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The dispersion curves of four lowest plate acoustic waves (PAWs) in the ZX cut of a periodically poled lithium niobate (PPLN) wafer are computed numerically and investigated experimentally. Experiment is in agreement with simulated dispersion curves. Calculations and measurements reveal the stop-bands in the dispersion curves of the PAW modes in PPLN despite a wafer that has uniform mechanical properties. At a specific wave number, within the frequencies of the stop-bands, the acoustic modes do not propagate.

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[1] Green's function analysis of ideal hard surface circular waveguides is proposed with application to excitation and scattering problems. A decomposition of the hard surface waveguide into perfect electric conductor and perfect magnetic conductor waveguides allows the representation of dyadic Green's function in terms of transverse electric (TE) and transverse magnetic (TM) waveguide modes, respectively. In addition, a term corresponding to a transverse electromagnetic (TEM) mode is included in the representation of the Green's dyadic. The TEM term is extracted in closed form from the eigenmode expansion of TM and TE modes in the zero-cutoff limit. The electric field distribution due to an arbitrarily oriented electric dipole source is illustrated for representative TM, TE, and TEM modes propagating in the ideal hard surface circular waveguide. The derived Green's function is used in the method of moments analysis of an ideal hard surface waveguide excited by a half-wavelength strip dipole antenna. In addition, the scattering of the TEM mode by a thin strip is studied in the ideal hard surface circular waveguide.

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