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Fig. 1 (a) Confocal optical microscopy image of LSFL prefabricated on silicon surface. (b) Experiment setup of the collinear pump-probe imaging system. (c) Spectra of the white-light pulse with (red solid curve) and without (black dotted curve) the short-wave-pass filter. (d) Laser spot area on the object plane with and without the concave lens.

Credit: OEAS

A new publication from Opto-Electronic Science; DOI 10.29026/oes.2024.230013 discusses pump-probe imaging and formation mechanisms of femtosecond laser-induced high spatial frequency periodic structures on silicon surfaces.

Femtosecond laser-induced periodic surface structures (LIPSS), especially high spatial frequency LIPSS (HSFL), have become an effective method for large-scale nano fabrication. These periodic nanostructures have significant applications in surface structural color, information storage, enhanced emission and absorption, anisotropic conductivity, and the regulation of hydrophobic and hydrophilic characteristics.

However, because of the smaller structure size of HSFL, its morphology becomes more irregular and even submerged due to the hydrodynamic effects of the molten layer, which greatly limiting its functional display. In order to fundamentally regulate the dynamic process and prepare high-quality HSFL, it is necessary to understand the formation mechanism.

SEM, TEM, and AFM are widely used to study the HSFL after solidification, but these methods cannot observe the transient nanostructures during the formation of LIPSS. The mechanism of HSFL formation remains unclear and is a challenging topic.

The authors of this article developed a collinear pump-probe imaging method with high spatial resolution (<300 nm) and high temporal resolution (<0.5 ps). The silicon surface with low spatial frequency LIPSS (LSFL) has been prepared in advance, and the ultrafast dynamics of the formation of HSFL under the irradiation of a single femtosecond laser pulse has been observed and analyzed. The prepared sample and experimental system are shown in Figure 1.

Figure 2 shows the ultrafast dynamics of the formation of HSFL irradiated by a single pulse with a fluence of 0.82 J/cm2. The results indicate that the evolution of the surface morphology under femtosecond laser irradiation underwent the following five stages: (A) 0-100 ps: LSFL begins to split and become uniform, (B) 150-350 ps: Uniform HSFL, (C) 400-700 ps: Uniform HSFL degenerates into an irregular LSFL, (D) 750-850 ps: LSFL undergoes secondary splitting and evolves into weakly uniform HSFL, and (E) 900 ps to solidification: The HSFL formed during the second splitting evolves into LSFL. The evolution process of surface morphology under laser irradiation with different fluences is basically similar.

After carefully analyzing the dynamic process of the HSFL formation, and making a lot of numerical simulations, researchers propose a mechanism for the HSFL formation, as shown in Figure 3. During the process of preparing LSFL on silicon, nano voids or amorphous molten silicon are formed in the middle of the ripple ridges. Under pump laser irradiation, amorphous silicon is excited and quickly becomes nano plasma. The local field enhancement in nano voids and nano plasma leads to the splitting of LSFL ripples, and the thermodynamic effect drives the homogenization of the splitting LSFL and evolves into HSFL.

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Professor Jia received his doctor's degree from Tongji University in 2000, and then worked as a postdoctoral fellow at the Shanghai Institute of Optics and Precision Mechanics, Chinese Academy of Sciences. In 2001, he served as an associate researcher at Shanghai Institute of Optics and Mechanics. In 2005, he became a special researcher at the Institute of Solid State Physics at the University of Tokyo, Japan. Since 2007, he has been a professor at East China Normal University.

Professor Tianqing Jia's group mainly focuses on laser precision machining, such as femtosecond laser-induced periodic nanostructures and ultrafast dynamics of femtosecond laser ablation. In the past decade, they have developed equipment for laser processing and online testing of cooling holes in flame tubes and turbine blades.

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Opto-Electronic Science (OES) is a peer-reviewed, open access, interdisciplinary and international journal published by The Institute of Optics and Electronics, Chinese Academy of Sciences as a sister journal of Opto-Electronic Advances (OEA, IF=9.682). OES is dedicated to providing a professional platform to promote academic exchange and accelerate innovation. OES publishes articles, reviews, and letters of the fundamental breakthroughs in basic science of optics and optoelectronics.

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Han RZ, Zhang YC, Jiang QL et al. Ultrafast dynamics of femtosecond laser-induced high spatial frequency periodic structures on silicon surfaces. Opto-Electron Sci 3, 230013 (2024). doi: 10.29026/oes.2024.230013

Opto-Electronic Science

10.29026/oes.2024.230013

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