1. Introduction

Impurity-sensitized luminescence phenomenon by energy transfer in various combinations of activated ions codoped laser materials has been extensively studied from the viewpoint of both applications and basic interests. Energy transfer is the process of transfer of excited state energy from a sensitizer to an activator. The rate of energy transfer depends on the extent of overlap of emission spectrum of the sensitizer with absorption spectrum of the activator, the relative orientation of the interacting dipoles and the sensitizer-activator distance [1].

Recently, Ni²⁺-doped transparent glass ceramics (GCs) with broadband and long near-infrared emission excited by 980 nm laser diode (LD) have attracted considerable attention for their potential applications in broadband optical amplifiers and tunable lasers [2-5]. The detailed spectral studies indicated that octahedrally coordinated Ni²⁺ ions incorporated into nanocrystals of transparent GCs are responsible for these useful broadband emissions. Unfortunately, the absorption of Ni²⁺ in the near-infrared in the transparent GCs was not strong for the very low doping concentration (typically ~0.1 mol%), which deteriorates the pumping efficiency and infrared optical properties of Ni²⁺. Yb³⁺ ion, which is one of the best well known sensitizer, has an intense emission near 1020 nm under excitation of a common commercial 980 nm InGaAs LD because of its strong absorption band matching well with the emission wavelength of the 980 nm InGaAs LD. It is interesting that the absorption band of octahedral Ni²⁺ in the near-infrared in transparent GCs and emission of Yb³⁺ overlap quite well. Therefore, it is possible to observe an efficient energy transfer from Yb³⁺ to Ni²⁺ in Yb³⁺/Ni²⁺codoped transparent GCs.

In this work, we choose MgO-Al₂O₃-Ga₂O₃-SiO₂-TiO₂ (MAGST) GCs containing spinel nanocrystals as the Yb³⁺/Ni²⁺ codoped hosts. Compared with the conventional MgO-Al₂O₃- SiO₂-TiO₂ GCs, Ga₂O₃ plays a very important role in the crystallization of MAGST glass during heat treatment. If Ga₂O₃ is not existed in MAGST GCs, two phases are precipitated in GCs, and moreover, the spinel crystalline phase is the secondary phase and very small. Single spinel phase is precipitated in glass ceramics when Ga₂O₃ is introduced; meanwhile the spectral properties of GCs with Ga₂O₃ are largely improved. The efficient energy transfer from Yb³⁺ to Ni²⁺ in Yb³⁺/Ni²⁺ codoped transparent MAGST GCs was studied. The spectra properties of Yb³⁺/Ni²⁺ codoped transparent MAGST GCs with different Yb₂O₃ concentration were also presented.

2. Experimental

Glasses with compositions of (100-y) (16.7MgO • 16.7Al₂O₃ • 8.3Ga₂O₃ • 50SiO₂ • 8.3TiO₂ • xNiO) • yYb₂O₃ (y=0, 0.25, 0.5, 0.75, 1.00 and 1.25 when x=0.3; y=0.25, 0.5, 0.75, 1.00 and 1.25 when x=0) (in mol%) were prepared in an electric furnace by using conventional melt-quenching method. Analytical reagents of SiO₂, Al₂O₃, Ga₂O₃, MgO, TiO₂ and high purity (99.99%) Yb₂O₃ and NiO were selected as raw materials. The raw materials were mixed thoroughly in an alumina mortar and melted in the corundum crucible at 1580 °C for 2 h in the ambient atmosphere. The melts were cast onto a stainless steel plate and then annealed at 650 °C for 2 h in air. According to differential scanning calorimetry (DSC) analysis, transparent GCs were obtained by further heat-treating the glasses at 950 °C for 2 h in air. The glasses and GCs were all polished and cut into pieces in the dimensions of 7mm×7mm×1.5mm.

3. Results and discussion

XRD patterns of the glasses with broad halo characteristics implied that they were vitreous. In contrast, sharp diffraction peaks in MAGST GCs without and with Yb₂O₃ were clearly observed in Fig. 1. The precipitated nanocrystals could be identified as MgAl₂O₄ spinel solid solutions consisting of Mg, Al, Ga, Ni, Ti and O. The detailed phase analysis and structure characteristic of the GCs can be found in Ref. [6]. From the obtained XRD patterns, the size of precipitated nanocrystals are calculated by using the Scherrer equation and the crystallinity of GCs is estimated by the ratio of integrated the area of the peaks and the total area of the XRD pattern from 10° to 85°. The results are shown in the inset of Fig. 1. It could be seen that the size of nanocrystals slightly increased in GCs with higher Yb₂O₃ compared to that in Yb³⁺- undoped GC, but it was not beyond 6 nm. The crystallinity of GCs did not show apparently variation with Yb₂O₃ and was about 40.0 %.

