Laser Gain without Inversion, in a Solid

Early descriptions of lasers emphasized that a majority of participating atoms in a laser medium needed to have undergone a "population inversion." That is, most of the atoms had to be in an excited state, the better to be stimulated into emitting light and contributing to a growing pulse of laser light. But this "gain" can be achieved without inversion. Experiments have shown that by coherently controlling the electrons in ground-state atoms through a process called electromagnetically induced transparency, the electrons could mostly be kept from absorbing laser light being developed among the small number of atoms in a sample actually in an excited state.

This gain-without-inversion (GWI) phenomenon has now been demonstrated in a solid material for the first time. Speaking at last week's Institute of Physics Condensed Matter and Materials Physics Conference in Exeter, U.K., Chris Phillips of Imperial College London said that his lab achieved GWI in an array of semiconductor nanostructures -- in effect, artificial atoms. Not only gain, but slowing of light can be achieved in the Imperial College solid state arrangement, making it possibly useful for future quantum information applications.

See also Frogley et al., Nature Materials, March 2006