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Eu3+ oxyfluoroborate glass
Chemical Physics Letters 414 (2005) 222–225 www.elsevier.com/locate/cplett
Frequency upconversion involving quartets of ions in a Pr3+/Eu3+ oxyfluoroborate glass Anita Rai a, S.B. Rai
b,*
, K.K. Mahato
b
a
b
Department of Chemistry, Jagatpur P.G. College, Varanasi, India Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi, Uttar Pradesh 221005, India Received 16 May 2005; in final form 15 July 2005 Available online 16 September 2005
Abstract Upconversion emission from 3P0 level of Pr3+ has been observed on excitation with 496.5 nm line of Ar+ laser when Pr3+ and Eu are codoped in oxyfluoroborate glass. No fluorescence is observed when only Pr3+ is present in the glass but even a small amount of Eu3+ leads to this fluorescence (3P0 ! 3H4, 3P0 ! 3F2) along with fluorescence from 5D0 level of Eu3+. The observed fluorescence of Pr3+ has been explained as being due to energy transfer from a pair of Eu3+ ions to a pair of Pr3+ ions. Ó 2005 Elsevier B.V. All rights reserved. 3+
1. Introduction Recently, a study of the optical spectra of Pr3+ ion in oxyfluoroborate glass [1] revealed that the Pr3+ doped glass gives no fluorescence attributable to Pr3+ (3PJ) if the exciting wavelength is P496.5 nm. In contrast, a similar glass doped with Eu3+ fluoresces strongly even for such excitation [2]. During the course of studies some oxyfluoroborate glasses were doped with different relative proportions of Eu3+ and Pr3+. The objective was to see if the well-known energy transfer [ET] between dissimilar ions has a significant effect on the fluorescence. It is observed that excitation of Eu3+ + Pr3+ codoped glass with 496.5 nm line leads to the appearance of not only fluorescence lines due to Eu3+ but also due to Pr3+. It is surmised that the 3P0 level of the Pr3+ is populated through energy transfer from Eu3+ ions (excited by the incident radiation). The emission from Pr3+ is due to 3P0 ! 3H4 transition at 484.0 nm and 3P0 ! 3F2 transition at 641.6 nm. Thus the incident light is upconverted by Pr3+ ions. A similar frequency upconversion through energy transfer in Nd3+/Pr3+ ions codoped in *
Corresponding author. Fax: +91 542 2368468. E-mail address: [email protected] (S.B. Rai).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.08.050
fluoroindate glass has been reported recently by Falcao-Filho et al. [3].
2. Experimental The glass containing the Eu3+ or Pr3+ or both was prepared as described in earlier papers [1,2]. The chemical composition of the glass with only one dopant is ð70 xÞH3 BO3 þ 20Li2 CO3 þ 10LiF þ xðEu2 O3 or Pr2 O3 Þ and for the codoped glasses ð70 x yÞH3 BO3 þ 20Li2 CO3 þ 10LiF þ xPr2 O3 þ yEu2 O3 where x = 1.0 mol% and y = 0.1, 0.5 and 1.0 mol%. The absorption spectra of all the three types of glasses were recorded in the spectral region 300– 2500 nm using a Cary 2390 UV–Vis–NIR spectrophotometer. A comparison of the spectra for glasses with Pr3+ or Eu3+ alone shows that in the codoped glass both types of ions are fairly well distributed. Changes in the concentration of each rare earth ions caused expected changes in the observed intensity of the absorption.
A. Rai et al. / Chemical Physics Letters 414 (2005) 222–225
For fluorescence excitation two spectral lines one at 457.9 nm and other at 496.5 nm of Ar+ laser were used. The fluorescence was dispersed using a 0.5 m-Spex monochoromator in the spectral region 450–700 nm. We have also measured the lifetime of 5D0 level of Eu3+ in the absence as well as in the presence of Pr3+.
