Abstract

Based on the radial finite-difference time domain method we numerically show that the illumination of a conical glass microaxicon of base radius $7\mu \text{m}$ and height $6\mu \text{m}$ (numerical aperture $\text{NA}=0.6$) by a radially polarized annular ${\text{R-TEM}}_{01}$ laser mode of wavelength $\lambda =1\mu \text{m}$ produces, 20 nm apart from the cone apex, a sharp focus of transverse diameter $\text{FWHM}=0.30\lambda$ and longitudinal (axial) width ${\text{FWHM}}_z=0.08\lambda$. The focal spot area at half-maximum intensity is $\text{HMA}=0.071{\lambda }^2$. For comparison the focus diameter reported here is 1.7 times smaller than the diameter of the minimal diffraction spot (Airy disk, $\text{NA}=1$) of $\text{FWHM}=0.51\lambda$, with its area being 2.87 times smaller than that of the Airy disk $\text{HMA}=0.204{\lambda }^2$. Also it is smaller than the diameter of a focal spot formed by a lens with $\text{NA}=0.6$ in an immersion medium $n=1.5$, $\text{FWHM}=0.51\lambda /n\text{\hspace{0.17em} NA}=0.55\lambda$, smaller than the diffraction limit in the medium ($n=1.5$, $\text{NA}=1$) $\text{FWHM}=0.51\lambda /n=0.33\lambda$ and smaller than the focusing limit for the Bessel beam $\left(\text{NA}=1\right)$ $\text{FWHM}=0.36\lambda$.

© 2010 Optical Society of America

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