Er3+-doped transparent glass ceramics containing micron-sized SrF2 crystals for 2.7 μm emissions

Er3+-doped transparent glass ceramics containing micron-sized SrF2 crystals were obtained by direct liquid-phase sintering of a mixture of SrF2 powders and precursor glass powders at 820 °C for 15 min. The appearance and microstructural evolution of the SrF2 crystals in the resulting glass ceramics were investigated using X-ray diffraction, field-emission scanning electron microscopy and transmission microscopy. The SrF2 crystals are ~15 μm in size and are uniformly distributed throughout the fluorophosphate glass matrix. The glass ceramics achieve an average transmittance of 75% in the visible region and more than 85% in the near-IR region. The high transmittance of the glass ceramics results from matching the refractive index of the SrF2 with that of the precursor glass. Energy dispersive spectroscopy, photoluminescence spectra, and photoluminescence lifetimes verified the incorporation of Er3+ into the micron-sized SrF2 crystals. Intense 2.7 μm emissions due to the 4I11/2 → 4I13/2 transition were observed upon excitation at 980 nm using a laser diode. The maximum value of the emission cross section of Er3+ around 2.7 μm is more than 1.2 × 10−20 cm2, which indicates the potential of using transparent glass ceramics containing micron-sized SrF2 crystals for efficient 2.7 μm lasers and amplifiers.

Scientific RepoRts | 6:29873 | DOI: 10.1038/srep29873 As is well known, some disadvantages of melt-quenching method, such as high preparation temperature and energy consumption, uncontrollable crystal formation, and long and complex heat treatment, severely hinder the GCs-based applications [14][15][16] . On the other hand, the optical properties in these GCs are achieved only through a careful partial crystallization of the nanometer scale grains. However, network modifiers and impurities in the glass matrix tend to accumulate on the surface of the nanocrystals because of the high surface energy, which may change the properties of the nanocrystals 17 . Meanwhile, defect energy levels are easily introduced into the forbidden band of nanocrystals because of large surface defects, resulting in defect luminescence 18 . Large grains can prevent the problems encountered in nanocrystals because of a small surface-to-volume ratio. Fan et al. 9 obtained intense IR fluorescence around 2.7 μ m in Er 3+ -doped transparent GC containing large CaF 2 crystals by a technique similar to the liquid-phase-sintering method, which implies that Er 3+ -doped transparent GCs containing large fluoride grains exhibit a high potential for mid-infrared applications. Additionally, abovementioned method possesses low preparation temperature and precise crystal phase control that compete with melt-quenching method. Unfortunately, a complex and time-consuming preparation process including coprecipitation, ball-milling and highly precise temperature and time control is requirement. Most importantly, long-time sintering is necessary for abovementioned method to remove pores and achieve the growth of crystals, which causes strong glass components volatilization. The changes of glass components will increase the differences in the refractive index between crystalline phase and glass phase and scattering losses. Meanwhile, glass components volatilization entails a certain degree of randomness, which is difficult to accurately adjust the refractive index of glass matrix. Measures must be taken to address these problems.
In order to avoid the difficulties of 6,9,12 , a liquid-phase sintering route was designed by tuning components of PG and optimizing the preparation process in foundation of existing technique 9,19 , which is directly sintering a mixture of commercial fluoride powders and PG powders prepared by grinding at low temperature for a short time. This liquid-phase-sintering route has several advantages over existing GCs preparation method such as: precise control over the crystal phase and refractive index, short preparation time, low preparation temperature and energy consumption, and simple and low-cost synthetic approaches, which results in a high financial and energetic saving.
In this study, based on remarkable optical performances and high potential for mid-infrared applications of SrF 2 crystal and GCs containing SrF 2 crystals 20-22 , Er 3+ -doped transparent GCs containing micron-sized SrF 2 crystals were fabricated by abovementioned liquid-phase sintering route and characterized. To the best of our knowledge, we obtained, for the first time, transparent GCs containing micron-sized SrF 2 crystals and 2.7 μ m emissions in GCs containing SrF 2 . Intense 2.7 μ m emissions generated by GCs have been discussed in detail for possible applications in mid-infrared lasers and amplifiers.

