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Journal of Alloys and Compounds 538 (2012) 220–223
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Specific features of fluorescence kinetics of Pr+3 doped BiB3O6 glasses L.R. Jaroszewicz a, A. Majchrowski a, M.G. Brik c, N. AlZayed e, W. Kuznik b,c, I.V. Kityk d,⇑, S. Kłosowicz a a
Institute of Applied Physics, Military University of Technology, Kaliskiego 2, 00-908 Warsaw, Poland Chemical Department, Silesian University of Technology, Strzody 9, 44-100 Gliwice, Poland c Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia d Electrical Engineering Department, Czestochowa University of Technology, Armii Krajowej 17, Czestochowa, Poland e Physics & Astronomy Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia b
a r t i c l e
i n f o
Article history: Received 2 May 2012 Received in revised form 12 May 2012 Accepted 17 May 2012 Available online 7 June 2012 Keywords: Kinetics of fluorescence Rare earth doped glasses Borate glasses
a b s t r a c t Praseodymium-doped BiB3O6 glass was synthesized and a wide range of its optical properties was studied, including absorption, excitation and high resolution time resolved luminescence spectra. Particular interest was devoted to study of their time decay kinetics of induced fluorescence under influence of the third harmonic Nd:YAG laser illumination (355 nm). Role of borate glass matrices contributing through the electron–phonon interaction to the time-decay kinetics of fluorescence is discussed. Comparison with other glass matrices is performed both following the Judd–Ofelt analysis as well as experimental decay times for principal fluorescence lines. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Studies of the BiB3O6 (BiBO) glasses are of an essential interest due to promising nonlinear optical and optoelectronic optical fibers features [1–3]. Following the Raman data, the principal structural fragments of such glasses are BO3 and BO4 sub-units [4], which possess relatively high phonon frequencies (up to 1600 cm1, giving substantial contribution to electron–phonon anharmonic interactions). Particular interest for nonlinear optics present the BiBO particles doped with the rare earth ions [5]. One can expect that incorporation of the rare earth ion fluorescent chromophore to glass leads to more effective overlapping between the localized d–f rare earth states and the surrounding ligands with respect to the crystals [6]. The main reason for that is the partial disorder, which in turn effectively interacts with the inserted rare earth ions through the high frequency phonons due to presence of light borate complexes. Particularly important are the third order tensor anharmonic interactions. Pr3+ ions are of a special interest for the borates due to the high efficiency of their interactions with the matrices [7,8]; the disorder in the surrounding matrix plays here a decisive role. Additionally due to effective interaction with the anharmonic phonon subsystems, such kinds of physical mechanisms may be crucial for transfer of the excited energy and its wavelength transformation. The processes related with the kinetics of the highly localized rare earth’s ions electronic states may play a crucial role in understand⇑ Corresponding author. E-mail addresses:
[email protected],
[email protected] (I.V. Kityk). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.05.071
ing of the impurity-matrix interactions, which is important for the use in the optical fiber technique. In the present work we apply time-resolved fluorescent technique together with the Judd–Ofelt theoretical analysis for characterization of the time kinetics of de-occupation for the particular levels. Comparison of the theoretical and the experimental data will be presented and the origin of the effect will be discussed. 2. Materials and methods 2.1. Glasses preparation and doping Very large viscosity of molten BiBO caused by high molar content of B2O3 [9], making growth of this single crystals difficult, allows on the other hand relatively easy BiBO glass formation. Moreover, during crystallization of BiBO from melts containing rare earth ions, crystals of rare earth borates instead of BiBO doped with these ions are obtained [9]. In case of BiBO glass formation relatively high amounts of RE ions can be introduced into starting melt without forming any unwanted phases. Synthesis of BiBO:Pr glass was carried out with use of stoichiometric amounts of B2O3 (Merck Suprapur), Bi2O3 (Aldrich, 99.999%) and Pr6O11 (Aldrich 99.99%) taking into account that praseodymium ions substituted bismuth ions forming mixture with composition Bi1xPrxB3O6 (x = 0.01; 0.04). Bi2O3 and Pr6O11 were dried before synthesis for 24 h at 500 °C and 1000 °C, respectively. Boron oxide was molten in the first step of the synthesis to secure known composition of glass. The 40 40 mm platinum crucible was used. Water, which is easily absorbed in boron oxide, was removed due to melting of B2O3 and keeping it at elevated temperatures up to 1000 °C for several hours. The mass of B2O3 was found by weighing the crucible. In the next step proper amounts of dried Bi2O3 and Pr6O11 were added to the crucible. The synthesis was carried out at 850 °C. To make the melt homogeneous, a platinum stirrer was used. The stirring was carried out for several hours until the melt became totally transparent. In the final step the melt was poured onto Pt/Au5% glass forming plate and rapidly cooled below 400 °C. Plates were cut out from formed glass with use of wire saw and polished before optical measurements.
