Damage in the germanium output mirror of a TEA CO2 laser shows a periodicity of 10.6 μm, the laser wavelength. A mechanism is proposed in which scattered and cavity radiation interfere. An interference mechanism is confirmed by experiments outside of the laser cavity. Periodic damage is obtained in thin films irradiated at angles other than the normal where the fringe spacing is modified.

Vapour cavities in liquid flows have long been associated with cavitation damage to nearby solid surfaces and it is thought that the final stage of collapse, when a highspeed liquid jet threads the cavity, plays a vital role in this process. The present study investigates this aspect of the motion of laser-generated cavities in a quiescent liquid when the distance (or stand-off) of the point of inception from a rigid boundary is between 0.8 and 1.2 times the maximum radius of the cavity. Numerical simulations using a boundary integral method with an incompressible liquid impact model provide a framework for the interpretation of the experimental results. It is observed that, within the given interval of the stand-off parameter, the peak pressures measured on the boundary at the first collapse of a cavity attain a local minimum, while at the same time there is an increase in the duration of the pressure pulse. This contrasts with a monotonic increase in the peak pressures as the stand-off is reduced, when the cavity inception point is outside the stated interval. This phenomenon is shown to be due to a splash effect which follows the impact of the liquid jet. Three cases are chosen to typify the splash interaction with the free surface of the collapsing cavity: (i) surface reconnection around the liquid jet; (ii) splash impact at the base of the liquid jet; (iii) thin film splash. Hydrodynamic pressures generated following splash impact are found to be much greater than those produced by the jet impact. The combination of splash impact and the emission of shock waves, together with the subsequent re-expansion, drives the flow around the toroidal cavity producing a distinctive double pressure peak.

We report the first systematic investigation into the properties of ultraviolet-laser-induced periodic surface structures that can be produced on nominally smooth surfaces. The study investigates the dependence of the patterns that are produced on Ge and Al by a KrF 249-nm laser on incident fluence and both the polarization and angle of incidence of the light. At high Auence we show that the results are consistent with the assumption that the surface melts uniformly, whereas the patterns formed at low Auence can be explained on the basis of the localized melting of the surface. For P-polarized light we find two dominant patterns, one perpendicular to the polarization and one parallel to it. For S-polarized light we report the first observation of two new patterns, one parallel and one perpendicular to the polarization, of which the latter is explained by invoking a new formation mechanism. The results show that for P-polarized light at large angles of incidence the ripple spacing shows large deviations from its previously expected value. The results are shown to be in excellent agreement with a previously developed first-principles theory. In addition, we report surface structures that are consistent with the idea of capillary waves being launched and subsequently frozen on the surface of the material.

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