Experimental
Sample preparation. Fluorophosphate (FP) glass, which is a promising laser material, has remarkable spectral properties, good chemical and mechanical stability, low OHcontent, and a low melting temperature 23 . In particular, the refractive index of FP glass can be readily tuned to match that of SrF 2 , yielding highly transparent GCs containing large SrF 2 crystals. In our experiment, Er 3+ doped FP glass with a nominal molar composition of 35CaF 2 -18MgF 2 -10SrF 2 -18BaF 2 -3NaF-6AlF 3 -7Na(PO 3 ) 3 -2.7Al(PO 3 ) 3 -0.3ErF 3 , denoted as precursor glass (PG), was prepared by the conventional melting-quenching technique. The refractive index of FP glass was matched with that of the SrF 2 crystal using the GE-SYSTEM software. Samples were prepared using analytical-grade NaF, MF 2 (M:Ca, Sr, Ba), AlF 3 , ErF 3 , Na(PO 3 ) 2 and Al(PO 3 ) 3 by the liquid-phase sintering method. A GC with a composition of 45SrF 2 -55 PG (wt%) was prepared from SrF 2 and PG powders. SrF 2 powders (4.5 g) were blended directly with PG powders (5.5 g) by grinding in an agate mortar for 5 min. The mixture was melted in an electric furnace at 820 °C for 15 min. The melts were then quenched in stainless steel molds and pressed into thin plates. The samples were annealed at 380 °C (Tg = 392 °C) for 2 h to reduce internal stress, when Er 3+ :SrF 2 GCs were obtained.

Sample characterization. The crystallization behavior of the samples was investigated using a Philips
PANalytical X'Pert X-ray diffraction (XRD) system at 40 kV and 40 mA with a Cu target X-ray tube. The transmittance characteristics were investigated by a Lambda 1050 UV/VIS/NIR spectrophotometer (Perkin Elmer) for wavelengths ranging from 300 to 3000 nm. The thermal behavior of the PG was characterized using thermogravimetry (TG) and differential thermal analysis (DTA) at a heating rate of 5 K/min in air from 100 °C to 1000 °C using an EXSTAR SII:TG/DTA7300 from Japan. The refractive index of the samples was obtained by the prism minimum deviation method using a WYV V-Prism refractometer (CANY, Shanghai, China) with an accuracy of n D ± 5 × 10 −5 . The microstructural evolution of the samples was studied by a field-emission scanning electron microscope (FESEM, HitachiS-4800, Japan) equipped with an energy-dispersive X-ray spectrometer (EDS) and transmission microscope (Olympus). The emission spectrum with excitation of a diode laser at 980 nm was recorded using by a PMT detector (R928) at 25 °C.

Results and Discussion
XRD measurements were performed to examine the crystallization behavior. Fig. 1 shows the XRD pattern of the Er 3+ -doped PG, Er 3+ -doped GC, and SrF 2 powders, respectively. Only two broad humps are observed in the XRD pattern of the PG, indicating that no spontaneous crystallization occurs in PG. After liquid-phase sintering at 820 °C for 15 min, two broad envelopes of glass diffraction and some intense sharp peaks of the crystalline SrF 2 cubic phase emerge in the XRD curve of GC, which confirms that SrF 2 crystals were successfully incorporated into PG. Interestingly, a small right shift of the diffraction peaks is observed in the GC compared to the SrF 2 powders. This is attributed to a decrease in the interplanar spacing (d) according to the Bragg equation, which implies Scientific RepoRts | 6:29873 | DOI: 10.1038/srep29873 ion doping in SrF 2 crystals. Notably, the diffraction peaks of GCs are sharper than the peaks for the SrF 2 powders, indicating that growth of crystalline SrF 2 occurred during the preparation process.
