Hello, I'm studying basic nonlinear optics and I would like to solve a couple doubts about (basic) photon interaction. Let a monocromatic (of frequency ω) electromagnetic field propagate through a nonlinear medium and let the third(and higher)-order terms in the relation between the polarization density P and the electric field E be negligible. I've read* that the second harmonic generation is based on two photons of frequency ω combining to produce a photon of frequency 2ω. I would like to understand the practical meaning of "combining" in this context: perhaps it's like "colliding"? May anyone help me? Thanks in advance. *Saleh, Teich - "Foundamentals of Photonics"