This paper reports the growth and spectral properties of Er3+-doped and Er3+/Yb3+-codoped Li3Ba2La3(WO4)8 crystals. The Er3+: Li3Ba2La3(WO4)8 crystal with dimensions of 56 mm×28 mm×9 mm and Er3+/Yb3+: Li3Ba2La3(WO4)8 crystal with dimensions of 52 mm×24 mm×8 mm were obtained by the top-seeded solution growth (TSSG) method. Thermal expansion coefficients and thermal conductivity of both crystals were measured. The spectroscopic characterizations of both crystals were investigated. The spectroscopic analysis reveals that the Er3+/Yb3+: Li3Ba2La3(WO4)8 crystal has much better optical properties than the Er3+: Li3Ba2La3(WO4)8 crystal, thus it may become a potential candidate for solid-state laser gain medium material.
Citation: Xiao B, Lin Z, Zhang L, Huang Y, Wang G (2012) Growth, Thermal and Spectral Properties of Er3+-Doped and Er3+/Yb3+-Codoped Li3Ba2La3(WO4)8 Crystals. PLoS ONE 7(7): e40631. doi:10.1371/journal.pone.0040631
Editor: Richard G. Haverkamp, Massey University, New Zealand
Received: February 2, 2012; Accepted: June 11, 2012; Published: July 13, 2012
Copyright: © 2012 Xiao et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work is supported by the National Natural Science Foundation of China (No. 61108054) and the National Natural Science Foundation of Fujian Province (No. 2011J01376), respectively. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Er3+ is a well-known active ion for the solid-state laser in near infrared and up-conversion emission –. The 4I13/2→4I15/2 transition has attracted much attention because its eye-safe emission around 1.55 µm has potential use in optical communication, range finding and medical treatment , , . The green output emission of Er3+ ions has already been used in various fields, such as data storage and laser display , . Unfortunately, the optical absorption band of the excited energy level (4I11/2) is weak, which means Er3+ ions cannot be effectively pumped. This problem is normally solved by adding a certain amount of Yb3+ sensitizing ions, since Yb3+ ions have a broad and high absorption band around 980 nm and the energy transfer from Yb3+ to Er3+ ions is efficient , , , . Laser oscillation has been observed in several Er3+ and Yb3+ codoped laser hosts, such as YAG, Y2SiO5 , YCa4O(BO3)3 , GdCa4O(BO3)3 , YVO4 , YAl3(BO3)4 , and NaCe(WO4)2 . Among them, the slope efficiencies of Er3+/Yb3+ codoped YCa4O(BO3)3 and GdCa4O(BO3)3 crystals are the highest, and exhibit a better thermal property than phosphate glass , . However, the full widths at half the maximum (FWHM) of absorption bands around 980 nm of the Er3+/Yb3+ codoped YCa4O(BO3)3 (4 nm) and GdCa4O(BO3)3 (3 nm) crystals are narrow , , , . The narrow absorption bands need crucially temperature controlling, because the emission wavelength of the pumping diode changes at 0.2–0.3 nm/°K with the operating temperature of the laser device , . As a consequence, it is necessary to explore novel materials with large absorption bandwidths for solid-state laser application.
Li3Ba2Ln3(WO4)8 (Ln = La-Lu, Y) belongs to the monoclinic system with space group C2/c, which was firstly discovered by our group . Due to the existence of a statistical distribution of Ln and Li atoms, these crystals have a high structure disorder, which results in the absorption and emission lines broadening homogeneously when rare-earth ions are doped and occupy the positions of Ln3+ ions . Li3Ba2La3(WO4)8 (hereafter denoted as LBLW) is a member of this family. In this work, the thermal expansion coefficients and thermal conductivity of Er3+: LBLW and Er3+/Yb3+: LBLW single crystals grown by TSSG method were measured. The room-temperature polarized absorption and fluorescence spectra as well as the up-conversion mechanism of both kinds of crystals were reported and analyzed.