SEM images showing the morphology of the SrF 2 powders and the fractured surface of the Er 3+ -doped GC containing SrF 2 are shown in Fig. 2(a,b), respectively. As shown in Fig. 2(a), the average size of the SrF 2 powders is ~300 nm. In Fig. 2(b), we can see that the fractured surface of the GC is rough, and large amounts of isolated particles are embedded in the glass matrix. No pores are observed in the magnified SEM image (Fig. 2(b), inset, lower left), which indicates that PG has the ability to remove pores. Pores can be easily removed by dissolution in the liquid glass phase and outward diffusion to the environment because of the low viscosity of FP glass. Based on the XRD pattern shown in Fig. 1, these isolated particles are considered as SrF 2 crystals. To confirm this conclusion, elemental mapping was used to discern these isolated particles, as presented in Fig. 2(b). The analysis shows that Sr, F, Ca and Er signals are detected in the isolated particle of GC, whereas the presence of Sr and F is stronger than other elements. This result indicates that the isolated particle is SrF 2 and that Er 3+ ions together with Ca 2+ ions are incorporated into the SrF 2 crystal lattice. The extra Ca is attributed to ion exchange during liquid-phase sintering. This observation supports the abovementioned doping assumption. The substitution of the larger Sr 2+ (1.12 Å radius) with the smaller Ca 2+ (1.00 Å radius) and Er 3+ (1.00 Å radius) during the preparation process shrinks the crystal lattice. As a result, the interplanar spacing of SrF 2 decreases, as shown in Fig. 1.  Some information on the quality of the isolated particles, such as size and distribution, can be obtained by direct imaging using the transmission microscope. Fig. 2 (d) displays the transmission microscope micrograph of the interior of the GC. The nearly spherical particles observed in the transmission micrograph correspond to SrF 2 , as identified by the XRD and EDS results. It can be seen that many SrF 2 particles that are ~15 μ m in size are randomly distributed in the glass matrix. For comparison, the transmission microscope micrograph of PG is shown in Fig. 2(c). There is no SrF 2 phase and only the glass phase is observed. Note that the size of the SrF 2 particles inside the GC is a few tens of micrometers, while that of the SrF 2 powders is hundreds of nanometers; this result is consistent with that of XRD pattern analysis which shows the growth of the SrF 2 crystals during liquid-phase sintering.
The transmittance is a very important factor in the assessment of the optical performance of transparent GCs. Based on the Rayleigh-Ganz particle scattering theory 24 , the grain size in a large number of GCs is limited to less than the corresponding wavelength to maintain high transparency. Generally, GCs are translucent or even opaque when the size of the grains reaches the micron level because of strong light scattering 9 , which indicates a poor potential for optical applications. Interestingly, a transparent Er 3+ -doped GC containing micron-sized SrF 2 crystals was obtained by the abovementioned liquid-phase sintering route, as shown in the inset of Fig. 3, which contained crystals with diameters of approximately 15 μ m. It can be seen that the average transmittance of the GC is about 75% in the visible region and more than 85% in the near-IR region. The transmittance is ~83% around 2.7 μ m. The high transparency of the samples is attributed to the matching of the refractive index between the SrF 2 crystals and PG. In order to examine this hypothesis, the refractive index of the SrF 2 single crystal and PG was measured as 1.442 and 1.447, respectively. This result agrees with the conclusion from the in-line transmittance spectra analysis. The stretching vibration of the free [OH − ] groups shows a strong absorption band at 2.5-3.0 μ m, which affects the 2.7 μ m emissions of Er 3+ ions 25,26 . A small [OH -] group absorption peak (~2.85 μ m) is observed in GCs, indicating that the samples possess a strong ability for water removal. Owing to the similar ionic sizes of F − and OHions and the isoelectronic properties, F − ions can efficiently replace OH − ions during liquid-phase sintering 27 .
