In this article, we report the detailed spectroscopic studies of high Er3+content (50%) transparent YAG ceramic co-doped with nominal Cr3+ content (0.1 mol %). Various radiative and non-radiative spectroscopic properties such as radiative decay time, fluorescence branching ratio, emission/absorption cross sections, internal radiative quantum yields of the infrared and the upconverted emission bands are explored using standard experimental and theoretical methods and compared with YAG single crystal. Results show that although the non-radiative losses are high for 50% Er doped ceramic; several radiative spectral properties are almost in agreement with those for the single crystal YAG. Furthermore, because of the low dopant concentration of Cr3+, the sensitizing effect of Cr3+ was not observed.
© 2011 OSA
In recent years, highly transparent polycrystalline rare-earth element doped Y3Al5O12 (YAG) ceramics (especially Nd: YAG) has become a significant laser media with the improved fabrication technologies and the proper diode laser excitation system. YAG single crystal has already proved its potential as a laser material especially doped with Nd3+ [1–3]. However, because of the difficulty to make large size single crystalline material using conventional single crystal growth methods, ceramic technology has been adopted with which one can make homogeneous large size YAG at comparatively lower cost. It has been proved that YAG ceramics materials exhibit excellent laser performance similar to YAG single crystals [3–10]. Among the rare-earth doped YAG-materials, high content Er3+:Y3Al5O12 (Er: YAG [10–15] single crystal and ceramics are important laser media to obtain laser emission at 3 μm, which is widely utilized by the medical community. Since this emission at 3 μm fortuitously overlaps with one of the few transition bands in the water-vapor spectrum, it is useful for meteorological applications. Laser experiment on 3 μm emission on 50% Er: YAG crystal has been reported by different groups [10–12].
Recently Zhou et al. reported the synthesis and upconversion spectral studies of Er doped YAG ceramic where the grain size was estimated to be 30 μm . Upconversion spectral studies were also reported in Er and Er: Yb: YAG ceramics by other groups [14–17]. Sardar et.al. reported the optical studies and comparison of various spectroscopic parameters of high Er content (50%) YAG ceramic with low Er content single crystal YAG .
In this paper, we report the synthesis and spectroscopic characteristics of high Er3+ content (50%) transparent YAG ceramic co-doped with nominal 0.1% Cr3+.The purpose of this study is to explore the near infrared emission mechanisms in Er3+ by pumping directly the Er3+ or by exciting the Cr3+ to its absorption band at 407 nm and utilize the purposed material as laser active medium. Introduction of Cr3+ was found to have influence on the emission characteristics of several rare earths such as Nd3+ [19–22] and Er3+  by suitable energy transfer mechanism which is determined by the concentrations of Cr and the rare earths [19–23].
To the best of our knowledge this is the first report on the detail spectroscopic investigation of a high Er content (50%) YAG co doped with nominal Cr3+ (0.1 mol%).
2.1 Synthesis and characterization
Transparent ceramics of Er: Cr: YAG (Er = 50% and Cr = 0.1% mol %) have been prepared by nanocrystalline assisted advanced ceramic laser technology , and then sliced and polished for optical measurements. The microstructure of the sample was characterized by scanning transmission electron microscope (FE-STEM) FEI Quanta 200 3D operated at 25 kV in low vacuum mode. The crystal structure of the sample has been analyzed using X-ray powder diffraction (XRD) at 40 KV and 30 mA in the parallel beam configuration using RIGAKU Ultima IV X-ray diffractometer with Cu Kα (λ = 1.5 A).
