A sharp-tipped gold nanocone and the vertically aligned metallic tip of a near-field optical microscope together form a three-dimensional optical antenna with a highly controllable gap. Confocal measurements with different laser modes show the efficient axial excitation of the cones with a longitudinally polarized field. In the antenna configuration, extremely strong field enhancement up to a factor of 100 is obtained by tuning the gap between the two sharp tips down to few nanometers.
Cones composed of noble metals with dimensions of the order of 100 nm, especially gold cones, are extremely well suited for optical near-field investigations using visible light. Their plasmon resonance frequency lies within the range of visible light frequencies, and upon illumination, strong electric field enhancement can be observed in the direct vicinity of the cone tip. For this purpose, a method to fabricate well-controlled nanocones with sharp tips is required. We present a top-down process in which arrays of cones are dry-etched from a metal stack on silicon. During the ion milling step, patterned hydrogen silsesquioxane resist is used as an etch mask. The resulting cones have tunable base diameters around 150 nm and tip radii down to less than 10 nm. Their optical characteristics are investigated by means of apertureless optical near-field microscopy, in which field enhancement at the cone tip is observed.
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