Generating and converting special focal field patterns in a 4Pi focusing system illuminated by azimuthally polarized vortex beam.

Based on the Richards–Wolf vector diffraction theory, the strong focusing properties of a 4Pi focusing system under the illumination of the azimuthally polarized Laguerre–Gaussian vortex beam, which is modulated by the diffracted optical elements (DOE), are investigated numerically. We mainly study the influence trend of three parameters, which are the ratio (β ), the numerical aperture ( NA ) of the objective lens and the fourth ring parameter of the DOE, on the focal field in the 4Pi focusing system. The results show that three kinds of focusing patterns (the class optical needle, the flattop optical field and the dark channel) can be obtained according to the influence trend of the three parameters on the focal field. And three focal patterns can be converted by changing three parameters. The conversion between these special focal field patterns has potential application value in the optical micro-control field. [ABSTRACT FROM AUTHOR]