Second-harmonic diffraction field in nonlinear propagation of transversely limited surface acoustic wave beams

The study of the Fresnel field of the second harmonic of a finite width surface acoustic wave, parametrically generated through the nonlinear interaction on a LiNbO3 substrate, is performed both theoretically and experimentally. Theoretically, a model has been developed which computes the acoustic field at each point as given by the sum of all the second-harmonic plane wave fronts nonlinearly generated by the interaction of any two plane wave fronts present in the fundamental wave angular spectrum. Experimentally, a phase contrast optical probing technique has been used, which permits separating the contributions of the second-harmonic field from that of the fundamental wave.Theoretical and experimental transversal amplitude profiles of the surface acoustic wave second-harmonic field are presented, relative to the case of an acoustic beam launched from a 24-wavelength-wide transducer on the free surface of a LiNbO2 sample, at the fundamental frequency of 34.5 MHz.