Up-conversion materials doped with rare-earth ions have been a significant interest in recent years due to various potential applications, such as biological labeling, three-dimensional displays, solar cells, etc. [1-3]. Oxyfluoride glass ceramics have been widely investigated as host materials for rare-earth ions recently, because they have the advantages of not only low phonon energies but also high chemical and mechanical stabilities [4]. CaF₂ is a nontoxic optical material with a wide spectral region of transparency, high solubility of rare-earth ions [5], and a good match of refractive index with aluminosilicate glass [6]. Doped rare-earth ions are confined in CaF₂ nanocrystals with much less phonon energy, yielding large quantum efficiency [7]. A solar-cell material with a high band gap, such as amorphous silicon, loses the near-infrared portion of the solar spectrum, above 730 nm, but multiply doped systems may improve up-conversion characteristics, because a rare-earth ion has many absorption levels in the near-infrared. Most previous studies on up-conversion processes have been reported for singly or doubly doped materials. Er³⁺ ions showing green and red up-conversion emissions are known as the most efficient up-conversion activators [8], and Pr³⁺ ions have many energy levels close to those of Er³⁺ ions, enabling energy transfer between these two species. Yb³⁺ ions are well-known sensitizers that have a high absorption cross section around 980 nm and transfer energy efficiently to Er³⁺ or Pr³⁺ ions [9, 10].

There have been only a few investigations of triply doped glass ceramics. The up-conversion properties of a system triply doped with Er³⁺, Pr³⁺, and Yb³⁺ was only investigated in tellurite glass, as far as we know [11]. In the present study, we report the infrared-to-visible up-conversion energy transfers in Er³⁺, Pr³⁺, and Yb³⁺ triply doped transparent glass ceramics containing CaF₂ nanocrystals, and compare the results to those in double doped systems under excitation at 980 nm. We also compare the up-conversion emissions of triply doped glass ceramics under excitation at 980 nm to those under simultaneous excitations at 980 and 1550 nm.

Oxyfluoride glass hosts were prepared with the composition (in mol%) 45SiO₂-22Al₂O₃-11CaO-22CaF₂. Dopants were 1%Er-2%Yb (ErYb), 1%Pr-2%Yb (PrYb), 1%Er-1% Pr-2%Yb (ErPrYb), and 1%Er-1%Pr-10%Yb (ErPrYb(10)). For each batch, the raw material was mixed and melted in a covered platinum crucible in air at 1400°C for 1 h, and then cast into an iron mold, followed by annealing at 530°C for 10 h to release the inner stress. The temperatures of the glass transition temperature and the onset of crystallization were measured by differential thermal analysis (DTA, SDT2960). The glass samples were then annealed precisely at 760°C for 4 h to induce crystallization, and polished for optical measurements. Oxyfluoride precursor glass and glass ceramic samples are denoted as G and GC respectively. We confirmed the formation of CaF₂ crystal phases by x-ray diffraction (XRD, RIGAKU, SmartLab) analysis. The up-conversion and down-conversion luminescences were measured with a monochromator (DK240) and photomultiplier tube (PMT) under diode-laser excitations at 365, 980, and 1550 nm.

Figure 1 shows the XRD patterns of triply doped samples.

The ErPrYb_G shows broad humps due to the amorphous structure. The background patterns of the ErYb_GC, PrYb_GC, and ErPrYb_GC treated at 760°C show peaks easily assigned to cubic CaF₂ (JCPDS 35-0816). We estimate the average nanocrystal size as 40-50 nm from XRD line- broadening measurements, using Sherrer’s equation [12],

where D (hkl) is the crystal size in the direction (hkl), λ is the wavelength of the x-rays, θ is the angle of diffraction, β is the full width at half maximum (FWHM) of the diffraction peak, and K is the instrument’s constant (0.9). The absorption spectrum of the glass reveals three kinds of dopants, as shown in Fig. 2. All three ions have absorptions at 980 nm.

