INHOMOGENEOUS BROADENING BY NUCLEAR-SPIN FIELDS - A NEW LIMIT FOR OPTICAL-TRANSITIONS IN SOLIDS

Using a low-strain, isotopically pure crystal of YLiF4, we have measured extremely narrow inhomogeneous linewidths (as low as 40 MHz) for the optical transitions of Er3+ impurities. Inhomogeneous broadening due to strains and other defects is so low that a new mechanism limits the inhomogeneous linewidth: local magnetic fields due to fluorine nuclear spins. These ultranarrow lines enabled us to make the first direct measurement of the isotope shifts of f-f transitions in a solid, and we find for the isotopes Er-164, Er-166, Er-168, and Er-170 a shift of 76 +/- 2 M Hz/unit mass on the I-4(15/2)(1) --> F-4(9/2)(1) transition. We show that this is consistent with a mechanism of coupling to zero-point vibrations.