Crystal structures on a copper thin film with a surface of periodic self-organized nanostructures induced by femtosecond laser pulses

Crystal structures on a single-crystalline copper film with a periodic self-organized surface induced by femtosecond laser pulses with fluences of 0.08--0.64 J/cm² have been analyzed by transmission electron microscopy. It was found that the crystal structures depend on laser fluence: polycrystalline structures are formed at fluences less than 0.2 J/cm², an amorphous state forms around 0.3 J/cm², and polycrystalline structures form again at fluences greater than 0.35 J/cm². The energy spectrum of ions emitted during the formation of periodic structures on the surface shows that the energy of copper ions is high enough to transform the crystal to an amorphous state. A conceptual mechanism for crystal structure transformation by the penetration of energetic ions generated in the process of self-organization of periodic structures is proposed.