Abstract

In atoms and molecules, nonresonant second-order and third-order optical processes mediated by virtual electronic excitations are described by their second-order (beta) ijk(-(omega) 3;(omega) 1,(omega) 2) and third-order (gamma) ijkl(-(omega) 4;(omega) 1,(omega) 2,(omega) 3) optical susceptibilities, respectively. In general, the real population of the initial state for the virtual electronic excitations can be either the usual singlet ground state S0 or an optically pumped excited state Sn. The authors have found for quasi-one (1-D) and quasi-two (2-D) dimensional chain-like and disc-like structures, compared to the ground state, the nonresonant excited state (beta) ijkSn(-(omega) 3;(omega) 1,(omega) 2) or (gamma) ijklSn(-(omega) 4;(omega) 1,(omega) 2,(omega) 3) can markedly increase, and even change sign, when the first (S1) or second (S2) (pi) -electron excited state is optically pumped and populated for timescales sufficiently long to allow nonresonant measurements of (beta) ijkSn(-(omega) 3;(omega) 1,(omega) 2) or (gamma) ijklSn(-(omega) 4;(omega) 1,(omega) 2,(omega) 3).

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