Dipolar interaction assisted effects on second harmonic generation

Measurements of the second order nonlinear susceptibility by second harmonic generation (SHG) signal were made on the 2-methyl 4-nitroaniline/poly(methyl methacrylate) guest/host system as a function of the nonlinear optical (NLO) chromophore concentration and the poling field strength. The polar orientational order parameters (POP) L33 and L31 (defined in the text) were calculated from the SHG signals. The POPs were found to depend on the NLO chromophore concentration. The concentration dependence of POP is described in terms of Kirkwood’s g factor that describes the orientational pair correlation. The ratio of polar order parameters (L33/L31), measured in the weak poling field condition, is 3, independent of the NLO chromophore concentration; this result is in agreement with the result of a recent calculation. The usual Onsager local field factor, derived without considering orientational pair correlations, was shown to be inadequate. The static dielectric constant of the guest/host system is extracted from the order parameter, using the Kirkwood–Frölich theory of dielectrics.