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Fibers 2013, 1(2), 11-20; doi:10.3390/fib1020011
Abstract: This work reports the enhanced emission at 2.7 µm in Er3+/Ho3+-codoped fluorotellurite glass upon a conventional 980 nm laser diode. The significantly reduced green upconversion and 1.5 µm emission intensity in Er3+/Ho3+-codoped samples are observed. The results suggest that the Er3+: 4I13/2 state can be efficiently depopulated via energy transfer from Er3+ to Ho3+ and the detailed energy transfer mechanisms are discussed qualitatively. The energy transfer efficiency from Er3+: 4I13/2 to Ho3+: 5I7 is calculated to be as high as 67.33%. The calculated emission cross-section in Er3+/Ho3+-codoped fluorotellurite glass is 1.82 × 10−20 cm2. This suggests that Er3+/Ho3+-codoped fluorotellurite glass is a potential material for 2.7 µm fiber laser.
Owing to the increased interest in mid-infrared laser fiber (2–5 µm) used in laser surgery and remote chemical sensing fields, considerable researches have been performed to searching for new materials to use as hosts for mid-infrared laser hosts especially for Er3+ 2.7 µm [1,2]. Among many alternatives, fluoride fibers have emerged as natural candidates for mid-infrared laser materials because of their low phonon energy which decreases the rate of phonon-assisted nonradiative transitions [3,4]. However, fluoride fibers suffer from poor thermal stability and require complex fabrication route. Usually, the mid-infrared emission of Er3+ can hardly be observed in oxide glasses owing to the large phonon energy. However, it is well known that oxide glasses are more chemically and thermally stable. Among all the oxide glasses, tellurite glasses emerge as good candidates for mid-infrared fiber laser materials because of their lowest phonon energy (760 cm−1) among all the oxide glasses with large refractive index and a broad transmission window (0.4–6 μm) [5,6,7].
Er3+ is an ideal luminescent center for 2.7 µm mid-infrared emission corresponding to the 4I11/2→4I13/2 transition. However, Er3+ suffers from self-terminating of the 4I11/2 level because of the much shorter lifetime of the emitting level (4I11/2) as compared to the terminal laser level (4I13/2). Fortunately, codoping with other ions such as Pr3+, Nd3+, Tm3+ and Ho3+ have been proved to be feasible alternatives to enhance the 2.7 µm emission [8,9,10,11]. The strong OH− absorption around 3 µm is another important fact to obtain efficient Er3+ 2.7 µm emission. As is reported before , the addition of fluoride in the tellurite glasses was proved to be an effective way to reduce OH−1 groups and increase the radiative transition probabilities of 2.7 µm emission. Therefore, we prepare the Er3+/Ho3+-codoped fluorotellurite glass and evaluate the spectroscopic parameters based on the absorption spectra using the Judd-Ofelt theory. The detailed energy transfer processes based on the measured upconversion, near-infrared and mid-infrared fluorescence spectra are also discussed.
2. Experimental Section
The investigated fluorotellurite glasses in this study have the following molar compositions: 85TeO2-10PbF2-5LaF3-1ErF3-xHoF3(x = 0, 2), hereafter named TF glass. The samples were prepared using high-purity of powder. Well-mixed, 25 g batches of the samples were placed in an aluminum crucible and melted at 900 °C for 30 min. Then the melts were cast on a preheated steel plate and annealed for 3 h. at a temperature 10 °C below the Tg before they were naturally cooled to room temperature. The annealed samples were polished with a thickness of 1 mm for the optical property measurements.
The absorption spectra were recorded by a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrophotometer in the wavelength range of 400–1700 nm. The fluorescence spectra were measured with an Edinburg FLSP920 type spectrometer upon excitation at 980 nm. The fluorescence lifetime was measured with the instrument FLSP920 (Edinburgh instruments Ltd., UK). All the measurements were carried out at room temperature.