Materials and Methods
1. Crystal Growth
The Er3+: LBLW and Er3+/Yb3+: LBLW crystals were grown by the top-seeded solution growth (TSSG) method from a flux of Li2WO4. The crystal growth was carried out in a vertical tubular furnace. The schematic diagram of crystal growth apparatuses is same as that in Ref. . The furnace temperature was controlled by an AL-708 controller with controlling accuracy of ±0.1 K. The raw materials of Er3+: LBLW and Er3+/Yb3+: LBLW were synthesized by the solid-state reaction. The chemicals used were WO3, Li2CO3, BaCO3, La2O3, Er2O3 and Y2O3 with the purity of 99.99%. The solutions were composed of 25 mol% of solute (LBLW) and 75 mol% of solvent (Li2WO4).The crystal growth procedure is similar to that in Ref . When the growth ended, the crystals were drawn out of the solution and cooled down to room temperature at a cooled rate of 15 K/h. Fig. 1 shows the grown Er3+: LBLW and Er3+/Yb3+: LBLW crystals with dimensions of 56 mm×28 mm×9 mm and 52 mm×24 mm×8 mm, respectively.
The concentrations of rare earth ions were determined to be 0.41 at.% Er3+ in Er3+: LBLW crystal and 0.48 at.% Er3+ and 3.18 at.% Yb3+ in Er3+/Yb3+: LBLW crystal by the inductively coupled plasma atomic emission spectrometry (ICP-AES, Ultima2, Jobin-Yvon).
2. Thermal Properties
The thermal expansion of crystal is an important thermal factor for the crystal growth , . The thermal expansion coefficients were measured using a thermal expansion dilatometer (NETZSCH DIL 402 PC). The linear thermal expansion coefficient is defined as:(1)where L0 is the initial length of the sample at room temperature, and is the change in length when the temperature changes . Since the LBLW crystal with monoclinic is of anisotropy, the thermal expansion coefficient αij is a second rank tensor with four nonzero components in the orthogonal crystallo-physical axes (a, b, c*) . Thus, in order to obtain thermal expansion ellipsoid, the measurement should be carried out along at least four different directions. Therefore, four rectangular samples were cut from both the Er3+-doped and Er3+/Yb3+-codoped LBLW crystals, of which three were along the crystallographic a-, b- and c*-axis and the fourth, namely c’, was cut with the anti-clockwise angle (φ) 45° with respect to the c-axis. During the measurement, the samples were heated at a heating rate of 5 K/min in the range of 300∼1100 K in the air atmosphere.
The processes to determine the thermal expansion tensor in both crystals is similar, therefore here, for brevity, we mainly discuss the Er3+-doped one. The measured thermal expansion ratios versus T are shown in Fig. 2 (a). It can be found that when the temperature is below 450 K, the value of rise nonlinearly with the temperature. This may be due to the error caused by the thermal dilatometer at temperature below 450 K , . By linear fitting of the curves above 450 K, the values of the thermal expansion coefficients along a, b, c* and c’ axes are derived as αa = 11.3×10−6 K−1, αb = 8.07×10−6 K−1, αc* = 8.82×10−6 K−1 and αc’ = 9.81×10−6 K−1, respectively. The values of the diagonal elements in the crystallo-physical axes are α11 = αa, α22 = αb and α33 = αc*. α13 = α31 can be deduced from the equation ,(2)
The next step is to find the values of the principal thermal expansion. For a monoclinic crystal, one of the principal axes (XII) of the thermal expansion ellipsoid coincides with the crystallographic b-axis. The other two principle axes (XI, XIII) which can be calculated from the secular equation det(αij-λδij) = 0  are in the (0 1 0) plane. For Er3+-doped LBLW crystal, the eigenvalues are α’11 = 11.33 K−1 and α’33 = 8.80 K−1, and the linear thermal expansion tensor in the principal axes is(4)the angle ρ between the crystallo-physical c*-axis and principal XIII axis can be evaluated by(5)the minus value of ρ denotes the clockwise angle from c*-axis to the XIII axis (see Fig. 2 (b)).
The values of the linear thermal expansion coefficients along the optical indicatrix axes are more important in practice because the laser elements are normally cut along these axes. The orientation of the optical indicatrix axes (X, Y, Z) with respect to the crystallographic axes (a, b, c) is from that of Ref : (a, X) = 19° and (c, Z) = 20° (see Fig. 2 (b)). Using the detailed procedure described in Ref. , the ellipsoid in the optical indicatrix axis can be determined as(6)
The linear thermal expansion coefficient for the both Er3+-doped and Er3+/Yb3+-doped crystals along the directions of crystallo-physical axes (a, b, c*), principal axes (XI, XII, XIII) and optical indicatrix axes (X, Y, Z) are included in Table 1. The values of αb/αa and αa/αc* are 0.71 and 0.78, respectively. The thermal expansion exhibits a larger anisotropy than Li3Ba2La3(MoO4)8 crystal , which means the LBLW crystal is easier to crack during the cooling process. Therefore, a slow annealing rate should be applied in the crystal growth procedure.
