Periodic surface structures on Ni–Fe film induced by a single femtosecond laser pulse with diffraction rings

Abstract

This Letter reports the formation of periodic surface structures on Ni-Fe film irradiated by a single femtosecond laser pulse. A concave lens with a focus length of −150 mm is placed in front of an objective (100×, NA ¼ 0.9), which transforms the Gaussian laser field into a ring distribution by the Fresnel diffraction. Periodic ripples form on the ablation area after the irradiation of a single femtosecond laser pulse, which depends on the laser polarization and laser fluence. We propose that the ring structure of the laser field leads to a similar transient distribution of the permittivity on the sample surface, which further launches the surface plasmon polaritons. The interaction of the incident laser with surface plasmon polaritons dominates the formation of periodic surface structures.OCIS codes: 220.4241, 160.3900, 240.6700, 320.7090. doi: 10.3788/COL201715.022201.Laser-induced periodic surface structures (LIPSSs) have been studied intensely in the last five decades [1][2][3][4][5][6][7][8] . Initial reports on the formation of LIPSSs were usually irradiated by a continuous wave (CW) or long duration laser pulses [nanosecond (ns) laser]. The periods of these structures (are usually ripples) were approximately equal to the laser wavelength. These periodic ripples were attributed to the interference between the incident laser and the scattered light caused by surface defects [1][2][3][4] . Low spatial frequent LIPSSs (LSFL) have been observed in semiconductors, dielectrics, and metals after the irradiation of femtosecond (fs) laser pulses . Laser polarization, fluence, and the number of laser pulses have been identified as the key parameters for the LSFL formation. Experimental results indicated that the dynamics of a fs laser-induced LSFL were rather different from the near-wavelength ripples induced by a CW and a ns laser. For a normal incident laser with a wavelength λ, the ripple periods changed in the range of 0.5λ-0.95λ [15]