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Spectroscopic characterization of orthorhombic δ-BiB3O6 phase nonlinear single crystal doped with Pr3+ ions
Author’s Accepted Manuscript Spectroscopic characterization of orthorhombic δBiB3O6 phase nonlinear single crystal doped with Pr3+ ions M. Kowalczyk, M. Kaczkan, A. Majchrowski, M. Malinowski www.elsevier.com/locate/jlumin
PII: DOI: Reference:
S0022-2313(18)31742-3 https://doi.org/10.1016/j.jlumin.2018.11.030 LUMIN16100
To appear in: Journal of Luminescence Received date: 21 September 2018 Revised date: 14 November 2018 Accepted date: 15 November 2018 Cite this article as: M. Kowalczyk, M. Kaczkan, A. Majchrowski and M. Malinowski, Spectroscopic characterization of orthorhombic δ-BiB3O6 phase nonlinear single crystal doped with Pr3+ ions, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.11.030 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.
Spectroscopic characterization of orthorhombic δ-BiB3O6 phase nonlinear single crystal doped with Pr3+ ions
M. Kowalczyka*, M. Kaczkana, A. Majchrowskib, M. Malinowskia a
Institute of Microelectronics and Optoelectronics, Warsaw University of Technology,
Koszykowa 75, 00-662 Warsaw, Poland, b
Institute of Applied Physics, Military University of Technology, Gen. Witolda Urbanowicza
2, 00-908 Warsaw, Poland
*
Corresponding author: [email protected]
Abstract The spectroscopic properties of 1at % Pr3+ doped δ-BiB3O6 (BIBO) single crystal were investigated. The absorption, emission and excitation spectra were studied at low (10 K) and at room temperature, with the emission and absorption spectra showing clear polarization dependence. The strongest observed luminescence band corresponds to the 1D2 → 3H4 transition at 610 nm. Energy levels of Pr3+ in δ-BIBO host were assigned on the basis of low temperature absorption and emission measurements. The number of crystal field levels derived implies that Pr3+ ions are located in one crystal site in the BIBO host. Decay time of the praseodymium 3P0 emission was unusually short and the 1D2 decay was non-exponential. The modified Judd-Ofelt intensity formalism was used to analyze the experimental data. The parameters, branching ratios and electric dipole transition probabilities were determined. The stimulated emission cross-sections for the 5D0 3HJ transitions were also evaluated in this system.
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Keywords: Judd-Ofelt formalism; Pr3+ ions in borate hosts; δ-BiB3O6; Pr3+ luminescence; Pr3+ spectroscopic properties
Introduction
Much effort has gone into developing and studying new rare-earth (RE) activated nonlinear optical materials, in crystalline, glassy, ceramic or nano-powder form, for multifunction photonic sources, efficient second-harmonic generation (SHG) and other parametric processes as well as quantum memories [1–3]. Experimental results show that borate type compounds constitute a valuable source of nonlinear optical crystals with excellent transmission ranges, extending from the ultraviolet (UV) into the infrared (IR) region, good nonlinear properties and good mechanical and chemical stability [4,5]. The analysis of the photo luminescent properties of RE doped BiB3O6 (BIBO) phosphors is rather scarce in the literature, however you can find examples indicating a large potential of these materials in the scope of optoelectronics and laser technology. As far as developed solid state lasers are concerned, the Nd3+ ions doped laser crystals have played a significant role. Monoclinic Nd3+-doped BIBO, as a new potential self-frequency conversion laser crystal was studied by Brenier et al. [6]. The authors indicated that to obtain laser oscillations much higher concentrations of neodymium are needed and that important technological progress is still required to increase this concentration. It was found that much larger concentrations of Nd3+ ions could be obtained in orthorombic -BIBO than in polymorph of bismuth triborate. Spectroscopic properties of Nd3+:-BIBO crystal favor lasing at 1.3 m, where this crystal possesses near non-critical phase matching [7]. Also of note is that crystals doped with triply ionized praseodymium ions (Pr3+) possess the spectral emission lines in the visible (VIS) and the near infrared (NIR) spectral 2
range [8]. Thus, the Pr3+ was used as an activator in a variety of host materials useful in many phosphors and different types of light emitting devices. In particular, the luminescence properties of Pr3+ doped materials were extensively investigated for applications including fiber optical communication [9], LEDs [10], field-emission display devices and photoluminescence devices [11]. It has been shown recently that the Pr3+ ion in oxide crystals facilitates laser operation in the VIS region under semiconductor diode blue laser pumping[12,13]. Furthermore, it is possible to reach the UV part of the spectrum simply by frequency doubling. Therefore, further investigations of Pr3+ doped -BIBO crystalline phase appear to be warranted. The results of spectroscopic studies of single -BIBO crystals were reported in [14] where it was shown that as a result of differences in crystal structure this phase of BIBO can be doped more easily by RE ions in comparison with the -phase. Composites containing nano-crystallites of novel rare earth doped borate -BIBO and polyvinyl alcohol (PVA) polymer were reported in [15]. The results of this study confirmed the role of the Ag-NPs in influencing the third-order susceptibilities. In the work of Plaza and Aragó the properties of BIBO glasses prepared at different thermal conditions were analyzed [16]. Kuznik et al. [17] studied second harmonic generation in praseodymium doped BIBO glass and its dependence on the degree of crystallinity. Comparisons with other glass matrices were performed following both the Judd–Ofelt analysis as well as experimental decay times for principal fluorescence lines in [18]. The goal of the investigations presented here was to study the spectroscopic properties of the new praseodymium system, Pr3+ activated δ-BIBO single crystal, because this system has never been characterized before and then to compare it with other Pr3+ borates.
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Experimental 1. Crystal growth According to the literature orthorhombic δ-BIBO phase is one of three stable polymorphs of BIBO (BiB3O6) that can be obtained in the form of bulk single crystals from stoichiometric melt [19]. The best known monoclinic α-BIBO phase, having strong nonlinear optical properties, can be grown from melt at temperatures above 710°C [20], monoclinic γBIBO phase crystallizes in the temperature range 680-710°C [21], while below 680°C δBIBO single crystals can be grown from strongly supercooled melt [22]. Despite the higher density of the γ-BIBO phase, compared to α-BIBO phase, it can be easily doped with Rare Earth (RE) ions. -BIBO crystallizes in the orthorhombic space group Pca21 and its structure contains BO4 tetrahedra, which share common corners and form layers [23]. Due to the similarity of ionic radii of the Pr3+ and Bi3+ ions (1.179 and 1,17 Å, correspondingly) [24], the only suitable lattice site for this substitution is the Bi3+ site, therefore we synthesized the starting material corresponding to BIBO composition in which the normal amount of Bi3+ ions was replaced with Pr3+ ions (1at.%). BIBO:Pr3+ single crystals were grown by means of Kyropoulos method on [001] oriented seed, similar to the method described by Aleksandrovsky et al. [22]. The growth was carried out in a two-zone resistance furnace enabling low temperature gradients and the seed was rotated at a rate of 5 rpm. No pulling was used. A BIBO:Pr3+ crystal grew in the volume of the melt and was confined with crystallographic faces. It was found that the main face was formed by (100) plane, a detailed description of the morphology of an as-grown δ-BIBO single crystal can be found elsewhere [24].