The thermal conductivity coefficient (κ) of both Er3+-doped and Er3+/Yb3+-doped crystals were measured by the laser-flash method (Model NETZSCH LFA 457, Germany) in the temperature range 350–700 K. Four samples along a, b, c and c* crystallographic directions for each crystal were prepared for thermal conductivity measurements. The dimension of the samples was about 6 mm×6 mm×2 mm. Fig. 3 shows the evolution of κ with temperature of both kinds of crystals. The average values of thermal conductivity at 400 K are 0.95 and 0.94 Wm−1K−1 for Er3+-doped and Er3+/Yb3+-codoped LBLW, respectively. Compared with other typical tungstate crystals, such as KGd(WO4)2 (≈3.3 Wm−1K−1) , KY(WO4)2 (≈2.7 Wm−1K−1)  and KLu(WO4)2 (≈3.3 Wm−1K−1) , the thermal conductivity of the LBLW crystal is very low. The low thermal conductivity may be related to the disordered structure of LBLW crystal which can increase the probability of phonon-phonon scattering. In fact, NdGd(WO4)2 with disordered structure also has very low thermal conductivity (≈1.2 Wm−1K−1) .
3. Spectral Properties
Two cubic samples with dimensions of 7.4 mm×3.8 mm×5.8 mm and 7.2 mm×2.4 mm×4.7 mm were cut from the Er3+: LBLW and Er3+/Yb3+: LBLW crystals, respectively. Each face of samples was perpendicular to one of the optical indicatrix axes. All the surfaces of these cuboids were polished for spectral experiments. The polarized absorption spectra from 300 nm to 1700 nm were measured using a Perkin-Elmer UV-VIS-NIR spectrometer (Lambda 900). The polarized fluorescence spectra were recorded by a spectrophotometer (FLS920, Edinburgh) equipped with a xenon lamp as the excitation source. Two photomultiplier tubes (PMT) (Hamamatsu R955 and R5509) were used as the detectors in the VIS and NIR regions, respectively. Furthermore, the up-conversion spectroscopic experiments were carried out by a monochromator (Triax550, Jobin-Yvon) excited at 976 nm with a diode laser, and the power range of the diode emission was from 40 to 1400 mW. The signals were detected with a PMT (R943-02, Hamamasu). All measurements were performed at room temperature.
The absorption spectra of the Er3+: LBLW and Er3+/Yb3+: LBLW crystals at room temperature are shown in Fig. 4. These sharp absorption lines are attributed to the Er3+ ions except the broad absorption band at 900–1050 nm, which is the overlap of the 4I15/2→ 4I11/2 transition of Er3+ ions and the 2F7/2→ 2F5/2 transition of Yb3+ ions. In comparison with Er3+: LBLW crystal, such broad and strong absorption band around 900–1050 nm was mainly attributed to the 2F7/2→2F5/2 transition of Yb3+ ions. The absorption coefficients for Er3+/Yb3+: LBLW crystal are 1.76 cm−1 at 980 nm, 2.54 cm−1 at 974 nm and 1.80 cm−1 at 978 nm for E||X, E||Y and E||Z respectively. They are roughly ten times as large as those of the Er3+: LBLW crystal (0.15 cm−1, 0.14 cm−1 and 0.22 cm−1 for E||X, E||Y and E||Z, respectively). Therefore, the crystal co-doped with Yb3+ ions can significantly increase the absorption of the pump energy if pumped at around 980 nm. It should be also noted that the FWHMs of Er3+/Yb3+: LBLW crystal around 980 nm are 35 nm, 38 nm and 34 nm for E||X, E||Y and E||Z, respectively, and these values are larger than those of Er3+/Yb3+: YCa4O(BO3)3 and Er3+/Yb3+: GdCa4O(BO3)3 crystals , . The broad absorption bands which can relax the requirement of accurate temperature control of diode laser make Er3+/Yb3+: LBLW crystal suitable for diode laser pumping.
