Vector spherical harmonic expansion of a field

Question

For a scalar function defined on the surface of a unit sphere, it is easy to expand it in terms of spherical harmonics, with the algorithm clearly defined, eg on wikipedia. \ However, to investigate, say, shear wave velocity variations in 3D, apart from the velocity values in each direction on the unit sphere, I'd also need another UNIT vector specifying the polarisation of the wave at each direction. Scalar SH clearly are not able to do that, I've seen the concept of vector spherical harmonics which I think would enable the description I wanted, but the introductory materials are scarce to find. Could anyone please point out a way to incorporate the polarisation vector into the SH expansion for me, thanks very much!