Objective Femtosecond laser- induced subwavelength periodic surface structures (LIPSS) have the capability to modify the physicochemical properties of material interfaces. Furthermore, they exhibit the advantages of fast processing speed and no requirements for expensive equipment. Hence, this promises wide application in multidisciplinary fields such as surface wetting property study, surface coloring, surface- enhanced Raman spectral sensing, and increased material surface biocompatibility. Compared with other traditional fabrication methods, direct laser writing is more efficient and reliable. Specifically, a femtosecond laser direct- writing technique is used to construct regular ripples on a 6H-SiC surface. Researchers have fabricated multiple types of microstructures on SiC surfaces. However, to date, the repeatable erasable properties of LIPSS on 6H-SiC surfaces have not been reported, and the physical mechanisms of LIPSS formation on the surfaces of semiconductor materials should be further explored. Therefore, in this study, the direct writing of subwavelength LIPSS and ablative spots on a 6H-SiC surface are investigated. Additionally, the repeatable erasable properties of LIPSS are systematically analyzed under two different polarization conditions using a femtosecond laser. Finally, the physical mechanism of LIPSS formation on the surfaces of the semiconductor materials is discussed. Methods A Ti:sapphire femtosecond laser amplifier is employed as an irradiation source to deliver horizontally polarized pulse trains with a repetition rate of 1 kHz, central wavelength of 800 nm, and time duration of 50 fs. The maximum pulse energy delivered by the laser system is 2 mJ. A half- wave plate is inserted before the high reflectivity mirror to change the laser polarization. After passing through the high reflectivity mirror, the laser beam is aligned for collinear propagation and then focused by an objective lens (4x , numerical aperture of 0.1) at a normal incidence. A bulk 6H-SiC plate ( 20 mm x 20 mm x 1 mm) is mounted on a three- dimensional (3D) precision translation stage. SiC is chosen because of its attractive properties such as high thermal conductivity, high breakdown electric field strength, and large band gap, with many potential applications. The sample is placed approximately 300 mu m away before reaching the focus, which leads to a Gaussian laser spot with a diameter of approximately 60 mu m on the surface, and the sample is translated at a fixed speed of 0.1 mm/s for the fabrication of LIPSS. Before and after the experiments, the sample surfaces are ultrasonically cleaned in an acetone solution. After laser micromachining, the surface morphological features are analyzed using scanning electron microscope (SEM). Results and Discussions In this study, femtosecond laser is used to induce ablative holes, ablative circular spots, and subwavelength LIPSS on a 6H-SiC surface. When the distance between the laser focus and sample surface increases, the diameter of the ablative spot gradually increases, while the ablative strength decreases (Fig. 2). As the defocusing distance increases, a subwavelength LIPSS is induced on the edges of the ablative spots [Figs. 2(e) and (f)]. When the laser power is increased from 4 mW to 14 mW, the conditions for the formation of regular ripples correspond to 6-9 mW (Fig. 4). These experimental results reveal that the formation of LIPSS is strongly dependent on the power of the incident laser. Additionally, to study the repeatable erasable properties of LIPSS on 6H-SiC samples, one-dimensional (1D) LIPSS is induced in the vertical direction first, followed by direct horizontal writing. The experimental results show that secondary irradiation with another femtosecond laser with different polarization directions at the location of the first LIPSS can simultaneously erase the previously formed LIPSS and induce a new LIPSS at the same time (Fig. 5). Conclusions It is determined that in addition to ablative holes and ablative circular spots, 1D LIPSS can be produced on the surface of a 6H-SiC sample under the irradiation of near infrared polarized femtosecond laser pulse. The size of the ablative spots induced by the femtosecond laser is mainly affected by the defocusing distance, and the LIPSS is generated at the edges of the ablative spots when the defocusing distance is large. By modulating the incident laser power, the optimal laser power range for forming a regular LIPSS is obtained in the range of 6-9 mW on the 6H-SiC surface. Additionally, the experimental results show that the LIPSS on the surface of the 6H-SiC material has repeatable erasable characteristics. This implies that secondary irradiation with another femtosecond laser with different polarization directions at the location of the first LIPSS can erase the previously formed LIPSS and rewrite a new LIPSS simultaneously. Regarding the physical mechanism of the formation of LIPSS on the surface of the SiC material, the preliminary analysis results show that when the incident laser power is high, the material removal by the femtosecond laser is based on plasma ejection, and the subsequent thermal melting effect. When the incident laser power is similar to the ablation threshold value of the sample, the surface plasma wave excitation and its effects play a dominant role. Hence, surface plasma waves can be stimulated on a sample surface under femtosecond laser irradiation. Periodic ripple structures are formed by the redistribution of the incident laser energy by the excited surface plasma wave, resulting in fringe- like energy concentrations. These results are significant for the controllable fabrication and formation of LIPSS.
