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Multichannel emission from Pr3+ doped heavy-metal oxide glass B2O3–PbO–GeO2–Bi2O3 for broadband signal amplification
Author’s Accepted Manuscript Multichannel emission from Pr3+ doped heavymetal oxide glass B2O3-PbO-GeO2-Bi2O3 for broadband signal amplification Alvaro Herrera, Carlos Jacinto, Ariel R. Becerra, Paulo L. Franzen, Naira M. Balzaretti www.elsevier.com/locate/jlumin
PII: DOI: Reference:
S0022-2313(16)30437-9 http://dx.doi.org/10.1016/j.jlumin.2016.08.019 LUMIN14180
To appear in: Journal of Luminescence Received date: 5 April 2016 Revised date: 3 August 2016 Accepted date: 8 August 2016 Cite this article as: Alvaro Herrera, Carlos Jacinto, Ariel R. Becerra, Paulo L. Franzen and Naira M. Balzaretti, Multichannel emission from Pr3+ doped heavymetal oxide glass B2O3-PbO-GeO2-Bi2O3 for broadband signal amplification, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.08.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Multichannel emission from Pr3+ doped heavy-metal oxide glass B2O3PbO-GeO2-Bi2O3 for broadband signal amplification Alvaro Herreraa, Carlos Jacintob, Ariel R. Becerrac, Paulo L. Franzena, Naira M. Balzarettia* a
Instituto de Física, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500 - Caixa Postal 15051 - CEP 91501-970, Porto Alegre - RS, Brazil b
Grupo de Fotônica e Fluidos Complexos, Instituto de Física, Universidade Federal de Alagoas, 57072900, Maceió - AL, Brazil c
Grupo Integrar, Departamento de Física, Universidad de Pamplona, Ciudad Universitaria, Colombia
* corresponding author: [email protected]
Abstract Pr2O3 doped 26.66B2O3-52.33PbO-16GeO2-4Bi2O3-1Pr2O3 (BPGBPr) glass was synthesized by melt quenching technique. X-ray diffraction confirmed the amorphous nature of the glass and the presence of different vibrational groups was identified by Raman spectroscopy. Differential thermal analysis indicated that the glass transition and crystallization temperature were ~354C and ~521C respectively, indicating good thermal stability. Visible and near infrared absorption spectra were measured and used to evaluate the Judd-Ofelt intensity parameters to calculate the radiative properties for the emission levels of Pr3+. Photoluminescence spectra were recorded in the visible and infrared regions at temperatures between 16 and 300 K. The spectroscopic results indicated that BPGBPr can be useful as a material for broadband optical amplifier in the region of ~1450 cm-1. keywords: heavy metal oxide glasses; praseodymium; Judd Ofelt theory; photoluminescence; NIR broadband
1. Introduction Spectroscopic properties of rare earth (RE) ion doped glasses are attractive for the development of many optoelectronic devices such as lasers and amplifiers, optical sensors, high density memories and so on [16]. The optical properties of RE ions, specially the luminescence efficiency, are strongly influenced by the structural environment and phonon energies (PE) of the host glass [7]. For example, glasses with low PE can provide high fluorescence quantum efficiency, which is important for lasers and optical fiber amplifiers, but most of the hosts with low PE have, for instance, low thermal and mechanical stability [8]. Glasses containing heavy metals, on the other hand, have been studied for their promising applications in optoelectronic devices due to its low PE (~800 cm-1) and large refractive index (~2.0) compared to silicates, borates and phosphates glasses [9–11]. In particular, B2O3-PbO-GeO2-Bi2O3 (BPGB) system exhibits high chemical and good thermal stability, and it is transparent in the visible and infrared regions, being suitable for RE doping for different applications in optoelectronics [12–14].
Co-doping with different RE ions have been used for improving the efficiency of solid state lasers and luminescent solar concentrators, as well as, to get broader bandwidth for network traffic [1,15]. However, according to Guan et al. [16], it is difficult to find the best concentration of RE ions which is suitable to control the energy transfer between them and to achieve the super broadband emission.. Among RE ions, Pr3+ has important luminescence features due to the 4f2 configuration with 91 degenerate energy levels [17] given rise to multichannel emission in the VIS and NIR regions. Different glass families doped with Pr3+ have been studied for applications in solid state lasers, optical amplifiers, optical temperature sensors and optical fibers operating at the E-S-L bands [18–21]. For certain glass systems Pr3+ presents four level laser action associated with the transitions from 3P0 in the visible region [22]. The standard Judd–Ofelt theory (J-O) has been used successfully in the characterization of radiative transitions of RE doped glasses [23,24]. However it is known that the application of J-O theory to Pr3+ ion doped in some host glasses leads to negative values of the phenomenological 2 intensity parameter due to the small energy difference between the ground state configuration 4f2 and the first excited state configuration of 4f15d1 [25,26]. To overcome this problem, the hypersensitive 3H4→3P2 transition has not been taken into account for calculating J-O parameters or modifications of the conventional J-O theory have been used [27,28] . In the present work, glass with composition 26.66B2O3-52.33PbO-16GeO2-4Bi2O3-1Pr2O3, hereafter named as BPGBPr, was synthesized and their structural and optical properties were investigated. Photoluminescence (PL) was studied in the visible and NIR regions under excitation at 488 nm as a function of temperature. The obtained results indicate the potential application of this system for broadband infrared emission in the E-, S-, C- and L-bands.
2. Experimental The glass sample with composition 26.66B2O3-52.33PbO-16GeO2-4Bi2O3-1Pr2O3 was prepared by the traditional melt quenching technique. The starting materials B 2O3 and Bi2O3 from Alfa-Aesar (99.99%), and GeO2, PbO, and Pr2O3 from Sigma Aldrich (99.99%), were mixed in an agate mortar for 30 minutes and heated at 1250C for 1 h in a Pt crucible. The melt was poured onto a brass mold preheated at 330°C to avoid thermal shock and annealed for 2 h below the glass transition temperature to release mechanical stress. Finally, the glass was slowly cooled down to room temperature, cut and polished for optical measurements. The amorphous nature of the sample was confirmed by X-ray powder diffraction using a diffractometer Siemens model D500. The thermal stability was verified by DTA (Differential Thermal Analysis) using a Shimadzu analyzer DTA 50 in the temperature range 20 -700 °C at a rate of 10°C/min, under Ar atmosphere. The refractive index (nD, nF and nC) was measured using a Sopra GES- 5E ellipsometer, where nD, nF and nC are the linear refractive index at the standard wavelengths 587.56, 461.3, and 656.27 nm, respectively. The density was calculated by the Archimedes method using an analytical balance Shimadzu AUW220D (0.1mg/0.01 mg) and distilled water as the immersion liquid. Room temperature absorption spectra in the UV-VIS-NIR region were recorded using high-performance spectrometer PerkinElmer LAMBDA 1050 with a spectral bandwidth resolution of 0.05 nm. Raman spectra were recorded in a Horiba Jobin Yvon
iHR320 system using a 632.8 nm He-Ne excitation laser and a liquid nitrogen CCD detector. Photoluminescence (PL) spectra were measured in a CryLas system with a CW excitation laser operating at 488 nm. The PL signal was dispersed by an Acton SP2300 monochromator and detected by a Pixis 256E CCD. The measurements were performed in an optical He closed cycle cryostat enabling PL measurements in the range of 4.5-300 K. The decay time was measured using an electro optic modulator to pulse the CW laser. The signal was dispersed by a SP2300 monochromator and detected by a Hamamatsu R955 photomultiplier tube. The detected signal was fed to a SR430 multichannel analyzer and transferred to a computer running acquisition software.
