Universal phase relation between longitudinal and transverse fields   observed in focused terahertz beams
  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. 
S. Winnerl, R. Hubrich, M. Mittendorff, H. Schneider, M. Helm
physics.optics