The absorption spectra were measured in the 300–1700 nm range using a UV-Vis-NIR spectrophotometer (Cary, Model 14R) in the transmission mode. The upconversion and near-infrared (NIR) emission spectra are recorded under the excitation at 967 nm band of Ti-Sapphire laser (Spectra Physics, Model 3900S) pumped by an Nd: YAG laser (Spectra Physics, Model Millennia). The NIR emission was also collected under the 407 nm excitation from a laser diode (Nichia Ltd). The emission from the sample was collected with a 1.25 m single grating scanning monochromator (SPEX, Model 1250M) and detected by a liquid nitrogen cooled InGaAs detector (DSS-1GA020L, Electrooptic System Inc) for the NIR and photo multiplier tube at −950V bias (Model 1911, Horiba) for the visible. The fluorescence spectrum for NIR transitions was scanned using a 600 grooves/mm diffraction grating blazed at 1.5 µm and 1200 grooves/mm diffraction grating blazed at 0.5 µm for the visible region, with a spectral resolution of 0.01 nm. The detector signal was processed in a computer coupled to the data acquisition system through a lock in amplifier (Stanford Research System, Model SR510,). The entire system was controlled though the data acquisition software Synerjy (Origin Lab), and HORIBA Jobin-Yvon). The decay profiles corresponding to 1571 nm, 557 nm and 680 nm were recorded by pulsing the excitation laser with a mechanical chopper (Terahertz Technology, Model C-995) and connecting the detector directly to a 155 MHz oscilloscope (Tektronix, Model 2445A). In all emission and decay experiments, the sample was excited near the surface to avoid the re-absorption losses. All the emission and decay time measurements were performed at room temperature.
3. Results and discussion
3.1. Phase and morphology
Figure 1 shows the X-ray powder diffraction patterns of transparent ceramic sintered at 1750 °C and is compared with standard JCPDS data of YAG (01-073-3184). It is clear that the patterns were basically consistent with YAG phase and no additional phases were observed. XRD results reveal that the polycrystalline transparent YAG ceramic sample has a cubic phase structure with unit cell parameters a = b = c = 11.99 Å.
Figure 2 shows the scanning electron microscope (SEM) micrograph of the ceramic with a photograph of mirror-polished Er: Cr: YAG ceramic in the inset. According to the SEM analysis, specimen is very compact and almost contains no pores. Based on SEM analysis, we did not observe any other phases along the grain boundaries and in the inner grains. The average grain size of the Er: YAG ceramics is about 6.9 μm. Such narrow grain boundary ensures the low scattering in the material.
The mirror-polished ceramic sample of thickness 3.81 mm possesses transparency up to nearly 84% at 1571 nm with estimated scattering losses of 0.011 cm−1. The reflectivity was calculated to be 0.097 (i.e.9.7%) at 1571 nm under the assumption of normal incidence.
3.2 Radiative properties
The room temperature UV-VIS-NIR absorption spectrum of the sample is shown in Fig. 3 with their spectral band assignments. The Fresnel surface reflection losses have been subtracted in all absorption measurements. It was found that all absorption bands of Er3+ in this YAG host is similar to those of previously reported garnet hosts [24–27]. The spectrum is well resolved so that almost every Stark components corresponding to different manifold of Er3+ are observed.
Since the concentration of Cr3+ is too low and its absorption bands are overlapping with Er3+, it is difficult to resolve the Cr3+ absorption bands corresponding to 4T1 at 456 nm, where the other band of Cr3+ corresponding to 4T2 at 600 nm is absent.
The absence of Cr3+absorption and emission band at 600 nm band clearly indicates that at low dopant concentration, Cr3+ absorption bands are either absent or totally overwhelmed by the highly doped Er3+ comparing with nominal presence of Cr (0.1 mol%).
The indices of refraction of the ceramic were found from . A least-squares fitting to the Sellmeier’s dispersion equationEq. (1) to find the refractive indices of the YAG at all wavelengths of interest.
The absorption bands of Er3+ ion correspond to transitions from the 4I15/2 ground state to the various excited levels. The data from these absorption spectra can be used to calculate the radiative transition probabilities, fluorescence branching ratios and radiative lifetimes of different transitions.
The measured absorption line strength (Smeas) for the electric dipole transition of each band was determined experimentally from the area under the absorption band and can be expressed in terms of absorption coefficient α (λ) by the following equation :29] and Ofelt  as31]. The three Judd–Ofelt parameters were obtained by fitting the measured dipole strength (Eq. (1)) to the theoretical dipole strength (Eq. (2)) using the least square fitting procedure. The experimental and calculated electric dipole line strengths of the different absorption bands observed are collected in Table 1 along with the RMS deviation of the fit. The calculated values of the phenomenological Judd-Ofelt parameters obtained for the Er3+ in the present system are Ω2 = 0.42 × 10−20 cm2, Ω4 = 0.70 × 10−20 cm2, Ω6 = 0.47 × 10−20 cm2 with the RMS error of 0.047 × 10−20 cm2. These values are in agreement with the values reported earlier for high Er doped single crystals  as shown in Table 2 .Eq. (2) and presents a host dependence through the Ωt parameters. Smd can be calculated with the expression [29,30]:
The radiative lifetime () for an excited state (J) is calculated byEq. (6) is taken over all terminal states J’.
