Oscillations, Waves, and Interactions - GWDG

242 A. Vogel, I. Apitz, V. Venugopalan
Figure 15. Dark-field Schlieren images of the acoustic transients and ablation plume during
skin ablation with a 200-µs Er:YAG laser pulse (Φ = 20 J/cm 2 , spot size 2.3 mm)
photographed (a) 22.4 µs and (b) 40 µs after the onset of the laser pulse. The images show
acoustic transients arising from individual spikes in the free-running laser irradiation, and
the plume containing vapour and tissue fragments. (Reprinted from Ref. [76] with permission.
Copyright 2006 Optical Society of America).
5.2 Primary material ejection in microsecond ablation
Free-running lasers typically provide pulse durations longer than 100 µs. Thus, unlike
nanosecond ablation, plume formation and expansion occurs largely during the laser
irradiation. As a result, the ablation plume influences the energy deposition of the
laser radiation into the tissue target, and the plume dynamics is also influenced by the
interaction of the laser beam with the ejected material. Nevertheless, the succession of
a sub-ablative phase, development of a vapour plume, and material ejection is similar
as with nanosecond pulses even though it occurs on a much longer time scale [99].
However, the heating rates available from microsecond laser pulses are generally much
smaller than those available from nanosecond laser pulses of moderate to high radiant
exposures. These lower heating rates are not sufficient to generate the temperatures
necessary to dissociate ECM molecules and are only able to produce supercritical
water at very large radiant exposures.
Free-running laser emission is characterized by intensity fluctuations during the
laser pulse (“spiking” behaviour). These intensity peaks modulate the vaporization
and material ejection rates [78,100] as well as the emission of acoustic transients
generated during the ablation process [101,102]. The intensity spikes of the laser pulse
are coupled with the generation of individual transients as shown in Fig. 15.
The mechanisms leading to material ejection are the same as for nanosecond pulses:
a phase explosion for mechanically weak materials and a succession of phase explosion
and confined boiling for mechanically stronger tissues. Previously it was believed
that the generation of a phase explosion requires pulse durations in the nanosecond
range [41]. However, using time-resolved photography, Nahen and Vogel [99] demon-