Generation of an axially super-resolved quasi-spherical focal spot using an amplitude-modulated radially polarized beam

Abstract

An axially super-resolved quasi-spherical focal spot can be generated by focusing an amplitude-modulated radially polarized beam through a high numerical aperture objective. A method based on the unique depolarization properties of a circular focus is proposed to design the amplitude modulation. The generated focal spot shows a ratio of x.y.z ̈ù 1.1.1:48 for the normalized FWHM in three dimensions, compared to that of x.y.z ̈ù 1.0:74.1:72 under linear polarization (in the x direction) illumination. Moreover, the focusable light efficiency of the designed amplitude-modulated beam is 65%, which is more than 3 times higher than the optimized case under linear polarization and thus make the amplitude-modulated radial polarization beam more suitable for a wide range of applications. © 2011 Optical Society of America.