1. Introduction

Energy transfer between activated ions has been extensively studied for decades because of its important role in the design of new laser and optoelectronic materials. The fundamental energy transfer process involves excitation of a sensitizer and excited energy transfer in whole or in part to an activator by nonradiative and/or radiative processes. For activator doped laser materials, energy transfer by introducing one sensitizer provides an efficient way in the selection of pump source and increase of pump efficiency. Many impurities have been confirmed to act as excellent sensitizers. Transition-metal ions, for example, are desirable sensitizers for many laser applications because their broad, strong absorption bands in the visible region can efficiently couple to conventional flashlamp pumping sources [1–3]. Cr³⁺ ion, in particular, has been widely used as an efficient sensitizer in combination with rare-earth (RE) ions. The broad ⁴T₂→⁴A₂ emission of Cr³⁺ in the red and near-infrared range overlaps strongly with absorption transitions of many RE ions. Therefore, the pumping and laser efficiency of these RE ions can be largely improved. Since the demonstration of improved near-infrared lasing efficiency of Nd³⁺ and Er³⁺ in garnets [1, 2], considerable studies of energy transfer from Cr³⁺ to RE ions in many hosts have been reported [4–7].

Ni²⁺-doped transparent glass ceramics (GCs) have attracted considerable attention for their potential applications in broadband optical amplifiers and tunable lasers in recent years, since this kind of Ni²⁺ doped hybrid optical materials exhibits broad and long emission in the wavelength region covering the whole optical communication windows excited by 980 nm laser diode [8–11]. The detailed structural and spectral studies have identified that octahedral Ni²⁺ ions in nanocrystals of transparent GCs are responsible for these useful broadband emission. It is known that Cr³⁺-doped transparent GCs, in which Cr³⁺ ions occupy the octahedral sites of nanocrystals, broadly and strongly absorbs in the visible region yet fluoresce in the red and near-infrared regions [12, 13]. Recently, we prepared transparent Cr³⁺ and Ni²⁺ single-doped ZnO-Al₂O₃-SiO₂ (ZAS) GCs. It is interesting to notice that the near-infrared emission of Cr³⁺ with 532 nm excitation has a good overlap with the near-infrared absorption of Ni²⁺ in the ZAS GCs. Hence, if Cr³⁺ and Ni²⁺ ions were introduced into the ZAS GCs together, it could be expected that Ni²⁺ ions would be activated by Cr³⁺ sensitizing.

In this paper, we demonstrate efficient energy transfer from Cr³⁺ to Ni²⁺ in Cr³⁺/Ni²⁺ co-doped transparent ZAS GCs by steady state and time-resolved (TR) emission spectra, and discuss the energy transfer mechanism.

2. Experimental

A ZAS glass with composition of 39.3SiO₂ • 26.7Al₂O₃ • 21.3ZnO • 6.3TiO₂ • 6.4ZrO₂ • 0.1NiO • 0.3Cr₂O₃ (in wt%) was prepared in an electric furnace by conventional melt-quenching method. For comparison, NiO and Cr₂O₃ single-doped glasses with corresponding concentration were also prepared. Analytical reagents of SiO₂, Al₂O₃, ZnO, TiO₂, ZrO₂, Cr₂O₃ and high purity NiO (99.99%) were selected as raw materials. The raw materials were mixed thoroughly in an alumina mortar and melted in a platinum crucible at 1580 °C for 2 hours in the ambient atmosphere. The melts were cast onto a stainless steel plate and then annealed at 650 °C for 2 hours in air. According to the differential scanning calorimetry results, transparent GCs was obtained by heat-treating the glasses at 1000 °C for 2 hours in a small programmed electric furnace. We prepared GCs with different NiO (0.05~0.5 wt%) and Cr₂O₃ (0.05~0.7 wt%) concentration. The spectral measurement results showed that the GC doped with 0.1 wt% NiO and 0.3 wt% Cr₂O₃ had the optimal optical properties. Therefore, we only discuss optical properties of the GC doped with 0.1 wt% NiO and 0.3 wt% Cr₂O₃ in this work. Before subjected to measurement, the glass and GC samples were all cut into pieces with the size of 7 mm×7 mm×2 mm and polished.

