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\email{[email protected]}  \keywords{shear wave, laser, soft solid, elastography}  \maketitle  When a laser beam of sufficient energy is incident on a medium, the absorption of the electromagnetic radiation leads to an increase of the local temperature. There is consequently a local dilatation, and the resulting displacement can propagate as elastic waves. Elastic waves can be separated in two components in a bulk: compression waves, corresponding to a curl-free propagation; and shear waves, corresponding to a divergence-free propagation. This phenomenon has been notably observed in metals. Measuring the compression and shear waves can be used as a method of inspection to reveal potential cracks in the solid. 
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The laser beam was emitted by a Nd:YAG laser (EverGreen 200, Quantel, Les Ulis, France), which produced a 200 mJ, 5 mm in diameter Q-switched pulse at a central wavelength of 532 nm during 10 ns. The absorption of the laser beam by the medium led to a local increase of temperature. The resulting temperature distribution equation in a semi-infinite space with a boundary plane at $z=0$ is given by $\nabla^2 T - \frac{1}{\kappa} \frac{\partial T}{\partial t} = - \frac{A}{K}$, where $T$ is the temperature distribution, $A$ the heat produced per unit volume per unit time and $K$ and $\kappa$ are respectively the thermal conductivity and diffusivity \cite{ready2012effects}.  However, in a non-metallic solids, the absorption coefficient $\gamma$ is relatively small, so that the radiation is able to penetrate into the bulk of the material - contrary to metal where all the radiation is absorbed within a few nanometres. This parameter needs to be compared to the thermal diffusion path, given by $\sqrt(4 \kappa t)$. $\kappa$ is approximately equal to 10${^6}$ m$^2$.s$^{-1}$ for water, the main component of biological tissues; for a 10 ns laser pulse, the thermal diffusion path is then equal to 0.01 to 0.1 $\micro$m. $\micro$ m.  $\gamma^{-1}$ of water is equal to 0.1 m, which is a million times higher; even for melanin and haemoglobin, highly absorbing at 532 nm, $\gamma^{-1}$ is respectively equal to 10 and 100 $\micro$m, $\micro$ m,  far higher than the thermal diffusion path. The thermal conductivity effects are consequently negligible, and increase of temperature lies in laser absorption zone. The local increase of temperature can lead to two main effects creating elastic waves: (1) Thermoelastic expansion and (2) Ablation of medium. In metals, transition from first to second regime occurs approximately about 10$^7$ W.cm$^{-2}$. This is equal to the energy of the laser we used, so the predominant regime in our experiment cannot be determined yet. 
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In both cases, the absorption of the laser by the phantom leads to a local displacement which can propagate as elastic wave in the medium. To observe the elastic wave, the medium was scanned with a 5 MHz ultrasonic probe made of 128 elements and a Verasonics scanner (Verasonics V-1, Redmond, WA, USA). The probe was used in ultrafast mode \cite{bercoff2004supersonic}, acquiring 1000 ultrasound frames per second. Due to the presence of graphite particles, the medium presented a speckle pattern on the ultrasound image. Tracking the speckle spots with an optical flow technique (Lucas-Kanade method) allowed to compute one component of the displacement in the medium (``Z-displacement'' or ``Y-displacement''). The laser beam was triggered 10 ms after the beginning of the ultrasound acquisition.