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Spectral analysis of Pr3+ doped germanate glasses modified by BaO and BaF2
Journal of Luminescence 171 (2016) 138–142
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Spectral analysis of Pr3 þ doped germanate glasses modified by BaO and BaF2 Joanna Pisarska a,n, Wojciech A. Pisarski a, Dominik Dorosz b, Jan Dorosz b a b
University of Silesia, Institute of Chemistry, Szkolna 9 Street, 40-007 Katowice, Poland Bialystok University of Technology, Faculty of Electrical Engineering, Wiejska 45D Street, 15-351 Bialystok, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 20 April 2015 Received in revised form 7 November 2015 Accepted 16 November 2015 Available online 23 November 2015
Luminescence properties of Pr3 þ ions in germanate glasses modified by BaF2 were investigated. Several luminescence bands originating to transitions from the 3P0 state to the lower-lying states of Pr3 þ were registered under 450 nm excitation. The spectral analysis suggests that the positions of luminescence bands and their relative intensities are changed significantly with BaF2 content. The relative integrated luminescence intensities of 3P0-3H4 transition (blue) to the 3P0-3F2 transition (red) of Pr3 þ ions strongly depend on fluoride modifier BaF2 in glass composition. The luminescence bands of Pr3 þ ions are shifted to shorter wavelengths with increasing BaF2 content. & 2015 Elsevier B.V. All rights reserved.
Keywords: Glasses Rare earth ions Luminescence Spectral properties
1. Introduction The energy level scheme of trivalent praseodymium belonging to 4f2 electronic configuration contains several metastable levels that provide the possibility of simultaneous luminescent transitions in the blue, orange, red and near-infrared spectral ranges [1–3]. The Pr3 þ ions in some glass matrices exhibit mainly efficient visible emission. Several observed luminescence bands correspond to transitions originating from 3P0 and 1D2 excited states of Pr3 þ [3]. Under the commercial blue LED excitation, the radiant flux and the quantum yield for red luminescence of Pr3 þ ions in heavy metal germanium tellurite glasses are solved to be 219 μW and 11.8%, respectively. Moreover, 85.24% photons of the luminescence in the visible spectral region are demonstrated to be located in 600–720 nm wavelength range, which indicate that Pr3 þ -doped heavy metal germanium tellurite glass is promising for irradiative light source in minimally invasive photodynamic therapy surgery [4]. The second group of glass materials exhibits broadband nearinfrared luminescence of Pr3 þ covering a wavelength range from 1.2 μm to 1.7 μm, which is important for optical fiber amplifiers operating at O-, E-, S-, C-, and L-band [5]. Among several inorganic glass systems, glasses containing CdF2 and/or PbF2 are classified as toxic raw materials and consequently they are being often eliminated from various practical applications due to their hazardous effect on health and environment, but at the same time these n
Corresponding author. E-mail address: [email protected] (J. Pisarska).
http://dx.doi.org/10.1016/j.jlumin.2015.11.023 0022-2313/& 2015 Elsevier B.V. All rights reserved.
fluoride components were established to play important role in glass formation and further strengthening of glass host network [6]. Alternatively, lead- and cadmium-free glasses are proposed for potential applications in photonics [7]. Recently, luminescence properties of trivalent Pr3 þ ions in lead-free [8–17] and lead based [18–24] glass host matrices were reported. Here, our research has been focused on excitation and luminescence of germanate glasses doped with Pr3 þ . The spectroscopic properties of Pr3 þ ions in glass samples modified by BaO and/or BaF2 are presented and discussed in details.
