Wave modes of different polarization exhibit unique wave characteristics in media. Mode polarization has been selectively controlled for different applications (e.g. optical devices, non-destructive inspections, and medical imaging). To control the polarization or convert a polarization to another, different interfacial layers have been considered. Due to multiple reflection and transmission at the interface, the resulting wave physics is very complicated. Unlike electromagnetic waves, mechanical waves intrinsically possess two different polarizations (i.e. longitudinal and transverse modes), and the modes could even be entangled in anisotropic media. In this study, we theoretically and numerically demonstrated the mode-conversion physics through a monolayer slab in an elastic medium, and the resonance nature based on the multiple internal reflections of the two coupled eigenmodes will be presented.