Full length articleFemtosecond laser surface structuring of silicon with Gaussian and optical vortex beams
Introduction
In the field of femtosecond (fs) laser surface processing, the formation of quasi-periodic surface structures is a striking feature of increasing interest due to the crucial role played by the surface morphology in regulating several material properties [1], [2], [3]. The process of surface structures generation is boundary-less in terms of materials (metals, semiconductors and dielectrics), and shows interesting outlooks in the fabrication of functional surfaces, whose characteristics (e.g., optical, wetting, hydrophobic, sensing, contaminants or pathogens adhesion, and antimicrobial efficacy) depend on the peculiar nano- and micro-structure of the surface [4], [5], [6], [7], [8], [9], [10], [11]. The features of the produced periodic surface structures vary with wavelength and polarization of the laser light and material properties. The typical quasi-periodic surface structures induced by fs laser pulse irradiation are generally indicated as ripples or (low-frequency) laser-induced periodic surface structures (LIPSS). The ripples are characterized by a sub-wavelength period and a preferential orientation. In metals and semiconductors, they are typically directed along the normal to laser polarization, meanwhile for dielectrics (fused silica and crystalline SiO2, e.g.) a spatial disposition parallel to the laser polarization is observed [1], [12], [13]. Recently, the development of other, supervening quasi-periodic surface structures, named as grooves, was reported in semiconductors (Si and InP, e.g.) irradiated by a large number of laser pulses, and at higher fluence than ripples [14], [15], [16], [17], [18], [19], [20]. Grooves are microstructures with preferential orientation along laser polarization, and a period larger than laser wavelength. Moreover, all surface structures produced by fs laser structuring are always decorated with nanoparticles produced during the laser ablation process [1].
Several mechanisms have been considered for the formation of ripples on metals and semiconductors irradiated with fs pulses, which include inhomogeneous energy deposition due to the interference between the incident beam and a scattered interface field, excitation of surface plasmon polaritons (SPPs), self-organization of surface instabilities, hydrodynamics, etc. [see e.g. Refs. [1], [2], [3] and papers therein quoted]. However, no widespread consensus has been reached yet on the mechanism leading to ripples formation [1], [21], while grooves generation attracted more interest only very recently [14], [15], [16]. In this respect, silicon is often the case study material, since it shows all kinds of surface structures and presents always interest because of its usefulness in electronic industry.
Here, we report an experimental study of the surface structures produced on a crystalline silicon target irradiated, in air, with Gaussian and Optical Vortex (OV) beams with fs pulse duration. OV are light beams carrying Orbital Angular Momentum characterized by a helical wave-front and controllable polarization patterns [22], [23], that can offer the possibility of generating more complex surface structures [24], [25], [26], [27], [28], [29], [30]. Our experimental findings suggest that the use of fs OV beams allows one to further extend the possibilities offered by the more standard Gaussian beam approach in tailoring the morphological features of the surface structures.
Section snippets
Experimental methods
The fs laser pulses are provided by a regenerative Ti:Sapphire amplifier delivering linearly polarized pulses with ≈35 fs duration at a central wavelength of 800 nm. A combination of half wave-plate and polarizer was used to control the pulse energy. Experiments were carried out with Gaussian and OV beams. The beam with a Gaussian spatial intensity profile is provided by the fundamental output of the laser source. The OV beam is generated by exploiting an efficient beam converter based on a
Results and discussion
In the following, we first illustrate the typical morphological features developed on the surface irradiated by Gaussian and OV beams. Then, we analyze the relative prominence of the two surface structures in the irradiated area, as well as the variation of the typical spatial period of ripples and grooves, as a function of the pulse energy, E0, and number of pulses, N. Finally, we discuss a physical mechanism interpreting the grooves generation as due to a progressive aggregation and melting
Summary
We have experimentally investigated the process of fs laser surface structuring of silicon with both Gaussian and OV beams, in air. The different spatial profiles of the two beams allow evidencing that the surface develops a texture that depends on the local value of the laser pulse fluence and number of laser pulses, as well as on the local SoP. Sub-wavelength ripples, aligned along a direction normal to the laser polarization, mainly forms in peripheral regions of the beam characterized by
Acknowledgements
A.R. acknowledges funding from the European Union (Programme FP7-PEOPLE-2012-CIG, Grant Agreement No. PCIG12-GA-2012-326499-FOXIDUET). Z.S. thanks the Tianjin Municipal Education Commission for providing his scholarship grant. This work was supported by the European Research Council (ERC), under grant no. 694683 (PHOSPhOR).
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