Modeling the sharp focus of a radially polarized laser mode using a conical and a binary microaxicon

Abstract

Based on the radial finite-difference time domain method we numerically show that the illumination of a conical glass microaxicon of base radius 7 μm and height 6 μm (numerical aperture NA=0.6) by a radially polarized annular R-TEM01 laser mode of wavelength λ=1 μm produces, 20 nm apart from the cone apex, a sharp focus of transverse diameter FWHM=0.30λ and longitudinal (axial) width FWHMz=0.08λ. The focal spot area at half-maximum intensity is HMA=0.071λ2. For comparison the focus diameter reported here is 1.7 times smaller than the diameter of the minimal diffraction spot (Airy disk, NA=1) of FWHM=0.51λ, with its area being 2.87 times smaller than that of the Airy disk HMA=0.204λ2. Also it is smaller than the diameter of a focal spot formed by a lens with NA=0.6 in an immersion medium n=1.5, FWHM=0.51λ/n NA=0.55λ, smaller than the diffraction limit in the medium (n=1.5, NA=1) FWHM=0.51λ/n=0.33λ and smaller than the focusing limit for the Bessel beam (NA=1)FWHM=0.36λ.