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Femtosecond optical reflectivity measurements of La 2-x Sr x CuO 4 , La 2 CuO 4+y , Bi 2 Sr 2 CuO 6+z and Bi 2 Sr 2 CaCu 2 O 8+δ thin films and single crystal samples indicate qualitative changes with fluence. At the lowest fluencies, there is a power law divergence in the relaxation time. The divergence has an onset temperature of 55±15K, independent of whether the sample is in the superconducting or normal states. At slightly higher fluencies, still perturbative, the additional response does not exhibit this power law divergence. At quite high fluencies-no longer perturbative-the metallic samples exhibit oscillations in the reflectivity amplitude. The period of these oscillations varies with the probe wavelength but not with the pump wavelength. The oscillations exhibit a decay time as long as 10 nsec.

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We present a time-resolved magneto-optical (MO) imaging study of high-temperature superconductor (HTS) in high-frequency alternating current (AC) regime. The evolution of the magnetic flux density distribution in YBa 2 Cu 3 O 7-δ (YBCO) thin film samples is studied in small steps of the phase of the applied AC current. The flux distribution at 10 K exhibits instabilities including flux jumps and flux creep. A quantitative analysis of the data allows us to obtain the current density evolution. The current profile changes considerably with the phase differently from the prediction of the critical state model. These observations can be explained by the higher self-field at the sample edge and the effects of flux creep.