Abstract
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.
Highlights
Near-field scanning optical microscopy (NSOM) aims for the experimental investigation of near-field distributions around nanostructures [1,2,3,4] mainly because resolving subwavelength features via the far field is not possible due to the Abbe diffraction limit [5,6]
We demonstrate experimentally and numerically that in fiber tips as they are used in NSOMs azimuthally polarized electrical fields (|Eazi|2 / |Etot|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)
In conclusion we have demonstrated the first fiber tip source for azimuthally polarized light (|Eazi|2 / |Etot|2 ≈55% ± 5%) based on a custom processed NSOM tip with a tip aperture diameter d = 1.4 μm < λ0 = 1.55 μm
Summary
Near-field scanning optical microscopy (NSOM) aims for the experimental investigation of near-field distributions around nanostructures [1,2,3,4] mainly because resolving subwavelength features via the far field is not possible due to the Abbe diffraction limit [5,6]. There have been first demonstrations showing that the aperture of an NSOM probe can be engineered to selectively detect certain field components, e.g. the magnetic moment of the electromagnetic field of a sample that was scanned [11,12,13] and in-plane electric fields were selectively detected with deep-subwavelength resolution [14]. It has been shown numerically that corrugated metal-coated silica tips [15,16] can act as efficient plasmonic concentrators and can in principle focus externally generated azimuthally polarized light down to a subwavelength spot (FWHM ≥ λ0/19) of a longitudinally polarized magnetic field [17]. Light leaving the fiber tip at its very end is confined to a tiny spot that is smaller than the wavelength and can be positioned with the resolution of a few nanometers This makes the presented optical setup an easy to reproduce source for azimuthally polarized coherent light. By fiber-optical integration of all relevant parts this system is kept stable over long timescales
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