3. Results and Discussion
3.1. Absorption Spectra and Judd-Ofelt Analyses
Figure 1 shows the absorption spectra of Er3+ singly and Er3+/Ho3+-codoped TF glasses. All the intrinsic absorption transitions of Er3+ and Ho3+ in the region from 300 to 1700 nm are retained and labeled in Figure 1. The strong absorption around 980 nm of the Er3+/Ho3+-codoped sample indicates that this glass can be excited efficiently by a 980 nm laser diode (LD). It can be seen that Er3+: 4F9/2, Ho3+: 5F5 and Er3+: 4S3/2, Ho3+: (5S2 + 5F4) are very close, which show that the energy transfer processes in Er3+/Ho3+-codoped TF glasses are expected to be efficient.
The Judd-Ofelt theory [13,14] has been commonly applied to determine the important spectroscopic and laser parameters of rare earth doped glasses. Judd-Ofelt theory has been described in other literature in detail . Basically, the Judd-Ofelt analyses were applied using the experimental oscillator strengths of the absorption bands obtained from absorption spectra. Judd-Ofelt intensity parameters and oscillator strengths provide indirect information of the symmetry and bonding of rare-earth polyhedra within the matrix. Experimental (fmea) and theoretical (fcal) oscillator strength for representative transitions of Er in TF glass are tabulated in Table 1. Then the Judd-Ofelt intensity parameters, Ωλ (λ = 2, 4, 6), can be derived using a least-square fitting approach and are shown in Table 2. Generally, Ωλ is closely related to the structure change of the sites of rare earth ligand and the basicity of the glass network. It is hypersensitive to the change of composition of host materials. As is shown in Table 2, the calculated Ω2 of Er3+ and Ho3+ in the present glass is lower than that other oxide glasses since the addition of fluoride in the tellurite glass can reduce the covalency and ligand of the glass matrix. The Ωλ parameters follow the trend Ω2 > Ω4 > Ω6 in present TF glass. It should be mentioned that the root-mean-square is 2.98 × 10−6 for Er3+/Ho3+-codoped TF glass. The larger value of the fitting is due to the overlap of energy levels of Er3+ and Ho3+ we select to calculate the intensity parameters Ωλ.
|Absorption||Wavelength (nm)||Oscillator strength (10−6)|
Table 3 represents the radiative transition probabilities (Ar), branching ratios (β) and radiative lifetime (τr) of certain levels of Er3+ ions which are calculated using the above obtained Judd-Ofelt intensity parameters. It is shown that the radiative probabilities Ar of Er3+: 4I11/2→4I13/2 transition is 53.26 s−1 for Er3+/Ho3+-codoped samples.
|Transitions||Aed (s−1)||Amd (s−1)||Er3+/Ho3+||τ (ms)|
3.2. Fluorescence Spectra Analyses and Energy Transfer Mechanisms
Figure 2 presents the mid-infrared and near-infrared emission spectra of Er3+ and Er3+/Ho3+-codoped TF glasses under excitation at 980 nm. The emission at 2.7 µm corresponds to the transition of the Er3+: 4I11/2→4I13/2. The emissions at 1.53 and 2.05 µm come from the transition of Er3+: 4I13/2→4I15/2 and Ho3+: 5I7→5I8, respectively. 2.7 µm emission can be observed in both kinds of samples. However, the intensity of the 2.7 µm increases with the addition of the HoF3, which demonstrates the effective sensitization of Ho3+ ions.
From the experimental phenomenon and theoretical data, possible mechanisms  for the emission bands are discussed based on the simplified energy levels of Er3+ and Ho3+ presented in Figure 3. Ions on the Er3+: 4I15/2 state are excited to the 4I11/2 state by ground state absorption (GSA) when the sample is pumped by 980 nm LD. The involved energy transfer mechanisms processes based on the 4I11/2 level are as follows: excited state absorption (ESA1), Er3+: 4I11/2 + a photon→Er3+: 4F7/2; ETU1, Er3+: 4I11/2 + 4I11/2→4I15/2 + 4F7/2; ETU3, 4I11/2(Er3+) + 5I6(Ho3+)→4I15/2(Er3+) + 5F4(Ho3+); 4I11/2→4I13/2 transition with 2.7 µm emission; ET1(a nonresonant process), from the Er3+: 4I11/2 level to the Ho3+: 5I6 level, energy excess (1470 cm—1) is given to the matrix; non-radiatively relaxation to the Er3+: 4I13/2 level.
