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Generation and subwavelength focusing of longitudinal magnetic fields in a metallized fiber tip

We demonstrate experimentally and numerically that in fiber tips as they are used in NSOMs azimuthally polarized electrical fields (|E$_{\text{azi}}$|$^2$/|E$_{\text{tot}}$|$^2$ $\approx$ 55% $\pm $ 5% for 1.4μm tip aperture diameter and λ$_0$ = 1550nm), respectively subwavelength confined (FWHM $\approx$ 450nm $\approx$ λ$_0$/3.5) magnetic fields, are generated for a certain tip aperture diameter (d = 1.4μm). We attribute the generation of this field distribution in metal-coated fiber tips to symmetry breaking in the bend and subsequent plasmonic mode filtering in the truncated conical taper.