The ~2.0μm emission characteristics of Ho3+ both by direct excitation and through Yb3+ sensitization in barium-tellurite glass are reported. The radiative properties of active ions have been evaluated by applying Judd-Ofelt theory on the measured absorption spectrum. A significant enhancement of Ho3+ emission (2.0μm) observed with 12 fold decrease of Yb3+ emission (1008nm) in co-doped sample entrenched the efficient energy transfer from Yb3+:2F5/2→Ho3+:5I6. The host phonon assistance in the energy transfer process has been conferred by using Dexter model. Comparatively better emission properties (Arad, Δλeff, σem) reveal that, the present material could be promising for laser emission at ~2.0μm.
©2011 Optical Society of America
Solid-state lasers emitting in the ~2.0 μm eye-safe region are promising candidates for various applications in laser medicine surgery, remote chemical sensing, eyesafe laser radar and monitoring of atmospheric pollutions [1–5]. Ever since Johnson  has reported the first laser action at 2.0 μm from Ho3+ doped CaWO4 crystal in 1962, an intense level of research has been focused on the spectroscopic and lasing properties of Ho3+ doped various crystals and glasses [7,8]. To date, ~2 μm solid-state lasers have utilised silica, fluoride and germanate glasses in addition to various crystals as the rare-earth ion host materials. However, yet another material, in which the study of solid-state laser performance is worthwhile, is the tellurite family of glasses. Tellurite glasses exhibit high linear and nonlinear refractive indices, good resistance to corrosion, relatively high mechanical stability, low cut-off phonon energy of around 750 cm−1 among oxide systems which makes their transmission range extend into the mid-IR up to about 5 - 6 μm and it has good capability to accommodate lanthanide dopants. The low phonon energy and broad transmission range of tellurite glasses allow the observation of laser emissions from rare earth ions in a wide optical range. These features, coupled with good resistance to corrosion and high mechanical stability, make tellurite glasses promising hosts for mid-IR lasers at ~2.0 μm wavelength. The most common sensitizer for Ho3+ doped glasses/crystals is Tm3+. The great advantage of sensitization with Tm3+ is the quantum efficiency of 2 that leads to a theoretical maximum achievable efficiency of 75% under 785 nm pumping with lasing at 2.1 μm . Unfortunately Tm3+ has an absorption maximum in at 785 nm which does not correspond well to the emission wavelength of common and powerful laser diodes. On the other hand Yb3+ is also an efficient sensitizer for Ho3+ glass/crystal lasers  which has a broad absorption band lying in the emission range of commercially available InGaAs diode lasers that can be used as excitation sources to obtain an efficient energy transfer from Yb3+to Ho3+.
There are innumerable studies on Er3+/ Tm3+ or Yb3+ co-doped tellurite glasses of different compositions as 1.54 μm or 1.4 μm amplifiers for optical communication [11,12] and also as IR to visible frequency upconversion host materials [13,14]. However, there are very few reports on NIR emission at ~2 μm from Ho3+ singly or Yb3+ co-doped tellurite glasses . Earlier, Debnath et. al., have reported upconversion visible emission and spectroscopic properties of Er3+ doped barium tellurite glasses [14,16] but there are no investigations on Ho3+ fluorescence in this glass system. Hence the present paper mainly aims to report on the studies of the 2.0 μm emission characteristics of Ho3+ ions both by direct excitation and by sensitised excitation through energy transfer from Yb3+ ions in the co-doped barium-tellurite glass. Additionally, a systematic characterisation of spectroscopic properties of Ho3+ ions in the present glass by applying the Judd-Ofelt theory has been dealt along with the physical, optical and structural analysis.
