“…The generation of surface and guided waves is favored when the shape of the pump laser spot on the sample surface is not a disc but made instead of periodically spaced optical lines, thus concentrating most of the input energy into controlled phonon wave vectors along one direction on the surface. Suitable transient gratings can be obtained by shaping the pump pulses with optical masks, creating two-beam interference on the surface, or by using electrically-addressed spatial light modulators [59] , [60] . This technique was used to characterize thin liquid and solid films and it could also be used to generate bulk shear waves [61] .…”