- Email: [email protected]
The blue up-conversion luminescence of vitroceramics codoped with Tm3+ and Yb3+ ions pumped with 680 nm
Volume 203, number 2,3
CHEMICAL PHYSICS LETTERS
19 February 1993
The blue up-conversion luminescence of vitroceramics codoped with Tm3+ and Yb3+ ions pumped with 680nm Xu Wu, J.P. Denis, G. &en, Ph. Goldner, M. Genotelle and F. PellC Lkboratoire de Physico-Chimie des MalPriaux,CNRS, I Place Arisfide Briand, 92190 Meudon. France Received 29 October 1992, in final form 4 December1992
A study of the spectroscopic properties of PbFS+ GeO, t WO, vitroceramics doped with Tm’+ and codoped with Yb” and Tm’+ ions upon 680 nm dye laser light excitation is made. The up-conversion emission and excitation spectrahave been measured at room temperature. An enhancement of blue emission centered at 478 nm in the codoped sample is observed. We suggest that this result is due to the interaction between Yb’+ and Tm3+ ions. When the excitation power of-680 nm light is increased, a three-photon absorption process happens for another blue (45 I nm) emission.
1. Introduction
2. Experiments
The vitroceramics are being studied for the use of up-conversion luminescence materials. These compounds have some advantages because they present a high transparency from the UV to the TRand a relatively large amount of trivalent rare-earth ions can be introduced into the host. Auzel et al. [l-3] have investigated the up-conversion process for blue emission of Tm3+ ions and energy transfer between Yb3+ and Tm” ions under infrared excitation in PbF2 t GeOl: Yb203, Tm203 compounds. The process is related to a three-photon absorption for the blue ( ‘G9-3H6) emission band. Recently, up-conversion of red light into blue light in fluoride glasses doped with Tm3+ ions has been reported [4]. This indicates that because the blue emission might result from a two-step process, the efficiency of up-conversion is higher. &en et al. [ 5 ] have reported the enhancement of blue emission of Tm3+ ions is due to the energy transfer upon red excitation in the fluoride glasses. In this work, we have studied the properties of blue up-conversion emission of Tm3+ ions and the mechanism of energy transfer between Yb3+ and Tm3+ ions in vitroceramics doped with Tm3+ and codoped with Yb3+ and Tm3+ ions pumped with 680 nm at room temperature.
The sample was prepared by mixing together glassforming oxides GeOz and WO, with lead fluoride and highly purified rare-earth fluoride (99.995%). The mixture was heated and melted inside a muffle furnace at 950°C for 1 hr. The sample was then obtained by pouring the melt. The vitroceramics prepared for this investigation were -69.8PbF, +ZOGeO, + 10W03 +0.2TmF3, -69.8PbF2 + (20- fx)GeO, t (IO- fx)WO, +xYbF3+0.2TmF3, withx=8, 10and 15, - 64.8PbF2 t lOGe0, + 5W0,
+ 20YbF3+ 0.2TmF3 . Emission spectra upon 680 nm excitation and excitation spectra from 620 to 700 nm were obtained by using Ar? laser pumped dye laser (Coherent 5920) supplied with DCM as excitation means. The emission signal was measured with a JobinYvon HR 1000 spectrometer and a R649 Hamamatsu photomultiplier. The intensity of exciting light at the sample position was detected with a Coherent 200 power meter.
0009-2614/93/S 06.00 D 1993 Elsevier Science Publishers B.V. All rights reserved.
211
Volume 203, number 2,3
I9 February 1993
CHEMICAL PHYSICS LETTERS
3. Experimental results and discussion
The up-conversion emission spectrum of the sample doped with 0.2 mol% Tm3+ under 680 nm dye laser light excitation in the range 420-520nm is shown in fig. 1a. Two main emission bands centered at 45 1 and 478 nm are observed. According to the energy level diagram of the Tm3+ ion, these emission bands correspond to the ‘D,-3H, and ‘G,-3H, transitions respectively; in addition, one other weak emission band can be discerned, viz. the ‘D2-3H6 at 363 nm. Fig. lb shows the up-conversion emission spectrum of the sample codoped with 10 mol% Yb3+ and 0.2 mol% Tm3+ under the same light excitation. The spectral shape and position of the emission bands are consistent with those obtained for single-doped sample, but intensity of emission band at 478 nm is much stronger than that obtained in single-doped sample. The intensity ratio of 478 nm emission of the codoped sample to the same emission of the singledoped sample is about 240.
