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Abstruct-Spontaneous, highly periodic, often permanent surface gratings or "ripples" can develop on the surface of almost any solid or liquid material illuminated by a single laser beam of sufficient intensity, under either pulsed or CW conditions. The grating periods are such that the incident laser beam is diffracted into a tangential wave which skims just along or under the illuminated surface. These spontaneously appearing surface ripples are generated by a runaway growth process analogous to stimulated Brillouin or Raman scattering or smallscale self focusing, but having many of the same properties as Wood's anomalies in diffraction gratings. Hence, it seems appropriate to refer to these spontaneous surface structures as "stimulated Wood's anomalies."

We report experimental results on spontaneous periodic surface structures or ripples which occur on the surface of crystalline or ion-implanted semiconductors, using high-power picosecond laser pulses. We suggest that these surface ripples develop as the result of an exponentially growing interaction between the incident laser wave front and a scattered surface optical wave.

We report our femtosecond time-resolved measurements on the photoresponse of an epitaxial YBa 2 Cu 3 O 7Ϫx ͑YBCO͒ thin-film photodetector, patterned into a microbridge geometry. By varying the current-voltage biasing conditions between the superconducting and resistive ͑hot spot͒ states, we observed transients that correspond to the nonequilibrium kinetic-inductance and the nonequilibrium electron-heating response mechanisms, respectively. The two-temperature model and the Rothwarf-Taylor theory have been used to simulate the measured wave forms and to extract the temporal parameters. The electron thermalization time and the electron-phonon energy relaxation time were determined by the electron temperature rise and decay times, which were found to be 0.56 and 1.1 ps, respectively, in the resistive state. We have also measured the ratio between the phonon and electron specific heats to be 38, which corresponds to a phonon-electron scattering time of 42 ps. No phonon-trapping effect ͑typical for low-temperature superconductors͒ was observed in YBCO, in the superconducting state, so the quasiparticle lifetime was given by the quasiparticle recombination time, estimated from the Rothwarf-Taylor equations to be below 1 ps.