Spectral hole burning and excited electrons in ${\mathrm{Sm}}^{2+}$-doped ${\mathrm{Al}}_2{\mathrm{O}}_3{-\mathrm{S}\mathrm{i}\mathrm{O}}_2$ glasses

Persistent spectral hole burning in the $f\to f$ transition and changes in the $f\to d$ transition spectra upon laser irradiation were investigated in ${\mathrm{Sm}}^{2+}$-doped ${\mathrm{Al}}_2{\mathrm{O}}_3{-\mathrm{S}\mathrm{i}\mathrm{O}}_2$ glasses prepared by a sol-gel process. The hole was burned in the ${}^7{F}_0{\to }^5{D}_0$ line of ${\mathrm{Sm}}^{2+}$ 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 $\sim 12\%$ 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 $\sim 1{\mathrm{W}/\mathrm{m}\mathrm{m}}^2,$ indicating a single-photon hole-burning process. On the other hand, laser irradiation with a wavelength corresponding to the energy of the ${4f}^6\to {4f}^{5}5d$ 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 ${4f}^{5}5d$ 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.