I think d(phi) = “value” as the relative fixed phases of the fields E1 ... En can be well undestood and explained if we compare this case with energy exchange between a driving force and a spring. The relative phase will provide the direction of flow of energy. By building the quantity P power = dE/dt we can find in order for energy to flow from driver to the oscillating mass (undampled) the phase difference is essential. Same way + or - pi/2 in SFG or other cases can be undestood.
I think d(phi) = “value” as the relative fixed phases of the fields E1 ... En can be well undestood and explained if we compare this case with energy exchange between a driving force and a spring. The relative phase will provide the direction of flow of energy. By building the quantity P power = dE/dt we can find in order for energy to flow from driver to the oscillating mass (undampled) the phase difference is essential. Same way + or - pi/2 in SFG or other cases can be undestood.