Ablation-cooled material removal with ultrafast bursts of pulses

Ablation cooling is demonstrated as an effective means of removing material using successive bursts of laser pulses with short intraburst delay times; the technique allows the overall pulse energy to be decreased, overcoming negative thermal effects during the ablation process. Laser processing is widely used in both hard and soft materials applications. Now Ömer Ilday and colleagues, using custom-built laser technology able to deliver specific pulse sequences and repetition rates, have identified a regime of laser–material interactions that they designate the 'ablation-cooled regime'. In this regime, heat build-up due to processing is removed more quickly than it can diffuse through the material. As a result, the overall energy of the laser pulse is decreased, as are the negative thermal effects on the processed material. This is particularly important for surgical applications, and the authors demonstrate decreased energy and damage for a range of materials, including dentine and brain tissue. The use of femtosecond laser pulses allows precise and thermal-damage-free removal of material (ablation) with wide-ranging scientific1,2,3,4,5, medical6,7,8,9,10,11 and industrial applications12. However, its potential is limited by the low speeds at which material can be removed1,9,10,11,13 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 powers6,13,14. Here we circumvent this limitation by exploiting ablation cooling, in analogy to a technique routinely used in aerospace engineering15,16. 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 parameters17,18. 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 bulk9,11.