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Ultrafast `pulsetrain-burst' machining has proven to deliver qualitative advantages for the processing of hard materials. This approach to tailoring the delivery of radiant exposure uses microsecond bursts of picosecond or femtosecond pulses. The mixed-timescale mode reaps the rewards of providing laser-material interactions at ultrashort timescales, while allowing for some control of heat and stress dissipation over longer timescales. As a result, our group has been able to ablate hard materials such as aluminum [1], as well as optically transparent materials like fused silica [2,3] beyond the capabilities of more conventional methods that employ slower repetition rates. It is well known that gross differences in the end results of materials-processing are the result of the manner in which the laser fluence is delivered - wavelength, continuous-wave vs. pulsed, pulse shape, and the pulse duration itself all have significant impact, and each parameter-value has significant advantages. For brittle materials, where limited tolerance to heat and tensile stress sets limits on the etch-depth possible for multi-kHz repetition rates, 'pulsetrain-burst' machining has enabled us to drill deeper and more cleanly, while eliminating damage due to thermal cycling and over-pressure shocks.
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