PATTERN-FORMATION IN LASER-INDUCED MELTING

A laser, with an incident intensity of the order of a kilowatt per square centimeter, focussed onto a semiconductor film can produce an ordered or a disordered pattern of coexisting solid-melt regions. Recent experiments which show such pattern formation are reviewed. To understand these patterns it is necessary to consider (i) the effects of coherent optical interference for, small laser beam diameters, and (ii) the statistical mechanical aspects of the nonequilibrium phase transition for large beam diameters. The concepts related to these two cases are reviewed. In the first case, the electrodynamic interference effects lead to ordered patterns whose wavelength is locked with that of incident laser radiation. In the second ease, thermal effects dominate and yield disordered patterns. To understand th;ese disordered patterns, a continuum model based on higher reflectivity of the melt has recently been constructed by us. This model and the results obtained from it are reviewed in some detail.