3. Results and discussion The fluorescence spectrum of Pr3+ (1.0 mol%) on excitation with 457.9 nm laser radiation (300 mW) is shown in Fig. 1a. It shows five fluorescence peaks at 464.0, 484.0, 528.2, 609.0 and 641.6 nm, which are as3 3 signed respectively to P1 ! 3H4, P0 ! 3H4, 3 3 1 3 3 3 3 P1 ! H5, D2 ! H4 ( P0 ! H6) and P0 ! 3F2 transitions. The peaks due to 3P0 ! 3H4 and 1D2 ! 3H4
223
transitions are relatively intense. The Pr3+ doped glass however, when excited with radiation of wavelength P496.5 nm (same laser power) does not show any fluorescence (see Fig. 1b). Increasing the incident laser power upto 500 mW has no effect and no fluorescence is seen if kexc P 496.5 nm. The Eu3+ (1.0 mol%) doped glass under similar irradiation i.e. at kexc = 457.9 nm or at kexc = 496.5 nm of Ar+ laser (300 mW) gives identical and intense fluorescence spectrum in the region 555–720 nm with fluorescence peaks at 578.5, 591.2, 593.0, 613.9, 650.0 and 701.0 nm (see Fig. 1c). These peaks are assigned to 5D0 ! 7F0, 5D0 ! 7F1(1), 5 D0 ! 7F1(2), 5D0 ! 7F2, 5D0 ! 7F3 and 5D0 ! 7F4 transitions respectively. A glass codoped with 1.0 mol% of Pr3+ and 1.0 mol% of Eu3+ was then examined under similar conditions of irradiation i.e. with the same two exciting lines viz. 457.9
Fig. 1. (a) Fluorescence spectrum of Pr3+ using 457.9 nm line, (b) spectrum of Pr3+ on excitation with 496.5 nm line, (c) fluorescence spectrum of Eu3+ on excitation with 496.5 nm line and (d) fluorescence spectrum of Pr3++ Eu3+ on excitation with 496.5 nm line of Ar+ laser.
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A. Rai et al. / Chemical Physics Letters 414 (2005) 222–225
and 496.5 nm of Ar+ laser. It is now seen that both exciting radiations excite fluorescence emission in both Eu3+ and Pr3+ (see Fig. 1d). The experiment was repeated using glasses containing 1.0 mol% Pr3+ + 0.1 mol% Eu3+; as well as those containing 1.0 mol% Pr3+ + 0.5 mol% Eu3+. For excitation by 457.9 nm line, as expected fluorescence due to Eu3+ and Pr3+ both is seen and an increasing concentration of Eu3+ increases the corresponding fluorescence. It is found that for 496.5 nm excitation the Pr3+ fluorescence increases in intensity as Eu3+ concentration increases. This is an evidence of the role of excitation energy transfer from Eu3+ ions to the codoped Pr3+ ions. The Pr3+ fluorescence lines viz. 3P0 ! 3H4 at 484.0 nm and 3P0 ! 3F2 at 641.6 nm are seen in all the three types of codoped glasses indicating the crucial role of the Eu3+ ions in exciting the Pr3+ ions. Increase in Eu3+ concentration increases the excitation transfer hence there is an increase in the Pr3+ fluorescence. The energy levels of Pr3+ and Eu3+ ions are shown in Fig. 2 (for some of the levels not observed in our studies energy values have been taken from Dieke [4]). It appears that the incident photon at 496.5 nm (20140 cm1) is absorbed by Eu3+ ions, which are excited to 5D1 level at 19025 cm1. The excess energy (1115 cm1) is taken up by the lattice (in borate based glasses a number of absorption frequencies in the range of 1500–800 cm1 are known). The excited Eu3+ ions rapidly relax non-radiatively to the 5D0 level. The measured lifetime of this level (present work) for 0.5 mol% concentration of Eu3+ is 1.4 ms, which is quite large,
Fig. 2. Energy level diagram of Pr3+ and Eu3+ depicting energy transfer.
and the 5D0 level may serve as a reservoir of excited Eu3+ ions. The excitation energy of the 5D0 level of the Eu3+ ion is 17 286 cm1 which is insufficient to excite the 3P0 level of the Pr3+ ion at 20 660 cm1 through direct one to one energy transfer. However, since the energy level 5D0 of Eu3+ is close to 1D2 level of Pr3+ (16 420 cm1) these Eu3+ ions may transfer their energy conveniently to 1D2 level of Pr3+ at (16 420 cm1), the excess energy of 866 cm1 again going into the glass. Though, we have not measured the lifetime of 1D2 level in the present study, earlier work has yielded a value between 200 and 400 ls in different glass hosts indicating that Pr3+ ion may accumulate in this level. However, excitation of Pr3+ ions to the 1D2 level would not result in upconverted fluorescence at 484.0 nm. Two possibilities are worth considering in this connection. 1. Two excited Eu3+ ions in the 5D0 level transfer their energy simultaneously to a single Pr3+ in ground state so as to populate high energy levels of Pr3+ which through relaxation populates the 3PJ level which then decays to 3P0 level to yield the 3P0 ! 3H4 and 3 P0 ! 3F2 transitions. The process involves two Eu3+ and one Pr3+ ion. 2. Two excited Eu3+ ions both in the 5D0 level transfer their energies to excite two unexcited Pr3+ ions to the 1D2 level. The two excited Pr3+ ions in 1D2 level then redistribute their combined energy in such a manner that one of them is excited to 3PJ level while the other goes down to the 1G4 level. The excess energy equivalent to 866 cm1 is transferred to the lattice. An alternative way may be that the excited Pr3+ ion in 1D2 level decays to the ground state dividing its excitation energy between two Pr3+ ions one of which is excited to 1G4 level (at 9733 cm1) and the other to 3F4 level (6700 cm1). The 3F4 level relaxes to the ground state while the excitation energy is
Fig. 3. A plot of log (upconversion intensity) versus log (laser power).