3. Results and discussions
3.1 Structural characterization The X-ray diffraction pattern, Raman spectrum and DTA curve for BPGBPr glass are shown, respectively, in Fig 1a, 1b and 1c. XRD exhibits a broad diffuse scattering at small angles, confirming a long range structural disorder characteristic of amorphous network. DTA curve shows the temperatures corresponding to the glass transition (Tg), softening (Ts), crystallization (Tc) and melting (Tm), listed in Table 1. The thermal stability parameter defined as T = Tc - Tg gives information about the stability of the glass against crystallization [29]. A glass composition is considered thermally stable when this parameter is larger than 100ºC [30], which is the case BPGBPr sample (T =167 °C). Raman spectrum for the BPGBPr glass exhibits a low intensity band at ~552 cm-1 related to the stretching of the symmetric Ge-O-Ge bond [31] and the band at ~755 cm-1 is related to the symmetric Ge-O bond [31,32]. The bands at ~ 922 and ~1040 cm-1 are assigned to ortho, penta and diborate groups [31–33] . The broad band at ~1292 cm-1 is related to BO2O- connected to BO4 units [12,31,33]. The most intense band is at 755 cm1, which corresponds to a low PE compared to the most intense band of silicate systems, which is at ~1150 cm-1 [34]. The low PE of the BPGBPr glass may be suitable for enhancing luminescence of Pr3+ ions.
INSERT FIGURE 1 From the values of density (), refractive index (nD), Pr3+ ions concentration (N) and average molecular weight, it was calculated, using standard equations, the polaron radius (rp), the mean inter-ionic distance (ri), the molar volume (Vm), the molar refraction (Rm) and the molar electronic polarizability ( m) [35]. The magnitude of the optical electronegativity () indicates the nature of chemical bonding in the materials. Materials with high values of are considered predominantly ionic while those with low values of Δ are considered predominantly covalent [36, 37]. The calculation of was obtained through the expression
9.8e nD [37]. The second-order refractive index n2 for glasses can be
given, approximately, by
n2 391 1013
nD 1 [38], where is the Abbé number, v5 / 4
(nD 1) (nF nC ) . According to Vogel et al. [39], the third-order nonlinear optical susceptibility
(3) is given by the expression
3
nD n2 . Large values of 3 are important for optical signal 12
processing devices. The values of the physical properties of BPGBPr glass are summarized in Table 1.The values of n2 and 3 are similar to those of other glasses reported in the literature [40] and the low value of indicates predominantly covalent bonding in this glass.
Table 1. Measured and calculated physical properties for BPGBPr glass. Density (g/cm3)
6.541 0.001
Refractive index
nD =1.983 nF = 1.991 nC = 1.973
Tg (C)
354
Tc (C)
521
Tm (°C)
677
Ts(°C)
406
Tc - Tg (C)
167
Abbe number ()
54.611
Concentration(x 1020ions/cm3)
2.264
Optical electronegativity ()
1.349
Polaron radius rp (Å)
6.612
Mean inter-ionic distance ri (Å)
16.40
Molar volume Vm
(cm3/mol)
22.477
Molar refraction Rm (cm3/mol)
11.110
Molar electronic polarizability m (x10-24 cm3)
4.404
Second-order refractive index, n2 (x10-13)
2.589
Third-order nonlinear optical susceptibility 3 (x10-13)
0.136
3.2 Optical absorption and Judd-Ofelt parameters
The room temperature optical absorption spectrum for BPGBPr glass in the visible and near-infrared regions is shown in Fig 2. The spectrum presents four absorption bands in the visible region and four in the infrared, centered at 446, 473, 485, 594, 1009, 1445, 1533 and 1943 nm. These absorption bands correspond to transitions from 3H4 to the excited states 3P2, (3P1,3I6), 3P0, 1D2, 1G4, 3F4, 3F3 and 3F2, and can be used to calculate the spontaneous transition probabilities within the 4f 2 configuration of Pr3+ by means of the Judd-Ofelt theory from the experimental oscillator strengths [41]. Table 2 presents the experimental and calculated oscillator strengths and deviation, rms, for BPGBPr glass both considering and not considering the effect of the hypersensitive transition 3H4→3P2. INSERT FIGURE 2
Table 2. Experimental and calculated oscillator strengths for BPGBPr glass considering and not considering the effect of the hypersensitive transition 3H4→3P0. Transitions 3H4→
Oscillator strength (x10-6)
f i exp
f i cal
f i cal (*)
3
P2
8.48
5.40
3
P1, 3I6
2.37
2.44
2.50
3
P0
2.03
2.40
2.46
1
D2
2.88
1.60
1.35
3
F4
2.85
5.66
4.64
3
F3
8.62
8.85
7.59
3
F2
3.13
3.04
3.20
rms = 2.19 x 10-6
rms = 1.51 x 10-6
(*) calculated without the hypersensitive transition 3H4→3P2. Table 3 shows the J-O parameters obtained for BPGBPr sample compared to values found in the literature for other glass hosts doped with Pr3+ ions. In this work it was used refractive index nD and the reduced matrix elements of the tensor operators following Weber [42]. The obtained values using standard J-O theory are positive, even when considering the hypersensitive transition, and the rms values shown in Table 2 are small for both cases, so there is no need to exclude the hypersensitive transition 3H4→3P2 from the calculations [26,41,43]. It has been assumed that 2 increases with the asymmetry of the vicinity of the rare earth ion and with the degree of covalence of the lanthanide-ligand bonds while 6 decreases with the degree of covalence. The parameter 4 is related to bulk properties of the samples [44]. Comparing the values for 2 shown in Table 3, the results found for BPGBPr are very similar to the values reported for fluoridate and PbO-Al2O3-SiO2 glasses doped with Pr3+ [45,46], and smaller than the values reported for borophosphate, lead telluroborate and LBTAF glasses [47, 48, 17], suggesting a relatively low asymmetry in the environment around Pr3+ ions in BPGBPr glass, consistent with the relatively large values of 6 and obtained in this work. Table 3. J-O parameters obtained for BPGBPr glass compared to values reported in the literature for other glass hosts doped with Pr3+.
