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Increase of the 800 nm excited Tm3+ blue upconversion emission in fluoroindate glasses by codoping with Yb3+ ions
Optical Materials 22 (2003) 327–333 www.elsevier.com/locate/optmat
Increase of the 800 nm excited Tm3þ blue upconversion emission in fluoroindate glasses by codoping with Yb3þ ions I.R. Martın a
a,*
, J. Mendez-Ramos a, V.D. Rodrıguez a, J.J. Romero b, J. Garcıa-Sole b
Dpto. Fısica Fundamental y Experimental, Electr onica y Sistemas, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain b Dpto. Fısica de Materiales, Universidad Aut onoma de Madrid, 28049 Madrid, Spain Received 28 March 2002; accepted 25 October 2002
Abstract Intense Tm3þ blue upconversion emission has been observed in Tm3þ –Yb3þ codoped fluoroindate glasses under direct excitation into Tm3þ ions with a diode laser at 796 nm. The dependence of the intensity of this upconversion emission on the Tm3þ and Yb3þ concentration has been studied in order to determine the optimum ion concentrations and the involved mechanisms. The blue upconversion emission is highly increased with the Yb3þ concentration. For fixed Yb3þ concentrations, the maximum upconversion efficiency was obtained with Tm3þ concentration of about 0.5 mol%. For this optimum Tm3þ concentration, the upconversion emission intensity is increased by a factor about 100 by codoping with 2.25 mol% of Yb3þ . The results are well explained by a proposed rate equation model. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.70.)a; 78.55.)m; 32.50.þd Keywords: Upconversion; Fluoroindate glasses
1. Introduction There is currently great interest to develop efficient and long-life blue laser sources to increase the density on optical written disks. It is possible to obtain these light sources using materials capable of converting near infrared light of com-
*
Corresponding author. Fax: +34-922-318228. E-mail address: [email protected] (I.R. Martın).
mercial diode lasers (around 650 or 800 nm) into shorter wavelengths. This upconversion process requires the sequential absorption of two or more photons by the material, so an efficient upconversion mechanism requires a long metastable intermediate level. This condition is well satisfied by Rare Earths as doping in fluoride glasses due to the low energy phonons of these matrixes. Therefore, many upconversion studies have been devoted in fluoride glasses doped with Er3þ or Ho3þ ions in order to convert the 800 nm excitation into green emission [1–5].
0925-3467/03/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-3467(02)00292-6
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I.R. Martın et al. / Optical Materials 22 (2003) 327–333
On the other hand, Tm3þ ions are also good candidates to produce blue emission under excitation in the red or near infrared due to the position of their levels. The Tm3þ ions upconversion emission has been more frequently obtained by energy transfer from Yb3þ ions in codoped glasses. However, different works have studied the upconversion efficiencies and mechanisms under direct Tm3þ ions excitation at about 650 ð3 H6 ! 3 F2 Þ or 680 nm ð3 H6 ! 3 F3 Þ [6–12], even a few of them have studied the dependence of the upconversion emission, after excitation in these wavelength, on the Yb3þ concentration in Tm3þ –Yb3þ codoped samples [8,11,12]. In Ref. [8], a decrease of the 650 nm excited upconversion Tm3þ luminescence was observed when codoping with Yb3þ , this quenching is due to losses by energy transfer from Tm3þ to Yb3þ . The same result was obtained in Ref. [11] under excitation at 657 nm but under excitation at 683 nm the Tm3þ upconversion emission was increased by codoping with Yb3þ [11,12]. Transfer from Tm3þ to Yb3þ ions and backtransfer processes were invoked in order explain this increase. However, to the authors knowledge, there is only one work [13] in which blue upconversion emission has been observed by excitation around 800 nm in single Tm3þ ions doped matrices. This mechanism is interesting because it can be excited by commercial diode laser but it would be expected not to be efficient due to the short lifetime of the 3 H5 level, which is an intermediate level in the upconversion mechanism proposed in that work. Nevertheless, the possibility of increasing the upconversion efficiency by codoping with Yb3þ has been explored in fluoride crystals [14]. In this work it is suggested that the involved upconversion process may serve as a pump mechanism enabling blue laser emission, if the Yb3þ and Tm3þ concentrations were optimized. Finally, among the fluoride glasses, the fluoroindate glasses are promising matrix for upconversion processes [8,15–18] because of their large transparency window (250 nm to 8 lm) and the capability of incorporating large concentrations of Rare Earth ions. So, in this work, the blue upconversion emission observed in Tm3þ –Yb3þ codoped fluoroindate glasses under direct excitation
of the Tm3þ ions at 796 nm, using a commercial diode laser, is analyzed as a function of dopant concentrations.
2. Experimental The samples used in this study were prepared with the following composition in mol%: (40 x y) InF3 , 20ZnF2 , 20SrF2 , 20BaF2 , xTmF3 and yYbF3 , with x and y in the range 0–2.5. Upconversion emission spectra were obtained by exciting the samples with light from a diode capable of greater than 150 mW output power at 796 nm. This laser light was focused on the sample with a 50 mm lens. Fluorescence was detected through a 0.25 m monochromator with a photomultiplier. The emission spectra were corrected by the system spectral response. A calibrated pyroelectric detector has been utilized to analyze the power of the incident radiation. The measurements of the emission decays of the 3 H4 level of the Tm3þ ions were carried out with a pulsed jet dye laser as excitation source. This laser was pumped by the 532 nm pulsed light from a doubled Nd-YAG laser. The decay curves of the 1 G4 level were obtained under direct excitation into this level using a flash lamp. In both decay experiments the fluorescence was recorded using a digital storage oscilloscope controlled by a personal computer.
