The fabrication of large-scale periodic nanostructures on surfaces is of paramount importance in nanotechnology and materials science. The common techniques such as photolithography, electron-beam lithography, imprint lithography and laser interference lithography require either high-cost, complex systems or offer limited flexibility.
As an alternative, femtosecond laser-induced periodic surface structuring provides a flexible and potentially very low-cost method. Its applications have significant potential in medicine, optics, tribology, and biology, among other areas. However, when irradiated by a large intense laser spot, the periodic structures usually exhibit an uncontrollable regularity, forming bifurcated patterns, thus limiting their widespread application. The irregularity usually originates from numerous independent branching seeds. The usual solution to this stubborn problem is to utilize the quasi-direct laser writing technique, that is, by limiting the laser beam size (diameter of < 10 wavelengths) and scanning the beam or samples using 2D translation stages.
In a new paper published in Light: Advanced Manufacturing, a team of scientists, led by Professor Min Qiu from Westlake University, China, have experimentally demonstrated an optical localization-induced nonlinear competition mechanism to solve this problem. The experiments were carried out on amorphous silicon films. The required femtosecond laser fluence is nearly one order of magnitude below the ablation threshold of silicon. Thus, the surface nanostructures are produced by oxidation rather than ablation. This oxidation-related nonlinear competition mechanism ensures self-selection of a single seed to initiate an array of bifurcated-free gratings under stationary irradiation with a large lasers pot (diameter > 100 wavelengths). Surprisingly, some unconventional complex patterns, such as radial, annular, and spiral gratings, can also be easily produced by structured light fields with unprecedented regularity. Their diameters reach up to >500 μm.
These scientists summarize the underlying physics of this mechanism:
"The growth process of oxidation-induced periodic nanostructures is beyond the ablative process. The ablation usually results in residual heat and ablative debris, which disturb the propagation of surface electromagnetic waves and distort the nanostructures. The oxidation process, occurring at much lower threshold and producing less material ejection, solves these two problems."
"Moreover, we found that the near-field enhancement plays an important role, which is usually ignored. The field enhancement is nonlinearly dependent on the particle sizes.
The larger nanoparticles acquire a higher fieldenhancement and thus a faster growth speed, forming a positive feedback. This feedback allows the self-selection of a single seed to initiate an array of bifurcated-free one-dimensional nanograting under stationary irradiation with a large laser spot. We refer to this nonlinear feedback as an optical localization-induced nonlinear competition mechanism."
"As a further step, we demonstrated the use of laser direct writing to control the laser-induced self-organization process. The former technique produces seeding structures, which guide the self-organized growth process."
"The size of the produced periodic nanostructures is limited by our laser pulse energy, which is 200 μJ. If one has much higher laser energy, the produced patterns can be much larger."
"Our method opens a new avenue to control the laser-induced periodic surface structuring by laser direct writing. It allows one to produce large-scale high-quality thin-film nanogratings." The scientist forecast.