Linear and nonlinear optical properties of RbTiOPO4–KTiOPO4 alloys studied from first principles

Abstract

We investigate the linear optical properties of RbxK1−xTiOPO4 alloy crystals (x = 0, 0.25, 0.5, 0.75, 1) in the framework of the Bethe–Salpeter approach and the second-order optical response of these crystals within the independent particle approximation. The independent particle spectrum of KTiOPO4 (KTP) shows that this crystal absorbs light in the near-UV region. By solving the full Bethe–Salpeter equation, it is seen that the excitonic effects lead to an overall increase of the intensity and a redshift of about 1.6 eV in the absorption onset from the near-UV region to the visible region, reflecting the significant role of the electron–hole (e)–(h) coulomb interaction. It is shown that the spatial distribution of the e–h pair dominating the absorption onset is very localized around the titanium atoms. The absorption spectra at different Rb substitutions indicate an insignificant variation in the energy of the absorption onset due to the small differences in the corresponding bandgaps. The second-harmonic susceptibility tensor components of KTP/RbTiOPO4 (RTP) alloys at λ=1064 nm are evaluated and compared with experimental results. We show that Rb substitution modifies the maximum values of deff in Rb0.5K0.5TiOPO4 and RTP. Our results provide a valuable theoretical framework for tuning the optical properties of the KTP based crystals through doping for optoelectronic applications.