[adrian_b]

Right. This is how you make a laser.

When passing through a material that is at thermal equilibrium, light is attenuated, because a part of it is absorbed. On the other hand, if the material is not at thermal equilibrium and you prepare it to have a negative temperature (by "pumping", so that more molecules/atoms/ions are in states with higher energy than they are in states with lower energy), then when light passes through the material it can be amplified (by stimulated emission), instead of being attenuated, like through a material at positive temperature.

Of course, in any laser/maser material, only a very small fraction of its constituents have an energy distribution corresponding to a negative temperature, there is no known method that could force a whole piece of a material to have a negative temperature (because the energy would redistribute spontaneously inside the material towards what corresponds to a positive temperature, faster than energy could be provided from the outside; lasers use some special energy states that have a low probability of decaying spontaneously, so they persist enough time after pumping).