Oscillations, Waves, and Interactions - GWDG

240 A. Vogel, I. Apitz, V. Venugopalan
Figure 14. Sequence of phase transitions and corresponding plume constituents in ablation
at radiant exposures well above threshold for (a) water ablation, and (b) ablation of
mechanically strong tissues such as, for example, skin.
The high volumetric energy density in the target material produced in Q-switched
Er:YAG laser ablation results in a very large initial expansion velocity of the ablation
plume that drives the emission of an equally fast shock wave. Shock front velocities
are usually on the order of 2000–4000 m/s for both IR and UV wavelengths [37,
79–81], i. e., they reach values up to Mach 12. Measured shock wave and plume
velocities correlate with the water content of the samples because lower water content
results in smaller volumetric energy densities and less vigorous ablation. By contrast,
the velocity of particulate fragments is larger for mechanically strong tissues (up
to 1700 m/s for skin) than the velocity of droplets ejected in water ablation (up
to 280 m/s) [37]. This is because the temperature required for thermal dissociation
of the tissue matrix into volatile products is higher than the temperature required
for complete vaporization of water. Therefore, tissue fragments become visible early
in the ablation process when the ablation front has reached a depth at which the
temperature is below the level required for thermolysis. At this time, the pressure
driving the ejection is still very high. By contrast, droplet ejection starts only once
the temperature at the ablation front has reached a lower level corresponding to the
onset of a phase explosion. This results in smaller velocities for the droplet ejection.
The ablation plume exhibits complex dynamics. The plume expansion is nearly
spherical during the initial phases of expansion but begins to propagate preferentially
in the forward direction after 1–2 µs. For small radiant exposures, the interaction
of the piston-like forward movement with the ambient air at rest results in ring
vortex formation [37,82]. For larger radiant exposures, a region of high density and
pressure is created at the contact front between plume and surrounding air. The
molecules and molecular clusters propagating with the plume possess a non-zero
average velocity. When they collide with air molecules that are, on average, at rest,
they are partially reflected back into the plume. As visible in Fig. 12, this reflection
leads to the formation of an internal shock wave that begins to propagate toward
the target surface when the rarefaction from the plume expansion has reduced the