Local field and optical second harmonics generation in polar Langmuir-Blodgett films

A.N. Vtyurin and A.N. Botvich

L.V.Kirensky Institute of Physics and Krasnoyarsk Technical University, Krasnoyarsk, 660036, Russia

Langmuir-Blodgett (LB) technique becomes now an effective tool producing thin molecular films of desired composition and structure. Its development gave rise to numerous experimental investigations of LB films as nonlinear optical wave guides devices. It's obvious that long organic molecules of LB films may be modified easily to increase their nonlinear optical parameters up to very high values, and well ordered structure of the films provides them with high macroscopic nonlinearities, so this comparatively simple technology gives us a means to synthesize rather complex structures of desirable properties.

It's clear that for such dense and highly anisotropic medium molecular packing and intermolecular interactions should effects strongly on the total macroscopic parameters. Therefore to design LB structure of desirable optical parameters one should connect molecular characteristics, structure of the film under investigations and total macroscopic parameters. For this reason we've tried to simulate effects on molecular and supramolecular structures on internal fields and macroscopic nonlinear properties of LB films.

Several types of highly nonlinear molecules have been chosen for investigations, namely: homogeneous unimolecular stack, stacks of different molecules with alternating orientations, single molecule nonlinear layer on a passive buffer stack.

Calculated nonlinear susceptibilities are in reasonable agreement with available experimental values. Uniform stacks show strong dependences on their thicknesses with odd-even oscillations; that explains rather unstable experimental results obtained with such films. Alternating stacks give nonlinear susceptibilities practically independent on their thickness starting from two-layer structure. Thickness of the buffer layer effects on the film nonlinearity only been of the same order as the thickness of nonlinear cover. Macroscopic properties of all simulated films converge very quickly to the values of the bulk medium, giving practically their bulk values for 10- to 20-layer films. It's interesting to point out, that good agreement has been obtained assuming molecular parameters independent on molecule's position in the film. It seems to show that effects of LB films structure on their nonlinear optical parameters are mainly connected with specifics of their local field distribution.