A. Rai et al. / Chemical Physics Letters 414 (2005) 222–225
transferred to the Pr3+ ion in 1D2 level raising the latter to 3P2 state. The 3P2 level relaxes to 3P0 to give upconverted fluorescence. Both of these processes involve four ions, two of Eu3+ and two of Pr3+.
225
(2) is more likely as the mechanism for populating the 3 PJ level of Pr3+ in glasses, codoped with Eu3+.
Acknowledgement If the population of the 3PJ level of Pr3+ is through channel (1), the fluorescence would show the character of a two-photon process. However, if it is through channel (2) then even though two photons are involved initially the fluorescence will show a behavior more akin to a one-photon process. The fluorescence emission of the glass containing 1.0 mol% Pr3+ + 0.5 mol% Eu3+ was recorded at various powers of the exciting 496.5 nm line. The intensity of all the lines (due to Eu3+ and Pr3+) is found to increase with pump power. A plot of the log of upconverted fluorescence intensity (I) for the 3P0 ! 3H4 transition in Pr3+ versus the log of the input pump power (P) is shown in Fig. 3. It gives a slope of 1.4. This shows that channel
Authors are grateful to Department of Science and Technology, New Delhi for financial assistance.
References [1] K.K. Mahato, S.B. Rai, D.K. Rai, Phys. Stat. Solidi 174 (1999) 277. [2] K.K. Mahato, A. Rai, S.B. Rai, Spectrochim. Acta A 60 (2004) 979. [3] E.L. Falcao-Filho, C.B. de Araujo, Y. Messaddeq, J. Appl. Phys. 92 (2002) 3065. [4] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystal, John Wiley and Sons Inc, NY, 1968.
Frequency upconversion involving quartets of ions in a Pr3+/Eu3+ oxyfluoroborate glass Anita Rai a, S.B. Rai
b,*
, K.K. Mahato
b
a
b
Department of Chemistry, Jagatpur P.G. College, Varanasi, India Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi, Uttar Pradesh 221005, India Received 16 May 2005; in final form 15 July 2005 Available online 16 September 2005
Abstract Upconversion emission from 3P0 level of Pr3+ has been observed on excitation with 496.5 nm line of Ar+ laser when Pr3+ and Eu are codoped in oxyfluoroborate glass. No fluorescence is observed when only Pr3+ is present in the glass but even a small amount of Eu3+ leads to this fluorescence (3P0 ! 3H4, 3P0 ! 3F2) along with fluorescence from 5D0 level of Eu3+. The observed fluorescence of Pr3+ has been explained as being due to energy transfer from a pair of Eu3+ ions to a pair of Pr3+ ions. Ó 2005 Elsevier B.V. All rights reserved. 3+
1. Introduction Recently, a study of the optical spectra of Pr3+ ion in oxyfluoroborate glass [1] revealed that the Pr3+ doped glass gives no fluorescence attributable to Pr3+ (3PJ) if the exciting wavelength is P496.5 nm. In contrast, a similar glass doped with Eu3+ fluoresces strongly even for such excitation [2]. During the course of studies some oxyfluoroborate glasses were doped with different relative proportions of Eu3+ and Pr3+. The objective was to see if the well-known energy transfer [ET] between dissimilar ions has a significant effect on the fluorescence. It is observed that excitation of Eu3+ + Pr3+ codoped glass with 496.5 nm line leads to the appearance of not only fluorescence lines due to Eu3+ but also due to Pr3+. It is surmised that the 3P0 level of the Pr3+ is populated through energy transfer from Eu3+ ions (excited by the incident radiation). The emission from Pr3+ is due to 3P0 ! 3H4 transition at 484.0 nm and 3P0 ! 3F2 transition at 641.6 nm. Thus the incident light is upconverted by Pr3+ ions. A similar frequency upconversion through energy transfer in Nd3+/Pr3+ ions codoped in *
Corresponding author. Fax: +91 542 2368468. E-mail address: [email protected] (S.B. Rai).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.08.050
fluoroindate glass has been reported recently by Falcao-Filho et al. [3].
