Ultrafast Bessel beams for high aspect ratio taper free micromachining of glass

Although ultrafast lasers have demonstrated much success in structuring and ablating dielectrics on the micrometer scale and below, high aspect ratio structuring remains a challenge. Specifically, microfluidics or lab-on-chip DNA sequencing systems require high aspect ratio sub-10 μm wide channels with no taper. Micro-dicing also requires machining with vertical walls. Backside water assisted ultrafast laser processing with Gaussian beams allows the production of high aspect ratio microchannels but requires sub-micron sample positioning and precise control of translation velocity. In this context, we propose a new approach based on Bessel beams that exhibit a focal range exceeding the Rayleigh range by over one order of magnitude. An SLM-based setup allows us to produce a Bessel beam with central core diameter of 1.5 μm FWHM extending over a longitudinal range of 150 μm. A working window in the parameter space has been identified that allows the reliable production of high aspect ratio taper-free microchannels without sample translation. We report a systematic investigation of the damage morphology dependence on focusing geometry and energy per pulse.