Hyperfine and isotope structure in optical spectra of rare earth ions in crystals

Full text: In this lecture, a brief review is given of the work on hyperfine and isotope structure in optical spectra of rare earth of ions in crystals which was carried out in the author's group since 1985. We have discovered and studied in detail the hyperfine structure in optical absorption spectra of LiMF4:Ho3+ (M=Y, Lu) single crystals by means of high resolution (0.004 cm-1 ) Fourier transform spectroscopy. The calculations of the spectral envelopes of the 5I8 -o-tilde 5IJ (J=4,5,6,7) transitions have been performed taking into account the crystal field as well as the magnetic dipole and electric quadrupole hyperfine interactions. The comparison between simulated in this way and measured spectra enabled us to account for all the peculiarities of the observed hyperfine structure. Recently, we have revealed the hyperfine structure in the infrared spectra of LiYF4 : Er3+ coming from the 167Er isotope. We have found that each hyperfine component in the spectra of low-strain LiYF4:Ho3+ consists of several closely spaced (0.01 -- 0.04 cm-1) equidistant lines and proved that this structure was caused by isotopic disorder in the lithium sublattice. Quantitative theory of isotope effects in the spectra of rare-earth ions in crystals has been developed by B.Z. Malkin on the basis of semiphenomenological models of the crystal field, lattice dynamics, and electron-phonon interaction in rare-earth insulators and has been applied to the case of LiHoF4 . The calculated isotope shifts are in a qualitative agreement with the measured ones for LiYF4:Ho3+. More work, both experimental and theoretical, is necessary to achieve a better understanding of isotope spectral effects. The first steps in this direction are reported here