Existence of harmonic metal thickness mode propagation for longitudinal leaky waves

In Plessky et al. (2001), an approximate model for the propagation of longitudinal leaky waves is described. This model is based on the assumption that this wave mode is due to a coupling by the substrate between quarter-wavelength resonance modes of successive electrodes in a grating. Based on this assumption, we tried to find out whether similar behavior is possible by coupling other electrode resonance modes, like a harmonic of the original quarter-wavelength mode. It is shown that longitudinal leaky waves can exist on YZ lithium niobate not only in the vicinity of the thickness of aluminum electrodes equal to 8% of the electrical period but also for thicknesses multiplied by 3 or 5., i.e. for which the operating frequency is in the vicinity of the 3rd or 5th harmonic electrode resonance modes, respectively. This possibility was verified by FEM/BEM simulations as well as by test devices. These results, confirming the original assumption, are reported and discussed. It is also shown by using FEM/BEM simulations, that the LL-waves can exist in split-finger configurations (in an IDT with 4 electrodes per period), showing strong resonance properties. This serves to illustrate that the strong reflections and the Bragg stop band observed in standard 2 electrodes-per-wavelength IDT are not mandatory for the existence of LL-waves.