PLASMA MEDIATED ABLATION OF BIOLOGICAL TISSUES WITH NANOSECOND-TO-FEMTOSECOND LASER-PULSES - RELATIVE ROLE OF LINEAR AND NONLINEAR ABSORPTION

Plasma mediated ablation of collagen gels and porcine cornea was studi ed at various laser pulse durations in the range of 1 ns-300 fs at 105 3-nm wavelength, It was found that pulsed laser ablation of transparen t and weakly absorbing gels is always mediated by plasma, On the other hand, ablation of strongly absorbing tissues is mediated by plasma in the ultrashort-pulse range only, Ablation threshold along with plasma optical breakdown threshold decreases with increasing tissue absorban ce for subnanosecond pulses, In contrast, the ablation threshold was f ound to be practically independent of tissue linear absorption for fem tosecond laser pulses, The mechanism of optical breakdown at the tissu e surface was theoretically investigated, In the nanosecond range of l aser pulse duration, optical breakdown proceeds via avalanche ionizati on initiated by heating of electrons contributed by strongly absorbing impurities at the tissue surface. In the ultrashort-pulse range, opti cal breakdown is initiated by multiphoton ionization of the irradiated medium (six photons in case of tissue irradiated at 1053-nm wavelengt h), and is less sensitive to linear absorption, High-quality ablation craters viith no thermal or mechanical damage to surrounding material were obtained with subpicosecond laser pulses, Experimental results su ggest that subpicosecond plasma mediated ablation can be employed as a tool for precise laser microsurgery of various tissues.