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L.R. Jaroszewicz et al. / Journal of Alloys and Compounds 538 (2012) 220–223 2.2. Measurements
BiBO: Pr 4% Photoluminescence (PL) excitation and emission spectra as well as luminescence decay kinetics were examined. A pulsed (pulse duration 10 ns, energy about 1 lJ) third harmonic generation of nanosecond Nd:YAG laser (355 nm) and frequency repetition of 10 kHz was used as the excitation source for measurements of PL emission spectra and decay kinetics. The laser beam was focused in a backscattering geometry (with angle lying about 75–85 degree) onto the area of approx-
1.0
PL [arb.un]
3 H6
3P 0
0.5
1.0
0.6
[μ
s]
20 40 60 80 100 120
m
e
3P 1 3P 0
0.4
550
600
650
Ti
Intensity [arb. un.]
0.0
3P 2
0.8
700
λ [nm] Fig. 3. Time-resolved photoluminescence spectrum of BiBO:Pr 4% glass.
0.2
250
300
350
400
450
500
[λ ]nm
Table 1 JO data of BiBO Pr 1%. Peak
Centre [1/cm] ([nm])
OSexp, in 106
OScalc, in 106
Final electronic state
A B C D E
5181 (1930) 6636 (1507) 9832 (1017) 16924 (591) 21957 (455)
2.84 10.34 0.24 1.80 16.82
2.85 10.18 0.46 2.39 16.82 Rms = 0.456
3
Fig. 1. Excitation spectrum of BiBO:Pr 1% measured at 603 nm.
BiBO: Pr 1%
Absorbance [arb. un.]
2
,
3F 3F 4 3
3P 2
1
F2 F4, 3F3 1 G4 1 D2 3 P0,1,2 3
3F 2
,
3P 3P 1 0 1D 2
1G 4
0 600
800 1000 1200 1400 1600 1800 2000 2200
Table 2 JO data of BiBO Pr 4%. Peak
Centre [1/cm] ([nm])
OSexp, in 106
OScalc, in 106
Final electronic state
A B C D E
5112 (1956) 6638 (1506) 9952 (1005) 16912 (591) 22006 (454)
2.17 5.15 0.22 0.90 8.59
2.17 5.09 0.23 1.18 8.59 Rms = 0.203
3
λ [nm]
F2 F4, 3F3 G4 1 D2 3 P0,1,2 3 1
Fig. 2a. Absorption spectra of the studied glass samples. Baseline-corrected UV–Vis absorption spectrum of BiBO:Pr 1%.
12.5
BiBO: Pr 4%
Absorbance [arb. un.]
4 3
,
3P 2
3F 3F 4 3
,
3P 3P 1 0
3F 2
2 1
1D 2
1G 4
log(PL intensity) [arb.un]
5
BiBO: Pr 1%, τ = 26.3 μs BiBO: Pr 4%, τ = 21.4 μ s
12.0 11.5 11.0 10.5 10.0 9.5 9.0 8.5
0
0
600
800 1000 1200 1400 1600 1800 2000 2200 λ [nm]
Fig. 2b. Baseline-corrected UV–Vis absorption spectrum of BiBO:Pr 4%.
20
40
60
80
100
Time [μs] Fig. 4. Luminescence lifetimes determined from integration of the time-resolved spectra.
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Table 3 Comparison of Judd–Ofelt parameters and luminescent properties of Pr3+ ions in BiBO glass (this work) and other materials. Matrix material/s in [ls], O in [1020cm2]
BiBO glass, this work, 1% Pr
BiBO glass, this work, 4% Pr
GeO2–PbO–PbF2 oxyfluoride glass [15]
La2CaB10O19 crystal [16]
Ca4GdO(BO3)3 crystal1 [8]
MgTP glass [17]
CdS nanoparticles in sol–gel glasses [18]
Li2O–Al2O3– ZrO2–SiO2 glasses [19]
O2 O4 O6 3 P0 calc. Lifetime, ls
9.97 5.37 15.72 12
9.86 2.94 7.41 17
0.66 12.49 3.17 14
8.63 0.97 10.12 28
7.35 1.15 3.09 7.3
2.69 11.87 8.39 14
10.58 7.4 0.95 155
2.34 5.27 29.43 26
imately (with beam diameter about 1 mm) on the surface of a sample. The spectra were recorded by a spectrograph (Andor SR-303i) equipped with an intensified charge coupled device (Andor DH-501) with spectral resolution up to 1 nm. This detector allowed recording the time-resolved PL spectra with time resolution of 5 ns. Decay kinetics was estimated by integration of time-resolved spectra and fitting a single exponential function. The photoluminescence excitation spectra were recorded with a 150 W Hamamatsu xenon lamp (stabilized up to 0.2% by power) with a monochromator (MDR-23, spectral width 2 nm) allowing tuning the wavelength of the source. The Jasco V-570 spectrophotometer was used to record the UV–VIS absorption spectrum.
comparison with other crystalline and glass matrices indicates that the X2 JO parameter is one of the higher among the presented matrices with except of the CdS nanoparticles in sol–gel glasses [18]. The similar behavior is observed for the X4 and X6 JO parameters, however for the case of GeO2–PbO–PbF2 oxyfluoride glass [15], MgTP glass [17] and Li2O–Al2O3–ZrO2–SiO2 glasses [19]. However, generally the efficient JO parameters are indicating a promising use of such materials in different fluorescent optoelectronic devices.