The fluorescence branching ratios β (J→J’) are determined from the radiative decay rates by using the following expression:Table 3 , along with the estimated fluorescence branching ratios (β) and radiative decay time (τrad).
The room temperature emission cross section (σe) of the intermanifold transitions can be obtained using the following expression :
Using this procedure emission cross-section obtained for various observed emission bands are 2.2 × 10−20 (557 nm), 2.5 × 10−20 (680 nm) and 5.2 × 10−20 cm2 (1571 nm). Calculated JO Parameters (Ωt, t = 2, 4, 6), Fluorescence lifetime (τf), Internal quantum yield (η) are reported in Table 2. It should be noted that these values are almost in agreement with the values reported for high Er content single crystals [11,26] as shown in Table 2.
3.3. Infrared and upconversion luminescence spectra
Figure 4 and Fig. 5 shows the near-infrared emission (NIR) of the transparent ceramic composition under the 407 and 967 nm excitation, where NIR emission was observed centered at 1571 nm with a spectral bandwidth of 300 nm. The spectra reported in Fig. 4 are in different scale to show the difference in fluorescence intensity under 967 and 407 nm excitation. Spectrum in Fig. 4 also shows that fluorescence intensity under 967 nm excitation is more than the 407 nm excitation under the identical measurement condition with the same power (60 mW), which is due to the larger excited state non-radiative losses at 407 nm excitation. As a whole fluorescence spectrum obtained under 407 nm and 967 nm excitation are shown in Fig. 5 and Fig. 6 respectively. The intense upconversion emission bands as shown in Fig. 6 were observed at 545 nm (green), 557 nm (green), 656 nm (red), 674 nm (red), and 680 nm (red) under 967 nm excitation; which are assigned to the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, 4F9/2 → 4I15/2 transitions and their Stark components to ground state respectively. Green emission centered at 557 nm has been observed under the 407 nm excitation, where the red emission is absent in the spectrum as shown in Fig. 5. The fluorescence branching ratios of the upconversion bands are 15% (545 nm), 37% (557 nm) and 48% (680 nm) respectively.
The mechanisms for the IR and upconversion visible emission bands under different excitation wavelength are explained with the help of the energy level diagram shown in Fig. 7 . The process of both visible emissions can be explained on the basis of various mechanisms such as two photon absorption (TPA), excited state absorption (ESA), and energy transfer (ET). TPA is a nonlinear process where the visible photons are created by the simultaneous absorption of two IR photons and mediated through a real or virtual intermediate level. TPA is relevant when excitation light sources used have high power that is sufficiently enough to create virtual intermediate levels in materials with high TPA cross section. In the case of trivalent Er3+, TPA can occur through the real state 4I11/2 because of the presence of such matching energy level. When the 4I11/2 level is excited by 967 nm directly through Er3+, part of the excitation energy at the 4I11/2 level relaxes non-radiatively to the 4I13/2 level. The radiative transition via 4I13/2 →4I15/2 leads to a NIR intense emission centered at 1571 nm emission.
At the same time, 4F7/2 is populated through 4I11/2 by TPA process. Since the lifetime of 4I11/2 level of Er3+ in ceramic is high as reported in Table 3, which is enough and efficient for TPA process. The populated 4F7/2 then relaxes non-radiatively to the next lower 2H11/2 giving rise to the green emission at 545 nm through the 2H11/2→4I15/2 transition. Part of the population accumulated in the 4S3/2 level through non-radiative decay from 2H11/2 relaxes to the 4I15/2 through the radiative decay giving the emission band at 557 nm.
