Second-Order Nonlinearity of Mixtures Including <i>p</i>-Nitroaniline Derivatives

Abstract

We have demonstrated the evaluation of nonlinear optical properties and the determination of the molecular orientation of the mixed crystals of p-nitroaniline (p-NA) and N-ethyl-4-nitroaniline (NENA) in powder form. Although neither material shows second-order nonlinearity due to the crystalline structure, the obtained mixture shows high optical nonlinearity because it includes the mixed crystals, which are noncentrosymmetric. We measured the second-order nonlinear optical efficiency of the mixture using the second-harmonics with evanescent wave (SHEW) technique. The SHEW technique can be applied to the compound in powder form, so that not only the nonlinear optical property of a mixture, which consists of several materials, but also that of mixed crystals, which are difficult to make in a certain size of single crystal for the usual measurement such as the Maker fringe measurement, are able to be characterized. The efficiency of the mixture corresponded well with the area of the X-ray diffraction peak that appeared after mixing, i.e., the amount of the new mixed crystal. This indicates that the SHEW technique can be used as a monitor of the crystalline structure. With the results of the X-ray powder diffraction, the angular averaged second-order nonlinear coefficient of the newly formed mixed crystal was obtained to be 86 ± 15 pm/V. This result suggests that molecules are aligned almost parallel in the mixed crystal.