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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. These waves are detected by acoustic transducers, and time of flight measurements 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 \cite{16674205}.  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. Shear waves have drawn an increasing interest in medical imaging, with the development of shear wave elastography techniques for the last two decades \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 between different tissues  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. We also propose a model for the underlying physical phenomenon. physics.  We 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 Q-switch Nd:YAG laser (EverGreen 200, Quantel, Les Ulis, France), which produced a pulse of energy $E$ = 200 mJ at a central wavelength of 532 nm during 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. 
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