Ultrafast laser ablation of 10-nm self-supporting membranes by two-beam interference processing

Significance 

Nanoengineering has undergone tremendous technological advancement characterized by developing more powerful and efficient fabrication techniques to meet the application demands. Most of these techniques, such as three-dimensional nanofabrication and direct lithography, use light. To this note, the majority of the reported electron phase elements were fabricated using a combination of focused ion beam and advanced manipulation techniques. Unfortunately, these techniques are generally expensive and time-consuming. Moreover, the large momentum transfer to the materials by the ion beams makes it difficult to optimize the processing parameters without breaking the membrane. In an attempt to overcome these challenges, researchers have recently identified ultrafast laser ablation as a promising alternative owing to the nonthermal processing nature that enables the treatment of nanomembranes.

Previously, laser ablation technique was used to fabricate holographic diffraction gratings to generate electron vortex beams in transmission electron microscopy. The method was observed to allow the processing of two-dimensional periodic structures with remarkably better processing speed than focused ion beam milling. Motivated by these results, a team of Tohoku University researchers: Dr. Yuuki Uesugi, Ryota Fukushima (Master Student), Professor Yuichi Kozawa, and Professor Shunichi Sato, explored and demonstrated the potential application of ultrafast laser ablation techniques in processing nm-thick self-supporting membranes. The authors aimed to obtain critical design parameters in the processing of nanomembranes as well as to reduce the grating spacing to achieve a larger diffraction angle desirable for most practical applications. Their work is currently published in the research journal, Optics Express.

Briefly, self-supporting membranes with thicknesses 10 and 30 nm were used to fabricate gratings with a diameter of 16 µm via a single-shot laser irradiation technique. The fabricated geometry was measured by varying the input pulse energy. Also, the relationship between the resulting geometry and the distribution of the superposed input pulses was examined. Finally, the applicability of the ultrafast laser ablation technique was validated by comparing it to conventional focused ion beam milling in terms of processing speed and ease of designing the processing parameters.

According to the findings, laser fluence was identified as the most critical parameter in nanomembrane processing. The fabricated geometry was found to depend on the intensity distribution of the input pulses. At a shorter wavelength, the scale of the periodic spacing was significantly reduced to the sub-micrometer scale resulting in a grating spacing of 0.75 µm. The decrease in the grating space suggested an increase in the diffraction angle preferable for most practical applications. Compared to conventional focused ion beam milling, ultrafast laser ablation offered remarkably high processing speeds and ease of designing processing parameters.

In summary, the research team reported the ultrafast ablation of 10-nm thick self-supporting membranes via a two-beam interference processing technique. Based on the experimental results, laser fluence was the critical design parameter in nanomembrane processing. Furthermore, the ablation technique outperformed the conventional focused ion beam milling in processing speed and ease of designing processing parameters. The results showed that the fabricated gratings are potential candidates for manufacturing membrane-based micro-electro-mechanical and electron-optical phase systems. In a statement to Advances in Engineering, the authors said that the presented method provided useful insights that can be used to verify the various unexplained laser processing dynamics.

Ultrafast laser ablation of 10-nm self-supporting membranes by two-beam interference processing - Advances in Engineering

About the author

Yuuki Uesugi received his PhD in 2016 from Graduate School of Advanced Sciences of Matter, Hiroshima University, Japan. He is currently an assistant professor at Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Japan, and also affiliated to PRESTO of Japan Science and Technology Agency.

His research focuses on the physics of light-electron interaction for electron microscopy and accelerator applications, which includes the use and development of nanofabricated electron-optical devices.

About the author

Ryota Fukushima received his MSc in 2020 from Department of Materials Science, Graduate School of Engineering, Tohoku University. He is currently working at Amada Co., Ltd.

About the author

Yuichi Kozawa received the Ph.D. degrees in Engineering from Tohoku University in 2008. In 2008, he joined IMRAM, Tohoku University as an assistant professor and has worked as an associate professor since 2016.

About the author

Shunichi Sato received his Bachelor’s degree in Science in 1981, Master’s degree in Engineering in 1983, and Doctorate in Engineering in 1992 from Tohoku University. He started his academic career with laser application to bioscience and medicine, and expanded his interest to materials science. He became Professor of IMRAM, Tohoku University in 2003. He has believed in the extreme potential of light and is trying to explore its hidden peculiarities.

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Reference

Uesugi, Y., Fukushima, R., Kozawa, Y., & Sato, S. (2020). Ultrafast laser ablation of 10-nm self-supporting membranes by two-beam interference processingOptics Express, 28(18), 26200-26206.

Go To Optics Express

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