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Abstract
A Tb3+:KYW crystal was grown by the modified Czochralski technique. Polarized ground state absorption and fluorescence spectra, as well as a fluorescence decay curve, were recorded at room temperature. Radiative properties such as emission probabilities, branching ratios, and radiative lifetime were investigated within the theory of 4f–4f transition intensity in the case of a strong configuration interaction. The limitations of visible laser operation of Tb3+-doped double tungstate crystals are discussed.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Tb3+ is an interesting dopant ion for solid-state optical materials for fluorescence applications due to its intense emission in the green spectral range [1]. Moreover, due to its high Verdet-constant, materials containing Tb3+ are frequently used as Faraday rotators [2]. In addition, efficient laser operation was reported in different Tb-doped fluoride crystals [3] at room temperature. An obvious disadvantage of the Tb3+ ion are low absorption and stimulated emission cross sections in the visible (∼10−21 cm2 in fluoride crystals [3]), since corresponding transitions are spin-forbidden. This problem can be overcome by using long crystals with a high terbium ion concentration. Therefore, there is an interest to investigate the spectroscopic properties of the Tb3+ ion in highly doped oxide matrices, especially in those with a low crystal field symmetry.
The family of potassium double tungstates α-KRE(WO4)2 (RE = lanthanide or Y) activated by trivalent rare-earth ions represents a group of well-known host materials for solid-state lasers [4]. These crystals accommodate high concentrations of active rare earth ions without significant fluorescence quenching due to relatively large distances between the respective lattice sites [5]. In addition, these crystals possess relatively high values of absorption and emission cross sections when doped with rare-earth ions. Recently the spectroscopic properties of Tb-doped double tungstate crystals were discussed e.g. for Tb:KLu(WO4)2 [6], and Tb:KYb(WO4)2 [7]. Also regarding low-doped Tb:KY(WO4)2 and stoichiometric KTb(WO4)2 some spectroscopic investigations were reported in the past [8,9]. However, these investigations were very basic and none of these reports presents, e.g., values for the transition cross sections. Thus, no substantiated predictions regarding the applicability of Tb:KY(WO4)2 as laser materials are possible based on existing work.
In the present work, the laser-related spectroscopic properties of Tb3+-doped KY(WO4)2 (Tb:KYW) are studied. Polarization dependent absorption and fluorescence spectra, as well as the fluorescence decay curve of the 5D4 level, are recorded and the transition cross sections and the fluorescence lifetime, respectively, are determined. The theory of 4f–4f transition intensities taking into account the influence of excited configurations [10] is applied to determine the radiative properties of Tb3+ ions. Based on these results, we discuss the prospects and limitations for obtaining laser action in Tb3+-doped double tungstates crystals.
2. Experimental details
A Tb3+ (53 at.%):KYW crystal was grown from a potassium ditungstate (K2W2O7) flux at a low temperature gradient (∼ 0.1 K/cm) using a [010]-oriented seed KY(WO4)2 crystal [11]. The pulling rate was 0.12–0.2 mm/h at the crystal rotation velocity of 40–60 rpm. The terbium concentration N0 was investigated by electron-probe microscope analysis (EPMA) and calculated to be 31.8×1020 cm−3 using the measured volumetric crystal density of 6.96 g/cm3. Terbium ions substitute yttrium ions in the KYW crystal and thus occupy sites with a C2 local point symmetry. The ionic radius of the Tb3+ ion (1.04 Å) is close to one of Y3+ (1.019 Å) for eight-fold coordination [12]. The as-grown pale green-yellow colored crystal (see Fig. 1a) was clear and had a good optical quality (without background losses).
Fig. 1. Tb3+(53 at.%):KYW crystal boule (a); crystallographic and optical indicatrix axes (I2/c setting) (b).
KYW is a biaxial crystal with a monoclinic structure (space group $C_{2h}^6$ - 2/m, Z = 4). Consequently, the optical properties of KYW crystals are different for directions along the optical indicatrix axes Np, Nm, Ng. The principal axis Np coincides with the crystallographic b axis ([010]). The other two axes lie in the a-c plane. The principal axis Ng is rotated at 17° clockwise from the crystallographic c axis (see Fig. 1b). Its orientation was determined using a polarizing microscope POLAM-L213-BE (LOMO).
