Investigation of polarization effects for high-numerical-aperture first-order Laguerre-Gaussian beams by 2D scanning with a single fluorescent microbead.

The focal intensity distribution of strongly focused (NA=0.9) first order Laguerre-Gaussian doughnut beams is investigated experimentally for three different polarizations: linear, and left-handed circular and right-handed circular. The investigations are done by 2-dimensional scanning the focal plane with of a 100nm diameter fluorescent microbead, and measuring the fluorescence signal. The results are shown to be in excellent agreement with theoretical predictions, and demonstrate the superiority of one of the circular polarizations to achieve a sharp dark central spot.