 

Fig. 1. XRD patterns of Yb³⁺/Ni²⁺ codoped MAGST GCs with different Yb₂O₃ concentration (mol%): (a) 0, (b) 0.25, (c) 0.50, (d) 0.75, (e) 1.00 and (f) 1.25. The inset shows the dependence of nanocrystal size and crystallinity of GCs on Yb₂O₃ concentration.

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Fig. 2 shows the absorption spectra of Ni²⁺-doped and Yb³⁺/Ni²⁺ codoped MAGST GCs (0.3 mol% NiO and 1.00 mol% Yb₂O₃) measured on JASCO V-570 spectrophotometer and emission spectra of Yb³⁺ in MAGST GC (1.00 mol% Yb₂O₃) obtained on ZOLIX SBP300 spectrophotometer with 980 nm LD excitation. For Ni²⁺-doped GC, two evident absorption bands centered at 602 and 978 nm could be ascribed to the spin-allowed transitions of the ³ A ₂(³ F) ground state to the ³ T ₁(³ F) and ³ T ₂(³ F) excited states of octahedral Ni²⁺, and a weak absorption band centered at 756 nm could be attributed to spin-forbidden transition of ³ A ₂(³ F)→¹ E(¹ D) of octahedral Ni²⁺ [2-5]. In the absorption spectrum of Yb³⁺/Ni²⁺ codoped MAGST GC, an intense absorption band from ² F _7/2 → ² F _5/2 transition of Yb³⁺ was observed except the absorption bands from Ni²⁺. It is interesting to notice that the intense Yb³⁺ (² F _5/2 → ² F _7/2) emission overlaps quite well with the ³ T ₂(³ F) absorption band of Ni²⁺ (Fig. 2). According to the theory of sensitized luminescence, it can be expected that energy transfer from Yb³⁺ to Ni²⁺ is possible in the transparent GCs.

 

Fig. 2. Absorption spectra of Ni²⁺-doped and Yb³⁺/Ni²⁺ codoped MAGST GCs (0.3 mol% NiO and 1.00 mol% Yb₂O₃) and emission spectra of Yb³⁺ in MAGST GC (1.00 mol% Yb₂O₃) with 980 nm LD excitation.

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It should be pointed out that according to the absorption spectra of Yb³⁺/Ni²⁺ codoped GC, Ni²⁺ ions are incorporated into the spinel nanocrystals of GCs [4], whereas Yb³⁺ ions are not included in the nanocrystals and still resided in the glass matrix of GCs. This is because Yb³⁺ ions usually can be incorporated into crystals or nanocrystals of GCs with cubic lattice site (such as PbF₂, CaF₂), whereas spinel nanocrystals only have tetrahedral and octahedral lattice sites. Moreover, the ionic radius of Yb³⁺ (0.99 Å) is much larger than those of Mg²⁺ (0.72 Å) and Al³⁺ (0.54 Å) in MgAl₂O₄ spinel, thus if Yb³⁺ ions incorporate into spinel nanocrystals of GCs, large lattice distortion would occur, which would result in the large shift of diffraction peak positions in XRD patterns. However, the diffraction peak positions in the XRD patterns of GCs with and without Yb₂O₃ hardly showed a shift. Therefore, it is reasonable to conclude that Yb³⁺ ions are still in the glass matrices of GCs. Another proof is related to emission spectra and fluorescence lifetime of Yb³⁺ in Yb³⁺-doped MAGST glasses and GCs. Luminescence properties of rare-earth (RE) ions depend strongly on their local environment [7], and therefore it can be expected that differences in the spectroscopic properties of RE ions are expected if they are placed in a glassy or in a crystalline surrounding in GCs. Generally speaking, if RE ions are incorporated into nanocrystals of GCs they usually show narrower emission spectra, more intense emission intensity and longer lifetime compared to in glasses [7]. Fig. 3 shows the emission spectra of 1.00mol% Yb₂O₃ doped MAGST glass and GC excited by 980 nm LD. The full width at half maximum of the emission spectra of Yb³⁺ in glass and GC is all about 95 nm, which is broader than that in some Yb³⁺-doped crystals [8, 9] and comparable to those in some Yb³⁺-doped glasses [10, 11]. Moreover, zero-phonon line often observed in Yb³⁺-doped crystals is not existed. The emission intensity of Yb³⁺ in GC is largely reduced compared to that in glass, and the measured fluorescence lifetime of Yb³⁺ is also decreased from 280 µs in glass to 175µs in GC. The above experimental results indicate that Yb³⁺ ions are not incorporated into spinel nanocrystals of GCs. The decrease of emission intensity and lifetime of Yb³⁺ in GC is caused by crystallization of glass, which results in the distance of Yb³⁺ shortening in the glass matrices of GCs compared to in glasses and thus energy transfer among Yb³⁺ and radiative reabsorption are improved [10-12].

