Micro-processing of transparent material by modified Bessel beams generated with spatially displaced axicons

The Bessel-Gauss beams being distinguished by an elongated focal zone have attracted great interest and potential in various laser micro-machining applications. The ability to create high width/depth ratio modifications inside transparent materials has opened new perspectives, ranging form micro-channel fabrication [1] to efficient cutting of various glasses [2] . The mostly used optical element to produce Bessel beam is an axicon. However, the shape of the axicon must be almost ideal, because any irregularities of its surface and especially a rounded tip greatly reduces the quality of the generated beam by inducing unwanted axial intensity modulation [3] . Hence, alternatives have emerged to substitute glass axicons to increase the overall quality of the beam or create more complex patterns, these are diffractive optical elements, sub-wavelength metasurfaces or liquid crystals (spatial light modulators) [4] , [5] to name a few. Ability to choose the Bessel beam generation method allows one to have a high level of freedom in application of them in various specific manufacturing processes. However, the price or low optical damage threshold of the beam shaping elements may prevent from wide applicability. Alternative beam shaping method can be utilised - the so-called geometric phase optical elements (GPOEs). These elements are based on the anisotropy of nanogratings inscribed by laser inside the fused silica [6] . The control of orientation and retardance of nanogratings enables to create various phase retarders that can work as custom flat beam shaping optical elements.