Optical magnetic field mapping using a subwavelength aperture
Spotlight summary: Most optical measurements rely on the optical electric-field inducing a response in a detector material. In many simple cases, this knowledge of the electric field is sufficient to infer the properties of the optical magnetic field. Increasingly however, the magnetic-electric relationship is nontrivial and the magnetic field is of interest in its own right (particularly in the characterization of metamaterials). Unfortunately, direct measurements of the magnetic field are challenging due to the limited range of detector materials with a significant magnetic response at optical frequencies. In this paper Kihm et al. demonstrate a method for mapping the magnetic field by using an aperture to couple the magnetic field to standard electric-field detectors.
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.
Technical Division: Light–Matter Interactions
ToC Category: Physical Optics
|OCIS Codes:||(260.2110) Physical optics : Electromagnetic optics|
|(310.6628) Thin films : Subwavelength structures, nanostructures|
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