Sesquioxide crystals were identified as promising gain media for high-power ultrafast lasers, and the broadening of their fluorescence spectra could shorten the achievable laser pulses, corresponding to the enhancement of the peak powers. In this study, we demonstrate the broadening of the fluorescence spectra of the sesquioxides with the electron-phonon coupling effect and ligand engineering associated with the advantages of the Lu2O3 host in the phonon energy and insensitivity to doping concentrations. The Yb:LuxY2-xO3 (0 <= x <= 2) sesquioxide crystals were grown and characterized. The introduction of Y3+ ions broadens the fluorescence spectra by approximately two times those of Yb:Lu2O3, which could be attributed to the strong electron-phonon coupling effect and octahedral distortion. This work would provide a series of novel laser materials for high-power ultrafast lasers and pave a new route for designing functional materials aimed at the enhancement of the given performance.