Optical Materials 29 (2007) 1231–1235 www.elsevier.com/locate/optmat
Energy transfer in Pr3+–Yb3+ codoped oxyfluoride glass ceramics S. Gonza´lez-Pe´rez a
a,*
, F. Lahoz a, J.M. Ca´ceres b, V. Lavı´n a, I. da Silva c, J. Gonza´lez-Platas c, I.R. Martı´n a
Dpto. de Fı´sica Fundamental y Experimental, Electro´nica y Sistemas, Universidad de La Laguna 38206, La Laguna S/C de Tenerife, Spain b Dpto. de Edafologı´a y Geologı´a, Universidad de La Laguna 38206, La Laguna S/C de Tenerife, Spain c Dpto. de Fı´sica Fundamental I, Universidad de La Laguna 38206, La Laguna S/C de Tenerife, Spain Received 14 October 2005; received in revised form 6 February 2006; accepted 16 February 2006 Available online 7 September 2006
Abstract A study of the optical properties of Pr3+–Yb3+ ions in oxyfluoride glass and glass ceramics was carried out. The cross section emissions at 1300 nm corresponding to the 1G4 ! 3H5 transition of Pr3+ ions were obtained. The ceramming process reduced the linewidth of this transition by about 12% and the emission stimulated cross section was increased by a factor of 1.4 compared to the precursor glasses. The mean wavelength is nearly constant (about 1325 nm) above room temperature. After analyzing the luminescence decay curves of Yb3+ ions, it was found that the majority of the optically active ions in the glass ceramic were located in the fluoride nanocrystals and the rest in the glassy phase. The shortening of distances between the ions turns the energy transfer process into Yb3+ ! Pr3. However, emission at 1300 nm Pr3+ was concentration quenched. 2006 Elsevier B.V. All rights reserved. PACS: 42.70.a; 81.07.Bc; 42.62.Fi Keywords: Glass ceramics; Energy transfer; Praseodymium; Ytterbium
1. Introduction Energy transfer processes in rare earth (RE) doped glasses have been widely investigated in recent years because of the important role that they play in the design of laser and optoelectronic devices [1]. One of the most important applications is to find an efficient 1.3 lm amplifier that could be used at the second telecommunications window in which most local fiber networks operate [2,3]. It is well known that the Pr3+: 1G4 ! 3H5 transition gives rise to an emission band of around 1300 nm. Direct pumping to the 1G4 excited state at 1017 nm seems to be the only efficient mechanism to populate this level. However, this introduces a practical problem because of the low efficiency of this transition and the lack of cheap high power laser diodes at this wavelength. In several systems, *
Corresponding author. E-mail address:
[email protected] (S. Gonza´lez-Pe´rez).
0925-3467/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.02.022
the Yb3+ ion is one of the most widely used lanthanide elements as a sensitizer [4,5]. Due to its electronic configuration it has a simple energy level scheme that consists of an excited 2F5/2 level, 104 cm1 above the ground state. The absorption band is located around 980 nm with a large cross section [7]. By codoping samples with the Yb3+ and Pr3+ ions, the intensity of the Pr3+: 1G4 ! 3H5 emission increases, through Yb3+ ! Pr3+ energy transfer process [4–7]. Oxyfluoride glass ceramics are a two-phase material (one aluminosilicate glassy phase and another fluoride nanocrystalline phase) that have proved to be an interesting matrix in the field of optical material engineering. They have an easy fabrication and manipulation process in air atmosphere. Moreover, they have interesting optical properties because of their low-energy phonons that reduce the multiphonon relaxation and improve the emission cross sections of the RE ions. During the sample preparation, the ions are incorporated into the fluoride nanocrystalline
S. Gonza´lez-Pe´rez et al. / Optical Materials 29 (2007) 1231–1235
phase increasing the probability of the energy transfer processes due to the shortening of distances between the RE ions [8,9]. In this work, we studied an Yb3+ and Pr3+ codoped sample incorporated in an oxyfluoride glass ceramics sample. Continuous wave and time resolved spectroscopy was used to analyze the efficient energy transfer processes in the nanocrystals where the RE ions are incorporated. The discussion on the experimental results is led to obtain efficient emission at 1.3 lm.
