Abstract

We have observed extremely fast nonradiative relaxation (picosecond to femtosecond) between closely spaced crystal field states derived from the ⁴G_5/2,²G_7/2 Stark manifold of Nd³⁺ in glassy media. The relaxation is significantly faster than that predicted by an indiscriminate application of the energy-gap law, and it is highly wavelength-dependent. Femtosecond accumulated-photon-echo techniques were used at 13 K, revealing single and nonexponential echo decays. Relaxation was found to involve single phonon processes. Generally, the dephasing rate increased as higher-lying crystal field states were excited, reflecting the larger number of available terminal levels for the relaxation and the phonon density of states. However, a detailed plot of the dephasing rate versus wavelength revealed considerable unexpected structure. As higher-lying states were excited, the relaxation rate did not increase in the expected monotonic fashion. Surprisingly, the relaxation rate decreased in various spectral regions as the excitation energy was increased. By using T₂-selective dynamic-excitation spectroscopy, we assigned these regions to the spectral locations of the inhomogeneously broadened crystal field states in the Stark manifold. Site-dependent electron-phonon interaction and a strong correlation between the inhomogeneous broadening of the crystal field states are deduced from the data.

© 1990 Optical Society of America