If the intensity of a light beam is strong enough, the electric field gets strong enough to spark the vacuum. That is, you get electron/positron pairs just like you get electron/ion pairs if you spark the air.
One way to think about it is that there are always "virtual particles" in the vacuum but normally they have a subtle effect because they come into existence and then disappear. If the electric field is strong you can separate them entirely from each other before they recombine, and in doing so pay the energy debt to make the particles "real".
This does not affect the "fabric" of space time in the sense of changing the metric or connectivity, or doing anything interesting with gravity.
It does recreate conditions at the point where the temperature was about 1 MeV in the big bang though.
It is different from this sort of thing though:
http://cms.web.cern.ch/news/lhc-photon-collider
in that the photons in the light beam are not high energy photons, but instead are low energy photons, just a whole lot of them. Thus it is probing strongly nonlinear conditions (e.g. they are breaking the "electromagnetic field" so to speak)
It is very interesting physics but unlikely to change how we see the world.