Investigation of the second-harmonic generation containing two linearly polarized states in KTiOPO4crystals

Abstract

Abstract KTiOPO4 is a positive biaxial crystal with very good nonlinear optical properties. According to the second-order nonlinear optical rule on a biaxial crystal, under the usual conditions of type II phase-matching, a slow ray (e ω 1) and a fast ray (e ω 2) at a fundamental frequency (the refractive index relationship is n e1 > n e2) can only be transformed into a fast ray (e2ω 2) at a doubled frequency, i.e. e ω 1 + e ω 2 → e2ω 2. However, under a d.c. field applied along the c-axis of the crystal, not only is the e2ω 2 ray generated, but a slow ray (e2ω 1) at the doubled frequency is also generated. The new e2ω 1 ray is also linearly polarized. The 90° angle between the polarization direction of the e2ω 1 ray and that of the e2ω 2 ray is independent of thickness in the transmission direction of the SHG element. KTiOPO4 is a typical quasi-one-dimensional ionic conductor. The phenomena investigated are caused by the quasi-one-dimensional ionic current, and can be explained using the quantum theory of...