Gain without inversion in semiconductor nanostructures

When Einstein showed that light amplification needed a collection of atoms in ‘population inversion’ (that is, where more than half the atoms are in an excited state, ready to emit light rather than absorb it) he was using thermodynamic arguments 1 . Later on, quantum theory predicted 2 , 3 that matter–wave interference effects inside the atoms could, in principle, allow gain without inversion (GWI). The coherent conditions needed to observe this strange effect have been generated in atomic vapours 4 , but here we show that semiconductor nanostructures can be tailored to have ‘artificial atom’ electron states which, for the first time in a solid, also show GWI. In atomic experiments, the coherent conditions, typically generated either by coupling two electron levels to a third with a strong light beam 2 , 3 or by tunnel coupling both levels to the same continuum (Fano effect 5 ), are also responsible for the observation of ‘electromagnetically induced transparency’ (EIT) 6 . In turn, this has allowed observations of markedly slowed 7 and even frozen 8 light propagation. Our ‘artificial atom’ GWI effects are rooted in the same phenomena and, from an analysis of the absorption changes, we infer that the light slows to ∼ c /40 over the spectral range where the optical gain appears.