Regular articleSpectroscopic properties of Yb3+, Ho3+-doped Y3Al5O12 single crystals grown by the micro-pulling-down method
Graphical abstract
Photograph of Yb,Ho:YAG crystals grown by the μ-PD method. (a) Yb = 0.05, (b) 0.1, (c) 0.15.
Introduction
Nowadays, considerable attention was paid to eye-safe laser near 2 μm from Ho3+ doped materials, which was widely used in the fields of remote sensing, precision guidance, material processing, optical communications and medicine surgery [1], [2], [3], [4]. In 1962, Johnson et al. reported the laser operation at 2.0 μm from Ho3+ doped CaWO4 crystal for the first time [5]. So far, a great deal of research has been focused on the spectroscopic and laser properties of Ho3+ doped host materials. In fact, it is extraordinary tough to obtain laser action in the 2 μm region, owing to the lack of commercial high-power laser diode corresponding to the absorption of Ho3+ ions. Fortunately, the matter can be settled by co-doping with Yb3+ ions, and the first laser oscillation of Yb,Ho-doped YSGG crystal were realized in 1993 [6]. The relatively strong absorption band at 980 nm of the transition 2F7/2 → 2F5/2 of Yb3+ ions is suitable for the AlGaAs diode pumping, which generally improves the absorption and pumping efficiency of the Ho3+ ion near the NIR range [7], [8]. Moreover, as a consequence of the efficient energy transfer from Yb3+ ions to Ho3+ ions, the upper level (5I6) of Ho3+ is pumped via Yb3+ (2F7/2 → 2F5/2), thus, Yb3+-sensitized Ho3+ doped materials are also used to generate laser operation around 2.0 μm. Until now, plenty of Yb3+, Ho3+ co-doped host materials generating 2.0 μm laser radiation have been developed extensively, such as Y2O3 [9], (YLa)2O3 [10], NaLa(MoO4)2 [11], LuVO4 [12], YAG [13] and so on.
Yttrium aluminium garnet (YAG, molecular formula is Y3Al5O12) crystal belongs to the cubic system, with the space group Oh(10)-Ia3d [14]. The radius of Y3+ is close to that of rare earth ions, therefore it is possible to dope a certain number of trivalent rare earth ions into the dodecahedral lattice. Since the phenomenon of laser oscillation in YAG crystal was firstly reported in 1964, YAG has been widely used as a promising laser gain medium due to its high thermal conductivity, ideal optical and excellent chemical properties [15]. Meanwhile, YAG crystal can provide a good crystal field environment for the activated ions, which make YAG crystal become a potential laser medium. The μ-PD technique has the capability to grow crystal fibers with short growth period, high component uniformity and cost-effectiveness [16], [17]. It is the front and hot spot in the fields of laser materials due to its good heat dissipation, large specific surface area, high aspect ratio and little nonlinear gain coefficient. Until now, to the best of our knowledge the growth and spectral properties of Yb,Ho:YAG crystal has never been reported by the μ-PD technique. Hence, it is a promising technique to systematically demonstrate the relationship between the ratio of Yb3+ in (Ho0.005YbxY0.995-x)3Al5O12 single crystals and the fluorescence properties.
In this paper, a sequence of Yb,Ho:YAG crystals have been successfully grown by the μ-PD technique. The room temperature spectral characteristics of Yb,Ho:YAG crystals were investigated. The possible energy transfer and upconversion mechanisms were discussed in detail.
Section snippets
Experimental procedures
YAG crystals doped with a fixed Ho3+ content of 0.5 at.% and various Yb3+ contents were grown by the μ-PD technique with an RF heating system. A stoichiometric mixture of Ho2O3, Yb2O3, Y2O3, and Al2O3 powders with high purity of 99.999% was used as the raw materials. The powders were dried and weighed according to the formula (Ho0.005YbxY0.995-x)3Al5O12 (x = 0.05, 0.10, 0.15). After the weighted powders were ground and mixed, the compounds were pressed into rod shape under 160 MPa for 2 min by
Absorption spectra
The room-temperature absorption spectra of the Yb,Ho:YAG crystals in the range 400–2200 nm are presented in Fig. 3. It can be found that eight prominent absorption peaks are located at 419, 453, 485, 537, 635, 936, 1129 and 1909 nm. Assignments of the absorption bands are shown by the transitions from the ground state 5I8 to the excited state of Ho3+ ions. The intense absorption in the 850–1050 nm wavelength is attributed to the 2F7/2 → 2F5/2 transition of Yb3+ ion. The introduction of Yb3+ and
Conclusion
A series of Yb,Ho:YAG single crystals (Ho = 0.5 at.%, Yb = 5, 10 and 15 at.%) have been successfully grown by the μ-PD technique. The XRD patterns show that the Yb3+ and Ho3+ doping ions well replace the positions in the host matrix structure. The absorption spectra, fluorescence spectra and fluorescence decay curve were recorded at room temperature. The effect of Yb3+ doping concentration on the structure, spectra and fluorescence lifetime was analyzed. The emission bands of Yb,Ho:YAG crystal
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by the National Key Research and Development Program of China (2016YFB1102303).
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