‘G,-‘H,
Fig. 2a reports the 478 nm emission intensity versus concentration of Yb’+ ions in codoped sample under 680 nm light excitation. It can be seen that the intensity first shows an increase and then a decrease with concentration of Yb’+ ions, and the critical concentration of 478 nm emission for Yb3* ions is about 15 mol%. In addition to the blue up-conversion emission spectrum, a red emission band centered at 780nril which is attributed to the 3F4-3H6 transition of Tm3+ ions is also observed for both compounds. The intensity of this emission band varies linearly with the 680nm excitation power. The dependence of the red emission intensity on the concentration of Yb3+ ions is presented in fig. 2b. The intensity decreases rapidly up to 15 mol% and then the decrease becomes slow above 15 mol%. The measurement of the excitation spectra in the red region for up-conversion emission is important to determine the excitation route in the up-conversion process. The excitation spectra of 478 nm emission for both samples are shown in fig. 3. There are three excitation bands peaking at 648, 655 and 680nm in the spectra. These bands are due to the excitation of Tm3+ ions from its ground state to excited states 3F, and 3F3.The intensity of excitation band at 680 nm for the codoped sample is much larger than in the single-doped sample. Two blue emission bands are obtained under 680nm light excitation and the dependence of the two bands is quadratic on the excitation power for
I
N.1 10.
‘ [ \ f *
0
:o-
IO
LOO
L50 WAVELENGTH
550
500 fnm)
Fig. I. Blue up-conversion emission spectra of both samples under 680 nm dye laser light excitation. (a) Single-doped sample (0.2 mol% Tm3+); (b) codoped sample (lOmol% Yb3+ and 0.2 mol% Tm3+).
212
1
\
“----:-
_-(b, \
Ial
i 01
0
10
20
I mol.%
YbF,
CONCENTRATION
Fig. 2. Dependence of emission intensities of Tm’+ ions on the Yb3+ concentration for 0.2 mol% of Tm3+ under 680 nm light excitation. (a) For 478 nm up-conversion emission (‘Gd-3H6); (b) for 780nm emission (‘F.,--‘H,).
CHEMICAL PHYSICS LETTERS
Volume 203, number 2,3
7
I
I
a
680
650
620
WAVELENGTH
(nm)
Fig. 3. Excitation spectra for 478 nm up-conversion emission of Tm’+ ions in both samples. (a) Single-doped sample (0.2 mol% Tm”+); (b) codoped sample (lOmol% Yb3+ and 0.2mol% Tm’+).
40
F
X103cm.’
‘16
t 30-
: 2QW w _ z w IO-
Ob Yb”’
Tm3’
Fig. 4. Energy levels diagram of Tm3+ and Yb’+ ions and energy transfer process between two ions under 680 nm light excitation.
the single-doped sample. It means that up-conversion processes involve a two-photon absorption. In the first step, Tm3+ ions are directly excited into the 3F3level upon 680 nm excitation. Then they may relax to the 3F4and 3H4 levels. Finally, these excited Tm3+ ions reach the ‘D2 and ‘Gq levels by secondphoton absorption, respectively (fig. 4).
I9 February 1993
Tm3+ ions are also excited into the 3Fzlevel upon 655 nm excitation. Then they relax rapidly via 3F3to ‘F4 and 3H4 levels. Blue up-conversion emission bands from ‘D, and ‘G, levels are measured when the second photon is absorbed while the ion is in the 3F.,or 3H, levels, It is in agreement with excitation spectrum of 478 nm emission. When Yb3+ ions are codoped into the compound, the stronger blue emission centered at 478 nm is observed and the emission intensity increases with concentration of Yb3+ up to 15 mol%. In addition, the broad excitation band peaking at 680 nm is obtained in the excitation spectrum of 478 nm emission and its intensity follows a square law with the excitation intensity. This phenomenon relates to the interaction between Yb3+ and Tm3+ ions. We suggest that an excited Tm3+ ion in the 3F4level transfers its energy to a nearby Yb3+ ion exciting it to the 2Fs,2level, then the excited Yb3+ ion transfers its energy back to a Tm3+ ion. This process can excite the Tm3+ ion into the 3H4 level directly or through the 3HSlevel [ 61. Finally, the blue emission from the ‘G, level is observed when the second exciting photon is absorbed while the ion is in the ‘H4 level (fig. 4( 1) ). The interaction between Tm3+ and Yb3+ ions is also confirmed by the fact that the red emission ( 3F4-3H6) intensity of Tm3+ ions decreases with the concentration of Yb3+ ions. For the codoped sample, if the power of 680 nm excitation light is changed, the intensity ratio of 45 1 and 478 nm emission bands is different. For example, when the power is increased from 60 to 350 mW, the ratio varies from 0.06 to 0.25 (fig. 5). The 45 1 nm emission intensity (I,) varies with the excitation intensity (IE) following as I.&; when the power is above 200mW, n is about 2.6. This fact means that an extra energy transfer process implying three-photon absorption happens. Moreover the intensity ratio of two blue emission bands does not change with varying 680 nm excitation power for the single-doped sample. The process has been proposed that a part of the population of ‘Gq level is excited into ‘Dz level by means of energy transfer from excited Yb3+ ions (fig. 4(2)). In summary, the stronger blue emission peaking at 478 nm excited with 680 nm dye laser light is observed in vitroceramics codoped with Yb3+ and Tm3+ ions. The emission intensity increases with the 213
Volume 203, number 2,3
CHEMICAL PHYSICS LETTERS
r
I9 February 1993
of Yb3+ ions up to 15 mol%. We suggest that these experimental results are due to the energy transfer between Yb3+ and Tm3+ ions. In addition, the up-conversion process involving absorption of three photons has been presented for ‘Dz level of Tm3+ ions.
