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 (broken vertical bar E-azi broken vertical bar(2) / broken vertical bar E-tot broken vertical bar(2) approximate to 55% +/- 5% for lambda(0) = 1550 nm), respectively subwavelength confined (FWHM approximate to 450 nm approximate to lambda(0)/3.5) magnetic fields, are generated for a certain tip aperture diameter (d = 1.4 mu m). 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. (C) 2014 Optical Society of America