Vibronics in optical spectra of Yb3+ and Ce3+ in YAG and Y2O3 ceramics

Abstract

The optical spectra of Yb3+ in YAG and Y2O3 and Ce3+: YAG show complex structure with contributions of vibronic satellites that extend beyond the spectral range delineated by the pure electronic transitions. The rather strong vibronic satellites are connected to the enhanced electron-phonon interaction of the RE3+ ions at the ends of 4fn series. Thus, tentatively we suggest that some of reported satellites in infrared absorption of Ce3+ in YAG crystals could be vibronics. The optical spectra of our Yb-doped YAG and Y2O3 ceramics are similar to those reported for single crystals. The vibronic lines intensities comparable with other lines of Yb makes separation of the zero-phonon lines from vibronics very difficult, particularly for Y2O3 which has two cationic sites. Measurements of optical spectra of Yb- and (Nd,Yb)-doped ceramics under direct excitation in Yb3+ or via energy transfer from Nd3+ and comparison with the phonon spectra enables such separation and identification of Stark level structures. The particularly large intensities of the vibronic lines in vicinity of the zero-phonon lines can be connected to near-resonant effects between the phonon energies and the Stark splitting of manifolds, that are also responsible for the broadening of electronic lines corresponding to transitions to high energy levels of 2F5/2,7/2 manifolds (even at 10 K by phonon emission). This analysis indicates that the observed one-micron spectra of Yb-doped YAG and Y2O3 ceramics belong exclusively to Yb3+.