3+ 2
Yb : F
2
5/2
-> F
7/2
Intensity (arb. units)
1232
3+ 1
3
Pr : G4 -> H5
x1000
2. Experimental The sample composition used in this work was the following in mol%: 30 SiO2, 15 Al2O3, 29 CdF2, 22 PbF2, 1.4YF3, 0.1PrF3 and 2.5YbF3. The final glass was obtained by melting the sample composition at 1050 C for 2 h and at the end casting the melt into a slab on a stainless steel plate at room temperature. The transparent oxyfluoride glass ceramic was obtained by thermal treatment of the precursor glass at 470 C for 36 h to precipitate nanocrystallites without loss of transparency. Emission spectra were obtained using a Ti-sapphire pumped by an Ar laser. A cryostat in the range from 150 K to 625 K was used for the temperature experiments. Fluorescence was detected through TRIAX monochromator with an InGaAs detector and finally amplified by a lock-in amplifier. The emission spectra were corrected by the spectral response of the equipment. Luminescence decay measurements were obtained by exciting the samples with light from a Qswitched Nd-YAG laser and the signal was acquired by a digital oscilloscope.
ð1Þ
where k0 is the peak position of the emission band, Dkeff is the effective linewidth of the band, n is the refractive index, c is the speed of light, and A(aJ, bJ 0 ) is the spontaneous emission probability of the 1G4 ! 3H5 transition. This value was obtained using the optical parameters given in Ref. [11] calculated with a modified theory developed by Kornienko, Kaminskii and Dunina (K-K-D). This work proved that this modified treatment gives better results in
1200
1400
1600
Fig. 1. Emission spectrum obtained at room temperature under excitation at 920 nm in a Pr3+ 0.1 mol%–Yb3+ 2.5 mol% codoped glass ceramic sample.
2
1
F
G4
5/2
980 nm 1300 nm 3 3
F
7/2
The fluorescence spectrum of Pr3+–Yb3+ codoped sample under excitation at 920 nm is shown in Fig. 1. The larger band around 1000 nm is due to the Yb3+: 2F5/2 ! 2F7/2 transition and the smaller band around 1300 nm is due to the Pr3+: 1G4 ! 3H5 transition (see energy level diagram shown in Fig. 2). The Pr3+: 1G4 ! 3H5 emission band obtained in the glass ceramic sample is compared with that obtained in the glass sample (Fig. 3). The integrated emission cross section for this transition was calculated using the parameters given in Table 1 and the following equation [10]: k40 AðaJ ; bJ 0 Þ 8pcn2 Dkeff
1000
Wavelength (nm)
2
3. Results and discussion
rðk0 Þ ¼
800
Yb
H5 H4
3+
Pr
3+
3
Fig. 2. Energy level diagram and energy transfer mechanism between Yb3+ and Pr3+ ions.
the case of the Pr3+ than the standard Judd–Ofelt theory. According to this theory, the equation for the spontaneous emission probability of an electric-dipole transition is: AED ðaJ ; bJ 0 Þ ¼
8p2 m2 e2 n2 fED ðaJ ; bJ 0 Þ mc3
ð2Þ
where m is an average frequency, m and e are the electron mass and charge, respectively, c the speed of the light, and n the refraction index. The term fED is the oscillator strength and is given by the following equation in this theory: fED ðaJ ; bJ 0 Þ ¼
8p2 mm v 3hð2J þ 1Þn2 ED X Xk jhaJ kU k kbJ 0 ij2 ½1 þ 2aðEðaJ Þ k¼2;4;6
þ EðbJ 0 Þ 2Eð4f N ÞÞ
ð3Þ
S. Gonza´lez-Pe´rez et al. / Optical Materials 29 (2007) 1231–1235
GC
G
1250
1300
1350
1400
1450
1500
Wavelegth (nm) Fig. 3. Emission spectra obtained at room temperature under excitation at 920 nm in Pr3+ 0.1 mol%–Yb3+ 2.5 mol% codoped glass and glass ceramic samples.
