  We use finite-difference time-domain modelling to investigate plasma
generation induced by multi-photon absorption of intense laser light in
dielectrics with tiny inhomogenities. Plasma generation is found to be strongly
amplified around nanometer-sized inhomogeneities as present in glasses. Each
inhomogeneity acts as the seed of a plasma structure growing against the
direction of light propagation. Plasma structures originating from randomly
distributed inhomogeneities are found to interact strongly and to organize in
regularly spaced planes oriented perpendicularly to the laser polarization. We
discuss similarities between our results and nanogratings in fused silica
written by laser beams with spatially homogeneous as well as radial and
azimuthal polarization.
