Laser-induced periodic surface structures (LIPSS) provide a direct laser writing method for fabricating nano-gratings on sample surfaces. These periodic nano-structures efficiently modify the properties of materials and have many applications in surface coloring, large-area grating, birefringence optical elements, data storage, and surface wettability.
The formation mechanism of low-spatial-frequency LIPSS (LSFL) with a spatial period Λ > λ∕2, where λ is the laser wavelength, was widely accepted as a result of the laser energy periodic distribution induced by surface plasmon polaritons (SPPs).
During LSFL formation on the semiconductor and metal surfaces induced by traditional femtosecond laser pulses (with a repetition frequency of 1 kHz), the deposited debris and the residual heat significantly affected the SPPs excitation, propagation, and light field distribution during the subsequent laser irradiation, resulting in significant distortions and bifurcations on LSFL. Three major challenges exist in the fabrication of regular and uniform LSFL: enhancing the periodic energy deposition, reducing the residual heat, and avoiding the deposited debris.
In order to solve the problems, Prof. Tianqing Jia's group from the State Key Laboratory of Precision Spectroscopy at East China Normal University, China, has proposed and demonstrated extremely regular periodic surface structures efficiently induced by temporally shaped femtosecond laser, which was published in Photonics Research, Vol. 9, Issue 5, 2021 (Yuchan Zhang, Qilin Jiang, Kaiqiang Cao, Tianqi Chen, Ke Cheng, Shian Zhang, Donghai Feng, Tianqing Jia, Zhenrong Sun and Jianrong Qiu. Extremely regular periodic surface structures in a large area efficiently induced on silicon by temporally shaped femtosecond laser[J]. Photonics Research, 2021, 9(5): 050839).
Schematic diagram of regular and deep periodic surface structures induced by temporally shaped femtosecond laser pulse.
4f configuration zero-dispersion pulse shaping system is able to generate nearly arbitrarily shaped ultrafast optical wave forms, which means is capable to control the ultrafast process of the interaction between femtosecond laser pulse and materials.
Based on 4f configuration zero-dispersion pulse shaping system, a Fourier transform limit (FTL) pulse is shaped into a pulse train with varying intervals in the range of 0.25–16.2 ps using periodic π-phase step modulation. Under the irradiation of the shaped pulse with an interval of 16.2 ps, extremely regular LSFL are efficiently fabricated on silicon.
The ultrafast imaging results demonstrated that the transient LIPSS began to form on the sample surface after several to tens of picoseconds (ps) after the laser irradiated on the Si surface. Therefore, there are transient LIPSS on Si surfaces after irradiation by the two main sub-pulses of 16.2 ps.
When the subsequent sub-pulse reaches the surface, the surface changes to a metal-like state and effectively supports the excitation of SPPs. The transient LIPSS induced by the previous sub-pulses enhance the excitation of SPPs and the periodic laser field. The substrate in the surface region remains at very high temperature when the subsequent subpulse reaches the Si surface. Part of the material is further excited and ejected from the surface, and it removes the deposited heat (ablation-cooling effect).
The ejected materials by the previous sub-pulse, including plume and debris, will be further excited by the subsequent sub-pulses, and the debris is further ionized and vaporized into aerosol, resulting in less deposited particles. The SPPs and the periodic laser energy deposition are enhanced, and the residual heat is reduced. Therefore, regular and deep LSFL can be induced on a Si surface by the shaped pulse of 16.2 ps.
The fabrication efficiency, LSFL depth, and regularity using a shaped pulse of 16.2 ps are significantly better than those by Fourier transform limit (FTL) femtosecond laser pulse. The scan velocity for fabricating regular LSFL is 2.3 times faster, while the LSFL depth is 2 times deeper, and the diffraction efficiency is 3 times higher than those of LSFL using the FTL pulse. The large-area LSFL demonstrate a very bright and pure structural color from blue to red, observed at different angles.
The enhancement of SPPs and the reduced of residual heat are two main factors which affect the formation of LIPSS. According to previous reports, the transient LIPSS became deep and clear at a delay time of 30–300 ps after irradiated by a single femtosecond laser pulse. Kerse et al. predicted and experimental demonstrated that the ablation–cooling effect was very clear at a repetition rate higher than GHz (corresponding to an interval less than 1 ns).
Thus, the group estimate that the optimum interval of sub-pulse is in a range of 30–300 ps. Due to the limitation of the size of optical table, the interval of sub-pulses cannot be increased any longer. The group are trying best to construct an F-P cavity to generate a pulse train with a larger interval, which will be constructed in the near future.
Prof. Jianrong Qiu, an optics expert from the same optics group, says: "Temporally shaped femtosecond pulse has potential to control the ultrafast dynamics during the interaction between femtosecond laser and materials. This work is of great significance to promote LIPSS to industrial application."