Spectroscopy Using Sum- and Difference-Frequency Generation in Molecular Solids

Both sum- and difference-frequency generation in organic materials are demonstrated and explored under conditions where all three waves (ω₁ and ω₂ ingoing and ω₁±ω₂ outgoing) are resonant with molecular transitions. The experiments are carried out in centrosymmetric media in which a small electric field is used to distinguish the polar sublattices. Unique electric field dependences of the parametric signals were discovered. The spectroscopic line narrowing and dephasing-induced coherent-emission resonances of these χ⁽²⁾ processes make them useful new spectroscopic methods.