These diagrams are essentially a shorthand for the complex nested integrals that define the 3rd-order response 5. Why "Fixed" Matters: The Practical Path

In the textbook, you will see lots of vector math: $\veck_sig = \pm \veck_1 \pm \veck_2 \pm \veck_3$.

Consider a standard three-pulse photon echo (a (\chi^(3)) experiment):