  We directly observe longitudinal electromagnetic fields in focused freely
propagating terahertz beams of radial and linear polarization. In accordance
with theory, the longitudinal fields are phase shifted by a value of pi/2 with
respect to the transverse field components. This behavior is found for all
frequency components of single cycle THz radiation pulses. Additionally we show
that the longitudinal field of a radially polarized THz beam has a smaller spot
size as compared to the transverse field of a linearly polarized beam, that is
focused under the same conditions.
