The development of innovative photonic devices and metamaterials with tailor-made functionalities depends critically on our capability to characterize them and understand the underlying light-matter interactions. Thus, imaging all components of the electromagnetic light field with nanoscale resolution is of paramount importance in this area. Nowadays, the electric and the vertical magnetic field components of light can be measured with sub-wavelength resolution. This is achieved by scanning the sample surface with specific probes in a method known as scanning near-field optical microscopy (SNOM). However, within this toolbox, an unambiguous way of visualizing the horizontal magnetic field component has been missing.
We have answered this challenge by demonstrating experimentally that a hollow-pyramid circular aperture probe SNOM can directly image the horizontal magnetic field of light in simple plasmonic antennas – rod, disk and ring. These results are also confirmed by numerical simulations, showing that the probe can be approximated, in the first order, by a magnetic point-dipole source. This approximation substantially reduces the simulation time and complexity and facilitates the otherwise controversial interpretation of near-field images. Further, we use the validated technique to study complex plasmonic antennas and to explore new opportunities for their engineering and characterization. The applicability of this methodology is currently being extended beyond plasmonics structures.
Thus, the presented hollow-pyramid circular aperture based SNOM approach complements the existing techniques for imaging the different electromagnetic field components, by providing an opportunity to explore the tangential magnetic field of light with sub-wavelength resolution.