INVESTIGATION OF THE 2ND-HARMONIC GENERATION CONTAINING 2 LINEARLY POLARIZED STATES IN KTIOPO4 CRYSTALS

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(1)(omega)) and a fast ray (e(2)(omega)) at a fundament al frequency (the refractive index relationship is n(e1) > n(e2)) can only be transformed into a fast ray (e(2)(2 omega)) at a doubled frequ ency, i.e, e(1)(omega) + e(2)(omega) --> e(2)(2 omega). However, under a d.c. field applied along the c-axis of the crystal, not only is the e(2)(2 omega) ray generated, but a slow ray (e(1)(2 omega)) at the do ubled frequency is also generated. The new e(1)(2 omega) ray is also l inearly polarized. The 90 degrees angle between the polarization direc tion of the e(1)(2 omega) ray and that of the e(2)(2 omega) ray is ind ependent of thickness in the transmission direction of the SHG element . KTiOPO4 is a typical quasi-one-dimensional ionic conductor. The phen omena investigated are caused by the quasi-one-dimensional ionic curre nt, and can be explained using the quantum theory of photons.