The fabrication of subwavelength nanostructures induced by femtosecond laser irradiation is critical to modern nanophotonics and has extensive applications across various fields. This study investigates the subwavelength periodic structural characteristics and formation mechanism of 4H-SiC material surfaces induced by delayed triple femtosecond laser pulses. By controlling the polarization direction and delay time of the three incident laser beams, subwavelength nanostructures of varying scales and dimensions are produced on the sample surface. Experimental results indicate that by adjusting the number of laser pulses and the delay time, the spatial period of the induced nanostructures can shift from high spatial frequency to low spatial frequency. In addition, by modulating the polarization direction between the three laser beams, two-dimensional low-spatial-frequency nano-square and nano-rhombic structures are generated on the sample surface. To explain the physical mechanism behind the formation of these subwavelength nanostructures, a transient temperature grating model is proposed. This model suggests that transient temperature gratings (or transient refractive index gratings) play a pivotal role in the laser irradiation process. The evolution of these gratings and subsequent energy relaxation ultimately result in permanent nanostructures. The surface roughness caused by laser irradiation further enhances the excitation of transient temperature gratings, promoting the formation of these permanent nanostructures. This research provides valuable insights into the controllable fabrication and underlying mechanisms of laser-induced periodic surface structures (LIPSSs).

In this study, we use a Ti: sapphire femtosecond laser amplifier (Spectra-Physics HP-Spitfire) as the irradiation source, delivering horizontally polarized pulse trains at a 1 kHz repetition rate, with a central wavelength of 800 nm and a pulse duration of 50 fs. The maximum pulse energy delivered by the system is 2 mJ. The laser output is split into three beams (P₁, P₂, P₃) using a beam splitter (BS). To fine-tune the laser processing parameters, laser power meters (PMs), half-wave plates (HWPs), and delay lines are introduced into the optical paths of P₁ and P₃. After passing through the delay lines, the three laser beams are aligned for collinear propagation and focused onto the sample using a 4× objective lens at normal incidence. A bulk 4H-SiC plate (20 mm×20 mm×1 mm) is mounted on a three-dimensional precision translation stage. The sample is positioned approximately 300 μm before the focal point, resulting in a Gaussian laser spot with a diameter of approximately 60 μm on the surface. During line scanning, a scanning speed of 0.1 mm/s is used, leading to a pulse overlap of 600 per scan. The sample surface is ultrasonically cleaned in acetone before and after the experiments. Surface morphology is analyzed using scanning electron microscopy (SEM) after the laser processing.

The laser-induced surface periodic structures are modified by controlling the laser processing parameters. When two laser beams with a fixed delay irradiate the sample, the quasi-periodic and periodic nano-grating structures exhibit low spatial frequency (LSF) characteristics (Fig. 3), in contrast to the single-beam and zero-delay double-beam laser results. Experimental findings suggest that the delay time between the beams significantly influences the formation of LSF nano-gratings. Using delayed triple femtosecond laser pulses, one-dimensional nano-grating ripples with LSF properties are fabricated, and a branching phenomenon is observed, where one LSF stripe splits into three high-spatial-frequency (HSF) stripes at the ripple edges (Fig. 4). The stripe bifurcation is associated not only with transient gratings but also with the energy deposition from femtosecond laser pulses. When the three laser beams have perpendicular polarization directions, a two-dimensional square-like structure forms on the surface [Fig. 5(b)], while a two-dimensional rhombus micro bump structure emerges when the polarization directions are set at θ₁=θ₂=60° [Fig. 5(c)]. These observations suggest that the polarization direction of the incident laser determines the orientation of the transient temperature gratings, thus shaping the final nanostructures.

In this paper, we explore the characteristics and formation mechanisms of one-dimensional nano-grating and two-dimensional nanostructures on the surface of 4H-SiC using triple time-delayed femtosecond laser pulses. Initially, linearly polarized femtosecond lasers induce HSF grating-like structures on the surface. Then, one-dimensional nano-grating structures with LSF characteristics are created through delayed double-beam laser pulses, with experimental results showing that as the delay time increases, the spatial period of the grating transitions from HSF to LSF. Lastly, by modulating the polarization direction and delay time of the three lasers, LSF nano-grating structures and two-dimensional square and rhombus patterns are fabricated. We demonstrate that two-dimensional periodic surface structures can be fabricated on the surface of 4H-SiC using three temporally delayed pulsed laser beams. Regarding the transition mechanism of the nano-grating period from HSF to LSF, preliminary analysis suggests that the incidence of two or three delayed laser beams increases the electron density in the irradiated area, reducing the dielectric constant in the localized air layer containing free electrons on the sample surface. This process ultimately induces the formation of LSF nano-grating ripple structures. A physical model based on surface plasmon polaritons (SPPs) and transient temperature grating is proposed to explain the structure formation process. The combined effect of the three laser beams induces transient temperature gratings, whose evolution and energy relaxation promote the formation of two-dimensional nanostructures and increase surface roughness. These findings provide essential insights into the fabrication of LIPSSs and their potential applications in direct laser fabrication of nano-devices.