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Intense 2.7 μm emission of Er3+ was achieved in transparent glass ceramics containing orthorhombic Vernier phase lutetium oxyfluoride nanocrystals, which was prepared through conventional melt-quenching and a subsequent thermal treatment method. X-ray diffraction and transmission electron microscopy analysis confirmed the precipitation of orthorhombic Vernier phase lutetium oxyfluoride nanocrystals in glass ceramics. The preferential incorporation of Er3+ ions into nanocrystals was evidenced by TEM element mapping, stark-split absorption band, and prolongation of luminescence lifetimes. Upon 980 nm laser excitation, enhanced 2.7 μm emission originating from Er3+:4I11/2 → 4I13∕2 transition was obtained in glass ceramics via increasing the heat treatment temperature, which resulted from the inhibited non-radiative relaxation from 4I11/2 level and favored energy transfer upconversion process between the localized Er3+ ions. Our results indicate that lutetium oxyfluoride nanocrystal embedded glass ceramic is a promising candidate for 2.7 μm laser.
© 2016 Optical Society of America
1. Introduction
Recently, mid-infrared lasers operating at around 3 μm from rare earth (RE) ions have gained much attention due to their potential applications in laser surgery, pump sources, remote sensing and atmosphere pollution monitoring, etc [1–3]. Among them, Er3+ ion is especially attractive owing to the corresponding 4I11/2 → 4I13/2 (2.7 μm) radiative transition, which can be readily triggered by the commercially available high power 980 nm laser diode (LD) [4,5]. However, the narrow energy gap (~3600 cm−1) between 4I11/2 and 4I13∕2 levels of Er3+ makes 2.7 μm luminescence more susceptible to the phonon vibration of host matrix [6]. So far, various glass hosts such as fluoride and heavy metal oxide systems have been extensively investigated to achieve efficient 2.7 μm emission [7–9]. Although the low phonon energy host like fluoride glass promotes the reduction of nonradiative relaxation rates, its poor chemical and mechanical stability remains a challenge for practical application [4,10].
On the other hand, transparent oxyfluoride glass ceramics (GCs) represent a promising host matrix for RE ions. They are characterized with good luminescent property due to the precipitated fluoride nanocrystals with low phonon energy in which rare earth ions prefer to reside, and high chemical and mechanical stabilities ascribing to the oxide glass matrix [11–13]. Up to now, some investigations on the 2.7 μm emission of Er3+ have been reported in oxyfluoride glass ceramics [14–18]. Efficient 2.7 μm emissions were achieved in these systems which indicate the potential application of oxyfluoride glass ceramics in mid-infrared lasers. However, majority of these results were realized in glass systems containing toxic PbF2 nanocrystals. Therefore, further investigation on lead free systems with favorable mid-infrared luminescent property is needed to take the advantage of the unique properties of glass ceramics.
More recently, lanthanide doped Lu-based hosts have aroused increasing attention due to their excellent properties. For example, upconversion emission in NaLuF4 nanocrystals was found more efficient than in NaYF4 nanocrystals [19], and Lu-based crystals such as LuVO4 and LuLiF4 presented excellent luminescence and laser performance [20,21]. Oxyfluoride glass ceramics containing Lu-based fluoride nanocrystals also exhibit favorable luminescent properties [22–24]. All of the above results indicate that intense 2.7 μm emission could be expected in Lu-based oxyfluoride glass ceramics. Moreover, the well match of ionic radius and valence between Lu3+ (0.861 Å) and Er3+ (0.89 Å) could guarantee the efficient incorporation of Er3+ into fluoride nanocrystals [25]. In present work, a novel transparent glass ceramics containing lutetium oxyfluoride nanocrystals were fabricated and then characterized by X-ray diffraction and transmission electron microscopy analysis. The effect of thermal treatment temperature on the spectroscopic properties was studied. The character of 2.7 μm as well as upconversion and near-infrared emissions was investigated upon excitation with 980 nm LD and the mechanisms were discussed.
