An optical waveguide with a nonlinear optical susceptibility inversion structure in the thickness direction

Abstract

A nonlinear optical susceptibility (χ(2)) inversion structure in the thickness direction (NIST) is proposed for an optical waveguiding layer to achieve efficient second-harmonic generation (SHG) in the form of Cerenkov radiation. Using a coupled-mode analysis, it is found that the SHG efficiency of the Langmuir–Blodgett (LB) film waveguide incorporating NIST is several tens of times larger than that of the conventional LB film waveguide. 2-docosylamino-5-nitropyridine (DCANP) was used as the LB film material. High precision is not so strongly required for the inversion position in the waveguiding layer thickness as for the total thickness of the waveguiding layer, if the refractive indices are almost uniform in the layer. A guiding principle is shown for selecting suitable materials for the substrate and the clad to achieve the highest efficiency with Cerenkov angle approaching zero. A waveguide structure was designed using a kind of perfluoropolymer for the clad. Several fabrication methods are proposed for the NIST waveguide. An enhancement of 20–30 times in the efficiency of Cerenkov radiation SHG due to the NIST is also experimentally demonstrated using DCANP-LB film as the waveguiding layer.