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Controlling the sign of quantum interference by tunnelling from quantum wells

Abstract

The sign of the interference (constructive or destructive) between quantum-mechanical paths depends on the phase difference between the paths. In the Fano effect1 two optical paths from the ground state of a system — one direct and one mediated by a resonance — to a state in an energy continuum interfere to produce an asymmetric absorption spectrum that 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 ° phase difference between the paths. Similar interference effects occur when two absorption paths are mediated by two different states, and they provide the basis for lasers that operate without a population inversion2,3,4,5,6,7. Here we report the control, by quantum mechanical tunnelling, of interference in optical absorption. The two intermediate states are resonances that arise from the mixing 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 tunnelling by reversing the position of the barrier with respect to the two quantum wells changes the interference from destructive to constructive, as predicted theoretically. This effect might provide a way to make semiconductor lasers without population inversion8.

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Figure 1: Schematic band diagram of the quantum well structures used to demonstrate quantum interference in optical absorption from the ground state |1〉 to the two continuum resonant states |2〉 and |3〉 which are predominantly broadened by tunnelling.
Figure 2: Calculated conduction-band diagram of a portion of the two grown semiconductor structures consisting of a deep 7.3-nm-thick GaAs well coupled to a shallow 6.8-nm-thick Al0.165Ga0.835 As well by a 2.5-nm-thick Al0.33Ga0.67 As barrier.
Figure 3: Top: computed absorption for the two structures with the two different locations of the tunnel barrier coupling the step-coupled well to the continuum, as shown by the schematic band diagrams (insets).

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Acknowledgements

We thank S. Harris, A. Imamoglu, R. J. Ram, A. Tredicucci and C. Gmachl for enlightening discussions, and the authors of ref. 11 for making their manuscript available before publication. The work was supported in part by DARPA/ARO.

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Correspondence to Federico Capasso.

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Faist, J., Capasso, F., Sirtori, C. et al. Controlling the sign of quantum interference by tunnelling from quantum wells. Nature 390, 589–591 (1997). https://doi.org/10.1038/37562

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