Near-IR Luminescence of Rare-Earth Ions (Er³⁺, Pr³⁺, Ho³⁺, Tm³⁺) in Titanate–Germanate Glasses under Excitation of Yb³⁺

Abstract

Inorganic glasses co-doped with rare-earth ions have a key potential application value in the field of optical communications. In this paper, we have fabricated and then characterized multicomponent TiO₂-modified germanate glasses co-doped with Yb³⁺/Ln³⁺ (Ln = Pr, Er, Tm, Ho) with excellent spectroscopic properties. Glass systems were directly excited at 980 nm (the ²F_7/2 → ²F_5/2 transition of Yb³⁺). We demonstrated that the introduction of TiO₂ is a promising option to significantly enhance the main near-infrared luminescence bands located at the optical telecommunication window at 1.3 μm (Pr³⁺: ¹G₄ → ³H₅), 1.5 μm (Er³⁺: ⁴I_13/2 → ⁴I_15/2), 1.8 μm (Tm³⁺: ³F₄ → ³H₆) and 2.0 μm (Ho³⁺: ⁵I₇ → ⁷I₈). Based on the lifetime values, the energy transfer efficiencies (η_ET) were estimated. The values of η_ET are changed from 31% for Yb³⁺/Ho³⁺ glass to nearly 53% for Yb³⁺/Pr³⁺ glass. The investigations show that obtained titanate–germanate glass is an interesting type of special glasses integrating luminescence properties and spectroscopic parameters, which may be a promising candidate for application in laser sources emitting radiation and broadband tunable amplifiers operating in the near-infrared range.

1. Introduction

Numerous inorganic glass materials are fabricated and widely applied industrially. Novel glass host matrices are still developed and must fulfill the rising demand for good-quality optical components and devices such as active optical fibers, solid-state laser sources, broadband near-IR fiber amplifiers, photonic integrated devices, and so on [1,2,3,4]. Systematic studies clearly demonstrate that positions and spectral linewidths for characteristic luminescence bands of lanthanides ions can be tuned according to the chemical compositions of glass and glass-ceramic matrices and dopant ions [5,6,7,8,9]. One of the most perspective options to remedy the drawbacks and improve the photoluminescence properties of lanthanide ions is a modification of the glassy network with suitable additives. In this context, TiO₂-modified glasses [10,11] have gained much importance due to their interesting properties that make them promising candidates for luminescent sources and optical devices. The beneficial effect of these additives is the improvement of the thermal and chemical stability of glasses [12,13,14]. It is also assumed that presence of titanium dioxide in matrices with low phonon energy may significantly broaden and enhance the luminescence bands of lanthanides ions.

From the accumulated experience and literature data [15,16], it is known that the trivalent ytterbium ions have been extensively studied for use as efficient emitters of radiation in the infrared range. It should be noted that Yb³⁺ ions with a broad absorption band in the wavelength region of 860–1060 nm and a relatively long fluorescence lifetime (1–2 ms) can be excellent sensitizers to activate other lanthanide ions for luminescence [17,18]. For this reason, co-doping of materials with Yb³⁺ ions allows for the efficient pumping of around 980 nm using a commercially available diode [19,20]. From the experimental approach, it can be concluded the near-IR radiative transitions of lanthanide ions are greatly dependent on the reduction of matrix phonons to achieve high luminescence efficiency. Indeed, in the past few years, subsequent research on germanate glass remains a perspective option as an oxide glass host matrix for lanthanide ions thanks to its favorable properties, such as smaller multiphonon relaxation probabilities due to relatively low phonon energy (~800 cm⁻¹), high transparency from the visible to the infrared region and presence of non-linear optical effects [21,22]. One should remember that the following lanthanides Ln³⁺ ions such as Pr³⁺, Er³⁺, Tm³⁺, and Ho³⁺ are proposed as co-activators for photoluminescence in Yb³⁺/Ln³⁺-doubly doped glasses owing to their favorable location of energy levels and the possibility of radiative transitions at the infrared [23]. Among them, Tm³⁺ [24] has gained increasing attention because the near-IR emission associated with the ³F₄ → ³H₆ transition at 1800 nm is useful as a medical light source. The Er³⁺ ions [25] are widely used in materials for optical applications. Main ⁴I_13/2 → ⁴I_15/2 near-IR transition of Er³⁺ at 1500 nm corresponds to the C + L telecommunication window. Next, Ho³⁺ ions [26] are one of the interesting dopants appropriate for laser sources operated at 2000 nm owing to the ⁵I₇ → ⁷I₈ transition. Although such interesting observations, insufficient attention has been paid to the effects of TiO₂ on near-IR emission properties of low-phonon germanate glasses co-doped with lanthanides. From this point of view, it is interesting to find out how emission bands of selected lanthanide ions located in the near-IR range are changed with different GeO₂:TiO₂ molar ratios in chemical composition under Yb³⁺ ions excitation.

