Azimuthally modulated axicon vortical beams for laser microprocessing

Non-diffracting beams have contributed greatly to recent developments in high power applications such as laser micromachining. Well known for their elongated focal region (also known as a focal line) are zeroth-order Bessel and Bessel vortical beams which allow achieving high aspect ratio modifications inside transparent materials. A superposition of several optical Bessel vortices of different topological charges allows generating high aspect ratio beams with complexly engineered transverse intensity patterns. Non-diffracting Bessel vortices commonly are created using a conical prism — an axicon. Therefore, they can be referred to as axicon beams. In this work, we have studied the formation of high power and energy axicon vortical beams both numerically and experimentally. We have inscribed sub-wavelength birefringent structures inside fused silica glass to create geometrical phase elements for the azimuthal modulation of the spatial spectra of a zeroth-order axicon (Bessel) beam which allows creating a high power superposition of several Bessel vortices. To prove the concept, we apply azimuthally modulated axicon vortical beams to create modifications inside fused silica glass samples.