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Embedded germanium nanocrystals (NCs) in a silica host matrix aretheoretically analyzed to identify their third-order bound-statenonlinearities. A rigorous atomistic pseudopotential approach is used fordetermining the electronic structure and the nonlinear opticalsusceptibilities. This study characterizing the two-photon absorption,nonlinear refractive index, and optical switching parameters reveals the fullwavelength dependence from static up to the ultraviolet spectrum and the sizedependence up to a diameter of 3.5 nm. Similar to Si NCs, theintensity-dependent refractive index increases with decreasing NC diameter. Onthe other hand, Ge NCs possess about an order of magnitude smaller nonlinearsusceptibility compared to Si NCs of the same size. It is observed that thetwo-photon absorption threshold extends beyond the half band-gap value. Thisenables nonlinear refractive index tunability over a much wider wavelengthrange free from two-photon absorption.

Bound-state third-order optical nonlinearities of embedded germanium nanocrystals