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Fig. 1. Experimental setup of non-get in touch with microsphere femtosecond laser irradiation and the fabricated nano-structures. Credit: Compuscript Ltd

In current decades, the improvement of nano-fabrication technologies is driven by the require to improve the density of elements and functionality, which calls for higher accuracy in material processing and the capability of manufacturing in an atmospheric atmosphere. Compared to other sophisticated processing procedures, ultrafast laser processing has been recognized as 1 of the most extensively applied tools for micro/nano-structuring.

Nevertheless, the essential challenge of ultrafast laser processing to generate very little attributes is the optical diffraction limit. The heat impacted zone by way of these strategies is nonetheless a great deal bigger than the nano-structures, which mainly exhibit &gt300 nm melting zone.

Making use of a dielectric microsphere as a close to-field lens for super-resolution nano-imaging and nano-fabrication has attracted good investigation interest. The optical phenomenon recognized as photonic nano-jet can contribute to laser beam focusing to overcome the diffraction limit. To improve the microsphere ultrafast laser processing throughput, the self-assembly process and micro-lens arrays lithography have been created to fabricate surface patterns at a rapidly speed and low price.

In addition to nano-hole structures accomplished by get in touch with mode, the microsphere femtosecond laser fabrication can also understand arbitrary structures on sample surfaces in non-get in touch with mode. By lifting the microsphere to type a gap among the sample and the microsphere, the functioning distance can be elevated to various micrometers.

This approach leads to the microsphere functioning in far field. In this case, the function size of surface structures can only be lowered to ~300 nm by the 405 nm lamp, 512 nm, and 800 nm femtosecond laser irradiation, which is nonetheless far from the optical diffraction limit. As a result, how to realize a great balance among the functioning distance and function size is a crucial situation for microsphere assisted laser fabrication.

To overcome these complications, the investigation group of Prof. Minghui Hong from Xiamen University and the National University of Singapore, and Prof. Tun Cao from Dalian University of Technologies jointly reported an ultrafast laser processing technologies primarily based on non-get in touch with microspheres, realizing Opto-Electronic Advances.

In non-get in touch with mode, the microsphere is placed on a specially created holder, and the nano-structures can be obtained by flexibly controlling of microsphere in x-y-z scanning. In this case, the distance among the microsphere and the sample is in the order of microns. By way of the femtosecond laser irradiation of microsphere, this new technologies enables the higher speed machining of finer function nano-structures in non-get in touch with mode in different situations.

Fig. two. Formation mechanism of microsphere assisted femtosecond laser irradiation. Credit: Compuscript Ltd

The researchers also analyzed and explained the forming mechanism of these nanostructures. By theoretical calculation, the focused spot size of the incident laser passing via the 50 µm microsphere is only ~678 nm. Due to the nonlinear effects of ultrafast laser, which includes two-photon absorption and leading threshold impact, the function of nano-structures can be lowered down to sub-50 nm. Consequently, the surface nano-structures are attributed to the co-impact of the microsphere focusing, the two-photon absorption, and the leading threshold impact of the ultrafast laser irradiation.

This process delivers a new concept for ultrafine laser surface nano-machining, and its machining efficiency and machining freedom are anticipated to be additional optimized and enhanced by way of microsphere array and microsphere engineering.

Extra information and facts:
Zhenyuan Lin et al, Microsphere femtosecond laser sub-50 nm structuring in far field by way of non-linear absorption, Opto-Electronic Advances (2023). DOI: ten.29026/oea.2023.230029

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