Hole-burning in rare earth doped materials

Tohru Suemoto, Institute for Solid State Physics, the University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan The spectral hole-buming in rare-earth doped crystals has been usually ascribed to the optical pumping of the hyper-fine sublevels. In contrast to the crystals, the holes in glasses have quite different character; the hole width is broad and the thermal stability is high. The hole buming mechanism in glass is believed to be a small lattice rearrangement around the rare-earth ions.' In other words, the rareearth ions couple with the neighboring two level systems (TLS) of structural origin. In spite of apparent success of this interpretation, we have had no direct evidence for the role of TLS on the hole properties. To clarify this point, we made a comprehensive study in several inorganic materials containing various degree of disorder, which we can control intentionally. The homogeneous width of one of the 3H, 'D, transitions of Pr3+ ion was compared for silicate glass, highly disordered mixed crystal (Y203 ZrO,)? and Y 2 0 3 crystals containing various amount of MgO. The temperature dependence of the homogeneous width was determined in a wide temperature range by combining fluorescence line narrowing and hole buming measurements. As shown in Fig. 1, it was found that the effect of disorder appears below about 20 K as T-linear components. The crossover from Raman process at higher temperature to the Tlinear dependence at lower temperatures is most clearly seen in the sample of Y20, with 1 mol% MgO. The magnitude of this component is roughly proportional to the degree of disorder. The result suggests that the whole temperature behavior of the homogeneous width can be understood by combining Raman process and TLS theory, without assuming other sophisticated processes such as a coupling to fractions. In the system of (MgO), (Y203)1-~ , we can observe real persistent holes due to lattice rearrangement at 20 K, where the optical pumping mechanism has practically no contribution. The buming efficiency shows a clear relationship with x. In this system, one oxygen vacancy is introduced for two Mg" ions substituted in the Y3* site. Examinin some other aliovalent ions (Ca2+, Bag, Zr4+), we concluded that the TLS due to oxygen vacancies play an important role in hole buming process! In Fig. 2 shown is the hole area in germanate glasses at a fixed buming time as a function of sodium ion concentration? The burning efficiency increases dramatically by adding sodium ions. Here the sodium ions are found to TEMPERATURE (K)