Creation of periodic subwavelength ripples on tungsten surface by ultra-short laser pulses
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Formation of periodic subwavelength ripples on a metallic tungsten surface is investigated through a line-scribing method under the irradiation of 800 nm, 50 fs to 8 ps ultra-short laser pulses. The distinctive features of the induced ripple structures are described in detail with different laser parameters. Experimental measurements reveal that with gradual decrease of the laser fluence, the pulse duration or the scanning speed, the ripple period is inclined to reduce but the ripple depth tends to become pronounced. Theoretical analyses suggest that the transient dielectric function change of the tungsten surface mainly originates from the nonequilibrium distribution of electrons due to the d-band transitions. A sandwich-like physical model of air–plasma–target is proposed and the excitation of a surface plasmon polaritonic (SPP) wave is supposed to occur on the interface between the metallic target and the electron plasma layer. Formation of ripples can be eventually attributed to the laser–SPP interference. Theoretical interpretations are consistent with the experimental observations.