In this work, the effects of direct femtosecond laser nanostructuring on monocrystalline silicon (Si) wafer immersed in liquid environment were studied. Ultrashort laser exposure induced nonequilibrium states in materials triggers the morphology self- organization driven by excited electromagnetic and hydrodynamic processes, which ultimately give rise to the formation of regular nanogratings with different periodicity (Λ1=250 nm to 300 nm, Λ2=70±10 nm) and randomly arranged spike structures. The type of the nanotextures can be controlled by varying the laser fluence and polarization distribution of the incident laser beam studied for the cases of linearly/circularly polarized Gaussian laser beams as well as azimuthally polarized cylindrical vector beam. Through laser-induced interface chemical reactions stimulated by adding the appropriate salt/acid/molecular precursors in the process of liquid-phase Si texturing, it becomes possible to functionalize the obtained nanotextures with mono- and multi-metallic nanoparticels and/or photoluminescent chemosensing molecular probes. Adjusting laser processing parameters and component functionalizing solutions has providing a flexible approach for large-scale manufacturing that can be realized for diverse applications such as light harvesting, chemosensing, optical detection, heterogeneous catalysis and microfluidics.
We present the results of direct laser-induced periodic surface structuring of semiconductors thin films (a-Si, a-Ge) deposited on glass substrate at different ambient environments (air, vacuum, nitrogen) resulting in regular gratings with the period of 600 nm to 900 nm at the laser wavelength of 1026 nm oriented either along (a-Si) or transverse (a-Ge) to the linear laser polarization direction. The processing speed has a different effect on morphology of obtained structures: on a-Si film, an increase of scanning speed leads to the reorientation of gratings and reduction of their period, while on a-Ge, the uniformity degradation and increase of the period are observed. Changing the ambient atmosphere from air to nitrogen and vacuum, when writing structures on a-Ge, helps to minimize the uniformity degradation and obtain highly regular nanogratings.
Recently, highly regular thermochemical laser-induced periodic surface structures (TLIPSS) have become the subject of active studies. TLIPSS are formed in the interference maxima due to the local oxidation of the material irradiated with ultrashort laser pulses and are characterized by the elevation of the relief that forms parallel oxide protrusions. The gas surrounding is expected to affect the morphology and chemical composition of the resulting TLIPSS; however, such effects were rarely studied so far. Here we present the results of the TLIPSS fabrication on glass-supported Si-Ti bilayer films using an astigmatic Gaussian IR femtosecond beam both in air and a nitrogen-rich atmosphere. The formation of ordered TLIPSS with the period of ≈ 920 nm is observed at slow scanning speeds (∼1 μm/s) and low fluences in a nitrogen-rich atmosphere. Raman spectroscopy revealed the presence of TiO2 (rutile) peaks, as well as bands centered at 280 cm-1 and 320 cm-1, which can be related to TiN in amorphous and polycrystalline phase.
Recently, chiral mass transfer on the surface of plasmonic-active metals appeared upon their ablation with vortex laser pulses was found to be driven by a helical-shape temperature and corresponding surface tension gradients rather than optical angular momentum transfer from the incident beam. Here, we demonstrate that using of perfect optical vortices with different topological charges for direct single-pulse laser ablation of noble-metal films don't allow to change the helicity of produced nanoneedles (also called nanojets). Meanwhile, the chirality of laser-induced nanojets can be tuned in a wide range of parameter by properly designing and tailoring the spiral-shape intensity patterns. Such optimization of the laser intensity profile governing the helical movement of the transiently molten metal allows to produce nanostructures with controlled chirality suited for various nanophotonics and biosensing applications.
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