PREPARATION AND CHARACTERIZATION OF A NOVEL LANGMUIR-BLODGETT-FILM OPTICAL WAVE-GUIDE WITH NONLINEAR OPTICAL SUSCEPTIBILITY INVERSION

We demonstrate that a novel Langmuir-Blodgett (LB) film structure with a second-order non-linear optical susceptibility inversion structure in the thickness direction (NIST) can be prepared by the substrate inversion method in which the substrate is turned upside down during the film deposition process. In an NIST structure, the sign of the non-linear optical susceptibility is designed to be inverted at the position where the electric field of the second harmonic wave has a node in the thickness direction of the LB film optical waveguide. A 2-docosylamino-5-nitropyridine LB film is used for the preparation of an NIST structure. From an investigation of the structure using the Maker fringe method, polarized UV absorption spectroscopy and X-ray diffraction, it was indicated that the LB film with an NIST structure was a highly ordered Y-type LB film (bilayer spacing, 4.45 nm) and that the difference from the usual Y-type structure was that the non-linear optical susceptibility was inverted at a designed position in the thickness direction. The interference pattem of the absorption spectrum indicates that the optical scattering due to disorder in the film was weak enough to allow good transparency in the thickness direction. Coherent green second harmonic generation light was observed both from the NIST structure and from the normal structure optical waveguides. The measured Cerenkov angle was about 5-degrees for both of the waveguides. This suggests that the homogeneity in the thickness direction of the linear optical constants of the optical waveguide with the NIST structure is in fair agreement with that with the normal structure.