Dispersion of plate acoustic waves in periodically poled lithium niobate.

Abstract

The phase and group velocity dispersion of the plate acoustic waves (PAWs) in ZX-cut periodically poled lithium niobate (PPLN) wafer is calculated theoretically and verified experimentally. The dispersion curves of eight lowest PAW modes are calculated with the help of the finite element method. The PAWs in PPLN have some peculiarities in comparison with the case of a single crystal waveguide. First, some PAW modes may reflect out of the multidomain structure, and so a band gap may occur in the PAW spectrum. For example, a zero antisymmetric mode A0 has 0.2-MHz-wide band gap at frequency where its wavelength is equal to the period of a domain pattern. Second effect includes a variation of a cutoff frequency of PAW. The experimental dispersion curves are in agreement with calculations. The phononic band gap of A0 mode and the cutoff frequency shift of first antisymmetric mode A1 are confirmed by our experiments. The group velocity is calculated from the experimental phase-velocity dispersion curves. The calculated and measured group velocities are consistent. It is shown that the group velocity of A0 mode approaches zero within the band gap.