  The use of photonic structures in the terahertz (THz) spectral region may
enable the essential characteristics of confinement, modal control, and
electric field shielding for very high gradient accelerators based on
wakefields in dielectrics. We report here an experimental investigation of THz
wakefield modes in a 3D photonic woodpile structure. Selective control in
exciting or suppressing of wakefield modes with non-zero transverse wave vector
is demonstrated by using drive beams of varying transverse ellipticity.
Additionally, we show that the wakefield spectrum is insensitive to the offset
position of the strongly elliptical beam. These results are consistent with
analytic theory and 3D simulations, and illustrate a key advantage of wakefield
systems with Cartesian symmetry, the suppression of transverse wakes by
elliptical beams.
