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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. Due to thermal effects, displacements occur in the medium which medium. These displacements  can then  propagate as elastic waves. Elastic waves can be are  separated in two components in a bulk: compression waves, corresponding to a curl-free propagation; and shear waves, corresponding to a divergence-free propagation \cite{aki2002quantitative}.This phenomenon has been notably observed in metals.  Measures of the compression and shear waves is notably used as a method of inspection to reveal potential cracks in the solid. a solid such as a metal.  In a medical context, induction of compression waves by laser has been studied for the last ten years, with the development of photoacoustic imaging \cite{Xu_2006}. In this technique, a laser beam is absorbed by the tissue, which induces by thermal expansion compression waves, which waves. These waves  are themselves acquired detected  by acoustic transducers. Time transducers, and time  of flight measurements allows then allow  to find the source of the waves and thus, to map optical absorption of the tissues \cite{22442475}. As the optical absorption coefficient of the tissue depends on the optical wavelength, different structures can be observed by tuning properly the laser wavelength. For example, oxygenated and de-oxygenated haemoglobin can be discriminated in this way. Noninvasive imaging of hemoglobin concentration and oxygenation in the rat brain using high-resolution photoacoustic tomography\cite{16674205}. The frequency of the elastic waves used in photoacoustic imaging are typically of a few megahertz. At this frequency, shear waves are quickly attenuated, typically over a few microns in soft tissues, so only compression waves can propagate over a few centimeters. way \cite{16674205}.  We hypothesized in this study that a laser beam The elastic waves used  in photoacoustic imaging are typically of  a soft tissue can nevertheless induce few megahertz. At this frequency,  shear waves are quickly attenuated, typically over a few microns  in addition to soft tissues, so only  compression waves. waves can propagate over a few centimeters.  Shear waves have drawn an increasing interest in medical imaging, with the development for the last two decades of shear wave elastography techniques \cite{muthupillai1995magnetic}, \cite{sandrin2002shear}. As its names indicates, this term covers the techniques used to measure or map the elastic properties of biological tissues using shear wave propagation. The shear modulus, directly proportional to Young's modulus in soft tissues, varies indeed of several orders of magnitude in human body and potentially offers an excellent contrast. As a shear wave propagates in an organ at a speed proportional to the square root of the shear modulus, measuring its speed throughout the organ allows to compute the shear modulus of the tissue. Shear waves can be induced by an external vibrator \cite{muthupillai1995magnetic}, a focused acoustic beam \cite{sarvazyan1998shear}, \cite{11937286}, the Lorentz force\cite{grasland2014elastoEMarticle}, or natural body displacements \cite{gallot2011passive}. Shear wave elastography techniques have been successfully applied for the detection of various pathologies in organs such as the liver \cite{sandrin2003transient}, the breast \cite{goddi2012breast}, \cite{sinkus2005viscoelastic}, the prostate \cite{cochlin2002elastography}, \cite{12878247}, the bladder \cite{25574440} and the eye cornea \cite{tanter2009high}, \cite{22627517}. In this study, we show that shear waves can be induced in soft tissues by a laser beam, with beam. We also propose  a model of for  the underlying physical phenomenon. We also finally  applied the technique in a biological tissue to evaluate its potential application in shear wave elastography. In the first experiment, illustrated by Figure \ref{Figure1}, we used a laser beam emitted by a Nd:YAG laser (EverGreen 200, Quantel, Les Ulis, France), which produced a Q-switched pulse of energy $E$ = 200 mJ at a central wavelength of 532 nm during 10 20  ns in a beam of section $S$=20 mm$^2$. We defined Z as the laser beam axis, and the laser beam impact on the medium is the origin of coordinates (0,0,0). The laser beam was absorbed in a 4x8x8 cm$^3$ tissue-mimicking phantom made of water and of 5\% polyvinyl alcohol, 1 \% black graphite powder and 1\% salt. A freezing/thawing cycle was applied to stiffen the material to a value of 15$\pm$5 kPa \cite{17375819}. The laser is absorbed in the medium with an exponential decay of the optical intensity $I(z)=I_0 \exp(- \gamma z)$ along medium depth $z$, where $I_0=\frac{1}{S}\frac{d E}{dt}$ is  the incident intensity distribution at the surface (the reflection on the black mat medium being neglected) and $\gamma$ the absorption coefficient of the medium. !!!We measured the fraction of light which go through different thicknesses of the medium with a laser beam power measurement device (): it indicated that $\gamma \approx$ ??? m$^{-1}$ in our sample,!!! meaning that most of the radiation is absorbed in the first hundred of micrometers.  %Even if the sample is mainlyeFor low concentration medium, $\gamma$ is hard to calculate in our case, as the sample is composed of different materials, but the graphite, even in low concentration, absorbate much more than other components, so we can approximate $\gamma \approx \gamma_{graphite}$. For graphite particles of 1.85 $\mu$m at a concentration of 10 g.L$^{-1}$, the order of magnitude of $\gamma$ is 10$^4$ m$^{-1}$, meaning that most of the radiation is absorbed in the first hundred of micrometers of sample.  This is quite higher than metals where the radiation is absorbed within a few nanometres. The absorption of the laser beam by the medium gives then rise to an absorbed optical energy $q = \gamma I$. Assuming that all the optical energy is converted to heat, a local increase of temperature appears. Temperature distribution $T$ in absence of melting convection and of phase transition  can be computed using heat equation: \begin{equation}  \frac{\partial T}{\partial t} = \kappa \nabla ^2 T + \frac{q}{\rho C}  %\label{eq:eqChaleur}  \end{equation}  where $\rho$ is the density, $C$ the heat capacity and $\kappa$ the thermal diffusivity.If melting temperature is reached, a part of the absorbed heat will melt the solid without increase of temperature.  The thermal diffusion path, equal to $\sqrt{4\kappa \tau}$, with $\tau$ = 20 ns the laser emission duration and $\kappa$ = 1.43 10$^{-7}$ m$^2$.s$^{-1}$ for water \cite{Blumm_2003}, is approximately equal here to 0.1 $\mu$m. As $\gamma^{-1} \gg \sqrt{4\kappa t}$, propagation of heat is negligible during laser emission, and term $k \nabla ^2 T$ so that equation \ref{eq:eqChaleur}  can be neglected during this time. simplified as $\frac{\partial T}{\partial t} = \frac{q}{\rho C}$.  The local increase of temperature can lead to two main effects creating elastic waves: (1) thermoelastic expansion and (2) ablation of medium.  In the thermoelastic regime, a local dilatation of the medium occurs. In Adopting  a unidimensional analysis along $z$ (as the depth of absorption is small compared to the beam diameter), the stress $\sigma_{zz}$ can be written \cite{scruby1990laser}: \begin{equation}  \sigma_{zz} = (\lambda + \mu) \frac{\partial u_z}{\partial z} - (3\lambda + 2\mu) \frac{\alpha E}{\rho C S \zeta}  \label{eq:stressUnidim} 
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