2. Experimental Germanate glasses with the following chemical composition: 60GeO2–(30 x)BaO–xBaF2–9.5Ga2O3–0.5Pr2O3, where x ¼0, 5, 10, 20, 30 (given in mol%) and 60GeO2–(30 x)BaO–xBaF2–9.9Ga2O3– 0.1Pr2O3 (x ¼0 and 5), were prepared by mixing and melting appropriate amounts of metal oxides and BaF2 of high purity (99.99%, Aldrich Chemical Co.). In order to prepare the glass samples, appropriate amounts of all components were mixed homogeneously. Due to the hygroscopicity of the fluoride component and, in order to minimize the adsorbed water content, batches of 5 g were weighted and stored in glove box, in a protective atmosphere of dried argon. Then, they were melted at 1200 °C for 0.45 h. Transparent glassy plates of 10 10 mm dimension were obtained. Each glass sample of 2 mm in thickness was polished for optical measurements. The nature of the studied samples was identified using the X-ray diffraction analysis (X’Pert
J. Pisarska et al. / Journal of Luminescence 171 (2016) 138–142
Fig. 1. Typical excitation spectrum for Pr3 þ ions in germanate glass.
139
Fig. 2. Luminescence spectra for Pr3 þ ions in germanate glasses in the absence (solid line) and presence of BaF2 (dashed line).
X-ray diffractometer). Absorption spectra were recorded using a Varian 5000 UV–VIS–NIR spectrophotometer. Excitation and luminescence measurements were performed on a PTI QuantaMaster QM40 coupled with tunable pulsed optical parametric oscillator (OPO), pumped by a third harmonic of a Nd:YAG laser (Opotek Opolette 355 LD). The luminescence was dispersed by double 200 mm monochromators. The luminescence spectra were recorded using a multimode UVVIS PMT (R928) and Hamamatsu H10330B-75 detectors controlled by a computer. All measurements were carried out at room temperature.
3. Results and discussion Germanate glasses singly doped with Pr3 þ ions were studied using excitation and luminescence spectroscopy. Fig. 1 presents typical excitation spectrum for trivalent Pr3 þ ions in germanate glass. The spectrum was monitored at 620 nm emission wavelength. In 420–510 nm ranges, the excitation spectrum consists of three characteristic bands, which correspond to transitions originating from the 3H4 ground state to the higher-lying 3P2, 3P1 þ 1I6 and 3P0 states of Pr3 þ . The energy gaps between excited states of Pr3 þ are very small and the excitation energy transfers very fast from the 3P2 state via 3P1 þ 1I6 states to the 3P0 state by nonradiative relaxation. Next, the 3P0 excited state is depopulated giving several radiative transitions to the lower-lying states of Pr3 þ . Fig. 2 shows luminescence spectra for Pr3 þ doped germanate glasses in the absence and presence of BaF2. The glass samples were excited at 450 nm (3P2 state of Pr3 þ ). The spectra consist of several emission bands, which are due to the 3P0-3H4, 3 P0-3H5, 1D2-3H4, 3P0-3H6 and 3P0-3F2 transitions of trivalent Pr3 þ , respectively. The broad luminescence band at 525 nm– 560 nm can contain a contribution from 3P1-3H5 transition, whereas the shoulder at about 473 nm is due to 3P1-3H4 transition of Pr3 þ . All transitions are schematized on the energy level scheme of Pr3 þ (Fig. 3). Generally, visible luminescence originating from the 3P0 state of Pr3 þ ions can be successfully observed. The relative integrated luminescence band intensities of Pr3 þ ions are different for glass samples without and with BaF2. You can evidently see that the changes in luminescence intensities are significant, when even a small amounts of BaF2 (5 mol%) is added to the germanate glass host. Moreover, the luminescence spectra
Fig. 3. Energy level scheme for Pr3 þ ions in germanate glass. All transitions are also indicated.