2. Experimental The glass containing the Eu3+ or Pr3+ or both was prepared as described in earlier papers [1,2]. The chemical composition of the glass with only one dopant is ð70 xÞH3 BO3 þ 20Li2 CO3 þ 10LiF þ xðEu2 O3 or Pr2 O3 Þ and for the codoped glasses ð70 x yÞH3 BO3 þ 20Li2 CO3 þ 10LiF þ xPr2 O3 þ yEu2 O3 where x = 1.0 mol% and y = 0.1, 0.5 and 1.0 mol%. The absorption spectra of all the three types of glasses were recorded in the spectral region 300– 2500 nm using a Cary 2390 UV–Vis–NIR spectrophotometer. A comparison of the spectra for glasses with Pr3+ or Eu3+ alone shows that in the codoped glass both types of ions are fairly well distributed. Changes in the concentration of each rare earth ions caused expected changes in the observed intensity of the absorption.
A. Rai et al. / Chemical Physics Letters 414 (2005) 222–225
For fluorescence excitation two spectral lines one at 457.9 nm and other at 496.5 nm of Ar+ laser were used. The fluorescence was dispersed using a 0.5 m-Spex monochoromator in the spectral region 450–700 nm. We have also measured the lifetime of 5D0 level of Eu3+ in the absence as well as in the presence of Pr3+.
3. Results and discussion The fluorescence spectrum of Pr3+ (1.0 mol%) on excitation with 457.9 nm laser radiation (300 mW) is shown in Fig. 1a. It shows five fluorescence peaks at 464.0, 484.0, 528.2, 609.0 and 641.6 nm, which are as3 3 signed respectively to P1 ! 3H4, P0 ! 3H4, 3 3 1 3 3 3 3 P1 ! H5, D2 ! H4 ( P0 ! H6) and P0 ! 3F2 transitions. The peaks due to 3P0 ! 3H4 and 1D2 ! 3H4
223
transitions are relatively intense. The Pr3+ doped glass however, when excited with radiation of wavelength P496.5 nm (same laser power) does not show any fluorescence (see Fig. 1b). Increasing the incident laser power upto 500 mW has no effect and no fluorescence is seen if kexc P 496.5 nm. The Eu3+ (1.0 mol%) doped glass under similar irradiation i.e. at kexc = 457.9 nm or at kexc = 496.5 nm of Ar+ laser (300 mW) gives identical and intense fluorescence spectrum in the region 555–720 nm with fluorescence peaks at 578.5, 591.2, 593.0, 613.9, 650.0 and 701.0 nm (see Fig. 1c). These peaks are assigned to 5D0 ! 7F0, 5D0 ! 7F1(1), 5 D0 ! 7F1(2), 5D0 ! 7F2, 5D0 ! 7F3 and 5D0 ! 7F4 transitions respectively. A glass codoped with 1.0 mol% of Pr3+ and 1.0 mol% of Eu3+ was then examined under similar conditions of irradiation i.e. with the same two exciting lines viz. 457.9
Fig. 1. (a) Fluorescence spectrum of Pr3+ using 457.9 nm line, (b) spectrum of Pr3+ on excitation with 496.5 nm line, (c) fluorescence spectrum of Eu3+ on excitation with 496.5 nm line and (d) fluorescence spectrum of Pr3++ Eu3+ on excitation with 496.5 nm line of Ar+ laser.
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A. Rai et al. / Chemical Physics Letters 414 (2005) 222–225
and 496.5 nm of Ar+ laser. It is now seen that both exciting radiations excite fluorescence emission in both Eu3+ and Pr3+ (see Fig. 1d). The experiment was repeated using glasses containing 1.0 mol% Pr3+ + 0.1 mol% Eu3+; as well as those containing 1.0 mol% Pr3+ + 0.5 mol% Eu3+. For excitation by 457.9 nm line, as expected fluorescence due to Eu3+ and Pr3+ both is seen and an increasing concentration of Eu3+ increases the corresponding fluorescence. It is found that for 496.5 nm excitation the Pr3+ fluorescence increases in intensity as Eu3+ concentration increases. This is an evidence of the role of excitation energy transfer from Eu3+ ions to the codoped Pr3+ ions. The Pr3+ fluorescence lines viz. 3P0 ! 3H4 at 484.0 nm and 3P0 ! 3F2 at 641.6 nm are seen in all the three types of codoped glasses indicating the crucial role of the Eu3+ ions in exciting the Pr3+ ions. Increase in Eu3+ concentration increases the excitation transfer hence there is an increase in the Pr3+ fluorescence. The energy levels of Pr3+ and Eu3+ ions are shown in Fig. 2 (for some of the levels not observed in our studies energy values have been taken from Dieke [4]). It appears that the incident photon at 496.5 nm (20140 cm1) is absorbed by Eu3+ ions, which are excited to 5D1 level at 19025 cm1. The excess energy (1115 cm1) is taken up by the lattice (in borate based glasses a number of absorption frequencies in the range of 1500–800 cm1 are known). The excited Eu3+ ions rapidly relax non-radiatively to the 5D0 level. The measured lifetime of this level (present work) for 0.5 mol% concentration of Eu3+ is 1.4 ms, which is quite large,
Fig. 2. Energy level diagram of Pr3+ and Eu3+ depicting energy transfer.
