The need to increase component density, as well as add performance, are long-standing drivers behind the development of nanofabrication technology. Both qualities require a high level of accuracy in materials processing and manufacturing capability. Ultrafast laser processing is used extensively for micro- and nanostructuring — although the optical diffraction limit presents a challenge to the use of ultrafast laser processing to fabricate extremely small features. When ultrafast laser processing techniques are used, the heat-affected zone is much larger than the nanostructures. In addition to having the ability to achieve nanohole structures by contact mode, microsphere femtosecond laser fabrication can be used to realize arbitrary structures on sample surfaces in noncontact mode. By lifting the microsphere to form a gap between the sample and the microsphere, the working distance can be increased to several microns — a step that leads to the microsphere working in far field. In this case, the feature size of surface structures can only be reduced to about 300 nm, which is still far from the optical diffraction limit. As a result, a balance between the working distance and the feature size is an important issue affecting microsphere-assisted laser fabrication. Researchers led by professor Minghui Hong from Xiamen University and the National University of Singapore and professor Tun Cao from Dalian University of Technology have developed an ultrafast laser processing technique to address this challenge. The technique uses noncontact microspheres to fabricate nanoscale patterns on the surface of phase change materials. In noncontact mode, the microsphere is placed on a specially designed holder, and the nanostructures are obtained through flexible control of the microsphere in x-y-z scanning. Through femtosecond laser irradiation of the microsphere, this approach enables the high-speed machining of fine-featured nanostructures in noncontact mode, under various conditions. Formation mechanism of microsphere-assisted femtosecond laser irradiation. Courtesy of Opto-Electronic Advances. The researchers used the technique to fabricate sub-50-nm nanostructures directly on thin films via microsphere femtosecond laser irradiation in far field and in ambient air. They tuned the feature size of the structures by varying laser fluence, scanning speed, and thin-film thickness. Linear fitting analyses based on the ablation width at different film thicknesses indicated that the feature size could be reduced to about 15 nm at a film thickness of about 10 nm. The researchers used the arbitrary surface structuring of nanostructures to demonstrate the flexible patterning capability of the microsphere femtosecond laser irradiation technique. According to the researchers, both the fabrication process and the excellent performance of the surface grating structures demonstrated the large area processing ability and high uniformity of the technique. This in turn showed that the technique can be used to achieve surface nanocreation with excellent performance, and that it is a feasible way. The researchers said that the ability to reduce the features of the nanostructures to under 50 nm was due to the nonlinear effects of the ultrafast laser, which include the combined effects of two-photon absorption, microsphere focusing, high-repetition-rate femtosecond-laser-induced incubation, and the top threshold effect of the ultrafast laser irradiation. The minimum feature size can be made as small as about 30 nm by manipulating the film thickness. The researchers calculated that, theoretically, the focused spot size of the incident laser passing through the 50-µm microsphere is only about 678 nm. Tests to fabricate a nanograting yielded a component that demonstrated strong beam diffraction performance. According to the researchers, the nanoscale resolution provided by the technique could be useful for next-generation laser nanolithography. In addition, the microsphere femtosecond laser irradiation technique could establish an approach to ultrafine laser surface nanomachining. The research team believes that the efficiency and freedom of the machining technique could be further improved by using a microsphere array, and through microsphere engineering. The research was published in Opto-Electronic Advances (www.oejournal.org/article/doi/10.29026/oea.2023.230029).
