- Email: [email protected]
Cooperative luminescence in Yb3+ : LiNbO3
Journal of Luminescence 87}89 (2000) 1036}1038
Cooperative luminescence in Yb> : LiNbO E. Montoya*, O. Espeso, L.E. BausaH Departamento Fn& sica de Materiales, Universidad Auto& noma de Madrid, Cantoblanco, 28049 Madrid, Spain
Abstract Up-converted green luminescence is observed in Yb> : LiNbO under excitation in the near-infrared region. On the basis of the emission and excitation spectra, decay time measurements, and excitation intensity dependence it is concluded that this luminescence is due to a cooperative process consisting on the simultaneous emission of a visible photon by a pair of interacting Yb> ions. The process points out the presence of Yb> pairs within the LiNbO matrix which is also evidenced from the dependence of the cooperative emission on the Yb concentration in the sample. 2000 Elsevier Science B.V. All rights reserved. Keywords: Cooperative luminescence; Yb>; LiNbO
1. Introduction LiNbO is one of the most deeply studied materials in optoelectronics. It has excellent electro-optic, acoustooptic and nonlinear coe$cients, and it has been used in active and passive waveguides and self-frequency doubling lasers [1]. Towards optically active devices, it has been extensively doped with trivalent rare earth and transition metal ions [2]. One of these impurities, Yb>, is receiving an increasing attention because its laser properties challenge those of Nd> activated lasers. Indeed, Yb> : LiNbO has been spectroscopically characterized [3,4] and recently, infrared laser action as well as e$cient self-frequency doubling have been demonstrated by the authors in Yb> : LiNbO : MgO single crystals [5]. Yb> has a 4f con"guration and hence its level scheme is particularly simple consisting of a F ground and of a F excited state manifolds separ ated by approximately 10 cm\. The Yb> ion #uoresces e$ciently since non-radiate relaxation over large inter-manifold gap is negligible in most instances and the quantum defect (pump-emission di!erence) is small. In addition, the 4f electrons in this system are somewhat less shielded than in other ions of the series and thus, they show a higher tendency to interact with the lattice and
* Corresponding author. Fax: 34-913978579. E-mail address: [email protected] (E. Montoya)
with neighboring ions } a requisite for cooperative interactions. Thus, in the case of relatively concentrated crystals inter-ionic coupling can lead to transitions in which ions combine simultaneously to produce cooperative absorptive or emissive transitions. Much of the interest in this phenomena has been centered on the up-conversion of near-infrared pump radiation to visible light, particularly since the advent of powerful near-infrared semiconductor laser arrays. Indeed, up-converted cooperative luminescence in Yb> was reported in YPO as early as 1970 [6]. Under near-infrared excitation, we have observed that ytterbium-doped LiNbO exhibits a relatively strong green luminescence due to a cooperative e!ect. The analysis of the spectroscopic results lead to conclude that this phenomenon corresponds to the simultaneous de-excitation of two interacting ytterbium ions resulting in the emission of a visible photon.
2. Experimental Yb> : LiNbO crystals with di!erent Yb concentra tion were grown in our laboratory by the Czochralski pulling method. one millimeter-thick plates were cut and polished to optical quality perpendicular to the c-axis. Emission spectra were taken under excitation with a Ti : sapphire laser (Spectra Physics 3900). A Si photodiode, whose spectral response was corrected, was used
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 5 3 1 - 1
E. Montoya et al. / Journal of Luminescence 87}89 (2000) 1036}1038
for the detection. Decay times measurements were obtained by mechanically chopping the laser beam at a rate which ensured that the decay times were not a!ected.