concentration
References
LOO
450 WAVELENGTH
500
I i5(
(nml
Fig. 5. Blue up-conversion emission spectra of Tm3+ ions in the sample containing lOmol% Yb3+ and 0.2mol% Tmr+ under 680 nm light excitation. The power of 680 nm excitation light is (a) k60mW; (b) P=240mW; (c) k350mW.
214
[ 1] F. Auzel, P. Pecile and D. Morin, J. Electrochem. Sot. 122 (1973) 101. [2] F. Auzel, P.A. Santa-Cruz and G.F. de Sk, Rev. Phys. Appl. 20 (1987) 273. [3] O.L. Malta, P.A. Santa-Cruz, CF. de SBand F. Auzel, J. Solid StateChem. 68 (1987) 314. [4] E.W.J.L. Oomen, J. Luminescence 50 (1991) 317. [5] G. Gzen, J.P. Denis, Ph. Goldner, Xu Wu, M. Genotelle and F. Pelle, Appl. Phys. Letters, submitted for publication. [6]F.Auzel,Proc.IEEE61 (1973)758.
CHEMICAL PHYSICS LETTERS
19 February 1993
The blue up-conversion luminescence of vitroceramics codoped with Tm3+ and Yb3+ ions pumped with 680nm Xu Wu, J.P. Denis, G. &en, Ph. Goldner, M. Genotelle and F. PellC Lkboratoire de Physico-Chimie des MalPriaux,CNRS, I Place Arisfide Briand, 92190 Meudon. France Received 29 October 1992, in final form 4 December1992
A study of the spectroscopic properties of PbFS+ GeO, t WO, vitroceramics doped with Tm’+ and codoped with Yb” and Tm’+ ions upon 680 nm dye laser light excitation is made. The up-conversion emission and excitation spectrahave been measured at room temperature. An enhancement of blue emission centered at 478 nm in the codoped sample is observed. We suggest that this result is due to the interaction between Yb’+ and Tm3+ ions. When the excitation power of-680 nm light is increased, a three-photon absorption process happens for another blue (45 I nm) emission.
1. Introduction
2. Experiments
The vitroceramics are being studied for the use of up-conversion luminescence materials. These compounds have some advantages because they present a high transparency from the UV to the TRand a relatively large amount of trivalent rare-earth ions can be introduced into the host. Auzel et al. [l-3] have investigated the up-conversion process for blue emission of Tm3+ ions and energy transfer between Yb3+ and Tm” ions under infrared excitation in PbF2 t GeOl: Yb203, Tm203 compounds. The process is related to a three-photon absorption for the blue ( ‘G9-3H6) emission band. Recently, up-conversion of red light into blue light in fluoride glasses doped with Tm3+ ions has been reported [4]. This indicates that because the blue emission might result from a two-step process, the efficiency of up-conversion is higher. &en et al. [ 5 ] have reported the enhancement of blue emission of Tm3+ ions is due to the energy transfer upon red excitation in the fluoride glasses. In this work, we have studied the properties of blue up-conversion emission of Tm3+ ions and the mechanism of energy transfer between Yb3+ and Tm3+ ions in vitroceramics doped with Tm3+ and codoped with Yb3+ and Tm3+ ions pumped with 680 nm at room temperature.
