Asymmetric Shaping for Ultrafast Elliptical Bessel-like Beams

Abstract

The generation of an elliptical Bessel–Gauss beam has become a topic of interest in ultrafast laser processing of transparent materials because of its nearly non-diffractive elliptical central core. These beams can show potential in generating anisotropic structures down to the nanoscale and in producing asymmetries in the induced fields of thermo-mechanical constraints relevant for material structuring. However, maintaining the central core ellipticity is a challenge that requires further analysis, notably in the propagation behavior of phase anisotropies during the conical interference. This paper presents the controlled generation and propagation of a highly elliptical Bessel–Gauss beam using asymmetric phase-modulation technique. The study involves engineering different asymmetric phase holograms and analyzing their performances in terms of the non-diffractive property and uniformity of the generated beams. We indicate the presence in specific cases of diffraction and its influence on the invariance of the beam shape. The simulation results are in excellent agreement with the experimental results, which verifies the accuracy and reliability of our approach.

Axicon-lens-doublet focusing for the fabrication of ultra-high-aspect-ratio structures through silicon with infrared ultrafast lasers

“…20 However, this image-relay procedure involves important physical limitations as the generation of a high-angle micro-Bessel beam is always limited by the damage threshold and entrance pupil diameter of the last focusing element. 18,19 Toward the aim to produce extremely high-angle infrared Bessel-like beams, we have investigated an alternative simple method that can be widely implemented. Our approach is based on applying a controlled perturbation strategy, which involves introducing conical wavefront from an axicon to the spherical wavefront generated by a focusing lens in such a way that the different angular components do not all collapse at the same distance (defined by lens geometry) but are distributed more in space along the pre-focal region.…”

“…Our approach involves the use of an axicon-lens doublet to produce extremely high-angle pseudo-Bessel beam which is otherwise hardly accessible by standard imagerelay methods. 18,19 Using temporally stretched picosecond pulses combined with our focusing doublet we show how this allows efficient fabrication of 1-mm long reproducible ultra-high-aspect-ratio optical through-silicon modifications. These feature a uniform micrometric diameter with a high positive index change that opens its potential for light-guiding functionalities in Si.…”