This paper concerns novel multicomponent titanate–germanate glasses belonging to the low-phonon oxide glass family. Glass samples were successfully synthesized using the conventional high-temperature melting technique. The optical properties of glass series containing two network-formers, TiO₂ and GeO₂, were characterized using luminescence spectroscopy. In our studies, Yb³⁺ plays an important role, such as a sensitizer to activate selected lanthanide ions. Our attention has been paid to titanate–germanate glass systems co-doped with Yb³⁺/Er³⁺, Yb³⁺/Ho³⁺, Yb³⁺/Tm³⁺, Yb³⁺/Pr³⁺, and their energy transfer process. Near-infrared luminescence spectra and decay curves were examined for samples where GeO₂ was substituted by TiO₂. In the studied glass systems, molar ratios are changed from GeO₂:TiO₂ = 5:1 up to GeO₂:TiO₂ = 1:5. Based on the measured values of luminescence lifetimes, the energy transfer efficiencies Yb³⁺ → Ln³⁺ (Ln = Pr Er, Tm, and Ho) were determined for glass samples differing in TiO₂ content. The influence of TiO₂ on structural properties has already been carried out on the previous titanate–germanate glasses published recently [27].

2. Materials and Methods

The investigated TiO₂-modified germanate glasses co-doped with lanthanides ions with chemical composition (given in mol%): xTiO₂-(60−x)GeO₂-30BaO-9.5Ga₂O₃-0.5Yb₂O₃ and xTiO₂-(60−x)GeO₂-30BaO-9.4Ga₂O₃-0.5Yb₂O₃-0.1Ln₂O₃, (where Ln = Er, Tm, Pr, Ho, and x = 10, 20, 30, 40, 45, 50) were obtained by traditional melt quenching technique. In present research, glasses containing various molar ratios GeO₂:TiO₂ are equal to 5:1, 2:1, 1:1, 1:2, 1:3, and 1:5 and glass codes are as follows: 5Ge-1Ti, 2Ge-1Ti, 1Ge-1Ti, 1Ge-2Ti, 1Ge-3Ti, and 1Ge-5Ti. All of the glass components used during synthesis were of high purity (99.99%) from Aldrich Chemical Co. (St. Louis, MO, USA). Appropriate precursor metal oxides were mixed in an agate mortar. After homogenization of the components, 5 g glass bathes were placed in a platinum crucible (Łukasiewicz Research Network, Institute of Ceramics and Building Materials, Cracow, Poland). In the present procedure, the melting conditions were T = 1250 °C for 60 min in an electric furnace. Finally, each glass sample was cooled to room temperature and polished to meet the requirements for optical measurements. A series of transparent glass samples with the dimensions 12 mm × 12 mm and thickness ±3 mm was successfully prepared to determine their optical properties. The luminescence measurements of glasses were performed on a Photon Technology International (PTI) Quanta-Master 40 (QM40) UV/VIS Steady State Spectrofluorometer (Photon Technology International, Birmingham, NJ, USA) supplied with a tunable pulsed optical parametric oscillator (OPO) pumped by the third harmonic of an Nd:YAG laser (Opotek Opolette 355 LD, OPOTEK, Carlsbad, CA, USA). The laser system was coupled with a 75 W xenon lamp, a double 200 mm monochromator, and a Hamamatsu H10330B-75 detector (Hamamatsu, Bridgewater, NJ, USA). The emission spectra were recorded with a spectral resolution of 0.5 nm. Decay curves were recorded by a PTI ASOC-10 (USB-2500) oscilloscope with an accuracy of ±2 μm and have been measured under excitation wavelengths 980 nm and monitoring emission wavelength 1030 nm. In order to compare the emission intensity under the same experimental conditions, measurements of glass systems were carried out at the same slit settings. Measurements were performed at room temperature.

3. Results and Discussion

3.1. Optical Absorption Properties

In this study, measurements of the absorption spectra of glass systems 1Ge:1Ti co-doped with Yb³⁺/Ln³⁺ (Ln = Er, Pr, Tm, Ho) were carried out and presented in Figure 1.

