Laser Induced Periodic Surface Structure.

The phenomenon by which a single, high intensity beam of coherent, polarized radiation induces periodic structure on the surfaces of many materials was studied from both an experimental and theoretical point of view. The morphology of the microstructure induced on three semiconductor and three metal surfaces by radiation at both 1.06 (mu)m and 0.53 (mu)m wavelengths was investigated as a function of the incident angle, polarization and fluence of the laser beam. By observing the Fraunhofer diffraction of a weak probe laser beam from the restructured surfaces, previously unappreciated symmetries in the Fourier spectrum of the induced roughness were revealed. For a given set of irradiation parameters, the diffraction patterns showed that the surface topography induced on the different materials is remarkably similar, which indicates that the phenomenon is quite generally associated with the interaction of radiation and material surfaces. The transient specular and first order diffraction of a weak probe beam from a germanium sample, while a high intensity pulse of 1.06 (mu)m radiation produced periodic structure on the surface, were time resolved and correlated to provide a "picture" of the semiconductor surface while the structure was forming. At low fluences near the minimum threshold required to induce any structure, the 1.06 (mu)m radiation caused the surface to melt along periodic strips which, upon resolidification, rose above the ambient surface level due to surface tension effects. At higher fluences, the 1.06 (mu)m radiation melted a uniform layer of germanium and proceeded to induce capillary waves on the air/liquid interface which were frozen as the surface resolidified. The periodic surface structure is shown to result from inhomogeneous energy deposition caused by interference of the incident radiation with fields scattered by the initial randomly rough surfaces. A detailed electrodynamic calculation of the Fourier components of inhomogeneous energy deposition as a function of the incident angle and polarization of the radiation is in good agreement with the Fourier spectrm of the induced microstructure. The periodic melting of the surface is shown to provide a feedback mechanism by which the transient periodic structures grow.