1. Introduction

Optical glasses and glass fibers doped with lanthanide ions, emitting radiation in the visible range by means of frequency up-conversion, have found numerous practical applications [1,2]. Universal demand for compact glass fiber sources and amplifiers necessitates the pursuit for brand new glass materials containing lanthanide elements. Incorporation of Tb³⁺ and Yb³⁺ ions to various host matrices is important in relation to applications in modern optics. Two quite different excited state relaxation processes can be observed for Yb³⁺/Tb³⁺ doubly doped systems, i. e. down-conversion and up-conversion. In the down-conversion luminescence processes, Tb³⁺(⁵D₄) → 2Yb³⁺(²F_5/2), two infrared photons are emitted for each Tb³⁺ ion de-excited by the energy transfer to two Yb³⁺ ions. These phenomena, so called quantum-cutting effects, have been analysed for different optical systems [3–5]. The quantum cutting material placed on the front surface of a conventional silicon solar cell is able to split a high-energy photon into two near-infrared (NIR) photon that match the energy gap of silicon (1.12 eV), and therefore enhance the conversion efficiency of silicon solar cells [6]. The second relaxation deals with up-conversion luminescence. Green emission due to main ⁵D₄ → ⁷F₅ transition of Tb³⁺ is successfully detected under near-infrared excitation of Yb³⁺ by 976 nm diode laser line. The up-conversion luminescence processes were examined for Yb³⁺/Tb³⁺ ions in single crystals [7], glasses [8] and glass fibers [9] as well as glass-ceramics containing fluoride nanocrystals [10,11]. A special attention has been paid to inorganic glasses containing Tb³⁺ and Yb³⁺ ions. Cooperative energy transfer between Tb³⁺ and Yb³⁺ ions occurs in tellurites [12], fluorophosphates [13] borates [14] as well as silica based glasses prepared by conventional high-temperature melting technique [15] and low-temperature sol-gel methods [16], but not specifically in lead-free germanate glasses. Spectroscopic properties of Yb³⁺/Tb³⁺ ions in oxyfluoride lead germanate glasses, glass fibers and glass-ceramics were examined in details [17,18]. Germanate-based glasses, owing to their good capacity for dissolving lanthanides [19,20] and relatively low phonon energies (850 cm⁻¹) enable to effective conversion of IR to VIS. It is derived from the fact that high concentration of RE ions (donor and acceptor) ensure efficient optical pump absorption, energy transfer and in result emission from acceptor ion. Therefore, germanate glasses are an attractive alternative for tellurite and heavy metal glasses. Moreover, glass fibers can be quite easily formed from precursor germanate glasses, which is important from the optical [21] and medical [22] point of view. Lanthanide-doped germanate glasses were studied for fiber lighting in minimally invasive surgery [23–25]. Generally, spectroscopic properties of germanate glass fibers singly [26] or doubly [27] doped with lanthanide ions are well documented in literature. Their favorable material properties and high stability make it possible to form them into fiber-optic structures.

In this work, the effects of optical pumping by NIR diode-laser and cooperative energy transfer processes between Yb³⁺ and Tb³⁺ in germanate glass are presented. The results of the conducted research into optimization of Yb³⁺/Tb³⁺ dopants concentration in a glass from GeO₂-Ga₂O₃-BaO system, ultimately aiming at maximization of up-conversion emission intensity, are discussed. The Yb³⁺→Tb³⁺ energy transfer efficiency is analyzed, along with the energy transfer coefficient C_f determination by means of the fitting method. To the best of our knowledge, cooperative energy transfer processes were not examined for lead-free germanate glasses containing Yb³⁺ and Tb³⁺ ions.

2. Experimental

Glasses with (1-y)50GeO₂-25GaO-10BaO-15Na₂O-0.7Yb₂O₃-yTb₂O_3, (y = 0.07, 0.15, 0.35, 0.7, 1.0) composition (in mol%) were melted from spectrally pure (99,99%) raw materials. The homogenized set was placed in a platinum crucible and melted in an electric furnace in T = 1500°C for 30 minutes. The molten glass was poured out onto a brass plate and then exposed to the process of annealing in air atmosphere at 610°C for 12 h to remove thermal strains. The glass samples were cutted and polished in order to carry out the optical measurements. Finally, a series of samples with the dimensions of 6x6x2mm³ were prepared. Emission spectra were measured at a station equipped with a Stellarnet Green-wave spectrometer and a pumping LIMO32-F200-DL980-LM laser diode (λ_p = 976 nm) with an optical fiber output having the maximum optical power P = 30W. A system PTI QuantaMaster QM40 coupled with tunable pulsed optical parametric oscillator (OPO), pumped by a third harmonic of a Nd:YAG laser (Opotek Opolette 355 LD) was used for luminescence decay measurements. The laser system was equipped with a double 200 mm monochromator, a multimode UV-VIS PMT (R928) and Hamamatsu H10330B-75 detectors controlled by a computer. Luminescence decay curves were recorded and stored by a PTI ASOC-10 [USB-2500] oscilloscope with an accuracy of ± 1 µs. The procedure of energy transfer coefficient determination was based on fitting of calculations and experimental results by least squares method. The calculated values were optimized by minimizing (simplex method) the sum of the squares o deviations of the calculated results, from experimental ones. The quality of the fits was characterized by the root-mean-squares deviation (RMS).