Absorption spectra measured for representative titanate–germanate glasses consist of the characteristic bands corresponding to transitions originating from the ground state to higher-lying excited states of selected lanthanide ions. The glass sample co-doped with Yb³⁺/Pr³⁺ ions (Figure 1a) shows four weakly intense absorption bands in the 350–800 nm range. The two most intense absorption bands due to ³H₄ → ³F₃ and ³H₄ → ³F₂ transitions are well observed in the infrared spectral range. Interestingly, literature data indicate that on the band edge, due to ³H₄ → ³F₃, a weakly separated absorption band at about 1400 nm associated with the ³H₄ → ³F₄ transition is detected [28]. Next, it is evidently seen for titanium germanium glass co-doped with Yb³⁺/Er³⁺ that the absorption bands (Figure 1b) due to the transition from the ⁴I_15/2 state of Er³⁺ are well observed at 380 nm (⁴G_11/2), 407 nm (²H_9/2), 489 nm (⁴F_7/2), 522 nm (²H_11/2), 652 nm (⁴F_9/2) and band centered in the near-infrared range at 1530 nm due to ⁴I_15/2 $\to$ ⁴I_13/2 transition [29]. Figure 1c shows the absorption spectrum of the Yb³⁺/Tm³⁺ co-doped titanate–germanate sample. The five absorption bands at 471 nm, 685 nm, 790 nm, 1210 nm, and 1690 nm correspond to the transitions from the ground state ³H₆ to excited stated ¹G₄, ³F₂ + ³F₃, ³H₄, ³H₅, and ³F₄, respectively [30]. In turn, the absorption spectrum measured for Yb³⁺/Ho³⁺ co-doped glass is shown in Figure 1d. The results show that the obtained glass characterizes seven absorption bands in the spectral region of 350–2200 nm. The spectrum exhibits the bands due to the following absorption transitions: ⁵I₈ $\to$ ⁵G_4,5, ⁵I₈ $\to$ ⁵G₆, ⁵I₈ $\to$ ⁵F_2,3, ⁵I₈ $\to$ ²S₂+⁵F₄, ⁵I₈ $\to$ ⁵F₅ in the visible range and ⁵I₈ $\to$ ⁵I₆, ⁵I₈ $\to$ ⁵I₇ in the NIR region [31]. For all glass samples, note that the main peak in the absorption spectra was concentrated at 980 nm, defining the lowest Yb³⁺: ²F_5/2 Stark splitting energy level is the most intense; therefore, in the following section, the excitation line at 980 nm had been selected to investigate the near-infrared luminescence properties of the fabricated glasses co-doped with Yb³⁺/Pr³⁺, Yb³⁺/Er³⁺, Yb³⁺/Tm³⁺, Yb³⁺/Ho³⁺, where Yb³⁺ plays an important role of emission sensitizer for lanthanides ions.

3.2. Near-Infrared Luminescence Properties

Trivalent ytterbium (Yb³⁺) has a simple energy-level structure, i.e., the ²F_7/2 ground level and the ²F_5/2 excited level, with an energy separation between them of about 10,000 cm⁻¹ [32]. The Yb³⁺ ion has been demonstrated to be an excellent emission sensitizer for other lanthanides due to its effective absorption cross-section at 980 nm [33]. In the presented work, four titanate–germanate glass systems co-doped with Yb³⁺/Pr³⁺, Yb³⁺/Er³⁺, Yb³⁺/Tm³⁺, and Yb³⁺/Ho³⁺ varying with TiO₂ referred to as 5Ge-1Ti, 2Ge-1Ti, 1Ge-1Ti, 1Ge-2Ti, 1Ge-3Ti, and 1Ge-5Ti were selected and their near-IR emission properties under direct excitation of Yb³⁺ at wavelength 980 nm were compared. In addition, the interactions between Yb³⁺ and the second lanthanide ion and their mechanisms are discussed and presented schematically for each glass system on diagrams of the energy levels in order to understand the energy transfer processes. One of the interesting aspects of the ongoing research focusing on the properties of co-doped glasses is the observation of the sensitization of near-IR emission of Pr³⁺ at 1.35 μm under excitation Yb³⁺. Figure 2 presents near-IR luminescence spectra for Yb³⁺/Pr³⁺ co-doped titanate–germanate glasses varying with TiO₂.

