Annealing effects on optical properties of natural alexandrite

Natural alexandrite (BeAl 2 O 4 :Cr 3+ ) crystals are investigated as regards the effects of annealing on their optical properties. Optical absorption spectra are measured from the ultraviolet (190 nm) to the near infrared (900 nm), for a sample subjected to consecutive annealing processes, where time and temperature are varied. Besides this, luminescence spectra are simultaneously obtained for this sample, excited with a Kr + laser source, tuned on an ultraviolet multi-line mode (337.5, 350.7 and 356.4 nm). We observe from absorption as well as from emission data that annealing mainly influences the distribution of Cr 3+ and Fe 3+ ions, located on sites of a mirror plane (C s symmetry), which are responsible for the optical properties of alexandrite. The results obtained lead to the conclusion that annealing induces a modification of the population of Cr 3+ on C s sites as well as on sites located on an inversion plane (C i ). Annealing could improve the optical properties of this material, as regards its application as a tunable laser.