Abstract

Photon echo measurements made at 2 °K on the ³H₄ - ¹D₂ transition in Pr³⁺:LaF₃ show that magnetic dipolar couplings between the Pr and F nuclei account for the 56 kHz homogeneous linewidth of this transition.¹ The homogeneous broadening arises from the enhanced ¹⁴¹Pr nuclear moment (I=5/2) interacting with the local field fluctuations of the ¹⁹F nuclear moments undergoing mutual spin flip transitions. Such resonant fluctuations should, in the absence of a fluorine spin diffusion barrier, produce a homogeneous linewidth of about 200 kHz which is, in fact, roughly what is observed for the inhomogeneous broadening of the Pr³⁺ hyperfine levels and is considerably broader than that obtained by the photon echo measurements. Shelby et al² proposed a simple model analogous to the spin diffusion barriers responsible for narrowing the homogeneous lines in certain electron paramagnetic resonance transitions³. In such systems, the field produced by the electron magnetic dipole moment (2-3 orders of magnitude larger than the enhanced nuclear moment associated with the ground state of Pr³+ in LaF₃) de-tunes the nearest neighbors from each other, prohibiting mutual spin flips among them. Thus, the fields produced by the neighboring spins are static and their interaction with the paramagnetic ion contributes to the inhomogeneous linewidth and not to homogeneous broadening. The de-tuned neighbors are referred to as the "frozen core." Application of this picture to the case of Pr³⁺:LaF₃ is supported by the calculation of Devoe et al.⁴ Their Monte Carlo calculation of the Pr³⁺ optical dephasing times shows that the observed homogeneous linewidth could be explained by deleting the nearest neighbor F spins from the lattice of rapidly fluctuating fluorine moments, in effect, treating them as part of a frozen core.

© 1991 Optical Society of America