Table 1 Judd–Ofelt parameters (Ref. [11]) and emission cross sections calculated for the Pr3+: 1G4 ! 3H5 transition Pr3+: 1G4 ! 3H5
X2
X4
X6
k (nm)
A (s1)
Dkeff (nm)
r (1020 cm2)
Glass ceramic Glass
0.14 0.25
4.05 3.65
17.10 13.95
1306 1325
1081 888
100 112
1.4 1.0
where a ¼ 12 ½Eð4f 1 5d 1 Þ Eð4f N Þ with E(4f15 d1) and E(4fN) being the mean energies of the ground and first opposite-parity configurations, respectively, and E(aJ) and E(bJ 0 ) the energies of the aJ and bJ 0 multiplets of the (4fN) configuration (N = 2 for the Pr3+ion). The doubly reduced matrix elements hkUkki are almost independent of the host, and the elements calculated by Weber for Pr3+ [12] were used. The values of the intensity parameters X2,4,6 for glass and glass ceramic cases were taken from Ref. [11]. It should be noted that when the glass ceramic samples are mentioned, the environment of the ions is different in this case because of its double nature. Moreover, in the glass ceramic sample, the ions are not only optically active in the crystalline phase but are also active in the glassy phase. Nevertheless, the majority are found in the crystalline phase. Therefore, the Judd–Ofelt parameters for the glass ceramic sample correspond to an average of 80% of the nanocrystal phase and 20% of the glassy phase. The fED(aJ, bJ 0 ) was calculated for the 1G4 ! 3H5 transition, then introduced in Eq. (2) to get AED(aJ, bJ 0 ) and finally this spontaneous emission probability was inserted in Eq. (1) to get the integrated emission cross section. It can be seen in the figure that the mean wavelength emission shifts to a shorter wavelength (from 1341 nm to 1325 nm) and is shortest for the glass ceramic sample, and the calculated spontaneous emission probability increases (from 888 s1 to 1081 s1) during ceramming processes. With these values the integrated emission cross sec-
tion obtained from Eq. (1) increases from 1.0 · 1020 cm2 for the glass to 1.4 · 1020 cm2 for the glass ceramic. It is important to take into account that there is uncertainty in the spontaneous emission probabilities (as well as in the emission cross sections), see Ref. [11]. However, with respect to the Pr3+: 1G4 ! 3H5 transition, the spontaneous emission probability depends on the square doubly reduced matrix elements jhkU 2kij2 = 0.3073, jhkU 4kij2 = 0.0715, jhkU 6kij2 = 0.3344 and on the Judd–Ofelt parameters given in Table 1. Therefore, it can be seen in Eqs. (2) and (3) that the value of this probability basically depends on the product: X6jhkU 6kij2. Therefore, the 20% increment observed from glass to glass ceramic for the value of this probability could be justified by the change in the X6 parameter (see Table 1). The observed increment in this parameter is due to a change of the lanthanide local structure towards a more ionic environment, as is found in fluoride nanocrystals. Fig. 4 shows the temperature dependence of the integrated intensity of the Yb3+ and Pr3+ emission bands. It can be seen that the Yb3+ emission decreases as the temperature increases. The phonon density rises when the temperature increases and therefore the transfer probability is higher. Consequently, the radiative emission of Yb3+ ions decreases. This behaviour is due to the fact that energy transfer processes from Yb3+ ! Pr3+ ions are non resonant and are assisted by phonons. There is an energy gap of about 390 cm1 between the emission of the Yb3+ and the absorption of the Pr3+ that nearly corresponds to the energy of one active phonon in this matrix. Therefore, the increase in the Pr3+ emission intensity at high temperatures could be expected due to this energy transfer behaviour. However, the experimental data show a decrease of the 1G4 ! 3H5 intensity as the temperature increases. This experimental behaviour could be explained on the basis of the different cross relaxation channels which affect the 1000 900
3+ 2
Yb : F
2
5/2
-> F
7/2
800
Intensity (a.u.)
