1. Introduction

Cubic YAG crystal has been widely recognized to be doped with various optically active ions, especially the rare earth element (Re) mainly due to the similar radius between Y³⁺ and Re³⁺ ions in host structure [1–8]. Depending on the doping ions, this type of crystal materials could be used in various application fields such as laser gain medium, window material, phosphor and scinllator materials because of their excellent optical, mechanical, and thermal properties. With the rapid development of sintering technology, high optical merit YAG ceramics have attracted much interest for materials engineers. In comparison to the glasses and single crystal counterparts [9,10], polycrystalline ceramic possesses not only the promising advantages in fabrication but also the unique microstructure which endows it with great properties.

Driven by the first report of Er³⁺ ions doped silicate glasses as laser materials in 1965 [11], YAG transparent ceramics (TC) doped with Er³⁺ ions has been intensively investigated as an eye-safe laser gain medium mainly due to the 1.5 μm emission from the ⁴I_13/2 to ⁴I_15/2 transition in the optical communication window [12–15]. However, the fact of lower absorption cross section of Er³⁺ ions implies that single-doped Er:YAG TC is not much efficient for the high power laser output. To overcome this shortcoming, the work of searching efficient sensitizers for Er³⁺ is of much significance. In the Yb, Er co-doped YAG system [16,17], non-radiative process of energy transfer from Yb³⁺ to Er³⁺ ions occurs, where Yb³⁺ ions, as donors, firstly absorb pumping energy and then partially transfer energy to Er³⁺ ions (the acceptors). This process is called the direct energy transfer process. Some authors have also reported the energy back transfer mechanism [18], which, however, is not dominant in this co-doped system. Obviously, Yb³⁺ ions serving as an efficient sensitizer for Er³⁺ions in host plays a significant role in the mid-infrared luminescence. However, there is rarely up-to-date report related to its in-depth luminescence mechanisms being published before.

In this letter, optical grade Yb³⁺ and Er³⁺ co-doped YAG TC is prepared via using high-temperature vacuum sintering technology. Cubic phase structure and full dense microstructure have been determined by means of XRD and SEM technologies, which endows our ceramic with high optical transmittance in visible region. The measured transmittance spectrum presents the featured absorption peaks for both Yb³⁺ and Er³⁺ ions in host which is in accordance with the obtained excitation spectrum. From both measured photoluminescence (PL) and time resolved photoluminescence (TRPL) spectra, it reveals that the doping concentration of Yb³⁺ ions shall be less than 5.5at.% for 0.5at.% Er doped YAG TC. Both ZPLs and sidebands deriving from Er³⁺ ions have been firstly discussed in this co-doped system. It is of much significance that anti-stokes sideband shows much stronger luminescence intensity than Stokes one, which is so much abnormal that it differs completely from that in most luminescent materials. This obtained result reveals the dominance of absorbing phonon energy in electron-phonon coupling (EPC) process under weak crystal field of YAG host.

2. Material preparation and characterization

To fabricate Yb sensitized Er:YAG TC, commercially available powders of α-Al₂O₃(99.99%, Sumitomo Chemical Co. Ltd, Japan), Y₂O₃(99.99%, Alfa Aesar, United States), Yb₂O₃(99.99%, Alfa Aesar, United States) and Er₂O₃(99.99%, CIAC, China) were used as the starting materials with tetraetheoxysilane (99.999 + %, Alfa Aesar, United States) and Oleic Acid (99%, Alfa Aesar, United State) acting as the sintering aid and dispersant agent. All powders were weighted in accordance with the chemical composition of (Yb_xEr_yY_(1-x-y))₃Al₅O₁₂ (x = 0.095, 0.075 and 0.055, y = 0.005) and then homogeneously mixed in ethanol for 20h in planetary-milling machine with the weight ratio of powder: solvent: mill-balls of 1:2.5:6. The obtained slurry was dried, then ground and sieved through 200-mesh screen. The powders were uniaxially pressed into Φ20 mm disks at about 2 MPa and then de-bindered at 700 °C for 10h in air. After operated under cold-isostatical press at 200 MPa, the obtained green bodies were sintered at 1720 °C for 10 h with the vacuum condition of 5 × 10⁻⁷ torr. The sintered ceramic was mirror-polished on both surfaces be 2.2 mm thickness, and annealing treatment was finally carried out at 1450 °C for 10 h in oxygen atmosphere.

The phase crystal structure were analyzed by X-ray powder diffractometer (Bruker D2, Germany) with Cu-Kα radiation (λ = 1.54056Å) as the radiation source. Obtained XRD data was utilized as an initial base for Rietveld refinement via using the GSAS package. The particle grain size and surface morphology of the ceramics were characterized with a JSM-6700F scanning electron microscopy (SEM). The optical transmittance spectrum was conducted via Lambda-900 UV/VIS/NIR spectrophotometer. Time resolved photoluminescence spectra of post-annealing specimen at 1532 nm were recorded using a spectrophotometer (Edinburgh, FL920), with a microsecond flash lamp (Edinburgh, μF900) being used as the exciting source and 917 nm as the exciting wavelength. The signals are detected with an NIR PMT (Hamamatsu, R5509). By fitting the curves, the lifetime at 1532 nm was determined. Under the excitation close to the surface perpendicular to the detecting direction, both excited and emission spectra were measured at room temperature.

