Intermolecular interactions in linear and nonlinear susceptibilities: beyond the local-field approximation

Reduced equations of motion for material and radiation field variables in a molecular crystal are presented that allow us to calculate linear- and nonlinear-optical susceptibilities, accounting in a systematic way for intermolecular interactions. These equations are derived starting from the multipolar (μ · D) Hamiltonian, and to second order in the molecular dipole they reduce to the Bloch–Maxwell equations with local-field corrections. The dielectric function obtained through our approach incorporates retarded interactions in a consistent way and is compared with the existing exciton polariton theory, which is based on the minimal coupling (p · A) Hamiltonian. We find that, unlike with the conventional polariton theory, spontaneous emission is not suppressed in an infinite crystal.