G4 -> H 5
Cross Section (*10-22 cm2) 1200
3
1
1233
700 600 500
3+ 1
3
Pr : G4 -> H5
400
x500
300 0
100
200
300
400
500
600
700
T (K) Fig. 4. Temperature dependence of the integrated emission bands of Yb3+: 2F5/2 ! 2F7/2 ions and Pr3+: 1G4 ! 3H5 ions obtained in a Pr3+ 0.1 mol%–Yb3+ 2.5 mol% codoped glass ceramic sample. The dashed lines are guides for the eyes.
S. Gonza´lez-Pe´rez et al. / Optical Materials 29 (2007) 1231–1235
1234 1
1
3
3
3
ð G4 ; H4 Þ ! ð F2 ; F2 Þ ð1 G4 ; 3 H4 Þ ! ð3 H6 ; 3 F2 Þ ð1 G4 ; 3 H4 Þ ! ð3 H5 ; 3 F4 Þ are non resonant (assisted by phonons), so a temperature dependent energy transfer probability is to be expected. The temperature dependence of the mean wavelength of the emission band at 1300 nm is shown in Fig. 5. This mean wavelength corresponds to the centre of gravity of this emission band. As can be seen, this shifts to lower wavelengths while the temperature is increased. So, above room temperature it has a value shorter than 1330 nm which is better for the second optical telecommunication window when compared to the mean wavelengths obtained in other matrixes (tellurite glasses [6], fluorindogallate glasses [7] or chacogenide glasses [14]). Room temperature fluorescence decays from Yb3+ ions were obtained in codoped samples under excitation at 1064 nm. The decays obtained in the glass and glass ceramics samples codoped with 0.1 mol% of Pr3+ and 2.5 mol% of Yb3+ ions are shown in Fig. 6. The glass codoped sample shows a non-exponential behaviour at room temperature which is characteristic of an energy transfer mechanism from Yb3+ to Pr3+ ions. Therefore, this non-exponential fluorescence decay curve is well described by the Inokuti– Hirayama formula for a dipole–dipole interaction [15]: t t1=2 IðtÞ ¼ Ið0ÞExp QG ð4Þ sG sG where sG is the intrinsic lifetime of Yb3+ ions taken as 800 ls. This value was taken from the results given by
1370
Mean Wavelength (nm)
3+ 1
3
Pr : G4 -> H5
1360
1350
1340
1330
1320 0
100
200
300
400
500
600
1
Intensity (arb. units)
G4 level. In a similar matrix [13], the authors have shown the decrease of the lifetime of the 1G4 level with the Pr3+ concentration indicating that the cross relaxation processes are very important even for the Pr3+ samples used in this work (0.1 mol%). Moreover, some of the possible cross relaxation channels given by
3+
3+
3+
3+
Pr 0.1-Yb
Pr 0.1-Yb
0
50
100
2.5 G
2.5 GC
150
t (μs) Fig. 6. Fluorescence decay curves obtained from Yb3+ ions in Pr3+ 0.1 mol%–Yb3+ 2.5 mol% codoped samples under excitation. The solid lines correspond to the fits indicated in the text.
