Material Requirements For All-Optical Devices:Nonlinear Properties of Poly-4BCMU

The non-resonant third order nonlinearity of conjugated polymers appears to be potentially useful for all-optical devices in waveguide formats.[l,2] This nonlinearity manifests itself as an intensity-dependent refractive index which leads to a nonlinear phase shift over some propagation distance. Device research over the last few years has shown that there are certain minimum requirements for the nonlinear phase shift that need to be achieved over one absorption length of the material.[l,3] There are two principal sources of absorption, the usual linear absorption which is independent of fluence, and two photon absorption for which the absorption scales linearly with intensity. Thus the usefulness of a nonlinear material for all-optical switching devices can be evaluated from a limited number of material parameters, namely n2 (in n = n0 + n2I where I is the local intensity), α0 which is the low power absorption coefficient and β which is the two photon coefficient (in α = α0 + βI). The problem for a given material is to identify spectral regions over which the minimum required phase shift can be achieved.