Hyperfine structure of Ho3+ levels and electron–phonon coupling in YPO4 single crystals

High resolution spectroscopy (the finest being 0.01 cm-1) was applied in the 75–25 000 cm-1 and 9–300 K ranges to a 1 mol% holmium doped YPO4 single crystal with two purposes: (1) to study the hyperfine splitting of Ho3+ energy levels of interest for possible quantum manipulation media and (2) to analyze the electron–phonon interaction. The hyperfine structure was clearly revealed for a high number of lines in a wide wavenumber range (up to 21 500 cm-1) and for a large number of multiplets. Several hyperfine patterns were monitored, differing in the number of components (a maximum of 16 could be easily distinguished in a single beautiful pattern), in their separation, and in their relative statistical weight. These features were all understood in terms of a crystal-field model, whose results are in good agreement with experiments and account for the involved level symmetry, the type of transitions (electric and magnetic dipole allowed), and the contribution of a second-order (pseudoquadrupolar) hyperfine coupling between close levels. The electron–phonon interaction, investigated through the thermally induced line shift, was critically discussed in the framework of single phonon coupling and of two phonon Raman scattering models.