Conditions for tighter focusing and higher focal depth of radially polarized vector beam

The radially polarized vector beam has attracted much attention recently and was also used to obtain a smaller focal spot. In this paper, highly focusing properties of radially polarized vector beam are investigated by comparing them with those of linearly polarized beam. A condition was found for tighter focusing of radially polarized vector beam. The focal spot of radially polarized vector beam is not always smaller than that of linearly polarized beam. Even if only a longitudinal field component is considered, in fact, the condition for tighter focusing of radially polarized vector beam is very complicated. Therefore, more attention should be paid to the smaller focal spot generation by means of radially polarized vector beam in practical use. In addition, the focal depth of radially polarized beam decreases on increasing numerical aperture under condition of small radius ratio, and increases on increasing radius ratio. The focal depth difference between these two kinds of beams shrinks upon increasing radius ratio and numerical aperture.