Are the new particles created from the photons... conservation of energy/mass from E = m * c^2?
I think the energy would come from what was previously the zero-point energy of the empty space (sometimes called vacuum energy). The theory is that even empty space (without even photons passing through) has a whole bunch of energy associated with it, but kind of equal and opposite, so it doesn’t seem to.
Not from the photons per se. Rather the laser is used to free matter and antimatter from the quantum stew we’ve all heard so much about.