2. Sample Preparation and Characterization
The Ho3+/Yb3+ co-doped, Yb3+ singly doped and undoped barium tellurite glasses having chemical composition (mol %) 80 TeO2 – 15 (BaF2 + BaO) – (5-x-y) La2O3 – x Ho2O3 – y Yb2O3, (where x = 0, 1 and y = 0, 1) were prepared by melt quenching method. The size of batches was approximately 10 g, and all the chemicals used were of AR grade (Sigma-Aldrich) with 99.99% purity. The batches were melted using pure platinum crucible as a container in an electrical furnace at 700-750°C for 1 hour with intermittent stirring with thin platinum rod for attaining the homogeneity. The homogeneous melt was poured onto a pre heated graphite mould to form clear glass followed by annealing at 230-250 °C for 12 hours to avoid thermal stress.
The density of the glasses was measured by Archimedes method using distilled water as buoyancy liquid at room temperature. Refractive indices of glasses were measured at five different wavelengths (473 nm, 532 nm, 632.8 nm, 1060 nm and 1552 nm) on Metricon M2010 Prism Coupler equipped with respective laser sources. The FTIR reflectance spectra of the glasses were recorded on a FTIR spectrometer (Model: Series 1615, Perkin Elmer, Norwalk, CT) in the wavenumber range of 400–2000 cm−1 at a 15° angle of incidence. The measurement of room temperature absorption spectra was performed with optically polished plate shaped samples of co-doped glass (average thickness 1.58 mm) and base glass as a reference on an UV-VIS-NIR absorption spectrophotometer (Model: 3001, Shimadzu, Japan). The emission, excitation spectra and fluorescence decay kinetics of the samples were recorded on spectrofluorimeter (Model: Quantum Master enhanced NIR from PTI, USA) fitted with double monochromators on both excitation and emission channels. The instrument was equipped with LN2 cooled gated NIR photo-multiplier tube (Model: NIR-PMT-R 1.7, Hamamatsu) as well as InGaS detectors for acquiring both steady state spectra and phosphorescence decay. For decay measurements, a 60 W Xenon flash lamp was employed as an excitation source.
3. Results and Discussion
3.1. Physical, Optical and Structural Properties
The measured densities of Yb3+/Ho3+ co-doped and Yb3+ singly doped barium-tellurite glasses along with the average molecular weight of the respective glasses have been used to estimate the rare earth ion concentration (NRE) and the results are presented in Table 1 . The measured refractive indices (n) of both the glasses at five different wavelengths were fitted to sellmeier dispersion equation to determine the linear refractive indices at nD, nF and nC to calculate the abbe number (ν). The non-linear refractive index (n2) is an important parameter for Raman amplifier application has also been computed for both the glasses by using relevant expression  and used for the estimation of the third-order nonlinear susceptibility (χ3) yet another important optical parameter for ultra-fast optical switching in optical communication systems. The computed values of all the physical and optical parameters were presented in Table. 1.Figure 1 presents the FTIR reflectance spectra of Yb3+ singly doped and Yb3+-Ho3+ co-doped tellurite glasses. For both the samples, the spectra comprise identical nature suggesting that the incorporation of Ho3+ in glass network does not bring significant change in the glass structure. The spectra revealed a broad band with superimposition of three reflectance bandscentred at 732 cm−1, 680 cm−1 and 595 cm−1 which have been identified due to the Te – O- stretching vibrational band in [TeO3] trigonal pyramid, Te – Oaxial stretching vibrational band of [TeO4] trigonal bi-pyramid and [TeO3+1] polyhedra respectively [18–20]. It is observed that, in the present barium tellurite glass these vibrational bands have shifted to lower wavenumber compared to the other tellurite glass systems, this may be due to the fluoride anion effect in the network. Further, it is interesting to note that in this system the magnitude of stretching vibrational mode arising from non-bridging oxygen units belonging to TeO3 trigonal pyramids (732 cm−1) is at lowest and that of connecting bridging bonds of TeO4 bi-pyramides (680 cm−1 and 595 cm−1) is at highest. This is considered to be good indication of stability or resistant to devitrification. From the FTIR reflectance spectra it is clear that, the vibrational mode at 595 cm−1 is found to be dominant over the other modes and hence it can be considered as the phonon energy of the present glass. To the authors’ knowledge, this is the lowest phonon energy reported so far for any tellurite glass system.
