Spectral hole burning and excited electrons in Sm2+-doped Al2O3-SiO2 glasses

Persistent spectral hole burning in the f-->f transition and changes in the f-->d transition spectra upon laser irradiation were investigated in Sm2+-doped Al2O3-SiO2 glasses prepared by a sol-gel process. The hole was burned in the F-7(0)-->D-5(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 similar to 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 similar to 1 W/mm(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)-->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.