Controlling laser-induced periodic surface structures on semiconductor materials is of significant importance for micro/nanophotonics. We here demonstrate a new approach to form the unusual structures on 4H-SiC crystal surface under irradiation of three collinear temporally delayed femtosecond laser beams (800 nm wavelength, 50 fs duration, 1 kHz repetition), with orthogonal linear polarizations. Different types of surface structures, two-dimensional arrays of square islands (670 nm periodicity) and one-dimensional ripple structures (678 nm periodicity) are found to uniformly distribute over the laser-exposed areas, both of which are remarkably featured by the low spatial frequency. By altering the time delay among three laser beams, we can flexibly control the transition between the two surface structures. The experimental results are well explained by a physical model of the thermally correlated actions among three laser-material interaction processes. This investigation provides a simple, flexible, and controllable processing approach for the large-scale assembly of complex functional nanostructures on bulk semiconductor materials.

The acquiring of superhydrophilic surfaces attracts the strong interest in self-cleaning, anti-fogging and anti-icing fields based on the unique features. However, the persistent time of superhydrophilic surfaces is still facing a big challenge because of easily adsorbing hydrophobic groups. Here, we propose a strategy to achieve a superhydrophilicity persisting for an unprecedently long time on sapphire surfaces, by compounding the femtosecond laser-induced hierarchical structures and the subsequent varnish of TiO2. The superhydrophilic effect (with a contact angle of CA = 0°) created by our method can be well prolonged to at least 180 days, even for its storage in air without additional illumination of UV lights. Based on comprehensive investigations, we attribute the underlying mechanisms to the coordination of laser-induced metal ions on the material surface via TiO2 doping, which not only prevents the adsorption of the nonpolar hydrocarbon groups, but also modulates the photo-response properties of TiO2. In addition, further experiments demonstrate the excellent anti-fogging properties of our prepared samples. This investigation provides a new perspective for further enhancing the durability of superhydrophilicity surfaces.