3.2. Optical Absorption Spectra and Judd Ofelt analysis
The room temperature absorption spectra of Yb3+/Ho3+ co-doped tellurite glass sample with base glass corrected and uncorrected along with base glass sample have been presented in Fig. 2 . The spectrum of base glass shows good transparency with no characteristic peaks in the measured wavelength range of 420 nm – 2500 nm having strong band gap absorption starting at 410 nm. While the rare earth co-doped sample exhibits a number of distinct absorption bands in the Vis-IR range at 420, 452, 488, 540, 644, 1154 and 1950 nm for Ho3+ ions and at 978nm for Yb3+ ions. Depending upon their spectral peak energies [21,22], peaks are assigned to the transitions from ground state 5I8 to (5G, 3G) 5, 5G6, 5F3, (5F4 + 5S2), 5I6 and 5I7 excited states of Ho3+ ions and 2F7/2→2F5/2 for Yb3+ ions respectively. It is observed that the intensity of the 5I8→5G6 transition is highest among the detected peaks. This is due to its hypersensitive nature in 4f 10 electronic configuration of Ho3+ ions with invariably obeying ΔS = 0, ΔL = 2 and ΔJ = 2 spectral rule possessing strong dependence on the ligand field environment surrounding the rare earth ion. In general, magnitude of the hypersensitivity of a transition mainly linked to the ligand covalency, crystal field symmetry and it is correlated strongly with the Ω2 intensity parameter.The Judd-Ofelt intensity parameters (Ω2, Ω4, Ω6) were calculated using relevant expressions , considering integrated absorption coefficient values of well defined absorption bands of Ho3+ ions along with the refractive indices of the glass at the concerned absorption peak wavelengths that derived from dispersion curve (Table 2 ). To calculate the integrated absorption coefficient of the bands, base glass corrected absorption spectrum has been used. A set of equations depending on the number of absorption bands considered for the simulation were generated from the measured electric dipole line strength and by employing the least square fitting method on those equations, the J-O intensity parameters (Ωt = 2,4,6) are attained. The best-fitted values obtained are Ω2 = 12.73 × 10−20 cm2, Ω4 = 7.06 × 10−20 cm2, Ω6 = 3.94 × 10−20 cm2 respectively. By using these J-O intensity parameters the calculated electric dipole line strengthsfor the respective absorption bands were determined. Among all the absorption transitions of Ho3+ ions, 5I8 → 5I7 transition has the considerable contribution of magnetic dipole nature. Hence magnetic dipole line strength (Smd) for this transition has also been calculated and the value is given in Table. 2. By using and values, measured (Pmea) and calculated (Pcal) oscillator strengths have been obtained for each transition and are listed in Table. 2. Further to validate the quality of fitting, the root mean square deviation for the measured and the calculated values of electric dipole line strengths (rms-ΔSed) and oscillator strengths (rms-ΔP) have been examined. The minimum the deviation, the more accurate are the obtained values. In the present case, the obtained results demonstrated good reliability of the data.
Table 3 presents a comparative data of J-O intensity parameters of Ho3+ ions obtained in the present glass system with that of various crystal/glass hosts reported in the literature. In the present glass system the Ωt parameters follow the trend Ω2 > Ω4 > Ω6 which is similar to the most of the tellurite glasses. Generally, Ω2 is closely related to the ligand symmetry and degree of covalency of the host material. The larger Ω2 value represents a higher degree of covalency of the glass network bonding and lower symmetry . The 2 µm emission transition 5I7→5I8 (Ho3+) is mainly affected by Ω6 intensity parameter due to its larger U(6) matrix element and as the radiative spontaneous emission probability (Ar) is proportionately dependent on Ω6 intensity parameter .
It should be mentioned that the Ω6 value in the present glass is higher than the other hosts except ZnBS glass. Similarly, it is observed that the spectroscopic quality factor (χ = Ω4/Ω6), which is an important predictor of stimulated emission is found to be almost similar to any other crystal/glass except YAG crystal and chalcogenide glass where is it higher (Table 3).
