Mechanism of formation of hexagonal symmetry nanostructures on metal surfaces by the sequence of doubled ultrashort pulses of ortogonal polatization

An experimental results causing the ordered hexagonal symmetry surface nanostructures formation on metals under the sequence of scanned doubled collinear femtosecond pulses of orthogonal polarization and variable time-delay have been analized. It was shown that the structures production caused by the dynamic grating formation by the first pulse, excitation of two paires of surface plasmon polaritons in symmetric to the polarization vector directions of second pulse directions with 60 degrees angle between formed respective dynamic gratings and forth-fold first grating degeneration. So three gratings of three-flold symmetry about surface normal creates experimentally observable resonant surface relief of hexagonal symmetry.