-20
Glass composition
2
Judd-Ofelt Parameter (10 cm ) Ω2
Ω4
Ω6
0.70
2.96
7.03
0.67
1.99
3.75
Borophosphate [45] (standard J-O)
3.12
7.21
5.81
Lead telluroborate [46] (modified J-O)
1.19
3.59
6.09
LBTAF [17] (excluding hypersensitive transition)
2.42
2.54
4.23
Fluoridate [47] (normalized J-O)
0.70
5.30
5.00
PbO-Al2O3-SiO2 [48] (standard J-O)
0.70
1.33
5.15
BPGBPr (This work) (standard J-O) BPGBPr (This work) (excluding hypersensitive transition)
The radiative transitions probabilities (Arad), radiative lifetime (rad) of emitting states and branching ratios ( R) for the 3P0 and 1D2 states of Pr3+:B2O3-PbO -GeO2-Bi2O3 glass presented in Table 4 were obtained using the J-O parameters both considering and not considering the hypersensitive transition. The relatively large branching ratios for the transitions 3P0→3H4 (~490 nm) and 1D2→3H4 (~606 nm), together with the 3P0→3H6 (~624 nm) transition, make this glass potentially useful for applications in the visible range. 3.3 Photoluminescence at low and room temperatures Under resonant excitation with the 3P0 level at 488 nm laser radiation, it was observed an efficient emission spectrum extending from orange-red to NIR regions. Figure 3a shows the emission spectrum of the sample in the range from 500 to 900 nm from low to room temperature (16 - 300 K), indicating the corresponding transitions. The inset of Fig. 3a shows the deconvolution of de emission bands at 606 nm and 624 nm. Figure 3b shows the spectrum in the NIR range, from 900 to1300 nm, while the inset shows the spectrum from 1300 to 1700 nm. Figure 4 presents the energy levels of Pr3+ ion. It should be noticed that the level 3P0 is excited directly under pumping at 488 nm but six of the nine emission bands observed are related to transitions from 1D2 level. It is worth mentioning that radiative emission for 3P0→1D2 transition is spin forbidden and, according to the results shown in Table 4, the branching ratio is, in fact, very low for this transition. Chen et al. [49] proposed that the level 1D2 could be populated under excitation at 488 nm by two different ways, as schematized in Fig. 4: one option is via non-radiative decay assisted by phonons directly from 3P0 to 1D2. The energy gap between these two levels is around ~3783 cm-1. Considering that the maximum vibration frequency of BPGB glass is 755 cm-1 (Fig. 1b), 5 phonons would be necessary to bridge this transition. In general, if the energy gap is more than 5 times the energy of the highest energy phonon, phonon assisted non-radiative relaxation process does not occur. The multiphonon relaxation rate (WMPR) for an excited state due to stimulated emission of phonons can be
obtained from the energy gap law given by WMPR = exp[-(E - hf)] [50] where E is the energy difference between the relaxing level and the very next level, hf is the maximum optical phonon energy of the host glass, and and are parameters that depend on the glass. Since the glass composition used in the present work has not been investigated in this context so far, it was assumed the same parameters and as for telluride glass investigated by Layne et al. [51], with phonons of ~760 cm-1, similar to the values measured for BPGB samples. In this context, the multiphonon relaxation rate for BPGB sample is 2 x 10-3 s-1, which is very low compared to other glasses, confirming the low probability of non radiative decay assisted by phonons for 3P0→1D2 transition. The second option to populate 1D2 level is via the cross relaxation (CR) process [3P0, 3H4] → [3H6, 1D2]. As shown in Fig. 4 and Table 4 the energy mismatch between the transitions 3P0→3H6 and 1D2→3H4 is ~500 cm-1 and, therefore, the CR process is probably more efficient than the multiphonon relaxation process. Chen et al. [49] showed that the origin of red emission (around 600 nm) of Pr 3+ doped LiSrPO4 phosphor upon excitation into the 3P2 level (which is equivalent to excitation into the 3P0 level) is from the 1
D2 level as well as the 3P0. According to Zhou et al. [52], the cross relaxation process is enhanced for 1%
Pr3+ doping compared to lower dopant concentrations for fluorotelluride glasses. INSERT FIGURE 3 INSERT FIGURE 4 Table 4. Emission probabilities Arad (s-1) , branching ratio R (%) and radiative lifetime rad (s) for the BPGBPr glass considering, and not, the contribution of the hypersensitive transition 3H4→3P0. BPGBPr
E (cm ) -1
Transition 3 from P0 →
Arad
Arad (*)
R
R (*)
1
3783
2.580
2.469
0.07
0.01
1
10698
783.421
526.692
2.23
2.26
3
13625
4617.431
3104.286
13.15
13.33
3
15466
3156.145
3020.881
9.00
12.97
3
16000
8686.896
4633.835
24.75
19.89
3
20408
17848.247
11999.329
50.85
51.52
1
6882
334.629
223.207
10.88
11.34
3
9596
417.358
359.853
13.57
18.28
3
10308
78.791
59.639
2.56
3.03
3
11534
390.232
266.430
12.69
13.53
D2 G4 F4 F2 H6 H4
rad
rad (*)
28.49
42.94
325.27
508.12
Transition 1 from D2 → G4 F4 F3 F2
3
12285
357.901
231.861
11.64
11.78
3
14204
19.399
12.231
0.63
0.62
3
16501
1476.027
814.796
48.01
41.40
H6 H5 H4
(*) calculated without the hypersensitive transition 3H4→3P2.
