The near-infrared emission intensity of Ni2+ in Yb3+/Ni2+ codoped transparent MgO-Al2O3-Ga2O3-SiO2-TiO2 glass ceramics could be enhanced up to 4.4 times via energy transfer from Yb3+ to Ni2+ in nanocrystals. The best Yb2O3 concentration was about 1.00 mol%. For the Yb3+/Ni2+ codoped glass ceramic with 1.00 mol% Yb2O3, a broadband near-infrared emission centered at 1265 nm with full width at half maximum of about 300 nm and lifetime of about 220 µs was observed. The energy transfer mechanism was also discussed.
©2008 Optical Society of America
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 .
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.
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. . 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. 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.
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 , 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 , 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 . 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. 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.
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  (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 . 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 . But energy transfer rate is well known to be proportional to the spectral overlap and interactions between sensitizer and activator or species . 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 2F7/2→2F5/2 (see Fig. 2). Ni2+ is excited by the Yb3+→Ni2+ energy transfer 2F5/2,3A2(3F)→3T2(3F), 2F7/2, which enables Ni2+ emit a 1265 nm photon making the transition 3T2(3F)→3A2(3F).
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.
This work was financially supported by National Natural Science Foundation of China (Grant No.50672087 and No. 60778039), National Basic Research Program of China (2006CB806000) and National High Technology Program of China (2006AA03Z304). This work is also supported by Program for Changjiang Scholars and Innovative Research Team in University.
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