That’s a linear mixing, non-linear allows for the absorption of two photons followed by the emission of one with twice the energy. That allows for a laser pumpe

That’s a linear mixing, non-linear allows for the absorption of two photons followed by the emission of one with twice the energy. That allows for a laser pumped frequency doubler. Actually most consumer oriented lasers (for example green lasers) work like this, but I don’t know if it works for frequencies as high as soft x-rays. https://en.m.wikipedia.org/wiki/Second-harmonic_generation https://en.m.wikipedia.org/wiki/Second-harmonic_generation