The band at ~1042 nm shown in Fig. 3b is assigned to an overlap of the transitions 1D2→3F3,4 since the energy difference between them in the absorption spectrum are in the same NIR emission range, at 10308 and 9598 cm-1, respectively. The same emission band was observed by Yu et al. [53] in oxyfluoride glass doped with Pr3+ when excited at 585 nm, in resonance with 1D2 level. The relatively low intensity of the band around 650 nm observed in this work compared to values reported in the literature for other glasses could be related to the fact that the hypersensitive transition 3P0→3F2 is strongly influenced by the polarizability and structure of the glass host [54-56]. The intensity of the luminescence spectrum changes slightly from 16 to 300 K, as shown in Figs. 3a and 3b, revealing good stability of the BPGBPr glass up to room temperature. Fig. 3b shows that the intensity of the emission corresponding to 1D2→3F4 slightly increased for lower temperatures. This behavior is consistent with the idea that the level 1D2 is mainly populated via nonresonant energy transfer processes, which are usually strongly dependent on temperature. Figure 5 shows a broad band in the NIR region, extending from 1300 nm up to 1700nm, similar to the one observed by Zhou and Pun [57] for Pr3+ doped bismuth gallate glass excited at 488 nm. The shoulder at ~1345 nm is assigned to 1G4→3H5 transition, important for optical fiber amplifiers operating at the second telecommunication window [58,59]. The bands at ~1397 and 1454 nm correspond to 1D2→1G41 and 1D2→1G4 transitions, according to the absorption spectrum shown in Fig 2, where a splitting of 1D2 level is observed at ~950 and 1025 nm. The broad shoulders at ~1548 and 1598 nm are related to 3F4,3F3→ 3H4 transitions, previously reported in the literature [58,60]. The inset of Fig. 3b exhibits the temperature behavior of this band. According to Varshni [61], a slight red shift of the emission bands should be observed for increasing temperature. However, a small blue shift was observed for the transitions 3P0→3H6, 1D2→3H6 , and 1D2→3F2 for higher temperatures. Jung et al. [62] examined the emission of non-stoichiometric phosphor CaAlSiN3:Eu2+ and they found that the Eu2+ activator site in an Al-rich local environment, corresponding to a smaller average distance between Ca and N, was responsible for a lower energy emission peak (red shift) while the activator site in a Si-rich local environment, corresponding to a longer average distance between Ca and N, was responsible for a higher energy emission peak (blue shift). Therefore, the red or blue shift of the emission band would probably be related to the size of the polyhedron around the Eu2+ activation sites [61,63]. The small blue-shift observed for Pr3+ in this work could be related to a B-rich local environment.
INSERT FIGURE 5
The fluorescence decay profile of the overlapped 3P0→3H6 and 1D2→3H4 transitions in the orange-red region under excitation at 488 nm is shown in Fig. 6. According to Zhang et al. [56], generally there is only one broad emission band of Pr3+ doped glasses centered at 600 nm, assigned to 1D2→3H4 transition. However, they observed for Pr3+ doped fluorotelluride glass that this broad emission band consists of two peaks at 602 and 615 nm, respectively, very similar to the result shown in Fig. 3a. According to them, the 1
D2 emission quenches much faster than those of 3P0 due to the cross-relaxation between 1D2→1G4 and
3
H4→3F4 transitions (shown in Fig. 4). The energy gap between 1D2→1G4 matches closely with 3H4→3F4,
increasing the probability of cross-relaxation process to occur. Since the gap between 602 and 615 nm (1D2→3H4 and 3P0→3H6 transitions) is quite small, it is extremely difficult to distinguish the two emission peaks during the fluorescence decay measurements and, therefore, the lifetime of 1D2 could not be measured accurately. Therefore, the average lifetime was estimated using second order exponential fitting [64]. The value obtained from the fit of the experimental data was exp = 22.04 s, while the radiative lifetime obtained from J-O theory was rad = 28.49 s taking into account the hypersensitive transition and 42.94 s without taking it into account. The agreement is considerably better if the contribution of the hypersensitive transition is considered. In this case, the fluorescence quantum efficiency for the 3P0 level (considering the influence of 1D2 level) is = exp/rad = 0.77, which is large compared with those of other hosts: for lead-indium-phosphate glasses with 1.0 wt.% of Pr2O3, = 0.55 [65] and for SrAl12O19 crystal doped with a small Pr3+ concentration (0.1%), = 0.31 [66]. INSERT FIGURE 6 Table 5 presents the stimulated emission cross section
( p )( J , J ' ) at room temperature for transition
between the J and J' states for the emission bands observed in this work, where λp is the wavelength corresponding to the peak and
Δeff is the effective line width calculated by the ratio between the area
under the emission spectrum and its average height.
Table 5 - Stimulated emission cross section for BPGBPr glass in the VIS and NIR regions. Transitions
p (nm)
Δeff (nm)
1
D2→3H4
p J , J 1020
606
18.47
0.37
3
3
624
18.26
2.43
3
3
649
7.51
2.51
1
3
870
34.48
0.22
1
3
1042
58.06
0.29
1
1
1450
140.26
0.36
P0→ H6 P0→ F2 D2→ F2 1
3
D2→ F4/ G4→ H4 D2→ G4
The larger values obtained for the stimulated emission cross sections correspond to transitions 1D2→3H4, 3
P0→3H6, 3P0→3F2 and 1D2→1G4. Therefore, the BPGBPr glass is a potential candidate for applications
requiring orange-red emissions at 606, 624 and 649 nm as well as for NIR emission at 1042 and 1450 nm. The band at 1450 nm , shown in detail in Fig. 5, has a large FWHM of ~140 nm under 488 excitation compared to other glass hosts [21,67] and covers the E-, S-, C- and L- communication band edges, which make it potentially useful for broad band signal amplification applications.
4. Conclusions 3+
Pr -doped B2O3-PbO-GeO2-Bi2O3 glass was obtained by conventional melt quenching technique. The Judd-Ofelt intensity parameters were calculated, as well as, the radiative transition rates, branching ratios, radiative lifetimes and stimulated emission cross-sections. Multichannel transition emissions were observed in the VIS and NIR regions. The broadband centered at 1450 nm presents FWHM ~140 nm under 488 nm excitation, which may be useful for broadband signal amplification applications. The quantum efficiency was determined to be 0.77 for the visible range.
Acknowledgements The authors would like to thank the Brazilian agencies CAPES, CNPq, and FAPERGS for financial support of this work.
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Figure Captions Figure 1 - (a) X-ray diffraction, (b) Raman spectrum and (c) differential thermal analysis of the BPGBPr sample.
Figure 2 - Absorption spectrum of the BPGBPr sample in the visible range, with the identification of the transition levels. The inset shows the spectrum in the NIR region.
Figure 3 - (a) Luminescence spectrum of the BPGBPr sample in the visible range measured at low and room temperature. The transition levels are identified. The inset shows the deconvolution of the bands related to 1D2 3H4 and 3P0 3H6 transitions. (b) Luminescence spectrum in the NIR range as a function of temperature. The inset shows the spectrum of the broad band centered at ~1450 nm as a function of temperature. The excitation was at 488 nm.
