I think I understand why two level laser is not possible if we use optical pumping. However, I don't understand why we can't create laser with two energy levels that are pumped by non optical pump? Why we can't create population inversion that way? For example, if we could somehow pump electrons from level 1 to level 2 with some collision processes then we could create population inversion on the level 2, right? Then electrons from level 2 would be able to go back to level 1 with stimulated emission. Why is this not possible?