Pol Grasland-Mongrain edited The_two_regimes_have.tex  over 8 years ago

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The two regimes have two opposite displacements \cite{undefined}. As previously described, the thermoelastic regime acts as local dipoles of force parallel to the surface. Thus, the medium stretches locally parallel to the surface, resulting two strong opposite displacements along Y axis (parallel to the surface) and a weak displacement outside the medium along Z axis (normal to the surface). In the ablative regime, the point force in the medium displaces locally at the surface of the medium along Z axis, resulting in weak displacements along Y axis. Initial displacement along Z axis, as seen at $t$=0.8 ms in Figure \ref{Figure2}, is negative, i.e., the displacement is inside the medium along Z axis. Moreover, the displacement along Y axis (approx. 1.5 $\mu$m) is quite smaller than the one along Z axis (approx. 3 $\mu$m). We can then conclude to The underlying phenomenon is consequently mostly  an ablative regime. This conclusion  is confirmed by the observation of a disk of paler color of the same size as the beam diameter at the impact location of the laser on the phantom, which could correspond to a vaporization or a melting of a fraction of the material. Shear wave amplitude has been shown to be linearly dependent on laser beam power in thermoelastic and ablative regime. However, at higher amplitude (about 10$^9$ W.cm$^{-2}$ in metals \cite{scruby1990laser}), a plasma occurs and lead to a non-linear amplitude of the displacement with laser energy. The dependence of the generated shear waves with laser energy has thus been investigated by increasing the laser beam energy from 10 to 200 mJ. Results are illustrated in Figure \ref{Figure4}-(A). We observe an linear relationship between shear wave amplitude and laser beam power, with a correlation coefficient of 0.9386. At an energy smaller than 50 mJ, shear wave amplitude becomes smaller than ambient noise. Due to this linear dependency, the plasma regime can be discarded.