In high Er content sample the green emission is also influenced due to the energy transfer (ET) as illustrated in the energy level diagram. Two erbium ions interacting as follows under 967 nm excitation: 4I11/2 (Er3+) + 4I11/2 (Er3+) → 4F7/2 (Er3+) + 4I15/2 (Er3+) contribute greatly to populating the 4F7/2 level. The red emission is mainly due to the cross relaxation and energy transfer process according to the equation 4I11/2(Er3+) + 4I13/2(Er3+)→4I15/2(Er3+) + 4F9/2(Er3+). Ultimately all these mechanisms lead to the radiative transition from the 2H11/2, 4S3/2 and 4F9/2 to the 4I15/2 level which emits green at 545 nm, 557 nm and red at 680 nm respectively. The absence of red emission under 407 nm excitation as shown in Fig. 5 also confirms that energy transfer and cross relaxation are the required and dominant phenomena for the red emission process.
In order to understand the contribution of the two-photon process we studied the pump power dependence of the green and red emission intensity. In an upconversion mechanism, the emission intensity Iup is proportional to the nth power of the excitation intensity P i.e., Iup ∝ Pn, where n is the number of IR photons absorbed to produce an up-conversion photon. A plot of log Iup vs. log P yields a straight line with slope n and is shown in Fig. 8 for the most intense 557 and 680 nm upconversion emissions. The values of the slopes obtained are 1.7 and 1.9 for the respective intense green and red bands, which confirms the two-photon contribution to the red and green emissions.
3.4 Non radiative processes and internal quantum yield
Figure 9 shows the decay profiles for the 1571 nm emission of ions in the highly Er3+ doped transparent ceramic fitted by single exponential function with a correlation factor of 0.99998 under 967 nm excitation. Figure 10 shows the semi-log plot for the decay profile for 557 and 680 nm of Er3+ ions in the co-doped sample.
A quantitative way of measuring the internal quantum yield (η) for a particular emission band is done through the fluorescence lifetime measurements. Non-radiative processes such as multiphonon relaxation, vibrational losses by hydroxyl and other functional groups and energy transfer interaction quench the fluorescence intensity and the efficiency. The observed lifetime of the emission can be written as 
The internal quantum efficiency (η) can be evaluated from the ratio of the fluorescence to radiative decay time . The fluorescence lifetime obtained for the 1571 nm emission in the present transparent ceramic composition was 4.81 ms, which is within the range of fluorescence lifetime observed for single crystal viz. 7.14 ms at 40 mol. % and 1.95 ms at 60 mol % . The decay time for the present sample is as expected with high dopant concentration, where rate of migration of the excitation energy is dominant. With the calculated radiative decay time of 8.46 ms, a radiative internal quantum efficiency of 56% was obtained for the Er (50%) Cr (0.1%) YAG ceramic sample. Similarly the fluorescence lifetime for green (557 nm) and red (680 nm) were measured to be 265 μs ( ± 0.124 μs) and 264 μs ( ± 0.126 μs) with internal quantum efficiency 24.8% and 24.17% respectively. However the influence of other non-radiative interactions detracts the efficiency less than 100% as noticed in other samples. The higher value of calculated radiative lifetime was expected in the highly Er (50%) doped YAG ceramic compared to 29% Er doped single crystal as reported in Table 2.
We performed experiments for characterizing the Er: Cr: YAG (Er = 50% and Cr = 0.1% mol %) doped transparent YAG ceramic structurally, optically and compared with Er3+ doped single crystal. SEM measurement shows that the average grain size is 6.9 μm. Various spectroscopic parameters such as Judd-Ofelt parameters, emission cross sections, branching ratios, etc. determined for the transparent ceramic sample are very much similar to high Er content YAG single crystal samples. The internal quantum yield for NIR (1571 nm), green (557 nm), and red (680 nm) were calculated to be 56%, 24.8%, and 24.17% respectively for Er: Cr: YAG ceramic. The fluorescence lifetime for the transition4I13/2 →4I15/2 (Er3+) is 4.81 ms, which is more than the 60% Er3+ doped YAG but less than 40% Er3+ doped YAG single crystal. The short florescence lifetime for the highly doped sample was as expected where various non-radiative processes are more dominant such as energy transfer. Because of the low concentration, the effect of Cr3+ (0.1%) as a sensitizer ion was not prominent.
This work was supported by the National Science Foundation Partnerships for Research and Education in Materials (PREM) Grant No. DMR-0934218.
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