Persistent spectral hole burning in the $f\ensuremath{\rightarrow}f$ transition and changes in the $f\ensuremath{\rightarrow}d$ transition spectra upon laser irradiation were investigated in
Sm2+-doped ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}{\ensuremath{-}\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ glasses prepared by a sol-gel process. The hole was burned in the ${}^{7}{F}_{0}{\ensuremath{\rightarrow}}^{5}{D}_{0}$ line of
Sm2+ ions using a DCM dye laser at 77 K and the dependence of the burning efficiency on laser power and burning time was measured. The hole depth increased with increasing laser irradiation time, reaching up to $\ensuremath{\sim}12%$ of the total intensity within a few hundred seconds. Neither an antihole around the burned hole nor a change in the fluorescence line narrowing spectrum were observed after hole burning. The hole depth linearly increased with increasing the incident laser power up to $\ensuremath{\sim}1{\mathrm{W}/\mathrm{m}\mathrm{m}}^{2},$ indicating a single-photon hole-burning process. On the other hand, laser irradiation with a wavelength corresponding to the energy of the ${4f}^{6}\ensuremath{\rightarrow}{4f}^{5}5d$ transition resulted in a decrease of both the absorption and fluorescence intensities but no formation of a hole. It was concluded that the electrons excited in the
4f55d level were further excited into the conduction band of the host matrix by a two-photon absorption process and then captured in the trapping center.