Minimized spot of annular radially polarized focusing beam

TLDR

An apodization function accounting for diffraction effects was introduced, yielding good agreement with experimental data. Experimentally, a radially polarized annular beam produced a tight focal spot of 0.0711 λ² at an annular factor of 0.91, but further aperture reduction did not sharpen the focus, indicating diffraction effects must be considered for small beam structures.

Abstract

We have experimentally demonstrated the measurement of a tighter focal spot generated by a radially polarized narrow-width annular beam with the double-knife-edge method. The reconstructed spot profiles indicate that sharper focus cannot be achieved by shrinking the annular aperture further. The smallest focal spot (0.0711λ(2)) is obtained in experiment with an annular factor of 0.91. An apodization function has been introduced with the consideration of the diffraction effect, which achieves good agreement with the experimental data. Our result shows that the diffraction effect should be considered with small topography structures of the incident beam.