3. Results and discussion 3.1. Fluorescence quenching From the energy level diagram of Tm3þ ions (see Fig. 1), it is possible to identify different deexcitation mechanisms by cross relaxation for the 3 H4 and upper emitting levels, which produce quenching of the luminescence and gain in importance when the Tm3þ concentration is increased. These cross relaxation channels have been observed in different matrix doped with Tm3þ ions [19–21] and the ones coming from the 3 H4 and 1 G4 levels are especially important for the upconversion processes [22–24].
I.R. Martın et al. / Optical Materials 22 (2003) 327–333
329
Fig. 1. Energy level diagram of Tm3þ and Yb3þ ions in fluoroindate glasses. Upconversion excitation mechanism under excitation at 796 nm is indicated.
In a previous work on fluoroindate glasses doped with Tm3þ ions, a decrease in the lifetime of the 3 H4 level was observed when the Tm3þ concentration increased over about 0.1 mol% [20]. This effect is due to a cross relaxation process given by the channel
Fig. 2. Dependence of the 800 nm (Tm3þ : 3 H4 ! 3 H6 Þ and 980 nm ðYb3þ : 2 F5=2 ! 2 F7=2 Þ emission intensities on Tm3þ concentration in codoped fluoroindate glasses with 0.75 or 2.25 mol% of Yb3þ ions under excitation at 796 nm.
Tm3þ ð3 H4 Þ; Tm3þ ð3 H6 Þ ! Tm3þ ð3 F4 Þ; Tm3þ ð3 F4 Þ ð1Þ An additional cross relaxation channel from the excited 3 H4 level appears in Tm3þ –Yb3þ codoped fluoroindate glasses [18]. So, a shortening of the 3 H4 level decay is observed when the Yb3þ concentration is enlarged with a fixed Tm3þ concentration. This effect can be explained in basis to the following energy transfer channel: Tm3þ ð3 H4 Þ; Yb3þ ð2 F7=2 Þ ! Tm3þ ð3 H6 Þ; Yb3þ ð2 F5=2 Þ
ð2Þ
The two processes given by (1) and (2) decrease the emission coming from the 3 H4 level of the Tm3þ ions. The emission intensity at 800 nm, corresponding to the 3 H4 ! 3 H6 transition, has been measured in different Tm3þ –Yb3þ codoped glasses under direct excitation to the 3 H4 level at 796 nm (see Fig. 2). For fixed Yb3þ concentrations of 0.75 or 2.25 mol%, a maximum for the 3 H4 ! 3 H6 emission intensity is observed for a Tm3þ concen-
tration of 0.75 mol%. This result can be explained considering that the Tm3þ emission initially increases with the ions concentration but the quenching process (1) becomes important at high concentration. As it would be expected for similar conditions, a maximum is also obtained for the emission of the Yb3þ ions at 980 nm, due to the excitation of these ions increases with the Tm3þ population of the 3 H4 level by the quenching mechanism given by (2). In fluoroindate glasses doped with Tm3þ ions, efficient cross relaxation is also found in the deexcitation of the 1 G4 level [20]. On the other hand, the deexcitation of this level is not affected by codoping with Yb3þ ions. This result is very interesting in order to find efficient blue upconversion processes, because codoping with Yb3þ ions it would be possible to increase the upconversion blue emission coming from the 1 G4 level (as it will be shown in Section 3.2) without quenching of this emission by the Yb3þ ions.
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3.2. Blue upconversion emission By exciting at 796 nm (Tm3þ : 3 H6 ! 3 H4 Þ a fluoroindate glass doped with 0.75 mol% of Tm3þ and 2.25 mol% of Yb3þ , the upconversion spectrum showed in Fig. 3 is observed. When this experiment is carried out in fluoroindate glasses doped only with Tm3þ ions, very low upconversion emission intensities are obtained. In order to analyze the upconversion mechanism which populates the 1 G4 level, the dependence of the emission from this level at 475 nm ð1 G4 ! 3 H6 Þ on the excitation intensity at 796 nm and on the Tm3þ and Yb3þ ion concentrations has been studied. The upconversion intensity shows a quadratic dependence on the excitation intensity at 796 nm, indicating a two photons process. The dependence of the upconversion intensity on the Tm3þ and Yb3þ ion concentrations is shown in Fig. 4. The dependence on the Tm3þ concentration is similar to the results presented in Fig. 2, indicating that the quenching processes given by (1) play an important role. Whereas, a near linear dependence on the Yb3þ concentration is observed. The important increase with the Yb3þ concentration is specially remarkable, so, for samples doped with 0.75 mol% of Tm3þ and 2.25 mol% of Yb3þ the blue upconversion intensity is about 100 times larger than for Tm3þ single doped glasses with similar concentration.
Fig. 4. Dependence of the blue upconversion emission on Tm3þ concentration in codoped samples with 0.0, 0.1, 0.75 or 2.25 mol% of Yb3þ ions obtained under excitation at 796 nm (154 mW). The solid lines correspond to the theoretical dependence obtained from Eq. (6).
