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Fig. 1. Experimental setup of non-contact microsphere femtosecond laser irradiation and the fabricated nano-structures. Credit score: Compuscript Ltd

In latest many years, the event of nano-fabrication know-how is pushed by the necessity to enhance the density of elements and efficiency, which requires excessive accuracy in materials processing and the potential of producing in an atmospheric setting. In comparison with different superior processing strategies, ultrafast laser processing has been acknowledged as one of the crucial extensively used instruments for micro/nano-structuring.

Nevertheless, the important thing problem of ultrafast laser processing to supply extraordinarily small options is the optical diffraction restrict. The warmth affected zone by way of these strategies continues to be a lot bigger than the nano-structures, which largely exhibit >300 nm melting zone.

Utilizing a dielectric microsphere as a near-field lens for super-resolution nano-imaging and nano-fabrication has attracted nice analysis curiosity. The optical phenomenon generally known as photonic nano-jet can contribute to laser beam focusing to beat the diffraction restrict. To extend the microsphere ultrafast laser processing throughput, the self-assembly methodology and micro-lens arrays lithography have been developed to manufacture floor patterns at a quick velocity and low price.

Along with nano-hole buildings achieved by contact mode, the microsphere femtosecond laser fabrication also can understand arbitrary buildings on pattern surfaces in non-contact mode. By lifting the microsphere to kind a niche between the pattern and the microsphere, the working distance could be elevated to a number of micrometers.

This technique results in the microsphere working in far area. On this case, the function dimension of floor buildings can solely be lowered to ~300 nm by the 405 nm lamp, 512 nm, and 800 nm femtosecond laser irradiation, which continues to be removed from the optical diffraction restrict. Thus, the best way to obtain a very good steadiness between the working distance and have dimension is a crucial subject for microsphere assisted laser fabrication.

To beat these issues, the analysis group of Prof. Minghui Hong from Xiamen College and the Nationwide College of Singapore, and Prof. Tun Cao from Dalian College of Know-how collectively reported an ultrafast laser processing know-how primarily based on non-contact microspheres, realizing Opto-Digital Advances.

In non-contact mode, the microsphere is positioned on a specifically designed holder, and the nano-structures could be obtained by flexibly controlling of microsphere in x-y-z scanning. On this case, the gap between the microsphere and the pattern is within the order of microns. By way of the femtosecond laser irradiation of microsphere, this new know-how permits the excessive velocity machining of finer function nano-structures in non-contact mode in numerous circumstances.

Fig. 2. Formation mechanism of microsphere assisted femtosecond laser irradiation. Credit score: Compuscript Ltd

The researchers additionally analyzed and defined the forming mechanism of those nanostructures. By theoretical calculation, the targeted spot dimension of the incident laser passing by means of the 50 µm microsphere is barely ~678 nm. As a result of nonlinear results of ultrafast laser, together with two-photon absorption and prime threshold impact, the function of nano-structures could be lowered all the way down to sub-50 nm. Due to this fact, the floor nano-structures are attributed to the co-effect of the microsphere focusing, the two-photon absorption, and the highest threshold impact of the ultrafast laser irradiation.

This methodology supplies a brand new concept for ultrafine laser floor nano-machining, and its machining effectivity and machining freedom are anticipated to be additional optimized and improved by way of microsphere array and microsphere engineering.

Extra info:
Zhenyuan Lin et al, Microsphere femtosecond laser sub-50 nm structuring in far area by way of non-linear absorption, Opto-Digital Advances (2023). DOI: 10.29026/oea.2023.230029

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