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Transparent oxyfluoride glass ceramics (GCs) containing LaOF:Er3+ nanocrystals were synthesized. X-ray diffraction (XRD), Raman spectra and transmission electron microscope (TEM) confirmed the formation of LaOF nanocrystals in glass matrix. Energy dispersive spectrometer (EDS) results and Judd–Ofelt (J-O) intensity parameters demonstrated the incorporation of Er3+ into LaOF nanocrystals. Enhanced 2.7 μm emission was achieved upon excitation with a 980 nm laser diode (LD). Upconversion, near-infrared emissions, and energy transfer mechanisms were studied to elucidate 2.7 μm emission behaviors. The enhanced 2.7 μm emission is most likely ascribed to the incorporation of Er3+ ions into the low phonon energy LaOF nanocrystals. The excellent optical properties of this GC material indicate that it is a promising candidate for mid-infrared fiber lasers.
© 2016 Optical Society of America
1. Introduction
Recently, many efforts have been devoted to the development of mid-infrared (~3 μm) laser owing to its potential applications such as optical sensors, remote sensing, spectroscopy and medical diagnosis, and so forth [1–3]. Er3+ is a suitable candidate active ion for achieving above applications due to the transition of 4I11/2→4I13/2 and it can be readily pumped by low-cost, high-efficiency and commercially available 808 or 980 nm LDs [4–6].
On the other hand, appropriate host material is required to obtain efficient mid-infrared emission. It is well known that the 2.7 μm emission is strongly dependent on the phonon vibration of matrix materials, because the quite narrow energy gap between 4I11/2 and 4I13/2 levels makes such radiative transition susceptible to nonradiative decays with larger probability for hosts with higher phonon energy. Therefore, it is necessary to develop host materials with low phonon energy. So far, various materials have been widely investigated such as tellurite, germanate, fluoride glasses, single-crystals and GCs [7–13]. Among these materials mentioned above, GCs are one of the most important optical materials which combine the advantages of both glass (excellent fiber-drawing ability) and nanocrystals (strong crystal field). Simultaneously, oxyfluoride nanocrystal is considered as an ideal host due to its combination of the low phonon energy of fluoride and excellent mechanical and chemical stability of oxide [14,15]. However, to our best knowledge, there are no any reports about the mid-infrared emission of Er3+-doped GCs containing oxyfluoride nanocrystals.
In this paper, Er3+-doped transparent oxyfluoride GCs containing LaOF nanocrystals have been successfully prepared and enhanced 2.7 μm emission was obtained from Er3+-doped GCs. In addition, the influence of heat treatment on mid-infrared emission properties and the possible mechanisms have been discussed in detail.
2. Experimental
Glass sample with nominal molar composition of 63SiO2–15B2O3–16Na2O-5LaF3-1ErF3 was prepared by the conventional melt-quenching method. A mixture of SiO2 (A.R.), B2O3 (A.R.), Na2CO3 (A.R.), LaF3 (5N) and ErF3 (4N) was melted in a covered corundum crucible at 1400 °C for 1 h at ambient temperature. Then melts were cast on a preheated copper plate, and the obtained glasses were cut to the size of 10 mm × 10 mm and heat-treated at 600°C-640°C for 4 h to achieve GCs. For further optical measurements, the samples were optically polished with a thickness of 1.5 mm.
Differential scanning calorimetry (DSC) was carried out in a simultaneous thermal analyzer (STA449C NETZSCH) under N2 atmosphere with heating rate at 10 °C /min in order to determine the crystallization characteristic temperatures. XRD analysis was performed on a X’Pert PRO X-ray diffractometer with Cu Kα (λ = 1.5418 Å) radiation to identify the crystalline phase and estimate the nanocrystal grain size. Raman spectra in the range of 100-1600 cm−1 were obtained by a Raman spectrometer (Renishaw in Via, London, UK) with an excitation of 532 nm laser. High-resolution transmission electron microscope (HRTEM, 2100F, JEOL, Japan) equipped with energy-dispersive spectrometer (EDS) system is performed to analyze the microstructure and the elements distribution of GC samples. The absorption spectra were measured in the range of 500-3200 nm using a Perkin-Elmer Lambda 900/UV/VIS/NIR spectrophotometer. The emission spectra in the range of 450-700 nm, 1400-1700 nm and 2500-2850 nm were recorded by a computer controlled Triax 320 type spectrofluorimeter (Jobin-Yvon Corp.) upon excitation of a 980 nm LD. All the measurements were carried out at room temperature.
3. Results and discussions
The DSC trace of the as-prepared glass sample is presented in Fig. 1, where glass transition temperature (Tg) and onset crystallization temperature (Tx) is around 503°C and 627°C, respectively. Besides, two exothermic peaks corresponding to the crystallization of the glass matrix is around 647°C and 675°C .To obtain transparent oxyfluoride glass ceramics, the glass samples were heat-treated in the range of 600-640°C.
