Pol Grasland-Mongrain edited Introduction.tex  over 7 years ago

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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 in the local temperature. Due to thermal effects, displacements occur in the medium, which can then propagate as elastic waves. Elastic waves within a bulk can be separated into two components: compression waves, corresponding to a curl-free propagation, and shear waves, corresponding to a divergence-free propagation \cite{aki1980quantitative}. Measures of the transmission characteristics of compression and shear waves are useful for inspecting solids, such as a metal, to reveal potential cracks or defects \cite{Shan_1993}. defects.  In biological tissues, induction of compression waves by laser has been studied with the development of photoacoustic imaging \cite{Xu_2006}. Elastic waves used in photoacoustic imaging are typically of a few megahertz; at this frequency, shear waves are quickly attenuated in soft tissues, typically over a few microns, and only compression waves can propagate over a few centimeters. While the induction of surface acoustic waves by laser in soft tissues was recently demonstrated by Li et al. \cite{Li_2012}, \cite{Li_2014}, a similar phenomenon with shear waves in bulk medium has never been described. This is of great interest for the shear wave elastography technique. As its name indicates, shear wave elastography comprises the techniques used to map the elastic properties of soft media using shear wave propagation \cite{Plewes_1995}, \cite{muthupillai1995magnetic}, \cite{Catheline_1999}. These techniques typically use low frequency (50-500 Hz) shear waves so that their propagation can be observed over a few centimeters. The shear waves are currently generated using either an external shaker or a focused acoustic wave. However, alternative shear wave generation methods have drawn important attention recently. For example, it has been demonstrated that one can use natural motion of the medium \cite{Hirsch_2012}, \cite{Zorgani_2015}, the Lorentz force \cite{Basford_2005}, \cite{Grasland_Mongrain_2014}, \cite{Grasland_Mongrain_2016} or electrolysed-induced bubbles \cite{Montalescot_2016}. Compared to these generation sources, a laser presents the advantage of being fully remote, without need of coupling gel; and of being miniaturizable at low cost using an optical fiber.