The down-conversion spectra of the ErYb_GC, PrYb_GC, and ErPrYb_GC under excitation at 365 nm showed emissions tens of times stronger than those of the glasses with the same dopants, as shown in Fig. 3. The enhancement can be attributed to suppressed nonradiative relaxation of rare-earth ions incorporated into CaF₂ nanocrystals with very low phonon energy [13]. Besides, the structured spectral shape showing Stark levels in the green and red emissions indicates that Er³⁺ and Pr³⁺ ions are mainly located in crystalline domains, rather than glass domains, of the glass ceramics. In addition, the energy transfer among neighboring rare-earth ions inside CaF₂ nanocrystals can occur effectively [14], because rare-earth ions reside preferentially inside the nanocrystals rather than in the glass matrix. [15].

The spectrum of Pr³⁺ ions in glass is known to feature very broad emission in the region of 580-720 nm [16]. Our PrYb_GC also shows a broad red emission from Pr³⁺ ions, indicating the ³P₀→ ³H₆, ³F₂ transitions. As we can see in Fig. 3, ErYb_G has negligible red emission, because the ⁴F_9/2 level is populated nonradiatively from the ⁴S_3/2 state, and its emission cross section is smaller than that of ⁴S_3/2 [17]. However, in ErYb_GC, the green emission of the ²H_11/2, ⁴S_3/2→⁴I_15/2 transitions in the region of 520-560 nm and the red emissions of the ⁴F_9/2→⁴I_15/2 transitions show much enhancement, indicating effective energy transfer from the charge-transfer band to the high-energy states of Er³⁺ ions incorporated in CaF₂ nanocrystals. Furthermore, the red emission is stronger than the green. The ⁴F_9/2 emitting state can be also populated by the two cross relaxations (⁴I_11/2→⁴I_15/2 and ⁴I_13/2→⁴F_9/2) and (⁴F_7/2→⁴F_9/2 and ⁴I_11/2→⁴F_9/2). Besides, there can be energy transfer from Er³⁺ to Yb³⁺ ions via the ⁴I_11/2→⁴I_15/2 (Er³⁺) and ²F_7/2→²F_5/2 (Yb³⁺) transitions. Following the back-transfer process to Er³⁺ ions by the ⁴I_13/2→⁴I_9/2 (Er³⁺) and ²F_5/2→²F_7/2 (Yb³⁺) transitions can enhance the red emission.

ErPrYb_GC shows strong green (Er³⁺) and red (Pr³⁺) emissions. However, the red emission of the Er³⁺ ions is much reduced. The spectral shapes of the PrYb_GC and ErPrYb_GC are similar, but the difference of two normalized spectra clearly shows only a purely red emission band of Er³⁺ ions at 660 nm. Very effective energy transfer of the ⁴I_13/2 (Er³⁺)→³F_3,4 (Pr³⁺) and ⁴I_11/2 (Er³⁺)→¹G₄ (Pr³⁺) channels reduces the populations of the ⁴I_13/2 and ⁴I_11/2 states of the Er³⁺ ions. The resultant inactivity of the two cross relaxations that are effective in ErYb_GC produces weak red emission from the ⁴F_9/2 state [18-20].

Besides, the excitation energy at 365 nm is split between the Er³⁺ and Pr³⁺ ions, and the ⁴I_11/2→⁴I_15/2 transition energy of the Er³⁺ is transferred not only to Yb³⁺ (²F_7/2→²F_5/2) but also to Pr³⁺ (³H₄→¹G₄ ). Furthermore, the excited energy of the ²F_5/2 state (Yb³⁺) can be transferred to both Er3+ and Pr³⁺ ions. Those processes result in the greatly reduced red emission of Er³⁺ ions in ErPrYb_GC. In contrast, the effective energy transfer from Pr³⁺ (³P₀) to Er³⁺ (⁴F_7/2) ions [21] contribute to the increasing green emission of Er³⁺ ions in ErPrYb_GC.

