1. Introduction

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 [6] 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 [9]. 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 [10] 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 [15]. 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 (N_RE) 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 n_D, n_F and n_C 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 [17] 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 – O_axial 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.

Table 1. Physical and Optical properties of Yb³⁺/Ho³⁺ co-doped and Yb³⁺ singly doped tellurite glass: Density (ρ), Rare earth ion concentration (N_RE), linear refractive indices (n_D, n_F, n_C), Abbe number (ν), Non-linear refractive index (n₂), Third-order nonlinear susceptibility (χ³).

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Fig. 1 FTIR reflectance spectra of tellurite glasses.

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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 ²F_7/2→²F_5/2 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 (Ω_2, Ω_4, Ω₆) were calculated using relevant expressions [21], 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 $\left(S_{ed}^{mea}\right)$and by employing the least square fitting method on those equations, the J-O intensity parameters (Ω_{t = 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 strengths$\left(S_{ed}^{cal}\right)$for 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 (S_md) for this transition has also been calculated and the value is given in Table. 2. By using $S_{ed}^{mea}$ and $S_{ed}^{cal}$values, measured (P_mea) and calculated (P_cal) 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-ΔS_ed) 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.

 

Fig. 2 Absorption spectra of base glass corrected and not corrected Yb³⁺/Ho³⁺ co-doped tellurite glass along with base glass.

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Table 2. Electric dipole line strengths (measured: S_ed^mea, calculated: S_ed^cal), magnetic dipole strength (S_md), Total Oscillator strengths (measured: P_mea, calculated: P_cal), refractive index (n) of different absorption transitions and (Ω_{t = 2,4,6}) J-O intensity parameters of Ho³⁺ions in the co-doped glass.

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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 Ω_t 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 [22]. 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 (A_r) is proportionately dependent on Ω₆ intensity parameter [23].

Table 3. Comparative chart of J-O intensity parameters of Ho³⁺ ion in various hosts.

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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 Ω_t values, various important radiative properties such as spontaneous emission probability (A_r), branching ratios (β_R) and lifetime of the radiative transition (τ_r) were calculated and are listed in Table 4 . As the ⁵I₇→⁵I₈ transition of Ho³⁺ ions is magnetic dipole allowed and the contribution of A_md (77.15 s⁻¹ in the present glass) has also been consider for estimating the spontaneous emission probability (A_{r = ed + md}).

Table 4. Electric dipole line strength (S_ed), Refractive index (n), Spontaneous emission probability (A_r), Radiative rate (ΣA_r), Radiative lifetime (τ_r), Branching ratio (β_R), for fluorescent levels of Ho³⁺ ions in the present tellurite glass.

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3.3. Fluorescence Spectra

Figure 3 shows the NIR emission spectra of Yb³⁺/Ho³⁺ co-doped sample when excited by 980 nm through ²F_5/2 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⁺: ²F_5/2→²F_7/2, Ho³⁺: ⁵I₆→⁵I₈, Ho³⁺: ⁵I₅→⁵I₆ and Ho³⁺: ⁵I₇→⁵I₈ respectively.

 

Fig. 3 Fluorescence spectra of Yb³⁺/Ho³⁺ co-doped tellurite glass (Inset: Excitation spectra for λ_emi = 2050nm.)

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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 (σ_e) has been calculated by using the following expression [24]

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 [25]. 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 (σ_e) 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) [26].

Table 5. Comparative chart of peak wavelength (λ_P), concentration of Ho₂O₃ (in wt%), radiative transition probability (A_r), effective bandwidth (Δλ_p) and stimulated emission cross section (σ_e) for ~2.0µm emission of Ho³⁺ ions in various glass hosts.

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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 ²F_5/2 of Yb³⁺ ions when compared to the direct excitations of Ho³⁺. This is because of high absorption cross section of Yb³⁺ ions (σ_a = 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 ²F_5/2 → ²F_7/2 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⁺: ²F_5/2 → Ho³⁺: ⁵I₆. The partial energy level diagram shown in Fig. 5 explains the mechanism involved in the energy transfer based emission process.

 

Fig. 4 Emission spectra of Sensitizer, Yb³⁺ ions in absence (black) and in the presence (red) of activator, Ho³⁺ ions, (Inset: Decay profiles for the same.)

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Fig. 5 Partial energy level diagram of Yb³⁺/Ho³⁺ co-doped tellurite glass showing the energy transfer mechanism under 980nm (red) and direct 1191nm, 658nm excitations (black)

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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 (W_ET) and the energy transfer efficiency (η_ET) were estimated by using the following expressions [27]

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 (P_ET) exhibits a linear relationship with the spectral overlap integral as [28],

 

Fig. 6 Energy transfer probability (Vs) Phonon energy in the co-doped glass.

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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 [29],

 

Fig. 7 Emission and absorption cross-section spectra of Yb³⁺ and Ho³⁺ ions with corresponding phonon sidebands

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4. Conclusion

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.