 

Fig. 3. Emission spectra of 1.00mol% Yb₂O₃ doped MAGST glass and GC excited by 980 nm LD.

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Fig. 4 shows the emission spectra of Ni²⁺, Yb³⁺ single-doped and codoped GCs obtained on ZOLIX SBP300 spectrophotometer excited with 980 nm LD, and the dependence of the integrated intensity of Ni²⁺ emission on the Yb₂O₃ concentration is also presented in the inset of Fig. 4. To compare the relative fluorescence intensity between different GC samples, the excitation and detection systems were fixed, and the samples were set at the same place in the experimental setup. Compared with the Yb³⁺ single-doped GC, it can be seen that the emission intensity of Yb³⁺ at 1020 nm in the Yb³⁺/Ni²⁺ codoped GC was largely reduced. On the contrary, the emission intensity of Ni²⁺ around 1265 nm in the Yb³⁺/Ni²⁺ codoped GC was much more intense than that of the Ni²⁺ single-doped GC., The decrease in the luminescence intensity when the Yb₂O₃ concentration was 1.25 mol% could be caused by concentration quenching between Yb³⁺ ions duo to radiative re-absorption. From 0 to 1.00 mol% Yb₂O₃, there was an increment up to 4.4 times in the integrated intensity of Ni²⁺ emission (4.7 times in the emission intensity of Ni²⁺ at 1265 nm). This could be arised from an energy transfer from Yb³⁺ to Ni²⁺ in spinel nanocrystals. The emission decay curves of Ni²⁺-doped MAGST GCs without and with Yb₂O₃ could not be well fitted by the first exponential decay equation at room temperature so that the average lifetimes were obtained by using τ=∫tI(t)dt/∫I(t)dt. Fig. 5 presents the dependence of fluorescence lifetime of Ni²⁺ in Yb³⁺/Ni²⁺ codoped GCs and Yb³⁺ in Yb³⁺ single-doped and Yb³⁺/Ni²⁺ codoped GCs and full width at half maximum (FWHM) of Ni²⁺ emission on Yb₂O₃ concentration. As shown in Fig. 5, the FWHM of Ni²⁺ emission in MAGST GCs without and with Yb₂O₃ did not show evident fluctuation and was about 300 nm. For Ni²⁺ emission in Yb³⁺/Ni²⁺ codoped GCs, the lifetime showed slightly increased compared to that in Yb₂O₃ un-doped GCs. However, the decay lifetime of Yb³⁺ emission in Yb³⁺ single-doped and Yb³⁺/Ni²⁺ codoped GCs all decreased with Yb₂O₃ concentration, and the former was longer than the latter. For silicate glasses doped with higher Yb₂O₃, it has known that cooperative upconversion luminescence of two excited Yb³⁺, energy transfer among Yb³⁺, multiphonon relaxation duo to large phonon energy of silicate glasses, radiative re-absorption duo to high superposition between the absorption and emission bands of Yb³⁺ can significantly decrease the emission intensity and lifetime of Yb³⁺ [10-12]. In our case, the distance between Yb³⁺ in the glass matrices of GCs becomes shorter duo to crystallization of glasses, thus the above processes will be stronger compared to in glasses, which results in strong and monotonous reduction of Yb³⁺ lifetime in Yb³⁺-doped and Yb³⁺/Ni²⁺ codoped GCs. Actually, upconversion luminescence in Yb³⁺-doped MAGST GCs with higher Yb₂O₃ has been observed.