The up-converted emission spectra of ErYb_GC, PrYb_GC, ErPrYb_GC, and ErPrYb(10)_GC excited at 980 nm are shown in Fig. 4. Pr³⁺ ions showed little absorption at 980 nm, because the oscillator strength of the ¹G₄ state has its maximum at 1020 nm. However, Er³⁺ has a weak absorption band at 980 nm, and Yb³⁺ has much stronger absorption at this wavelength, because of high oscillator strength [22] and concentration. The up-conversion emission intensity of Er-doped CaF₂ was reported to be more than an order of magnitude weaker than that of Er-Yb codoped CaF₂ under 980 nm excitation [23]. In general, efficient energy transfer occurs in Er³⁺-Yb³⁺ codoped materials, owing to the large spectra overlap of both absorption and emission between these ions around 980 nm. The up-conversion mechanisms for green and red emissions of Er³⁺ in Er³⁺and Yb³⁺ codoped materials are well known [9], and are described in Fig. 5. Two excited-state absorptions, the ESA-1 (⁴I_13/2→⁴F_9/2) and ESA-2 (⁴I_11/2→⁴F_7/2) transitions, are enabled by the pump energy at 980 nm, and are responsible for the red and green emissions respectively. Besides, the cross relaxation (CR)-a of (⁴I_11/2→⁴F_7/2) and (⁴I_11/2→⁴I_15/2) populates the ⁴F_7/2 state, and the CR-b of (⁴I_13/2→⁴F_9/2) and (⁴I_11/2→⁴I_15/2) populates the ⁴F_9/2 state respectively. We expect relatively stronger red emission than green because the metastable ⁴I_13/2 state enables CR-b more than CR-a to increase the population of the ⁴F_9/2 state effectively. This reduced ratio of green:red is also driven by depopulation of the ⁴F_7/2 state and population of the ⁴F_9/2 by the CR-c of (⁴F_7/2→⁴F_9/2) and (⁴I_11/2→⁴F_9/2). However, the contribution of CR-c is relatively small, due to nonradiative relaxation from ⁴F_7/2 to ²H_11/2, although the CR-c process in Er-Yb codoped phosphors has been reported [24, 25].

The emission intensity from the PrYb_GC is weak, ~10^-5 times as strong as that from the ErYb_GC in Fig. 4. The emission bands at 480, 490, 525, 540, 600, 610, 640, 656, and 850 nm are attributed respectively to the ³P₁→³H₄, ³P₀→³H₄, ³P₁→³H₅, ³P₀→³H₅, ¹D₂→³H₄, ³P₀→³H₆, ³P₁→³F₂, ³P₀→³F₂, and ³P₁→¹G₄ transitions of Pr³⁺ [26]. The proposed up-conversion mechanism for the emissions of Pr³⁺ is also displayed in Fig. 5. The energy of excited Yb3+ is transferred to Pr³⁺ by (²F_5/2→²F_7/2) (Yb³⁺) and (³H₄→¹G₄) (Pr³⁺). The subsequent processes for Pr³⁺ are ESA-3 (³F_3,4→¹D₂) and ESA-4 (¹G₄→³P₀). However, energy transfer from Yb³⁺ to Pr³⁺ ions is less efficient than that from Yb³⁺ to Er³⁺ [27] at 980 nm, resulting in weak visible emission from Pr³⁺.

The ErPrYb_GC shows significant, yet weaker by a factor of over 400, up-conversion emission than the ErYb_GC, under 16-mW excitation at 980 nm. In the ErPrYb_GC, pump energy is mainly shared by Er³⁺ and Yb³⁺ ions, and their excitation energies are transferred to each other via Yb³⁺-Er³⁺ [9], Er³⁺-Pr³⁺ [19, 21], and Yb³⁺-Pr³⁺ [27] interactions.