The observed near-IR emission bands at about 1.35 μm correspond to ¹G₄ → ³H₅ transition of Pr³⁺. It should be particularly pointed out that the emission intensities of Pr³⁺ ions increased significantly with increasing TiO₂ concentration from 5Ge-1Ti to 1Ge-5Ti. Hence, it could be suggested that the introduction of titanium dioxide to germanate glass favor near-infrared luminescence attributed to the ¹G₄ → ³H₅ transition of Pr³⁺ under direct excitation of Yb³⁺. The observed ¹G₄ → ³H₅ transition of Pr³⁺ ions in titanate–germanate glass under excitation of Yb³⁺ is presented on the energy level diagram in Figure 2. It is clearly seen that both excited levels ²F_5/2 (Yb³⁺) and ¹G₄ (Pr³⁺) lie close to each other and the energy gap between them is very small. Moreover, the absorption cross-section at around 980 nm is larger for Yb³⁺ than Pr³⁺ ions, which is crucial for the pumping efficiency of praseodymium-doped fiber amplifiers PDFA [34,35]. Due to this fact, the energy transfer process Yb³⁺ → Pr³⁺ is nearly resonant and supposed to be much more efficient in titanate–germanate glass. Thus, we observe near-IR emission at 1.35 μm due to ¹G₄ → ³H₅ transition of Pr³⁺, which enhanced rapidly with increasing in TiO₂ content. Based on the above experiment analysis, it can be concluded that Yb³⁺/Pr³⁺ co-doped germanate glass in the presence of TiO₂ is promising for near-IR emission and sample 1Ge-5Ti seems to be a potential precursor active glass material to realize a fiber laser operating at 1.35 μm [36,37].

From the literature [38,39,40], it is well known that broadband near-infrared emission bands of Er³⁺ ions in inorganic glasses depend strongly on their chemical compositions. The rapid development of optical telecommunications requires the broadening of the near-IR range for erbium-doped fiber amplifiers (EDFA), in which signal transmission occurs at about 1500 nm. In fact, the EDFA systems based on silicate glasses [41,42] exhibit relatively narrow bandwidth, which contributes to the limited near-infrared broadband transmission. For that reason, many precursor glass systems singly doped with Er³⁺ and co-doped with Yb³⁺/Er³⁺ are still tested and selected in order to obtain enhanced near-IR luminescence in the so-called third telecommunication window. Figure 3 presents near-IR emission spectra measured in the wavelength range from 1400 nm to 1700 nm for Yb³⁺/Er³⁺ co-doped titanate–germanate glasses under excitation by a 980 nm line. Near-IR emission bands centered at about 1.53 μm correspond to the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺.

The intensity of the near-IR emission band at 1.5 μm is reduced from 5Ge-1Ti to 1Ge-2Ti and then increased with further increasing TiO₂ concentration up to glass sample 1Ge-5Ti. The emission linewidth for the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions, referred to as the full width at half maximum, is larger for sample 1Ge-5Ti (FWHM = 55 nm) than 5Ge-1Ti (FWHM = 38 nm). Schematic representation of energy levels, possible energy transfer between Yb³⁺ and Er³⁺ ions and the main near-IR laser transition of Er³⁺ ions at 1.5 μm are presented in Figure 3. Similar to the Yb³⁺/Pr³⁺ system, the excitation energy transfers resonantly very fast from the ²F_5/2 (Yb³⁺) to the ⁴I_11/2 (Er³⁺) due to a small energy mismatch between the interacting excited levels [43]. The absorption cross-section of Yb³⁺ ions at around 980 nm is higher by a factor of ten roughly than that of Er³⁺ [44], favoring an efficient Yb³⁺ → Er³⁺ energy transfer process. Next, the excitation energy relaxes nonradiatively to the ⁴I_13/2 state by multiphonon process and consequently, we observe ⁴I_13/2 → ⁴I_15/2 near-IR transition of Er³⁺.