Taking into account these results, the blue emission mechanism under infrared excitation at 796 nm can be based in the following (see Fig. 1). Under direct excitation, the Tm3þ ions are excited to the 3 H4 level and from this level some ions can transfer their energy to the Yb3þ ions. This transfer is nonresonant and the energy excess (about 1900 cm1 ) is given to the matrix. Finally, a transfer from a Yb3þ ion to a Tm3þ ion previously excited, can excites this ion from the 3 H4 level to the 1 G4 level and the energy excess (about 1000 cm1 ) is again given to the matrix. The model that describes this upconversion mechanism is based in the following rate equations: dA3 1 ¼ r/A þ WCR3 þ WTY A3 RUP A3 Y2 s3 dt ð3Þ
Fig. 3. Upconversion emission spectrum obtained at room temperature under excitation at 796 nm in a fluoroindate glass codoped with 0.75 mol% of Tm3þ and 2.25 mol% of Yb3þ .
dY2 1 ¼ þ WYT Y2 þ WTY A3 RUP A3 Y2 sY dt
ð4Þ
I.R. Martın et al. / Optical Materials 22 (2003) 327–333
dA4 1 ¼ þ WCR4 A4 þ RUP A3 Y2 s4 dt
ð5Þ
In these equations the populations of the j-level of the Tm3þ and Yb3þ ions are denoted by Aj and Yj , respectively, as indicated in Fig. 1. Neglecting depopulations of the ground levels for the used excitation intensity, the populations for these levels by unit volume correspond to the concentrations A and Y for the Tm3þ and Yb3þ ions, respectively. The absorption cross-section of Tm3þ ions is r and the incident pumping flux is /. The term sj indicates the lifetime of the j-level of the Tm3þ ions and the term sY is the lifetime of the excited Yb3þ ions. The cross relaxation rates from the A3 and A4 levels of the Tm3þ ions are characterized by the probabilities WCR3 and WCR4 , respectively, while the WTY and WYT parameters correspond to the probabilities for the Tm3þ $ Yb3þ transfer and backtransfer processes, respectively. Finally, RUP is related to the transfer probability from Yb3þ to Tm3þ ions in the excited 3 H4 level, in such a way that RUP A3 Y2 gives the rate for this transfer process. Under stationary regime these equations can be solved neglecting the upconversion term RUP A3 Y2 in Eqs. (3) and (4). This approximation is valid in experiments with no very high excitation intensities. In this case the population of the 1 G4 level is given by A4 ¼ h
1 sY
þ WYT
ih
WTY RUP ðr/AÞ2 1 s3
þ WCR3 þ WTY
i2 h
1 s4
þ WCR4
i ð6Þ
The transfer probabilities that appear in Eq. (6) can be calculated using the energy transfer parameters obtained in previous works [18,20] and presented in Table 1. The lifetimes of the levels
331
involved in the upconversion processes are also presented in this table. Using these values the transfer probabilities can be calculated from gT ð7Þ WT ¼ sð1 gT Þ where s is the lifetime of the involved donor level and gT is the transfer efficiency. Assuming a dipole–dipole interaction between donor and acceptor ions and using the Inokuti– Hirayama model, the transfer efficiency is given by [25,26] pffiffiffi gT ¼ px expðx2 Þ½1 erfðxÞ
ð8Þ where erfðxÞ is the error function and x is given by x¼
2p pffiffiffi 1=2 1=2 pACDA s 3
ð9Þ
where CDA is the donor–acceptor energy transfer parameter. When the migration processes between excited donor ions are important, the model of Parent et al. [27] would be used instead of the Inokuti– Hirayama model. In this case, characterizing the migration processes by the probability WD , we have obtained for the transfer efficiency gT the expression pffiffiffi 0 px expðx02 Þ½1 erfðx0 Þ þ sWD gT ¼ ð10Þ 1 þ sWD where x0 is given by 1=2 2p pffiffiffi 1=2 s 0 pACDA x ¼ 3 1 þ sWD
ð11Þ
As a particular case, if the migration between donor ions is negligible ðWD ¼ 0Þ, then the expression for the transfer efficiency given by Eq. (8) is obtained from Eq. (10). The Tm3þ upconversion intensity dependence on the Tm3þ and Yb3þ ion concentrations has been
Table 1 Data for fluoroindate glasses [17,19] used in Eqs. (7)–(11) to calculate the parameters that appear in Eq. (6) sY (ms)
s3 (ms)
s4 (ms)
WD (s1 ) (for Yb3þ )
CDA CCR3 (cm6 s1 )
2.1
1.9
0.73
1306
40
4:5 10
CCR4 (cm6 s1 ) 39
7:65 10
CTY (cm6 s1 ) 41
2:2 10
CYT (cm6 s1 ) 3:7 1040
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obtained using Eq. (6) and the fit to the experimental results is shown in Fig. 4. Migration processes between Yb3þ ions were taken into account using the proposed Eq. (10) to calculate the Yb3þ ! Tm3þ transfer efficiency. Whereas, Eq. (8) was used to obtain the Tm3þ ! Yb3þ transfer efficiency. A very good agreement between calculated and experimental results is observed in Fig. 4. It is outstanding that the proposed model accounts for the dependence of the Tm3þ upconversion intensity for fixed Yb3þ ion concentration and also accounts for the dependence on the Yb3þ ion concentration. So, for fixed Yb3þ concentrations, the optimum Tm3þ concentration is about 0.5 mol%, increasing lightly with the Yb3þ concentration. In Ref. [13] the authors analyze the blue upconversion emission obtained in ZnCl2 glasses doped with low Tm3þ ion concentration (0.12 mol%) under excitation at 795 nm. These authors proposed for the upconversion mechanism an excited state absorption pumping scheme. In this mechanism the 3 H5 level plays an important role as a temporal storage of excited Tm3þ ions. The population of this level is obtained by deexcitation from the 3 H4 level and it is expected to decrease by cross relaxation processes when the Tm3þ concentration is increased. Therefore, the upconversion efficiency is limited to the low values obtained with low Tm3þ concentrations. However, codoping with Yb3þ , although the Tm3þ concentration is limited to values about 0.5– 0.75 mol% due to the cross relaxation processes, the blue upconversion efficiency is strongly increased with the Yb3þ concentration.
