Femtosecond laser burst: a tool for regulating cross-scale micro/nanofabrication

Recently, a joint research team from Hangzhou Institute of Technology of Xidian University, Humboldt University and the Max Born Institute announced a breakthrough in fabricating high-precision tunable nanogratings through femtosecond laser bursts-induced oxidation of amorphous silicon films. This achievement overcomes the limitations of conventional processes and provides a pioneering solution for high-efficiency, high-quality, and controllable self-organized micro/nanofabrication driven by femtosecond lasers.
The related findings, entitled‘Burst Laser-Driven Plasmonic Photochemical Nanolithography of Silicon with Active Structural Modulation’, were recently published in Ultrafast Science, a subsidiary journal of Science.

Research Overview
Since their initial discovery in 1965, Laser-Induced Periodic Surface Structures (LIPSSs) have remained a focal point in laser micro/nanofabrication. Studies demonstrate that femtosecond laser ablation enables batch fabrication of LIPSS on the surface of material through non-contact processing. With its ultrashort pulse duration and minimal thermal effects, this technique is particularly suitable for large-area, high-throughput periodic nanolithography. However, the process faces persistent challenges: debris generated during laser ablation and residual heat accumulation significantly degrade the controllability of the fabrication process.
In response to the above bottlenecks, the development of non-ablative femtosecond laser processing technology provides a new approach to overcome the challenges in the quality and controllability of periodic nanolithography. Previous research has shown that the preparation of LIPSS by femtosecond bursts-induced oxidation on the sample surface can improve the processing controllability to a certain extent. However, it is still difficult to achieve flexible adjustment of the target structure.
To address this issue, this study innovatively used 40-MHz femtosecond laser bursts to fabricate periodic nanogratings. This approach not only produced nanoscale textures with better regularity but also successfully achieved active modulation of the LIPSS’s period and depth by varying the pulse number per burst (PpB).
Compared with the traditional technique using repetitive single-pulse, the sub-pulses with a higher repetition frequency in 40-MHz femtosecond bursts increase the thermal accumulation on the irradiated material surface, affecting the effective refractive index and absorption characteristics of the material, and then change the excitation and propagation of Surface Plasmon Polaritons (SPPs). Ultimately, these changes lead to alterations in the morphology of LIPSS. Based on the above principle, this study demonstrated the effect of PpB on the LIPSS’s morphology.  
Figures3B and 3C demonstrate the influence of femtosecond laser bursts on LIPSS’s period and modulation depth. Both parameters exhibit similar trends, showing linear growth with increasing PpB. However, when PpB exceeds 7, the LIPSS’s period and modulation depth cease to increase, with the latter even displaying a slight decline. This observation strongly suggests that excessive thermal accumulation suppresses the formation of LIPSS. In contrast, as shown in Figure 3D, the single-pulse mode demonstrates almost no capability to modulate the LIPSS’s period through adjustments in pulse energy.
To gain an in-depth understanding of the oxidative LIPSS generated on the surface of silicon, sample images observed by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) are presented in Figure4. These results collectively reveal that the experimentally fabricated LIPSS consist of a large number of oxidized nanoparticles, with their size increasing proportionally as PpB rises.
Finally, to demonstrate the feasibility of this process for large-scale nanolithography, the team fabricated nanogratings utilizing cylindrical lens focusing. As shown in Figure5, the gratings obtained under both modes exhibit excellent regularity, indicating that even when using burst mode, the thermal accumulation induced by the high-repetition-rate sub-pulses remains within a controllable threshold. This ensures no significant impact on the surface morphological accuracy or structural integrity of the devices.

Summary and Outlook
This novel processing method demonstrates significant potential for application in large-scale nanolithography, representing a critical breakthrough in the field of micro/nano-fabrication. It lays the groundwork for achieving more efficient and higher-quality nanolithography. Looking ahead to the broader optoelectronics industry, this technique also shows great application prospects.