The Bethe-Bouwkamp model provides a simple description of the optical field induced beyond a small aperture by an incident field. In particular, the model shows that an incident tangential magnetic field couples much more strongly than other field components to an electric-field detector placed directly above the aperture. Despite the fact that the Bethe-Bouwkamp result assumes an aperture size tending to zero, and an infinitely-thin perfectly-conducting aperture plane, the authors show that this principle of magnetic-field coupling via an aperture can be applied in practical measurements. The dominance of the tangential magnetic-field contribution (over the electric-field components and the normal magnetic-field) is clearly demonstrated experimentally; and by translating the aperture the tangential magnetic field can be spatially mapped.
After demonstrating the feasibility of their method using a small aperture in a gold film, the authors present a more versatile method for sampling the magnetic field. By applying focused ion beam milling, an aperture is formed in a near-field scanning optical microscopy probe, which can subsequently be used for high resolution Bethe-Bouwkamp-based magnetic-field measurement. Using this probe the authors scan both far-field light (radially- and azimuthally-polarized focused beams) and near-field light (surface plasmon polaritons). The presented data agree well with theoretical predictions and clearly demonstrate the comprehensive magnetic-field mapping capabilities offered by this new modality.
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