Efficient frequency doubling in Lithium Niobate waveguides by Reverse Proton Exchange

{We numerically investigate efficient frequency doubling of near infrared light in a coupled system of buried and surface waveguides obtained by Reverse Proton Exchange in z-cut Lithium Niobate. For a monomode TE surface guide at 1.32 mum and a highly multimode TM (buried) guide at 666nm and exploiting the d(31) nonlinear tensor element, for planar structures we calculated conversion efficiencies as high as 14\% mum/W cm, with a weak dependence on temperature. Noticeably, this geometry features the physical separation of the harmonics at the output.}