3. Results and discussion

Figure 1 shows the X-ray diffraction (XRD) patterns of the Cr³⁺/Ni²⁺ co-doped glass and GC samples. The glass is amorphous without any sharp diffraction peaks, whereas several sharp diffraction peaks are observed in the XRD pattern of GC. According to JCPDS standard cards, these diffractions peaks can be ascribed to ZnAl₂O₄ (JCPDS Card No. 05-0669) and ZrTiO₄ (JCPDS Card No. 07-0290) phases. It is noticed that the diffraction peaks of the (220) plane of ZnAl₂O₄ and (111) plane of ZrTiO₄ overlap each other in the angle range of 29°–33°. To estimate the size of precipitations using the Scherrer’s equation, these two diffraction peaks were split by Gauss fitting, as shown in the inset of Fig. 1. The strongest diffraction peaks around 2θ=36.9° and 30.5° corresponding to the respective (311) plane of ZnAl₂O₄ and (111) plane of ZrTiO₄ were selected for the calculation. The sizes of ZnAl₂O₄ and ZrTiO₄ crystalline phases are all about 11.5 nm. The similar size of ZnAl₂O₄ and ZrTiO₄ nanocrystals was also obtained in Ref. [14] for this component glass after heat treatment at 1000 °C, but the size in our case was larger than that in Ref. [14]. From the diffraction peak area it can be concluded that the volume percent of ZrTiO₄ phase is very small and ZnAl₂O₄ is the main crystalline phase. Transparent GCs are produced by controlled crystallization of parent glasses and the size of precipitations and crystallinity (volume percent of precipitations) are determined by glass compositions and thermal treatment conditions. Thus it can be expected that the Cr³⁺, Ni²⁺ single- and co-doped GCs have similar nanocrystal size and crystallinity for their same main compositions and thermal treatment conditions.

 

Fig. 1. XRD patterns of Cr³⁺/Ni²⁺ co-doped glass and GC.

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Fig. 2. Optical absorption spectra of Cr³⁺ (a), Ni²⁺ (b) and Cr³⁺/Ni²⁺ (c) doped glasses (solid line) and GCs (short dot line).

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Optical absorption spectra of Cr³⁺, Ni²⁺ single- and co-doped glass and GC samples are presented in Fig. 2. The absorption spectra of Cr³⁺-doped glass and GC were very similar to those of Ref. [14]. After crystallization, the ⁴T₂ and ⁴T₁ absorption bands of Cr³⁺ showed a large blue shift [Fig. 2(a)]. The ⁴T₂ absorption band shifted from 645 nm in glass to 535 nm in GC, whereas the ⁴T₁ absorption band shifted from 442 nm to the outside of the ultraviolet absorption edge and thus could not be observed in the absorption spectrum of GC. The most interesting was that an apparent absorption band near 628 nm was embedded into the wavelength region of the ⁴T₂ absorption in the absorption spectrum of the GC, which could be attributed to the ⁴A₂→²E absorption transition of Cr³⁺. It has been understood that Cr³⁺ ions in the GC were incorporated into the ZnAl₂O₄ nanocrystals [14]. For Ni²⁺-doped glass, three clear absorption bands near 436, 902 and 1700 nm [assigned in Fig. 2(b)] could be attributed to the absorption transitions of trigonal bipyramid fivefolded Ni²⁺ in glass, and a shoulder absorption band around 570 nm could be from tetrahedrally coordinated Ni²⁺ [9, 11]. After crystallization, the absorption spectrum of Ni²⁺ in GC was significantly different from that in glass: only two absorption bands near 590 and 1010 nm were observed. This large change in absorption spectra elucidates that the ligand field surrounding Ni²⁺ has changed after crystallization. Ni²⁺ ions in the GC can be considered to locate in spinel ZnAl₂O₄ nanocrystals and occupy octahedral sites [10]. The absorption spectra of Cr³⁺/Ni²⁺ co-doped glass and GC in Fig. 2(c) could be considered as the superposition of the corresponding absorption spectra of the Cr³⁺ and Ni²⁺ single-doped glasses and GCs. From the absorption spectra, it can be concluded that Cr³⁺ and Ni²⁺ ions in the GCs are incorporated into ZnAl₂O₄ nanocrystals.

