Ultrashort-pulse burst-mode materials processing and laser surgery

Abstract

Laser processing of materials and biological tissues has evolved in stages, since the earliest use of the laser for gross deposition of heat and for ablation. For instance, wavelength specificity was an early development that facilitated the treatment of certain biological tissues, while leaving others relatively unaffected. Ultrashort-pulse material ablation escapes the usual paradigm of heat diffusion because of the comparisons of scales: a rarefaction wave can cut through the thin layer of femtosecond-laser-heated material and carry away the absorbed energy before much heat can diffuse into the substrate. Burst-mode femtosecond laser ablation brings yet another paradigm, in which the laser fluence is divided over two disparate timescales: the ultrashort duration of a pulse, and the microsecond-scale duration of a burst. This division of timescales opens new avenues for control, because much of the governing physics is about the comparison of timescales-for instance the timescale of thermalization of heated electrons into the substrate lattice, the timescale of hydrodynamic ablation, as these compare to the time, or now times, over which laser energy is deposited. Applications in fused silica, in vitro cell-cultures prepared in hydrogels, and ex vivo articular cartilage help to show what is different, in the science of ultrashort-pulse burst-mode laser processing.