Colour Centre Bragg Grating Recording in Lithium Fluoride Thin Layers

摘要

We study the recording of permanent Bragg gratings on surface-coloured Lithium Fluoride crystals by using the interference pattern of a CW UV Argon ion laser operating at 244 nm. Gratings with spatial periods ranging from 400 nm to 1000 nm are written by using a phase-mask interferometer. They are stable up to several months after the writing process. Absorption and photoluminescence spectra show the bleaching of primary F and F—aggregate colour centres as a result of the process. Confocal microscopy is used to determine the pitch and the profile of the fluorescent gratings. The UV-induced optical bleaching in densely coloured LiF layers is responsible for the periodic spatial modulation of absorption and photoemission properties that characterises the grating. In the coloured surface layer, a reduction up to 50% of the initial refractive index increase has been estimated in the bleached areas.