We present a detailed characterization of enhanced 2.0 μm emission and energy transfer processes by codoping Ce3+ in ZBYA: Ho3+/Yb3+ glasses under 1550 nm excitation. The measured absorption and emission spectra show that Er3+ ions are efficiently excited by pumping and energy transfer from Er3+: 4I13/2 to Ho3+: 5I7 level. The 2.0 μm emission from the Ho3+: 5I7→5I8 transition is enhanced by codoping Ce3+ (< 0.5 mol %) ions in the Ho3+/Er3+ doped glasses. However, excess Ce3+ ions in the glass network negatively affect the mid-infrared emission. The upconversion luminescence is dominated by Er3+ (667 nm) red emission in the Ho3+/Er3+ doped sample, which is suppressed after introducing Ce3+ ions. The red emission is abnormally dominated by the Ho3+ (650 nm) emission when the ratio of the three ions (Ho3+: Er3+: Ce3+) is 1:1:0.5. These results indicate that Ce3+ ions can enhance Ho3+: 2.0 μm emission by suppressing the upconversion processes. The Ho3+/Er3+/Ce3+ triply-doped ZBYA glass is a promising material for 2.0 μm fiber laser applications.
© 2014 Optical Society of America
Recently, 2.0 μm laser materials have attracted extensive interest because of their applications in medical surgery, remote sensing, and eye-safe radar [1–6]. Among laser sources, solid-state materials doped with trivalent rare-earth ions, such as thulium (Tm3+) and holmium (Ho3+), represent the main area of development because of their reliability and compactness [7–9]. Ho3+ serves as the active ion emitting at 2.0 μm by 5I7→5I8 transition, which possess higher gain cross-section, longer radiative lifetime, and longer-operating laser wavelength than Tm3+ . However, the lack of effective pump sources for Ho3+ doped materials limits their development. Previous investigations have been trying to use Yb3+, Tm3+, and Er3+ ions to sensitize the Ho3+ ions under 800 or 980 nm LDs (laser diodes) [11–13]. In our current experiments, we found that intensive Ho3+: 2.0 μm emission is also observed in Ho3+/Er3+ doped materials under 1550 nm excitation. In theory, Ho3+ is more effectively sensitized by Er3+ ions under 1550 nm LD compared with 800 and 980 nm LDs.
Trivalent cerium (Ce3+) is an ideal candidate in enhancing Er3+ 1.5 μm emission by reducing the upconversion emission when pumped by a 980 nm LD [11, 14–16]. A small energy mismatch is found between the Er3+:4I11/2→4I13/2 and Ce3+: 2F7/2→2F5/2 energy gap. The addition of Ce3+ in the Er3+ doped materials results in rapid decay of more Er3+ ions in the 4I11/2 to the next lower level 4I13/2 through cross relaxation (CR) process (Er3+:4I11/2 + Ce3+ 2F5/2→Er3+:4I13/2 + Ce3+: 2F7/2). This conversion suppresses the excited state absorption (ESA) processes. Our work shows that strong red emission is obtained in the Ho3+/Er3+ materials under 1550 nm excitation. As well known, the Ho3+:5I6→5I7 shows energy gap that is similar to that of the Er3+: 4I11/2→4I13/2. Thus, the introduction of Ce3+ ions in the Ho3+/Er3+ doped materials can suppress the upconversion processes and enhance the 2.0 μm emission through CR processes (Er3+:4I11/2 + Ce3+ 2F5/2→Er3+:4I13/2 + Ce3+: 2F7/2 and Ho3+:5I6 + Ce3+: 2F5/2→Ho3+:5I7 + Ce3+: 2F7/2) . However, the 2.0 μm emission properties in the Ho3+/Er3+/Ce3+ doped materials have not been investigated.
Glasses have been known as convenient hosts for rare-earth ions and have been widely used because of their good mechanical and thermal stability, low synthesis cost, as well as possibility of pulling to fiber compared with crystals . To develop more efficient optical devices based on Ho3+, the host glass is another important factor that needs to be considered besides the sensitizer ions . Fluoride glasses have advantages of low phonon energy, high doping level, low viscosity, and wide transparency from UV to IR . We have reported the ZrF4-BaF2-YF3-AlF3 (ZBYA) system, which has low phonon energy, fine chemical ability and good optical properties . In current study, we report the sensitization effect of Ce3+ ions on Ho3+/Er3+ doped ZBYA glass. The luminescence characteristics and energy transfer processes are discussed.
