Fig. 2 | Nature Communications

Fig. 2

From: Direct observation of exciton–exciton interactions

Fig. 2

Origin of the direct-double-excitation EEI signal. Right: Energy scheme of a four-level system with indicated relaxation from its second excited state (red arrow). Left: Double-sided Feynman diagrams of the rephasing EEI signal for direct double excitation. Some of the diagrams are present twice (as indicated) due to multiple possible time orderings of two interactions with the same laser pulse. The diagrams are classified as stimulated emission (SE), ground-state bleach (GSB), and excited-state absorption (ESA) with numerical indices that label the states of the final transition. The population relaxation replaces two of the diagrams by a new pair as indicated by the red boxes

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