Modulation frequency doubling in the axially periodic dependence of the luminescence of F 3 + centers in LiF crystals

Abstract

A spatially periodic distribution of the luminescence intensity of F2 and F3+ color centers is observed in LiF crystals with induced anisotropy. The period of spatially periodic modulation of the luminescence intensity generally coincides with the period of the change in the polarization state of excitation light. For F2 centers, the experimental axial spatially periodic dependence of the luminescence intensity is close to the calculated dependence obtained in the description of the absorption and radiative transitions in the centers by linear oscillators oriented along six C2 axes of the cubic crystal. However, for F3+ centers, the same experiment shows a spatially periodic distribution of luminescence intensity with a modulation period shorter by a factor of 2. It is shown that this effect of modulation frequency doubling in the spatially periodic pattern of the luminescence of color centers arises due to the nonlinear dependence of the luminescence intensity on the excitation light intensity. For F3+ centers, upon luminescence excitation in the absorption band with λmax = 452 nm, the modulation frequency doubling is due to the saturation of the metastable triplet state of such a center.