Observation of enhanced biexcitonic effect in semiconductor nanocrystals

We have observed enhanced biexcitonic effect in CdS0.6Se0.4 nanocrystals by means of femtosecond two-photon pump-probe spectroscopy. In contrast to a weak biexcitonic photoinduced absorption in the presence of \ 1S(e) 1S(h)> or \ 1P(e) 1P(h)> excitons due to one-photon excitation. we have found a pronounced photoinduced absorption arising from a strong Coulomb coupling between a \ 1S(e) 1 S-h> exciton and a two-photon excited \ 1S(e) 1 P-h> exciton. We have also theoretically shown that \ 1S(e) 1S(h): 1S(e) 1P(h)> biexcitons have a large binding energy compared with \ 1S(e) 1S(h); 1S(e) 1S(h)> and \ 1S(e) 1S(h); 1P(e) 1P(h)> biexciton states.