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(azi)|2 / |E(tot)|2 ≈55% ± 5% for λ0 = 1550 nm), respectively subwavelength confined (FWHM ≈450 nm ≈λ0/3.5) magnetic fields, are generated for a certain tip aperture diameter (d = 1.4 μm). 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.