Light propagating in photonic crystals behaves in a way that is analogous to electrons travelling through a semiconductor crystal. In other words, periodic structures can be used to control the flow of light in a way quite similar to electrons in semiconductor electronic devices. Much of the research in the EU-funded project INDIGO (Interaction of nonlinearity and disorder: Gateways to optics) was directed at exploring these periodicity effects in photonic crystals. The focus was set on a little-studied phenomenon: the thermalisation of coupled light fields in the presence of cross-phase modulation and wave mixing. In the framework of nonlinear optics, this situation corresponds to the propagation of polarised light in birefringent materials. The INDIGO team used aluminium gallium arsenide as a sample material with cubic symmetry and fused silica as the corresponding example of an isotropic crystal. The evolution of the system to the final equilibrium state was shown to pass through an intermediate stage where the energy exchange between waveguides is negligible. On the other hand, the distribution of phase differences, intensity and polarisation state of each waveguide is strongly dependent on the material parameters and initial conditions. Project research was the first attempt to study polarisation effects on thermalisation in discrete optical waveguide systems. The findings are expected to have a significant impact on signal processing with fibre communication systems. They are also quite general and can be applied to other non-linearly coupled systems. As a possible continuation of this research, INDIGO scientists are considering a detailed study of the material dependence on final thermal state and a full thermodynamical analysis.
Nonlinear medium, light scattering, nonlinearity, photonic crystal, optics, thermalisation