Figure 4 - Schematic representation of the energy levels of Pr3+ and of the cross relaxation process (CR).
Figure 5 - Broad luminescence band observed in the NIR region and the communication band edges.
Figure 6 - Fluorescence decay profile of the 3P0 level under excitation at 488 nm.
PII: DOI: Reference:
S0022-2313(16)30437-9 http://dx.doi.org/10.1016/j.jlumin.2016.08.019 LUMIN14180
To appear in: Journal of Luminescence Received date: 5 April 2016 Revised date: 3 August 2016 Accepted date: 8 August 2016 Cite this article as: Alvaro Herrera, Carlos Jacinto, Ariel R. Becerra, Paulo L. Franzen and Naira M. Balzaretti, Multichannel emission from Pr3+ doped heavymetal oxide glass B2O3-PbO-GeO2-Bi2O3 for broadband signal amplification, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.08.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Multichannel emission from Pr3+ doped heavy-metal oxide glass B2O3PbO-GeO2-Bi2O3 for broadband signal amplification Alvaro Herreraa, Carlos Jacintob, Ariel R. Becerrac, Paulo L. Franzena, Naira M. Balzarettia* a
Instituto de Física, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500 - Caixa Postal 15051 - CEP 91501-970, Porto Alegre - RS, Brazil b
Grupo de Fotônica e Fluidos Complexos, Instituto de Física, Universidade Federal de Alagoas, 57072900, Maceió - AL, Brazil c
Grupo Integrar, Departamento de Física, Universidad de Pamplona, Ciudad Universitaria, Colombia
* corresponding author: [email protected]
Abstract Pr2O3 doped 26.66B2O3-52.33PbO-16GeO2-4Bi2O3-1Pr2O3 (BPGBPr) glass was synthesized by melt quenching technique. X-ray diffraction confirmed the amorphous nature of the glass and the presence of different vibrational groups was identified by Raman spectroscopy. Differential thermal analysis indicated that the glass transition and crystallization temperature were ~354C and ~521C respectively, indicating good thermal stability. Visible and near infrared absorption spectra were measured and used to evaluate the Judd-Ofelt intensity parameters to calculate the radiative properties for the emission levels of Pr3+. Photoluminescence spectra were recorded in the visible and infrared regions at temperatures between 16 and 300 K. The spectroscopic results indicated that BPGBPr can be useful as a material for broadband optical amplifier in the region of ~1450 cm-1. keywords: heavy metal oxide glasses; praseodymium; Judd Ofelt theory; photoluminescence; NIR broadband
1. Introduction Spectroscopic properties of rare earth (RE) ion doped glasses are attractive for the development of many optoelectronic devices such as lasers and amplifiers, optical sensors, high density memories and so on [16]. The optical properties of RE ions, specially the luminescence efficiency, are strongly influenced by the structural environment and phonon energies (PE) of the host glass [7]. For example, glasses with low PE can provide high fluorescence quantum efficiency, which is important for lasers and optical fiber amplifiers, but most of the hosts with low PE have, for instance, low thermal and mechanical stability [8]. Glasses containing heavy metals, on the other hand, have been studied for their promising applications in optoelectronic devices due to its low PE (~800 cm-1) and large refractive index (~2.0) compared to silicates, borates and phosphates glasses [9–11]. In particular, B2O3-PbO-GeO2-Bi2O3 (BPGB) system exhibits high chemical and good thermal stability, and it is transparent in the visible and infrared regions, being suitable for RE doping for different applications in optoelectronics [12–14].
Co-doping with different RE ions have been used for improving the efficiency of solid state lasers and luminescent solar concentrators, as well as, to get broader bandwidth for network traffic [1,15]. However, according to Guan et al. [16], it is difficult to find the best concentration of RE ions which is suitable to control the energy transfer between them and to achieve the super broadband emission.. Among RE ions, Pr3+ has important luminescence features due to the 4f2 configuration with 91 degenerate energy levels [17] given rise to multichannel emission in the VIS and NIR regions. Different glass families doped with Pr3+ have been studied for applications in solid state lasers, optical amplifiers, optical temperature sensors and optical fibers operating at the E-S-L bands [18–21]. For certain glass systems Pr3+ presents four level laser action associated with the transitions from 3P0 in the visible region [22]. The standard Judd–Ofelt theory (J-O) has been used successfully in the characterization of radiative transitions of RE doped glasses [23,24]. However it is known that the application of J-O theory to Pr3+ ion doped in some host glasses leads to negative values of the phenomenological 2 intensity parameter due to the small energy difference between the ground state configuration 4f2 and the first excited state configuration of 4f15d1 [25,26]. To overcome this problem, the hypersensitive 3H4→3P2 transition has not been taken into account for calculating J-O parameters or modifications of the conventional J-O theory have been used [27,28] . In the present work, glass with composition 26.66B2O3-52.33PbO-16GeO2-4Bi2O3-1Pr2O3, hereafter named as BPGBPr, was synthesized and their structural and optical properties were investigated. Photoluminescence (PL) was studied in the visible and NIR regions under excitation at 488 nm as a function of temperature. The obtained results indicate the potential application of this system for broadband infrared emission in the E-, S-, C- and L-bands.
2. Experimental The glass sample with composition 26.66B2O3-52.33PbO-16GeO2-4Bi2O3-1Pr2O3 was prepared by the traditional melt quenching technique. The starting materials B 2O3 and Bi2O3 from Alfa-Aesar (99.99%), and GeO2, PbO, and Pr2O3 from Sigma Aldrich (99.99%), were mixed in an agate mortar for 30 minutes and heated at 1250C for 1 h in a Pt crucible. The melt was poured onto a brass mold preheated at 330°C to avoid thermal shock and annealed for 2 h below the glass transition temperature to release mechanical stress. Finally, the glass was slowly cooled down to room temperature, cut and polished for optical measurements. The amorphous nature of the sample was confirmed by X-ray powder diffraction using a diffractometer Siemens model D500. The thermal stability was verified by DTA (Differential Thermal Analysis) using a Shimadzu analyzer DTA 50 in the temperature range 20 -700 °C at a rate of 10°C/min, under Ar atmosphere. The refractive index (nD, nF and nC) was measured using a Sopra GES- 5E ellipsometer, where nD, nF and nC are the linear refractive index at the standard wavelengths 587.56, 461.3, and 656.27 nm, respectively. The density was calculated by the Archimedes method using an analytical balance Shimadzu AUW220D (0.1mg/0.01 mg) and distilled water as the immersion liquid. Room temperature absorption spectra in the UV-VIS-NIR region were recorded using high-performance spectrometer PerkinElmer LAMBDA 1050 with a spectral bandwidth resolution of 0.05 nm. Raman spectra were recorded in a Horiba Jobin Yvon
iHR320 system using a 632.8 nm He-Ne excitation laser and a liquid nitrogen CCD detector. Photoluminescence (PL) spectra were measured in a CryLas system with a CW excitation laser operating at 488 nm. The PL signal was dispersed by an Acton SP2300 monochromator and detected by a Pixis 256E CCD. The measurements were performed in an optical He closed cycle cryostat enabling PL measurements in the range of 4.5-300 K. The decay time was measured using an electro optic modulator to pulse the CW laser. The signal was dispersed by a SP2300 monochromator and detected by a Hamamatsu R955 photomultiplier tube. The detected signal was fed to a SR430 multichannel analyzer and transferred to a computer running acquisition software.
