L.C. Andreani, D. Bajoni, I. Carusotto, M. Galli, L. O’Faolain, T.F. Krauss, for stimulating discussions concerning the physics and the realization of this proposal. This work was partly supported by NanoSci Era-NET project “LECSIN” coordinated by F. Priolo. One of the authors (S.F.) was supported by Fondazione Cariplo under project 2010-0523.

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