Published online by Cambridge University Press: 21 February 2011
The non-resonant third order nonlinearity of conjugated polymers appears tobe potentially useful for all-optical devices in waveguide formats.[l,2]This nonlinearity manifests itself as an intensity-dependent refractiveindex which leads to a nonlinear phase shift over some propagation distance.Device research over the last few years has shown that there are certainminimum requirements for the nonlinear phase shift that need to be achievedover one absorption length of the material.[l,3] There are two principalsources of absorption, the usual linear absorption which is independent offluence, and two photon absorption for which the absorption scales linearlywith intensity. Thus the usefulness of a nonlinear material for all-opticalswitching devices can be evaluated from a limited number of materialparameters, namely n2 (in n = n0 + n2Iwhere I is the local intensity), α0 which is the low powerabsorption coefficient and β which is the two photoncoefficient (in α = α0 + βI). The problem for agiven material is to identify spectral regions over which the minimumrequired phase shift can be achieved.