TRANSIENT GRATINGS BY 4F-]5D EXCITATION OF RARE-EARTH IMPURITIES IN SOLIDS

We have generated transient population gratings in CaF2:Eu2+ and LiYF4:Nd3+ by utilizing 355 nm excitation from a pulsed laser source. The scattering from the gratings was observed with the aid of a helium-neon laser operating at 632.8 nm. The grating is the result of a 4f --> 5d excitation mechanism for both crystals. For the case of CaF2:Eu2+, the grating is absorptive in origin, and is due to the excitation of the 4f7 --> 4f65d transition by the crossed pump beams. A unique situation is encountered for the LiYF4:Nd3+ system, in which the gratings is created via a two-step two-photon excitation process, described by the transition sequence: 4I9/2(4f3) --> 4D3/2(4f3) --> 4f25d. Following excitation into the 4f2(5)d manifold, a portion of the Nd3+ ions rapidly relax into the high-lying 2G9/2(4f3) state. The grating arises from the difference in the optical properties of the 2G9/2 and 4I9/2 states.