Abstract
We report on the formation of slantwise-oriented periodic subwavelength ripple structures on chromium surfaces irradiated by single-beam femtosecond laser pulses at normal incidence. Unexpectedly, the ripples slanted in opposite directions on each side the laser-scanned area, neither perpendicular nor parallel to the laser polarization. The modulation depth was also found to change from one ripple to the next ripple. A theoretical model is provided to explain our observations, and excellent agreement is shown between the simulations and the experimental results. Moreover, the validity of our theory is also confirmed on bulk chromium surfaces. Our study provides insights for better understanding and control of femtosecond laser nanostructuring.
Highlights
During the last several years, the research of femtosecond laser-induced periodic surface structures (Fs-LIPSSs), or the ripple structures, has attracted tremendous attention because of the abundant scientific issues involved [1,2,3]
To elucidate our experimental observations, we proposed the following physical scenario: In our experiments, which are based on multi-pulse femtosecond laser irradiation processes, the pristine surface of the metal film was modified by the preceding incident femtosecond laser pulses, leading to a rough, shallow crater with the modulation depth gradually reducing from the beam center to the peripheral regimes, as shown in Figure 4a, where the inclined surface was created on the laser irradiation area
We comprehensively studied the generation of slantwise-oriented subwavelength periodic ripple structures on the surfaces of chromium material by normal incidence of linearly polarized femtosecond laser pulses
Summary
During the last several years, the research of femtosecond laser-induced periodic surface structures (Fs-LIPSSs), or the ripple structures, has attracted tremendous attention because of the abundant scientific issues involved [1,2,3]. When the linearly polarized single-beam femtosecond laser pulses are used to irradiate materials, the induced ripple structures are either parallel or perpendicular to the direction of the laser polarization [13,14,15,16]. The ripple structures induced by femtosecond lasers presented an unusual feature of slantwise orientation, which is neither perpendicular nor parallel to the laser polarization direction [17,18,19,20,21,22]. Our research group generated a series of v-like structures, called a herringbone pattern, on copper [23] Such anomalous phenomena indicate the physical complexities during the ripple surface structure formation, which is significant for femtosecond laser nanoprocessing. A comprehensive underlying mechanisms of LIPSS orientation is still lacking
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