2.3. Judd–Ofelt analysis The Judd–Ofelt parameters were found by modified JO procedure described in Ref. [10], in which the theoretical Oscillator Strengths (OS) are defined as follows:
fthe
" # EJ 0 2E0f 8pmcmv X ¼ 1þ Xk hf N ½c; S; LJkU k kf N ½c0 ; S0 ; L0 J0 i2 3hð2j þ 1Þ k¼2;4;6 Esd þ E0f
where m is the mass of an electron, c is the speed of light, h denotes the Planck constant, v stands for the local field correction v ¼ ðn2 þ 2Þ2 =9n, n is the refractive index of the material, the bra- vector denotes initial state and –ket vector represents the final state. kU k k stands for the reduced matrix elements of the unit tensor operators between the initial and final state of the transition. The terms EJ’, Ef0 and E5d, which are not present in the conventional JO approach, are the energy of the final state, the average energy of all 4f2 states of praseodymium ion and the energy of the lowest 4f15d1 state, respectively. Ef0 was taken as 10000 cm1 [11] and the value of E5d was assumed to be equal to 33000 cm1 [12], while the kU k k matrix was calculated using the free Pr3+ ion Hamiltonian parameters [13,14]. The experimental OS were determined by numerical integration of the absorption peaks assigned to the appropriate transitions. The calculated peak areas were substituted into the following equation, which yields the experimental OS:
fexp ¼ 4:32 109
Z
eðv Þdv
where eðv Þ stands for molar absorption coefficient at given frequency.
3. Results and discussion The UV–Vis absorption, excitation and time-resolved luminescence spectra were recorded (see Figs. 1–3 ). Following Figs. 2 and 3 one can clearly see three strong bands corresponding to the 3P0,1,2 transitions (at wavelength about 500 nm), 3F3,4 (at 1500 nm) and 3F2 (at 1900 nm) (all transition originate from the 3H4 ground state of Pr3+ ions). One can see substantial contribution to the broadening of the observed absorption lines due to the disordered glass matrices; in addition, a clear dependence of the primary and secondary maxima on the dopant’s concentration. The spectral resolution of the peaks indicate that the rare earth ions are likely situated in some fixed local site positions. More details are given in Tables 1 and 2. It was established that the emission lifetimes are strongly concentration-dependent, with s dropping from 26.3 ls (1% of Pr3+ dopant) to 21.4 ls (4% of the dopant) (see Fig. 4). It is in a contradiction with the JO analysis which predicts an increase of the corresponding values of s from 12 ls up to 17 ls. This may be a consequence of substantial contribution of the phonon subsystem to the observed fluorescence time kinetics (see Table 3). Additional reasons may be caused by multi-photon excitation, deformation of the local ligand surrounding coordination, etc.). Moreover, the
4. Conclusions In the present work we apply the time-resolved fluorescent technique together with the Judd–Ofelt theoretical analysis to the BiB3O6 glasses doped with the Pr3+ ions. Following the time kinetics for the particular levels we have established that there exist strong bands corresponding to the 3P0,1,2 transitions (at wavelength about 500 nm), 3F3,4 (at 1500 nm) and 3F2 (at 1900 nm). One can see substantial role to the broadening of the lines due to the disordered glass matrices. The concentration of the dopant ions was shown to affect noticeably the intensities of the principal and secondary absorption maxima. Following the obtained emission spectra some role here is played by anharmonic phonon subsystem (due to asymmetry of the fluorescent shapes). The space homogeneity of the output fluorescence signal was varied not more than 2.4%. The spectral resolution of the peaks indicates that the rare earth ions are likely situated in some fixed local site positions. It was established that the emission lifetimes are strongly concentration-dependent, with s dropping from 26.3 ls (1% of Pr3+ dopant) to 21.4 ls (4% of the dopant); however, the JO analysis predicts an increase from 12 ls up to 17 ls. This may be a consequence of substantial contribution of the phonon subsystem to the observed fluorescence time kinetics. Moreover, the comparison of the obtained JO parameters with other literature data for various crystals and glasses doped with Pr3+ ions shows that in the samples studied in the present work the X2 JO parameter is one of the highest, which may have important consequences on the BiBO:Pr3+ applications. Acknowledgements This work was partially supported by the Polish Ministry of Sciences and Higher Education, Key Project POIG. 01.03.01-14-016/08 ‘‘New Photonic Materials and their Advanced Applications’’. References [1] [2] [3] [4]
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