Photon merging and splitting in electromagnetic field inhomogeneities

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We investigate photon merging and splitting processes in inhomogeneous, slowly varying electromagnetic fields.

• Our study is based on the three-photon polarization tensor following from the Heisenberg-Euler effective action.
• We put special emphasis on deviations from the well-known constant field results, also revisiting the selection rules for these processes.
• In the context of high-intensity laser facilities, we analytically determine compact expressions for the number of merged/split photons as obtained in the focal spots of intense laser beams.