Spectral hole burning and optical dephasing in disordered crystals - Pr³⁺:LiNbO₃ and Pr³⁺:Sr_.6Ba_.4Nb₂O₆ (SBN)

We have measured the homogeneous line widths of the 3H4(1) to 1D2(1) transition of Pr3+ ions in the ferroelectric, photorefractive crystals, strontium barium niobate and lithium niobate in the range 1.3-3 K using spectral hole burning and photon echoes. Both crystals show substitutional disorder and the presence of vacancies. A near-linear temperature dependence of the homogeneous line width is observed in both materials showing the presence of disorder modes or two-level systems (TLS). In the case of lithium niobate the density of states of the TLS is three orders of magnitude lower than in SBN and has not been detected in heat capacity measurements.