Further, by using the Ωt values, various important radiative properties such as spontaneous emission probability (Ar), branching ratios (βR) and lifetime of the radiative transition (τr) were calculated and are listed in Table 4 . As the 5I7→5I8 transition of Ho3+ ions is magnetic dipole allowed and the contribution of Amd (77.15 s−1 in the present glass) has also been consider for estimating the spontaneous emission probability (Ar = ed + md).
3.3. Fluorescence Spectra
Figure 3 shows the NIR emission spectra of Yb3+/Ho3+ co-doped sample when excited by 980 nm through 2F5/2 excited state of sensitizer, Yb3+ ions and by direct Ho3+ ion excitation through 5F5 level with 658nm and 5I6 level with 1191nm wavelength. These respective excitation wavelengths have been chosen from the recorded excitation spectrum of co-doped sample by monitoring 2050 nm emission of Ho3+ ions as seen in inset of Fig. 3. The emission peaks detected at 1008, 1209, 1487 and 2050 nm are assigned to the transitions of Yb3+: 2F5/2→2F7/2, Ho3+: 5I6→5I8, Ho3+: 5I5→5I6 and Ho3+: 5I7→5I8 respectively.
Further to mention that the emission peak at 1487 nm corresponding to the transition Ho3+: 5I5→5I6 was observed only under direct excitation (by 658nm) and not detected under sensitizer excitation with 980 nm due to the fact that the energy is not sufficient to excite to higher level (5F5). From the emission spectra it can be clearly seen that the intensity of the peak at 2050 nm (Ho3+: 5I7→5I8) is highest among all the measured emissions transitions. Hence, its stimulated emission cross-section (σe) has been calculated by using the following expression 
The stimulated emission cross-section, effective band width and spontaneous emission probability of Ho3+: 5I7→5I8 transition in the present barium tellurite glass is presented in Table 5 in comparison with the other host systems. It is evident from this data that the effective band width of 5I7→5I8 (2050 nm) fluorescence band is higher (160 nm) when compared to other glass hosts which can be attributed to the existence of varied structural units TeO4, TeO3+1 and TeO3 of tellurite glass. The different Te-O bond lengths associated with these three different structural units result in multiplicity of ion-host field strengths and therefore, yield a range of electro-static fields around a rare earth ion in a tellurite glass leading to an inhomogeneous broadening of the fluorescence band . This could result in an enhanced tuning range for solid state lasers utilising a tellurite glass material. Also, it is observed that, the value of stimulated emission cross section (σe) of this transition in present host has the higher (1.45 × 10−20 cm2) when compared to other glasses systems (Table 5) but, slightly lesser than that of chalcogenide glass (1.54 × 10−20 cm2) .
The larger emission cross-section is mainly due to the high spontaneous emission probability resulting from high refractive index. From Table 5 it is worthwhile to notice that the value of effective band width is higher than any other host material, which is highly useful for laser tuning. Another interesting observation was that, the emission intensity of Ho3+: 5I7→5I8 had increased 8 fold under the excitation with 980 nm through the energy level 2F5/2 of Yb3+ ions when compared to the direct excitations of Ho3+. This is because of high absorption cross section of Yb3+ ions (σa = 1.889 × 10−20cm2 in the present glass) accompanied with efficient sensitised energy transfer from Yb3+ to Ho3+ ions.