XRD patterns of the as-prepared glass and GCs are shown in Fig. 2. No apparent diffraction peaks are observed for the as-prepared glass, besides two broad humps, which is characteristic diffraction of the amorphous glass matrix. After a heat-treated process above 610°C for 4 h, strong diffraction peaks appear, which are in well agreement with the hexagonal LaOF (JCPDS: 06-0281). With the increase of heat-treated temperature, the diffraction peaks become more evident and sharper, which indicates crystalline size grows gradually. When the heat-treated temperature is elevated to 640°C, another crystalline phase LaF3 occurs.
Fig. 2 XRD patterns of as-prepared glass and resulting GCs obtained by heat treatment at 600°C-640°C for 4 h.
From the peak width of XRD pattern, the crystalline size of LaOF crystals in the GCs can be calculated on the basis of the Scherrer’s equation [16]:
where D is the grain size of nanocrystals, K is a dimensionless shape factor, λ is the wavelength of X-ray, θ is the angle of diffraction peak, β is the full-width at half maximum (FWHM) of the diffraction peak. The mean size of LaOF crystals from GC-610, GC-620, GC-630 and GC-640 are about 19 nm, 24 nm, 35 nm and 52 nm, respectively.Figure 3 illustrates the Raman spectra of undoped as-prepared glass and GCs excited by a 532 nm laser. For the as-prepared glass, the Raman spectra consists of several bands, the peaks located at 477 cm-1, 802 cm-1 and 1059 cm-1 can be assigned to Si-O-Si stretching or bending modes [17], the peaks located at 767 cm-1 and 1421 cm-1 belong to stretching vibrations of BO4 and BO3 units, respectively [18]. However, after heat treatment above 610 °C, the sharp peaks located at 253 cm-1, 296 cm-1 and 392 cm-1 are observed in the spectra, corresponding to the characteristic Raman bands of LaOF crystal [19], which indicates that LaOF nanocrystal has been formed in glass matrix. With the increase of heat-treated temperature, the intensity of the peaks tends to be stronger owing to the increasing crystallinity. When the heat-treated temperature increases up to 640°C, other sharp peaks located at 282 cm-1 and 363 cm-1 assigning to the characteristic Raman bands of LaF3 crystal are found in the spectra [20]. These results match well with XRD results as mentioned before.
Figure 4(a) shows the TEM image of GC heat-treated at 630°C for 4 h. The dark and quasi-spherical particles are homogeneously dispersed in glass matrix and the crystalline size is distributed in the range of 25–42 nm, which is consistent with our calculated result based on the XRD data. The inset in Fig. 4(a) is the corresponding selected area electron diffraction (SAED) image and the diffraction rings are consistent with the calculation of LaOF. The HRTEM image, as shown in Fig. 4(b), shows the resolved lattice fringes with a constant spacing of 0.33 nm, according to the (012) plane of LaOF nanocrystal. To detect the distribution of Er3+ ions in the GCs, the EDS spectra taken from the nanocrystals-rich region and glass matrix are shown in Fig. 4(c). La, O, F, and Er peaks are detected in the nanocrystals-rich region, while very weak La and Er signal are found in the glass matrix. These results indicate that Er3+ ions preferentially enter into the precipitated LaOF nanocrystals after crystallization. The two-dimensional mapping distribution of La, O, F, and Er image, illustrated in Fig. 4(d)-(h), shows that the distribution region of La and Er are almost overlapping, which further confirms that Er3+ ions have entered into the precipitated LaOF nanocrystals. Due to the charge match and similar ion radius of Er3+ and La3 + , Er3+ substitute for La3+ cites in LaOF nanocrystals.
Fig. 4 (a) TEM micrograph and corresponding SAED pattern, (b) HRTEM image, and (c) EDS spectra of GC sample heat-treated at 630oC for 4 h, (d)-(h) STEM–HAADF and the two-dimensional mapping distribution images of La, O, F, Er elements, respectively. The square 1 and square 2 in figure (d) indicate the nanocrystals-rich region and glass matrix, separately.
The absorption spectra of as-prepared glass and GCs in the range from 500 to 3200 nm are shown in Fig. 5. Five absorption bands at 521 nm, 652 nm, 800 nm, 976 nm and 1535 nm are found and attributed to the transitions of Er3 + ions from the ground state 4I15/2 to the excited states 2H11/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2, respectively. With the increase of heat-treated temperature, the baselines of the absorption curves become more precipitous toward to the shorter wavelength region. It can be ascribed to the transmittance loss causing by the Rayleigh scatting effect, which is resulted from the growth of LaOF nanocrystals. The inset of Fig. 5 shows that as-prepared glass and GCs have a good transparency, which begins to devitrification for GC heat treated at 640 oC due to the multiple scattering of nanocrystals with large size.
