  It has been known for a long time that light carries both linear and angular
momenta parallel to the direction of propagation. However, only recently it has
been pointed out that beams of light, under certain conditions, may exhibit a
transverse spin angular momentum perpendicular to the propagation direction.
When this happens, the electric field transported by the light rotates around
an axis transverse to the beam path. Such kind of fields, although deceptively
elusive, are almost ubiquitous in optics as they manifests in strongly focused
beams, plasmonic fields and evanescent waves. In this work we present a general
formalism describing all these phenomena. In particular, we demonstrate how to
mathematically generate a wave field possessing transverse spin angular
momentum density, from any arbitrarily given scalar wave field, either
propagating and evanescent.
