Formation of conical microstructures upon laser evaporation of solids

The formation and development of the large-scale periodic structures on a single crystal Si surface are studied upon its evaporation by pulsed radiation of a copper vapor laser (wavelength of 510.6 nm, pulse duration of 20 ns). The development of structures occurs at a high number of laser shots (~10) at laser fluence of 1--2 J/cm below optical breakdown in a wide pressure range of surrounding atmosphere from 1 to 10 Pa. The structures are cones with angles of 25, which grow towards the laser beam and protrude above the initial surface for 20--30 *mm. It is suggested that the spatial period of the structures (10--20 *mm) is determined by the capillary waves period on the molten surface. The X-ray diffractometry reveals that the modified area of the Si substrate has a polycrystalline structure and consists of Si nanoparticles with a size of 40--70 nm, depending on the pressure of surrounding gas. Similar structures are also observed on Ge and Ti.