d31 mode and d33 mode of piezoelectric materials is decided by the strain direction and the polarization direction.But the materials have natural polarization direction and the external electric field also affects the polarization direction.For example, a c-axis polarization AlN with IDTs electrode, there will be a SAW ,but the external field is in-plane ,then it is a d31 mode or d33 mode? Which kind of polarization direction dominates?