Ref. [16] for a 2.5%Yb3+ sample (Table 1) and it is about 800 ls. The transfer parameter QG is related with the Pr3+ concentration ion (CA) and the donor–acceptor transfer parameter CDA by QG ¼
4p 1=2 1=2 C A p1=2 C DA sG 3
ð5Þ
As can be seen in Fig. 6 a good fit is obtained with QG = 1.5 ± 0.1, indicating that the Pr3+ ions are excited from Yb3+ by an energy transfer process of dipole–dipole character. A two component decay curve is observed in the glass ceramic sample. A slower decay tail follows a very fast initial decay. Similar results were observed in the fluorescence decays of Nd3+ ions [17] and in Yb3+–Tm3+ ions [16] in the same glass ceramic matrix. The fluorescence decay observed in the glass ceramic can be explained on the basis of two different contributions to the emission. First, Yb3+ and Pr3+ ions are found in the fluoride nanocrystals and are separated by short distances, showing a high energy transfer probability which causes the fast initial decay. Second, Yb3+ and Pr3+ ions are located in the glassy phase, and are more isolated with longer distances between each other and therefore the decay constant is slower. Thus, the decay curves are fitted using the Inokuti–Hirayama formula [13] taking into account the two contributions, " 1=2 # t t IðtÞ ¼ I G ð0ÞExp QG sG sG " 1=2 # t t QGC þ I GC ð0ÞExp ð6Þ sGC sGC
700
T (K) 1
3
Fig. 5. Temperature dependence of the mean wavelength of G4 ! H5 emission band at 1300 nm obtained in a Pr3+ 0.1 mol%–Yb3+ 2.5 mol% codoped glass ceramic sample. The dashed line is a guide for the eyes.
In Ref. [16] a value of about 800 ls was obtained for the intrinsic lifetime of Yb3+ ions (sGC) in single doped samples, which is similar to the value obtained in glass samples. Therefore, the best fits to the curve shown in Fig. 6 were
S. Gonza´lez-Pe´rez et al. / Optical Materials 29 (2007) 1231–1235
obtained assuming a similar intrinsic lifetime for the Yb3+ in the glass and in the crystalline phase. The obtained values for the rest of the parameters are IG(0) = 0.23 ± 0.02, IGC(0) = 0.77 ± 0.02, QG = 0.3 ± 0.1 and QGC = 20 ± 2. The values obtained for the IG(0) and IGC(0) parameters indicate that about 77% of the ions are incorporated into the nanocrystals and the rest stay in the glassy phase, which agrees with the relative proportions obtained for other rare earth ions in the same matrix [15,16]. The value obtained for QG using Eq. (6) decreases by a factor of about 4.7 with respect to the value obtained in the glass sample. This result is due to the fact that most of the RE ions in the glass ceramics sample are incorporated into the nanocrystals. Therefore, the effective concentration of ions in the glassy phase in this sample decreases as the QG parameter is proportional to the effective concentration, see Eq. (5). A reduction from 1.4 to 0.3 ls1/2 is observed when moving from the precursor glass to the glass ceramic, reducing the Yb3+–Pr3+ transfer rate and therefore the QG parameter (proportional to the concentration by Eq. (5)). Besides which, it is interesting to note that the value for the QGC parameter is about 60 times larger than QG, which could be explained by the short distances among rare earth ions in the nanocrystals. This result is interesting because it could indicate a very efficient energy transfer process from Yb3+ to Pr3+ ions in the glass ceramics samples, as can be seen in the fast initial decay in the curve shown in Fig. 6. However, cross relaxation processes between Pr3+ ions produce a