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2. Experiment The absorption spectrum was taken at room temperature using a Perkin–Elmer Lambda 950 spectrophotometer. Optical measurements were carried out on a single crystal sample in the form of a polished parallelepiped cut perpendicular to the crystallographic axes. The morphology of the obtained Pr3+:-BIBO sample allowed absorption measurements with the light propagation direction along the b axis for two directions of light electric vector, namely, along a and c axes. Emission measurements were performed using a Photon Technology International spectrophotometer. Emission spectra were also measured using a CVI-480 grating monochromator followed by a PMT and SR-400 photon counting system. The samples were excited by a pulsed (10 ns pulse-width, repetition rate 10 Hz) tunable optical parametric oscillator (Continuum) pumped by a frequency-tripled Nd:YAG pulse laser (Continuum Surelite II). Fluorescence dynamics profiles were recorded with a SR-430 multichannel analyzer controlled with a PC computer. Sample cooling was provided by a closed cycle He optical cryostat which allowed the temperature to be varied between 10 and 300 K.
3. Results 3.1. Absorption Absorption spectra of the Pr3+:-BIBO crystal for ~E||a and ~E||c polarizations are shown in Fig. 1. Spectra consist of narrow bands corresponding to f–f transitions in Pr3+ ions, see inset in Fig.1. The absorption edge of the crystal is observed at about 315 nm giving a -BIBO gap of about 3,95 eV which corresponds well with observations presented in[15,23] and is close to the band gap Eg = 4.16 eV reported for Pr3+: -BIBO [14]. The polarized absorption spectra of Pr3+: -BIBO between 435 nm and 605 nm recorded at 10 K are presented in Fig. 2. Five well-distinguished groups of lines are characteristic of transitions within the 4f2 configuration between the lowest crystal field components of ground 3H4 level 5
and crystal field states of the excited 3PJ, 1I6 and 1D2 multiplets. The strong influence of incident light polarization on transition intensities is clearly observed. Polarizations E||c shows higher absorption coefficients, thus optical pumping along crystallographic c axis should be favorable. The analysis of the Stark components of the
2S+1
LJ levels of Pr3+ in
absorption at 10 K were conducted successfully. The energies of Stark levels of the ground 3
H4 multiplet were determined from the analysis of the low temperature emission lines related
to the 1D2→3H4 transition and are displayed in Table 1. The splitting of 6 cm-1 observed in absorption transitions suggests that the first Stark component in the ground 3H4 state has energy of 6 cm-1. Spectroscopic features of Pr3+ is strongly affected by the shape and the size of the coordination environment. -BIBO has sevenfold-coordinated Bi3+ ions [23], see Fig.3. However, according to Cong, et al [25], Bi3+ cations have an irregular nine-fold coordination environment (Bi–O, 2.26–3.03 Å) and the surrounding environment of the Bi3+ is anisotropic due to the existence of its 6s lone-pair electrons, the point symmetry is C2v. In the crystal field of a low symmetry, the Pr3+ levels are split into 2J+1 crystal field components, consequently there are three and five components in the 3P1, and 3P2,1D2 manifolds respectively, which agrees with the data presented in Table 1. It is also assumed that, because of the high coordination number, the local crystal field is weak. A weak crystal field results in a high energy position of the 4f5d Pr3+ configuration, suggesting that it lies above the 1S0 state. Strong polarization behavior of the absorption transitions observed for Pr3+:-BIBO suggests that the effective point symmetry of Pr3+ sites is higher than C1.
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Fig. 1 Absorption spectra of the Pr3+:-BIBO crystal for ~E||a and ~E||c polarizations registered at room temperature.
Fig. 2. Visible part of the absorption spectra of the Pr3+:-BIBO crystal for ~E||a and ~E||c polarizations registered at 10 K.
Fig.3 The coordination geometry of polyhedron around the Bi3+ ion in -BIBO crystal.
Fig. 4. The photoluminescence excitation (PLE) (blue line) and photoluminescence (PL) (red line) spectra of Pr3+:-BIBO at 300 K. Excitation was recorded at the 609.5 nm emission line and emission was recorded under 447 nm excitation.
3.2. Emission As can be seen in Fig.4, the photoluminescence excitation (PLE) spectrum (recorded at 609.5 nm emission, blue line) contains a broad band in the region of 260–380 nm and a series of narrow lines located in the range of 400–500 nm. As reported in [26] the broad bands in question are attributed to the 1S0 → 3P1 absorption transition of Bi3+. This transition is expected to have reasonable oscillator strength through the spin−orbit mixing that takes place between 3P1 and 1P1 [26]. The presence of similar bands, in the approximate region of ~220 - 400 nm, within the spectra of other Bi3+ materials was also reported [27]. The presence of Bi3+ absorption in the excitation spectra of Pr3+, as shown in Fig.4, indicates that some energy transfer from Bi3+ to Pr3+ ions also occurs. The narrow band transitions located above 400 nm belong to the intrinsic f–f transitions of Pr3+ and are attributed to the electronic
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transitions from 3H4 to 3PJ levels. This demonstrates that the investigated phosphor can be efficiently excited by near-ultraviolet and blue light. After excitation of the 3PJ levels by blue-violet radiation at 447 nm, room temperature, a detector response corrected emission spectrum of 1% Pr :BIBO was registered in the visible range and is also shown in Fig.4. The spectrum represents transitions from emitting 3P0 and 1
D2 multiplets to terminal energy (Stark) levels within the 3H4 multiplets. As can be seen from
Fig. 4, the spectrum is dominated by a red transition centered at 610 nm, but also a relatively weak emission originating from the 3P0 level was detected at wavelengths shorter than 600 nm. This red emission is attributed to transitions originating either from the 3P0 or 1D2 levels [28]. To clearly distinguish between the 3P0 and 1D2 lines, selectively excited emission spectra were recorded at 10 K. In Fig.5 part of the emission spectrum in the 600 nm region after selective excitation is shown. No emission from the 3P0→3H6 transition around 610 nm was observed. Thus, from Figs. 4 and 5 it can be concluded that the red emission originates mostly from the excited 1D2 state, populated by the non-radiative relaxation from the 3P0 state. Such fast nonradiative relaxation from 3P0 to 1D2 could be due to multiphonon relaxation, cross relaxation (CR) energy transfer or the charge transfer (CT) mechanism.
Fig.5. Low temperature emission spectrum in the 600 nm region after selective excitation into 3
P1 and 1D2 multiplets.
