Optical and nuclear spin spectroscopy in Pr3+:LaF3 by optical pumping

A Stark Modulated Optical Pumping (SMOP) technique was used as a probe of optical hole-burning to study the effects of nuclear spin interactions in Pr3+:LaF3. The results of two experiments are reported. In the first experiment, the SMOP technique is applied for optical detection of the NQR spectrum of the Pr3+ optical ground state. Frequency and linewidth variations of the Pr3+ ground state hyperfine levels for different positions within the main 3H4(Γ1)-1D2(Γ1) inhomogeneous optical line and satellite transitions of Pr3+:LaF3 are observed. A linear increase in the broadening of the Pr3+ hyperfine transitions, without a shift of central frequency, is measured as the laser is tuned toward the wings of the inhomogeneous optical transition. In the second experiment, direct evidence for the presence of a spin diffusion barrier or frozen core in Pr3+:LaF3 is observed by examining the cross relaxation between the Pr and F nuclei in a magnetic field chosen so that a pair of the optical ground state Pr3+ hyperfine energy levels matches the F splitting or a multiple of the F splitting. This level crossing condition allows resonant flip-flop interactions with the nearest neighbor frozen core fluorine spins to re-populate Pr3+ hyperfine levels emptied by laser hole-burning, and is detected as enhanced absorption of the laser beam. The coupling of core fluorine spins to bulk fluorine spins during Pr-F cross-relaxation is measured by NMR of the bulk fluorine spin magnetization. The rate of cross relaxation between the Pr spins and the bulk F spins measured in this way is at least three to four orders of magnitude slower than that expected in the absence of a spin diffusion barrier. This reduction of coupling indicates nearly complete de-tuning of the frozen core F spins immediately surrounding the Pr3+ ion, cutting off resonant coupling with the bulk F spins