Ni²⁺ shows no near-infrared luminescence in Ni²⁺ single- and Cr³⁺/Ni²⁺ co-doped glasses, because Ni²⁺ occupies tetrahedral or trigonal bipyramid fivefolded sites in glasses, which hinders its efficiency as luminescent centers. Here, we mainly discuss luminescent properties of Cr³⁺, Ni²⁺ single- and co-doped GCs. The near-infrared emission spectra of Cr³⁺, Ni²⁺ single- and co-doped GCs with 532 nm excitation are compared in Fig. 3. To compare the relative fluorescence intensity, the excitation and detection systems were fixed, and the samples were set at the same place in the experimental setup. We also observed an intense red emission at 707 nm in Cr³⁺ single- and Cr³⁺/Ni²⁺ co-doped GCs (see the inset of Fig. 3), which was the characteristic of zero-phonon line of ²E→⁴A₂ transition of octahedral Cr³⁺ [6, 12]. The fine structures of the red emission at 707 nm were related to the R-line, Cr³⁺-Cr³⁺ pair interaction and a phonon sideband [12]. In Fig. 3, the intense peak around 1064 nm and small hump near 1600 nm are the second and third order diffraction peaks of 532 nm pumping light, and the peak at 1415 nm (indicated by a black arrow) is the second order diffraction peak of the red emission of Cr³⁺ at 707 nm. Cr³⁺-doped GC exhibits a near-infrared emission centered at 960 nm in the 840–1200 nm wavelength region with 532 nm excitation, which could be ascribed to the ⁴T₂→⁴A₂ transition of octahedral Cr³⁺ in nanocrystals. For Ni²⁺-doped GC, the excitation wavelength of 532 nm locates in the front-end of ³T₁(³F) absorption band of Ni²⁺ in GC [Fig. 2(b)], and a near-infrared emission around 1200 nm is observed. This broadband emission spectrum is the same as that obtained by exciting the ³T₂(³F) absorption band of Ni²⁺ in the GC with 980 nm laser diode, so under the 532 nm excitation the excited photons in the ³T₁(³F) excited state firstly relaxes to the ³T₂(³F) excited state by multiphonon relaxation process and then the electric transition of ³T₂(³F)→³A₂(³F) occurs [15]. For Cr³⁺/Ni²⁺ co-doped GC, two most apparent characteristics are observed: (i) the emission intensity at 960 nm of Cr³⁺ is largely reduced compared with that in Cr³⁺-doped GC; (ii) the emission intensity at 1200 nm of Ni²⁺ is greatly increased (approximately 8 times) compared with that in Ni²⁺- doped GC. This phenomenon can be attributed to efficient energy transfer from Cr³⁺ to Ni²⁺ in nanocrystals of GCs.

To further characterize the energy transfer between Cr³⁺ and Ni²⁺ ions in the ZnAl₂O₄ nanocrystals of GCs, TR emission spectra were measured using single photon counting technique in an Edinburgh Instrument Ltd. (UK) spectrometer (Model FL920). Figure 4 presents the TR emission spectra of the Cr³⁺/Ni²⁺ co-doped GC at different time interval ranging from 0 to 982 µs. When the time interval is zero two emission peaks at 875 and 975 nm are clearly observed, and no apparent emission at 1200 nm is detected. The emission at 975 nm corresponding to the ⁴T₂→⁴A₂ transition of octahedral Cr³⁺ (for the large excitation wavelength interval the observed emission peak positions in TR emission spectra may have a little deviation with those in the steady state emission spectra of Fig. 3). The emission at 875 nm is not observed in the steady-state emission spectrum of Cr³⁺ in the GC, which might be attributed to the zero-phonon vibronic sideband induced by the ⁴T₂ vibrational transition of octahedral Cr³⁺ [6]. In the transient time phonon vibration is thermally activated, whereas in thermal balance in the steady state, thus the emission at 875 nm is intense in the TR emission spectra and could hardly be observed in the steady-state emission spectrum (Fig. 3). As the time interval is prolonged, it is interesting to notice that the relative emission intensity of Cr³⁺ descends monotonically and rapidly, meanwhile a broadband emission at 1200 nm from octahedral Ni²⁺ is observed. Its relative emission intensity increases till the time interval is 57 µs, and then descends. This phenomenon clearly shows the energy transfer from Cr³⁺ to Ni²⁺: before the time interval of 57 µs the Ni²⁺ emission intensity continuously ascends because of the efficient energy transfer from Cr³⁺ to Ni²⁺, and then descends since the Cr³⁺ emission intensity is largely reduced which results in low energy transfer.

 

Fig. 3. Steady-state near-infrared emission spectra of Cr³⁺ (solid line), Ni²⁺ (short dash line) and Cr³⁺/Ni²⁺ (short dot line) doped GCs with 532 nm excitation. The emission curves were normalized to the third order diffraction. The inset shows intense red emission at 707 nm from zero-phonon line of ²E→⁴A₂ transition of octahedral Cr³⁺ in Cr³⁺ (solid line) single- and Cr³⁺/Ni²⁺ (short dot line) co-doped GCs excited with 532 nm.

