The topography of laser-irradiated germanium

Considerable interest centres around the use of germanium as a reflector material in high-power carbon-dioxide lasers. An important problem is the damage caused to the mirror surface as the result of the dynamic development of imperfections. This damage may be caused by an interference process which has been described previously. A characteristic of the damage which occurs is the melting of the germanium followed by resolidification. The authors describe a certain type of melting pattern which depends upon density differences between the liquid and solid phases. The experimental phenomenon is first described with reference to germanium laser mirror damage and this is followed by a theoretical model of the resolidification of molten regions where a density difference occurs between the solid and liquid phases, as in paraffin wax for example.