The compositions of the glasses were (98-X) ZBYA-1ErF3-1HoF3-XCeF3 (X = 0, 0.25, 0.5, 1 designated as EH, EHC0.25, EHC0.5, and EHC1, respectively). The 1 mol % Ho3+ singly doped ZBYA glass with the composition of 99 ZBYA-1HoF3 was also prepared as a comparison and was designed as H. The samples were prepared using high-purity ZrF4, AlF3, YF3, BaF2, ErF3, HoF3, and CeF3 powders (99% to 99.99%). Well-mixed 25 g batches of the samples were placed in platinum crucibles and melted at approximately 1000°C for 30 min. The melts were then poured onto a preheated copper mold and annealed in a furnace around the glass transition temperature. The annealed samples were fabricated and polished to the size of 20 mm × 15mm × 1 mm for the optical property measurements.
The densities and refractive indices of the samples were measured through the Archimedes method using distilled water as an immersion liquid and the prism minimum deviation method respectively. The transmission spectra were obtained using a Nexus FT-IR spectrometer (Thermo Nicolet USA) in the range of 2500 nm to 10000 nm. Furthermore, absorption spectra were also obtained with the use of a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrophotometer in the range of 200 nm to 2100 nm. Emission spectra were determined with a Triax 320 type spectrometer (Jobin-Yvon Co., France) under 1550 nm LD. All of the measurements were conducted at room temperature.
3. Results and discussions
3.1 Transmittance spectra
The inset (b) of Fig. 1 shows the transmittance spectrum of a 1.0 mm thick Ho3+ doped ZBYA sample. As it can be seen the transmittance reaches as high as 87%. The 13% loss contains the Fresnel reflections dispersion, and absorption of the glass. It is important to mention that the OH- content in the glass is related to the emission efficiency of rare earth ions as the residual OH- groups will participate in the energy transfer (ET) of rare earth ions and reduce the intensity of emission [21, 22]. The content of OH- groups in the glass is expressed by the absorption coefficient of the OH- vibration band at 3 μm which is given by 24, 25]25, 26] because there is no relevant data reported about fluoride glass so far. The absorption coefficient and OH- content at 3 μm is 0.031 cm−1 and 3.8 × 1017 cm−3, respectively, which are much lower than those of fluorophosphate, tellurite and silicate glasses [25, 27]. The good IR transmission property proves that fluoride glass is a potential candidate for IR laser materials.
3.2 Absorption spectra
Absorption spectra of Ho3+ singly doped, Ho3+/Er3+ codoped and Ho3+/Er3+/Ce3+ tridoped fluoride glasses in the wavelength region 400-2100 nm are shown in Fig. 1. The corresponding Ho3+ and Er3+ absorption bands are also labeled. The characterized three bands of the Ho3+ singly doped sample center at 1150, 637, and 536 nm, corresponding to the absorptions starting from the ground state 5I8 to the excited states 5I6, 5F5, and 5F4 + 5S2, respectively. Meanwhile, the emerging three band of the Ho3+/Er3+ codoped sample located at 1550, 980, and 800 nm belong to the absorptions of the Er3+ ions. The absorption spectra remain the same when the Ce3+ ions are introduced into the Ho3+/Er3+ codoped glass network as shown in Fig. 1. However, it can be seen from inset Fig. 1(a) that the addition of Ce3+ ions results in red-shift of the UV-side absorption edge. This red-shift has been reported in the Ce3+ doped phosphate glasses and is assigned to the 4f→5d transition in Ce3+ ions which causes a broad absorption around 400 nm to 500 nm [15, 28]. The codoped and tridoped samples are effectively pumped by 800, 980, and 1550 nm LDs owing to the Er3+ ions.
Based on the measured absorption spectra, the absorption cross section (σa) can be deduced usingFig. 2. Evidently, the 1550 nm emission shows the largest absorption cross-section. In this study, the samples were pumped by 1550 nm LD.