The crystal structure was refined by the powder diffraction method, using a DRON - 3M diffractometer with a Cu K-α radiation source (λ = 0.15406 nm). The powder pattern of Tb:KYW shows that there are no impurity phases. Lattice parameters of a = 8.067(1) Å , b = 10.371(1) Å, c = 7.5624(8) Å, β = 94.675(5)° (I2/c setting) are slightly different from pure KY(WO4)2 [13] and KTb(WO4)2 [8] crystals as expected from the high Tb3+ concentration.
For the spectroscopic investigations a cuboid sample of Tb:KYW crystal was oriented along the three principal optical axes and had polished opposite planes Nm-Ng and Nm-Np to enable polarization dependent measurements. A Glan-Taylor polarizer was used to separate the light polarizations. The transmission spectra in polarized light were recorded by a Varian CARY 5000 spectrophotometer with a spectral bandwidth (SBW) of 0.1 nm in the UV and visible spectral range. The absorption spectra in the infrared (IR) region were obtained by a FTIR spectrometer Nexus470 (Thermo Nicolet) with a spectral resolution not less than 1.2 nm.
Fluorescence spectra were recorded under laser diode excitation at 380 nm in the range of 470–700 nm. The fluorescence emission was collected by a wide-aperture lens and focused onto the entrance slit of a monochromator MDR-23 (SBW 0.08–0.11 nm). The signal was detected with a PMT R9110 (Hamamatsu) associated with a SR830 Stanford lock-in amplifier. A gray body light source with a color temperature of 2900 K was used to correct the measured spectra with respect to detector response, grating efficiency and other optical element`s influence.
The decay curve of the upper 5D4 manifold was registered under 20 ns pulsed excitation at 488 nm from an optical parametric oscillator LT-2214 (LOTIS TII). The fluorescence emission at 544 nm was imaged onto the entrance slit of a monochromator MDR-12. The signal was detected by a fast Hamamatsu photodetector C5460 connected to an oscilloscope Tektronix TDS-3052B.
3. Results and discussion
3.1 Ground state absorption and f-f transition intensities
Polarization dependent absorption spectra of Tb:KYW were recorded in the spectral range between 360 nm and 500 nm covering the absorption into the emitting level 5D4 and above as well as in the infrared between 1.6 µm and 3.5 µm to investigate the energetic positions of the uppermost 5 (J = 0 - 4) of the 7 low energy 7FJ levels. The spectra of ground-state absorption (GSA) cross sections σGSA in the visible and IR spectral regions are shown in Fig. 2a and b. The absorption spectra of the Tb:KYW exhibit significant polarization anisotropy. In the visible spectral range, the characteristic 7F6 → 5D4 transition has a peak σGSA of 5.3×10−21 cm2 (for E||Nm) at 486.7 nm. For the investigated crystal, the length required for efficient absorption (i.e. 90% absorbed light at 486.2 nm) is below 10 mm. The increased absorption below 400 nm is attributed to the intervalence charge transfer (IVCT) Tb3+(4f8)W6+(5d0) → Tb4+(4f7)W5+(5d1) [14].
Among the triply ionized rare earth ions, Tb3+ has the lowest energetic separation between the ground state 7F6 of the 4f8-configuration and the lowest 5d level (see Fig. 3). Therefore, for this ion only host materials with a low crystal field depression (CFD) should be considered, as the CFD strongly influences the energetic position of the lowest 5d energy levels [15]. A too low energetic position of these lowest levels makes the crystal prone to excited state absorption in the visible, which is detrimental for laser operation [16]. However, for monoclinic double tungstates, the CFD value as well as the position of the 5d levels in KRE(WO4)2 cannot easily be experimentally determined, because even the lowest 5d level 9D of Tb3+ is located inside the conduction band of these host materials. This is due to their low bandgap energy of only ∼32000 cm−1 corresponding to ∼315 nm [17].
Fig. 3. Energy level scheme of the Tb3+ ion. Dashed vertical lines represent possible ESA transitions, dashed area – the energetic position of the IVCT band of KYW host, black and purple lines – levels of 4f8 and 4f75d1 configurations, respectively.
To determine the transition intensities of Tb3+ ions in KYW the theory of 4f–4f transition intensities for systems with strong configuration interaction (SCI) [10] was applied. This theory takes into account the influence of the low-lying levels of the excited configuration 4fN-15d1 at the energy Δ [18]. In this case the line strength SED, calc of the electric dipole (ED) transition from an initial manifold (J) to some terminal manifold (J′), can be evaluated by the equation:
Table 1. Experimental (fed, exp) and calculated (fed, SCI) oscillator strength
The intensity parameters Ωt were evaluated by the least-square fitting procedure and are equal to Ω2 = 8.078×10−20 cm2, Ω4 = 4.607×10−20 cm2, Ω6 = 3.586×10−20 cm2 and the energy Δ was determined to be 39125 cm−1. This value is higher than in several fluoride compounds [15], which is attributed the large distance between the doping ion and the surrounding O2- ligands of 2.28–2.71 Å [12]. The root mean square deviation was 0.74×10−6.
