Light confinement by structured metal tips for antenna-based scanning near-field optical microscopy

Scanning near-field optical microscopy (SNOM) aims at imaging nanostructured samples with sub-wavelength resolution. Tip-enhanced SNOM utilizes the strong local electromagnetic fields near a laser illuminated sharp metal tip to probe the near-field response of the sample. To achieve a clear image contrast, however, this near-field signal needs to exceed the background contribution resulting from simultaneous far-field excitation and far-field detection. For the resolution of tip-enhanced SNOM, the exact localization of the light emission plays a major role. In this work, the nearfield- to-far-field-ratio and the confinement of surface plasmon polaritons is improved for electrochemically etched conical gold tips by structuring them using multiple three-dimensional nanopatterning with a focused ion beam (FIB). For the first time, surface plasmon Bragg reflectors were fabricated all around the tip in a well-defined distance to the tip apex to mimic finite length antenna structures for which more efficient light confinement is expected. The design of the structures, the fabrication strategy, and the characterization of the resulting tips by scanning electron and optical microscopy is discussed. Photoluminescence spectra recorded before and after FIB modification indicate an increase of the light confinement of 60 %.