Simultaneous determination of average direction of molecular orientation and effective second order nonlinear optical constant (vertical bar d(eff)vertical bar) by phase measurements of second harmonic generation

We present a new analytical method of molecular orientation in films using a phase measurement of second harmonic generation (SHG) with a theoretical curve fitting. The new analytical method provides the average direction of molecules and the absolute value of the effective second-order nonlinear op tical constant (\d (eff)\) simultaneously. In conventional methods, the ave rage direction of molecules in a sample has been relatively determined with that in a standard sample having preknown molecular direction. The second harmonic (SH) interference fringe obtained between a reference having prekn own molecular direction and the sample has been compared with that between the reference and the standard sample. In the new method, however, the aver age direction of molecules can be obtained without the standard sample. The value of \d (eff)\ is calculated through the fitting process, the average direction of molecules is determined by the sign or the phase of d (eff) wh ich is introduced by a reference. Complete expression with physical meaning s is given for the valley points and the contrast of SH interference fringe . There, effect of a lens to focus a fundamental wave to a sample is also t aken into account. The new phase measurement of SHG with the curve fitting has been carried out with poled poly(methyl methacrylate) films doped with 2 wt % of p-nitroaniline (PNA/PMMA), which were used as a reference and als o as a sample. The accuracy of the new method has been demonstrated in comp arison of the value \d (eff)\ of the sample with that determined by Maker f ringe measurement. The determined average direction of molecules for the sa mple was confirmed by experimental geometry of the reference and the sample and also from the poling geometry for the sample. We have shown that the p hase measurement of SHG and the curve fitting are useful tools to investiga te the molecular orientation.