FIG. 2.

CTPL diagrams for the contributions to the pump-probe signal which contain ω₁+ω₂=ω_fg resonances. By convention, the last interaction is an absorption and occurs at the left branch of the loop. Diagrams i–iv represent k₁ (pump) and k₂ (probe)[v–vii are k₁ (probe) and k₂ (pump)]. The top row diagrams represent pump modulated probe (PMP) where the system ends in the singly excited state |e〉〈e|. The bottom row diagrams represent two-photon absorption (TPA) where the matter ends in the doubly excited state |f〉〈f|.