3. Results and discussion Fig. 1(a) shows the low-temperature near-infrared emission spectrum of Yb> (4 at%.) in LiNbO under excitation at 920 nm. It consists of four main bands located at 981, 1006, 1030, and 1060 nm, which are associated with the transitions from the lowest Stark energy level of the excited F state (0) to each of the four Stark levels of the F fundamental state (0,1,2,3, respec tively). Simultaneously, a green emission clearly visible with the naked eye was observed. The corresponding spectrum, shown in Fig. 1(b), is depicted in an energy range which is twice the energy of emission associated to single Yb> ions and. As it can be clearly observed, some of the peaks exactly lie at double the energy of the infrared emission peaks. In fact, it is composed of various combinations of these transitions. The low-temperature excitation spectra monitoring both the green emission and the near-infrared one were practically similar and essentially consisting of the three bands corresponding to the transitions from the fundamental Stark level of the F state to the three Stark levels of the F , in good agreement with the Yb> absorption spectrum. Thus, the
Fig. 1. Emission spectra of Yb>(4%) : LiNbO at 12 K. (a) Near infrared emission spectrum associated with the F PF transition of Yb> ion; (b) continuous line: Cooperative luminescence spectrum, dotted line: self-convolution of the near-infrared emission spectrum.
1037
spectrum in Fig. 1(b) can be, in principle, associated with a cooperative emission corresponding to the simultaneous annihilation of two excited states of two Yb> interacting ions resulting in the emission of a single photon with an energy equal to the sum of the energies of the excited ions. The cooperative emission spectrum can be simulated by the self-convolution of the infrared emission spectrum:
I (E)" I (E )I (E!E ) dE . !* '0 '0 The numerically obtained green spectrum is shown in dotted trace in Fig. 1(b). As can be observed, though the cooperative emission spectrum is slightly broader, the number and positions of all peaks is correctly reproduced. The assignment of the cooperative transitions can be based on the single crystal "eld transitions as shown in Fig. 1(b). In terms of the double de-excitations they can be related to F (0,0) to F (m,n) transitions. Fig. 2 shows the decays curves of the intensities of the green and near-infrared emission for the same sample. The single-ion emission shows a simple exponential decay with a time constant of 400 ls. The cooperative emission decays with a time constant of 210 ls, which is nearly the half of that for the single-ion luminescence. The dynamics of two center cooperative luminescence is ruled by pairs of ions. Thus, since the concentration of pairs of excited ions is regarded as being proportional to the square of the dopant concentration, at su$cient high excitation levels, the pair process will dominate at early times of the decay yielding a shortening in around half the e!ective lifetime for the pair emission. Similar results have been obtained for YbPO [6]. Fig. 3 shows the dependence of the intensity of the cooperative emission on the excitation power at 920 nm. As expected, a nonlinear behavior of the visible lumines-
Fig. 2. Decay time curves at 12 K for single-ion luminescence (solid circles) and for cooperative luminescence (open circles).
1038
E. Montoya et al. / Journal of Luminescence 87}89 (2000) 1036}1038
Fig. 3. Dependence of the intensity of the cooperative emission at 12 K on the excitation power.
cence intensity on the near-infrared excitation is obtained. The experimental results can be well "tted to a quadratic function, particularly for the higher-power range. Cooperative luminescence provides a very useful way to evidence the amount of Yb> pairing or clustering in our host crystal by using the ytterbium ion as a probe. In fact, for this process to happen, a typical critical distance around 5 As has been reported [7]. To con"rm the Yb> pairing within the LiNbO matrix, the dependence of the cooperative emission on the Yb content has been studied. To eliminate undesired e!ects due to the experimental conditions the measurements were taken under the same excitation power and the visible emission was normalized by the corresponding infrared one after correction by the detector response. The ratio of the intensity of the visible emission to that of the near-infrared one was in the order of 10\, which would correspond to the ratio of the two oscillator strengths. The results obtained for the di!erent Yb> concentrations are shown in Fig. 4. As observed, a linear I /I ratio is obtained, indicating that the cooperative !* '0 emission is indeed, quadratic on the dopant concentration. Previous RBS/channeling studies on these samples [8] have shown that, to the resolution of the technique, Yb> ions locate at the Li> octahedral site. Therefore, one possibility is that the cooperative emission could be originated by two Yb> ions both located in neigboring Li> sites since the separation between them is 3.76 As . However, from the point of view of charge compensation this is not the most favorable situation, and other type of pair structure cannot be disregarded. Additional experiments are now on their way to investigate this point by
Fig. 4. Dependence of the I /I ratio on Yb concentration in !* '0 the samples.
studying the in#uence of MgO as a co-dopant on the cooperative luminescence.