The Judd-Ofelt theory ,  has been widely used to analyze the spectroscopic properties of the rare earth ions except Yb3+ ion in crystals. The oscillator strength parameters Ωt (t = 2, 4, 6) can be fitted from the room-temperature absorption spectra, then the spontaneous emission probabilities, radiative lifetime and fluorescence branching ratios can be obtained. The detailed calculation procedure is similar to that reported in Ref . The reduced matrix elements values of unit tensor operators used in the calculation could be found in Ref , . Except for the two high absorption bands which centered at 524 nm and 379 nm, namely 4I15/2→2H11/2 and 4I15/2→4G11/2, respectively (see Fig. 4), all the other ones were chose to fit the oscillator strength parameters for E||X, E||Y and E||Z polarizations. Because those two transitions belong to hypersensitive transition , , they are sensitive to the variation of local structure around Er3+ ions. Here, only the spectrum of the Er3+: LBLW crystal was calculated for brevity. Table 2 lists the values of the measured (Smea) and calculated (Scal) line strengths, the intensity parameters ΩX,Y,Z for each polarization as well as the effective intensity parameters which are defined as Ωeff = (ΩX+ΩX+ΩX)/3. After obtaining the oscillator strength parameters ΩX,Y,Z for each polarization, the spontaneous emission probabilities of the electric- and magnetic-dipole transitions (named and respectively), fluorescence branching ratio β and radiative lifetime τr of some typical transitions could be gained. The values of these spectroscopic parameters are all outlined in Table 3.
The Er3+: LBLW crystal could not be efficiently excited by Xenon lamp because of the weak absorption at 976 nm. Moreover, considering the small phonon energy of the (WO4)2− groups (roughly 900 cm−1) , the multiphonon relaxation from the 4I11/2 to 4I13/2 multiplets of Er3+ ions was slow. Therefore, the emission band surrounding 1550 nm (4I13/2→4I15/2) for Er3+: LBLW crystal is too weak to be distinguished. Thus, the fluorescence spectra of the Er3+/Yb3+: LBLW crystal were only recorded (see Fig. 5). The stimulated-emission cross-sections were calculated by the Füchtbauer-Ladenburg (F-L) formula , ,(7)where Aq is the spontaneous emission probability for q polarization, Iq(λ) is the fluorescence intensity as a function of wavelength. The peak emission cross-sections are about 0.81×10−20, 1.23×10−20 and 0.84×10−20 cm2 for E||X, E||Y and E||Z respectively, which are comparable to other co-doped crystals, such as 1.89×10−20 cm2 for Er3+/Yb3+: KY(WO4)2 , 0.71×10−20 cm2 for Er3+/Yb3+: LaPO4  and 0.95×10−20 cm2 for Ce3+/Er3+ NaLa(MoO4)2 .
The Er3+ laser via the 4I13/2→4I15/2 transition operates in a quasi-three scheme, therefore the re-absorption losses should be considered. The useful laser wavelength could be evaluated by the so-called effective gain cross section .(8)here, is the emission cross-section, is the absorption cross-section and β is the population inversion of Er3+ ions. Results of the wavelength dependences around 1550 nm for several β values (β = 0.4, 0.5, 0.6, 0.7) are shown in Fig. 6. It can be noted that the wavelengths under the low population inversion, for all polarizations, are all located approximately 1590 nm. Additionally, a laser oscillating at shorter wavelength can also be realized by increasing the values of β.
Fig. 7 shows the up-conversion fluorescence spectra for Er3+: LBLW and Er3+/Yb3+: LBLW crystals in the range from 500 to 700 nm excited at 976 nm radiation of diode laser. Note that the fluorescence intensity of Er3+/Yb3+ co-doped LBLW crystal is much larger than that of Er3+-doped LBLW. This means there existed fast and efficient Yb3+→Er3+ energy transfer in Er3+/Yb3+: LBLW crystal. Fig. 8 displays the up-conversion mechanisms and simplified energy levels of Er3+ and Yb3+ ions in Er3+/Yb3+: LBLW crystal. Two different mechanisms, namely Er3+ excited state absorption (ESA) and a two-step Yb-Er energy transfer (ET), may exist in the up-conversion process , , .
