Oscillations, Waves, and Interactions - GWDG

224 A. Vogel, I. Apitz, V. Venugopalan
Figure 4. Variation of the thermoelastic stress prefactor A (see Eq. (7)) with pulse duration
relative to the stress propagation time across the heated volume t ∗ m. Results are shown for
a rectangular, biexponential, and Gaussian laser pulse shape and derived from the results
of Refs. [42] and [44].
where A = 1 and the duration of the thermoelastic stress transient ta scales with the
stress propagation time and ta ≈ (4–6/µaca). When the stress transient leaves the
heated volume, the peak stress drops to 0.5 σp.
In the limit t ∗ m → ∞, where there is no stress confinement, A → 0 and the duration
of the stress transient approaches that of the laser pulse. The variation of A with t ∗ m
for different temporal laser pulse shapes is shown in Fig. 4.
While thermal expansion of a heated volume generates compressive thermoelastic
stresses, subsequent propagation of these stresses results in transients that contain
both compressive and tensile components. Tensile stresses arise from the reflection
of the compressive stress waves at a boundary to a medium with lower acoustic
impedance (tissue–air, tissue–water) or from the three-dimensional characteristics
of acoustic wave propagation from a heated tissue volume of finite size [7,43,45–48].
Tensile stress wave generation originating from acoustic impedance mismatch at a
tissue surface is shown in Fig. 5.
4 Thermodynamics and kinetics of phase transitions
All ablation processes involve the fracture of chemical bonds and lead to the removal
of single molecules, molecular fragments, and molecular clusters. Bond fracture can
also produce the formation of voids (i. e., bubbles or cracks) that facilitate the ejection
of non-decomposed material fragments upon mechanical failure of the target. Vaporization,
molecular fragmentation, and void formation can all be viewed as phase
transitions that are accomplished via photothermal, photomechanical, and/or pho-