 

Fig. 4. Emission spectra of Ni²⁺ (solid line), Yb³⁺ (short dash line) and Yb³⁺/Ni²⁺ (short dot line) -doped MAGST GCs excited by 980 nm LD, and the inset shows the dependence of the integrated intensity of Ni²⁺ emission on the Yb₂O₃ concentration.

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Fig. 5. Dependence of fluorescence lifetime of Ni²⁺ in Yb³⁺/Ni²⁺ codoped MAGST GCs and Yb³⁺ in Yb³⁺ single-doped and Yb³⁺/Ni²⁺ codoped MAGST GCs and FWHM of Ni²⁺ emission on Yb₂O₃ concentration.

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Visible upconversion luminescence and enhanced near-infrared Cr⁴⁺ luminescence by energy transfer from near-infrared excited Yb³⁺ around 1µm to transition metal Cr³⁺, Mn²⁺, Ni²⁺ and Cr⁴⁺ ions in their combination codoped crystals have been extensively studied [13-16]. However, there has been no report about enhanced near-infrared luminescence from Ni²⁺ by Yb³⁺-sensitizing in GCs. In our case, the enhance luminescence of Ni²⁺ in Yb³⁺/Ni²⁺ codoped MAGST GCs can be ascribed to energy transfer from Yb³⁺ to Ni²⁺ in nanocrystals of GCs, which is a resonant and nonradiative process duo to the decrease of the lifetime of the sensitizer Yb³⁺ ions in Yb³⁺/Ni²⁺ codoped MAGST GCs compared with that in Yb³⁺ single-doped MAGST GCs, because a decrease in the sensitizer’s emission decay lifetime is a distinguishing feature associated with nonradiative energy transfer [1] (see Fig. 5). Hence, the energy transfer between Yb³⁺ and Ni²⁺ in nanocrystals of GCs can be treated using the Forster-Dexter theory of nonradiative energy transfer between impurities (sensitizer and activator) in solids [1]. However, for Ni²⁺ ions’ gathering in nanocrystals of GCs and not homogeneously distributed in the GCs it is a little difficult to calculate energy transfer rate according to the Forster-Dexter theory, which requires the homogeneous distribution of activators in a host matrix [1]. But energy transfer rate is well known to be proportional to the spectral overlap and interactions between sensitizer and activator or species [17]. Energy transfer rate is larger for band-to-band process duo to the larger spectral overlap. In addition, the strength of interaction is determined by the nature of optical transition. For example, it is larger for allowed transition (broad band) compared to forbidden transition (narrow line). Although Yb³⁺ absorption transition is forbidden in nature it can be broadened by variation in crystal field strength in glasses. Therefore energy transfer in Yb³⁺/Ni²⁺ codoped MAGST GCs occurs from a “forbidden” and broadened narrow band absorption for Yb³⁺ ions to an “allowed” broadband absorption for Ni²⁺ ions, and thus high rate is reasonable. The energy transfer mechanism that lead to enhanced near-infrared luminescence of Ni²⁺ in Yb³⁺/Ni²⁺ codoped MAGST GCs is suggested in Fig. 6. The main pumping channel is the Yb³⁺ duo to its intense absorption band ²F_7/2→²F_5/2 (see Fig. 2). Ni²⁺ is excited by the Yb³⁺→Ni²⁺ energy transfer ²F_5/2,³A₂(³F)→³T₂(³F), ²F_7/2, which enables Ni²⁺ emit a 1265 nm photon making the transition ³T₂(³F)→³A₂(³F).

 

Fig. 6. Energy level diagram of Yb³⁺/Ni²⁺ codoped MAGST GCs which exhibits Yb³⁺→Ni²⁺ energy transfer (ET). Dashed and solid lines indicate the respective nonradiative and radiative processes.

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4. Conclusion

Transparent MAGST GCs containing Yb³⁺ ions and Ni²⁺-doped spinel nanocrystals were synthesized. The near-infrared emission intensity from Ni²⁺ in GCs was enhanced up to 4.4 times by energy transfer from Yb³⁺ to Ni²⁺ in nanocrystals. The best Yb₂O₃ concentration was about 1.00 mol% that was determined by luminescence measurement. For the Yb³⁺/Ni²⁺ codoped GC with 1.00 mol% Yb₂O₃, broadband near-infrared emission centered at 1265 nm with FWHM of about 300 nm and lifetime of 220 µs was obtained. The energy transfer mechanism was also discussed. It is expected that the present Yb³⁺/Ni²⁺ codoped transparent GCs have potential applications in broadband optical amplifiers and tunable lasers.