Our previous studies [27] indicate that the introduction of TiO₂ to germanate glass resulted in higher asymmetry and better covalent bonds between rare earth and oxygens. In addition, these structural changes led to the site-to-site variation of the crystal field strength in the local environment of rare earths. It resulted in the inhomogeneous broadening of spectral lines corresponding to the presence of various sites for the optically active ions. As a consequence, the profiles of emission spectra and their values of FWHM are dependent on the symmetry, the ligand field strength, and the site-to-site variation of the Ln³⁺ local environment. This is the main reason that the spectral profiles of Er³⁺, i.e., the mission peak position, emission linewidth (FWHM), and the relative intensities of shoulders existing at about 1600 nm, are changed during the modification of glass matrices. In some cases, the glass modifiers have a minor influence on absorption properties but a strong impact on the emission cross-sections attributed to the ⁴I_13/2 → ⁴I_15/2 near-infrared transition of Er³⁺ ions. Recently, it was well demonstrated for Er³⁺ ions in silicate glass with various Al₂O₃ content [45] and Er³⁺/Yb³⁺ co-doped phosphate glass modified by Y₂O₃ [46].

Thulium is another well-known lanthanide dopant, which is introduced to various glass systems in order to generate near-IR luminescence at about 1.8 μm [47]. In particular, co-doping with Tm³⁺ and Yb³⁺ ions is an excellent way to achieve enhanced near-infrared emission by using a 980 nm wavelength as an excitation source [48]. Figure 4 shows near-IR emission spectra of series titanate–germanate glass samples co-doped with Yb³⁺/Tm³⁺. Near-IR emission bands centered at 1.8 μm are associated with ³F₄ → ³H₆ transition of Tm³⁺. The intensities of near-IR emission bands increase and then decrease with increasing TiO₂ concentration in the glass composition. The highest intensity of emission band related to the ³F₄ → ³H₆ transition of Tm³⁺ was observed for sample 1Ge-1Ti, where the molar ratio GeO₂ to TiO₂ is close 1:1. The energy level diagram for Yb³⁺/Tm³⁺ co-doped titanate–germanate glasses is shown in Figure 4.

In contrast to the Yb³⁺/Er³⁺ system, the energy mismatch between interacting ²F_5/2 (Yb³⁺) and ³H₅ (Tm³⁺) excited levels is much higher [49] and thus, non-resonant Yb³⁺ → Tm³⁺ energy transfer process in titanate–germanate glasses occurs. The excitation energy is transferred nonradiatively by multiphonon relaxation from the ³H₅ level to the lower-lying ³H₅ level generating near-IR emission at 1.8 μm due to ³F₄ → ³H₆ transition of Tm³⁺. Nearly the same mechanism was proposed for Yb³⁺/Ho³⁺ co-doped glass systems, which are also interesting from the optical point of view [50].

These glass systems present near-IR emission at 2 μm due to the ⁵I₇ → ⁵I₈ transition of Ho³⁺ [51,52]. The energy transfer mechanism has been mentioned in the literature by Wang et al. [53]. Upon excitation wavelength at 980 nm, the excited level of Yb³⁺ is well populated and then the excitation energy is transferred from the ²F_5/2 state of Yb³⁺ to the ⁵I₆ state of Ho³⁺. The energy transfer process Yb³⁺ → Ho³⁺ is non-resonant. In the next step, very fast multiphoton relaxation to the lower-lying ⁵I₇ level of Ho³⁺ is observed and consequently, we can observe near-IR emission at about 2000 nm associated with the ⁵I₇ → ⁵I₈ transition of Ho³⁺ [54,55]. Figure 5 presents near-IR luminescence spectra measured for titanate–germanate glasses co-doped with Yb³⁺/Ho³⁺. The emission bands are more intense for glass samples containing higher concentrations of TiO₂. All transitions are also indicated in the energy level diagram, which is shown in Figure 5.

