Fig. 4: Quantum view of two-dimensional terahertz spectroscopy.

Double-sided Feynman diagrams describing possible optical nonlinearities for the three-level ladder system shown. Diagrams are constructed according to the rules detailed in the main text, assuming an initial density matrix \(\left\vert 0\right\rangle \left\langle 0\right\vert\). In the case of two-pulse excitation, each pulse may act either once or twice (contribute one or two field interactions) prior to the time delays τ and t as indicated.