High-aspect-ratio, high-quality microdrilling by electron density control using a femtosecond laser Bessel beam

We propose an efficient microdrilling method of high-aspect-ratio, high-quality microholes in polymethyl methacrylate by controlling localized transient spatial electron density using single-pulse femtosecond laser Bessel beams. The microholes fabricated with diameters of 1.5–2.4 μm are taper-free, which are of much better quality in the entrances and sidewalls, as compared with those fabricated by Gaussian beams. The aspect ratio of the microholes is up to 330:1. It takes 42 min to fabricate a 501 × 501 microhole array (with 251,001 holes in total, about 100 holes per second under a repetition rate of 100 Hz) in a 1 cm × 1 cm area, which is very uniform in size and shape. For single-pulse drilling of a microhole array, the number of ultrahigh-aspect-ratio microholes processed per second is theoretically determined by the repetition rate. The liquid infiltration method and cross-sectional profile tests confirm hollow microhole drillings rather than material modifications. The theoretical simulation of optical intensity distribution and intensified charge-coupled device detection shows that fabricating such thin, long, uniform microholes using Bessel beams is attributed to electron density control by spatially shaping femtosecond laser pulses.