In order to achieve intense IR emission of rare-earth ions, the heavy doping of the activators such as Pr³⁺, Er³⁺, Ho³⁺, and Tm³⁺ is usually required. Remarkably interesting results were presented in work by Tu et al. [56], where they successfully developed heavily Tm³⁺-doped germanate glasses, promising for glass fibers. On the other hand, the concentrations of activators should be optimal and relatively low in order to reduce luminescence quenching. In our case, luminescence quenching in the studied glass samples is negligibly small because of the low concentrations of acceptors (Pr³⁺, Er³⁺, Ho³⁺, Tm³⁺) and the lack of energy transfer processes between pairs of Pr³⁺-Pr³⁺, Er³⁺-Er³⁺, Tm³⁺-Tm³⁺, and Ho³⁺-Ho³⁺ ions, respectively. The non-radiative transfer processes become dominant for glass samples with higher Ln³⁺ concentrations. These effects are especially stronger for glass systems with diagrams of excited states favoring the presence of cross-relaxation processes. Thus, the probabilities of these non-radiative relaxation processes increase and the luminescence is quenched due to the increasing interaction among the Ln³⁺ ions at higher concentrations. Our spectroscopic investigations indicate that the relative intensities of emission bands of rare-earth ions in germanate glasses are changed drastically with the presence of TiO₂. Figure 6 shows the integrated intensities of emission bands related to the main ¹G₄ → ³H₅ (Pr³⁺),⁴I_13/2 → ⁴I_15/2 (Er³⁺),³F₄ → ³H₆ (Tm³⁺) and ⁵I₇ → ⁵I₈ (Ho³⁺) near-IR transitions of rare-earth ions in the studied glass samples varying with TiO₂ content. For pairs Yb³⁺/Pr³⁺ and Yb³⁺/Ho³⁺, the integrated intensities of near-infrared emission bands located at 1.35 and 2 µm increase with increasing TiO₂ concentration. A completely different situation is observed for pairs of Yb³⁺/Er³⁺ and Yb³⁺/Tm³⁺ ions in titanate–germanate glasses. The integrated intensities of near-infrared emission bands due to the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions are reduced from 5Ge-1Ti to 1Ge-2Ti and then increase with further increasing TiO₂ content. Contrary to Yb³⁺/Er³⁺, the emission intensities of near-infrared bands related to the ³F₄ → ³H₆ transition of Tm³⁺ ions are enhanced to the 1Ge-1Ti system and then start to decrease with increasing TiO₂ content in the glass composition; however, the changes in emission intensities with TIO₂ content are non-linear for pair Yb³⁺/Tm³⁺.

To summarize this part of the research, the authors declare that near-IR emission studies will be devoted in the future to further optimization of the TiO₂ content of individual systems containing Yb³⁺/Ln³⁺ (Ln = Pr, Er, Tm, Ho). Obtained results for near-IR emission presented here will contribute to the fabrication of titanate–germanate optical fibers.

3.3. Luminescence Decays and Energy Transfer Efficiencies

The systematic studies indicate that luminescence lifetimes for excited states of Yb³⁺ in several low-phonon glass systems are completely different and depend significantly on the glass network-former and network-modifier added to the base composition [57,58]. To determine the efficiency of the energy transfer process between Yb³⁺ and Ln³⁺ ions (Ln = Pr, Er, Tm, Ho), the luminescence decays for titanate–germanate glasses were measured and analyzed. Figure 7 shows decay curves measured for co-doped samples under 980 nm excitation. Based on decays, luminescence lifetimes were determined and compared to Yb³⁺ singly doped glass samples. The results are given in

Table 1. Measured lifetimes for ⁵F₂ state of Yb³⁺ in single and co-doped titanate–germanate glasses.

			τm(ms)
TiO2 (mol%)	GeO2:TiO2	Yb3+	Yb3+/Pr3+	Yb3+/Er3+	Yb3+/Tm3+	Yb3+/Ho3+
10	5:1	1.21	0.63	0.65	0.70	0.83
20	2:1	1.10	0.55	0.59	0.59	0.75
30	1:1	1.00	0.48	0.52	0.51	0.61
40	1:2	1.91	0.44	0.51	0.51	0.56
45	1:3	0.86	0.42	0.50	0.49	0.54
55	1:5	0.84	0.40	0.49	0.48	0.52

.

In general, measured lifetimes are reduced from 5Ge-1Ti to 1Ge-5Ti with increasing TiO₂ concentration in the glass composition. The experimental values of emission lifetimes decrease from 0.63 ms (5Ge-1Ti) to 0.40 ms (1Ge-5Ti) for Yb³⁺/Pr³⁺ co-doped glass systems, 0.65 ms (5Ge-1Ti) to 0.49 ms (1Ge-5Ti) for Yb³⁺/Er³⁺ systems, 0.70 ms (5Ge-1Ti) to 0.48 ms for Yb³⁺/Tm³⁺ systems, and 0.83 ms (5Ge-1Ti) to 0.52 ms (1Ge-5Ti) for Yb³⁺/Ho³⁺ systems, respectively. Luminescence lifetimes measured for Yb³⁺ singly doped glasses and samples co-doped with Yb³⁺/Ln³⁺ were applied to calculate the energy transfer efficiencies [59]. The energy transfer efficiency η_ET between Yb³⁺ and lanthanides ions in fabricated glasses was evaluated by calculations with the formula given below:

$${\mathsf{\eta }}_{\mathrm{ET}}=1 -\frac{{\tau }_{\mathrm{Yb}\left(\mathrm{Ln}\right)}}{{\tau }_{\mathrm{Yb}}}$$

where ${\tau }_{\mathrm{Yb}\left(\mathrm{Ln}\right)}$ and ${\tau }_{\mathrm{Yb}}$ are the measured lifetimes for the ²F_5/2 level of Yb³⁺ ions in the presence and absence of acceptor Ln (where Ln = Pr, Er, Tm, Ho), respectively. The results are presented schematically in Figure 8.

