Brillouin surface wave excitations

Laser ultrasonic transducers are truly non-contact devices which effectively avoid acoustic coupling problems (e.g. damping in the transducer and couplant reflection and transmission losses at the interface). Most laser ultrasonic devices have been used for excitation and detection of bulk elastic waves in point source or planar geometry, but also surface acoustic (Rayleigh or Brillouin) waves. Unlike the bulk wave regime, only one sample side is needed for excitation and detection when surface waves are used. This not only renders the measurements easier, but also avoids the need for an accurate knowledge and uniformity of the sample thickness. In addition, the excitation laser can be focused using cylindrical lenses in order to obtain an excitation line. 

Brillouin light scattering [73-75]. The electrical field of the photons couples to the spin wave via spin-orbit interachon in this case. The possible wave vector transfer paraUel to the surface is, however, limited by the wave vector of the incident hght, which is of the order of 10 A. Thus, in this method, only modes with a very long wavelength compared to the lattice constant are excited. 

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