at about 590 nm referred as (*) are drastically changed. Luminescence band due to 1D2-3H4 transition of Pr3 þ is also located in this spectral region. This band was identified only for the oxide glass sample containing BaO. It is interesting to see that luminescence due to 1D2-3H4 transition of Pr3 þ ions is not observed for glass samples, where BaO was partially (or totally) substituted by BaF2 in glass composition. Fig. 4 shows the luminescence decay profiles of the excited states of Pr3 þ ions in germanate glass without BaF2 measured under different excitation and monitoring emission wavelengths. Luminescence decay curves for the glass sample excited at 450 nm (3P2 state of Pr3 þ ) depend critically on monitoring emission wavelengths (a) 620 nm and (b) 590 nm. The luminescence decay is longer for glass sample under monitoring emission wavelength 590 nm than 620 nm, respectively. The detailed analysis confirmed that longer decay is identical than that one obtained for the glass sample under direct excitation at 590 nm (1D2 state of Pr3 þ ). Based on decay measurements, luminescence lifetimes for 0.5 mol% of Pr3 þ ions in germanate glass containing BaO were determined. Measured lifetimes for the 3P0 and 1D2 states of Pr3 þ are close to 8 μs and 21 μs, respectively. Their values are in a good agreement with the experimental results 8 μs (3P0) and 32 μs (1D2) obtained for Pr3 þ ions (0.5 mol%) in heavy metal glasses containing PbO and TiO2 [25]. In contrast to oxide glass sample, only shorter decays (below 10 μs) from the 3P0 state of Pr3 þ ions in glass samples containing BaF2 were measured,
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Fig. 4. Luminescence decay curves for germanate glass without BaF2 under excitation 3P2 (450 nm) and 1D2 (590 nm) states of Pr3 þ .
Fig. 6. Relative integrated emission band intensities and spectral shift for luminescent transitions of Pr3 þ ions with BaF2 content.
Fig. 5. Luminescence spectra for Pr3 þ ions in germanate glasses with BaF2 content.
independently on monitoring emission wavelength conditions. The 3P0 lifetimes are relatively short in comparison to that one obtained for fluoroindate glass: at praseodymium concentrations lower than 0.5%, the 3P0 state was found to exhibit a luminescence lifetime longer than 40 μs [1], which is compatible with efficient laser emission. Our further studies clearly indicated that the luminescence decays from the 3P0 state of Pr3 þ ions in germanate glass are very fast and difficult to precisely measure as a function of BaF2 concentration. Fig. 5 presents emission spectra of Pr3 þ ions as a function of BaF2 concentration. In order to compare their intensities, the spectra were normalized to 3P0-3F2 transition of Pr3 þ .
The relative intensities of emission bands originating to transitions from the 3P0 excited state to the lower-lying states of Pr3 þ and their peak maxima are changed significantly with BaF2 content. The detailed spectral analysis suggests that two luminescent transitions, i.e. 3P0-3H4 (blue) and 3P0-3F2 (red) transitions located at about 490 nm and 645 nm are the most intense lines. The relative integrated luminescence intensities of 3P0-3H4 transition to the 3P0-3F2 transition of Pr3 þ depend strongly on fluoride modifier BaF2 in glass composition. The results are graphically presented in Fig. 6. The blue-to-red luminescence intensity ratios I490/I645 related to the 3P0-3H4 and 3P0-3F2 transitions of Pr3 þ are drastically reduced from 2.87 (without BaF2) to 1.08 (with 5% BaF2) and next increase with further increasing BaF2 concentration. The previously published results indicate that the blue-to-red (or alternatively orange-to-blue) luminescence intensity ratios of Pr3 þ ions are changed with glass host [26], activator concentration [27] and heat treatment process [28]. It is generally accepted that red luminescence dominates over blue luminescence similar to oxide hosts. Trivalent Pr3 þ exhibits dominant red luminescence and the relative intensity of transitions 3P0-3H4 (blue) as well as 1D2-3H4/3P0-3H6 (red) is almost equal in tellurite glass [26]. The situation is considerably changed, where some fluoride components were added to the oxide glass host matrices. Our previous studies indicate that radiative transitions of rare earth ions increase with substitution PbO by PbF2 in lead borate glass [29]. Here, the intensity of emission band due to 3P0-3H4 transition of Pr3 þ starts to enhance with increasing BaF2 content in glass composition. It could be attributed to the increased radiative transition from the 3P0 state to the 3H4 ground state as a consequence of a substantial decrease in nonradiative relaxation from the 3P0 state to the 1D2 state of Pr3 þ . Also, Fig. 6 presents graphically spectral shift for 3P0-3H4 and 3P0-3F2 luminescent
