Figure \ref{figElastoPVA} illustrates the resulting displacement amplitude maps observed along the ultrasound axis at 1.0, 1.5, 2.0, 2.5, and 3.0 ms after laser emission for two laser beam energies (10 and 200 mJ). Displacements reached an amplitude of 0.02 $\mu$m for the 10-mJ laser beam and 2.5 $\mu$m for the 200-mJ laser beam. They propagated at a velocity of 5.5$\pm$0.5 m.s$^{-1}$, which is typical for a shear wave, but far slower than the usual velocity of a compression wave (about 1500 m.s$^{-1}$ in soft tissues). Shear modulus can then be calculated using the relationship $v_s = \sqrt{\mu/\rho}$. Supposing the medium density, $\rho$, at 1000 kg.m$^{-3}$ (water density), the propagation velocity corresponds to a shear modulus of 30$\pm$5 kPa, which is in the range of the expected value for this phantom. Shear wave frequency spectrum was centered at 500 $\pm$ 50 Hz.

Careful observation reveals several differences in the propagation patterns of the two laser beam energies. At low energy, first central displacements are directed towards the outside of the medium (left arrow), and three half cycles are observed. Conversely, at high energy, first displacements are directed towards the inside of the medium (right arrow), and only two half cycles can be observed.

