A method for generating a three-dimensional dark spot using a radially polarized beam

A novel method is proposed for generating a three-dimensional dark spot surrounded uniformly and completely by light in all three dimensions. The superposition of a radially polarized beam with a circular p phase plate and the same radially polarized beam with a quadrant 0/pi phase plate can yield a better three-dimensional dark spot. The coherent and incoherent superpositions of these two beams are demonstrated and discussed in detail and the new effective full width at half maximum (FWHM) of a super-resolution focal spot is discussed. Compared with some former results, the dark spot yielded by this proposed method proved to be more uniformly surrounded by light. Only one type of polarized beam is applied. It facilitates the process of finding a suitable polarization for the pump beam in a super-resolution fluorescent system. Axial resolution has a significant improvement.