Plasmons in small protrusions of an otherwise flat gold film were studied with a scanning near-field optical microscope (SNOM). Individual protrusions were selected by approaching them with another medium in a controlled way. Extremely narrow resonances in terms of scattering intensity versus distance were observed. The results open new opportunities for local plasmon studies as well as for SNOM with enhanced sensitivity and resolution, e.g., operation in a novel topographic mode (optical tunnel microscopy).

• Received 12 October 1988

DOI:https://doi.org/10.1103/PhysRevLett.62.458