Microspheres non-contact ultrafast laser nanopatterning technology
(Nanowerk NewsIn recent decades, the development of nanofabrication technology has been driven by the need to increase component density and performance, which require high accuracy in material processing and manufacturability in atmospheric environments. Compared to other advanced processing methods, ultrafast laser processing has been recognized as one of the most widely used tools for micro/nano styling.
However, the main challenge of ultrafast laser processing to produce very small features is the optical diffraction limit. The heat affected zone by this technique is still much larger than that of the nanostructures, most of which exhibit >300 nm melting zones. Using dielectric microspheres as near-field lenses for high-resolution nanoscale imaging and nanofabrication has attracted great research interest. An optical phenomenon known as photonic nano-jets can contribute to focusing laser light to overcome diffraction limits.
To increase the throughput of ultrafast microsphere laser processing, self-assembly methods and micro-lens array lithography have been developed to fabricate surface patterns with fast speed and low cost. In addition to the nano-hole structures achieved by the contact mode, femtosecond microsphere laser fabrication can also realize arbitrary structures on the sample surface in the non-contact mode. By lifting the microspheres so that a gap forms between the sample and the microspheres, the working distance can be increased to several micrometers.
This strategy leads to far-field working microspheres. In this case, the surface structure feature size can only be reduced to ~300 nm by femtosecond laser irradiation of 405 nm, 512 nm, and 800 nm, which is far from the optical diffraction limit. Thus, how to achieve a good balance between working distance and feature size is an important issue for microsphere-assisted laser fabrication.
To overcome the above problems, Prof.’s research group. Minghui Hong from Xiamen University and the National University of Singapore, and Prof. Tun Cao from Dalian University of Technology co-reported on an ultrafast laser processing technology based on non-contact microspheres, realizing <50 nm functional nano-patterns on the surface of phase change materials. In the non-contact mode, the microspheres are placed on a specially designed stand and the nanostructures can be obtained by flexibly controlling the microspheres in an xyz scan.
In this case, the distance between the microspheres and the sample is on the order of microns. Through femtosecond laser irradiation of the microspheres, the new technology enables high-speed machining of nanostructures with finer features in a non-contact mode under a wide range of conditions.
The researchers also analyzed and explained the mechanism for the formation of these nanostructures. By theoretical calculations, the size of the focused spot of the incident laser passing through a 50 µm microsphere is only ~678 nm. Due to the nonlinear effects of ultrafast lasers, including two-photon absorption and upper-threshold effects, nanostructured features can be reduced to sub-50 nm. Therefore, the surface nanostructure is associated with the joint effect of microsphere focusing, absorption of two photons, and upper threshold effect of ultrafast laser irradiation.
This method provides a new idea for nano machining of ultrafine laser surfaces, and its machining efficiency and machining freedom are expected to be further optimized and improved through microsphere arrays and microsphere engineering.
The team published their findings at Advances in Opto-Electronics (“Sub-50 nm femtosecond laser microspheres in the far field via non-linear absorption”).