Waveguides photo-written by femtosecond laser filament in chalcogenide glass

It is well known that when intense femtosecond pulse propagates in a transparent medium, self-focusing effect can occur due to the nonlinear refractive index (NRI) of the medium. In case of positive NRI the beam is focused in the medium and can even collapse if a critical power is reached. Due to the highly concentrated intensities in a small volume, a plasma can then be generated by multiphoton excitation of electrons (in condensed matter). A transient negative variation of the refractive index is induced by the plasma and defocuses the beam. So it exists a dynamical equilibrium between these two effects and consequently the natural divergence of the beam can be compensated over distances much larger than the Rayleigh range, a phenomena known as femtosecond pulse filament [1, 2].