NONTHERMAL LASER-ABLATION MODEL FOR MICRO-SURGICAL APPLICATIONS

We have developed a non-thermal laser ablation model which may reduce thermal damage to neighboring structures. Based on this model, the thr ee critical parameters for a well controlled non-thermal microsurgery are (1) the laser wavelength with its photon energy matching closely t he bond dissociation energy, (2) the energy fluence must be above thre shold to avoid thermal process due to non-radiative relaxation from th e excited electronic states to vibrational, (3) ultra short laser puls es (few fs) to completely eliminate thermal and direct biomolecular re actions. in this model the UV laser photon dissociates the molecular b onds which leads to :he splitting of longer polymer chains into small fragments. The excess energy if any may appear as kinetic energy in th e polymer-fragments. The extreme rapidity of the bond breaking process reduces heat conduction. The model establishes a relationship between ablation depth per pulse, the absorption coefficient, the incident la ser energy fluence, and the threshold energy fluence. The ablation dep ths per pulse were calculated for the polymers Polymethyl methacrylate (PMMA) and polyimide for various commercially available UV lasers. It has been found that the minimum ablations depth occurs at 193 nm for both PMMA and polyimide. This assures a well defined incision with min imal thermal damage to the surrounding structures at this wavelength. There exists a definite threshold energy fluence for non-thermal ablat ion for any given biomolecule and below the threshold the non-radiativ e relaxation process may cause thermal ablation. New ultra fast lasers (few femtoseconds) (fs) will completely eliminate thermal diffusion a s well as direct biomolecular reactions.