Nonequilibrium Pattern Formation in Silicon during CW Laser Induced Melting.

Silicon films under intense, continuous laser irradiation (at lambda = 10.6 μm) develop into spatially inhomogeneous molten states. The steady states of this system were systematically studied and revealed a diverse array or reproducible, ordered and disordered states. We show that the ordering of the molten regions reflects the competition between spatially coherent energy deposition from the laser and heat flow from the illuminated region. The types of patterns produced range from periodic structures (gratings) with spatial periods of lambda, 2lambda , 3lambda etc. to disordered structures. Several distinct types of states could be identified on the basis of symmetry, long range order and the fraction of the surface that was molten. A specific pattern could be maintained by fixing the power deposited per unit area from the laser and the spot size, and with this identification it was possible to construct a nonequilibrium phase diagram or stability diagram for the structures. In addition to experimental observations, we present extensive theoretical work on the electrodynamics and heat flow that accounts for most of our observations. It is shown that the observed patterns concentrate the optical absorption into the molten portions of the film, in direct contrast to calculations of others. The work is also put into the context of nonequilibrium thermodynamics and pattern formation in far from equilibrium systems. Finally the issue of a potential evolution criterion for the pattern selection is addressed, along with the possible observation of long range ordering in a nonequilibrium system.