P1L-1 Leaky Wave Cuts of LiNbO3 and LiTaO3 with Minimized Propagation Loss Due to Thin Layer

It is well known that the propagation loss in leaky wave cuts on LiTaO₃ and LiNbO₃ depends on the geometry of Al electrodes. According to K. Hashimoto et al. [1997] the widely used cut of 36deg-LiTaO₃ optimal for zero thickness of metal fingers was replaced by 42degLiTaO₃ for Al electrodes with h/lambda=8% . Here we demonstrate theoretically that by introducing thin film (h_diel/lambda = 1%to 3%) sub-layer of hard material such as TiO₂ many combinations of cuts and Al electrode thickness can be optimised [Plessky, et al., 2003] in terms of minimization of propagation loss resulting from leakage of SAW energy into bulk by slow shear waves. High dielectric permittivity of mentioned materials is necessary to minimize the loss of coupling caused by separation of electrodes from the piezoelectric. For example, with sub-layer of 1.5% of TiO₂ classic 36deg cut of LiTaO₃ , according to this simulations, will show minimal "leaky" loss with 8% thick Al electrodes. Moreover, rarely used cut of 41deg-LiNbO ₃, having substantial "leaky" losses but extremely strong coupling, can be transformed into zero leaky loss substrate with 2.5% thick TiO₂ layer and 7.5% Al electrodes on top of it. The proposed method allows to get low propagation loss on substrate with desired coupling coefficient, which makes the design of low loss filters more flexible