We present a model for the description of a thin polymer layer as a set of noninteracting randomly oriented molecules in order to determine dependencies between third-order nonlinear tensor components. The model results in different numbers of independent tensor components for the three-dimensional and two-dimensional cases. Degenerate four-wave mixing measurements with different angles of incidence were experimentally performed to check relations between different tensor components in accordance with the sample thickness. Numerical calculations for diffraction in the Fraunhofer approximation were applied for degenerate four-wave mixing experiments on thin films in order to study the angle dependence of the signal beam. It was shown that the distribution has vividly unsymmetrical character. The difference in the nonlinear tensor values between the traditional and the proposed procedure is almost an order of magnitude.
S. Rozouvan and W. Schrepp, "Degenerate Four-Wave Mixing Measurements on Thin Polymer Films," Appl. Spectrosc. 54, 1399-1404 (2000)