For the Er3+/Yb3+ crystal, the green emissions of 530 and 553 nm (2H11/2→4I15/2 and 4S3/2→4I15/2, respectively) can be explained by the following steps: Firstly, the Er3+ ions were excited from ground state to the excited state 4I11/2 by means of ground state absorption (GSA) and by ET process from 2F5/2 level of Yb3+ to Er3+. The ET process is dominant because of the large absorption across-section around 980 nm of Yb3+ ions. Secondly, some Er3+ ions at the 4I11/2 level were promoted up to the higher 4F7/2 level by ET process from 2F5/2 level of Yb3+ or by ESA of Er3+ ions, then the ions at the 4F7/2 level relaxed non-radiatively to the lower levels 2H11/2 and 4S3/2 owning to the small energy gap between them. When the Er3+ ions at the 2H11/2 and 4S3/2 levels transited to the ground state, they produced 530 and 553 nm green emissions, respectively. The green emissions of the Er3+: LBLW crystal also experienced the above processes except the lack of ET process. Because the lifetime of the 4S3/2 level is much longer than that of the 2H11/2 level , more ions would non-radiatively decay to the 4S3/2 level. As a consequence, the intensity of 553 nm is stronger than 530 nm.
For the red emission of 661 nm (4F9/2→4I15/2), population on the 4F9/2 might be accumulated by two ways: ESA and ET process. Both ways excited Er3+ ions from 4I13/2 to 4F9/2. Besides, the ions at the 4S3/2 level also relaxed rapidly to the 4F9/2 level. The red emission intensity is also significantly weaker than that of Er3+/Yb3+ crystal because of lacking of ET process in the Er3+: LBLW crystal.
The dependence of integrated up-conversion fluorescence intensity on the excitation power at 976 nm for Er3+/Yb3+ crystal is shown in Fig. 9. According to the relation , where n is the number of photon involved in the up-conversion process and I is the excitation power. The slopes (for green and red light are all near 2) indicate that two photon processed populated the 2H11/2, 4S3/2 and 4F9/2 levels. However, due to the competition between the linear decay and the depletion of the intermediate excited states, the values of n may be lower than 2 (see Fig. 9) .
Results and Discussion
The Er3+: LBLW and Er3+/Yb3+: LBLW have been successfully grown by the TSSG method from the flux of Li2WO4. The thermal expansion coefficients in the optical indicatrix axes were αX = 11.17×10−6 K−1, αY = 8.07×10−6 K−1 and αZ = 8.94×10−6 K−1 for the Er3+: LBLW crystal, and αX = 11.18×10−6 K−1, αY = 8.01×10−6 K−1 and αZ = 9.22×10−6 K−1 for the Er3+/Yb3+: LBLW crystal. The anisotropy of thermal expansion indicates that the LBLW crystals are easier to crack; thus, slow cooling rate should be adopted after the crystals were withdrawn from the melt. The Er3+/Yb3+: LBLW crystal has broad absorption bands near 980 nm (35 nm, 38 nm and 34 nm for E||X, E||Y and E||Z, respectively), which make it very suitable for diode pumping. The effective J-O intensity parameters of the Er3+: LBLW were calculated to be = 11.94×10−20 cm2, = 1.60×10−20 cm2, = 0.78×10−20 cm2, respectively. Considering the re-absorption losses of the quasi-three scheme, the effective emission cross-section around 1550 nm was also calculated. Under the 976 nm excitation, the up-conversion emissions of three visible optical bands, corresponding to the 2H11/2→4I15/2, 4S3/2→4I15/2 and 4F9/2→4I15/2, respectively, for Er3+/Yb3+: LBLW crystal were observed. The investigation of up-conversion spectra denotes that the energy transfer between Yb3+ and Er3+ is efficient. The spectroscopic analysis reveals that the Er3+/Yb3+: LBLW crystal has much better optical properties than the Er3+: LBLW crystal. Therefore, the Er3+/Yb3+: LBLW crystal may become a potential candidate for solid-state laser gain medium material.
Conceived and designed the experiments: BX GW. Performed the experiments: BX ZL. Analyzed the data: BX GW. Contributed reagents/materials/analysis tools: YH LZ. Wrote the paper: BX GW.
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