J. Pisarska et al. / Journal of Luminescence 171 (2016) 138–142
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Considering the spectral overlap between the high-energy emission side and the low-energy absorption side of Pr3 þ : 3P023H4, some energy in the higher sub-levels of Pr3 þ : 3P0 can be absorbed by a nearby Pr3 þ in the ground state when they are in a close separation. It results in a depleting of the higher energy sub-levels of Pr3 þ : 3P0. These effects were well confirmed for Pr3 þ doped fluorotellurite glasses by the changes involved in the emission peak wavelength that presents spectral shift in function of activator concentration [30]. In contrast to 3P0-3H4 transition, luminescence band due to 3 P0-3F2 transition is shifted to shorter wavelengths, when BaO was partially (5 mol%) replaced by BaF2. High-energy shift is also observed with further increasing BaF2 content (Fig. 6). The later transition, i.e. 3P0-3F2 corresponds to ‘hypersensitive transition’ of Pr3 þ , which is related to the polarizability and structure of glass host [31]. It suggests that the presence of BaF2 promotes a strong modification of the germanate glass host structure and significantly contribute to the bonds between Pr3 þ and surrounding ligands (O 2, F ). Thus, the high-energy shift of luminescence bands due to the 4f–4f transitions of Pr3 þ increases with increasing BaF2 content. From the literature data it is well known that the 4f–4f transitions experience a high-energy shift compared to the transitions in the free-ion, i.e. to the ion in the gas phase. Jørgensen called this spectral shift the “nephelauxetic effect” and attributed it to a covalent contribution to the bonding between the rare earth ion and the surrounding ligands [32,33]. It was discussed by Koen Binnemans in an excellent review published recently [34]. This phenomenon is also observed for other optical glass systems containing Pr3 þ ions [35].
4. Conclusions
Fig. 7. Absorption and emission spectra for Pr3 þ ions in germanate glasses in the absence and presence of BaF2.
transitions of Pr3 þ ions with BaF2 content. Generally, all luminescent transitions of Pr3 þ are shifted to shorter wavelengths with increasing BaF2 concentration. However, it is worthy to notice that the relative positions of the emission bands of Pr3 þ ions in oxyfluoride glass samples with BaF2 are changed in comparison to oxide ones. The changes are drastically different for 3P0-3H4 transition than 3P0-3F2 transition. The luminescence bands due to 3 P0-3H4 transition of Pr3 þ are considerably shifted to longer wavelengths, when BaO was partially (5 mol%) substituted by BaF2. Next, they are shifted to shorter wavelengths with further increasing BaF2 concentration in glass composition. The strong decrease of the relative intensity and low-energy shift from 0 to 5 mol% BaF2 is rather surprising and reabsorption at the lower wavelength side of emission transition band, due to the superimposing of absorption and emission is quite possible. For that reason, the absorption and emission spectra for glass samples without BaF2 and 5 mol% BaF2 were compared. They are presented on Fig. 7. The spectroscopic analysis suggests that the absorption bands (especially 3H4-3P0 transition) for 0.5% Pr-doped samples without BaF2 and 5 mol% BaF2 are practically unchanged, whereas emission bands are drastically different (Fig. 7 top). In our opinion, these phenomena are rather related to the activator (Pr3 þ ) concentration. Further experiments indicate that these effects are not observed for glass samples without BaF2 and 5 mol% BaF2, when concentration of Pr3 þ is equal to 0.1 mol% (Fig. 7 bottom). Similar results were obtained for Pr3 þ ions in fluorotellurite glasses [30].
Pr3 þ -doped germanate glasses modified by BaF2 were prepared and next studied using luminescence spectroscopy. Luminescence bands originating to transitions from the 3P0 state to the lowerlying states of Pr3 þ were measured under excitation by 450 nm line. Two of them, 3P0-3H4 (blue) and 3P0-3F2 (red) transitions located at about 490 nm and 645 nm are the most intense emission lines. The detailed spectral analysis indicates that the positions of emission bands and their relative intensities are changed significantly with BaF2 content. The relative integrated intensities of 3P0-3H4 transition (blue) to the 3P0-3F2 transition (red) of Pr3 þ ions strongly depend on fluoride modifier BaF2 in glass composition. The luminescent transitions of Pr3 þ ions are shifted to shorter wavelengths with increasing BaF2 concentration.
