Fabrication of periodic structures in thin metal films by pulsed laser irradiation

We developed a technique for the fabrication of 1D and 2D periodic structures in thin gold and silver films by irradiation with two and four intersecting beams of a nanosecond pulsed laser. The periodically alternating intensity distribution caused by interference induces mass transfer in thin metal films. The mechanisms and kinetics of mass transfer depend on the film thickness, morphology, and laser intensity. In the films thinner than some transitional value, h*, redistribution of material between "hot" and "cold" regions on the substrate occurs as a result of melting and beading of the film in "hot" regions and subsequent motion of liquid drops towards "cold" regions in the optically induced temperature gradient. In the films thicker h* redistribution of the film material occurs by hydrodynamic flow of the molten film in the temperature gradient, with subsequent crystallization in the "cold" regions and formation of tall and narrow ridges or rims. Theoretical models were developed that allowed us to calculate the time dependent temperature distributions around irradiated regions of the film. From the temperature distributions we calculated the expected speed of liquid beads, as well the distances between the ridges, the rim diameters and found good agreement with the measured values.