4. Conclusions Tm3þ blue upconversion emission excited at about 800 nm has been obtained in fluoroindate glasses via an efficient mechanism by codoping with Yb3þ ions. The dependence of the upconversion efficiency on the Tm3þ and Yb3þ concentration is well described by a rate equation system which takes into account energy transfer Tm3þ ! Yb3þ and Yb3þ ! Tm3þ , cross relaxation processes between Tm3þ ions and migration between Yb3þ ions. As a result, the upconversion efficiency
can be strongly increased over the limit due to the Tm3þ cross relaxation processes by codoping with Yb3þ ions. It would be remarked that the studied process needs a diode laser at 800 nm, quite less expensive than the diode lasers at 1 lm used to excite the Yb3þ ions in the well known Yb3þ –Tm3þ system. Acknowledgements We would like to thank to Professor R. Alcala (Universidad de Zaragoza) for supplying the samples studied in this work, which was partially supported by ÔGobierno Aut onomo de CanariasÕ (PI 2001/048), ÔComisi on Interministerial de Ciencia y TecnologıaÕ (MAT 2001-3363) and ÔMinisterio de Educaci on, Cultura y DeportesÕ (Beca FPU: AP2000-1801). References [1] S. Tanabe, S. Yoshii, K. Hirao, N. Soga, Phys. Rev. B 45 (1992) 4620. [2] M. Shojiya, M. Takahashi, R. Kanno, Y. Kawamoto, K. Kadono, Appl. Phys. Lett. 65 (1994) 1874. [3] B.R. Reddy, S. Nash-Stevenson, P. Venkateswarlu, J. Opt. Soc. Am. B 11 (1994) 923. [4] M. Shojiya, M. Takahashi, R. Kanno, Y. Kawamoto, K. Kadono, Appl. Phys. Lett. 67 (1995) 2453. [5] H. Higuchi, M. Takahashi, Y. Kawamoto, K. Kadono, T. Ohtsuki, N. Peyghambarian, N. Kitamura, J. Appl. Phys. 83 (1998) 19. [6] S. Tanabe, K. Tamai, K. Hirao, N. Soga, Phys. Rev. B 47 (1993) 2507. [7] J.P. Jouart, M. Bouffard, G. Klein, G. Mary, J. Lumin. 60&61 (1994) 93. [8] S. Kishimoto, K. Hirao, J. Appl. Phys. 80 (1996) 1965. [9] X.B. Chen, G.Y. Zhang, Y.H. Mao, Y.B. Hou, Y. Feng, Z. Hao, J. Lumin. 69 (1996) 151. [10] S. Guy, D.P. Shepherd, M.F. Joubert, B. Jacquier, H. Poignant, J. Opt. Soc. Am. B 14 (1997) 926. [11] G. Ozen, J.P. Denis, Ph. Goldner, X. Wu, M. Genotelle, F. Pelle, Appl. Phys. Lett. 62 (1993) 928. [12] W. Xu, J.P. Denis, G. Ozen, A. Kermaoui, F. Pelle, B. Blanzat, J. Appl. Phys. 75 (1994) 4180. [13] B. Dussardier, J. Wang, D.C. Hanna, D.N. Payne, Opt. Mat. 4 (1995) 565. [14] X.X. Zhang, P. Hong, M. Bass, B.H.T. Chai, Phys. Rev. B 51 (1995) 9298. [15] N. Rakov, C.B. de Ara ujo, Y. Messaddeq, M.A. Aegerter, Appl. Phys. Lett. 70 (1997) 3084. [16] C.T.M. Ribeiro, A.R. Zanatta, L.A.O. Nunes, Y. Messaddeq, M.A. Aegerter, J. Appl. Phys. 83 (1998) 2256.
I.R. Martın et al. / Optical Materials 22 (2003) 327–333 [17] I.R. Martın, V.D. Rodrıguez, V. Lavın, U.R. RodrıguezMendoza, J. All. Comp. 275–277 (1998) 345. [18] I.R. Martın, V.D. Rodrıguez, V. Lavın, U.R. RodrıguezMendoza, Spect. Acta A 55 (1999) 941. [19] A. Brenier, C. Pedrini, B. Moine, J.L. Adam, C. Pledel, Phys. Rev. B 41 (1990) 5364. [20] I.R. Martın, V.D. Rodrıguez, R. Alcala, R. Cases, J. NonCryst. Sol. 161 (1993) 294. [21] C. Li, A. Lagriffoul, R. Moncorge, J.C. Souriau, C. Borel, Ch. Wyon, J. Lumin. 62 (1994) 157.
