Oscillations, Waves, and Interactions - GWDG

236 A. Vogel, I. Apitz, V. Venugopalan
At volumetric energy densities in excess of the spinodal limit at ambient pressure,
i. e., for T > 300 ◦ C, the superheated liquid is unstable and the onset of explosive
ablation need not be initiated by the tensile component of the thermoelastic stress.
Nevertheless, the thermoelastic stress transient can still contribute to material removal.
The magnitude of thermoelastic transients produced by a given temperature
rise under conditions of stress confinement is much larger than the saturation vapour
pressure resulting from the same temperature rise and for T > 1000 ◦ C may well
exceed 1 GPa. The compressive component of the thermoelastic stress wave upon
propagation will develop into a shock wave. The propagation of this shock wave into
the depth of the target along with energy dissipation at the shock front [71,72] results
in tissue heating at locations beyond those heated directly by the laser irradiation and
subsequent heat diffusion. Shock wave propagation thus serves as a form of convective
heat transfer that extends the ablation depth and increases ablation efficiency [73].
Experimental evidence for shock wave induced phase changes of water after laserinduced
breakdown was provided by Vogel and Noack [74]. For pulsed laser surface
ablation, temperatures in the shock wave region will usually be below the spinodal
limit since a pressure jump in the neighbourhood of 5 GPa is required to heat water
from room temperature to 300 ◦ C [71]. Nevertheless, the temperature rise can result
in ablation because the tensile component of the thermoelastic stress that follows the
shock wave will catalyze an explosive boiling process as described above. Convective
heat transfer will become important for ablation only for sufficiently large volumetric
energy densities and for very high degrees of stress confinement, i. e. mainly for
ultrashort laser pulses. We conclude that regardless of the volumetric energy density,
stress confinement invariably serves to lower the ablation threshold and increase
ablation efficiency [6,42,69,73,75].
5 Ablation plume dynamics
The phase transitions described in the previous section drive the formation of a plume
consisting of material removed from the ablation site. Usually, the ablation dynamics
and plume formation is not governed by just a single type of phase transition
but by an interplay of different transitions occurring at the target surface and in its
bulk. Moreover, the type and strength of the phase transition may change during
the laser pulse depending on the volumetric energy densities reached at each target
location when the phase change occurs. The characteristics of the ablation plume
reflect the underlying ablation dynamics and its analysis provides the insight necessary
to draw conclusions about the phase transitions involved in a given ablation
event. Furthermore, the plume dynamics influence the ablation process in various
ways. The primary ejection of ablation products perpendicular to the tissue surface
induces a recoil pressure that may produce additional, secondary material expulsion
and cause collateral effects in the bulk tissue. Flow components parallel to the tissue
surface that develop at later times may result in a redeposition of ablated material.
Scattering and absorption of the incident light by the ablation plume reduce the
amount of energy deposited in the target and limit the ablation efficiency at high
radiant exposures.