The sample was prepared by mixing together glassforming oxides GeOz and WO, with lead fluoride and highly purified rare-earth fluoride (99.995%). The mixture was heated and melted inside a muffle furnace at 950°C for 1 hr. The sample was then obtained by pouring the melt. The vitroceramics prepared for this investigation were -69.8PbF, +ZOGeO, + 10W03 +0.2TmF3, -69.8PbF2 + (20- fx)GeO, t (IO- fx)WO, +xYbF3+0.2TmF3, withx=8, 10and 15, - 64.8PbF2 t lOGe0, + 5W0,
+ 20YbF3+ 0.2TmF3 . Emission spectra upon 680 nm excitation and excitation spectra from 620 to 700 nm were obtained by using Ar? laser pumped dye laser (Coherent 5920) supplied with DCM as excitation means. The emission signal was measured with a JobinYvon HR 1000 spectrometer and a R649 Hamamatsu photomultiplier. The intensity of exciting light at the sample position was detected with a Coherent 200 power meter.
0009-2614/93/S 06.00 D 1993 Elsevier Science Publishers B.V. All rights reserved.
211
Volume 203, number 2,3
I9 February 1993
CHEMICAL PHYSICS LETTERS
3. Experimental results and discussion
The up-conversion emission spectrum of the sample doped with 0.2 mol% Tm3+ under 680 nm dye laser light excitation in the range 420-520nm is shown in fig. 1a. Two main emission bands centered at 45 1 and 478 nm are observed. According to the energy level diagram of the Tm3+ ion, these emission bands correspond to the ‘D,-3H, and ‘G,-3H, transitions respectively; in addition, one other weak emission band can be discerned, viz. the ‘D2-3H6 at 363 nm. Fig. lb shows the up-conversion emission spectrum of the sample codoped with 10 mol% Yb3+ and 0.2 mol% Tm3+ under the same light excitation. The spectral shape and position of the emission bands are consistent with those obtained for single-doped sample, but intensity of emission band at 478 nm is much stronger than that obtained in single-doped sample. The intensity ratio of 478 nm emission of the codoped sample to the same emission of the singledoped sample is about 240.
‘G,-‘H,
Fig. 2a reports the 478 nm emission intensity versus concentration of Yb’+ ions in codoped sample under 680 nm light excitation. It can be seen that the intensity first shows an increase and then a decrease with concentration of Yb’+ ions, and the critical concentration of 478 nm emission for Yb3* ions is about 15 mol%. In addition to the blue up-conversion emission spectrum, a red emission band centered at 780nril which is attributed to the 3F4-3H6 transition of Tm3+ ions is also observed for both compounds. The intensity of this emission band varies linearly with the 680nm excitation power. The dependence of the red emission intensity on the concentration of Yb3+ ions is presented in fig. 2b. The intensity decreases rapidly up to 15 mol% and then the decrease becomes slow above 15 mol%. The measurement of the excitation spectra in the red region for up-conversion emission is important to determine the excitation route in the up-conversion process. The excitation spectra of 478 nm emission for both samples are shown in fig. 3. There are three excitation bands peaking at 648, 655 and 680nm in the spectra. These bands are due to the excitation of Tm3+ ions from its ground state to excited states 3F, and 3F3.The intensity of excitation band at 680 nm for the codoped sample is much larger than in the single-doped sample. Two blue emission bands are obtained under 680nm light excitation and the dependence of the two bands is quadratic on the excitation power for
I
N.1 10.
‘ [ \ f *
0
:o-
IO
LOO
L50 WAVELENGTH
550
500 fnm)
Fig. I. Blue up-conversion emission spectra of both samples under 680 nm dye laser light excitation. (a) Single-doped sample (0.2 mol% Tm3+); (b) codoped sample (lOmol% Yb3+ and 0.2 mol% Tm3+).
212
1
\
“----:-
_-(b, \
Ial
i 01
0
10
20
I mol.%
YbF,
CONCENTRATION
Fig. 2. Dependence of emission intensities of Tm’+ ions on the Yb3+ concentration for 0.2 mol% of Tm3+ under 680 nm light excitation. (a) For 478 nm up-conversion emission (‘Gd-3H6); (b) for 780nm emission (‘F.,--‘H,).
CHEMICAL PHYSICS LETTERS
Volume 203, number 2,3
7
I
I
a
680
650
620
WAVELENGTH
(nm)
Fig. 3. Excitation spectra for 478 nm up-conversion emission of Tm’+ ions in both samples. (a) Single-doped sample (0.2 mol% Tm”+); (b) codoped sample (lOmol% Yb3+ and 0.2mol% Tm’+).
