Investigation of the second-harmonic generation containing two linearly polarized states in KTiOPO₄ crystals

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 (e^2ω ₂) at a doubled frequency, i.e. e ^ω ₁ + e ^ω ₂ → e^2ω ₂. However, under a d.c. field applied along the c-axis of the crystal, not only is the e^2ω ₂ ray generated, but a slow ray (e^2ω ₁) at the doubled frequency is also generated. The new e^2ω ₁ ray is also linearly polarized. The 90° angle between the polarization direction of the e^2ω ₁ ray and that of the e^2ω 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 photons.