3. Result and discussions

Figure 1(a) shows a photograph of 9.5at.% Yb, 0.5at.% Er co-doped YAG TC sample taken under the condition of natural light. Obviously, this ceramic exhibits a weak red color under the illumination of natural light, and it is totally different from that of single doped Yb: YAG TC [19]. This is mainly ascribed to the featured absorption of Er³⁺ ions in visible region. To evaluate the ceramic microstructure, SEM technology was employed as presented in Fig. 1(b). The surface morphology is observed with full dense microstructure. Distinct micro-grains appear with particle size ranging from 1~10 μm. Such a fine microstructure endows our ceramic with high optical transmittance, as seen from the measured transmittance spectrum of Fig. 2. The obtained result presents the optical transmittance of nearly 82% at 600 nm. Three strongest absorption peaks are observed to be located around 917nm, 940nm and 969nm respectively. They have been assigned to the electronic transition from ²F_7/2 to ²F_5/2 in Yb³⁺ ions [20]. In addition, numerous of featured absorption peaks have also been detected as highlighted in the light-red region of Fig. 2. They shall be attributed to the various transition processes between energy levels of Er³⁺ ions in host [21]. From this spectrum, it shows that Yb³⁺ ions absorption performs much stronger than that of Er³⁺ ions, which is tightly related to their doping concentration. To determine the influence of both Yb and Er doping ions on YAG host structure, a theoretical calculation was performed via applying Rietveld refinement method on the experimental XRD data of x = 0.095 TC sample. Herein, GSAS software package was used for the refinement work. As seen in Fig. 3, theoretical result is in excellent agreement with the measured XRD patterns. Crystalline structure of the TC sample was thus determined and can be uniquely characterized in a cubic space group of O_h¹⁰-Ia3d. Determined unit-cell parameters were tabulated in Table 1. It shows that the incorporation of Yb³⁺ and Er³⁺ ions into YAG host structure results in a slightly enlarged unit cell. From the consideration of ionic radius, this result is of some interest, and opposite to the previous report of lattice shrinkage. It may be ascribed to the following reasons. One is that some of rare earth ions crowed into the interstitial sites of lattice, and another reason is that Yb³⁺ ions or Er³⁺ ions may replace the Al³⁺ ions rather than Y³⁺ ions. Both above may conjunctly result in our obtained result.

 

Fig. 1 (a) Photograph and (b) SEM image of mirror-polished 9.5at.%Yb, 0.5at.%Er co-doped YAG transparent ceramic.

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Fig. 2 (a) Measured optical transmittance spectrum for 9.5at.% Yb, 0.5at.% Er co-doped YAG transparent ceramic. Light red-region shows the absorption peaks of Er³⁺ ions while blue-region absorption derives from Yb³⁺ ions. (b) Energy level diagram of Yb³⁺ and Er³⁺ions in YAG crystal.

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Fig. 3 Measured and calculated XRD patterns of 9.5at.% Yb, 0.5at.% Er:YAG TC sample.

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Table 1. Unit cell parameters obtained from Rietveld structural refinement of XRD data.

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In order to give an in-depth illustration on the photoelectronic process of the ceramic, room-temperature excited spectra was measured from 855 nm to 1020 nm with the monitor wavelength of 1532 nm, as depicted in Fig. 4. The strongest excited peak is observed at 917 nm, and this wavelength corresponds to the absorption transition from ²F_7/2 to ²F_5/2 of Yb³⁺ ions as mentioned above. Moreover, the fact of the similar energy value between Yb³⁺-²F_5/2 and Er³⁺-⁴I_11/2 manifolds implies that Yb³⁺ ions could behavior as an efficient sensitizer for Er³⁺ ion emission in the co-doped YAG ceramics, and it plays a significantly important role in the ~1.5 μm luminescence properties of our co-doped ceramic. Herein, we present three types of TC specimen with fixed Er³⁺ concentration of 0.5at.% but Yb³⁺ concentration being 9.5at.%, 7.5at.% and 5.5at.% respectively. With 917 nm laser as the excitation wavelength, TRPL spectra have been measured for these three samples and their luminescence lifetime are determined by fitting the obtained decay curves with one-exponential function, as shown in Fig. 5. The obtained result is comparable to that (7.53 ms) of previous work and it is greater than that (6.67 ms) of Er:YAG ceramic [16]. This clearly reveals the significance of Yb³⁺ ions serving as the sensitizer for Er³⁺ ions. Moreover, it is observed that the lifetime slightly decreases with increasing Yb³⁺ concentration, which may be attributed to both cross relaxation and the enhancement upconversion process [23]. The luminescence decay curves show us a rise at early short time and then decay exponentially at later times. The rise time at the beginning of the transient reveals the nature of emission at 1532 nm being partially due to the non-radiative process of energy transfer from Yb³⁺ to Er³⁺ ions. That is, when Yb³⁺ ions are excited, they partially pass energy to their nearby Er³⁺ ions by means from ²F_5/2 to ⁴I_11/2. Then, the population of ⁴I_11/2 decays fast to ⁴I_13/2 by non-radiative process and therefore increases the lifetime of Er³⁺-⁴I_13/2 excited state. Although some authors have previously stated the possibility of the energy back transfer from Er³⁺ to Yb³⁺ ions [18,24], this process, however, is not dominant due to the fact of rapidly decay of luminescence intensity in this co-doped system.

