Contributing bands to the optical absorption of reduced LiNbO₃: thermal and optical excitation

The effect of thermal and optical excitation on the optical absorption band of reduced LiNbO₃ has been investigated for crystals with different Li/Nb ratios. Fe- and Cu-doped samples were also studied. Although the main effect of the excitation is to enhance the infrared part of the spectra at the cost of the ultraviolet-visible region, a number of new features appear depending on stoichiometry and doping. In all cases, the effect of thermal excitation is reversible. The spectra for all undoped samples, before as well as after the various treatments were fitted with four gaussian bands peaking at 1.6, 2.3, 3.1 and 3.9 eV. Additional bands have to be used to fit the spectra for doped crystals (2.5 eV for Fe-doping and 2.8 eV and 3.3 eV for Cu-doping). The possible origin of the bands is discussed in relation to other available information, but a definite assignment cannot be offered yet.