and the 5D0 level may serve as a reservoir of excited Eu3+ ions. The excitation energy of the 5D0 level of the Eu3+ ion is 17 286 cm1 which is insufficient to excite the 3P0 level of the Pr3+ ion at 20 660 cm1 through direct one to one energy transfer. However, since the energy level 5D0 of Eu3+ is close to 1D2 level of Pr3+ (16 420 cm1) these Eu3+ ions may transfer their energy conveniently to 1D2 level of Pr3+ at (16 420 cm1), the excess energy of 866 cm1 again going into the glass. Though, we have not measured the lifetime of 1D2 level in the present study, earlier work has yielded a value between 200 and 400 ls in different glass hosts indicating that Pr3+ ion may accumulate in this level. However, excitation of Pr3+ ions to the 1D2 level would not result in upconverted fluorescence at 484.0 nm. Two possibilities are worth considering in this connection. 1. Two excited Eu3+ ions in the 5D0 level transfer their energy simultaneously to a single Pr3+ in ground state so as to populate high energy levels of Pr3+ which through relaxation populates the 3PJ level which then decays to 3P0 level to yield the 3P0 ! 3H4 and 3 P0 ! 3F2 transitions. The process involves two Eu3+ and one Pr3+ ion. 2. Two excited Eu3+ ions both in the 5D0 level transfer their energies to excite two unexcited Pr3+ ions to the 1D2 level. The two excited Pr3+ ions in 1D2 level then redistribute their combined energy in such a manner that one of them is excited to 3PJ level while the other goes down to the 1G4 level. The excess energy equivalent to 866 cm1 is transferred to the lattice. An alternative way may be that the excited Pr3+ ion in 1D2 level decays to the ground state dividing its excitation energy between two Pr3+ ions one of which is excited to 1G4 level (at 9733 cm1) and the other to 3F4 level (6700 cm1). The 3F4 level relaxes to the ground state while the excitation energy is
Fig. 3. A plot of log (upconversion intensity) versus log (laser power).
A. Rai et al. / Chemical Physics Letters 414 (2005) 222–225
transferred to the Pr3+ ion in 1D2 level raising the latter to 3P2 state. The 3P2 level relaxes to 3P0 to give upconverted fluorescence. Both of these processes involve four ions, two of Eu3+ and two of Pr3+.
225
(2) is more likely as the mechanism for populating the 3 PJ level of Pr3+ in glasses, codoped with Eu3+.
Acknowledgement If the population of the 3PJ level of Pr3+ is through channel (1), the fluorescence would show the character of a two-photon process. However, if it is through channel (2) then even though two photons are involved initially the fluorescence will show a behavior more akin to a one-photon process. The fluorescence emission of the glass containing 1.0 mol% Pr3+ + 0.5 mol% Eu3+ was recorded at various powers of the exciting 496.5 nm line. The intensity of all the lines (due to Eu3+ and Pr3+) is found to increase with pump power. A plot of the log of upconverted fluorescence intensity (I) for the 3P0 ! 3H4 transition in Pr3+ versus the log of the input pump power (P) is shown in Fig. 3. It gives a slope of 1.4. This shows that channel
Authors are grateful to Department of Science and Technology, New Delhi for financial assistance.
References [1] K.K. Mahato, S.B. Rai, D.K. Rai, Phys. Stat. Solidi 174 (1999) 277. [2] K.K. Mahato, A. Rai, S.B. Rai, Spectrochim. Acta A 60 (2004) 979. [3] E.L. Falcao-Filho, C.B. de Araujo, Y. Messaddeq, J. Appl. Phys. 92 (2002) 3065. [4] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystal, John Wiley and Sons Inc, NY, 1968.