3. Results and discussions
3.1 Structural characterization The X-ray diffraction pattern, Raman spectrum and DTA curve for BPGBPr glass are shown, respectively, in Fig 1a, 1b and 1c. XRD exhibits a broad diffuse scattering at small angles, confirming a long range structural disorder characteristic of amorphous network. DTA curve shows the temperatures corresponding to the glass transition (Tg), softening (Ts), crystallization (Tc) and melting (Tm), listed in Table 1. The thermal stability parameter defined as T = Tc - Tg gives information about the stability of the glass against crystallization [29]. A glass composition is considered thermally stable when this parameter is larger than 100ºC [30], which is the case BPGBPr sample (T =167 °C). Raman spectrum for the BPGBPr glass exhibits a low intensity band at ~552 cm-1 related to the stretching of the symmetric Ge-O-Ge bond [31] and the band at ~755 cm-1 is related to the symmetric Ge-O bond [31,32]. The bands at ~ 922 and ~1040 cm-1 are assigned to ortho, penta and diborate groups [31–33] . The broad band at ~1292 cm-1 is related to BO2O- connected to BO4 units [12,31,33]. The most intense band is at 755 cm1, which corresponds to a low PE compared to the most intense band of silicate systems, which is at ~1150 cm-1 [34]. The low PE of the BPGBPr glass may be suitable for enhancing luminescence of Pr3+ ions.
INSERT FIGURE 1 From the values of density (), refractive index (nD), Pr3+ ions concentration (N) and average molecular weight, it was calculated, using standard equations, the polaron radius (rp), the mean inter-ionic distance (ri), the molar volume (Vm), the molar refraction (Rm) and the molar electronic polarizability ( m) [35]. The magnitude of the optical electronegativity () indicates the nature of chemical bonding in the materials. Materials with high values of are considered predominantly ionic while those with low values of Δ are considered predominantly covalent [36, 37]. The calculation of was obtained through the expression
9.8e nD [37]. The second-order refractive index n2 for glasses can be
given, approximately, by
n2 391 1013
nD 1 [38], where is the Abbé number, v5 / 4
(nD 1) (nF nC ) . According to Vogel et al. [39], the third-order nonlinear optical susceptibility
(3) is given by the expression
3
nD n2 . Large values of 3 are important for optical signal 12
processing devices. The values of the physical properties of BPGBPr glass are summarized in Table 1.The values of n2 and 3 are similar to those of other glasses reported in the literature [40] and the low value of indicates predominantly covalent bonding in this glass.
Table 1. Measured and calculated physical properties for BPGBPr glass. Density (g/cm3)
6.541 0.001
Refractive index
nD =1.983 nF = 1.991 nC = 1.973
Tg (C)
354
Tc (C)
521
Tm (°C)
677
Ts(°C)
406
Tc - Tg (C)
167
Abbe number ()
54.611
Concentration(x 1020ions/cm3)
2.264
Optical electronegativity ()
1.349
Polaron radius rp (Å)
6.612
Mean inter-ionic distance ri (Å)
16.40
Molar volume Vm
(cm3/mol)
22.477
Molar refraction Rm (cm3/mol)
11.110
Molar electronic polarizability m (x10-24 cm3)
4.404
Second-order refractive index, n2 (x10-13)
2.589
Third-order nonlinear optical susceptibility 3 (x10-13)
0.136
3.2 Optical absorption and Judd-Ofelt parameters
The room temperature optical absorption spectrum for BPGBPr glass in the visible and near-infrared regions is shown in Fig 2. The spectrum presents four absorption bands in the visible region and four in the infrared, centered at 446, 473, 485, 594, 1009, 1445, 1533 and 1943 nm. These absorption bands correspond to transitions from 3H4 to the excited states 3P2, (3P1,3I6), 3P0, 1D2, 1G4, 3F4, 3F3 and 3F2, and can be used to calculate the spontaneous transition probabilities within the 4f 2 configuration of Pr3+ by means of the Judd-Ofelt theory from the experimental oscillator strengths [41]. Table 2 presents the experimental and calculated oscillator strengths and deviation, rms, for BPGBPr glass both considering and not considering the effect of the hypersensitive transition 3H4→3P2. INSERT FIGURE 2
Table 2. Experimental and calculated oscillator strengths for BPGBPr glass considering and not considering the effect of the hypersensitive transition 3H4→3P0. Transitions 3H4→
Oscillator strength (x10-6)
f i exp
f i cal
f i cal (*)
3
P2
8.48
5.40
3
P1, 3I6
2.37
2.44
2.50
3
P0
2.03
2.40
2.46
1
D2
2.88
1.60
1.35
3
F4
2.85
5.66
4.64
3
F3
8.62
8.85
7.59
3
F2
3.13
3.04
3.20
rms = 2.19 x 10-6
rms = 1.51 x 10-6
(*) calculated without the hypersensitive transition 3H4→3P2. Table 3 shows the J-O parameters obtained for BPGBPr sample compared to values found in the literature for other glass hosts doped with Pr3+ ions. In this work it was used refractive index nD and the reduced matrix elements of the tensor operators following Weber [42]. The obtained values using standard J-O theory are positive, even when considering the hypersensitive transition, and the rms values shown in Table 2 are small for both cases, so there is no need to exclude the hypersensitive transition 3H4→3P2 from the calculations [26,41,43]. It has been assumed that 2 increases with the asymmetry of the vicinity of the rare earth ion and with the degree of covalence of the lanthanide-ligand bonds while 6 decreases with the degree of covalence. The parameter 4 is related to bulk properties of the samples [44]. Comparing the values for 2 shown in Table 3, the results found for BPGBPr are very similar to the values reported for fluoridate and PbO-Al2O3-SiO2 glasses doped with Pr3+ [45,46], and smaller than the values reported for borophosphate, lead telluroborate and LBTAF glasses [47, 48, 17], suggesting a relatively low asymmetry in the environment around Pr3+ ions in BPGBPr glass, consistent with the relatively large values of 6 and obtained in this work. Table 3. J-O parameters obtained for BPGBPr glass compared to values reported in the literature for other glass hosts doped with Pr3+.
