Focused Azimuthally Polarized Vector Beam and Spatial Magnetic Resolution below the Diffraction Limit

Abstract: An azimuthally electric-polarized vector beam (APB), with a polarization vortex, has a salient feature that it contains a magnetic-dominant region within which electric field ideally has a null while longitudinal magnetic field is maximum. Fresnel diffraction theory and plane-wave spectral (PWS) calculations are applied to quantify field features of such a beam upon focusing through a lens. The diffraction-limited full width at half maximum (FWHM) of the beam's longitudinal magnetic field intensity profile and complementary FWHM (CFWHM) of the beam's annular-shaped total electric field intensity profile are examined at the lens's focal plane as a function of the lens's paraxial focal distance. Then, we place a subwavelength dense dielectric Mie scatterer in the minimum-waist plane of a self-standing converging APB and demonstrate for the first time that a very high resolution magnetic field at optical frequency is achieved with total magnetic field FWHM of 0.23{\lambda} (i.e., magnetic field spot area of 0.04{\lambda}^2) within a magnetic-dominant region. The theory shown here is valuable for development of optical microscopy and spectroscopy systems based on magnetic dipolar transitions which are in general much weaker than their electric counterparts.