Laser-induced periodic surface structure (LIPSS) is a type of periodic micro-nanostructure formed on solid surfaces irradiated by laser light. The generation of periodic micro-nano-structures can improve material surface properties such as optical properties, surface hydrophilicity/hydrophobicity, biomedical properties, and friction properties. Furthermore, this can lead to a wide range of applications in the field of material surface functionalization. Prefabricated structures on material surfaces have been shown to modulate the electromagnetic field energy distribution on material surfaces, which in turn affects the formation of LIPSS on material surfaces. In extant studies, only the influence of prefabricated structures on the electromagnetic field distribution has been discussed. However, there is a significant difference between the thermal effects produced by laser pulses with pulse widths in the order of nanoseconds and picoseconds on the surface of the material. Hence, the aim of this study is to investigate the mechanism of nanosecond- and picosecond laser-induced periodic structure formation assisted by the prefabricated structure on the surface of single-crystalline germanium.

First, the difference in surface temperature changes between picosecond and nanosecond laser irradiation of prefabricated structures is investigated using a two-temperature model. During laser irradiation of the target, the maximum temperature in the center of the groove surface will be higher than the melting point of the single-crystal germanium. This suggests the presence of a molten layer on the surface of the target in the process of laser irradiation. The semiconductor material is instantaneously transformed into a metallic state, which permits the excitation of surface plasma excitations. An interference model of surface plasmon excitations (SPP) is then used to explain the formation mechanism of LIPSS in semiconductor materials. By numerically solving Maxwell equations, the electromagnetic field energy distribution, i.e., the distribution of surface plasmonic excitons, is obtained when the laser irradiates the prefabricated grating.

At energy densities in the range of 0.12?0.14 J/cm², the bottom of the grating groove under nanosecond laser irradiation changes from none to the melting region as the laser energy density increases, while the picosecond laser melting region does not change significantly (Fig. 3). As the period of the prefabricated grating increases, the electric field intensity distribution changes with the groove width. The enhancement is larger when the groove width is equal to or close to an odd multiple of the SPP half-wavelength, whereas the enhancement is significantly weaker when the groove width is equal to or close to an even multiple of the SPP half-wavelength (Fig. 7). As the laser energy density increases, the fusion region on the surface of the prefabricated grating expands, resulting in a stronger electric field at the bottom of the prefabricated grating groove (Fig. 10). Different prefabricated grating heights result in varying electric field distributions at the bottom of the groove. For grating heights less than 100 nm, the periodic electric field distribution at the groove bottom is affected. Conversely, for heights exceeding 200 nm, the energy entering the bottom of the groove reduces, leading to a narrower range of periodic electric field distribution (Fig. 13). Additionally, the electric field intensity at the bottom of the groove decreases as the grating duty cycle increases beyond 0.5. When the duty cycle is less than 0.5, the electric field intensity decreases with an increase in duty cycle if the groove width is small. However, it increases if the groove width is large (Fig. 14). Incident laser wavelengths of 355 nm and 1030 nm create periodic electric field distributions on the grating ridges with periods of approximately 348 nm and 976 nm, respectively (Fig. 16). Furthermore, thermal effect analysis of different pulse widths shows that the parameter space where nanosecond lasers can produce LIPSS is narrower than that for picosecond lasers (Fig. 17).

In this study, the effects of different prefabricated structure parameters, laser thermal effects, and varying wavelengths of incident laser are examined on the electromagnetic field energy distribution on the surface of single-crystal germanium. The larger the fusion region on the prefabricated grating, the stronger the electric field intensity at the bottom of the groove, facilitating the formation of periodic structures on the material surface. At lower heights of the prefabricated grating, the electric field intensity at the bottom of the grating groove is influenced by scattering at the junctions of the grating sidewalls with the bottom and ridge. This interference disrupts the distribution of the periodic electric field at the bottom of the grating groove, reducing the probability of forming periodic structures there. Conversely, at higher grating heights, less energy reaches the bottom of the groove, making it difficult to generate periodic structures. A grating duty cycle in the range of 0.4 to 0.6 is typically more effective. For both ultraviolet and infrared incident lasers, the prefabricated surface produces a near-wavelength periodic electric field distribution, which can result in the formation of near-wavelength periodic structures on the surface of the material. The findings of this study may provide useful references for generating laser-induced periodic structures on semiconductor surfaces using prefabricated structures.