Acknowledgments The National Science Centre (Poland) supported this work under research project 2011/03/B/ST7/01743.
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Spectral analysis of Pr3 þ doped germanate glasses modified by BaO and BaF2 Joanna Pisarska a,n, Wojciech A. Pisarski a, Dominik Dorosz b, Jan Dorosz b a b
University of Silesia, Institute of Chemistry, Szkolna 9 Street, 40-007 Katowice, Poland Bialystok University of Technology, Faculty of Electrical Engineering, Wiejska 45D Street, 15-351 Bialystok, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 20 April 2015 Received in revised form 7 November 2015 Accepted 16 November 2015 Available online 23 November 2015
Luminescence properties of Pr3 þ ions in germanate glasses modified by BaF2 were investigated. Several luminescence bands originating to transitions from the 3P0 state to the lower-lying states of Pr3 þ were registered under 450 nm excitation. The spectral analysis suggests that the positions of luminescence bands and their relative intensities are changed significantly with BaF2 content. The relative integrated luminescence intensities of 3P0-3H4 transition (blue) to the 3P0-3F2 transition (red) of Pr3 þ ions strongly depend on fluoride modifier BaF2 in glass composition. The luminescence bands of Pr3 þ ions are shifted to shorter wavelengths with increasing BaF2 content. & 2015 Elsevier B.V. All rights reserved.
Keywords: Glasses Rare earth ions Luminescence Spectral properties
1. Introduction The energy level scheme of trivalent praseodymium belonging to 4f2 electronic configuration contains several metastable levels that provide the possibility of simultaneous luminescent transitions in the blue, orange, red and near-infrared spectral ranges [1–3]. The Pr3 þ ions in some glass matrices exhibit mainly efficient visible emission. Several observed luminescence bands correspond to transitions originating from 3P0 and 1D2 excited states of Pr3 þ [3]. Under the commercial blue LED excitation, the radiant flux and the quantum yield for red luminescence of Pr3 þ ions in heavy metal germanium tellurite glasses are solved to be 219 μW and 11.8%, respectively. Moreover, 85.24% photons of the luminescence in the visible spectral region are demonstrated to be located in 600–720 nm wavelength range, which indicate that Pr3 þ -doped heavy metal germanium tellurite glass is promising for irradiative light source in minimally invasive photodynamic therapy surgery [4]. The second group of glass materials exhibits broadband nearinfrared luminescence of Pr3 þ covering a wavelength range from 1.2 μm to 1.7 μm, which is important for optical fiber amplifiers operating at O-, E-, S-, C-, and L-band [5]. Among several inorganic glass systems, glasses containing CdF2 and/or PbF2 are classified as toxic raw materials and consequently they are being often eliminated from various practical applications due to their hazardous effect on health and environment, but at the same time these n
Corresponding author. E-mail address: [email protected] (J. Pisarska).
http://dx.doi.org/10.1016/j.jlumin.2015.11.023 0022-2313/& 2015 Elsevier B.V. All rights reserved.
fluoride components were established to play important role in glass formation and further strengthening of glass host network [6]. Alternatively, lead- and cadmium-free glasses are proposed for potential applications in photonics [7]. Recently, luminescence properties of trivalent Pr3 þ ions in lead-free [8–17] and lead based [18–24] glass host matrices were reported. Here, our research has been focused on excitation and luminescence of germanate glasses doped with Pr3 þ . The spectroscopic properties of Pr3 þ ions in glass samples modified by BaO and/or BaF2 are presented and discussed in details.
2. Experimental Germanate glasses with the following chemical composition: 60GeO2–(30 x)BaO–xBaF2–9.5Ga2O3–0.5Pr2O3, where x ¼0, 5, 10, 20, 30 (given in mol%) and 60GeO2–(30 x)BaO–xBaF2–9.9Ga2O3– 0.1Pr2O3 (x ¼0 and 5), were prepared by mixing and melting appropriate amounts of metal oxides and BaF2 of high purity (99.99%, Aldrich Chemical Co.). In order to prepare the glass samples, appropriate amounts of all components were mixed homogeneously. Due to the hygroscopicity of the fluoride component and, in order to minimize the adsorbed water content, batches of 5 g were weighted and stored in glove box, in a protective atmosphere of dried argon. Then, they were melted at 1200 °C for 0.45 h. Transparent glassy plates of 10 10 mm dimension were obtained. Each glass sample of 2 mm in thickness was polished for optical measurements. The nature of the studied samples was identified using the X-ray diffraction analysis (X’Pert
J. Pisarska et al. / Journal of Luminescence 171 (2016) 138–142
Fig. 1. Typical excitation spectrum for Pr3 þ ions in germanate glass.
