The remarkable transformation of nanoripples in glass-metal nanocomposite

We report the linear-to-circular transformation of high spatial frequency (HSF) ripples formed on the surface of a glass with embedded silver nanoparticles under irradiation with 120 fs long pulses at the wavelength of 800 nm. In the experiments, at the ablation threshold of 1.28 J/cm2, the HSF ripples with the period of ∼300 nm were oriented along the polarization of the laser beam. However, we found that the nanoripples pattern is gradually transforming from linear to an untypical circular one when the pulse energy increases. The ripples take nearly perfect circular shape at the pulse energy of ∼8.0 μJ, which corresponds to the fluence of 1.5 J/cm2.