As the 3P0 and 1D2 levels are separated by about 3500 cm-1, the probability of multiphonon relaxation process is not significant [27]. Therefore, it is concluded that, for populating the 1D2 level and resulting emissions, an energy transfer process is most likely. It was shown that in a number of praseodymium activated solids, concentration quenching of
8
the 3P0 fluorescence dynamics is related to the cross-relaxation (CR) mechanism of the type (3P0, 3H4) (3H6, 1D2). This energy transfer is responsible in most praseodymium compounds for a sharp decrease of the luminescence with increasing activator concentration and a reduction of the 3P0 lifetime. Moreover, it was reported recently that the intervalence charge transfer (IVCT) of metal-to-metal can also provide an efficient quenching channel [28]. Such a situation was observed in Pr3+ doped Bi2ZnOB2O6 microcrystals, where, due to strong 3P0 → 1D2 nonradiative relaxation of Pr3+ ion by low-lying CT states, only red 1D2 emission was detected [29].
3.3. Excited state dynamics Decays originating from the 3P0 and 1D2 levels were registered and are presented in Fig.6. 3P0 decay was measured at 483.8 nm where a resonant luminescence corresponding to the 3P0 3H4 transition occurs after excitation into the 3P2 levels at 447.6 nm. 1D2 decay was measured at 600 nm after direct excitation into the 1D2 levels at 594.7 nm. Non-exponentiality of the 1D2 decay was observed even at the relatively low dopant concentration of 1 at. % and at a low temperature indicating that energy transfer processes strongly contribute to the decay of this luminescent level. In addition, the multiphonon relaxation from the 1D2 to its next lower 1G4 manifold suggests that it could participate in the nonradiative process. Low temperature 3P0 lifetime, determined in 1% Pr3+ doped sample, was 0.08 s when the 1D2 decay time, determined from the long time part of the decay, was 55.3 s which is close to the value of 50 s found in -BIBO crystal [14]. The fluorescence quantum efficiencies were calculated using the expression f/r. The fluorescence quantum efficiencies of the 3P0 and 1D2 manifolds are 1% and 57% respectively. The main reason for the very low quantum efficiency of the 3P0 manifold is a high relaxation rate from the 3P0 to the 1D2 manifolds. A similar situation was found in several 9
praseodymium activated borates like Pr3+:YAl3(BO3)4 [30] and Pr3+:Sr3Y2(BO3)4 [31]. The low fluorescence quantum efficiency of the 1D2 manifold is due to the multiphonon relaxation and cross relaxation which is confirmed by the nonexponential character of this decay.
Fig.6 Low temperature fluorescence decay curves of Pr3+ :-BIBO crystal plotted in semilog scale. The Judd-Ofelt (J–O) theory [32,33] is a well-recognized method for analysis and comparison of the spectroscopic properties of RE3+ ions in dielectric matrixes. The experimental absorption oscillator strength fexp for transitions from the ground state 3H4 to the excited states of Pr3+ can be determined from the room temperature absorption spectrum using the expression:
∫
(1)
where i is the integrated absorption coefficient of the ith absorption band and is the Pr3+ concentration in [ions/cm3]. Because of the crystal morphology we used the J-O analysis of absorption spectra for two polarizations. The oscillator strengths of the electric dipole transitions can be obtained theoretically from three sets of J-O intensity parameters [32,33]. According to the modified J-O theory [34,35], for the case of a strong mixture between the 4f2 and 4f5d state, which is also the case of Pr3+ ion, the calculated oscillator strength of the electric dipole transition between two states can by expressed as:
(
)
∑ [
]
10
〈
[
] ‖
‖
[
] 〉
(2)
The reduced matrix elements used in our calculations were taken from the work of Carnall et al. [36]. The terms EJ’, Ef0 and E5d, which are not present in the conventional JO approach [32,33], 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 cm-1 [37] and E5d was set as 47393 cm-1 [38]. The phenomenological J-O parameters for Pr3+ ions are found by the least squares fitting between fexp and fcalc. During the calculations the deviation was minimized by the means of Root Mean Square (RMS) method, which can be formally expressed using the following formula:
√
∑
(
)
(3)
Nine experimental oscillator strength values were involved in the fitting procedure for each polarization. Values of the experimental fexp and calculated fcalc oscillator strengths for Pr3+ ions in δ-BIBO are listed in Table 2. The resulting set of Judd–Ofelt parameters was found to be; Ω2= 2.99 x 10-20 cm2, Ω4=7.02 x 10-20 cm2, Ω6=20.36 x 10-20 cm2 with RMS=0.665, see Table 2. Furthermore, a set of fundamental spectroscopic parameters like radiative transition probabilities, radiative lifetimes and branching ratios were derived from the J-O calculations and are presented in Table 3.
Electric-dipole transition probability AED could be written as: 11
(
∑
)
( ) |⟨
‖
( )‖
⟩|
(4)
This, together with the magnetic-dipole transition probability, which is:
(
)
|⟨
‖
‖
⟩|
(5)
and branching ratios β using the respective
allow the computation of radiative lifetimes formulas: ∑(
(6)
)
and ∑
(
)
(7)
Comparison of the J-O parameters calculated for several Pr3+ activated borate materials are shown in Table 4. It can be seen that Pr3+:δ-BIBO, like most borate crystals, is characterized by high values of Ω parameters which is consistent with the low local symmetry of the emitting center. An important parameter characterizing an optically active material is the stimulated emission cross-section σem. The product of the stimulated emission cross-section and lifetime is proportional to the gain of a medium P= σem × exp. Thus, a knowledge of σem is essential in evaluating laser or optical amplifier system parameters such as maximum gain, saturation power and optimum output mirror reflectivity. The stimulated emission crosssection can be expressed as:
(8)
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where λp is the emission peak wavelength, Δeff is the effective linewidth found by dividing the area of the emission band by its maximum height and A is the transition probability. The calculated values of stimulated emission cross-section for 1D2 → 3H4 transition is 4.01×10–20 cm2 and 2.17×10–20 cm2 for and polarizations respectively. The values obtained here are higher than those obtained for other Pr3+ activated borate hosts [40]. Furthermore, the optical gain parameter for the strongest red emission in BIBO is equal to: P= 20.35×10-19 cm2s which makes this system a good red laser candidate.
4. Conclusion Pr3+ doped -BIBO single crystals of optical quality were grown by means of the Kyropoulos method. The spectroscopic characteristics and excited state relaxation dynamics of Pr3+ ions in δ-BIBO host were studied at cryogenic and at room temperature. The presence of only one narrow line for the 3H4 → 3P0 transition of Pr3+ ion indicates that there is only one coordination environment of Pr3+ ion, which is consistent with the structural data. The phenomenological Judd–Ofelt intensity 2, 4 and 6 parameters were obtained from the polarized absorption spectra considering the influence of the 4f5d lowest energy level for the first time. The values of transition probability, stimulated emission cross-section and radiative lifetime of the 3P0 and 1D2 levels were determined. In the Pr3+ doped -BIBO system the emitting 1D2 level is populated efficiently by the non-radiative processes under pumping into the 3PJ levels. This non-radiative process enhances 1D2 → 3H4 transition of Pr3+ associated with emission in the red spectral range. The polarized emission cross sections of the 1D2 → 3H4 transition were estimated. The results show that the investigated system may be considered as a potential candidate for red laser and new red emitting phosphor. This work gives a deeper insight and understanding of the spectral 13
characteristics of Pr3+ ions in borate hosts, especially new orthorhombic phase of BIBO (BIBO). Declarations of interest: none.
