The present document relates to a method for focussing acoustic waves on an interface between two layers of a multilayer structure, the method comprising : - providing laser means able to emit a laser beam toward an exterior surface of the multilayer with a view to producing a longitudinal wave from the centre of a laser-beam spot projected onto the exterior surface and a transverse wave from the periphery of said spot, - determining the disance between said interface and the exterior surface, - determining one or more propagation velocities of the longitudinal wave and one or more propagation velocities of the transverse wave in the one or more layers passed through by said waves to reach said interface, - determining a radius (r) of the laser-beam spot to depending on the one or more propagation velocities and on said distance so that the time taken by the longitudinal wave to reach said interface is equal to three times the time taken by the transverse wave to reach said interface.