The Er3+: 4I13/2 level is populated owing to the nonradiative relaxation from the upper 4I11/2 level. There exist four main energy transfer processes for the Er3+: 4I13/2 level in present glass as follows: excited state absorption (ESA2), Er3+: 4I13/2 + a photon→Er3+: 4I9/2; ETU2 , Er3+: 4I13/2 + 4I13/2→4I15/2 + 4F9/2; ETU4, 4I13/2 (Er3+) + 5I6(Ho3+)→4I15/2(Er3+) + 5F5(Ho3+) ; 4I13/2→4I15/2 transition with 1.5 µm emission; ET2(a nonresonant process), from the Er3+: 4I13/2 level to the Ho3+: 5I7 level, energy excess (1398 cm−1) is given to the matrix. After ET2 process, 2.05 µm emission can be observed due to the Ho3+: 5I7→5I8. The ESA2 process populates the Er3+: 4F9/2 level which relaxes radiatively to the ground state with red emission around 660 nm and non-radiatively to the next lower Er3+: 4I9/2 level. The energy stored in the Er3+: 4I9/2 can partly be non-radiatively decayed to the Er3+: 4I11/2 level, which is beneficial to the 2.7 µm emission.
The visible emission spectra for Er3+ doped and Er3+/Ho3+-codoped glasses upon 980 nm excitation are shown in Figure 4. Three visible emission peaks centered at 525, 550 and 660 nm are observed. As discussed above, the stored energy in the 4F7/2 level after ESA1 process decays non-radiatively to the lower levels 2H11/2 and 4S3/2. Then the Er3+: (2H11/2 + 4S3/2)→4I15/2 transitions bring green emissions (525 and 548 nm). Meanwhile, the excited Ho3+ ions at (5S2 + 5F4) and 5F5 levels also generate 550 and 660 nm emissions, respectively. It is noted that the fluorescence intensity of red emissions becomes stronger when codoped with Ho3+ while the green emission becomes weaker. Because the energy gaps of the ET1 and ET2 process are relatively small, so both processes can happen efficiently. It is expected that the ESA1 and ESA2 processes will be reduced while the sample is codoped with Ho3+, so part of green and red emission can be attributed to the Ho3+ upconversion emissions. Since the energy gap of the ET2 process is smaller than that of ET1 process, the energy transfer efficiency of ET2 is expected to be higher than that of ET1, this is also proved from the extremely decreased 1.5 µm emission in the Er3+/Ho3+-codoped samples, consequently ions on the 5I7 level are much more than that on the 5I6 level. Then the 5F5 level that is populated through ETU4 generates stronger 660 nm emission while the 5S2 (5F4) levels that are populated through ETU3 generate weaker 550 nm emission.
3.3. Fluorescence Spectra Analyses and Energy Transfer Mechanisms
The energy transfer efficiency has been estimated from the measured lifetime of the 1.53 µm emission of Er3+ singly and Er3+/Ho3+-codoped samples by the following formula :
3.4. Cross-Sections Analyses
The emission cross section is an important factor for evaluating the emissive ability of luminescent center. The absorption and emission cross sections could be calculated from Füchtbauer–Ladenburg equation  and McCumber theory :
Enhanced 2.7 µm emission was obtained in Er3+/Ho3+-codoped fluorotellurite glass. This suggests that Ho3+ can be a feasible approach to obtain efficient Er3+ 2.7 µm emission pumped by common 980 nm LD in fluorotellurite glass for practical applications. It was also found that the green upconversion and 1.5 µm emissions extremely decreased in the Er3+/Ho3+-codoped glass. The energy transfer mechanisms between Er3+ and Ho3+ were discussed in detail and the energy transfer efficiency was calculated to be 67.33%. Larger absorption and emission cross sections of Er3+: 4I11/2→4I13/2 in TF glasses were obtained which were 1.54 × 10−20 cm2 and 1.82 × 10−20 cm2, respectively. These results suggest that Er3+/Ho3+-codoped fluorotellurite glass has potential applications for 2.7 µm fiber laser materials.
This research was financially supported by the Chinese National Natural Science Foundation (No. 51172252).
Conflicts of Interest
The authors declare no conflicts of interest.
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