Ripple formation on silver after irradiation with radially polarised ultrashort-pulsed lasers

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Abstract
We report on the morphological effects induced by the inhomogeneous absorption of cylindrically polarized femtosecond laser irradiation of silver (Ag) in sub-ablation conditions. A theoretical prediction of the role of surface plasmon excitation and thermal effects in the production of self-formed periodic ripples structures is evaluated. To this end, a combined hydrodynamical and thermoelastic model is presented to account for the influence of temperature-related lattice movements in laser beam conditions that are sufficient to produce material melting. The results indicate that material displacements due to hydrodynamics are substantially larger than strain-related movements, which also emphasises the predominant role of fluid transport in surface modification. Moreover, theoretical simulations highlight the influence of the polarisation state in the size of ripple periodicity for a specialized case of cylindrically polarized beams, the radially polarized beams. The results show that the ripple periodicity is larger if linearly polarized beams are used. This is the opposite trend to the behaviour for materials with decreasing electron-phonon coupling constant g with increasing electron temperature, which highlights the significant role of g.

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