Abstract
In the last few years femtosecond optical vortex beams with different spatial distributions of the state of polarization (e.g. azimuthal, radial, spiral, etc.) have been used to generate complex, regular surface patterns on different materials. Here we present an experimental investigation on direct femtosecond laser surface structuring based on a larger class of vector beams generated by means of a q-plate with topological charge q = +1/2. In fact, voltage tuning of q-plate optical retardation allows generating a family of ultrashort laser beams with a continuous spatial evolution of polarization and fluence distribution in the focal plane. These beams can be thought of as a controlled coherent superposition of a Gaussian beam with uniform polarization and a vortex beam with a radial or azimuthal state of polarization. The use of this family of ultrashort laser beams in surface structuring leads to a further extension of the achievable surface patterns. The comparison of theoretical predictions of the vector beam characteristics at the focal plane and the generated surface patterns is used to rationalize the dependence of the surface structures on the local state of the laser beam, thus offering an effective way to either design unconventional surface structures or diagnose complex ultrashort laser beams.
Highlights
IntroductionSurface morphology plays a crucial role in defining many properties (e.g., optical, mechanical, chemical, biological, wetting, etc.) of a material
Surface morphology plays a crucial role in defining many properties of a material
Going beyond standard radially and azimuthally polarized vortex beams, in this paper we report on direct fs laser surface structuring of silicon using a larger class of vector beams generated by means of a q-plate with topological charge q =+1/2, obtained by varying the optical birefringent retardation δ of such device[22]
Summary
Surface morphology plays a crucial role in defining many properties (e.g., optical, mechanical, chemical, biological, wetting, etc.) of a material. The creation of surface structures depends on several experimental parameters including, e.g., wavelength λ, fluence F, number N of femtosecond (fs) laser pulses hitting the target surface, and material properties, but the key-factor is related to the state of polarization (SoP) of the laser beam. This strong influence is evidenced by the preferential alignment as well as the degree of order/disorder of the emerging surface patterns. The various features of the surface patterns and the vector beams characteristics at the focal plane are compared, demonstrating how the spatial variation of the local state of the laser beam offers an effective way to both design unconventional, asymmetric surface structures and characterize complex ultrashort laser beams
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