Submicrometer surface structuring with a Bessel beam generated by a reflective axicon

In ultrashort pulse (USP) laser ablation, focus diameters in the range of >20 mm are common for microstructuring, but the demand for much smaller structure sizes is rising, especially in the fields of filter technology, surface functionalization, and electronics. However, strong focusing of a Gaussian beam near the diffraction limit is accompanied by a very limited depth of focus, which leads to an extreme increase in process sensitivity. It is often too challenging to meet the necessary precision requirements for the system technology. A potential solution to overcome the problem of the short focus depth is the usage of a nondiffracting Bessel beam that is well known for providing a depth of field in the mm range while allowing the diameter of the central processing spot to be below 1 mm. There are several ways to generate a Bessel beam, but only an axicon is suitable for efficient high-power USP ablation. However, even high-precision manufactured axicons have a round tip resulting in a highly oscillating intensity along the propagation axis. This characteristic is a major obstacle for reproducible and reliable laser nanostructuring of metals. For this reason, reflective axicons were newly introduced to the market. They generate a Bessel beam much closer to the ideal axial intensity distribution. In this paper, we compare the Bessel beam generated by a reflective axicon with that of a conventional axicon in an application-oriented setting. Furthermore, we demonstrate the enormous potential of Bessel beams for surface structuring.