2017, 66 (11): 110201.
The sub-diffraction-limit spatially structured light patterns have attracted more and more attention for their important applications in many frontier scientific fields. The present paper aims at developing sub-diffraction-limit spatially structured beam patterns which might have great potential to improve the light performance in fields such as super resolution imagery, optical tweezer, micro/nano lithography, etc. Here, a variety of spatially structured beam patterns are obtained by the phase modulation of polarized beams and studied in detail experimentally and numerically. Firstly, a new kind of phase plate, which combines the merits of circular and vortex 2 phase plates, is proposed based on the wave front design; it is composed of two spiral-shaped phase plates with their phases changing from 0 to 2 and - to , respectively. Later, the phase plate is applied to the circularly polarized Gaussian beam modulation in a high NA system. By combining a self-made circular with a commercial vortex 2 phase plate, the designed new phase plate is implemented in the experiment. The morphology of the spatially structured light pattern, which is generated on the focal plane, is observed by a CCD camera in the experiment. The beam pattern presents a donut shape on the focal plane, while the dimension of the donut-shaped pattern becomes smaller as the imaging plane axially deviates from the focal plane. It is found that the beam patterns captured in experiment highly consist with the numerical simulation results carried out by the vectorial diffraction integral theory. It can be deduced that the spatially structured beam is capillary-shaped. Meanwhile, at the two ends of the capillary-shaped beam, the inner diameter is smaller than the diffraction limitation. Furthermore, the structured beam pattern presents a spatial voxel distribution with center and axis symmetry. Finally, the characteristics of the spatially structured beam patterns, which are generated by modulating circular, linear, radial and azimuthal polarized beams with the new designed phase plate, are analyzed and discussed in detail. It is found that for circular, linear, radial and azimuthal polarization, the full widths at half maximum (FWHMs) of the minimum dark spots in the horizontal direction are 0.31, 0.32, 0.24 and 0.36, respectively. On the optical axis, the FWHMs of the dark spots created by linearly, radially and azimuthally polarized light, are 0.8, 0.78 and 0.76 , respectively, and no axial intensity is found with circularly polarized beam incidence.