Super-Resolution Scanning Near-Field Optical Microscopy

Scanning near-field optical microscopy (SNOM) is a method to obtain information about the optical properties of a sample at a lateral resolution below the diffraction limit of far-field microscopy. In SNOM, a light source of a dimension which is small compared to the wavelength of light and which is held at a small distance from the sample is scanned across the surface of the sample. The modulation by the sample of the light emitted from the source is recorded as a signal. As a general rule one may say that the size of the source and the distance to the sample limit the resolution of SNOM. A radiating self-emitting point dipole may be regarded as an idealized SNOM source. With such a source the resolution of SNOM imaging is expected to be limited by the distance of this dipole to the surface of the object [1]. It is difficult to design a light-emitting SNOM probe corresponding to a dipole at a distance of less than 10 nm from the object and it is therefore difficult to conceive SNOM imaging beyond a resolution of 10 nm. There have been, however, occasional reports of near-field optical imaging at a resolution in the range of 1-10 nm [2, 3]. In SNOM-images recorded with the tetrahedral tip (T-tip) a resolution in the range of 1-10 nm was obtained reproducibly on samples consisting of small grains of silver of a size of the order of 2-10 nm embedded in a flat surface of gold [3, 4]. An example of an image is shown in Fig. 1. In a different experiment we investigated a surface-embedded latex bead projection pattern [5] consisting of a flat surface of a polymer into which gold patches of a triangular shape of a size of about 50 nm and a thickness of 20 nm were embedded [6].