Our studies indicate that measured lifetimes decrease with increasing TiO₂ content, while changes in the energy transfer efficiency seems to be completely different. For all pairs of Yb³⁺/Ln³⁺ (Ln = Pr, Er, Tm, Ho), the energy transfer efficiency is the highest for the 1Ge-1Ti system, but the trend of η_ET values varying with TiO₂ content is not the same. For the pair of Yb³⁺/Pr³⁺, the values of η_ET increase to 1Ge-1Ti, whereas they are nearly independent for glasses with higher TiO₂ content. For pairs Yb³⁺/Er³⁺ and Yb³⁺/Tm³⁺, the energy transfer efficiency increases from 5Ge-1Ti to 1Ge-1Ti and then decreases to 1Ge-5Ti with further increasing TiO₂ concentration. For the pair of Yb³⁺/Ho³⁺, the values of η_ET are the highest for 1Ge-1Ti to 1Ge-2Ti glass systems, respectively; however, the changes of η_ET with TiO₂ content are non-linear. Our calculations indicate that the energy transfer efficiencies are changed from 31% for Yb³⁺/Ho³⁺ glass (5Ge-1Ti) to nearly 53% for Yb³⁺/Pr³⁺ glass (1Ge-5Ti). At this moment, it should also be mentioned that the up-conversion luminescence pathways [60] make an important contribution to the energy transfer processes and their efficiencies in Yb³⁺/Ln³⁺ (Ln = Pr, Er, Ho, Tm) co-doped glasses.

4. Conclusions

Multicomponent titanate–germanate glasses co-doped with Yb³⁺/Ln³⁺ (Ln = Pr³⁺, Er³⁺, Tm³⁺, Ho³⁺) were synthesized and then studied their near-IR luminescence properties. The spectroscopic properties of glasses have been examined under the excitation of Yb³⁺ ions by 980 nm. Obtained results were discussed based on the energy level diagrams for sensitizer (Yb³⁺) and acceptors (Pr³⁺, Er³⁺, Tm³⁺, Ho³⁺) and interactions between them. The near-IR luminescence bands corresponding to the ¹G₄ → ³H₅ (Pr³⁺), ⁴I_13/2 → ⁴I_15/2 (Er³⁺), ³F₄ → ³H₆ (Tm³⁺) and ⁵I₇ → ⁵I₈ transitions of lanthanide ions have been examined with TiO₂ concentration. Our investigations indicate that the intensities of emissions are dependent on titanium dioxide content. The resonant Yb³⁺ → Pr³⁺ and Yb³⁺ → Er³⁺ and non-resonant Yb³⁺ → Tm³⁺ and Yb³⁺ → Ho³⁺ energy transfer process in co-doped titanate–germanate is observed. The analysis of decay profiles allowed for the deeper optical characterization of the energy transfer processes between Yb³⁺ and Ln³⁺ ions (Ln = Pr, Er, Tm, Ho) and for establishing the relation between luminescence lifetimes and the role of titanium dioxide in germanate glasses. Based on decay measurements and values of luminescence lifetimes, the efficiencies of energy transfer were estimated. The values of η_ET are changed from 31% for Yb³⁺/Ho³⁺ to nearly 53% for Yb³⁺/Pr³⁺. For all studied pairs Yb³⁺/Ln³⁺, the maximal values of η_ET are 53% (Yb³⁺/Pr³⁺), 48% (Yb³⁺/Er³⁺), 49%, (Yb³⁺/Tm³⁺), and 40% (Yb³⁺/Ho³⁺). Further studies revealed that the luminescence lifetimes are reduced with increasing TiO₂ content, whereas the energy transfer efficiencies are changed completely different, depending on pair Yb³⁺/Ln³⁺ (Ln = Pr³⁺, Er³⁺, Tm³⁺, Ho³⁺) in titanate–germanate glass.