2. Experimental
The precursor glass (PG) sample was prepared with nominal molar composition of 40SiO2–20Al2O3–15Na2O–15NaF–10LuF3 by melt-quenching method in air atmosphere. Er3+ doping of 1 mol% was introduced by addition of ErF3 compound. Analytical reagent-grade SiO2, Al2O3, Na2CO3, NaF, and 4N purity of LuF3 and ErF3 were used as raw materials. Accurately weighed batch of 20g starting materials was well mixed in agate mortar and then melted in a covered corundum crucible at 1450°C for 1 h. Subsequently, the melt was quenched on a stainless steel plate to form transparent glass, and then the obtained glass was annealed at 400°C for 2h to release internal stresses. To form transparent glass ceramics, the PG sample was cut into pieces and then subjected to thermal treatment at 540°C, 550°C, 560°C, 570°C, and 580°C for 3h, respectively. The obtained glass ceramic samples were labeled as GC540, GC550, GC560, GC570, and GC580 correspondingly. Afterwards, GC and PG samples were polished for optical measurements.
To identify the crystalline phase, X-ray diffraction (XRD) measurements were performed on a powder diffractometer (X’Pert PROX, Cu-Kα) operated at 40 kV and 40 mA with the scan range of 10–90°. Transmission electron microscopy (TEM) observation of GC was carried out on a JEM-2010 instrument equipped with energy dispersive spectrometer system (JEOL, Japan) to reveal the microstructures. TEM specimens were prepared by directly drying a drop of a dilute ethanol dispersion solution of glass pieces on the surface of a holey copper grid. The absorption spectra were obtained on a Perkin–Elmer Lambda 900 UV/VIS/NIR spectrophotometer in the spectral range of 300–1650 nm with a resolution of 1 nm. Using a 980 nm laser diode (LD) as excitation source, the photoluminescence spectra were recorded on an iHR320 spectrometer (Jobin-Yvon Corp., Horiba Scientific). A digital oscilloscope (TDS3012C, Tektronix) equipped on the iHR 320 spectrometer was employed to collect the decay curves upon excitation with 808 nm or 980 nm pulsed LD. All the measurements were performed at room temperature.
3. Results and discussion
XRD patterns of PG and GCs are shown in Fig. 1. The precursor glass is completely amorphous with only two broad humps in the curve. While after thermal treatment, several intense diffraction peaks emerge in the curves, indicating the formation of crystalline phase in glass. Interestingly, the diffraction peaks cannot be indexed to any standard JCPDS data card of rare earth fluorides, but it is noticeable that these peaks are very similar to the diffraction patterns of orthorhombic Y5O4F7 (JCPDS No. 80-1124) phase, except the right shift of the experimental peaks. Considering the difference of ionic radius between Lu3+ (0.861 Å) and Y3+ (0.900 Å) [26], it is reasonable to suppose that the crystalline phase in glass ceramics is Vernier phase LunOn-1Fn + 2 (n = 5-10) (denoted as LOF). Actually, the diffraction peaks are consistent with previous reports of this phase in polycrystals, but we cannot ascribe the as prepared phase to any specific composition with an exact n value due to the little differences between the phases with different n values and the interference of the glass matrix in XRD patterns [27–31]. As the heat treatment temperature increases, the diffraction peaks become distinct and sharp, indicating that LOF nanocrystals grow gradually with enhanced crystalline quality. The photograph of all samples is shown in the inset of Fig. 1, GC samples fabricated through thermal treatment remain visually high transparent except the GC580 sample, which turns translucent due to multiple scattering of large size nanocrystals [32].