3.4. Energy transfer mechanism and Decay analysis
In order to understand the energy transfer process from Yb3+ to Ho3+, the emission characteristics of Yb3+ ions with and without Ho3+ ions, have been examined in the glass system undertaken. Figure 4 compares the room temperature emission spectra of Yb3+ singly doped and co-doped samples under 924 nm excitation. The second intense excitation peak at924 nm has been selected instead of 980 nm to excite upon Yb3+ ions, only to obtain a full gaussian shape of spectral transition 2F5/2 → 2F7/2 at 1008 nm and also because of relatively small energy separation between excited state multiplets at 924 nm and 980 nm. From this Fig. 4, it is clear that the relative peak intensity of Yb3+ at 1008 nm has significantly decreased up to 12 folds in the Yb3+/Ho3+ co-doped tellurite glass compared to Yb3+ singly doped glass, showing an evidence of energy transfer from Yb3+: 2F5/2 → Ho3+: 5I6. The partial energy level diagram shown in Fig. 5 explains the mechanism involved in the energy transfer based emission process.
It is observed that, the measured fluorescence decay of Yb3+ emission is independent of excitation wavelength (980 nm/954 nm/924 nm). By studying the decay kinetics of Yb3+ ions in the presence and absence of Ho3+ ions, the energy transfer rate (WET) and the energy transfer efficiency (ηET) were estimated by using the following expressions 
3.5. Phonon assisted energy transfer process
According to the Forster’s resonant energy transfer (FRET) theory, the extent of energy transfer depends on the spectral overlap of donor’s emission with acceptor’s absorption. Hence, in the case of resonant energy transfer among donor-acceptor, the energy transfer probability (PET) exhibits a linear relationship with the spectral overlap integral as ,Eq. (4) is generalized for phonon assisted energy transfer process by considering the energy of phonons involved in the energy transfer. Thus, transfer probability (PET) can be estimated from the phonon modified spectral overlap integral, I(Eph) as given by ,Fig. 6 . It can be seen that the normalized energy transfer probability increases with an increase in phonon energy and it reaches a maximum for the phonons with energy of around 1480 cm−1. For further increase in the phonon energy, the energy transfer probability decreases and diminishes. Thus, it is evident that, the energy difference between the energy levels of 2F5/2 (Yb3+) and 5I6 (Ho3+) has been bridged by the host phonons for an efficient energy transfer. As the phonon energy of present telluride glasses is 595 cm−1, about two to three phonons are required to bridge the energy gap. Though the high phonon energy hosts like silicate, phosphate or borate are suitable to promote the energy transfer from Yb3+ to Ho3+ with less number of phonons but they also possess higher non-radiative relaxation of 5I7 level of Ho3+ leading to less probability of 2 μm emission.
3.6. Energy transfer microparameters
The energy transfer microparameters for a donor - acceptor energy transfer can be estimated using Forster’s spectral overlap model given by ,30],Figure 7 presents the emission and absorption cross-section spectra of Yb3+ and Ho3+ ions respectively by using Eq. (7) with corresponding phonon sidebands. By applying Eq. (6) to the spectral overlap function in Fig. 7, the energy transfer microparameter has been calculated and found to be 1.39 × 10−40 cm6.sec−1. Such a reasonably good value of microparameter suggests the host phonon contribution in the energy transfer process.
A detailed spectroscopic analysis has been performed by applying the standard J-O theory and the energy transfer mechanism involved in 2.0µm emission of Ho3+ under Yb3+ sensitization upon 980 nm excitation in Yb3+/Ho3+ co-doped tellurite glass has satisfactorily been explained. The evaluated J-O intensity parameters have been used to estimate the oscillator strengths, radiative properties of emission transitions and also the spectroscopic quality factor which is an important predictor in understanding the suitability of this material as a laser material. Further, the important laser parameters such as the stimulated emission cross-section, effective band width and spontaneous emission probability of the Ho3+: 5I7→5I8 transition in the present glass have been found to be comparatively high. The energy transfer rate of 9557.3 s−1 with 85.8% efficiency and 8 fold increase in the 2.0µm emission under 980 nm excitation clearly indicates the sensitizer excitation is more effective than the direct excitation. A comparative assessment of the data suggests that the present glass having got low energy (595 cm−1) phonons could be identified as a promising optical material for an efficient NIR emission at 2.0 µm.
The authors would like to thank the Director, CGCRI for his continued support and encouragement. This work was carried out under an In-house project OLP 0288.
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