Pulse Number Dependence of a Femtosecond Laser-Induced Micro/Nanostructure Formed on the Surface of a 65 Mn Plate
D-Q. Yuan and J-T. Xu
Laser-induced periodic surface structures(LIPSS) with different spatial characteristics have been observed after multiple linearly polarized femtosecond laser pulse (130 fs, 800 nm and 1 kHz pulse repetition frequency) irradiation of a 65 Mn plate. Parallel periodic ripple structures were observed at a laser fluence of 0.5 J/cm2 with different numbers of pulses: 5, 6, 10, 20, 50, 200, 600 and 800, which lay parallel to the laser electric polarization field vector. Nano-rippled structures (period of around 100 nm) parallel to the direction of polarization of the laser beam were generated when using 5 pulses at 0.5 J/cm2, which were covered with many nanoparticles. On increasing the pulse number from 10 to 50, an uninterrupted periodic ripple structure was obtained (period of around 390 to 580 nm). In the range of 200 to 600 pulses, a consistent ripple structure interrupted with nano-holes, which evolved from nanoparticles, was obtained. The generation of a second-harmonic has been supposed to be the main mechanism in the formation of the ripples whose period was approximately half of the wavelength of the femtosecond laser beam. Because of the accumulation of the multiple pulses irradiation, the period of ripple structures as a function of pulse number was divided into two regions.
Keywords: 65 Mn, femtosecond laser, laser-induced periodic surface structures (LIPSS), microstructure, nanostructure