TEM measurement was also performed on GC570 sample to investigate the detailed microstructure of glass ceramic, and the images are illustrated in Fig. 2. Figure 2(a) shows the TEM image and corresponding selected area electron diffraction (SAED) pattern. LOF nanoparticles with quasi-spherical shape are distributed densely and homogenously in glass matrix with particle size mainly in the range of 16–28 nm (see Fig. 2(c)). The HRTEM image of an individual particle in Fig. 2(b) clearly displays the resolved lattice fringes of crystalline LOF nanocrystal and the associated interplanar spacing d value is about 0.31 nm, which is similar to the previous reports of orthorhombic LOF (d = 0.31nm) [27,28,30]. In order to understand the distribution of elements in glass, scanning transmission electron microscopy operated in the high-angle annular dark-field mode (STEM–HAADF) and associated two-dimensional element mapping of Lu, F, Er, O, and Na images are presented in Fig. 2(d)–2(i). Lu are clearly separated from the glass matrix and mainly localized in crystal phase, the segregation of F is not clear but some inhomogeneous regions could be found, although indistinct, to be overlapping with Lu element. Besides, the distribution of Er is almost consistent with Lu, indicating the preferred incorporation of Er3+ ions into the LOF nanocrystals. O and Na elements distribute equally among the particles and glass matrix due to their high content. It could be concluded from the Fig. 2(g) that the concentration of Er3+ ions in nanocystals is relatively high.
Fig. 2 (a) TEM image and SAED pattern, (b) HRTEM image of an individual LOF nanocrystal and (c) particle size distribution of nanocrystals in GC570; (d-i) STEM-HAADF image and associated Lu, F, Er, O, and Na two-dimensional element mapping, respectively.
Figure 3 depicts the absorption spectra of PG and GC samples in the wavelength region of 300–1650 nm. Typical absorption bands corresponding to the transitions from the ground state to excited states of Er3+ are labeled. The absorption transitions in GCs are similar to that of PG except for the red shift of ultraviolet absorption edge in GC560, GC570, and GC580 samples caused by Rayleigh scattering [33]. It is notable that compared to that of PG, the 1.53 μm (Er3+:4I15/2 → 4I13/2) absorption band (see the inset of Fig. 3) changes remarkably in GC samples. Stark splits appear which indicates the change of circumstance around Er3+ ions after crystallization [16].
Fig. 3 Absorption spectra of PG and GC samples. The inset shows the zoomed-in figure of the absorption band in the region from 1400 to 1650 nm of all samples.
Mid-infrared emission spectra around 2.7 μm in PG and GCs were recorded upon excitation with 980 nm LD and are shown in Fig. 4(a). It is obvious that all spectra corresponding to GC samples exhibit intense 2.7 μm emission, but the signal is too weak to be detected in PG. As depicted in the simplified energy-level diagram of Er3+ in Fig. 5, 2.7 μm fluorescence could be generated through 4I11/2 → 4I13/2 transition when 4I11/2 level is populated. However, the 4I11/2 level could be easily quenched in PG due to its large maximum phonon energy of silicate glass matrix (~1100 cm−1). As a result, the 2.7 μm emission signal is scarcely obtained. On the other hand, when glass samples are subjected to thermal treatment, Er3+ ions would preferentially incorporate into the precipitated LOF nanocrystals, where the non-radiative relaxation of 4I11/2 → 4I13/2 is restricted, leading to the generation of intense 2.7 μm emission in GCs. Additionally, due to the short Er-Er distance caused by high concentration of Er3+ in nanocrystals, the generation of 2.7 μm emission could be benefited from the process of ETU2:4I13/2 + 4I13/2 → 4I9/2 + 4I15/2 in Fig. 5, which transfers populations from 4I13/2 to the upper 4I9/2 level in GCs [34,35]. With the increase of heat treatment temperature, the luminescent intensity increases monotonously, implying that more Er3+ ions have incorporated into LOF crystalline phase.
Fig. 4 Emission spectra of the PG and GC samples in the (a) 2.7 μm and (b) 1.5 μm region excited by 980 nm LD. The inset of (b) shows the intensity of 1.5 μm as a function of heat treatment temperature.