139
Fig. 2. Luminescence spectra for Pr3 þ ions in germanate glasses in the absence (solid line) and presence of BaF2 (dashed line).
X-ray diffractometer). Absorption spectra were recorded using a Varian 5000 UV–VIS–NIR spectrophotometer. Excitation and luminescence measurements were performed on a PTI QuantaMaster QM40 coupled with tunable pulsed optical parametric oscillator (OPO), pumped by a third harmonic of a Nd:YAG laser (Opotek Opolette 355 LD). The luminescence was dispersed by double 200 mm monochromators. The luminescence spectra were recorded using a multimode UVVIS PMT (R928) and Hamamatsu H10330B-75 detectors controlled by a computer. All measurements were carried out at room temperature.
3. Results and discussion Germanate glasses singly doped with Pr3 þ ions were studied using excitation and luminescence spectroscopy. Fig. 1 presents typical excitation spectrum for trivalent Pr3 þ ions in germanate glass. The spectrum was monitored at 620 nm emission wavelength. In 420–510 nm ranges, the excitation spectrum consists of three characteristic bands, which correspond to transitions originating from the 3H4 ground state to the higher-lying 3P2, 3P1 þ 1I6 and 3P0 states of Pr3 þ . The energy gaps between excited states of Pr3 þ are very small and the excitation energy transfers very fast from the 3P2 state via 3P1 þ 1I6 states to the 3P0 state by nonradiative relaxation. Next, the 3P0 excited state is depopulated giving several radiative transitions to the lower-lying states of Pr3 þ . Fig. 2 shows luminescence spectra for Pr3 þ doped germanate glasses in the absence and presence of BaF2. The glass samples were excited at 450 nm (3P2 state of Pr3 þ ). The spectra consist of several emission bands, which are due to the 3P0-3H4, 3 P0-3H5, 1D2-3H4, 3P0-3H6 and 3P0-3F2 transitions of trivalent Pr3 þ , respectively. The broad luminescence band at 525 nm– 560 nm can contain a contribution from 3P1-3H5 transition, whereas the shoulder at about 473 nm is due to 3P1-3H4 transition of Pr3 þ . All transitions are schematized on the energy level scheme of Pr3 þ (Fig. 3). Generally, visible luminescence originating from the 3P0 state of Pr3 þ ions can be successfully observed. The relative integrated luminescence band intensities of Pr3 þ ions are different for glass samples without and with BaF2. You can evidently see that the changes in luminescence intensities are significant, when even a small amounts of BaF2 (5 mol%) is added to the germanate glass host. Moreover, the luminescence spectra
Fig. 3. Energy level scheme for Pr3 þ ions in germanate glass. All transitions are also indicated.
