Generation of Shock Waves in Confined Excimer Laser Ablation of Polyimide
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GENERATION OF SHOCK WAVES IN CONFINED EXCIMER LASER ABLATION OF POLYIMIDE ADRIAN D. ZWEIG AND T.F. DEUTSCH Wellman Laboratories of Photomedicine, Hospital, Boston, MA 02114
Massachusetts General
ABSTRACT We study acoustic effects associated with the ablation of polyimide under water confinement. Ablation launches a shock in the target. After the shock propagates into the surrounding water we measure its velocity with an optical probe system. In the water the shocks decay after propagating a few hundred microns. The peak shock pressure scales with the square root of the laser fluence. Our observations can be explained by a simple model that interprets the generated pressures as being due to the heating of a confined ideal gas.
INTRODUCTION A shock wave differs qualitatively from an acoustic pressure transient. It is characterized by a supersonic propagation speed and a very steep front that for all practical purposes can be treated as a discontinuity in pressure, density, particle velocity and internal energy. At a shock front the pressure rises a few kbars within a distance of some nanometers, resulting in pressure transients with risetimes of only a few picoseconds [I]. A shock also changes the thermodynamic state of the medium in which it propagates in a different way than an adiabatic process does. During shock propagation all the energy dissipation occurs within the shock front, which is very thin. Because of this, shock waves are unique, very intense sources of energy that are possibly relevant for causing photoacoustic damage to cells, macromolecules and tissue. The aim of this study was to investigate the conditions under which shocks are generated by excimer-laser ablation of polymers and biological tissues. To date, only acoustic pressure transients of several hundred bars with risetimes of a few nanoseconds have been measured [2,3]. In order to study the acoustic effects associated with UVlaser ablation we investigated the ablation of a thin (25/um) sheet of polyimide immersed in distilled water. Using an optical probe system we measure the velocities of the laser-induced stress waves in the water as a function of incident laser fluence. The measured supersonic velocities indicate the formation of shock waves. Because the equation of state of water is known, the corresponding peak pressures follow directly from the jump conditions at the shock front. Our observations are explained by a simple ideal-gas model. We show that shocks are generated by the expansion of the hot gaseous ablation products.
Mat. Res. Soc. Symp. Proc. Vol. 236. (11992 Materials Research Society
48
MATERIALS AND METHODS The setup is illustrated in Fig. 1. For ablation we use a XeCl excimer laser (Lambda Physik model EMG 103 MSC) emitting pulses of a duration of 20 ns at 308 nm wavelength. To select an uniform part of the laser beam we place a 5-mm aperture in its path. The beam is attenuated by filters and focused by an aspheric lens with a focal length of 18 mm into a cuvette that is filled with distilled water. As targets we
