Coherent Superposition of Multi - Exciton Complexes in Semiconductor Nanocrystals

Very efficient multi-exciton generation has been recently observed in nanocrystals where an optically excited electron-hole pair with an energy greater than the bandgap ($E_g$) produces one or more additional electron-hole pairs [1,2]. We present a theory of multiple exciton generation in nanocrystals. We have shown that very efficient and fast exciton generation in nanocrystals occurs by the optical excitation of a coherent superposition of multi-exciton states by a single photon. This model explains ultrafast dynamics of optical bleaching that arises from state filling including quantum beats between the multi-exciton states. We have also shown that although highly efficient multiple exciton generation begins at photon energy $3E_g$, the threshold of multiple exciton generation is $2E_g$ not, $3E_g$ as was suggested previously. \newline 1. R. Schaller and V. Klimov, Phys. Rev. Lett. {\bf 92}, 186601 (2004). \newline 2. R. J. Ellingson, M. C. Beard, P. Yu, O. I. Micic, A. J. Nozik, A. Shabaev, and Al. L. Efros, submitted.