CONTROLLING THE SIGN OF QUANTUM INTERFERENCE BY TUNNELING FROM QUANTUM-WELLS

The sign of the interference (constructive or destructive) between qua ntum-mechanical paths depends on the phase difference between the path s, In the Fano effect(1) two optical paths from the ground state of a system-one direct and one mediated by a resonance-to a state in an ene rgy continuum interfere to produce an asymmetric absorption spectrum t hat falls to zero near the absorption maximum. Zero absorption occurs as the wavelength is scanned across the resonance, at a photon energy corresponding to a 180 degrees phase difference between the paths, Sim ilar interference effects occur when two absorption paths are mediated by two different states, and they provide the basis for lasers that o perate without a population inversion(2-7). Here we report the control , by quantum mechanical tunnelling, of interference in optical absorpt ion. The two intermediate states are resonances that arise from the mi xing of the states in two adjacent semiconductors quantum wells, which are broadened by tunnelling into the same energy continuum through an ultra-thin potential-energy barrier, Inverting the direction of tunne lling by reversing the position of the barrier with respect to the two quantum wells changes the interference from destructive to constructi ve, as predicted theoretically, This effect might provide a way to mak e semiconductor lasers without population inversions(8).