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[28] P. Boutinaud, P. Putaj, R. Mahiou, E. Cavalli, A. Speghini, M. Bettinelli, Quenching of Lanthanide Emission by Intervalence Charge Transfer in Crystals Containing Closed Shell Transition Metal Ions, Spectrosc. Lett. 40 (2007). [29] K. Jaroszewski, M. Chrunik, P. Głuchowski, E. Coy, B. Maciejewska, R. Jastrzab, A. Majchrowski, D. Kasprowicz, Photoluminescence properties of Pr3+ doped Bi2ZnOB2O6 microcrystals and PMMA-based composites, Opt. Mater. (Amst). 62 (2016) 72–79. doi:10.1016/J.OPTMAT.2016.09.059. [30] M.H. Bartl, K. Gatterer, E. Cavalli, A. Speghini, M. Bettinelli, Growth, optical spectroscopy and crystal field investigation of YAl3(BO3)4 single crystals doped with tripositive praseodymium, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 57 (2001) 1981–1990. doi:10.1016/S1386-1425(01)00484-X. [31] Q. Wei, X.Z. Li, Z.J. Wang, X.F. Long, Growth and spectroscopic properties of Pr3+ doped Sr3Y2(BO3)4 crystal, Mater. Res. Innov. 13 (2009) 2–6. doi:10.1179/143307509X402093. [32] B.R. Judd, Optical absorption intensities of rare-earth ions, Phys. Rev. 127 (1962) 750– 761. doi:10.1103/PhysRev.127.750. [33] G.S. Ofelt, Intensities of Crystal Spectra of Rare‐ Earth Ions, J. Chem. Phys. 37 (1962) 511–520. doi:10.1063/1.1701366. [34] A.A. Kornienko, A.A. Kaminskii, E.B. Dunina, Dependence of the Line Strength of f–f Transitions on the Manifold Energy. II. Analysis of Pr3+in KPrP4O12, Phys. Status Solidi. 157 (1990) 267–273. doi:10.1002/pssb.2221570127. [35] E.B. Dunina, A.A. Kornienko, L.A. Fomicheva, Modified theory of f-f transition intensities and crystal field for systems with anomalously strong configuration interaction, Cent. Eur. J. Phys. 6 (2008) 407–414. doi:10.2478/s11534-008-0084-3. [36] W.T. Carnall, P.R. Fields, K. Rajnak, Electronic Energy Levels in the Trivalent
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Lanthanide Aquo Ions. I. Pr 3+, Nd 3+, Pm 3+, Sm 3+, Dy 3+, Ho 3+, Er 3+, and Tm 3+, J. Chem. Phys. 49 (1968) 4424–4442. doi:10.1063/1.1669893. [37] P. Goldner, F. Auzel, Comparison between standard and modified Judd - Ofelt theories in a Pr3+-doped fluoride glass, Acta Phys. Pol. A. 90 (1996) 191–196. doi:10.12693/APhysPolA.90.191. [38] D. Wang, S. Huang, F. You, S. Qi, G. Zhang, Temperature-Dependent Emission of Pr3+-Doped LaB3O6 under Vacuum Ultraviolet Excitation, J. Rare Earths. 25 (2007) 139–142. doi:10.1016/S1002-0721(07)60061-1. [39] H.Y.P. Hong, S.R. Chinn, Influence of local-site symmetry on fluorescence lifetime in high-Nd-concentration laser materials, Mater. Res. Bull. 11 (1976) 461–468. doi:10.1016/0025-5408(76)90224-5. [40] F. Xiong, X. Lin, Z. Luo, Q. Tan, E. Ma, Y. Huang, Spectroscopic properties of Pr3+ ions in biaxial LaB3O6 crystal, J. Appl. Phys. 99 (2006) 064905(1-8). doi:10.1063/1.2182079.
[41] M. Malinowski, M. Kowalska, R. Piramidowicz, T. Lukasiewicz, M. Swirkowicz, A. Majchrowski, Optical transitions of Pr3+ ions in Ca4GdO(BO3)3 crystals, in: J. Alloys Compd., Elsevier, 2001: pp. 214–217. doi:10.1016/S0925-8388(01)01113-6. [42] D. Kasprowicz, M.G. Brik, K. Jaroszewski, T. Pedzinski, B. Bursa, P. Głuchowski, A. Majchrowski, E. Michalski, Spectroscopic properties of Bi2ZnOB2O6 single crystals doped with Pr3+ ions: Absorption and luminescence investigations, Opt. Mater. (Amst). 47 (2015) 428–434. doi:10.1016/j.optmat.2015.06.016. [43] M. V. V. Kumar, K. R. Gopal, R. R. Reddy, G. V. L. Reddy, N. S. Hussain, B. C. Jamalaiah, Application of modified Judd-Ofelt theory and the evaluation of radiative properties of Pr3+-doped lead telluroborate glasses for laser applications, J. Non Cryst.
19
Solids. 364 (2013) 20–27. doi:10.1016/j.jnoncrysol.2012.11.049. [44] T. Satyanarayana, M.G. Brik, N. Venkatramaiah, I. V. Kityk, K.J. Plucinski, V. Ravikumar, N. Veeraiah, Influence of Crystallization on the Luminescence Characteristics of Pr3+ Ions in PbOSb2O3B2O3 Glass System, J. Am. Ceram. Soc. 93 (2010) 2004–2011. doi:10.1111/j.1551-2916.2010.03696.x.
20
Fig. 1.
Fig. 2.
21
Fig. 3.
Fig. 4.
22
Fig. 5.
Fig. 6.
23
Table 1. Experimental energy level scheme for Pr3+ in -BiB3O6 observed at 10 K. Stark splitting of the 3H4 was determined from the emission spectra.
Experimental energy level scheme for Pr3+ in -BiB3O6 observed at 10 K
2S+1
LJ
E [cm-1]
E [cm-1]
Sub-levels (exp./theor.)
3
P2
22338, 22413, 22488, 22646, 22699
361 5/5
3
P1
20915, 21147, 21172,
257 3/3
1
I6
20752, 21311, 21330, 21494, 21574, 21693, 21765, 22101
3
P0
20672,5
1
D2
16664, 16814, 16926, 17052, 17224
560 5/5
3
H4
0, 6, 78, 132, 149, 233, 307, 385, 570, 607
607 9/9
1349 8/12 1/1
24
Table 2. Experimental and calculated oscillator strengths for respective transitions and resulting JuddOfelt parameters for δ-BiB3O6: 1% Pr3+ sample.
Experimental and calculated oscillator strengths for respective transitions
[nm]
Wavenumber [cm-1]
Transition
fexp.
fexp.
fexp. mean
fcalc.