Wei 6 and Fan 9 have pointed out that the efficient 2.7 μ m emissions of Er 3+ -doped GCs containing fluoride crystals is attributed to the Er 3+ ions entering the fluoride crystal lattices. Thus, Er 3+ ions in crystals embedded  into the glass matrix play a crucial role in mid-infrared emissions. In order to confirm that the Er 3+ ions are incorporated into the SrF 2 lattices during the GC preparation process, the fluorescence spectra at 1.53 μ m was investigated, as presented in Fig. 4. Under 980 nm laser excitation, both PG and GC show 1.53 μ m emission bands attributed to the transition of Er 3+ : 4 F 13/2 → 4 I 15/2 28 . Notably, the GC exhibits stronger 1.53 μ m emissions than the PG, and the integrated intensity of the GC is about 2.15 times higher than that of the PG. Decay lifetimes of the Er 3+ : 4 S 3/2 , 2 H 11/2, 4 F 9/2 , and 4 I 13/2 level in the prepared samples excited at 980 nm were also recorded, as shown in Table.1. Obviously, the decay lifetimes originated from the Er 3+ : 4 S 3/2 , 2 H 11/2, 4 F 9/2 , and 4 I 13/2 level in the GC are much longer than that of PG. The changes observed in both the photoluminescence (PL) spectra and PL lifetimes indicate that the doped Er 3+ ions are embedded in the low-phonon-energy SrF 2 crystals during liquid-phase sintering.
The mid-infrared emissions of the Er 3+ -doped PG and Er 3+ -doped GC containing SrF 2 crystals were investigated, as presented in Fig. 5. An intense fluorescence around 2.7 μ m is observed in GC and no mid-infrared emissions are generated in PG, indicating that the mid-infrared emissions result from the Er 3+ ions in the low-phonon-energy SrF 2 distributed in the glass matrix rather than the Er 3+ ions in the glass matrix. Meanwhile, the 4 I 11/2 → 4 I 13/2 transition shows an evident Stark splitting in the GC, implying that Er 3+ ions are incorporated into the SrF 2 lattices and the 4 I 11/2 → 4 I 13/2 transition is influenced by the crystal field of SrF 2 29,30 . Mid-infrared fluorescence quenching of Er 3+ in PG results from strong nonradiative decay due to multiphonon relaxation 9 , which is attributed to a large amount of high-phonon-energy phosphate in PG. To estimate the 2.7 μ m emission properties, the emission cross section of GC is calculated as follows 31 : where A rad is the radiative transition probability at 2.7 μ m and ∫ λ λ λ I d ( ) is the normalized line-shape function of the emission spectrum. The inset in Fig. 5 shows the emission cross section of the GC. We can see that the maximum emission cross section is more than 1.2 × 10 −20 cm 2 , which is obviously larger than that of the Er 3+ -doped oxyfluoride tellurite glass (4.5 × 10 −21 cm 2 ) 32 , ZBLAN glass (5.7 × 10 −21 cm 2 ) 23 , FP glass (6.57 × 10 −21 cm 2 ) 23 , germanate glass (8.44 × 10 −21 cm 2 ) 27 , bismuthate glass (9.53 × 10 −21 cm 2 ) 11 , and GCs containing NaYF 4 (7.05 ± 0.25 × 10 −21 cm 2 ) 6 . A large emission cross section indicates that the GC would be a promising candidate for IR lasers and amplifiers.