Fig. 5 Absorption spectra of as-prepared glass and GCs heat-treated at 600°C-640°C for 4 h. Inset shows the images of as-prepared glass and GCs with 1.5 mm thickness.
The J-O intensity parameters Ωλ (λ = 2, 4, 6) can be calculated from the measured absorption spectra and their values are presented in Table 1. According to J-O theory, Ωλ are useful parameters for the investigation of the local structure and bonding in the vicinity of rare earth ions. Generally, Ω2 is sensitive to the environmental configuration symmetry of rare earth ions, and it decreases with the host changing from oxides to fluorides, while Ω4 and Ω6 are dependent on bulk properties. It can be seen from Table 1 that the value of Ω2 decreases gradually with the increase of heat-treated temperature indicating Er3+ ions are successfully incorporated into LaOF nanocrystals. On the other hand, the value of Ω6, which is proportional to the rigidity of the host, increases in the transition from glass to GCs indicating the physical properties of samples are improved gradually [21].
Table 1. J–O intensity parameters of Er3+ in different samples
To investigate the emission characteristics of as-prepared glass and GCs, mid-infrared, upconversion, and near-infrared emission spectra of the samples under a 980 nm LD excitation are displayed in Fig. 6. As shown in Fig. 6(a), the 2.7 μm mid-infrared emission corresponding to Er3+: 4I11/2→4I13/2 transition can hardly be detected in the as-prepared glass. However, with the increase of heat-treated temperature, enhanced 2.7 μm emission can be observed in the GCs, indicating the enhancement of 2.7 μm emission originated mainly from the Er3+ ions in LaOF nanocrystals. It is well known that the intensity of mid-infrared emission is very sensitive to the multiphonon relaxation rate of RE ions which strongly depends on the phonon energy of their matrix. Usually, the lower the phonon energy of host is, the smaller the multiphonon relaxation probability is. The maximum phonon energy in silicate oxide and borate oxide glass is about 1100cm−1 and 1400cm−1, respectively [22,23], while, the maximum phonon energy of LaOF is about 400cm−1 [24], which can be confirmed by the Raman spectra in Fig. 3. The maximum phonon energy of LaOF is much lower than that in the as-prepared glass. Therefore, the 2.7 μm emission of Er3+ in the LaOF nanocrystals will be stronger than that in the as-prepared glass. On the other hand, the increasing heat-treated temperature induces the better crystallinity of host matrix, there is an increasing possibility for Er3+ ions to be incorporated into the LaOF nanocrystals. Thus the 2.7 μm emission intensity of the GCs increase significantly with increasing crystallization temperature.
Fig. 6 (a) 2.7 μm, (b) upconversion, and (c) 1.53 μm emission spectra of as-prepared glass and GCs heat-treated at 600°C-640°C for 4 h pumped by a 980 nm LD.
In order to further understand the emission behavior of Er3+ ions, upconversion and near- infrared emission spectra have been measured, which is illustrated in Figs. 6(b) and (c). It is clear from Fig. 6(b) that the upconversion emission can hardly be detected in the as-prepared glass, while three intense emission bands centered at 526 nm, 540 nm and 659 nm corresponding to Er3+: 2H11/2→4I15/2, 4S3/2→4I15/2 and 4F9/2→4I15/2 transitions are observed in the GCs, which further indicates the incorporation of Er3+ ions into the LaOF nanocrystals. Figure 6(c) shows the near-infrared emission spectra of Er3+: 4I13/2→4I15/2 transition. Enhanced 1.53 μm emission are observed both in as-prepared glass and GCs. The upconversion and near-infrared emission intensity increase gradually with the increase of heat-treated temperature and exhibits similar trend with mid-infrared emission, which can be explained by the same reasons mentioned above.
The absorption (σabs) and emission (σem) cross sections in GC-640 sample at 2.7 μm are depicted in Fig. 7(a). The detailed calculation processes can refer to Ref [25]. It can be observed that the peak absorption and emission cross sections of the GC sample are 0.69 × 10-20 cm2 and 0.72 × 10-20 cm2, respectively. Higher emission cross section is extremely useful for better laser actions. It is found that the calculated emission cross section for the present sample is higher than those of fluorophosphate (0.65 ×10-20 cm2) [26], ZBLAN (0.54 × 10−20 cm2) [27] glass and comparable with that of fluoroaluminate-tellurite (0.75 ×10-20 cm2) [28] glass. On the basis of σabs and σem, gain cross section (G) was calculated to evaluate the mid-infrared gain properties. The gain cross section can be defined as [29]
where P is the population inversion given by the ratio between the population of Er3+: 4I11/2 level and the total Er3+ concentration. Figure 7(b) displays the gain cross sections at 2.7 μm for a set of P values starting from 0 to 1 with an increasing step of 0.1. A positive gain appears when P is more than 0.5, which implies that a low pumping threshold can be achieved for the Er3+: 4I11/2→4I13/2 laser operation.Fig. 7 (a) Absorption and stimulated emission cross sections, and (b) gain cross sections at 2.7 μm in GC sample heat-treated at 640°C for 4 h.
