Generation of sub-half-wavelength non-diffracting beams with tunable depth of focus by focusing azimuthally polarized beams

We theoretically demonstrate a non-diffracting beam with a long depth of focus and a sub-half-wavelength spot size can be generated by tight focusing of a narrow annulus of azimuthally polarized beam with a phase mask and a high-numerical-aperture parabolic mirror. In this paper, numerical verifications are implemented in the calculation of the electromagnetic fields near the focus based on the vector diffraction theory. We present the expression of the approximate relationship between the angular thickness Delta 0 and the depth of focus. We verify that the depth of focal line can be tunable by changing the value of Delta 0.