-20
Glass composition
2
Judd-Ofelt Parameter (10 cm ) Ω2
Ω4
Ω6
0.70
2.96
7.03
0.67
1.99
3.75
Borophosphate [45] (standard J-O)
3.12
7.21
5.81
Lead telluroborate [46] (modified J-O)
1.19
3.59
6.09
LBTAF [17] (excluding hypersensitive transition)
2.42
2.54
4.23
Fluoridate [47] (normalized J-O)
0.70
5.30
5.00
PbO-Al2O3-SiO2 [48] (standard J-O)
0.70
1.33
5.15
BPGBPr (This work) (standard J-O) BPGBPr (This work) (excluding hypersensitive transition)
The radiative transitions probabilities (Arad), radiative lifetime (rad) of emitting states and branching ratios ( R) for the 3P0 and 1D2 states of Pr3+:B2O3-PbO -GeO2-Bi2O3 glass presented in Table 4 were obtained using the J-O parameters both considering and not considering the hypersensitive transition. The relatively large branching ratios for the transitions 3P0→3H4 (~490 nm) and 1D2→3H4 (~606 nm), together with the 3P0→3H6 (~624 nm) transition, make this glass potentially useful for applications in the visible range. 3.3 Photoluminescence at low and room temperatures Under resonant excitation with the 3P0 level at 488 nm laser radiation, it was observed an efficient emission spectrum extending from orange-red to NIR regions. Figure 3a shows the emission spectrum of the sample in the range from 500 to 900 nm from low to room temperature (16 - 300 K), indicating the corresponding transitions. The inset of Fig. 3a shows the deconvolution of de emission bands at 606 nm and 624 nm. Figure 3b shows the spectrum in the NIR range, from 900 to1300 nm, while the inset shows the spectrum from 1300 to 1700 nm. Figure 4 presents the energy levels of Pr3+ ion. It should be noticed that the level 3P0 is excited directly under pumping at 488 nm but six of the nine emission bands observed are related to transitions from 1D2 level. It is worth mentioning that radiative emission for 3P0→1D2 transition is spin forbidden and, according to the results shown in Table 4, the branching ratio is, in fact, very low for this transition. Chen et al. [49] proposed that the level 1D2 could be populated under excitation at 488 nm by two different ways, as schematized in Fig. 4: one option is via non-radiative decay assisted by phonons directly from 3P0 to 1D2. The energy gap between these two levels is around ~3783 cm-1. Considering that the maximum vibration frequency of BPGB glass is 755 cm-1 (Fig. 1b), 5 phonons would be necessary to bridge this transition. In general, if the energy gap is more than 5 times the energy of the highest energy phonon, phonon assisted non-radiative relaxation process does not occur. The multiphonon relaxation rate (WMPR) for an excited state due to stimulated emission of phonons can be
obtained from the energy gap law given by WMPR = exp[-(E - hf)] [50] where E is the energy difference between the relaxing level and the very next level, hf is the maximum optical phonon energy of the host glass, and and are parameters that depend on the glass. Since the glass composition used in the present work has not been investigated in this context so far, it was assumed the same parameters and as for telluride glass investigated by Layne et al. [51], with phonons of ~760 cm-1, similar to the values measured for BPGB samples. In this context, the multiphonon relaxation rate for BPGB sample is 2 x 10-3 s-1, which is very low compared to other glasses, confirming the low probability of non radiative decay assisted by phonons for 3P0→1D2 transition. The second option to populate 1D2 level is via the cross relaxation (CR) process [3P0, 3H4] → [3H6, 1D2]. As shown in Fig. 4 and Table 4 the energy mismatch between the transitions 3P0→3H6 and 1D2→3H4 is ~500 cm-1 and, therefore, the CR process is probably more efficient than the multiphonon relaxation process. Chen et al. [49] showed that the origin of red emission (around 600 nm) of Pr 3+ doped LiSrPO4 phosphor upon excitation into the 3P2 level (which is equivalent to excitation into the 3P0 level) is from the 1
D2 level as well as the 3P0. According to Zhou et al. [52], the cross relaxation process is enhanced for 1%
Pr3+ doping compared to lower dopant concentrations for fluorotelluride glasses. INSERT FIGURE 3 INSERT FIGURE 4 Table 4. Emission probabilities Arad (s-1) , branching ratio R (%) and radiative lifetime rad (s) for the BPGBPr glass considering, and not, the contribution of the hypersensitive transition 3H4→3P0. BPGBPr
E (cm ) -1
Transition 3 from P0 →
Arad
Arad (*)
R
R (*)
1
3783
2.580
2.469
0.07
0.01
1
10698
783.421
526.692
2.23
2.26
3
13625
4617.431
3104.286
13.15
13.33
3
15466
3156.145
3020.881
9.00
12.97
3
16000
8686.896
4633.835
24.75
19.89
3
20408
17848.247
11999.329
50.85
51.52
1
6882
334.629
223.207
10.88
11.34
3
9596
417.358
359.853
13.57
18.28
3
10308
78.791
59.639
2.56
3.03
3
11534
390.232
266.430
12.69
13.53
D2 G4 F4 F2 H6 H4
rad
rad (*)
28.49
42.94
325.27
508.12
Transition 1 from D2 → G4 F4 F3 F2
3
12285
357.901
231.861
11.64
11.78
3
14204
19.399
12.231
0.63
0.62
3
16501
1476.027
814.796
48.01
41.40
H6 H5 H4
(*) calculated without the hypersensitive transition 3H4→3P2.