4. Conclusion We have demonstrated the existence of cooperative luminescence in Yb> : LiNbO . The observation of this luminescence can be understood on the basis of the relatively strong interaction between the 4f electrons of neighboring Yb> ions in a covalent host lattice such as LiNbO . The short distances at which this process should occur evidence for the "rst time the presence of ion pairing in this important nonlinear host.
References [1] A. CoH rdova-Plaza, M. Digonnet, H.J. Shaw, IEEE J. Quantum Electron. 13 (1987) 262. [2] J. GarcmH a-SoleH , L.E. BausaH , D. Jaque, E. Montoya, H. Murrieta, F. Jaque, Spectrochim. Acta A 54 (1998) 1571. [3] E. Montoya, J.A. Sanz-GarcmH a, L.E. BausaH , Spectrochim. Acta A 54 (1998) 2081. [4] E. Montoya, A. Lorenzo, L.E. BausaH , J. Phys.: Condens. Matter 11 (1999) 311. [5] E. Montoya, J. Capmany, L.E. BausaH , T. Kellner, A. Diening, G. Huber, Appl. Phys. Lett. 74 (1999) 3113. [6] E. Nakazawa, S. Shionoya, Phys Rev. Lett. 25 (1970) 1710. [7] F. Auzel, D. Meichenin, F. PelleH , P. Goldner, Optical Materials 4 (1994) 35. [8] A. Lorenzo, H. Ja!rezic, G. Boulon, J. GarcmH a-SoleH , Appl. Phys. Lett 67 (1995) 1.
Cooperative luminescence in Yb> : LiNbO E. Montoya*, O. Espeso, L.E. BausaH Departamento Fn& sica de Materiales, Universidad Auto& noma de Madrid, Cantoblanco, 28049 Madrid, Spain
Abstract Up-converted green luminescence is observed in Yb> : LiNbO under excitation in the near-infrared region. On the basis of the emission and excitation spectra, decay time measurements, and excitation intensity dependence it is concluded that this luminescence is due to a cooperative process consisting on the simultaneous emission of a visible photon by a pair of interacting Yb> ions. The process points out the presence of Yb> pairs within the LiNbO matrix which is also evidenced from the dependence of the cooperative emission on the Yb concentration in the sample. 2000 Elsevier Science B.V. All rights reserved. Keywords: Cooperative luminescence; Yb>; LiNbO
1. Introduction LiNbO is one of the most deeply studied materials in optoelectronics. It has excellent electro-optic, acoustooptic and nonlinear coe$cients, and it has been used in active and passive waveguides and self-frequency doubling lasers [1]. Towards optically active devices, it has been extensively doped with trivalent rare earth and transition metal ions [2]. One of these impurities, Yb>, is receiving an increasing attention because its laser properties challenge those of Nd> activated lasers. Indeed, Yb> : LiNbO has been spectroscopically characterized [3,4] and recently, infrared laser action as well as e$cient self-frequency doubling have been demonstrated by the authors in Yb> : LiNbO : MgO single crystals [5]. Yb> has a 4f con"guration and hence its level scheme is particularly simple consisting of a F ground and of a F excited state manifolds separ ated by approximately 10 cm\. The Yb> ion #uoresces e$ciently since non-radiate relaxation over large inter-manifold gap is negligible in most instances and the quantum defect (pump-emission di!erence) is small. In addition, the 4f electrons in this system are somewhat less shielded than in other ions of the series and thus, they show a higher tendency to interact with the lattice and
* Corresponding author. Fax: 34-913978579. E-mail address: [email protected] (E. Montoya)
with neighboring ions } a requisite for cooperative interactions. Thus, in the case of relatively concentrated crystals inter-ionic coupling can lead to transitions in which ions combine simultaneously to produce cooperative absorptive or emissive transitions. Much of the interest in this phenomena has been centered on the up-conversion of near-infrared pump radiation to visible light, particularly since the advent of powerful near-infrared semiconductor laser arrays. Indeed, up-converted cooperative luminescence in Yb> was reported in YPO as early as 1970 [6]. Under near-infrared excitation, we have observed that ytterbium-doped LiNbO exhibits a relatively strong green luminescence due to a cooperative e!ect. The analysis of the spectroscopic results lead to conclude that this phenomenon corresponds to the simultaneous de-excitation of two interacting ytterbium ions resulting in the emission of a visible photon.
