Infrared to ultraviolet measurements of two-photon absorption and n 2 in wide bandgap solids

The bound electronic nonlinear refractive index, n₂, and two-photon absorption (2PA) coefficient, β, are measured in a variety of inorganic dielectric solids at the four harmonics of the Nd:YAG laser using Z scan. The specific materials studied are: barium fluoride (BaF₂), calcite (CaCO₃), potassium bromide (KBr), lithium fluoride (LiF), magnesium fluoride (MgF₂ ), sapphire (Al₂O₃), a tellurite glass (75%TeO₂+20%ZnO+5%Na₂O) and fused silica (SiO₂). We also report n₂ and β in three second-order, χ⁽²⁾, nonlinear crystals: potassium titanyl phosphate (KTiOPO₄ or KTP), lithium niobate (LiNbO₃), and β-barium berate (β-BaB₂O ₄ or BBO). Nonlinear absorption or refraction can alter the wavelength conversion efficiency in these materials. The results of this study are compared to a simple two-parabolic band model originally developed to describe zincblende semiconductors. This model gives the bandgap energy (E_g) scaling and spectrum of the change in absorption. The dispersion of nl as obtained from a Kramers-Kronig transformation of this absorption change scales as E_g^-1 . The agreement of this theory to data for semiconductors was excellent. However, as could be expected, the agreement for these wide bandgap materials is not as good, although general trends such as increasing nonlinearity with decreasing bandgap energy can be seen