[10-6]
[10-6]
[10-6]
[10-6]
[10-20 cm2]
Pol. 90
Pol. 180
449
22296
3
P2
-
-
-
-
Ω2= 2.99
469
21321
3
P1
1.1648
0.7438
0.9516
0.5815
Ω4= 7.02
485
20618
3
P0
1.5492
0.4660
1.0058
1.0184
Ω6=20.36
595
16806
1
D2
0.2311
0.2688
0.2502
0.1297
RMS=0.665
1012
9881
1
G4
0.0059
0.0069
0.0064
0.0165
1450
6898
3
F4
0.1491
0.2259
0.1884
0.1875
1535
6516
3
F3
0.3503
0.3929
0.3714
0.3709
1961
5100
3
F2
0.1927
0.1257
0.1594
0.1597
2201
4543
3
H6
0.0227
0.0155
0.0195
0.0247
25
26
Table 3. The spontaneous emission probabilities, fluorescence branching ratios and radiative lifetimes of δ-BiB3O6: 1% Pr3+ crystal determined by the modified JO theory.
The spontaneous emission probabilities, fluorescence branching ratios and radiative lifetimes of δBiB3O6: 1% Pr3+
Transition P0-1D2
A [ED]
A [MD]
β
τR [s]
1E-4
3
3
10.84 0
3
2327.27 0
0.0181 7.8*10-6
3
9875.75 0
0.0769
3
0 0
0
3
17663.51 0
0.1376
3
33354.01 0
0.2598
3
0 0
0
3
65135.87 0
0.5074
3
128367.25 -
1
841.39 0
1
2059.90 0
P0-1G4 P0-3F4 P0-3F3 P0-3F2 P0-3H6 P0-3H5 P0-3H4 P0- SUM
D2-1G4 D2-3F4 D2-3F3
P0
-
0.0750
217.43 6.06
0.0199
1
989.63 4.82
0.0887
D2-3H6
1
1083.17 0
0.0966
1
71.07 0
0.0063
1
5938.27 0
0.5296
1
1120.86 10.88
D2-3H5 D2-3H4 D2-SUM
D2
0.1837 89.2*10-6
1
D2-3F2
1
27
-
Table 4. Comparison of J-O parameters for Pr3+ ion in various borate hosts.
Comparison of J-O parameters for Pr3+ ion in various borate hosts
Host
2 [10-20 cm ]
4
6
R [s]
[10-20 cm2]
[10-20 cm2]
3
f [s]
1
P0
D2
3
P0
Reference
1
D2
2
- BiB3O6
2.99
7.02
20.36
BiB3O6 glass
9.97
5.37
15.72
Sr3Y2(BO3)4
10.57
2.94
17.87
11.1
100.3
0.022
9.5 [31]
LaB3O6 monoclinic
1.47
1.24
12.34
44.0
278.0
0.028
21.7 [40]
Ca4GdO(BO3)3
0.879
13.208
6.219
10.7
193.1
Bi2ZnOB2O6
1.323
1.967
1.467
27.0
399.0
0.86
3.51 [42]
telluroborate glass
3.07
3.36
8.62
39.0
388.0
12.0
152 [43]
LABG glass
11.526
5.219
23.543
28
7.8
89.2
0.08
12.0
50.8 this work 21.4 [16]
27 [41]
[44]
PII: DOI: Reference:
S0022-2313(18)31742-3 https://doi.org/10.1016/j.jlumin.2018.11.030 LUMIN16100
To appear in: Journal of Luminescence Received date: 21 September 2018 Revised date: 14 November 2018 Accepted date: 15 November 2018 Cite this article as: M. Kowalczyk, M. Kaczkan, A. Majchrowski and M. Malinowski, Spectroscopic characterization of orthorhombic δ-BiB3O6 phase nonlinear single crystal doped with Pr3+ ions, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.11.030 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.
Spectroscopic characterization of orthorhombic δ-BiB3O6 phase nonlinear single crystal doped with Pr3+ ions
M. Kowalczyka*, M. Kaczkana, A. Majchrowskib, M. Malinowskia a
Institute of Microelectronics and Optoelectronics, Warsaw University of Technology,
Koszykowa 75, 00-662 Warsaw, Poland, b
Institute of Applied Physics, Military University of Technology, Gen. Witolda Urbanowicza
2, 00-908 Warsaw, Poland
*
Corresponding author: [email protected]
Abstract The spectroscopic properties of 1at % Pr3+ doped δ-BiB3O6 (BIBO) single crystal were investigated. The absorption, emission and excitation spectra were studied at low (10 K) and at room temperature, with the emission and absorption spectra showing clear polarization dependence. The strongest observed luminescence band corresponds to the 1D2 → 3H4 transition at 610 nm. Energy levels of Pr3+ in δ-BIBO host were assigned on the basis of low temperature absorption and emission measurements. The number of crystal field levels derived implies that Pr3+ ions are located in one crystal site in the BIBO host. Decay time of the praseodymium 3P0 emission was unusually short and the 1D2 decay was non-exponential. The modified Judd-Ofelt intensity formalism was used to analyze the experimental data. The parameters, branching ratios and electric dipole transition probabilities were determined. The stimulated emission cross-sections for the 5D0 3HJ transitions were also evaluated in this system.
1
Keywords: Judd-Ofelt formalism; Pr3+ ions in borate hosts; δ-BiB3O6; Pr3+ luminescence; Pr3+ spectroscopic properties
Introduction
Much effort has gone into developing and studying new rare-earth (RE) activated nonlinear optical materials, in crystalline, glassy, ceramic or nano-powder form, for multifunction photonic sources, efficient second-harmonic generation (SHG) and other parametric processes as well as quantum memories [1–3]. Experimental results show that borate type compounds constitute a valuable source of nonlinear optical crystals with excellent transmission ranges, extending from the ultraviolet (UV) into the infrared (IR) region, good nonlinear properties and good mechanical and chemical stability [4,5]. The analysis of the photo luminescent properties of RE doped BiB3O6 (BIBO) phosphors is rather scarce in the literature, however you can find examples indicating a large potential of these materials in the scope of optoelectronics and laser technology. As far as developed solid state lasers are concerned, the Nd3+ ions doped laser crystals have played a significant role. Monoclinic Nd3+-doped BIBO, as a new potential self-frequency conversion laser crystal was studied by Brenier et al. [6]. The authors indicated that to obtain laser oscillations much higher concentrations of neodymium are needed and that important technological progress is still required to increase this concentration. It was found that much larger concentrations of Nd3+ ions could be obtained in orthorombic -BIBO than in polymorph of bismuth triborate. Spectroscopic properties of Nd3+:-BIBO crystal favor lasing at 1.3 m, where this crystal possesses near non-critical phase matching [7]. Also of note is that crystals doped with triply ionized praseodymium ions (Pr3+) possess the spectral emission lines in the visible (VIS) and the near infrared (NIR) spectral 2
range [8]. Thus, the Pr3+ was used as an activator in a variety of host materials useful in many phosphors and different types of light emitting devices. In particular, the luminescence properties of Pr3+ doped materials were extensively investigated for applications including fiber optical communication [9], LEDs [10], field-emission display devices and photoluminescence devices [11]. It has been shown recently that the Pr3+ ion in oxide crystals facilitates laser operation in the VIS region under semiconductor diode blue laser pumping[12,13]. Furthermore, it is possible to reach the UV part of the spectrum simply by frequency doubling. Therefore, further investigations of Pr3+ doped -BIBO crystalline phase appear to be warranted. The results of spectroscopic studies of single -BIBO crystals were reported in [14] where it was shown that as a result of differences in crystal structure this phase of BIBO can be doped more easily by RE ions in comparison with the -phase. Composites containing nano-crystallites of novel rare earth doped borate -BIBO and polyvinyl alcohol (PVA) polymer were reported in [15]. The results of this study confirmed the role of the Ag-NPs in influencing the third-order susceptibilities. In the work of Plaza and Aragó the properties of BIBO glasses prepared at different thermal conditions were analyzed [16]. Kuznik et al. [17] studied second harmonic generation in praseodymium doped BIBO glass and its dependence on the degree of crystallinity. Comparisons with other glass matrices were performed following both the Judd–Ofelt analysis as well as experimental decay times for principal fluorescence lines in [18]. The goal of the investigations presented here was to study the spectroscopic properties of the new praseodymium system, Pr3+ activated δ-BIBO single crystal, because this system has never been characterized before and then to compare it with other Pr3+ borates.