2. Experimental Yb> : LiNbO crystals with di!erent Yb concentra tion were grown in our laboratory by the Czochralski pulling method. one millimeter-thick plates were cut and polished to optical quality perpendicular to the c-axis. Emission spectra were taken under excitation with a Ti : sapphire laser (Spectra Physics 3900). A Si photodiode, whose spectral response was corrected, was used
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 5 3 1 - 1
E. Montoya et al. / Journal of Luminescence 87}89 (2000) 1036}1038
for the detection. Decay times measurements were obtained by mechanically chopping the laser beam at a rate which ensured that the decay times were not a!ected.
3. Results and discussion Fig. 1(a) shows the low-temperature near-infrared emission spectrum of Yb> (4 at%.) in LiNbO under excitation at 920 nm. It consists of four main bands located at 981, 1006, 1030, and 1060 nm, which are associated with the transitions from the lowest Stark energy level of the excited F state (0) to each of the four Stark levels of the F fundamental state (0,1,2,3, respec tively). Simultaneously, a green emission clearly visible with the naked eye was observed. The corresponding spectrum, shown in Fig. 1(b), is depicted in an energy range which is twice the energy of emission associated to single Yb> ions and. As it can be clearly observed, some of the peaks exactly lie at double the energy of the infrared emission peaks. In fact, it is composed of various combinations of these transitions. The low-temperature excitation spectra monitoring both the green emission and the near-infrared one were practically similar and essentially consisting of the three bands corresponding to the transitions from the fundamental Stark level of the F state to the three Stark levels of the F , in good agreement with the Yb> absorption spectrum. Thus, the
Fig. 1. Emission spectra of Yb>(4%) : LiNbO at 12 K. (a) Near infrared emission spectrum associated with the F PF transition of Yb> ion; (b) continuous line: Cooperative luminescence spectrum, dotted line: self-convolution of the near-infrared emission spectrum.
1037
spectrum in Fig. 1(b) can be, in principle, associated with a cooperative emission corresponding to the simultaneous annihilation of two excited states of two Yb> interacting ions resulting in the emission of a single photon with an energy equal to the sum of the energies of the excited ions. The cooperative emission spectrum can be simulated by the self-convolution of the infrared emission spectrum:
I (E)" I (E )I (E!E ) dE . !* '0 '0 The numerically obtained green spectrum is shown in dotted trace in Fig. 1(b). As can be observed, though the cooperative emission spectrum is slightly broader, the number and positions of all peaks is correctly reproduced. The assignment of the cooperative transitions can be based on the single crystal "eld transitions as shown in Fig. 1(b). In terms of the double de-excitations they can be related to F (0,0) to F (m,n) transitions. Fig. 2 shows the decays curves of the intensities of the green and near-infrared emission for the same sample. The single-ion emission shows a simple exponential decay with a time constant of 400 ls. The cooperative emission decays with a time constant of 210 ls, which is nearly the half of that for the single-ion luminescence. The dynamics of two center cooperative luminescence is ruled by pairs of ions. Thus, since the concentration of pairs of excited ions is regarded as being proportional to the square of the dopant concentration, at su$cient high excitation levels, the pair process will dominate at early times of the decay yielding a shortening in around half the e!ective lifetime for the pair emission. Similar results have been obtained for YbPO [6]. Fig. 3 shows the dependence of the intensity of the cooperative emission on the excitation power at 920 nm. As expected, a nonlinear behavior of the visible lumines-
Fig. 2. Decay time curves at 12 K for single-ion luminescence (solid circles) and for cooperative luminescence (open circles).