 

Fig. 4 Room temperature measured excited spectrum of 9.5at.% Yb, 0.5at.% Er co-doped YAG transparent ceramic monitored at 1532 nm.

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Fig. 5 Normalized luminescence decay curves of Yb, Er co-doped YAG TC monitored at the wavelength of 1532nm. The green lines represent the fitting results of decay curves with one-exponential function.

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To provide an in-depth insight into the influence of Yb³⁺ active concentration on luminescence intensity, room-temperature luminescence spectra have been measured on the above mentioned three specimen by using CW 940 nm laser as the excitation source, and the spectral range is set from 1400 nm to 1600 nm, as presented in Fig. 6. Numerous of emission peaks appear in the spectra and all of them are assigned to the electronic transition from ⁴I_13/2 to ⁴I_15/2 in Er³⁺ ions. It can be clearly observed that mid-infrared PL intensity deteriorates with the rise of Yb³⁺ content, which is probably due to the decreasing of emission efficiency accompanied by the enhancement of upconversion luminescence. This phenomenon indicates that the optimum sensitizer concentration of Yb³⁺ ions shall be less than 5.5at.% for 0.5at.% Er³⁺ ions in YAG host. Moreover, two strongest emission peaks are observed at wavelengths of ~1527 nm and ~1532 nm respectively. The energy interval between them is measured to be ~2.1 meV which is comparable to the value (~2.3 meV) between adjacent peaks in the Strokes phonon sidebands, as shown in the inset figure of Fig. 6. Such a small energy value corresponds to the splitting of upper energy level ⁴I_13/2 induced by weak crystal field distortion. Due to the spin-allowed electric-dipole transition from ⁴I_13/2 to ⁴I_15/2 in Er³⁺ ions, both peaks arises from the pure electronic transition, and could be considered as zero phonon lines (ZPLs) [25,26]. Herein, they are denoted as R1 and R2 as depicted in the shaded region of Fig. 6. From the measured luminescence spectra, Stokes and anti-Stokes phonon sidebands are observed and nearly systematically located on both sides of ZPLs. Interestingly, anti-Stokes sidebands perform much stronger intensity than Stokes ones which reveals the dominance of absorbing phonon modes in the room-temperature EPC process. This phenomenon, however, is completely different from our observation in most of luminescent materials [27–29], and the underlying physical mechanism remains to be discovered in future. Furthermore, three main phonon modes in the EPC interaction could be identified from the measured phonon sidebands. The involved phonon energies have been determined to be ~6.9 meV (ν₁), ~22 meV (ν₂), and ~41 meV (ν₃), and they probably derive from the internal lattice vibrations. With a further insight into the anti-Stokes sideband, a series of periodical emission peaks appear mainly due to the complicated acoustic phonon coupling process, and the line-shapes of both Stokes and anti-Stokes sidebands reflect the density of state of acoustic phonons in YAG crystal.

 

Fig. 6 Room temperature measured emission spectra of 9.5at.% Yb, 0.5at.%Er co-doped YAG ceramics. The inset figure shows the Stokes sideband of the studied TC.

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In conclusion, highly optical-grade Yb, Er co-doped YAG ceramic is successfully fabricated via high-temperature solid-state raction method under vacuum condition. Full dense microstructure with cubic phase endows our sample with great optical transmittance of ~82% at 600 nm. The sensitization role of Yb³⁺ ions in co-doped TC is mainly discussed, and its best concentration is less than 5.5at% for 0.5at.% Er³⁺:YAG ceramic. From the measured luminescence spectra in this co-doped system, both ZPLs and sidebands arising from active center Er³⁺ ions are firstly discussed and the weak EPC interaction has been determined from the phonon sidebands. The most significant finding is that anti-Stokes sidebands show much stronger intensity than Stokes one, which completely differs from our observation in most of luminescent materials. This obtained result implies the dominance of absorbing phonon energy in EPC process under weak crystal field of YAG host.