EFFECTS OF THE ORIENTATIONAL PAIR CORRELATION ON 2ND-ORDER NONLINEAR OPTICAL COEFFICIENTS

The orientational order parameter associated with the second order nonlinear optical (NLO) coefficient is calculated by using a statistical mechanics method that includes the angular dependent intermolecular interaction potential. It is shown that in addition to the alignment of dipoles due to the poling field, the orientational pair correlation (OPC) factor is also important in determining the orientational order parameter. When OPC is included, the order parameter becomes a function of the NLO chromophore concentration. The concentration dependent part of the order parameter is shown to be proportional to the Kirkwood g factor associated with the orientational correlation of the electric dipoles. In the weak field limit, it is shown that the ratio of two nonlinear optical coefficients, chi33/chi31, remains equal to 3, independent of the specific nature of the anisotropic intermolecular potential.