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Fig. 4. (Color online) Time-resolved emission spectra of Cr³⁺/Ni²⁺ co-doped GC taken at different time interval (between 0 and 982 µs). The spectra are scaled and vertically shifted for better visualization. Measurements were performed at room temperature and the excitation wavelength was 532 nm.

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Examining the emission spectrum of Cr³⁺ in GC (Fig. 3) and absorption spectrum of Ni²⁺ in GC [Fig. 2(b)], it can be seen that the spectral overlap between the Cr³⁺ emission and Ni²⁺ absorption is almost entirely due to the Cr³⁺:⁴T₂ emission and almost unrelated to the Cr³⁺:²E emission. Therefore, the energy transfer can be considered to originate mainly from the ⁴T₂ state of Cr³⁺. The energy transfer mechanism that leads to enhanced near-IR emission of Ni²⁺ at 1200 nm in Cr³⁺/Ni²⁺ co-doped ZAS GC is demonstrated in Fig. 5. The main pumping channel is the Cr³⁺ duo to its intense ⁴T₂ absorption band (see Fig. 2). Ni²⁺ is efficiently excited by the Cr³⁺→Ni²⁺ energy transfer ⁴T₂, ³A₂(³F)→³T₂(³F), ⁴A₂, which enables Ni²⁺ emit a 1200 nm photon making the transition ³T₂(³F)→³A₂(³F). The decay curves of the red emission at 707 nm of Cr³⁺ in Cr³⁺-doped and Cr³⁺/Ni²⁺ co-doped GCs were measured by exciting the samples using 532 nm Xenon lamp at room temperature. These two decay curves could not be well fitted by the first exponential decay, thus the average decay lifetime τ_m was obtained by using τ_m=∫tI(t)dt/∫I(t)dt. A decrease in the Cr³⁺ lifetime of ~244 µs for Cr³⁺-doped GC to ~105 µs for Cr³⁺/Ni²⁺ co-doped GC was observed. This decrease in lifetime of Cr³⁺ indirectly indicates that the energy transfer from Cr³⁺ to Ni²⁺ should be nonradiative, because a decrease in the sensitizer’s emission decay lifetime is a distinguishing feature associated with nonradiative energy transfer. On the other hand, the absorption dip around 1010 nm duo to radiative absorption by the near-infrared absorption band of Ni²⁺ was not observed in the TR emission spectra, which also favored that the energy transfer from Cr³⁺ to Ni²⁺ was nonradiative. But the definite energy transfer mechanism still need to be further studied. The energy transfer efficiency η from Cr³⁺ to Ni²⁺ could be estimated according to the red emission decay lifetime of Cr³⁺ using the following formula:

where τ_Cr and τ_Cr(0) are the red emission decay lifetime of Cr³⁺ ions in GCs with and without Ni²⁺ ions. The calculated η is about 57%. It is well known that energy transfer rate is proportional to the spectral overlap and interactions between sensitizer and activator or species [16]. Generally speaking, energy transfer rate is larger for band-to-band transfer 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). In our case, the Cr³⁺ emission (Fig. 3) and Ni²⁺ absorption [Fig. 2(b)] are all allowed and broadband transitions and their overlap is very well, thus high energy transfer rate can be expected.

 

Fig. 5. Energy level diagram of Cr³⁺/Ni²⁺ co-doped ZAS GCs which exhibits Cr³⁺→Ni²⁺ energy transfer (ET). Solid and dash arrow lines represent the respective radiative processes and nonradiative processes. The ²E energy level is embedded into the ⁴T₂ energy level.

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4. Conclusion

Transparent ZAS GCs containing Cr³⁺/Ni²⁺ co-doped ZnAl₂O₄ nanocrystals were synthesized. Ni²⁺ single-doped ZAS GCs showed a near-infrared emission at 1200 nm with 532 nm excitation, whereas its emission intensity could be improved approximately 8 times by Cr³⁺ codoping. This enhanced emission could be attributed to an efficient energy transfer from Cr³⁺ to Ni²⁺ in the GCs, which was confirmed by the TR emission spectra. The energy transfer efficiency was about 57%. It is suggested that Cr³⁺/Ni²⁺ co-doped transparent ZAS GCs have potential applications in broadband optical amplifiers and tunable lasers.