3
Experimental 1. Crystal growth According to the literature orthorhombic δ-BIBO phase is one of three stable polymorphs of BIBO (BiB3O6) that can be obtained in the form of bulk single crystals from stoichiometric melt [19]. The best known monoclinic α-BIBO phase, having strong nonlinear optical properties, can be grown from melt at temperatures above 710°C [20], monoclinic γBIBO phase crystallizes in the temperature range 680-710°C [21], while below 680°C δBIBO single crystals can be grown from strongly supercooled melt [22]. Despite the higher density of the γ-BIBO phase, compared to α-BIBO phase, it can be easily doped with Rare Earth (RE) ions. -BIBO crystallizes in the orthorhombic space group Pca21 and its structure contains BO4 tetrahedra, which share common corners and form layers [23]. Due to the similarity of ionic radii of the Pr3+ and Bi3+ ions (1.179 and 1,17 Å, correspondingly) [24], the only suitable lattice site for this substitution is the Bi3+ site, therefore we synthesized the starting material corresponding to BIBO composition in which the normal amount of Bi3+ ions was replaced with Pr3+ ions (1at.%). BIBO:Pr3+ single crystals were grown by means of Kyropoulos method on [001] oriented seed, similar to the method described by Aleksandrovsky et al. [22]. The growth was carried out in a two-zone resistance furnace enabling low temperature gradients and the seed was rotated at a rate of 5 rpm. No pulling was used. A BIBO:Pr3+ crystal grew in the volume of the melt and was confined with crystallographic faces. It was found that the main face was formed by (100) plane, a detailed description of the morphology of an as-grown δ-BIBO single crystal can be found elsewhere [24].
4
2. Experiment The absorption spectrum was taken at room temperature using a Perkin–Elmer Lambda 950 spectrophotometer. Optical measurements were carried out on a single crystal sample in the form of a polished parallelepiped cut perpendicular to the crystallographic axes. The morphology of the obtained Pr3+:-BIBO sample allowed absorption measurements with the light propagation direction along the b axis for two directions of light electric vector, namely, along a and c axes. Emission measurements were performed using a Photon Technology International spectrophotometer. Emission spectra were also measured using a CVI-480 grating monochromator followed by a PMT and SR-400 photon counting system. The samples were excited by a pulsed (10 ns pulse-width, repetition rate 10 Hz) tunable optical parametric oscillator (Continuum) pumped by a frequency-tripled Nd:YAG pulse laser (Continuum Surelite II). Fluorescence dynamics profiles were recorded with a SR-430 multichannel analyzer controlled with a PC computer. Sample cooling was provided by a closed cycle He optical cryostat which allowed the temperature to be varied between 10 and 300 K.
3. Results 3.1. Absorption Absorption spectra of the Pr3+:-BIBO crystal for ~E||a and ~E||c polarizations are shown in Fig. 1. Spectra consist of narrow bands corresponding to f–f transitions in Pr3+ ions, see inset in Fig.1. The absorption edge of the crystal is observed at about 315 nm giving a -BIBO gap of about 3,95 eV which corresponds well with observations presented in[15,23] and is close to the band gap Eg = 4.16 eV reported for Pr3+: -BIBO [14]. The polarized absorption spectra of Pr3+: -BIBO between 435 nm and 605 nm recorded at 10 K are presented in Fig. 2. Five well-distinguished groups of lines are characteristic of transitions within the 4f2 configuration between the lowest crystal field components of ground 3H4 level 5
and crystal field states of the excited 3PJ, 1I6 and 1D2 multiplets. The strong influence of incident light polarization on transition intensities is clearly observed. Polarizations E||c shows higher absorption coefficients, thus optical pumping along crystallographic c axis should be favorable. The analysis of the Stark components of the
2S+1
LJ levels of Pr3+ in
absorption at 10 K were conducted successfully. The energies of Stark levels of the ground 3
H4 multiplet were determined from the analysis of the low temperature emission lines related
to the 1D2→3H4 transition and are displayed in Table 1. The splitting of 6 cm-1 observed in absorption transitions suggests that the first Stark component in the ground 3H4 state has energy of 6 cm-1. Spectroscopic features of Pr3+ is strongly affected by the shape and the size of the coordination environment. -BIBO has sevenfold-coordinated Bi3+ ions [23], see Fig.3. However, according to Cong, et al [25], Bi3+ cations have an irregular nine-fold coordination environment (Bi–O, 2.26–3.03 Å) and the surrounding environment of the Bi3+ is anisotropic due to the existence of its 6s lone-pair electrons, the point symmetry is C2v. In the crystal field of a low symmetry, the Pr3+ levels are split into 2J+1 crystal field components, consequently there are three and five components in the 3P1, and 3P2,1D2 manifolds respectively, which agrees with the data presented in Table 1. It is also assumed that, because of the high coordination number, the local crystal field is weak. A weak crystal field results in a high energy position of the 4f5d Pr3+ configuration, suggesting that it lies above the 1S0 state. Strong polarization behavior of the absorption transitions observed for Pr3+:-BIBO suggests that the effective point symmetry of Pr3+ sites is higher than C1.
6
Fig. 1 Absorption spectra of the Pr3+:-BIBO crystal for ~E||a and ~E||c polarizations registered at room temperature.
Fig. 2. Visible part of the absorption spectra of the Pr3+:-BIBO crystal for ~E||a and ~E||c polarizations registered at 10 K.
Fig.3 The coordination geometry of polyhedron around the Bi3+ ion in -BIBO crystal.
Fig. 4. The photoluminescence excitation (PLE) (blue line) and photoluminescence (PL) (red line) spectra of Pr3+:-BIBO at 300 K. Excitation was recorded at the 609.5 nm emission line and emission was recorded under 447 nm excitation.