luminescence quenching from the 1G4 level due to the high concentration of the rare earth ions in the nanocrystals. Different RE dopant concentrations have to be tried in order to prevent the 1G4:Pr3+ concentration quenching, but keeping the high Yb3+ ! Pr3+ energy transfer rate. 4. Conclusions The oxyfluoride glass ceramic matrix was codoped with Yb3+ and Pr3+ ions. Emission at 1300 nm coming from Pr3+ ions was detected under excitation to the Yb3+ ions. The cross section of this emission band increased by a factor of about 1.4 with respect to the precursor glass. Moreover, the mean wavelength is nearly constant (about 1325 nm) above room temperature, which is a very interesting result for telecommunication devices. This relatively low value is advantageous for amplifier applications when compared with chalcogenide and tellurite. By analyzing the luminescence decay curves of Yb3+, it can be concluded that most of the optically active ions (about the 80%) in the glass ceramic samples are located
1235
in fluoride nanocrystals and the rest in the glassy phase. The shortening of distances between the ions in the nanocrystals gives rise to an energy transfer process between ions and therefore excitation of Pr3+ ions occurs through energy transfer by Yb3+ ions. The high local concentration of Pr3+ ions in the samples also increases the cross relaxation processes between them, and as a result the radiative emission at 1300 nm is quenched. In order to improve the efficiency of this emission, a detailed study of the doping concentration needs to be carried out and this will be the aim of future projects. Acknowledgement This work was supported by ‘Comisio´n Interministerial de Ciencia y Tecnologı´a’ under Project Mat 2004-06868 and by Universidad de La Laguna (SEGAI). References [1] S. Tanabe, K. Susuki, N. Soga, T. Hanada, J. Opt. Soc. Am. B 11 (1994) 933. [2] D.W. Hewak, B.N. Samson, J.A. Medeiros Neto, R.I. Laming, D.N. Payne, Electron. Lett. 30 (12) (1994) 968. [3] S. Tanabe, T. Hanada, M. Watanabe, T. Hayashi, N. Soga, J. Am. Ceram. Soc. 78 (1995) 2917. [4] Y. Hou, Y. Li, X. Chen, G. Zhang, Y. Wang, J. Non-Cryst. Solids 260 (1999) 54. [5] S. Tanabe, T. Kouda, T. Hanada, Opt. Mater. 12 (1998) 35. [6] S. Tanabe, T. Kouda, T. Hanada, J. Non-Cryst. Solids 274 (2000) 55. [7] E. Pecorato, D.F. de Sousa, R. Lebullenger, A.C. Herna´ndez, L.A.O. Nunes, J. Appl. Phys. 86 (1999) 3144. [8] J. Me´ndez-Ramos, V. Lavı´n, I.R. Martı´n, U.R. Rodrı´guez-Mendoza, V.D. Rodrı´guez, A.D. Lozano-Gorrı´n, P. Nu´n˜ez, J. Appl. Phys. 89 (2001) 5307. [9] J. Me´ndez-Ramos, V. Lavı´n, I.R. Martı´n, U.R. Rodrı´guez-Mendoza, V.D. Rodrı´guez, A.D. Lozano-Gorrı´n, P. Nu´n˜ez, J. Appl. Phys. 94 (2003) 2295. [10] R.R. Jacobs, M.J. Weber, IEEE J. Quantum Electron. QE-12 (1976) 102. [11] R.T. Ge´nova, I.R. Martı´n, U.R. Rodrı´guez-Mendoza, F. Lahoz, A.D. Lozano-Gorrı´n, P. Nu´n˜ez, J. Gonza´lez-Platas, V. Lavı´n, J. All. Comp. 380 (2004) 167. [12] M.J. Weber, J. Chem. Phys. 48 (1968) 4774. [13] P.A. Tick, N.F. Borrelli, L.K. Cornelius, M.A. Newhouse, J. Appl. Phys. 78 (1995) 6367. [14] Y. Ohishi, A. Mori, T. Kanamori, K. Fujiura, S. Sudo, Appl. Phys. Lett. 65 (1994) 13. [15] M. Inokuti, F. Hirayama, J. Chem. Phys. 43 (1965) 1978. [16] F. Lahoz, I.R. Martı´n, J. Me´ndez-Ramos, P. Nu´n˜ez, J. Chem. Phys. 120 (2004) 6180. [17] M. Abril, J. Me´ndez-Ramos, I.R. Martı´n, U.R. Rodrı´guez-Mendoza, V. Lavı´n, A. Delgado-Torres, V.D. Rodrı´guez, P. Nu´n˜ez, A.D. Lozano-Gorrı´n, J. Appl. Phys. 95 (2004) 5271.