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[22] A. Brenier, C. Garapon, C. Madej, C. Pedrini, G. Boulon, J. Lumin. 62 (1994) 147. [23] J.M. Dyson, S.M. Jaffe, H. Eilers, M.L. Jones, W.M. Dennis, W.M. Yen, J. Lumin. 60&61 (1994) 668. [24] M.F. Joubert, S. Guy, C. Linares, B. Jacquier, J.L. Adam, J. Non-Cryst. Sol. 184 (1995) 98. [25] Th. Forster, Z. Naturforsch. 4a (1949) 321. [26] M. Inokuti, F. Hirayama, J. Chem. Phys. 43 (1965) 1978. [27] C. Parent, C. Lurin, G. Le Flem, P. Hagenmuller, J. Lumin. 36 (1986) 49.
Increase of the 800 nm excited Tm3þ blue upconversion emission in fluoroindate glasses by codoping with Yb3þ ions I.R. Martın a
a,*
, J. Mendez-Ramos a, V.D. Rodrıguez a, J.J. Romero b, J. Garcıa-Sole b
Dpto. Fısica Fundamental y Experimental, Electr onica y Sistemas, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain b Dpto. Fısica de Materiales, Universidad Aut onoma de Madrid, 28049 Madrid, Spain Received 28 March 2002; accepted 25 October 2002
Abstract Intense Tm3þ blue upconversion emission has been observed in Tm3þ –Yb3þ codoped fluoroindate glasses under direct excitation into Tm3þ ions with a diode laser at 796 nm. The dependence of the intensity of this upconversion emission on the Tm3þ and Yb3þ concentration has been studied in order to determine the optimum ion concentrations and the involved mechanisms. The blue upconversion emission is highly increased with the Yb3þ concentration. For fixed Yb3þ concentrations, the maximum upconversion efficiency was obtained with Tm3þ concentration of about 0.5 mol%. For this optimum Tm3þ concentration, the upconversion emission intensity is increased by a factor about 100 by codoping with 2.25 mol% of Yb3þ . The results are well explained by a proposed rate equation model. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.70.)a; 78.55.)m; 32.50.þd Keywords: Upconversion; Fluoroindate glasses
1. Introduction There is currently great interest to develop efficient and long-life blue laser sources to increase the density on optical written disks. It is possible to obtain these light sources using materials capable of converting near infrared light of com-
*
Corresponding author. Fax: +34-922-318228. E-mail address: [email protected] (I.R. Martın).
mercial diode lasers (around 650 or 800 nm) into shorter wavelengths. This upconversion process requires the sequential absorption of two or more photons by the material, so an efficient upconversion mechanism requires a long metastable intermediate level. This condition is well satisfied by Rare Earths as doping in fluoride glasses due to the low energy phonons of these matrixes. Therefore, many upconversion studies have been devoted in fluoride glasses doped with Er3þ or Ho3þ ions in order to convert the 800 nm excitation into green emission [1–5].
0925-3467/03/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-3467(02)00292-6
328
I.R. Martın et al. / Optical Materials 22 (2003) 327–333
On the other hand, Tm3þ ions are also good candidates to produce blue emission under excitation in the red or near infrared due to the position of their levels. The Tm3þ ions upconversion emission has been more frequently obtained by energy transfer from Yb3þ ions in codoped glasses. However, different works have studied the upconversion efficiencies and mechanisms under direct Tm3þ ions excitation at about 650 ð3 H6 ! 3 F2 Þ or 680 nm ð3 H6 ! 3 F3 Þ [6–12], even a few of them have studied the dependence of the upconversion emission, after excitation in these wavelength, on the Yb3þ concentration in Tm3þ –Yb3þ codoped samples [8,11,12]. In Ref. [8], a decrease of the 650 nm excited upconversion Tm3þ luminescence was observed when codoping with Yb3þ , this quenching is due to losses by energy transfer from Tm3þ to Yb3þ . The same result was obtained in Ref. [11] under excitation at 657 nm but under excitation at 683 nm the Tm3þ upconversion emission was increased by codoping with Yb3þ [11,12]. Transfer from Tm3þ to Yb3þ ions and backtransfer processes were invoked in order explain this increase. However, to the authors knowledge, there is only one work [13] in which blue upconversion emission has been observed by excitation around 800 nm in single Tm3þ ions doped matrices. This mechanism is interesting because it can be excited by commercial diode laser but it would be expected not to be efficient due to the short lifetime of the 3 H5 level, which is an intermediate level in the upconversion mechanism proposed in that work. Nevertheless, the possibility of increasing the upconversion efficiency by codoping with Yb3þ has been explored in fluoride crystals [14]. In this work it is suggested that the involved upconversion process may serve as a pump mechanism enabling blue laser emission, if the Yb3þ and Tm3þ concentrations were optimized. Finally, among the fluoride glasses, the fluoroindate glasses are promising matrix for upconversion processes [8,15–18] because of their large transparency window (250 nm to 8 lm) and the capability of incorporating large concentrations of Rare Earth ions. So, in this work, the blue upconversion emission observed in Tm3þ –Yb3þ codoped fluoroindate glasses under direct excitation
of the Tm3þ ions at 796 nm, using a commercial diode laser, is analyzed as a function of dopant concentrations.
