Yb3+ energy levels in α-Al2O3

The incorporation of rare earth ions into α-Al2O3 is difficult due to the large difference between the rare earth and the Al3+ ionic radii. Recently, rare earth doped α-Al2O3 has been synthesized via the sol–gel technique. In this process, rare earth doped γ-Al2O3 is produced first, which can then, upon sintering, be converted into the α-phase. It has to be noted that this technique leads to a number of different rare earth containing phases in addition to the desired phase in which the rare earth ions substitutes for an Al3+ ion, particularly in heavily doped samples. Also, the sol–gel produced α-Al2O3 is polycrystalline, which prohibits an investigation of the polarization of optical transitions. Here, we report on optical studies of Yb3+ doped single crystalline Al2O3, grown by the laser heated pedestal growth technique. This technique, in which a focused CO2 laser beam is used to melt polycrystalline precursors, produces single crystal fibers with typically 1 mm diameter and 15 mm length. Single crystal seeds enable the growth of oriented samples. Using absorption, photoexcitation, and emission measurements, the energy level diagram of Yb3+ in α-Al2O3 : Yb3+has been determined. We note that only a small amount of Yb3+ions enter the lattice substitutionally, and that higher dopant concentration again leads to the presence of a variety of Yb containing phases.