Abstract
Metal-coated dielectric tetrahedral tips (T-tip) have long been considered to be interesting structures for the confinement of light to nanoscopic dimensions, and in particular as probes for scanning near-field optical microscopy. Numerical investigations using the Finite-Difference Time-Domain (FDTD) method are used to explore the operation of a T-tip in extraction mode. A dipole source in close proximity to the apex excites the tip, revealing the field evolution in the tip, the resulting edge and face modes on the metal-coated surfaces, and the coupling from these modes into highly directional radiation into the dielectric interior of the tip. These results are the starting point for illumination-mode numerical investigations by a Volume Integral equation method, which compute the field distribution that develops in a T-tip when a Gaussian beam is incident into the tip, and which show that a highly confined electric field is produced at the apex of the tip. The process of light confinement can be considered as a superfocussing effect, because the intensity of the tightly confined light spot is significantly higher than that of the focussed yet much wider incident beam. The mechanism of superfocussing can be considered as a dimensional reduction of surface plasmon modes, where an edge plasmon is the most important link between the waveguide-modes inside the tip and the confined near field at the apex.
Highlights
Metal-coated dielectric tetrahedral tips (T-tip) have long been considered to be interesting structures for the confinement of light to nanoscopic dimensions [1,2,3,4], and in particular as probes for scanning near-field optical microscopy (SNOM) [2, 3]
Metal-coated glass fragments of a tetrahedral shape have been used to image the photonic nanopatterns related to the surface plasmon excitation of metal nanostructures down to a resolution of 30 nm [5, 6]
The wavelength is 633 nm, and the relative permittivities of the glass and the gold are given by ε1/ε0 = 2.25 and Figure 4 shows the distribution of the optical intensity in cross-sections through the T-tip in the (x, y = 0, z)-plane and the (x = 0, y, z)-plane for different conditions
Summary
Metal-coated dielectric tetrahedral tips (T-tip) have long been considered to be interesting structures for the confinement of light to nanoscopic dimensions [1,2,3,4], and in particular as probes for scanning near-field optical microscopy (SNOM) [2, 3]. A dipole source in close proximity to the apex of the tip excites the tip, revealing the field evolution in the tip, the resulting edge and face modes on the metalcoated surfaces, and the coupling from these modes into highly directional radiation into the dielectric interior of the tip. These results are the starting point for the illuminationmode numerical investigations of Sect. The optical properties of a metal-coated tetrahedral tip will be compared to the numerical results
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