Nonlinear Optical Susceptibilities of a One-Dimensional Semiconductor

By use of the Keldysh nonequilibrium Green's-function methods, analytic expressions are computed for the nonlinear optical susceptibilities for the one-dimensional semiconductor model of a conducting polymer. The model has no adjustable parameter. The results for third-harmonic generation are in excellent agreement with experiments on polyacetylene. Further nonlinear optical effects, which should be experimentally observable, are predicted. The physical significance of these results is discussed.