Fig. 5 Simplified energy-level diagram of Er3+ and possible mid-infrared and upconversion mechanism excited with 980 nm LD.
Near-infrared emission spectra in the range of 1400–1700 nm are also measured upon excitation with 980 nm LD to elucidate the fluorescent behavior of the lower state (4I13/2) of 2.7 μm transition. The result in Fig. 4(b) shows that intense 1.5 μm emissions (Er3+:4I13/2 → 4I15/2) are obtained both in PG and GC samples. Consistent with the absorption spectra in Fig. 3, the split peak at around 1560 nm could be observed, and the shoulder in the range of 1580–1600 nm becomes flat in GCs. Differed from the 2.7 μm emission, the intensity of 1.5 μm emission decreases in GCs compared to that of PG (see the inset in Fig. 4(b)). As shown in Fig. 5, the 4I13/2 level is mainly populated from 4I11/2 level through non-radiative relaxation when pumped by 980 nm LD [36,37]. However, the population of 4I13/2 level would be reduced in GCs due to the enhanced ESA and suppressed non-radiative relaxation in 4I11/2 level, which resulted in decreased 1.5 μm emission intensity. In addition, the ETU2 and ETU3 processes could also reduce the population in 4I13/2 level.
To further investigate the mechanism of infrared emissions, the emission decay curves of Er3+:4I13/2 (monitored at 1530 nm) and 4I11/2 (monitored at 980 nm) levels pumped by 808 nm pulsed LD were also measured and given in Fig. 6. The average lifetimes were determined by fitting with single or second-order exponential functions. The dependence of lifetime values on heat treatment temperature is plotted in Fig. 6(a). The values for both levels increase obviously in GCs comparing with PG. The lifetime of 4I13/2 level [Fig. 6(b)] increases from 6.2 ms in PG to 7.43 ms in GC540, and then it is further prolonged to 9.19 ms in GC570 and quenched to 8.72 ms in GC580. For 4I11/2 level [Fig. 6(c)], the lifetime increases from 0.3 ms in PG to 1.23 ms in GC580. The results clearly evidence that phonon energy of the environment around Er3+ ions has decreased in GCs, and Er3+ ions have increasingly incorporated in nanocrystals along with the elevation of heat treatment temperature. The prolonged lifetime of Er3+:4I11/2 level after thermal treatment is mainly responsible for the observation of 2.7 μm emission in GC samples. On the other hand, 1.5 μm emission could also benefit from the increased lifetime of 4I13/2 level, but the effects of reduced population from 4I11/2 level seems more efficient.
Fig. 6 (a) Dependence of lifetime values on heat treatment temperature for Er3+:4I13/2 and 4I11/2 levels. Decay curves corresponding to (b) 4I13/2 and (c) 4I11/2 levels in PG and GC samples excited with 808 nm LD.
In order to further understand the luminescent character of Er3+ ions in present system, the visible upconversion luminescence was also investigated. Figure 7(a) shows the upconversion emission spectra in the range of 450–750 nm pumped with 980 nm LD. Two intense emission bands centered at around 546 nm and 669 nm with obvious stark splitting are observed in GC samples, corresponding to the 2H11/2,4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+ respectively, while the emissions are rather weak in PG. The emission intensity increases with the elevation of heat treatment temperature, which can be assigned to the same reason as 2.7 μm emission. It is notable that the red emission is dominant over the green one in all GC samples. It is well known that 2H11/2/4S3/2 level could be populated by multi-phonon relaxation from 4F7/2 level, which is promoted through steps of ground state absorption (GSA:4I15/2 + one photon → 4I11/2) and then excited state absorption (ESA:4I11/2 + one photon → 4F7/2) and energy transfer upconversion (ETU1:4I11/2 + 4I11/2 → 4F7/2 + 4I15/2) (as shown in Fig. 5). However, the population of 4F9/2 level through relaxation from 4S3/2 level is inefficient in GCs due to the large energy gap between 4S3/2 and 4F9/2 levels [13]. Therefore, considering that Er3+ ions are concentrated in LOF nanocrystals (see in Fig. 2(g)), it is reasonable to believe that interactions such as cross relaxation (CR) of CR:4F7/2 + 4I11/2 → 4F9/2 + 4F9/2 and energy transfer upconversion of ETU3:4I11/2 + 4I13/2 → 4F9/2 + 4I15/2 could be mainly responsible for the efficient population on 4F9/2 level [38, 39]. The observation of dominant red upconversion emission suggests that the Er3+-Er3+ distance is short and these interactions are efficient in GCs.