The band at ~1042 nm shown in Fig. 3b is assigned to an overlap of the transitions 1D2→3F3,4 since the energy difference between them in the absorption spectrum are in the same NIR emission range, at 10308 and 9598 cm-1, respectively. The same emission band was observed by Yu et al. [53] in oxyfluoride glass doped with Pr3+ when excited at 585 nm, in resonance with 1D2 level. The relatively low intensity of the band around 650 nm observed in this work compared to values reported in the literature for other glasses could be related to the fact that the hypersensitive transition 3P0→3F2 is strongly influenced by the polarizability and structure of the glass host [54-56]. The intensity of the luminescence spectrum changes slightly from 16 to 300 K, as shown in Figs. 3a and 3b, revealing good stability of the BPGBPr glass up to room temperature. Fig. 3b shows that the intensity of the emission corresponding to 1D2→3F4 slightly increased for lower temperatures. This behavior is consistent with the idea that the level 1D2 is mainly populated via nonresonant energy transfer processes, which are usually strongly dependent on temperature. Figure 5 shows a broad band in the NIR region, extending from 1300 nm up to 1700nm, similar to the one observed by Zhou and Pun [57] for Pr3+ doped bismuth gallate glass excited at 488 nm. The shoulder at ~1345 nm is assigned to 1G4→3H5 transition, important for optical fiber amplifiers operating at the second telecommunication window [58,59]. The bands at ~1397 and 1454 nm correspond to 1D2→1G41 and 1D2→1G4 transitions, according to the absorption spectrum shown in Fig 2, where a splitting of 1D2 level is observed at ~950 and 1025 nm. The broad shoulders at ~1548 and 1598 nm are related to 3F4,3F3→ 3H4 transitions, previously reported in the literature [58,60]. The inset of Fig. 3b exhibits the temperature behavior of this band. According to Varshni [61], a slight red shift of the emission bands should be observed for increasing temperature. However, a small blue shift was observed for the transitions 3P0→3H6, 1D2→3H6 , and 1D2→3F2 for higher temperatures. Jung et al. [62] examined the emission of non-stoichiometric phosphor CaAlSiN3:Eu2+ and they found that the Eu2+ activator site in an Al-rich local environment, corresponding to a smaller average distance between Ca and N, was responsible for a lower energy emission peak (red shift) while the activator site in a Si-rich local environment, corresponding to a longer average distance between Ca and N, was responsible for a higher energy emission peak (blue shift). Therefore, the red or blue shift of the emission band would probably be related to the size of the polyhedron around the Eu2+ activation sites [61,63]. The small blue-shift observed for Pr3+ in this work could be related to a B-rich local environment.
INSERT FIGURE 5
The fluorescence decay profile of the overlapped 3P0→3H6 and 1D2→3H4 transitions in the orange-red region under excitation at 488 nm is shown in Fig. 6. According to Zhang et al. [56], generally there is only one broad emission band of Pr3+ doped glasses centered at 600 nm, assigned to 1D2→3H4 transition. However, they observed for Pr3+ doped fluorotelluride glass that this broad emission band consists of two peaks at 602 and 615 nm, respectively, very similar to the result shown in Fig. 3a. According to them, the 1
D2 emission quenches much faster than those of 3P0 due to the cross-relaxation between 1D2→1G4 and
3
H4→3F4 transitions (shown in Fig. 4). The energy gap between 1D2→1G4 matches closely with 3H4→3F4,
increasing the probability of cross-relaxation process to occur. Since the gap between 602 and 615 nm (1D2→3H4 and 3P0→3H6 transitions) is quite small, it is extremely difficult to distinguish the two emission peaks during the fluorescence decay measurements and, therefore, the lifetime of 1D2 could not be measured accurately. Therefore, the average lifetime was estimated using second order exponential fitting [64]. The value obtained from the fit of the experimental data was exp = 22.04 s, while the radiative lifetime obtained from J-O theory was rad = 28.49 s taking into account the hypersensitive transition and 42.94 s without taking it into account. The agreement is considerably better if the contribution of the hypersensitive transition is considered. In this case, the fluorescence quantum efficiency for the 3P0 level (considering the influence of 1D2 level) is = exp/rad = 0.77, which is large compared with those of other hosts: for lead-indium-phosphate glasses with 1.0 wt.% of Pr2O3, = 0.55 [65] and for SrAl12O19 crystal doped with a small Pr3+ concentration (0.1%), = 0.31 [66]. INSERT FIGURE 6 Table 5 presents the stimulated emission cross section
( p )( J , J ' ) at room temperature for transition
between the J and J' states for the emission bands observed in this work, where λp is the wavelength corresponding to the peak and
Δeff is the effective line width calculated by the ratio between the area
under the emission spectrum and its average height.
Table 5 - Stimulated emission cross section for BPGBPr glass in the VIS and NIR regions. Transitions
p (nm)
Δeff (nm)
1
D2→3H4
p J , J 1020
606
18.47
0.37
3
3
624
18.26
2.43
3
3
649
7.51
2.51
1
3
870
34.48
0.22
1
3
1042
58.06
0.29
1
1
1450
140.26
0.36
P0→ H6 P0→ F2 D2→ F2 1
3
D2→ F4/ G4→ H4 D2→ G4
The larger values obtained for the stimulated emission cross sections correspond to transitions 1D2→3H4, 3
P0→3H6, 3P0→3F2 and 1D2→1G4. Therefore, the BPGBPr glass is a potential candidate for applications
requiring orange-red emissions at 606, 624 and 649 nm as well as for NIR emission at 1042 and 1450 nm. The band at 1450 nm , shown in detail in Fig. 5, has a large FWHM of ~140 nm under 488 excitation compared to other glass hosts [21,67] and covers the E-, S-, C- and L- communication band edges, which make it potentially useful for broad band signal amplification applications.
4. Conclusions 3+
Pr -doped B2O3-PbO-GeO2-Bi2O3 glass was obtained by conventional melt quenching technique. The Judd-Ofelt intensity parameters were calculated, as well as, the radiative transition rates, branching ratios, radiative lifetimes and stimulated emission cross-sections. Multichannel transition emissions were observed in the VIS and NIR regions. The broadband centered at 1450 nm presents FWHM ~140 nm under 488 nm excitation, which may be useful for broadband signal amplification applications. The quantum efficiency was determined to be 0.77 for the visible range.
Acknowledgements The authors would like to thank the Brazilian agencies CAPES, CNPq, and FAPERGS for financial support of this work.
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Figure Captions Figure 1 - (a) X-ray diffraction, (b) Raman spectrum and (c) differential thermal analysis of the BPGBPr sample.
Figure 2 - Absorption spectrum of the BPGBPr sample in the visible range, with the identification of the transition levels. The inset shows the spectrum in the NIR region.
Figure 3 - (a) Luminescence spectrum of the BPGBPr sample in the visible range measured at low and room temperature. The transition levels are identified. The inset shows the deconvolution of the bands related to 1D2 3H4 and 3P0 3H6 transitions. (b) Luminescence spectrum in the NIR range as a function of temperature. The inset shows the spectrum of the broad band centered at ~1450 nm as a function of temperature. The excitation was at 488 nm.
Figure 4 - Schematic representation of the energy levels of Pr3+ and of the cross relaxation process (CR).
Figure 5 - Broad luminescence band observed in the NIR region and the communication band edges.
Figure 6 - Fluorescence decay profile of the 3P0 level under excitation at 488 nm.