at about 590 nm referred as (*) are drastically changed. Luminescence band due to 1D2-3H4 transition of Pr3 þ is also located in this spectral region. This band was identified only for the oxide glass sample containing BaO. It is interesting to see that luminescence due to 1D2-3H4 transition of Pr3 þ ions is not observed for glass samples, where BaO was partially (or totally) substituted by BaF2 in glass composition. Fig. 4 shows the luminescence decay profiles of the excited states of Pr3 þ ions in germanate glass without BaF2 measured under different excitation and monitoring emission wavelengths. Luminescence decay curves for the glass sample excited at 450 nm (3P2 state of Pr3 þ ) depend critically on monitoring emission wavelengths (a) 620 nm and (b) 590 nm. The luminescence decay is longer for glass sample under monitoring emission wavelength 590 nm than 620 nm, respectively. The detailed analysis confirmed that longer decay is identical than that one obtained for the glass sample under direct excitation at 590 nm (1D2 state of Pr3 þ ). Based on decay measurements, luminescence lifetimes for 0.5 mol% of Pr3 þ ions in germanate glass containing BaO were determined. Measured lifetimes for the 3P0 and 1D2 states of Pr3 þ are close to 8 μs and 21 μs, respectively. Their values are in a good agreement with the experimental results 8 μs (3P0) and 32 μs (1D2) obtained for Pr3 þ ions (0.5 mol%) in heavy metal glasses containing PbO and TiO2 [25]. In contrast to oxide glass sample, only shorter decays (below 10 μs) from the 3P0 state of Pr3 þ ions in glass samples containing BaF2 were measured,
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Fig. 4. Luminescence decay curves for germanate glass without BaF2 under excitation 3P2 (450 nm) and 1D2 (590 nm) states of Pr3 þ .
Fig. 6. Relative integrated emission band intensities and spectral shift for luminescent transitions of Pr3 þ ions with BaF2 content.
Fig. 5. Luminescence spectra for Pr3 þ ions in germanate glasses with BaF2 content.
independently on monitoring emission wavelength conditions. The 3P0 lifetimes are relatively short in comparison to that one obtained for fluoroindate glass: at praseodymium concentrations lower than 0.5%, the 3P0 state was found to exhibit a luminescence lifetime longer than 40 μs [1], which is compatible with efficient laser emission. Our further studies clearly indicated that the luminescence decays from the 3P0 state of Pr3 þ ions in germanate glass are very fast and difficult to precisely measure as a function of BaF2 concentration. Fig. 5 presents emission spectra of Pr3 þ ions as a function of BaF2 concentration. In order to compare their intensities, the spectra were normalized to 3P0-3F2 transition of Pr3 þ .
The relative intensities of emission bands originating to transitions from the 3P0 excited state to the lower-lying states of Pr3 þ and their peak maxima are changed significantly with BaF2 content. The detailed spectral analysis suggests that two luminescent transitions, i.e. 3P0-3H4 (blue) and 3P0-3F2 (red) transitions located at about 490 nm and 645 nm are the most intense lines. The relative integrated luminescence intensities of 3P0-3H4 transition to the 3P0-3F2 transition of Pr3 þ depend strongly on fluoride modifier BaF2 in glass composition. The results are graphically presented in Fig. 6. The blue-to-red luminescence intensity ratios I490/I645 related to the 3P0-3H4 and 3P0-3F2 transitions of Pr3 þ are drastically reduced from 2.87 (without BaF2) to 1.08 (with 5% BaF2) and next increase with further increasing BaF2 concentration. The previously published results indicate that the blue-to-red (or alternatively orange-to-blue) luminescence intensity ratios of Pr3 þ ions are changed with glass host [26], activator concentration [27] and heat treatment process [28]. It is generally accepted that red luminescence dominates over blue luminescence similar to oxide hosts. Trivalent Pr3 þ exhibits dominant red luminescence and the relative intensity of transitions 3P0-3H4 (blue) as well as 1D2-3H4/3P0-3H6 (red) is almost equal in tellurite glass [26]. The situation is considerably changed, where some fluoride components were added to the oxide glass host matrices. Our previous studies indicate that radiative transitions of rare earth ions increase with substitution PbO by PbF2 in lead borate glass [29]. Here, the intensity of emission band due to 3P0-3H4 transition of Pr3 þ starts to enhance with increasing BaF2 content in glass composition. It could be attributed to the increased radiative transition from the 3P0 state to the 3H4 ground state as a consequence of a substantial decrease in nonradiative relaxation from the 3P0 state to the 1D2 state of Pr3 þ . Also, Fig. 6 presents graphically spectral shift for 3P0-3H4 and 3P0-3F2 luminescent
