 (1997) 951. [9] S.R. Rotman, E. Luria, N. Yitzhaki, A. Eyal, Opt. Mater. 5 (1996) 1. [10] S.R. Rotman, Opt. Mater. 8 (1997) 11. [11] S.R. Rotman, E. Luria, J.A. Mares, G. Boulon, A. Brenier, L. Lou, J. Appl. Phys. 80 (2) (1996) 1094. [12] T. Foester, Ann. Physik. 2 (1948) 55. [13] D.L. Dexter, J. Chem. Phys. 21 (1953) 836. [14] M. Inokuti, F. Hirayama, J. Chem. Phys. 43 (1953) 1978.