Analogs of Brewster's angles for surface acoustic waves: the dependence on the strip shape

The analogs of Brewster's angles for surface acoustic waves (SAW) were found 30 years ago. Considering the reflection of classical Rayleigh waves in an isotropic half-space at oblique incidence by long topographic irregularities of small thickness, such as projections and grooves, it was found by a perturbation method that, independently from the shape of the irregularities, the reflection coefficient is equal to zero for some angle of incidence. The problem was never treated more accurately than by the first order perturbation method using the thickness/wavelength ratio as a small perturbation parameter. In this work, the impedance method is used for a correct investigation of the problem in the case of periodic projections. A strong dependence of the angle on the thickness/wavelength ratio is observed, if this parameter is more than 0.03. Moreover, a second such angle appears and both angles become closer and disappear for thick projections. The practically important cases of aluminum strips on piezoelectric STX-quartz and on 128 degrees YX-LiNbO3 crystals are also considered.