Langmuir-Blodgett Film Waveguide with the Nonlinearity Inversion in the Thickness Direction
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NOBUTOSHI ASAI, ICHIRO FUJIWARA, HITOSHI TAMADA, AND JUN'ETSU SETO Sony Corporation Research Center, 174 Fujitsuka-cho, Hodogaya-ku,Yokohama 240, Japan
ABSTRACT A second order nonlinear optical susceptibility inversion structure in the thickness direction (NIST) is proposed for enhancing the efficiency of Cerenkov radiation type phase matched second harmonic generation (CRPM-SHG). With the aid of the coupled-mode theory, it is analyzed that the efficiency is enhanced by 50 to 100 times by introducing the NIST structure to a waveguiding layer made of a 2-docosylamino-5-nitropyridine (DCANP) Langmuir-Blodgett (LB) film, and that the optical conditions for the most efficient CRPM-SHG are equivalent to those for guided mode SHG at the cutoff points of the modes. Several fabrication methods are possible for NIST waveguides. For the first time, a NIST waveguide has been fabricated using a DCANP-LB film by employing a novel and simple technique in which the substrate inversion is carried out during the dipping process. The inversion in nonlinearity was confirmed using the Maker fringe method and polarized absorption spectra. A NIST planar waveguide of 260 monolayers of DCANP (570 nm thick) was fabricated. Intense CRPM-SHG was observed from the waveguide when a YAG laser was coupled into it. The intensity was 20 to 30 times larger than that of a conventional waveguide of the same thickness.
INTRODUCTION Organic materials for nonlinear optical applications are currently the subject of intensive research because of their large second and third order susceptibilities. In particular, the application of second order nonlinearity for efficient second harmonic generation (SHG) is attracting much attention because it can be applied to the production of small size visible (blue and green) coherent light sources for use in highly density optical memories, small laser displays, high resolution laser printers and so on. For the second order nonlinearity to appear, materials must have a noncentrosymmetric structure. For high SHG conversion efficiencies, optical waveguide structures have many advantages since the waveguides are able to propagate the light beam with very high density over a long distance. The Langmuir-Blodgett (LB) deposition technique is attractive because it can be used prepare the noncentrosymmetric waveguide structure. While there have been many reports concerning nonlinear optical LB films, few have been discussed phase matched SHG using LB film waveguides[l,2,3]. In this paper, we propose a novel LB film structure for efficient Cerenkov radiation type phase matched SHG (CRPM-SHG) and demonstrate the principle using 2-docosylamino-5-nitropyridine (DCANP). 91 Mat. Res. Soc. Symp. Proc. Vol. 328. @1994 Materials Research Society
CONCEPT We have proposed a multilayer type optical waveguide structure in which the directions of the molecular dipole moments are oriented antiparallel between neighboring layers as shown in Fig. I. The diagonal components of X(2) also change sign due to the direction inversion of the dip
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