3.2 Radiative properties
The obtained fit parameters were used to calculate the radiative lifetime τrad of the 5D4 level and the branching ratios β [19]. The radiative lifetime of the 5D4 state of τrad = 466 μs compares well with the value of the fluorescence lifetime τmeas = 460 μs experimentally measured in a low doped Tb:KYW crystal [6]. The calculated branching ratios β are listed in Table 2.
Table 2. Measured (βJ′J) and calculated (βcalc) branching ratios for the 5D4 level in Tb:KYW crystal
Figure 4 represents the polarized averaged fluorescence spectrum of Tb:KYW, which was obtained by averaging of the polarized ones recorded under excitation at 380 nm and corrected to the spectral response of the fluorescence set-up. The observed emission bands are related to the radiative transitions from 5D4 manifold to the lower lying 7FJ levels. Manifold to manifold branching ratios for these transitions are found from:
The fluorescence lifetime of the emitting 5D4 level of Tb:KYW was measured at a wavelength of ∼545 nm. A single exponential decay function was used to fit the fluorescence curve (see Fig. 5) and the fluorescence lifetime was determined to be 350 µs, indicating a fluorescence quantum yield η of 76% despite the high doping concentration of 53 at.%. The relatively high value of η can be explained by the large energy gap (∼14500 cm−1) between the 5D4 level and lower lying 7FJ levels which prohibits multi-phonon relaxation.
Fig. 5. Fluorescence decay curve of the 5D4 level of Tb:KYW. The solid line represents a single-exponential fit curve.
Moreover, according to the Förster-Dexter theory [19] with the cross sections stated above, the critical distance for nonradiative dipole-dipole energy transfer is 3.8 Å, which is shorter than the minimal distance between Y3+-sites in KYW of 4.06 Å [12]. This strongly inhibits concentration quenching by energy migration in Tb:KYW.
The stimulated emission (SE) cross sections σSE for the three most intense transitions 5D4 → 7F5…3 were calculated by the Füchtbauer-Ladenburg formula [20]:
The obtained spectroscopic results would allow to conclude, that Tb:KYW crystals are suitable candidates for laser operation in the visible spectral range. However, in laser experiments under 2ω-OPSL pumping at 486.2 nm we did not obtain laser operation with this crystal. We believe that the laser oscillation is strongly suppressed by pump-induced IVCT, which is also evident in the absorption spectra in Fig. 1a. The same limitation was observed in Pr3+:KGd(WO4)2 crystals [22]. The reason for this behavior is found in the tendency of Pr3+ and Tb3+ to oxidize to the quadrivalent state in combination with the high oxidation state of the W6+-ion in tungstates, which gives rise to an electron exchange process between the rare earth ion and the tungsten ion. Based on these findings we can conclude that – despite opposing reports [7,9] – double tungstate crystals are in general unsuitable host materials for visible laser operation in the most promising visibly emitting laser ions Tb3+ and Pr3+.
4. Conclusion
A high quality single crystal of 53 at.% Tb3+-doped KY(WO4)2 was grown by modified Czochralski technique. The most suitable pumping transition 7F6 → 5D4 has a peak absorption cross section of 5.3×10−21 cm2 (for E||Nm polarization) at a wavelength of 486.7 nm. The highest stimulated emission cross sections of 9.0×10−21 cm2 (for E||Nm polarization) and 8.3×10−21 cm2 (for E||Np polarization) were found in the green spectral range at 549.3 nm and 542.7 nm, respectively. The 4f–4f transition intensities were calculated by applying the theory for a system with strong configuration interaction and the radiative lifetime of the upper state 5D4 was derived to be about 0.46 ms. The fluorescence lifetime and quantum efficiency of this level were found to be 0.35 ms and 76%, respectively. Despite these promising findings, laser experiments were not successful which we attribute to inter-valence charge transfer between Tb3+ and W6+. As this is an intrinsic problem in all Tb3+-doped double tungstates, we do not regard this material class to be useful for visible laser operation.
Funding
Bundesministerium für Bildung und Forschung (BMBF) (FKZ 13N14192).
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