Pattern formation during laser melting of silicon

Silicon films under intense, continuous laser irradiation at λ=10.6 μm develop into spatially inhomogeneous molten states. The systematic study of the different patterns as a function of laser intensity and spot size reveals a variety of ordered and disordered states. For small spot size, highly correlated, long-range periodic structures (gratings) with spatial periods of λ,2λ,3λ,. . . are observed for increasing intensity. For low intensities and large spot sizes, irregular molten and solid lamellae are observed with only short-range order. We show that the ordered and disordered molten structures reflect different levels of balance between spatially coherent energy deposition from the laser and heat flow from the illuminated region. Changes in the intensity or spot size can be used to alter the balance and induce morphological ‘‘phase transitions’’ between the different structures. We find that many of the characteristics of the patterns, including the spacings of the periodic structures and the size and separation of the lamellae, can be explained by the theory of the electrodynamic response of optically inhomogeneous surfaces presented in the preceding paper.