To illustrate the 2.7 μ m emission behavior, the energy level diagram and energy transfer mechanism of Er 3+ ions, as shown in Fig. 6, have been proposed. Electrons at the ground state absorb energy and jump into the 4 I 11/2 level via ground state absorption (GSA) upon excitation at 980 nm. Electrons in the excited state 4 I 11/2 will relax in two ways. Electrons cycle back to 4 I 13/2 by radiative or nonradiative relaxation. Radiative transitions of the electrons generate the 2.7 μ m emissions, while the nonradiative transitions cause fluorescence quenching. Meanwhile, electrons in the 4 I 13/2 level can further relax to the ground state, emitting at 1.53 μ m. Furthermore, the excited state absorption (ESA2: 4 I 13/2 + a phonon → 4 F 9/2 ) and the energy transfer up-conversion (UC) (ETU2: 4 I 13/2 + 4 I 13/2 → 4 I 9/2 + 4 I 15/2 ) process, which are conducive for population reduction of the 4 I 13/2 level and improving the 2.7 μ m  emissions, occur simultaneously in the electrons at the 4 I 13/2 level 6 . On the other hand, electrons in the 4 I 11/2 level are excited to the 4 F 7/2 level by the excited state absorption (ESA1: 4 I 11/2 + a phonon → 4 F 7/2 ) 33 or energy transfer UC (ETU1: 4 I 11/2 + 4 I 11/2 → 4 F 7/2 + 4 I 15/2 ) 34 . Then, the electrons in the 4 F 7/2 level relax nonradiatively to the 4 S 3/2 , 2 H 11/2 , or 4 F 9/2 level and cycle back to the ground state, generating green or red emissions.
Based on the 2.7 μ m emission behaviors, the ESA2 process is beneficial for electron accumulation in the 4 I 13/2 level and the 4 F 9/2 → 4 I 15/2 transition. Meanwhile, the ESA2 process can improve the 2.7 μ m emissions. Therefore, interactions occur between the UC emissions and the 2.7 μ m emissions of the Er 3+ ions. In order to understand the relationship between the mid-infrared emissions and the UC emissions of the Er 3+ ions, the UC emissions spectrum of GC and PG is shown in Fig. 7. Under 980 nm laser excitation, the emission peaks at 521, 540, and 653 nm in Fig. 7 are attributed to the transition of Er 3+ : 2 H 11/2 → 4 I 15/2 35 , 4 S 3/2 → 4 I 15/2 35 , and 4 F 9/2 → 4 I 15/2 36 , respectively. It can be seen that the intensity of the red light emissions is much stronger than that of the green light emissions in PG and GC, which is ascribed to the ESA2 process. It is noted that the GC exhibits a much stronger UC luminescence than the PG, and the integrated intensity of the GC is about 4.5 times higher than that of the PG. Meanwhile, the degree of growth of the red light emissions is larger than the green emissions in the GC. This result indicates that the low-phonon-energy SrF 2 crystals make the ESA2 process, as well as the ESA1 and EUT1 process, stronger. The UC emissions spectrum indicates that the incorporation of Er 3+ into the SrF 2 crystal lattices results in an enhancement of both the UC and mid-infrared emissions.

Conclusion
Er 3+ -doped transparent GCs containing micron-sized SrF 2 crystals were successfully prepared using a liquid-phase sintering route. The SrF 2 crystals were ~15 μ m in size and were uniformly distributed throughout the fluorophosphate glass matrix. The GCs achieved average transmittances of 75% in the visible region and more than 85% in the near-IR region by matching the refractive index of the precursor glass with that of the SrF 2 crystals. The EDS, PL spectra, and PL lifetime results indicate that Er 3+ ions were incorporated into the SrF 2 micron-sized crystals. Intense 2.7 μ m emissions derived from the transition of Er 3+ : 4 I 11/2 → 4 I 13/2 were observed upon excitation at 980 nm using a laser diode because of the incorporation of Er 3+ ions into the low-phonon-energy micron-sized SrF 2 crystals. The maximum emission cross-section of the Er 3+ : 4 I 11/2 → 4 I 13/2 transition is more than 1.2 × 10 −20 cm 2 at 980 nm excitation, implying that there is good potential for fabricating transparent GCs containing micron-sized SrF 2 crystals with good spectroscopic properties for mid-IR lasers.