Laser-induced periodic surface structure. III. Fluence regimes, the role of feedback, and details of the induced topography in germanium

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Abstract
The mechanisms responsible for the growth of periodic surface structure on Ge irradiated by 1.06-μm laser pulses from a single beam are investigated. Time-resolved diffraction of a weak cw probe beam from the developing structures, coupled with electron-microscope pictures of the resulting morphology, is used to identify four distinct regimes of ripple formation at different incident laser fluences. At low fluences, the structure develops when thin (<1 μM wide) isolated, molten strips resolidify on the solid substrate, while at high fluences, the structure results from freezing of capillary waves which are generated on the surface that the laser pulse has melted uniformly. Models are presented which clearly demonstrate how the incident electromagnetic field interacts with the evolving structures (in different fluence regimes) to provide feedback mechanisms which sustain their growth.