Reversal phenomenon in phase velocity of surface acoustic waves on free and metallized surfaces by proton-exchange

An interesting phenomenon, in the case of the phase velocity of surface aco ustic waves propagating on a proton-exchanged substrate, was observed exper imentally. The phase velocity on proton-exchanged 128 degrees Y-X LiNbO3 wa s measured as a function of the exchange depth using parallel comb filters. Unlike the ordinary dispersive phase velocity for the free surface case, t he phase velocity for the case of the metallized surface maintained a const ant value when the exchange depth was greater than a certain value. In this region, a reversal phenomenon was observed, indicating that the phase velo city in the case of the metallized surface became faster than that in the c ase of the free surface. This phenomenon is probably caused by the mode con version of the Rayleigh wave to some other mode in the metallized surface.