2. Experimental The samples used in this study were prepared with the following composition in mol%: (40 x y) InF3 , 20ZnF2 , 20SrF2 , 20BaF2 , xTmF3 and yYbF3 , with x and y in the range 0–2.5. Upconversion emission spectra were obtained by exciting the samples with light from a diode capable of greater than 150 mW output power at 796 nm. This laser light was focused on the sample with a 50 mm lens. Fluorescence was detected through a 0.25 m monochromator with a photomultiplier. The emission spectra were corrected by the system spectral response. A calibrated pyroelectric detector has been utilized to analyze the power of the incident radiation. The measurements of the emission decays of the 3 H4 level of the Tm3þ ions were carried out with a pulsed jet dye laser as excitation source. This laser was pumped by the 532 nm pulsed light from a doubled Nd-YAG laser. The decay curves of the 1 G4 level were obtained under direct excitation into this level using a flash lamp. In both decay experiments the fluorescence was recorded using a digital storage oscilloscope controlled by a personal computer.
3. Results and discussion 3.1. Fluorescence quenching From the energy level diagram of Tm3þ ions (see Fig. 1), it is possible to identify different deexcitation mechanisms by cross relaxation for the 3 H4 and upper emitting levels, which produce quenching of the luminescence and gain in importance when the Tm3þ concentration is increased. These cross relaxation channels have been observed in different matrix doped with Tm3þ ions [19–21] and the ones coming from the 3 H4 and 1 G4 levels are especially important for the upconversion processes [22–24].
I.R. Martın et al. / Optical Materials 22 (2003) 327–333
329
Fig. 1. Energy level diagram of Tm3þ and Yb3þ ions in fluoroindate glasses. Upconversion excitation mechanism under excitation at 796 nm is indicated.
In a previous work on fluoroindate glasses doped with Tm3þ ions, a decrease in the lifetime of the 3 H4 level was observed when the Tm3þ concentration increased over about 0.1 mol% [20]. This effect is due to a cross relaxation process given by the channel
Fig. 2. Dependence of the 800 nm (Tm3þ : 3 H4 ! 3 H6 Þ and 980 nm ðYb3þ : 2 F5=2 ! 2 F7=2 Þ emission intensities on Tm3þ concentration in codoped fluoroindate glasses with 0.75 or 2.25 mol% of Yb3þ ions under excitation at 796 nm.
Tm3þ ð3 H4 Þ; Tm3þ ð3 H6 Þ ! Tm3þ ð3 F4 Þ; Tm3þ ð3 F4 Þ ð1Þ An additional cross relaxation channel from the excited 3 H4 level appears in Tm3þ –Yb3þ codoped fluoroindate glasses [18]. So, a shortening of the 3 H4 level decay is observed when the Yb3þ concentration is enlarged with a fixed Tm3þ concentration. This effect can be explained in basis to the following energy transfer channel: Tm3þ ð3 H4 Þ; Yb3þ ð2 F7=2 Þ ! Tm3þ ð3 H6 Þ; Yb3þ ð2 F5=2 Þ
ð2Þ
The two processes given by (1) and (2) decrease the emission coming from the 3 H4 level of the Tm3þ ions. The emission intensity at 800 nm, corresponding to the 3 H4 ! 3 H6 transition, has been measured in different Tm3þ –Yb3þ codoped glasses under direct excitation to the 3 H4 level at 796 nm (see Fig. 2). For fixed Yb3þ concentrations of 0.75 or 2.25 mol%, a maximum for the 3 H4 ! 3 H6 emission intensity is observed for a Tm3þ concen-
tration of 0.75 mol%. This result can be explained considering that the Tm3þ emission initially increases with the ions concentration but the quenching process (1) becomes important at high concentration. As it would be expected for similar conditions, a maximum is also obtained for the emission of the Yb3þ ions at 980 nm, due to the excitation of these ions increases with the Tm3þ population of the 3 H4 level by the quenching mechanism given by (2). In fluoroindate glasses doped with Tm3þ ions, efficient cross relaxation is also found in the deexcitation of the 1 G4 level [20]. On the other hand, the deexcitation of this level is not affected by codoping with Yb3þ ions. This result is very interesting in order to find efficient blue upconversion processes, because codoping with Yb3þ ions it would be possible to increase the upconversion blue emission coming from the 1 G4 level (as it will be shown in Section 3.2) without quenching of this emission by the Yb3þ ions.
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3.2. Blue upconversion emission By exciting at 796 nm (Tm3þ : 3 H6 ! 3 H4 Þ a fluoroindate glass doped with 0.75 mol% of Tm3þ and 2.25 mol% of Yb3þ , the upconversion spectrum showed in Fig. 3 is observed. When this experiment is carried out in fluoroindate glasses doped only with Tm3þ ions, very low upconversion emission intensities are obtained. In order to analyze the upconversion mechanism which populates the 1 G4 level, the dependence of the emission from this level at 475 nm ð1 G4 ! 3 H6 Þ on the excitation intensity at 796 nm and on the Tm3þ and Yb3þ ion concentrations has been studied. The upconversion intensity shows a quadratic dependence on the excitation intensity at 796 nm, indicating a two photons process. The dependence of the upconversion intensity on the Tm3þ and Yb3þ ion concentrations is shown in Fig. 4. The dependence on the Tm3þ concentration is similar to the results presented in Fig. 2, indicating that the quenching processes given by (1) play an important role. Whereas, a near linear dependence on the Yb3þ concentration is observed. The important increase with the Yb3þ concentration is specially remarkable, so, for samples doped with 0.75 mol% of Tm3þ and 2.25 mol% of Yb3þ the blue upconversion intensity is about 100 times larger than for Tm3þ single doped glasses with similar concentration.
