Model of Laser-Induced Nano-Cavitation in the Surface of Gelatin Confronted with the Fast Rise Time of the Optical Attenuation Measurements

In the laser-polymer interaction a particular mechanism is evidenced when the polymer absorption coefficient is small. In this case the laser ablation is most of the time, but not necessarily, accompanied by intense bubbling, which results from the interplay between ablation gas, polymer melt and pressure wave. In this work, the inception of this phenomenon called cavitation, is monitored for the gelatin case (as a model polymer) by the fast extinction of a probe cwHeNe laser for several fluences of the KrF laser pulses (1.13, 0.65, 0.470 J/cm2) used to trigger ablation whose threshold is 0.5 J/cm2. A new model combining the time dependence of temperature, pressure and viscosity is developed for the simulation of this extinction due to the cavitation phenomenon and used to fit the experimental data. The model concludes that the cavity precursors with initial radius R0 = 3 nm and concentration n0 = 5×1013 cm-3 are suddenly expanded by the tension wave launched immediately after the absorption of the laser pulse. The probe laser radiation is attenuated by Rayleigh or Mie scattering with a rise time of the order of 110 ns, depending on the ablation fluence, and corresponding to the transit of the tension wave. The dynamics of the radius increase is given by the Rayleigh-Plesset equation and the thermodynamic kinetics of the growth is given by the classical theory of the homogeneous nucleation. The model shows that complete darkening of the sample surface is achieved only for a fluence larger than 1.1 J/cm2 and that the increase of the radius is limited to the range 3–100 nm, in this case. The approximations are discussed in the paper. In the experiments, the increase of the opacity of the sample surface occurs also at lower fluence (0.65 and 0.470 J/cm2) although with a slower dynamics. This reveals that another mechanism may be invoked to account for the slower opacity rise observed at lower fluences in the experimental data. The model excludes melting in these cases and indicates rather a solid phase transition due to the gas evolution.