Important spectroscopic and laser parameters of rare earth doped glasses have been commonly analyzed using Judd–Ofelt (J-O) theory by many researchers [2, 29]. Details of the theory and method have been well described in earlier studies [30–34], hence only the results are presented here. The intensity parameters Ωt, fexp, fcal of Ho3+-doped present glass were calculated from the absorption spectrum. The fexp, and fcal of the sample are shown in Table 1 and Ωt parameters together with the values reported for other Ho3+-doped host are summarized in Table 2.
As is shown in Table 1, good agreement is found between the calculated and experimental values. The room-mean-square error deviation of intensity parameters is 0.9 × 10−6, which indicates the validity of the J-O theory for predicting the spectral intensities of Ho3+ and the reliable calculations. The J-O intensity parameters provide information on the symmetry and bonding of the rare-earth polyhedral within the matrix . The disordered glass environment is important because of the effects on the luminescent properties of rare-earth ions and the intra-configurational optical transitions . The J-O intensity parameters of Ho3+ for the glass in the present work follow the trend of Ω4>Ω2 >Ω6, which is also reported in other fluoride glasses .
Ω2 is closely related with the hypersensitive transitions. For example, the larger line strength of the hypersensitive transition implies a larger Ω2 value. The hypersensitivity is related to the covalency parameter through the nephelauxetic effect and is attributed to the increasing polarizability of the ligands around the rare earth ions . The calculated Ω2 of Ho3+ in the present glasses is lower than those of the oxide glasses, because oxygen ions possess higher polarizability than fluoride ions caused by their lower electronegativity . This parameter is also affected by the asymmetry of the rare earth sites and is reflected on the crystal field parameters. Meanwhile, given that Smd (line strength for an electric dipole transition) is independent of ligand fields and Sed (line strength for the magnetic dipole transition) is a function of glass structure and composition, the relative contribution of the electric-dipole transition can be employed to obtain flat emission spectra. According to the J-O theory the line strength of the electric dipole components of 2.0 μm emission from Ho3+: 5I7→5I8 transition is expressed as:Table 2, Ω6 in the present glass is much higher than those in reported fluoride and oxide glasses. Therefore, this kind of fluoride glass is an appropriate host material to obtain flat emission spectrum from the Ho3+: 5I7→5I8 transition.
The host-dependent J-O intensity parameters can also be used to obtain the radiative parameters. The radiative properties including spontaneous transition probabilities, branching ratios, and radiative lifetimes of selected transitions of Ho3+ doped sample are listed in Table 3.The radiative lifetime of the 5I7→5I8 transition is 16.2 ms which is similar to that of fluoride glass (17.2 ms) . However, this radiative lifetime is much higher than those of FP (fluorophosphate) (5.6 ms) , germinate (0.36 ms)  and silicate (0.32 ms)  glasses. In general, the relatively longer radiation lifetime is beneficial to reduce the laser oscillation threshold. Therefore, this Ho3+ doped fluoride glass can be considered as an appropriate medium to achieve high-quality 2.0 μm laser. As shown in Table 3, the spontaneous transition probability A of the Ho3+: 5I7→5I8 transition in ZBYA glass is 61.44 S−1, whereas for 5I5→5I8 and 5F5→5I8 transitions, the values are as high as 135 and 2242 S−1, respectively. Thus, once the Ho3+ ions in the 5I7 level are populated to the upper levels (5I5 + 5I7) under 1550 nm LD, the ions decay directly to ground state instead of the 5I7 level. Ce3+ ions should be introduced to suppress the upconversion processes and populate the 5I7 level.
3.3 Fluorescence spectra
The IR photoluminescence spectra of Ho3+ singly doped, Ho3+/Er3+ codoped and Ho3+/Er3+/Ce3+ tridoped fluoride glasses in the wavelength region 1800 nm to 2200 nm are shown in Fig. 3. For the Ho3+ singly doped glass, the 2.0 μm emission from the 5I7→5I8 transition is absent because Ho3+ ions have no absorption under 1550 nm LD. Meanwhile, intensive 2.0 μm emission is observed in the Ho3+/Er3+ codoped sample. It is apparently that Er3+ ions are pumped by the excitation and transfer energy from Er3+: 4I13/2 level to Ho3+: 5I7 level (Fig. 5). The intensity of 2.0 μm emission is further enhanced by introducing 0.25 mol% Ce3+ ions into the Er3+/Ho3+ doped glass network. However, addition of more Ce3+ ions negatively affects the 2.0 μm emission.
