Observation of Nonlinear Harmonic Generation of Bulk Modes in SAW Devices

Nonlinear generation of harmonics is measured on the wafer level on temperature compensated surface-acoustic-wave resonators on lithium niobate. The second-harmonic response features strong peaks looking like multiple resonant modes. The product of the frequency difference between the successive peaks to the substrate thickness is found to remain almost constant. The results also show that the level of the harmonic signal strongly depends on the roughness of the back side of the substrate. Due to these findings, it was assumed that the signal is due to nonlinear generation of a bulk mode. This assumption is confirmed experimentally by measuring the second-harmonic wave field on the back side of the substrate with an interferometer. Possible nonlinear mechanisms explaining these results are discussed.