3.2. Emission As can be seen in Fig.4, the photoluminescence excitation (PLE) spectrum (recorded at 609.5 nm emission, blue line) contains a broad band in the region of 260–380 nm and a series of narrow lines located in the range of 400–500 nm. As reported in [26] the broad bands in question are attributed to the 1S0 → 3P1 absorption transition of Bi3+. This transition is expected to have reasonable oscillator strength through the spin−orbit mixing that takes place between 3P1 and 1P1 [26]. The presence of similar bands, in the approximate region of ~220 - 400 nm, within the spectra of other Bi3+ materials was also reported [27]. The presence of Bi3+ absorption in the excitation spectra of Pr3+, as shown in Fig.4, indicates that some energy transfer from Bi3+ to Pr3+ ions also occurs. The narrow band transitions located above 400 nm belong to the intrinsic f–f transitions of Pr3+ and are attributed to the electronic
7
transitions from 3H4 to 3PJ levels. This demonstrates that the investigated phosphor can be efficiently excited by near-ultraviolet and blue light. After excitation of the 3PJ levels by blue-violet radiation at 447 nm, room temperature, a detector response corrected emission spectrum of 1% Pr :BIBO was registered in the visible range and is also shown in Fig.4. The spectrum represents transitions from emitting 3P0 and 1
D2 multiplets to terminal energy (Stark) levels within the 3H4 multiplets. As can be seen from
Fig. 4, the spectrum is dominated by a red transition centered at 610 nm, but also a relatively weak emission originating from the 3P0 level was detected at wavelengths shorter than 600 nm. This red emission is attributed to transitions originating either from the 3P0 or 1D2 levels [28]. To clearly distinguish between the 3P0 and 1D2 lines, selectively excited emission spectra were recorded at 10 K. In Fig.5 part of the emission spectrum in the 600 nm region after selective excitation is shown. No emission from the 3P0→3H6 transition around 610 nm was observed. Thus, from Figs. 4 and 5 it can be concluded that the red emission originates mostly from the excited 1D2 state, populated by the non-radiative relaxation from the 3P0 state. Such fast nonradiative relaxation from 3P0 to 1D2 could be due to multiphonon relaxation, cross relaxation (CR) energy transfer or the charge transfer (CT) mechanism.
Fig.5. Low temperature emission spectrum in the 600 nm region after selective excitation into 3
P1 and 1D2 multiplets.
As the 3P0 and 1D2 levels are separated by about 3500 cm-1, the probability of multiphonon relaxation process is not significant [27]. Therefore, it is concluded that, for populating the 1D2 level and resulting emissions, an energy transfer process is most likely. It was shown that in a number of praseodymium activated solids, concentration quenching of
8
the 3P0 fluorescence dynamics is related to the cross-relaxation (CR) mechanism of the type (3P0, 3H4) (3H6, 1D2). This energy transfer is responsible in most praseodymium compounds for a sharp decrease of the luminescence with increasing activator concentration and a reduction of the 3P0 lifetime. Moreover, it was reported recently that the intervalence charge transfer (IVCT) of metal-to-metal can also provide an efficient quenching channel [28]. Such a situation was observed in Pr3+ doped Bi2ZnOB2O6 microcrystals, where, due to strong 3P0 → 1D2 nonradiative relaxation of Pr3+ ion by low-lying CT states, only red 1D2 emission was detected [29].
3.3. Excited state dynamics Decays originating from the 3P0 and 1D2 levels were registered and are presented in Fig.6. 3P0 decay was measured at 483.8 nm where a resonant luminescence corresponding to the 3P0 3H4 transition occurs after excitation into the 3P2 levels at 447.6 nm. 1D2 decay was measured at 600 nm after direct excitation into the 1D2 levels at 594.7 nm. Non-exponentiality of the 1D2 decay was observed even at the relatively low dopant concentration of 1 at. % and at a low temperature indicating that energy transfer processes strongly contribute to the decay of this luminescent level. In addition, the multiphonon relaxation from the 1D2 to its next lower 1G4 manifold suggests that it could participate in the nonradiative process. Low temperature 3P0 lifetime, determined in 1% Pr3+ doped sample, was 0.08 s when the 1D2 decay time, determined from the long time part of the decay, was 55.3 s which is close to the value of 50 s found in -BIBO crystal [14]. The fluorescence quantum efficiencies were calculated using the expression f/r. The fluorescence quantum efficiencies of the 3P0 and 1D2 manifolds are 1% and 57% respectively. The main reason for the very low quantum efficiency of the 3P0 manifold is a high relaxation rate from the 3P0 to the 1D2 manifolds. A similar situation was found in several 9
praseodymium activated borates like Pr3+:YAl3(BO3)4 [30] and Pr3+:Sr3Y2(BO3)4 [31]. The low fluorescence quantum efficiency of the 1D2 manifold is due to the multiphonon relaxation and cross relaxation which is confirmed by the nonexponential character of this decay.
Fig.6 Low temperature fluorescence decay curves of Pr3+ :-BIBO crystal plotted in semilog scale. The Judd-Ofelt (J–O) theory [32,33] is a well-recognized method for analysis and comparison of the spectroscopic properties of RE3+ ions in dielectric matrixes. The experimental absorption oscillator strength fexp for transitions from the ground state 3H4 to the excited states of Pr3+ can be determined from the room temperature absorption spectrum using the expression:
∫
(1)
where i is the integrated absorption coefficient of the ith absorption band and is the Pr3+ concentration in [ions/cm3]. Because of the crystal morphology we used the J-O analysis of absorption spectra for two polarizations. The oscillator strengths of the electric dipole transitions can be obtained theoretically from three sets of J-O intensity parameters [32,33]. According to the modified J-O theory [34,35], for the case of a strong mixture between the 4f2 and 4f5d state, which is also the case of Pr3+ ion, the calculated oscillator strength of the electric dipole transition between two states can by expressed as:
(
)
∑ [
]
10
〈
[
] ‖
‖
[
] 〉
(2)
The reduced matrix elements used in our calculations were taken from the work of Carnall et al. [36]. The terms EJ’, Ef0 and E5d, which are not present in the conventional JO approach [32,33], 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 cm-1 [37] and E5d was set as 47393 cm-1 [38]. The phenomenological J-O parameters for Pr3+ ions are found by the least squares fitting between fexp and fcalc. During the calculations the deviation was minimized by the means of Root Mean Square (RMS) method, which can be formally expressed using the following formula:
√
∑
(
)
(3)
Nine experimental oscillator strength values were involved in the fitting procedure for each polarization. Values of the experimental fexp and calculated fcalc oscillator strengths for Pr3+ ions in δ-BIBO are listed in Table 2. The resulting set of Judd–Ofelt parameters was found to be; Ω2= 2.99 x 10-20 cm2, Ω4=7.02 x 10-20 cm2, Ω6=20.36 x 10-20 cm2 with RMS=0.665, see Table 2. Furthermore, a set of fundamental spectroscopic parameters like radiative transition probabilities, radiative lifetimes and branching ratios were derived from the J-O calculations and are presented in Table 3.