To further understand the enhancement of Ho3+ 2.0 μm emission, it is necessary to investigate the upconversion processes of Ho3+/Er3+ codoped and Ho3+/Er3+/Ce3+ tridoped fluoride glasses. The upconversion luminescence spectra of the samples in the wavelength region of 500 nm to 850 nm are shown Fig. 4. Upconversion luminescence of the Ho3+ singly doped glass is also absent because no absorption is found under 1550 nm LD. Intensive red emission dominates the upvonversion luminescence of the codoped and tridoped samples because of the Er3+: 4F9/2 →4I15/2 and Ho3+: 5F5→5I8 transitions. As well known, Er3+: 4F9/2 →4I15/2 transition occurs at 667 nm and Ho3+: 5F5→5I8 transition locates at 652 nm. The shape of the red emission shows that the Er3+: 4F9/2 →4I15/2 transition dominates the red emission except for the EHC0.5 sample. The deconvolution of the red emission of the EHC0.5 sample is carried out and shown in Fig. 3(inset). The red emission origins from the Er3+: 4F9/2 →4I15/2 and Ho3+: 5F5→5I8 transitions and the Ho3+ 652 nm emission shows more intensive.
The involved energy transfer mechanisms are indicated in Fig. 5. For the Ho3+/Er3+ codoped sample, the Er3+ ions on the ground state jump to the 4I13/2 level by absorbing one 1550 nm photon. Some ions on the Er3+: 4I13/2 level can transfer energy to the Ho3+: 5I7 level and ions can also be pumped to the higher 4I9/2 level through the excited state absorption (ESA) process. Then the ions on the 4I9/2 level can decay radiatively or nonradiatively to the 4I11/2 level. The Er3+ ions on the 4I9/2 and 4I11/2 levels can continue absorbing another photon, which contributes to the population of 4S3/2 and 4F9/2 levels. The 4S3/2→4I15/2 and 4F9/2→4I15/2 transitions generate green and red emissions located at 550 and 667 nm, respectively. Here, the weak green emission indicates that most of the ions on the 4I9/2 level decay nonradiatively to the 4I11/2 level owing to its small energy gap. The Ho3+ ions on the 5I7 level can drop to ground state with the 2.0 μm emission. Meanwhile, Ho3+ ions can also jump to the higher level (5I5, 5F5) through ESA processes and generate red emission at approximately 650 nm. As shown in the spectra of the EH samples, the red emission is mostly from the Er3+:4F9/2→4I15/2 transition and the probability of ESA processes for the Ho3+ ions is small. The red emission is restrained trough the energy transfer processes (Er3+:4I11/2 + Ce3+:2F5/2→Er3+:4I13/2 + Ce3+:2F7/2 and Er3+:4F9/2 + Ce3+:2F5/2→Er3+:4I9/2 + Ce3+:2F7/2) after introducing 0.25 mol % Ce3+ ions in to the glass network. This effect is beneficial to 2.0 μm emission. When 0.5 mol % Ce3+ ions are introduced, more ions can be accumulated on the Ho3+: 5I7 level together with the long lifetime of the level, the ESA processes of Ho3+ become serious. Therefore, the Ho3+: 5F5→5I8 transition enhances and dominates the red emission. The 2.0 μm emission changes slightly. The emission spectra shows that if more Ce3+ ions are added in the glass, then the red and 2.0 μm emissions are both suppressed because of the energy transfer process between Ce3+ and the other two ions.
In order to certain the CR processes after adding Ce3+ ions more clearly, the spectra of 980, 1200, and 2708 nm emissions are tested and shown in Fig. 6(a). The 2708 nm emission comes from Er3+: 4I11/2 → 4I13/2 transition and it becomes weaker with the increasing of Ce3+ ions, which can directly prove the CR process between the Ce3+ and Er3+ ions. The emission spectra in the wavelength of 900 −1400 nm further certain the energy transfer process Er3+: 4I13/2→ Ho3+: 5I7 and the change of intensity of the two emissions accords well with the red emission. The decay curves of the samples at 1949 nm are plotted in Fig. 6(b). The Y-scale is the log values of the measured lifetimes and the decay curves recorded mainly belong to the single exponential function. Lifetime of the Ho3+: 5I7 level is reaches as high as 18.2 ms for Er3+/Ho3+ codoped sample. The lifetime becomes larger when introducing < 0.5mol % Ce3+ ions. The lifetime cures agree with the 2.0 μm emission spectra, which proves that the introduction of some Ce3+ ions is beneficial for the 2.0 μm emission and excess ions have negative effect.
