Terahertz second-harmonic generation by excited octupolar molecules

We present a unified theoretical model for the nonlinear terahertz response of an excited octupolar molecule in an electromagnetic field. The model incorporates strong vibronic interactions (the Jahn-Teller effect) and dipole-field interaction, enabling a comprehensive analysis of the zeroth (permanent), first, and second harmonics of the induced molecular dipole moment. A key finding is the contrasting response of the harmonics to rotation of the wave's polarization plane: the first-harmonic component of the dipole moment remains locked collinear to the electric field, whereas the zeroth- and second-harmonic components rotate in the opposite direction at twice the angle. This counter-rotation originates from the C3 symmetry of the molecule and provides for arbitrary mutual orientation of the first-harmonic component relative to the zeroth- and second-harmonic components, thereby enabling direct measurements of these two contributions. Moreover, the response exhibits strong resonances at well-defined frequencies governed by the vibronic interaction parameters, giving rise to clearly identifiable spectral features.