  Here, to the best of our knowledge, for the first time we report the in-depth
experimental study of high ultrafast laser ablation efficiency for processing
of copper and steel with single-pulses, MHz-, GHz- and burst in the burst
(biburst) regime. The comparison of burst, biburst, and single-pulse ablation
efficiencies was performed for beam-size-optimised regimes, showing the real
advantages and disadvantages of milling and drilling processing approaches.
Highly-efficient ultrashort pulse laser processing was achieved for ~1 um
optical wavelength: 8.8 um3/uJ for copper drilling, 5.6 um3/uJ for copper
milling, and 6.9 um3/uJ for steel milling. We believe that the huge
experimental data collected in this study will serve well for the better
understanding of laser burst-matter interaction and theoretical modelling.