Based on the upconversion spectra and energy transfer mechanism, the red emission of the Er3+ and Ho3+ ions belongs to three-photon processes. To confirm this conclusion, the dependence of the upconversion luminescence intensity on the incident pump power (log-log plot) is shown in Fig. 7. For the UC process, it is reported  that the emission intensity (I) increases proportional to the power of the infrared pump radiation (P) by the following relation, I ~Pn, where I is the fluorescent intensity, P is the pumped laser power and n is the number of pump photons absorbed per upconversion photons emitted. Figure 7 shows that the plot of ln(I) versus ln(P) yields a straight line with slope n. The values of n of present samples are also indicated in Fig. 7 and the calculated slopes (n) are 2.53 and 2.46 for EH and EHC0.25 samples, respectively, indicating more than two (possibly three) photons are involved to generate one up-converted photon. When Ce3+ ions are introduced into the Ho3+/Er3+ system, the n value becomes smaller, which means the red upconversion luminescence is governed by more two-photon processes. The measured value is not absolute equal to 3 because that besides the ESA process, complex energy transfer processes (ETU, CR) also coexist in the system [41, 42].
3.4 Cross sections and emission parameters
To compare the absorption of the pumping beam and analyze the possible energy transfer between the doped rare earth ions, the absorption and emission cross sections are calculated. According to , the emission cross-section σem can be calculated from the equation
The absorption and emission cross sections of Ho3+ in EHC0.25 sample are shown in Fig. 8(inset). The maximal value of the calculated emission cross-section spectrum centers at 2045 nm and reaches as high as 4.3 × 10−21 cm2. This value is higher than the reported ones of germinate (4.0 × 10−21 cm2) , gallate (3.8 × 10−21 cm2) , and aluminate (4.1 × 10−21 cm2)  glasses. The fluoride glasses possessing large emission cross section can be an excellent candidate in achieving intense 2.0 μm emission.
On the basis of the σabs and σem, it is interesting to calculated the wavelength dependence of net gain as a function of population inversion for the upper laser level in order to determine the gain property qualitatively Fig. 8, the gain coefficients with various P values ranging from 0 to 1 are calculated for 5I7 →5I8 transition of the Er3+/Ho3+/Ce3+ tridoped ZBYA glass. Evidently, the positive gain is obtained when P>0.4, similar to the case in Ho3+-doped other glasses . It is noted that the maximum gain coefficient of Ho3+: 5I7→5I8 transition is similar to the values of Ho3+ doped glass ceramics . Meanwhile, the gain coefficient increases and gain band extends to longer wavelengths with the increase in P value. This behavior is a typical characteristic of a quasi-three level system . This Er3+/Ho3+/Ce3 tridoped fluoride glass can be used as a suitable optical material to obtain infrared emission.
In conclusion, Ho3+ singly doped, Ho3+/Er3+ codoped and Ho3+/Er3+/Ce3+ tridoped ZBYA glasses that possess good IR transmission property are successfully prepared. The absorption spectra show strong 1550 nm absorption band in the Ho3+/Er3+ codoped glass and red-shift of the UV-side absorption edge after introducing Ce3+ ions. Intense 2.0 μm emission is achieved because of energy transfer process from Er3+:4I13/2 to Ho3+: 5I7 level and the intensity increases after introducing Ce3+ ions. The enhanced 2.0 μm emission is obtained because Ce3+ ions suppress the upconversion processes of Er3+ and Ho3+ ions. Furthermore, the energy transfer mechanisms, which include the shape and intensity of the red emission, are discussed based on the upconversion spectra. The results suggest that this Ho3+/Er3+/Ce3+ tridoped ZBYA system can be employed to improve the Ho3+ 2.0 μm fiber laser performance.
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