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Considering the spectral overlap between the high-energy emission side and the low-energy absorption side of Pr3 þ : 3P023H4, some energy in the higher sub-levels of Pr3 þ : 3P0 can be absorbed by a nearby Pr3 þ in the ground state when they are in a close separation. It results in a depleting of the higher energy sub-levels of Pr3 þ : 3P0. These effects were well confirmed for Pr3 þ doped fluorotellurite glasses by the changes involved in the emission peak wavelength that presents spectral shift in function of activator concentration [30]. In contrast to 3P0-3H4 transition, luminescence band due to 3 P0-3F2 transition is shifted to shorter wavelengths, when BaO was partially (5 mol%) replaced by BaF2. High-energy shift is also observed with further increasing BaF2 content (Fig. 6). The later transition, i.e. 3P0-3F2 corresponds to ‘hypersensitive transition’ of Pr3 þ , which is related to the polarizability and structure of glass host [31]. It suggests that the presence of BaF2 promotes a strong modification of the germanate glass host structure and significantly contribute to the bonds between Pr3 þ and surrounding ligands (O 2, F ). Thus, the high-energy shift of luminescence bands due to the 4f–4f transitions of Pr3 þ increases with increasing BaF2 content. From the literature data it is well known that the 4f–4f transitions experience a high-energy shift compared to the transitions in the free-ion, i.e. to the ion in the gas phase. Jørgensen called this spectral shift the “nephelauxetic effect” and attributed it to a covalent contribution to the bonding between the rare earth ion and the surrounding ligands [32,33]. It was discussed by Koen Binnemans in an excellent review published recently [34]. This phenomenon is also observed for other optical glass systems containing Pr3 þ ions [35].
4. Conclusions
Fig. 7. Absorption and emission spectra for Pr3 þ ions in germanate glasses in the absence and presence of BaF2.
transitions of Pr3 þ ions with BaF2 content. Generally, all luminescent transitions of Pr3 þ are shifted to shorter wavelengths with increasing BaF2 concentration. However, it is worthy to notice that the relative positions of the emission bands of Pr3 þ ions in oxyfluoride glass samples with BaF2 are changed in comparison to oxide ones. The changes are drastically different for 3P0-3H4 transition than 3P0-3F2 transition. The luminescence bands due to 3 P0-3H4 transition of Pr3 þ are considerably shifted to longer wavelengths, when BaO was partially (5 mol%) substituted by BaF2. Next, they are shifted to shorter wavelengths with further increasing BaF2 concentration in glass composition. The strong decrease of the relative intensity and low-energy shift from 0 to 5 mol% BaF2 is rather surprising and reabsorption at the lower wavelength side of emission transition band, due to the superimposing of absorption and emission is quite possible. For that reason, the absorption and emission spectra for glass samples without BaF2 and 5 mol% BaF2 were compared. They are presented on Fig. 7. The spectroscopic analysis suggests that the absorption bands (especially 3H4-3P0 transition) for 0.5% Pr-doped samples without BaF2 and 5 mol% BaF2 are practically unchanged, whereas emission bands are drastically different (Fig. 7 top). In our opinion, these phenomena are rather related to the activator (Pr3 þ ) concentration. Further experiments indicate that these effects are not observed for glass samples without BaF2 and 5 mol% BaF2, when concentration of Pr3 þ is equal to 0.1 mol% (Fig. 7 bottom). Similar results were obtained for Pr3 þ ions in fluorotellurite glasses [30].
Pr3 þ -doped germanate glasses modified by BaF2 were prepared and next studied using luminescence spectroscopy. Luminescence bands originating to transitions from the 3P0 state to the lowerlying states of Pr3 þ were measured under excitation by 450 nm line. Two of them, 3P0-3H4 (blue) and 3P0-3F2 (red) transitions located at about 490 nm and 645 nm are the most intense emission lines. The detailed spectral analysis indicates that the positions of emission bands and their relative intensities are changed significantly with BaF2 content. The relative integrated intensities of 3P0-3H4 transition (blue) to the 3P0-3F2 transition (red) of Pr3 þ ions strongly depend on fluoride modifier BaF2 in glass composition. The luminescent transitions of Pr3 þ ions are shifted to shorter wavelengths with increasing BaF2 concentration.
Acknowledgments The National Science Centre (Poland) supported this work under research project 2011/03/B/ST7/01743.
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