On the role of permanent dipoles in second harmonic generation

The significance of permanent electric dipoles in second harmonic generation is considered with regard to their role in both the non-linear susceptibility and also in the orientation of polar molecules. Under conditions of near-resonance with the first electronic excited state, the susceptibility is dominated by a two-level term governed by the difference between the ground and excited-state dipoles; this term also drives the harmonic conversion in electric field-induced second harmonic generation. If the resonant level is not the lowest electronic excited state, there is a secondary resonance feature which can contribute just as strongly as the two-level term, and which can dominate the harmonic conversion if the ground- and excited-state dipoles are either equal, or both zero.