Abstract
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.
Highlights
The science of laser ablation won its development and popularity mainly due to its unique applicability in various fields like materials etching or synthesis, micro/nano-systems production, biomedical microprocessing, etc
For the first time the transient attenuation induced by a short laser pulse is reported along with a cavitation model based on a combined interaction of the viscosity drop, the tension wave and the ablation gases
In the experiments dedicated to the study of the mechanism of laser-induced foaming of gelatin film, the fast rise of attenuation before the expansion of the foam is measured for a fluence F0 = 1.13 J/cm2 above the threshold
Summary
The science of laser ablation won its development and popularity mainly due to its unique applicability in various fields like materials etching or synthesis, micro/nano-systems production, biomedical microprocessing, etc. The curiosity of scientists was attracted by the so-called cleanliness of ultraviolet (UV) laser ablation (LA), a clean cut or etching, well defined by the small size of the beam spot on the surface of the material. Such approach was even proposed for subcellular nanosurgery by using the radiation femtosecond laser in Vogel (2005). In the experiments, this highly efficient phenomenon is achieved e.g. in the ArF laser ablation of cornea (λ = 193 nm)
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