Fig. 7 (a) Upconversion emission spectra in the range of 450-750 nm, and decay curves corresponding to the (b) Er3+:4S3/2 and (c) 4F9/2 states of PG and GC samples excited by 980 nm LD.
Additionally, upconversion decay curves of the Er3+:4S3/2 (monitored at 546 nm) and 4F9/2 levels (monitored at 669 nm) pumped by 980 nm pulsed LD were recorded, as shown in Fig. 7(b) and 7(c). The decay curves show second-order exponential decay model and the average lifetimes were calculated. Apparently, the decay lifetimes of both levels in GC are much longer than those in PG. The values increase from 0.16 ms and 0.49 ms in PG to 0.54 ms and 1.00 ms in GC540 for 4S3/2 and 4F9/2 levels, respectively. And then, the lifetime further increase to 0.87 ms for 4S3/2 and 1.34 ms for 4F9/2 level in GC570 with the rise of heat treatment temperature before quenching appears in GC580. Prolonged lifetime of corresponding levels guarantees the generation of intense visible upconversion emissions. The results also evidence that the population of 4F9/2 level is mainly from Er-Er interaction processes other than non-radiative relaxation from 4S3/2 level in GCs. Intense upconversion emission and longer lifetimes further confirm the good luminescent property of this kind of glass ceramic.
Finally, to estimate the possibility of the GC as mid-infrared laser material, the absorption (σab) and emission (σem) cross sections around the 2.7 μm in GC560 sample are calculated and illustrated in Fig. 8(a). The detailed calculation processes have been given in Ref [40]. The maximum value of emission cross section is 0.52 × 10−20 cm2, which is comparable to that of LiYF4 crystal (0.53 × 10−20 cm–2) [41] but smaller than that of fluoride glass (0.98 × 10−20 cm–2) [42]. Based on absorption and emission cross section spectra, gain cross section G(λ) can be estimated by the following equation [43]:
where P is the population inversion assigned to the concentration ratio of Er3+ ions in the 4I11/2 and 4I13/2 levels. The gain cross section as a function of wavelength with various P values is shown in Fig. 8(b). Apparently, the positive gain appears when P reaches 0.4, implying that a low pumping threshold could be achieved for the Er3+:4I11/2 → 4I13/2 transition laser operation.Fig. 8 (a) Calculated absorption and emission cross-section spectra, and (b) gain cross sections with various P values corresponding to the Er3+:4I11/2→4I13/2 transition in GC560 sample.
4. Conclusion
In summary, Er3+ doped lutetium oxyfluoride nanocrystals embedded transparent glass ceramics were prepared. The formation of LunOn-1Fn + 2 nanocrystals in glass matrix upon thermal treatment was confirmed by XRD and TEM analysis and the preferential incorporation of Er3+ ions in nanocrystals was directly evidenced by TEM element mapping result. Upon 980 nm LD excitation, efficient 2.7 μm emissions were achieved in GCs due to the prolonged lifetime of 4I11/2 level induced by the decreased phonon energy around Er3+ ions in lutetium oxyfluride nanocrystals. Our results indicate that this kind of glass ceramic might have potential application in the development of 2.7 μm mid-infrared laser.
Acknowledgments
This work is financially supported by NSFC (Grant Nos. 51125005, 51472088 and 51302086), and the Fundamental Research Funds for the Central Universities, SCUT.
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