Abstract
The formation of periodic surface structures is a general effect of femtosecond laser irradiation of solid targets showing promising interest in material science and technology. However, the experiments are typically carried out in air, a condition in which the target surface becomes densely decorated with nanoparticles that can influence the formation of the surface structures in the early stage of the irradiation process. Here we report an investigation of structures generation on a silicon surface irradiated in vacuum (10−5 mbar) with a low number of laser pulses (N ≤ 10) that exploits several microscopy techniques (optical, atomic force, electron and Raman). Our analyses allow identifying the creation of silicon phase-change gratings consisting of alternating amorphous and crystalline periodic lines, with almost no material removal, located at the periphery of a shallow ablation crater. These gratings originate from two different kinds of defects: (i) the first is characterized by a peculiar lobed shape that is produced by the first few laser pulses; (ii) the second is provided by the one-dimensional, linear singularity defined by the ablation edge of the nascent crater. Both kind of defects lead to grating structures extending outwards the amorphous central area of the crater along the direction of the laser polarization. Comparative analysis with the surface formed in air, in the same experimental conditions, evidences the important role played by nanoparticles densely decorating the target in air and the striking variation occurring in vacuum.
Highlights
Direct femtosecond laser surface structuring is a versatile method to tailor material properties and the formation of laser-induced periodic surface structures (LIPSS) is a striking and extensively studied phenomenon with several applications[1,2,3]
The most established picture considers that interference between incident laser light and surface waves[3,4,5,6,7], which can be either light scattered at a rough surface or surface plasmon polaritons (SPPs), causes a modulation of the light intensity distribution that is eventually imprinted onto the material surface
These LIPSS are generally dubbed as ripples and in metals and semiconductors are preferentially oriented along the direction normal to laser polarization[3,8]
Summary
The extended frame of stripes with different Si phases around the crater is not formed in air, but a ring of nanoparticles is present instead While this indicates that the formation of peculiar localized defects is possible in air, the scarce presence of Si phase gratings surrounding the amorphous spot evidences the significant role played by the nanoparticles in mediating surface structure development at the early stage of the process. Our multi-imaging approach evidences that, besides the typical topographic ripples present in a shallow crater, phase-change gratings consisting of alternating amorphous and crystalline periodic stripes, with no material removal, form at its periphery The source of these phase gratings has been identified in two kinds of surface defects: the edge of the nascent ablation crater and peculiar lobe-shaped structures generated in the very first laser pulses. The comparison with results obtained at atmospheric air pressure, in the same experimental conditions, have confirmed the important role played by nanoparticles decorating the samples surface in this case and the merits of vacuum processing
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