1038
E. Montoya et al. / Journal of Luminescence 87}89 (2000) 1036}1038
Fig. 3. Dependence of the intensity of the cooperative emission at 12 K on the excitation power.
cence intensity on the near-infrared excitation is obtained. The experimental results can be well "tted to a quadratic function, particularly for the higher-power range. Cooperative luminescence provides a very useful way to evidence the amount of Yb> pairing or clustering in our host crystal by using the ytterbium ion as a probe. In fact, for this process to happen, a typical critical distance around 5 As has been reported [7]. To con"rm the Yb> pairing within the LiNbO matrix, the dependence of the cooperative emission on the Yb content has been studied. To eliminate undesired e!ects due to the experimental conditions the measurements were taken under the same excitation power and the visible emission was normalized by the corresponding infrared one after correction by the detector response. The ratio of the intensity of the visible emission to that of the near-infrared one was in the order of 10\, which would correspond to the ratio of the two oscillator strengths. The results obtained for the di!erent Yb> concentrations are shown in Fig. 4. As observed, a linear I /I ratio is obtained, indicating that the cooperative !* '0 emission is indeed, quadratic on the dopant concentration. Previous RBS/channeling studies on these samples [8] have shown that, to the resolution of the technique, Yb> ions locate at the Li> octahedral site. Therefore, one possibility is that the cooperative emission could be originated by two Yb> ions both located in neigboring Li> sites since the separation between them is 3.76 As . However, from the point of view of charge compensation this is not the most favorable situation, and other type of pair structure cannot be disregarded. Additional experiments are now on their way to investigate this point by
Fig. 4. Dependence of the I /I ratio on Yb concentration in !* '0 the samples.
studying the in#uence of MgO as a co-dopant on the cooperative luminescence.
4. Conclusion We have demonstrated the existence of cooperative luminescence in Yb> : LiNbO . The observation of this luminescence can be understood on the basis of the relatively strong interaction between the 4f electrons of neighboring Yb> ions in a covalent host lattice such as LiNbO . The short distances at which this process should occur evidence for the "rst time the presence of ion pairing in this important nonlinear host.
References [1] A. CoH rdova-Plaza, M. Digonnet, H.J. Shaw, IEEE J. Quantum Electron. 13 (1987) 262. [2] J. GarcmH a-SoleH , L.E. BausaH , D. Jaque, E. Montoya, H. Murrieta, F. Jaque, Spectrochim. Acta A 54 (1998) 1571. [3] E. Montoya, J.A. Sanz-GarcmH a, L.E. BausaH , Spectrochim. Acta A 54 (1998) 2081. [4] E. Montoya, A. Lorenzo, L.E. BausaH , J. Phys.: Condens. Matter 11 (1999) 311. [5] E. Montoya, J. Capmany, L.E. BausaH , T. Kellner, A. Diening, G. Huber, Appl. Phys. Lett. 74 (1999) 3113. [6] E. Nakazawa, S. Shionoya, Phys Rev. Lett. 25 (1970) 1710. [7] F. Auzel, D. Meichenin, F. PelleH , P. Goldner, Optical Materials 4 (1994) 35. [8] A. Lorenzo, H. Ja!rezic, G. Boulon, J. GarcmH a-SoleH , Appl. Phys. Lett 67 (1995) 1.