Ablation-cooled material removal with ultrafast bursts of pulses

The use of femtosecond laser pulses allows precise and thermal damage -free removal of material (ablation) with wide-ranging scientificl(1,5), medical(6-11) and industrial applications(12). However, its potential is limited by the low speeds at which material can be removed(1,9-11,13) and the complexity of the associated laser technology. The complexity of the laser design arises from the need to overcome the high pulse energy threshold for efficient ablation. However, the use of more powerful lasers to increase the ablation rate results in unwanted effects such as shielding, saturation and collateral damage from heat accumulation at higher laser powers(6,13,14). Here we circumvent this limitation by exploiting ablation cooling, in analogy to a technique routinely used in aerospace engineering(13,16). We apply ultrafast successions (bursts) of laser pulses to ablate the target material before the residual heat deposited by previous pulses diffuses away from the processing region. Proof-of-principle experiments on various substrates demonstrate that extremely high repetition rates, which make ablation cooling possible, reduce the laser pulse energies needed for ablation and increase the efficiency of the removal process by an order of magnitude over previously used laser parameters(17,18). We also demonstrate the removal of brain tissue at two cubic millimetres per minute and dentine at three cubic millimetres per minute without any thermal damage to the bulk(9,11).