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.

Pronounced separation (750 nm) between two individual fluorescence spots in a novel super-resolution microscopy based on a two-color up-conversion fluorescence depletion technique has been investigated. This microscopy has the potential to achieve a spatial resolution (<300nm) of 1/2 the diffraction limit.

The vectorial Debye integral shows that tightly focused Laguerre-Gaussian (LG) beams have a residual intensity at the focal point for linear polarization, for a topological charge of m=1 and 2. We measured the shapes of linearly and circularly polarized LG beams and found that a central intensity appeared at m=1 and 2 for linear and right-handed circular polarization, however, it is completely canceled for left-handed circular polarization. In general, when the orbital angular momentum of the LG beam is parallel to the spin angular momentum of the photons, zero intensity is always achieved at the focus.

To generate a doughnut-shaped beam, i.e., the first order of a Bessel beam, a spiral phase plate with 8 divided etching areas is fabricated with an etching accuracy of better than 6 nm. The etching depth of each area is designed so that the phase distribution of the laser beam passing through has a phase difference of π at a symmetric position with respect to the optical axis. Using a laser beam with a wavefront aberration of 1/10λ, the phase distribution of the beam passing through the plate is measured by a Shack Hartman wavefront sensor. It has been found that the beam has a spiral phase change of 2π along the optical axis. The focused beam has a circular doughnut pattern, as predicted by a theoretical calculation, and we succeeded to generate the ideal first-order of a Bessel beam.

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