Emission dynamics of the 4F3/2 level of Nd3+ in YAG at low pump intensities

The room-temperature high-resolution luminescence decay investigation of YAG crystals doped with Nd3+ in concentrations up to 1.5 at.% shows that under nonselective weak 532-nm pump, which prevents the up conversion, the emission dynamics of F-4(3/2) level is influenced by a cross relaxation inside the system of Nd3+ ions, dominated by superexchange (R-0 = 5.43 Angstrom, L = 0.54 Angstrom) for the nearest-neighbor Nd3+ ion pair and by dipolar coupling (C-DA = 1.85 x 10(-40) cm(6) s(-1)) for all other pairs. These parameters are in strong contradiction with other recent reports. The energy-transfer parameters are verified in the quasiselectively excited decay of the first-, second-, and third-order Nd3+ ion pairs by using a proper adaptation of the theory of energy transfer for discrete, equiprobable, and random placement of Nd3+ ions in the crystalline lattice. They give also a very good description of the data on quantum efficiency and fractional thermal load for YAG:Nd3+ samples of various concentrations. The absence of the quadrupolar interaction in the mechanism of transfer is consistent with the restrictions imposed by the selection rules for the transitions involved in cross relaxation.