Characterization of vector diffraction-free beams
  It is observed that a constant unit vector denoted by $\mathbf I$ is needed to characterize a complete orthonormal set of vector diffraction-free beams. The previously found diffraction-free beams are shown to be included as special cases. The $\mathbf I$-dependence of the longitudinal component of diffraction-free beams is also discussed. 
Ting-Ting Wang, Shuang-Yan Yang, and Chun-Fang Li
physics.optics