  Third-order bound-charge electronic nonlinearities of Si nanocrystals (NCs)
embedded in a wide band-gap matrix representing silica are theoretically
studied using an atomistic pseudopotential approach. Nonlinear refractive
index, two-photon absorption and optical switching parameter are examined from
small clusters to NCs up to a size of 3 nm. Compared to bulk values, Si NCs
show higher third-order optical nonlinearities and much wider two-photon
absorption threshold which gives rise to enhancement in the optical switching
parameter.