According to discussions mentioned above, a reasonable energy level diagram and energy transfer sketch is proposed, which is displayed in Fig. 8. Firstly, electrons in 4I15/2 level are excited to 4I11/2 level by ground state absorption (GSA) under 980 nm LD pumping. On one hand, electrons in 4I11/2 level can decay radiatively or nonradiatively to the next 4I13/2 level and the radiative process generates the 2.7 μm emission. Hereafter, the electrons in 4I13/2 level relax radiatively to the ground state and 1.53 μm emission happens. Moreover, energy transfer upconversion (ETU2: 4I13/2 + 4I13/2→4I9/2 + 4I15/2) benefits to achieve population inversion between 4I11/2 and 4I13/2 level and thus improve 2.7 μm emission [30]. On the other hand, the excited electrons in 4I11/2 level can undergo excited state absorption (ESA1) or energy transfer upconversion process (ETU1: 4I11/2 + 4I11/2→4I15/2 + 4F7/2) making the electrons in 4F7/2 level populated [31]. Due to small energy gap between 4F7/2 and 2H11/2, 4S3/2 level, the electrons of 4F7/2 level can decay rapidly to the 2H11/2 and 4S3/2 level by multiphonon relaxation process. Thus, green light emissions corresponding to 2H11/2→4I15/2, 4S3/2→4I15/2 transitions occur. Then, electrons in 4S3/2 level relax quickly to 4F9/2 level and 4F9/2→4I15/2 transition gives rise to red light emission. Also, excited state absorption (ESA2) and cross relaxation (CR: 4I11/2 + 4I13/2→4F9/2 + 4I15/2) make a contribution to the red light emission. As for the as-prepared glass, the nonradiative relaxation in 4I11/2 level is so remarkable owing to the high phonon energy that upconversion and 2.7 μm emissions are too weak to be detected. While for the 1.53 μm emission (4I13/2→4I15/2), due to the relatively larger energy gap than 2.7 μm emission (4I11/2→4I13/2), the influence of matrix phonon energy on 1.53 μm emission is relatively small, obvious 1.53 μm emission is still detected in as-prepared glass. After heat treatment, Er3+ ions are embedded into the low phonon energy LaOF nanocrystals and consequently the nonradiative relaxation at 4I11/2 level is suppressed, which enhances the upconversion and 2.7 μm emission. In addition, due to the Er3+ ions preferentially enter into LaOF nanocrystals, the Er3+ ions concentration in LaOF nanocrystals is much higher than that in as-prepared glass, which will remarkably shorten the distance between adjacent Er3+ ions. Therefore, the energy transfer (ETU2) efficiency of the adjacent Er3+ ions increases due to the short distance between them, which also has an advantage on 2.7 μm emission.
4. Conclusion
In summary, transparent oxyfluoride GCs containing LaOF:Er3+ nanocrystals were successfully prepared by conventional melt-quenching and subsequent thermal treatment. The XRD, Raman spectra and TEM results confirmed that LaOF nanocrystals were precipitated in the glass matrix with a significant number of Er3+ ions incorporated after a proper heat-treatment. Enhanced 2.7 μm emission of Er3+: 4I11/2→4I13/2 transition was observed in the transparent GCs compared to that of as-prepared glass due to the incorporation of Er3+ ions into the low phonon energy LaOF nanocrystals. Upconversion, near-infrared emissions, and energy transfer mechanism were investigated to understand 2.7 μm emission behaviors. Furthermore, absorption, emission and gain cross sections at 2.7 μm were also analyzed, the maximum value of obtained absorption and emission cross sections are 0.69 × 10-20 cm2and 0.72 ×10-20 cm2, respectively. The desirable spectroscopic characteristics endow the potential applications of the GC samples in mid-infrared amplifiers and tunable lasers.
Acknowledgments
This work has been supported by the National Natural Science Foundation of China (61475047, 51302086); Guangdong Natural Science Foundation for Distinguished Young Scholars (2014A030306045); Pearl River S&T Nova Program of Guangzhou (2014J2200083); West Light Foundation from Chinese Academy of Science (CAS) of China, and the Fundamental Research Funds for the Central Universities (2015PT021).
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