Electric-dipole transition probability AED could be written as: 11
(
∑
)
( ) |⟨
‖
( )‖
⟩|
(4)
This, together with the magnetic-dipole transition probability, which is:
(
)
|⟨
‖
‖
⟩|
(5)
and branching ratios β using the respective
allow the computation of radiative lifetimes formulas: ∑(
(6)
)
and ∑
(
)
(7)
Comparison of the J-O parameters calculated for several Pr3+ activated borate materials are shown in Table 4. It can be seen that Pr3+:δ-BIBO, like most borate crystals, is characterized by high values of Ω parameters which is consistent with the low local symmetry of the emitting center. An important parameter characterizing an optically active material is the stimulated emission cross-section σem. The product of the stimulated emission cross-section and lifetime is proportional to the gain of a medium P= σem × exp. Thus, a knowledge of σem is essential in evaluating laser or optical amplifier system parameters such as maximum gain, saturation power and optimum output mirror reflectivity. The stimulated emission crosssection can be expressed as:
(8)
12
where λp is the emission peak wavelength, Δeff is the effective linewidth found by dividing the area of the emission band by its maximum height and A is the transition probability. The calculated values of stimulated emission cross-section for 1D2 → 3H4 transition is 4.01×10–20 cm2 and 2.17×10–20 cm2 for and polarizations respectively. The values obtained here are higher than those obtained for other Pr3+ activated borate hosts [40]. Furthermore, the optical gain parameter for the strongest red emission in BIBO is equal to: P= 20.35×10-19 cm2s which makes this system a good red laser candidate.
4. Conclusion Pr3+ doped -BIBO single crystals of optical quality were grown by means of the Kyropoulos method. The spectroscopic characteristics and excited state relaxation dynamics of Pr3+ ions in δ-BIBO host were studied at cryogenic and at room temperature. The presence of only one narrow line for the 3H4 → 3P0 transition of Pr3+ ion indicates that there is only one coordination environment of Pr3+ ion, which is consistent with the structural data. The phenomenological Judd–Ofelt intensity 2, 4 and 6 parameters were obtained from the polarized absorption spectra considering the influence of the 4f5d lowest energy level for the first time. The values of transition probability, stimulated emission cross-section and radiative lifetime of the 3P0 and 1D2 levels were determined. In the Pr3+ doped -BIBO system the emitting 1D2 level is populated efficiently by the non-radiative processes under pumping into the 3PJ levels. This non-radiative process enhances 1D2 → 3H4 transition of Pr3+ associated with emission in the red spectral range. The polarized emission cross sections of the 1D2 → 3H4 transition were estimated. The results show that the investigated system may be considered as a potential candidate for red laser and new red emitting phosphor. This work gives a deeper insight and understanding of the spectral 13
characteristics of Pr3+ ions in borate hosts, especially new orthorhombic phase of BIBO (BIBO). Declarations of interest: none.
14
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20
Fig. 1.
Fig. 2.
21
Fig. 3.
Fig. 4.
22
Fig. 5.
Fig. 6.
23
Table 1. Experimental energy level scheme for Pr3+ in -BiB3O6 observed at 10 K. Stark splitting of the 3H4 was determined from the emission spectra.
Experimental energy level scheme for Pr3+ in -BiB3O6 observed at 10 K
2S+1
LJ
E [cm-1]
E [cm-1]
Sub-levels (exp./theor.)
3
P2
22338, 22413, 22488, 22646, 22699
361 5/5
3
P1
20915, 21147, 21172,
257 3/3
1
I6
20752, 21311, 21330, 21494, 21574, 21693, 21765, 22101
3
P0
20672,5
1
D2
16664, 16814, 16926, 17052, 17224
560 5/5
3
H4
0, 6, 78, 132, 149, 233, 307, 385, 570, 607
607 9/9
1349 8/12 1/1
24
Table 2. Experimental and calculated oscillator strengths for respective transitions and resulting JuddOfelt parameters for δ-BiB3O6: 1% Pr3+ sample.
Experimental and calculated oscillator strengths for respective transitions
[nm]
Wavenumber [cm-1]
Transition
fexp.
fexp.
fexp. mean
fcalc.
[10-6]
[10-6]
[10-6]
[10-6]
[10-20 cm2]
Pol. 90
Pol. 180
449
22296
3
P2
-
-
-
-
Ω2= 2.99
469
21321
3
P1
1.1648
0.7438
0.9516
0.5815
Ω4= 7.02
485
20618
3
P0
1.5492
0.4660
1.0058
1.0184
Ω6=20.36
595
16806
1
D2
0.2311
0.2688
0.2502
0.1297
RMS=0.665
1012
9881
1
G4
0.0059
0.0069
0.0064
0.0165
1450
6898
3
F4
0.1491
0.2259
0.1884
0.1875
1535
6516
3
F3
0.3503
0.3929
0.3714
0.3709
1961
5100
3
F2
0.1927
0.1257
0.1594
0.1597
2201
4543
3
H6
0.0227
0.0155
0.0195
0.0247
25
26
Table 3. The spontaneous emission probabilities, fluorescence branching ratios and radiative lifetimes of δ-BiB3O6: 1% Pr3+ crystal determined by the modified JO theory.
The spontaneous emission probabilities, fluorescence branching ratios and radiative lifetimes of δBiB3O6: 1% Pr3+
Transition P0-1D2
A [ED]
A [MD]
β
τR [s]
1E-4
3
3
10.84 0
3
2327.27 0
0.0181 7.8*10-6
3
9875.75 0
0.0769
3
0 0
0
3
17663.51 0
0.1376
3
33354.01 0
0.2598
3
0 0
0
3
65135.87 0
0.5074
3
128367.25 -
1
841.39 0
1
2059.90 0
P0-1G4 P0-3F4 P0-3F3 P0-3F2 P0-3H6 P0-3H5 P0-3H4 P0- SUM
D2-1G4 D2-3F4 D2-3F3
P0
-
0.0750
217.43 6.06
0.0199
1
989.63 4.82
0.0887
D2-3H6
1
1083.17 0
0.0966
1
71.07 0
0.0063
1
5938.27 0
0.5296
1
1120.86 10.88
D2-3H5 D2-3H4 D2-SUM
D2
0.1837 89.2*10-6
1
D2-3F2
1
27
-
Table 4. Comparison of J-O parameters for Pr3+ ion in various borate hosts.
Comparison of J-O parameters for Pr3+ ion in various borate hosts
Host
2 [10-20 cm ]
4
6
R [s]
[10-20 cm2]
[10-20 cm2]
3
f [s]
1
P0
D2
3
P0
Reference
1
D2
2
- BiB3O6
2.99
7.02
20.36
BiB3O6 glass
9.97
5.37
15.72
Sr3Y2(BO3)4
10.57
2.94
17.87
11.1
100.3
0.022
9.5 [31]
LaB3O6 monoclinic
1.47
1.24
12.34
44.0
278.0
0.028
21.7 [40]
Ca4GdO(BO3)3
0.879
13.208
6.219
10.7
193.1
Bi2ZnOB2O6
1.323
1.967
1.467
27.0
399.0
0.86
3.51 [42]
telluroborate glass
3.07
3.36
8.62
39.0
388.0
12.0
152 [43]
LABG glass
11.526
5.219
23.543
28
7.8
89.2
0.08
12.0
50.8 this work 21.4 [16]
27 [41]
[44]