Laser-Induced Periodic Surface Structure on Solids: A Universal Phenomenon

Over the past eighteen years it has been observed that single, intense linearly polarized laser pulses could induce permanent ripples on the surface of various metal. s, semiconductors, and insulators. ' " It is generally considered that the pattern results from inhomogeneous energy deposition associated with the interference of the incident beam with a surface scattered field, but the nature of this field has been the subject of some controversy. ' "" In this Letter we report that l.aser-induced periodic surface structure (I IPSS) can be understood as a very general phenomenon; it can be expected on a wide variety of material. s, polariton active and polariton inactive, with different surface microroughness, and over a broad range of wavelengths. With respect to beam polarization, we have observed for the first time that LIPSS can be produced on metals and semiconductors by circularly, as well as l.inearl. y, polarized l.ight, contrary to earl. ier suggestions. '" We have observed LIPSS on Ge, Si, Al. , and brass using 15-nsec pulses with wavelength A. =0.53 or 1.06 p. m. Although it is possible to form ripples with single pulses, the patterns so formed can be influenced by the presence of isolated defects or scratches. For multiple shots at constant fluence (typical. ly 75 mJ/cm'), a steadystate pattern emerges after about twenty shots which is independent of the number of additional shots and the initial surface conditions. Only this case is discussed here. The role of defects in connection with single-pulse LIPSS is discussed elsewhere. " Hecentl. y, using linear l.y polarized, 1.06-JLt, m 15nsec pulses incident on Ge, we studied the ripple patterns induced by sand P-polarized light as a function of the angle of incidence 0.' It was noted that although the patterns in real space can be quite complex, their Fourier (~) spectra, as revealed by the Fraunhofer diffraction pattern of a cw Ar' laser, are quite simple; typical. l.y the damage spot size is 5 mm whereas the probe spot size is 1 mm. For 0=0, well-defined ripples of spacing X were formed with an orientation perpendicular to the polarization. In contrast, we report here that when circul. arly polarized l.ight is used at 8 = 0', well. -defined fringes are not formed, but instead the surface appears to be speckled, as Fig. 1(a) shows. The corresponding observed diffraction pattern is a uniformly bright circle (as shown schematically in Fig. 2) whose radius in Tc space is 27r/A. . This indicates that the speckled surface actually consists of fringes of spacing A. formed isotropically along the surface. For 6= 30, the circularly polarized beam produces a much more complex diffraction pattern as seen in Fig. 2. In Fig. 2 we summarize schematically the diffraction patterns as seen on the various material. s at the two different wavel.engths. All the observed diffraction patterns are located on parts of two intersecting circles in ~ space which satisfy