Laser gain occurs without inversion

Despite the thermodynamic argument that more than half of the atoms in a laser medium need to undergo population inversion for lasing to occur, quantum theory predicts that matter/wave interference effects inside the atoms can allow gain without inversion (GWI).

Jun 1st, 2006

Despite the thermodynamic argument that more than half of the atoms in a laser medium need to undergo population inversion for lasing to occur, quantum theory predicts that matter/wave interference effects inside the atoms can allow gain without inversion (GWI). Now, researchers at Imperial College (London, England) and the University of Neuchâtel (Neuchâtel, Switzerland) have demonstrated GWI for the first time in a solid.

They achieved the coherent conditions that are needed to observe this strange effect by using tailored electron states in “artificial atom” semiconductor nanostructures-layers so thin that they confine electrons in states whose energies can be chosen at will. By making two of the optical transitions overlap in energy, and using a coupling laser beam to make the electron wave functions interfere, they produced gain at an energy 30% above the coupling beam, even though the majority of the electrons were in the ground state. The physics are transferable to other materials and wavelengths and also allow observations of slowed-light propagation, useful for quantum computing and related applications. Contact Chris Phillips at chris.phillips@imperial.ac.uk.

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