Fig. 4. Dependence of the blue upconversion emission on Tm3þ concentration in codoped samples with 0.0, 0.1, 0.75 or 2.25 mol% of Yb3þ ions obtained under excitation at 796 nm (154 mW). The solid lines correspond to the theoretical dependence obtained from Eq. (6).
Taking into account these results, the blue emission mechanism under infrared excitation at 796 nm can be based in the following (see Fig. 1). Under direct excitation, the Tm3þ ions are excited to the 3 H4 level and from this level some ions can transfer their energy to the Yb3þ ions. This transfer is nonresonant and the energy excess (about 1900 cm1 ) is given to the matrix. Finally, a transfer from a Yb3þ ion to a Tm3þ ion previously excited, can excites this ion from the 3 H4 level to the 1 G4 level and the energy excess (about 1000 cm1 ) is again given to the matrix. The model that describes this upconversion mechanism is based in the following rate equations: dA3 1 ¼ r/A þ WCR3 þ WTY A3 RUP A3 Y2 s3 dt ð3Þ
Fig. 3. Upconversion emission spectrum obtained at room temperature under excitation at 796 nm in a fluoroindate glass codoped with 0.75 mol% of Tm3þ and 2.25 mol% of Yb3þ .
dY2 1 ¼ þ WYT Y2 þ WTY A3 RUP A3 Y2 sY dt
ð4Þ
I.R. Martın et al. / Optical Materials 22 (2003) 327–333
dA4 1 ¼ þ WCR4 A4 þ RUP A3 Y2 s4 dt
ð5Þ
In these equations the populations of the j-level of the Tm3þ and Yb3þ ions are denoted by Aj and Yj , respectively, as indicated in Fig. 1. Neglecting depopulations of the ground levels for the used excitation intensity, the populations for these levels by unit volume correspond to the concentrations A and Y for the Tm3þ and Yb3þ ions, respectively. The absorption cross-section of Tm3þ ions is r and the incident pumping flux is /. The term sj indicates the lifetime of the j-level of the Tm3þ ions and the term sY is the lifetime of the excited Yb3þ ions. The cross relaxation rates from the A3 and A4 levels of the Tm3þ ions are characterized by the probabilities WCR3 and WCR4 , respectively, while the WTY and WYT parameters correspond to the probabilities for the Tm3þ $ Yb3þ transfer and backtransfer processes, respectively. Finally, RUP is related to the transfer probability from Yb3þ to Tm3þ ions in the excited 3 H4 level, in such a way that RUP A3 Y2 gives the rate for this transfer process. Under stationary regime these equations can be solved neglecting the upconversion term RUP A3 Y2 in Eqs. (3) and (4). This approximation is valid in experiments with no very high excitation intensities. In this case the population of the 1 G4 level is given by A4 ¼ h
1 sY
þ WYT
ih
WTY RUP ðr/AÞ2 1 s3
þ WCR3 þ WTY
i2 h
1 s4
þ WCR4
i ð6Þ
The transfer probabilities that appear in Eq. (6) can be calculated using the energy transfer parameters obtained in previous works [18,20] and presented in Table 1. The lifetimes of the levels
331
involved in the upconversion processes are also presented in this table. Using these values the transfer probabilities can be calculated from gT ð7Þ WT ¼ sð1 gT Þ where s is the lifetime of the involved donor level and gT is the transfer efficiency. Assuming a dipole–dipole interaction between donor and acceptor ions and using the Inokuti– Hirayama model, the transfer efficiency is given by [25,26] pffiffiffi gT ¼ px expðx2 Þ½1 erfðxÞ
ð8Þ where erfðxÞ is the error function and x is given by x¼
2p pffiffiffi 1=2 1=2 pACDA s 3
ð9Þ
where CDA is the donor–acceptor energy transfer parameter. When the migration processes between excited donor ions are important, the model of Parent et al. [27] would be used instead of the Inokuti– Hirayama model. In this case, characterizing the migration processes by the probability WD , we have obtained for the transfer efficiency gT the expression pffiffiffi 0 px expðx02 Þ½1 erfðx0 Þ þ sWD gT ¼ ð10Þ 1 þ sWD where x0 is given by 1=2 2p pffiffiffi 1=2 s 0 pACDA x ¼ 3 1 þ sWD
ð11Þ
As a particular case, if the migration between donor ions is negligible ðWD ¼ 0Þ, then the expression for the transfer efficiency given by Eq. (8) is obtained from Eq. (10). The Tm3þ upconversion intensity dependence on the Tm3þ and Yb3þ ion concentrations has been
Table 1 Data for fluoroindate glasses [17,19] used in Eqs. (7)–(11) to calculate the parameters that appear in Eq. (6) sY (ms)
s3 (ms)
s4 (ms)
WD (s1 ) (for Yb3þ )
CDA CCR3 (cm6 s1 )
2.1
1.9
0.73
1306
40
4:5 10
CCR4 (cm6 s1 ) 39