40
F
X103cm.’
‘16
t 30-
: 2QW w _ z w IO-
Ob Yb”’
Tm3’
Fig. 4. Energy levels diagram of Tm3+ and Yb’+ ions and energy transfer process between two ions under 680 nm light excitation.
the single-doped sample. It means that up-conversion processes involve a two-photon absorption. In the first step, Tm3+ ions are directly excited into the 3F3level upon 680 nm excitation. Then they may relax to the 3F4and 3H4 levels. Finally, these excited Tm3+ ions reach the ‘D2 and ‘Gq levels by secondphoton absorption, respectively (fig. 4).
I9 February 1993
Tm3+ ions are also excited into the 3Fzlevel upon 655 nm excitation. Then they relax rapidly via 3F3to ‘F4 and 3H4 levels. Blue up-conversion emission bands from ‘D, and ‘G, levels are measured when the second photon is absorbed while the ion is in the 3F.,or 3H, levels, It is in agreement with excitation spectrum of 478 nm emission. When Yb3+ ions are codoped into the compound, the stronger blue emission centered at 478 nm is observed and the emission intensity increases with concentration of Yb3+ up to 15 mol%. In addition, the broad excitation band peaking at 680 nm is obtained in the excitation spectrum of 478 nm emission and its intensity follows a square law with the excitation intensity. This phenomenon relates to the interaction between Yb3+ and Tm3+ ions. We suggest that an excited Tm3+ ion in the 3F4level transfers its energy to a nearby Yb3+ ion exciting it to the 2Fs,2level, then the excited Yb3+ ion transfers its energy back to a Tm3+ ion. This process can excite the Tm3+ ion into the 3H4 level directly or through the 3HSlevel [ 61. Finally, the blue emission from the ‘G, level is observed when the second exciting photon is absorbed while the ion is in the ‘H4 level (fig. 4( 1) ). The interaction between Tm3+ and Yb3+ ions is also confirmed by the fact that the red emission ( 3F4-3H6) intensity of Tm3+ ions decreases with the concentration of Yb3+ ions. For the codoped sample, if the power of 680 nm excitation light is changed, the intensity ratio of 45 1 and 478 nm emission bands is different. For example, when the power is increased from 60 to 350 mW, the ratio varies from 0.06 to 0.25 (fig. 5). The 45 1 nm emission intensity (I,) varies with the excitation intensity (IE) following as I.&; when the power is above 200mW, n is about 2.6. This fact means that an extra energy transfer process implying three-photon absorption happens. Moreover the intensity ratio of two blue emission bands does not change with varying 680 nm excitation power for the single-doped sample. The process has been proposed that a part of the population of ‘Gq level is excited into ‘Dz level by means of energy transfer from excited Yb3+ ions (fig. 4(2)). In summary, the stronger blue emission peaking at 478 nm excited with 680 nm dye laser light is observed in vitroceramics codoped with Yb3+ and Tm3+ ions. The emission intensity increases with the 213
Volume 203, number 2,3
CHEMICAL PHYSICS LETTERS
r
I9 February 1993
of Yb3+ ions up to 15 mol%. We suggest that these experimental results are due to the energy transfer between Yb3+ and Tm3+ ions. In addition, the up-conversion process involving absorption of three photons has been presented for ‘Dz level of Tm3+ ions.
concentration
References
LOO
450 WAVELENGTH
500
I i5(
(nml
Fig. 5. Blue up-conversion emission spectra of Tm3+ ions in the sample containing lOmol% Yb3+ and 0.2mol% Tmr+ under 680 nm light excitation. The power of 680 nm excitation light is (a) k60mW; (b) P=240mW; (c) k350mW.
214
[ 1] F. Auzel, P. Pecile and D. Morin, J. Electrochem. Sot. 122 (1973) 101. [2] F. Auzel, P.A. Santa-Cruz and G.F. de Sk, Rev. Phys. Appl. 20 (1987) 273. [3] O.L. Malta, P.A. Santa-Cruz, CF. de SBand F. Auzel, J. Solid StateChem. 68 (1987) 314. [4] E.W.J.L. Oomen, J. Luminescence 50 (1991) 317. [5] G. Gzen, J.P. Denis, Ph. Goldner, Xu Wu, M. Genotelle and F. Pelle, Appl. Phys. Letters, submitted for publication. [6]F.Auzel,Proc.IEEE61 (1973)758.