This is fundamentally incorrect. Again, the distinction exists at the level that most people learn these materials, not at their academic level. I learned how second harmonic generation physics versus two photon physics from the people that actually invented these technologies. Both of them. I have published with two of them and one scientist has even been publishing in third harmonic physics for a few years because of third harmonics based on a laser he made and he proved it based on third harmonic effects theorized by the other guy. Your comment is really silly, and dumb and it shows how immature the posts on reddit are because frequently your 'guys' are all the same person.