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