Investigation of crystal field effects for the spectral broadening of Yb 3+ -doped Lu x Y 2- x O 3 sesquioxide crystals.

The investigation of crystal field effects is significant for elucidating the spectral characteristics of Yb-doped sesquioxide crystals for ultrafast laser generation. The narrow spectra of Yb-doped single sesquioxide crystals limit the generation of ultrafast lasers; in this study, the Y ions were introduced into LuO single crystals by the employment of ion replacement to broaden the spectra. To analyze the spectral broadening, the responsible crystal field parameters (CFPs) were calculated. The conversion of the host dominant ion and the distortion of the ligand affected the values and signs of the CFPs, and further determined the energy level splitting and fluorescence spectra. A linear relationship expressed by the semi-empirical equations for Yb-doped sesquioxide crystals was produced, which could be used for high throughput spectral prediction. Opposite variations of high- and low-frequency vibrational energies and the influence of the electron-phonon coupling on the spectra were also achieved. The redshift from the crystal field and the blueshift from the electron-phonon coupling make the optimal spectral broadening appear when = 1.19 in the Yb:Lu Y O crystals. The results of these analyses could provide some key clues for the development of Yb-doped crystals for the generation and amplification of ultrafast lasers.