3. Results and discussion

Figure 1 presents emission spectra of Tb³⁺ ions in germanate glasses under excitation of Yb³⁺ by diode laser with λ_p = 976 nm, P_pump = 2W. Inset shows the logarithmic dependence of emission intensity on pumping radiation power. All possible transitions are schematized on the energy level diagram of Tb³⁺ and Yb³⁺ ions in germanate glass (Fig. 2).

 

Fig. 1 Emission spectra for Yb³⁺/Tb³⁺ co-doped germanate glasses. Inset shows dependence of upconversion emission intensity of 0.7Yb³⁺/0.7Tb³⁺ co-doped germanate glass on excitation power (λ_p = 976 nm).

Download Full Size | PDF

 

Fig. 2 Simplified energy level diagram of Tb^3+/Yb³⁺ ion and possible upconversion luminescence mechanisms.

Download Full Size | PDF

The up-conversion spectra consist of seven emission bands related to ⁵D₄→⁷F_J (J = 6, 5, 4, 3) and ⁵D₃,(⁵G₆)→⁷F_J (J = 6, 5, 4) transitions of Tb³⁺. Terbium ions do not absorb directly the 976 nm pumping radiation. Instead, they become excited in the course of cooperative energy transfer from Yb³⁺ ions. As for the excited state ⁵D₄, it is populated in the course of cooperative energy transfer between a pair of excited Yb³⁺ and a neighboring Tb³⁺ by 2-photon process.

 

Fig. 3 Typical luminescence decay from the ²F_5/2 state of Yb³⁺ in glass co-doped with 0.7Yb₂O₃/0.7Tb₂O_3.

Download Full Size | PDF

Based on luminescence decay measurements for ²F_5/2 state of Yb³⁺ ions in glass samples without and with Tb³⁺, the Yb³⁺→Tb³⁺ energy transfer efficiency was determined. Dependence of ²F_5/2 (Yb³⁺) lifetime and the energy transfer efficiency with terbium concentration is presented in Fig. 4.The ²F_5/2 lifetime of Yb³⁺ is reduced from 882 μs (0.7Yb₂O₃) to 751 μs in the presence of Tb³⁺ (0.7Yb₂O₃/1Tb₂O₃). However, maximum upconversion luminescence was obtained in glass co-doped with 0.7Yb₂O₃/0.7Tb₂O₃. Increase in terbium content up to 1mol% results in concentration quenching of luminescence.

 

Fig. 4 Lifetime for ²F_5/2 state of Yb³⁺ and energy transfer efficiency as a function of Tb³⁺ concentration.

Download Full Size | PDF

Ignoring the reverse transfer (Tb³⁺→Yb³⁺), quantum efficiency of the Yb³⁺→Tb³⁺ energy transfer can be established according to the equation:

From literature data it is well known that the reverse energy transfer also call back transfer process is not neglected for some glasses containing Tb³⁺ and Yb³⁺ ions [31]. Here, the concentrations of rare earth ions are relatively low and participation of back transfer process in population of Yb³⁺ and Tb³⁺ levels is rather not very significant. The equations do not take into account the reverse energy transfer (Tb³⁺→Yb³⁺) whose coefficient is several orders of magnitude smaller than C_f. Similar phenomena were observed for silicate glasses, where the calculated back transfer coefficient (C_b) was seven orders smaller than Yb³⁺→Tb³⁺ energy transfer coefficient [32]. The standard procedure of energy transfer coefficient determination based on least squares fitting of theory and experimental results was used [32,33]. The experimental and calculated results from decay curve of Tb³⁺:⁵D₄→⁷F₅ obtained for sample with 0.7Yb₂O₃/0.7Tb₂O₃ are presented in Fig. 5.

 

Fig. 5 Experimental and calculated decay curve of Tb³⁺: ⁵D₄→⁷F₅.

Download Full Size | PDF

The ⁵D₄ luminescence lifetime of Tb³⁺ is equal to 1.98 ms. The Yb³⁺→Tb³⁺ energy transfer coefficient C_f close to 1.5∙10⁻³³ cm⁶/s was determined by means of the nonlinear least squares fitting method. Root mean square is 1.41∙10⁻⁵ [s], which proofs reliability of performed calculations. The C_f value is greater than in the case of silicate glasses [33].

4. Conclusions

Up-conversion luminescence processes in GeO₂-Ga₂O₃-BaO glass systems co-doped with Tb³⁺ and Yb³⁺ ions were studied. Emission bands due to ⁵D₄→⁷F_J (J = 6, 5, 4, 3) and ⁵D₃, ⁵G₆→⁷F_J (J = 6, 5, 4) transitions of Tb³⁺ ions are observed under direct diode-laser excitation of Yb³⁺ by 976 nm line. Energy transfer coefficient in glass co-doped with 0.7Yb₂O₃/0.7Tb₂O₃ amounts to C_f = 1.5∙10⁻³³ cm⁶/s while efficiency of the Yb³⁺→Tb³⁺ energy transfer is 12%.