A laser-induced periodic surface structure (LIPSS) created using pulsed lasers on a target surface has been used to develop various novel devices. Recently, LIPSS formed on silicon surfaces has gained interest in the surface customization optics, aerospace, silicon photonics (SiP), and biomedical fields because they provide antireflective, self-cleaning, and bioactive surfaces. However, the formation mechanism of LIPSS remains unclear, and the mechanism of interaction between femtosecond lasers and matter is not fully understood. Therefore, in this study, we investigate the evolution of femtosecond laser-induced periodic surface structures on monocrystalline silicon. Optimized laser processing parameters are used to process uniform LIPSS on the surface of monocrystalline silicon, and its application in structural coloration is demonstrated.

A femtosecond pulsed laser with a central wavelength of 1030 nm and a pulse duration of 300 fs was used to treat N-type monocrystalline silicon with a resistivity of 9.76?12.7 Ω·cm. A low-spatial-frequency LIPSS (LSFL), high-spatial-frequency LIPSS (HSFL), and short-period stripe structure formed by a split LSFL were investigated. The surface morphologies of the LIPSS formed on the silicon targets were characterized using field-emission scanning electron microscopy (SEM) (S-4800, Hitachi) and laser confocal microscopy (Leica DCM8Brochure CN) at room temperature.

At a laser fluence of 0.224 J/cm², a grid-like periodic surface structure forms at the center of the spot. With an increase of laser fluence to 0.252 J/cm², the grid-like periodic surface structure is formed at the contour of the laser crater (Fig. 4). As the laser fluence further increases to 0.420 J/cm², the area of the grid-like increases until the surface is overablated due to significant energy accumulation. Under irradiation with a femtosecond laser, the carrier concentration increases rapidly. Meanwhile, at the interface between air and the silicon target, the dielectric constant Re(ε)=1 in air, and the real part of the dielectric constant of the silicon surface in an excited state is approximately Re(ε)=-18.7, which satisfy the conditions for excited SPPs. The energy field generated by the interference of the SPPs and incident laser is interfered, and the silicon surface is melted and vaporized to achieve material ablation and form the LSFL. Because of the continuous accumulation of energy, the surface electric field is enhanced at the fringe edge of the LSFL. This leads to the melting of the fringes, and the molted materials flow into the LIPSS groove under the action of surface tension and the thermal capillary effect. In addition, the HSFL is formed because of self-organization driven by surface instability. Moreover, we observe a split LSFL by adjusting the laser fluence. When the laser fluence is set to 0.66 J/cm² at a repetition rate of 50 kHz, a short-period LIPSS is formed with a period of 448 nm instead of LSFL (Fig. 6). Notably, the short-period LIPSS is close to the HSFL period; however, its direction is perpendicular to that of the HSFL. In the experiments, the “split” LIPSS is formed when the laser fluence is higher than 0.66 J/cm², and the formation of split LIPSS is attributed to the second harmonic mechanism and self-organization. Structural color is induced on a silicon surface using LIPSS. “HEBUST” is written on the surface by femtosecond laser direct scanning. The letters “HEB” and “UST” after scanning have different colors owing to the application of different polarization directions in laser processing (“HEB” for horizontal polarization; “UST” for vertical polarization). The results demonstrate that LIPSS has broad applications in the fields of dynamic color control, anti-faking, and information storage based on structural color (Fig. 8).

In this study, a femtosecond laser with a wavelength of 1030 nm was used to generate an LIPSS on the surface of a single-crystal silicon target. Several periodic surface structures were investigated, and their formation mechanisms are discussed.

1) When the laser fluence is in the range of 0.224?0.364 J/cm² at a laser repetition rate of 100 kHz, a grid-like periodic surface structure is formed on the surface of the target, composed of LSFL and HSFL. The structure is formed by the synergistic interaction between surface plasmon polariton interference and the self-organization effect.

2) At a laser repetition frequency of 50 kHz, when the laser fluence is between 0.66 and 3.24 J/cm², a split LSFL is observed, which is significantly related to the laser parameters, including laser fluence and repetition frequency. However, further research is required to elucidate the mechanisms underlying this phenomenon.

3) Structural color is induced on the silicon surface using LIPSS. The variability in structural color is closely related to the orientation of the light source, laser polarization direction, and observation angle.