The Yb3+ to Er3+ energy transfer in YAl3(BO3)4 crystal
Introduction
The laser emission around wavelength of 1.55 μm, which is located in the “eye-safe” region [1] and plays an important role in the optical communication as amplifier [2], can be obtained from the 4I13/2 → 4I15/2 transition in the Er3+-doped materials. However, the Er3+-doped materials cannot be efficiently pumped by the InGaAs laser diodes because of the week absorption of Er3+ at the wavelength of 980 nm. Generally, the solution is adding another ion as a sensitizer to improve the pumping efficiency. As a sensitizer, a broad and high absorption band around the pump wavelength is necessary. Fortunately, Yb3+ ion can entirely satisfy these requirements. In the Yb3+ and Er3+ co-doped materials, the pumping energy can be absorbed by Yb3+ efficiently and transferred to the Er3+. Furthermore, the Yb3+ ion has only two energy levels, which makes it no excited state absorption or up-conversion losses in principle. So Er3+ and Yb3+ co-doped materials have received great attention as laser media at 1.55 μm [3], [4], [5], [6].
YAl3(BO3)4 (YAB) is a non-linear optical crystal with excellent chemical and physical properties [7], [8], which has been demonstrated as an efficient self-frequency-doubling (SFD) material [9], [10]. The structure of the crystal is trigonal with space group R32. The good laser results around 1.55 μm have been achieved in Yb3+ and Er3+ co-doped YAB crystal [11], [12], but the energy transfer properties constitute an important factor for optimization of these crystals. So in this paper, the energy transfer from Yb3+ to Er3+ in the YAB crystal is studied on the basis of the rate equations. The energy transfer coefficient and other related parameters have been determined. The fluorescence lifetime of the 2F5/2 level of Yb3+ ion has been measured and used to calculate the energy transfer efficiency. To make the analysis of energy transfer simply, the anisotropy of the crystal were not considered.
Section snippets
Experimental procedure
Yb3+ and Er3+ co-doped YAB crystals were grown by the top seeded solution method with a flux system of NaF–MoO3–B2O3. Details of the growth procedures have been given previously [13]. The Er3+ concentration in the melt was fixed at 1.3 at.% whereas the Yb3+ concentration was varied (6, 12, 20 at.%). The real concentrations of the Yb3+ and Er3+ ions in YAB crystal were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) and summarized in Table 1. In brief, the three
Spectroscopic properties and rate equation
Fig. 1 shows the schematic energy level diagrams of Yb3+ and Er3+ ions and the relevant transitions including the energy transfer processes. The energy gap between the 2F7/2 and 2F5/2 levels of Yb3+ ion (between 9613 and 10672 cm−1 [15]) matches well with that between the 4I15/2 and 4I11/2 levels of Er3+ ion (between 9895 and 10344 cm−1 [16]). Fig. 2 shows the good overlap between the normalized unpolarized absorption cross-section of Er3+ and emission cross-section of Yb3+. It means that, when
Conclusion
The Yb3+ to Er3+ energy transfer in YAB crystal have been investigated in this paper. The energy transfer coefficients (W25) have been determined by using the simplified rate equations and compared with those calculated by Tkachuk model. In the Yb, Er:YAB crystal, the energy back transfer is not considered because of the high phonon energy of borates, causing almost all the 4I11/2 population rapidly non-radiative transition to 4I13/2 level of Er3+ ions. The other parameters, such as
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