7:65 10
CTY (cm6 s1 ) 41
2:2 10
CYT (cm6 s1 ) 3:7 1040
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obtained using Eq. (6) and the fit to the experimental results is shown in Fig. 4. Migration processes between Yb3þ ions were taken into account using the proposed Eq. (10) to calculate the Yb3þ ! Tm3þ transfer efficiency. Whereas, Eq. (8) was used to obtain the Tm3þ ! Yb3þ transfer efficiency. A very good agreement between calculated and experimental results is observed in Fig. 4. It is outstanding that the proposed model accounts for the dependence of the Tm3þ upconversion intensity for fixed Yb3þ ion concentration and also accounts for the dependence on the Yb3þ ion concentration. So, for fixed Yb3þ concentrations, the optimum Tm3þ concentration is about 0.5 mol%, increasing lightly with the Yb3þ concentration. In Ref. [13] the authors analyze the blue upconversion emission obtained in ZnCl2 glasses doped with low Tm3þ ion concentration (0.12 mol%) under excitation at 795 nm. These authors proposed for the upconversion mechanism an excited state absorption pumping scheme. In this mechanism the 3 H5 level plays an important role as a temporal storage of excited Tm3þ ions. The population of this level is obtained by deexcitation from the 3 H4 level and it is expected to decrease by cross relaxation processes when the Tm3þ concentration is increased. Therefore, the upconversion efficiency is limited to the low values obtained with low Tm3þ concentrations. However, codoping with Yb3þ , although the Tm3þ concentration is limited to values about 0.5– 0.75 mol% due to the cross relaxation processes, the blue upconversion efficiency is strongly increased with the Yb3þ concentration.
4. Conclusions Tm3þ blue upconversion emission excited at about 800 nm has been obtained in fluoroindate glasses via an efficient mechanism by codoping with Yb3þ ions. The dependence of the upconversion efficiency on the Tm3þ and Yb3þ concentration is well described by a rate equation system which takes into account energy transfer Tm3þ ! Yb3þ and Yb3þ ! Tm3þ , cross relaxation processes between Tm3þ ions and migration between Yb3þ ions. As a result, the upconversion efficiency
can be strongly increased over the limit due to the Tm3þ cross relaxation processes by codoping with Yb3þ ions. It would be remarked that the studied process needs a diode laser at 800 nm, quite less expensive than the diode lasers at 1 lm used to excite the Yb3þ ions in the well known Yb3þ –Tm3þ system. Acknowledgements We would like to thank to Professor R. Alcala (Universidad de Zaragoza) for supplying the samples studied in this work, which was partially supported by ÔGobierno Aut onomo de CanariasÕ (PI 2001/048), ÔComisi on Interministerial de Ciencia y TecnologıaÕ (MAT 2001-3363) and ÔMinisterio de Educaci on, Cultura y DeportesÕ (Beca FPU: AP2000-1801). References [1] S. Tanabe, S. Yoshii, K. Hirao, N. Soga, Phys. Rev. B 45 (1992) 4620. [2] M. Shojiya, M. Takahashi, R. Kanno, Y. Kawamoto, K. Kadono, Appl. Phys. Lett. 65 (1994) 1874. [3] B.R. Reddy, S. Nash-Stevenson, P. Venkateswarlu, J. Opt. Soc. Am. B 11 (1994) 923. [4] M. Shojiya, M. Takahashi, R. Kanno, Y. Kawamoto, K. Kadono, Appl. Phys. Lett. 67 (1995) 2453. [5] H. Higuchi, M. Takahashi, Y. Kawamoto, K. Kadono, T. Ohtsuki, N. Peyghambarian, N. Kitamura, J. Appl. Phys. 83 (1998) 19. [6] S. Tanabe, K. Tamai, K. Hirao, N. Soga, Phys. Rev. B 47 (1993) 2507. [7] J.P. Jouart, M. Bouffard, G. Klein, G. Mary, J. Lumin. 60&61 (1994) 93. [8] S. Kishimoto, K. Hirao, J. Appl. Phys. 80 (1996) 1965. [9] X.B. Chen, G.Y. Zhang, Y.H. Mao, Y.B. Hou, Y. Feng, Z. Hao, J. Lumin. 69 (1996) 151. [10] S. Guy, D.P. Shepherd, M.F. Joubert, B. Jacquier, H. Poignant, J. Opt. Soc. Am. B 14 (1997) 926. [11] G. Ozen, J.P. Denis, Ph. Goldner, X. Wu, M. Genotelle, F. Pelle, Appl. Phys. Lett. 62 (1993) 928. [12] W. Xu, J.P. Denis, G. Ozen, A. Kermaoui, F. Pelle, B. Blanzat, J. Appl. Phys. 75 (1994) 4180. [13] B. Dussardier, J. Wang, D.C. Hanna, D.N. Payne, Opt. Mat. 4 (1995) 565. [14] X.X. Zhang, P. Hong, M. Bass, B.H.T. Chai, Phys. Rev. B 51 (1995) 9298. [15] N. Rakov, C.B. de Ara ujo, Y. Messaddeq, M.A. Aegerter, Appl. Phys. Lett. 70 (1997) 3084. [16] C.T.M. Ribeiro, A.R. Zanatta, L.A.O. Nunes, Y. Messaddeq, M.A. Aegerter, J. Appl. Phys. 83 (1998) 2256.
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