On the optimum form of an aperture for a confinement of the optically excited electric near field.

A triangular nanoaperture in an aluminium film was used previously as a probe in a scanning near-field optical microscope to image single fluorescent molecules with an optical resolution down to 30 nm. The high-resolution capability of the triangular aperture probe is because of a highly confined spot of the electric near field which emerges at an edge of the aperture, when the incident light is polarized perpendicular to this edge. Previous numerical calculations of the near-field distribution of a triangular aperture in a planar metal film using the field susceptibility technique yielded a nearly quantitative agreement with the experimental results. Using the same numerical technique we now explored the possibility for a further confinement of the electric near field and an increase in its intensity by modifications of the form of a triangular aperture. By introducing a kink on an edge pointing into the aperture, an arrow-shaped aperture is formed with one convex and three concave metal corners. It turns out that this form leads to a substantial further confinement of the near-field intensity at the convex corner. By extending the wings of this arrow-shaped aperture a further 5-fold increase of the intensity can be obtained without a deterioration of the confined spot.