Reduced energy transfer from Yb³⁺ to Er³⁺ in ErPrYb_GC, due to another transfer channel from Yb³⁺ to Pr³⁺ [27], weakens the ESA-1 and ESA-2 processes of Er³⁺. Besides, as mentioned, for down-conversion in ErPrYb_GC under 365-nm excitation, two efficient energy-transfer channels from Er³⁺ to Pr³⁺ ions [18], ⁴I_13/2(Er³⁺) + ³H₄(Pr³⁺) → ⁴I_15/2(Er³⁺) + ³F_3,4(Pr³⁺) and ⁴I_11/2 (Er³⁺) + ³H₄(Pr³⁺) → ⁴I_15/2(Er³⁺) + ¹G₄(Pr³⁺), make the ESA-1 and ESA-2 processes negligible. As we can see from the spectrum of PrYb_GC in Fig. 4, Pr³⁺ ions are not effective at producing up-conversion emission under 980-nm excitation. Thus, both green and red up-conversion emissions from ErPrYb_GC are dramatically reduced.4

However, the ratio of green:red emission of ErPrYb_GC is 10 times as large as that of ErYb_GC, which differs from the down-conversion seen in Fig. 3. The increased ratio of green:red emission in ErPrYb_GC can be explained mainly by suppression of the CR-b and ESA-1 processes, because of the reduced population of ⁴I_13/2 due to energy transfer from Er³⁺ (⁴I_13/2) to Pr³⁺ (³F_3,4) [19]. In addition, the population of the ¹G₄ state (Pr³⁺) can be increased by energy transfer from Yb³⁺(²F_5/2) and Er³⁺(⁴I_11/2). The probability of energy transfer from the ⁴I_9/2 or ⁴F_9/2 state of Er³⁺ is very low, because the large energy differences of those states from ¹G₄ (Pr³⁺) require many phonons to be involved. Increased population of the ¹G₄ state enables the ESA-4 process to populate the ³P₀ (Pr³⁺) state, followed by energy transfer from it to the ⁴F_7/2 (Er³⁺) state [21], resulting in enhanced green emission. Thus, the ratio of green:red emission can be enhanced.

Figure 4 also shows that the green (540 nm), red (656 nm), and near-infrared (850 nm) emissions of ErPrYb(10)_GC with 10 mol% Yb³⁺ ions respectively increase by factors of 90, 40, and 150compared to those of ErPrYb_GC with 2 mol% Yb³⁺ ions. Our up-conversion emission in the green and red regions showed no line broadening, even in 2%Er³⁺-10%Yb³⁺ codoped glass ceramics, indicating no additional inhomogeneous broadening caused by a doping concentration five times as high.

The huge enhancement seen just by increasing Yb³⁺ concentration by a factor of 5 is not easily understood. Moreover, in ErPrYb(10)_GC the ratio of green:red emission is improved more than twofold. Chen et al. [9] reported that the ratio of green:red emission in the ErYb system decreases with Yb³⁺ concentration, down to almost zero for 10% Yb³⁺ concentration, under 980-nm excitation in codoped ZrO₂ nanocrystals. The ratio of green:red emission for ErYb_GC in Fig. 4 is twice as large as in their report, for the 1%Er-2%Yb system. The Stark-splitting levels of Er³⁺ and Pr³⁺ are revealed in the emission bands, indicating that Er³⁺ and Pr³⁺ ions are located in the crystalline phase.

The up-conversion channels in ErPrYb(10)_GC are assumed to be about the same as in ErPrYb_GC. The huge enhancement of two emissions are ascribed to electrons accumulating in the ²F_5/2 state of highly concentrated Yb³⁺ ions, which are energy providers and intermediates, because they do not have any excited state in the visible except for ²F_5/2.

