Enhancement of the third-order nonlinear optical susceptibility in Si quantum wires

Recent observation of efficient light emission from porous silicon has attracted much attention and renewed interests in the study of nonlinear optical properties of nanometer-sized quantum systems. In this paper, we study the third-order nonlinear optical susceptibility of semiconductor quantum wires. The quantum wires are taken to be circular columns with a cross section size of {~}1 nm. The excitonic effects are taken to be the major electronic excitations. We find that the quantum confinement of the excitons greatly enhances the third-order optical nonlinear susceptibility in a quantum wire. The source of the enhancement is primarily the confinement-induced localization of excitons. The large enhancement of the third-order optical nonlinearity estimated here is consistent with the recent observation of the efficient infrared-up-conversion luminescence in porous silicon.