Optical methods offer an intrinsic high potential for experiments with excellent spatial and in particular also temporal resolution. Using the Faraday effect we carried out magneto-optical investigations of high-Tc superconductor thin films in a polarization microscope. Small defects in the superconducting material which possess a lower critical current density disturb the homogeneous penetration of magnetic flux into a sample when an external magnetic field is applied after zero field cooling. This is true even if the defects are below the sample surface or when the superconducting sample is covered by a thin layer of another material, e.g., gold, and can be used to characterize samples with diameters up to 3". For studies of the dynamics of magnetic flux in a superconducting sample, a pump-probe setup has been used. An instability which causes magnetic flux to enter the sample in dendritic form [1] is triggered by local heating with a focused ns or fs laser pulse. Part of the beam is separated by a beam splitter, passed through a variable delay line of suitable length and used for illumination of the sample. ForYBa2Cu307- 6 thin films a spreading velocity of (5 + 2) x 1o4 m/s is found which is an order of magnitude higher than the velocity of sound. The total area of the dendritic structure formed is found to depend linearly on the change Bext of the external magnetic flux density applied before the trigger pulse. No dependence on the pulse duration has been observed, suggesting a purely thermal nature of the trigger process
