Oscillations, Waves, and Interactions - GWDG

Dynamics of pulsed laser tissue ablation 237
Figure 12. Early phase of water ablation by a Q-switched Er:YAG laser pulse of 70 ns
duration, photographed using a novel white light Schlieren technique [76]. The irradiated
spot size was 700 µm, the radiant exposure 2.8 J/cm 2 (25× ablation threshold). All times
refer to the beginning of the laser pulse. The dynamics is characterized by vapour plume
formation, the emission of external and internal shock waves, droplet ejection, and the onset
of recoil-induced material expulsion.
To date, most investigations of the plume dynamics and acoustic phenomena associated
with pulsed laser ablation of biological tissues have been performed experimentally
by time-resolved photography, probe beam deflectometry, and spectroscopic
techniques as reviewed in Refs. [6] and [76]. Here, we focus on the description of the
plume dynamics itself rather than on the techniques of investigation. We first discuss
the dynamics for water ablation and then progress to the more complicated case
of tissue ablation where the primary ablation process and recoil-induced material
expulsion are modified by the tissue matrix.
5.1 Primary material ejection in nanosecond ablation
For Q-switched laser pulses of 50–100 ns duration, the rate of energy deposition is
extremely large. Close to threshold, the ablation process for liquids such as water
is typically characterized by non-equilibrium mass transfer [52] at the target surface
followed by a phase explosion of the superficial liquid layer [37]. However, when pulse
energies well above the ablation threshold are used, large volumetric energy densities
are produced in the target material that result in an ablation process characterized by
more vigorous types of phase transitions. To illustrate this, Fig. 12 shows the sequence