In general, Yb³⁺ ions have been reported as proper dopants for up-conversion, with concentrations as high as 10-25 mol% in materials such as CaF₂ [28], PbF₂ [29], and NaLuF₄ crystals [30], without concentration quenching under 980-nm excitation. The emission intensity is proportional to Iⁿ, where I is excitation intensity and n is the number of pump photons required.

The power dependence of the green and red emissions of ErPrYb_GC is shown in Fig. 6. The slope of 2.1 for the red emission implies that it originates mainly from a two-photon process, in the present power range. The slope of 2.3 for the green emission suggests that it has not only the two-photon channel of ⁴I_15/2→ ⁴I_11/2 and ESA-2, but also the three-photon channel of ⁴I_15/2→⁴I_11/2, ESA-1, and ESA-2 transitions, including nonradiative relaxations [31]. Energy transfer from the 3P0 (Pr³⁺) to ⁴F_7/2 (Er³⁺) states follows another three-photon channel, consisting of the ³H₄→¹G₄, ESA-3, and ESA-4 transitions [21]. Thus the slope of the green emission is less than 3.

Increasing the concentration of Yb³⁺ ions from 2% to 10% corresponds to a fivefold increase in absorption at 980 nm. The factor of 70 enhancement in up-conversion intensity seen in Fig. 6 may be explained by a more than quadratic dependence on the absorbed power. Moreover, we expect more efficient energy transfer from Yb³⁺ to Er³⁺ ions at higher concentrations in CaF₂, because the average distance between Yb³⁺ and Er³⁺ ions is reduced, and the rate of energy transfer between ions is increased [32].

Excitation at 1550 nm for Pr_GC, ErPr_GC, PrYb_GC, and ErPrYb _GC did not show any visible up-conversion emission. However, upon double excitation at both 980 nm (16 mW) and 1550 nm (8 mW), the ErPrYb_GC showed enhanced emissions from both Er³⁺ and Pr³⁺ ions, compared to those under single excitation at 980 nm, as shown in Fig. 7. The enhancement is assumed to originate from GSA (³H₄→³F_3,4) and ESA (¹D₂→³P₂) processes of 1550 nm excitation of Pr³⁺, as shown in Fig. 6. The relatively large enhancement of 850-nm emission (¹D₂→³H₆) in PrYb_GC under double excitation may be due to a more populated ¹D₂ state, thanks to ³H₄→³F_3,4 and ESA-3 transitions.

In down-conversion emission under excitation at 365 nm, two cross relaxations of (⁴I_11/2→⁴I_15/2 and ⁴I_13/2→⁴F_9/2) and (⁴F_7/2→⁴F_9/2 and ⁴I_11/2→⁴F_9/2) in ErYb_GC decreased the ratio of green:red emission of Er³⁺ ions. In contrast, the inactivity of two cross relaxations in ErPrYb_GC, due to energy transfer from Er³⁺ to Pr³⁺, increased the ratio. In up-conversion emission under 16-mW excitation at 980 nm, the strong up-conversion emission seen in ErYb_GC was reduced by more than a factor of 400 in ErPrYb_GC. This reduction was explained by diminished energy transfer from Yb³⁺ to Er³⁺, because of an additional energy-transfer channel from Yb³⁺ to Pr³⁺, on top of the direct energy transfer from Er³⁺ to Pr³⁺. Energy transfer from Pr³⁺(³P₀) to Er³⁺(⁴F_7/2) contributes to an increase in the green:red emission ratio. A huge enhancement of up-conversion intensity by increasing Yb³⁺ concentration fivefold was explained by quadratic dependence on the absorbed power plus enhanced efficiency in energy transfer from Yb³⁺ to Er³⁺, due to reduced Er³⁺-Yb³⁺ distance. We also compared the up-conversion emission of excitation at 980 nm to those of simultaneous excitations at 980 nm and 1550 nm in triply doped glass ceramics, and detailed up-conversion mechanisms were suggested, based on the observations.